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IR and Raman Spectroscopy: Principles & Spectral Interpretation
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INFRARED AND RAMAN SPECTROSCOPY PRINCIPLES AND SPECTRAL INTERPRETATION PETER LARKIN AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Elsevier 225 Wyman Street, Waltham, MA 02451, USA 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands Copyright Ó 2011 Elsevier Inc. All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier. com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein Library of Congress Cataloging-in-Publication Data Larkin, Peter (Peter J.) Infrared and raman spectroscopy: principles and spectral interpretation/Peter Larkin. p. cm. ISBN: 978-0-12-386984-5 (hardback) 1. Infrared spectroscopy. 2. Raman Spectroscopy. I. Title. QD96.I5L37 2011 535’.8’42ddc22 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-386984-5 For information on all Elsevier publications visit our web site at elsevierdirect.com Printed and bound in The USA 11 12 13 14 10 9 8 7 6 5 4 3 2 1 2011008524 To my wife and family Preface IR and Raman spectroscopy have tremendous potential to solve a wide variety of complex problems. Both techniques are completely complementary providing characteristic fundamental vibrations that are extensively used for the determination and identification of molecular structure. The advent of new technologies has introduced a wide variety of options for implementing IR and Raman spectroscopy into the hands of both the specialist and the nonspecialist alike. However, the successful application of both techniques has been limited since the acquisition of high level IR and Raman interpretation skills is not widespread among potential users. The full benefit of IR and Raman spectroscopy cannot be realized without an analyst with basic knowledge of spectral interpretation. This book is a response to the recent rapid growth of the field of vibrational spectroscopy. This has resulted in a corresponding need to educate new users on the value of both IR and Raman spectral interpretation skills. To begin with, the end user must have a suitable knowledge base of the instrument and its capabilities. Furthermore, he must develop an understanding of the sampling options and limitations, available software tools, and a fundamental understanding of important characteristic group frequencies for both IR and Raman spectroscopy. A critical skill set an analyst may require to solve a wide variety of chemical questions and problems using vibrational spectroscopy is depicted in Figure 1 below. Selecting the optimal spectroscopic technique to solve complex chemical problems encountered by the analyst requires the user to develop a skill set outlined in Figure 1. A knowledge of spectral interpretation enables the user to select the technique with the most favorable selection Software knowledge Instrument capability Instrument accessories Chemometric models Spectroscopist Sampling options Spectral interpretation Method validation Method development FIGURE 1 Skills required for a successful vibrational spectroscopist. (Adapted from R.D. McDowall, Spectroscopy Application Notebook, February 2010). ix x PREFACE of characteristic group frequencies, optimize the sample options (including accessories if necessary), and use suitable software tools (both instrumental and chemometric), to provide a robust, sensitive analysis that is easily validated. In this book we provide a suitable level of information to understand instrument capabilities, sample presentation, and selection of various accessories. The main thrust of this text is to develop high level of spectral interpretation skills. A broad understanding of the bands associated with functional groups for both IR and Raman spectroscopy is the basic spectroscopy necessary to make the most of the potential and set realistic expectations for vibrational spectroscopy applications in both academic and industrial settings. A primary goal of this book has been to fully integrate the use of both IR and Raman spectroscopy as spectral interpretation tools. To this end we have integrated the discussion of IR and Raman group frequencies into different classes of organic groups. This is supplemented with paired generalized IR and Raman spectra, use of numerous tables that are discussed in text, and finally referenced to a selection of fully interpreted IR and Raman spectra. This fully integrated approach to IR and Raman interpretation enables the user to utilize the strengths of both techniques while also recognizing their weaknesses. We have attempted to provide an integrated approach to the important group frequency of both infrared and Raman spectroscopy. Graphics is used extensively to describe the basic principles of vibrational spectroscopy and the origins of group frequencies. The book includes sections on basic principles in Chapters 1 and 2; instrumentation, sampling methods, and quantitative analysis in Chapter 3; a discussion of important environmental effects in Chapter 4; and a discussion of the origin of group frequencies in Chapter 5. Chapters 4 and 5 provide the essential background to understand the origin of group frequencies in order to assign them in a spectra and to explain why group frequencies may shift. Selected problems are included at the end of some of these chapters to help highlight important points. Chapters 6 and 7 provide a highly detailed description of important characteristic group frequencies and strategies for interpretation of IR and Raman spectra. Chapter 8 is the culmination of the book and provides 110 fully interpreted paired IR and Raman spectra arranged in groups. The selected compounds are not intended to provide a comprehensive spectral library but rather to provide a significant selection of interpreted examples of functional group frequencies. This resource of interpreted IR and Raman spectra should be used to help verify proposed assignments that the user will encounter. The final chapter is comprised of the paired IR and Raman spectra of 44 different unknown spectra with a corresponding answer key. Peter Larkin Connecticut, August 2010 C H A P T E R 1 Introduction: Infrared and Raman Spectroscopy Vibrational spectroscopy includes several different techniques, the most important of which are mid-infrared (IR), near-IR, and Raman spectroscopy. Both mid-IR and Raman spectroscopy provide characteristic fundamental vibrations that are employed for the elucidation of molecular structure and are the topic of this chapter. Near-IR spectroscopy measures the broad overtone and combination bands of some of the fundamental vibrations (only the higher frequency modes) and is an excellent technique for rapid, accurate quantitation. All three techniques have various advantages and disadvantages with respect to instrumentation, sample handling, and applications. Vibrational spectroscopy is used to study a very wide range of sample types and can be carried out from a simple identification test to an in-depth, full spectrum, qualitative and quantitative analysis. Samples may be examined either in bulk or in microscopic amounts over a wide range of temperatures and physical states (e.g., gases, liquids, latexes, powders, films, fibers, or as a surface or embedded layer). Vibrational spectroscopy has a very broad range of applications and provides solutions to a host of important and challenging analytical problems. Raman and mid-IR spectroscopy are complementary techniques and usually both are required to completely measure the vibrational modes of a molecule. Although some vibrations may be active in both Raman and IR, these two forms of spectroscopy arise from different processes and different selection rules. In general, Raman spectroscopy is best at symmetric vibrations of non-polar groups while IR spectroscopy is best at the asymmetric vibrations of polar groups. Table 1.1 briefly summarizes some of the differences between the techniques. Infrared and Raman spectroscopy involve the study of the interaction of radiation with molecular vibrations but differs in the manner in which photon energy is transferred to the molecule by changing its vibrational state. IR spectroscopy measures transitions between molecular vibrational energy levels as a result of the absorption of mid-IR radiation. This interaction between light and matter is a resonance condition involving the electric dipolemediated transition between vibrational energy levels. Raman spectroscopy is a two-photon inelastic light-scattering event. Here, the incident photon is of much greater energy than the vibrational quantum energy, and loses part of its energy to the molecular vibration with the 1 2 1. INTRODUCTION: INFRARED AND RAMAN SPECTROSCOPY TABLE 1.1 Comparison of Raman, Mid-IR and Near-IR Spectroscopy Raman Infrared Near-IR Ease of sample preparation Very simple Variable Simple Liquids Very simple Very simple Very simple Powders Very simple Simple Simple Polymers Very simple* Simple Simple Gases Simple Very simple Simple Fingerprinting Excellent Excellent Very good Best vibrations Symmetric Asymmetric Comb/overtone Group Frequencies Excellent Excellent Fair Aqueous solutions Very good Very difficult Fair Quantitative analysis Good Good Excellent Low frequency modes Excellent Difficult No * True for FT-Raman at 1064 nm excitation. remaining energy scattered as a photon with reduced frequency. In the case of Raman spectroscopy, the interaction between light and matter is an off-resonance condition involving the Raman polarizability of the molecule. The IR and Raman vibrational bands are characterized by their frequency (energy), intensity (polar character or polarizability), and band shape (environment of bonds). Since the vibrational energy levels are unique to each molecule, the IR and Raman spectrum provide a “fingerprint” of a particular molecule. The frequencies of these molecular vibrations depend on the masses of the atoms, their geometric arrangement, and the strength of their chemical bonds. The spectra provide information on molecular structure, dynamics, and environment. Two different approaches are used for the interpretation of vibrational spectroscopy and elucidation of molecular structure. 1) Use of group theory with mathematical calculations of the forms and frequencies of the molecular vibrations. 2) Use of empirical characteristic frequencies for chemical functional groups. Many empirical group frequencies have been explained and refined using the mathematical theoretical approach (which also increases reliability). In general, many identification problems are solved using the empirical approach. Certain functional groups show characteristic vibrations in which only the atoms in that particular group are displaced. Since these vibrations are mechanically independent from the rest of the molecule, these group vibrations will have a characteristic frequency, which remains relatively unchanged regardless of what molecule the group is in. Typically, group frequency 3 3000 X=Y=Z or X≡Y N=C=O, C≡N 2000 C=O acid ester ketone amide C=C olefinic, aromatic NH2 def, R CO2 -salt, C=N P OH or SH str NH str amines amides aliphatic CH str OH str alcohols phenols =CH and aromatic CH str Intensity HISTORICAL PERSPECTIVE: IR AND RAMAN SPECTROSCOPY CH2 CH3 1500 Wavenumbers(cm–1) C O C ethers esters C OH alcohols phenols S=O P=O C F =CH aromatic C Cl C Br 1000 500 FIGURE 1.1 Regions of the fundamental vibrational spectrum with some characteristic group frequencies. analysis is used to reveal the presence and absence of various functional groups in the molecule, thereby helping to elucidate the molecular structure. The vibrational spectrum may be divided into typical regions shown in Fig. 1.1. These regions can be roughly divided as follows: • • • • • XeH stretch (str) highest frequencies (3700e2500 cm1) XhY stretch, and cumulated double bonds X¼Y¼Z asymmetric stretch (2500e2000 cm1) X¼Y stretch (2000e1500 cm1) XeH deformation (def) (1500e1000 cm1) XeY stretch (1300e600 cm1) The above represents vibrations as simple, uncoupled oscillators (with the exception of the cumulated double bonds). The actual vibrations of molecules are often more complex and as we will see later, typically involve coupled vibrations. 1. HISTORICAL PERSPECTIVE: IR AND RAMAN SPECTROSCOPY IR spectroscopy was the first structural spectroscopic technique widely used by organic chemists. In the 1930s and 1940s both IR and Raman techniques were experimentally challenging with only a few users. However, with conceptual and experimental advances, IR gradually became a more widely used technique. Important early work developing IR spectroscopy occurred in industry as well as academia. Early work using vibrating mechanical molecular models were employed to demonstrate the normal modes of vibration in various molecules.1, 2 Here the nuclei were represented by steel balls and the interatomic bonds by helical springs. A ball and spring molecular model would be suspended by long threads attached to each ball enabling studies of planar vibrations. The source of oscillation for the ball and spring model was through coupling to an eccentric variable speed motor which enabled studies of the internal vibrations of molecules. When the oscillating frequency matched that of one of the natural frequencies of vibration for the mechanical model 4 1. INTRODUCTION: INFRARED AND RAMAN SPECTROSCOPY 3400 3300 3200 3100 3000 2900 2800 2700 CH3, CH2, CH, stretch C≡C H Arom-H C=CH2 C=CH R CH3 Arom-CH3 R CH2 R R O CH3 R2 N CH3 O=C H FIGURE 1.2 The correlation chart for CH3, CH2, and CH stretch IR bands. a resonance occurred and the model responded by exhibiting one of the internal vibrations of the molecule (i.e. normal mode). In the 1940s both Dow Chemical and American Cyanamid companies built their own NaCl prism-based, single beam, meter focal length instruments primarily to study hydrocarbons. The development of commercially available IR instruments had its start in 1946 with American Cyanamid Stamford laboratories contracting with a small optical company called PerkineElmer (PE). The Stamford design produced by PE was a short focal length prism IR spectrometer. With the commercial availability of instrumentation, the technique then benefited from the conceptual idea of a correlation chart of important bands that concisely summarize where various functional groups can be expected to absorb. This introduction of the correlation chart enabled chemists to use the IR spectrum to determine the structure.3, 4 The explosive growth of IR spectroscopy in the 1950s and 1960s were a result of the development of commercially available instrumentation as well as the conceptual breakthrough of a correlation chart. Appendix shows IR group frequency correlation charts for a variety of important functional groups. Shown in Fig. 1.2 is the correlation chart for CH3, CH2, and CH stretch IR bands. The subsequent development of double beam IR instrumentation and IR correlation charts resulted in widespread use of IR spectroscopy as a structural technique. An extensive user base resulted in a great increase in available IR interpretation tools and the eventual development of FT-IR instrumentation. More recently, Raman spectroscopy has benefited from dramatic improvements in instrumentation and is becoming much more widely used than in the past. HISTORICAL PERSPECTIVE: IR AND RAMAN SPECTROSCOPY References 1. Kettering, C. F.; Shultz, L. W.; Andrews, D. H. Phys. Rev. 1930, 36, 531. 2. Colthup, N. B. J. Chem. Educ. 1961, 38 (8), 394e396. 3. Colthup, N. B. J. Opt. Soc. Am. 1950, 40 (6), 397e400. 4. Infrared Characteristic Group Frequencies G. Socrates, 2nd ed.; John Wiley: New York, NY, 1994. 5 C H A P T E R 2 Basic Principles 1. ELECTROMAGNETIC RADIATION All light (including infrared) is classified as electromagnetic radiation and consists of alternating electric and magnetic fields and is described classically by a continuous sinusoidal wave like motion of the electric and magnetic fields. Typically, for IR and Raman spectroscopy we will only consider the electric field and neglect the magnetic field component. Figure 2.1 depicts the electric field amplitude of light as a function of time. The important parameters are the wavelength (l, length of 1 wave), frequency (v, number cycles per unit time), and wavenumbers (n, number of waves per unit length) and are related to one another by the following expression: n 1 ¼ n ¼ ðc=nÞ l where c is the speed of light and n the refractive index of the medium it is passing through. In quantum theory, radiation is emitted from a source in discrete units called photons where the photon frequency, v, and photon energy, Ep, are related by Ep ¼ hn where h is Planck’s constant (6.6256 1027 erg sec). Photons of specific energy may be absorbed (or emitted) by a molecule resulting in a transfer of energy. In absorption spectroscopy this will result in raising the energy of molecule from ground to a specific excited state E + – + – Time FIGURE 2.1 The amplitude of the electric vector of electromagnetic radiation as a function of time. The wavelength is the distance between two crests. 7 8 2. BASIC PRINCIPLES E2 Molecular energy levels Ep ( ) Ep = E2 – E1 E1 FIGURE 2.2 Absorption of electromagnetic radiation. as shown in Fig. 2.2. Typically the rotational (Erot), vibrational (Evib), or electronic (Eel) energy of molecule is changed by 6E: DE ¼ Ep ¼ hn ¼ hcn In the absorption of a photon the energy of the molecule increases and DE is positive. To a first approximation, the rotational, vibrational, and electronic energies are additive: ET ¼ Eel þ Evib þ Erot We are concerned with photons of such energy that we consider Evib alone and only for condensed phase measurements. Higher energy light results in electronic transitions (Eel) and lower energy light results in rotational transitions (Erot). However, in the gas-state both IR and Raman measurements will include Evib þ Erot. 2. MOLECULAR MOTION/DEGREES OF FREEDOM 2.1. Internal Degrees of Freedom The molecular motion that results from characteristic vibrations of molecules is described by the internal degrees of freedom resulting in the well-known 3n 6 and 3n 5 rule-of-thumb for vibrations for non-linear and linear molecules, respectively. Figure 2.3 shows the fundamental vibrations for the simple water (non-linear) and carbon dioxide (linear) molecules. The internal degrees of freedom for a molecule define n as the number of atoms in a molecule and define each atom with 3 degrees of freedom of motion in the X, Y, and Z directions resulting in 3n degrees of motional freedom. Here, three of these degrees are translation, while three describe rotations. The remaining 3n 6 degrees (non-linear molecule) are motions, which change the distance between atoms, or the angle between bonds. A simple example of the 3n 6 non-linear molecule is water (H2O) which has 3(3) 6 ¼ 3 degrees of freedom. The three vibrations include an in-phase and out-of-phase stretch and a deformation (bending) vibration. Simple examples of 3n 5 linear molecules include H2, N2, and O2 which all have 3(2) 5 ¼ 1 degree of freedom. The only vibration for these simple molecules is a simple stretching vibration. The more complicated CO2 molecule has 3(3) 5 ¼ 4 degrees of freedom and therefore four vibrations. The four vibrations include an in-phase and outof-phase stretch and two mutually perpendicular deformation (bending) vibrations. The molecular vibrations for water and carbon dioxide as shown in Fig. 2.3 are the normal mode of vibrations. For these vibrations, the Cartesian displacements of each atom in molecule 9 MOLECULAR MOTION/DEGREES OF FREEDOM (a) Water O H H O O H H νip νop H H νdef (b) CO2 O C νop O O C νip O O C νdef O O C O νdef FIGURE 2.3 Molecular motions which change distance between atoms for water and CO2. change periodically with the same frequency and go through equilibrium positions simultaneously. The center of the mass does not move and the molecule does not rotate. Thus in the case of harmonic oscillator, the Cartesian coordinate displacements of each atom plotted as a function of time is a sinusoidal wave. The relative vibrational amplitudes may differ in either magnitude or direction. Figure 2.4 shows the normal mode of vibration for a simple diatomic such as HCl and a more complex totally symmetric CH stretch of benzene. (a) Diatomic Stretch Equilibrium position of atoms Displacement Time (b) Totally symmetric CH stretch of benzene FIGURE 2.4 Normal mode of vibration for a simple diatomic such as HCl (a) and a more complex species such as benzene (b). The displacement versus time is sinusoidal, with equal frequency for all the atoms. The typical Cartesian displacement vectors are shown for the more complicated totally symmetric CH stretch of benzene. 10 2. BASIC PRINCIPLES 3. CLASSICAL HARMONIC OSCILLATOR To better understand the molecular vibrations responsible for the characteristic bands observed in infrared and Raman spectra it is useful to consider a simple model derived from classical mechanics.1 Figure 2.5 depicts a diatomic molecule with two masses m1 and m2 connected by a massless spring. The displacement of each mass from equilibrium along the spring axis is X1 and X2. The displacement of the two masses as a function of time for a harmonic oscillator varies periodically as a sine (or cosine) function. In the above diatomic system, although each mass oscillates along the axis with different amplitudes, both atoms share the same frequency and both masses go through their equilibrium positions simultaneously. The observed amplitudes are inversely proportional to the mass of the atoms which keeps the center of mass stationary X1 m2 ¼ X2 m1 The classical vibrational frequency for a diatomic molecule is: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 1 n ¼ Kð þ Þ 2p m1 m2 where K is the force constant in dynes/cm and m1 and m2 are the masses in grams and n is in cycles per second. This expression is also encountered using the reduced mass where 1 1 1 m1 m2 þ or m ¼ ¼ m1 þ m2 m m1 m2 In vibrational spectroscopy wavenumber units, n (waves per unit length) are more typically used m1 K m2 X1 –X2 Time Displacement FIGURE 2.5 Motion of a simple diatomic molecule. The spring constant is K, the masses are m1 and m2, and X1 and X2 are the displacement vectors of each mass from equilibrium where the oscillator is assumed to be harmonic. CLASSICAL HARMONIC OSCILLATOR 11 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 1 K n ¼ þ 2pc m1 m2 where n is in waves per centimeter and is sometimes called the frequency in cm1 and c is the speed of light in cm/s. If the masses are expressed in unified atomic mass units (u) and the force constant is expressed in millidynes/Ångström then: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 n ¼ 1303 K þ m1 m2 where 1303 ¼ [Na 105)1/2/2pc and Na is Avogadro’s number (6.023 1023 mole1) This simple expression shows that the observed frequency of a diatomic oscillator is a function of 1. the force constant K, which is a function of the bond energy of a two atom bond (see Table 2.1) 2. the atomic masses of the two atoms involved in the vibration. TABLE 2.1 Approximate Range of Force Constants for Single, Double, and Triple Bonds Bond type K (millidynes/Ångström) Single 3e6 Double 10e12 Triple 15e18 Table 2.1 shows the approximate range of the force constants for single, double, and triple bonds. Conversely, knowledge of the masses and frequency allows calculation of a diatomic force constant. For larger molecules the nature of the vibration can be quite complex and for more accurate calculations the harmonic oscillator assumption for a diatomic will not be appropriate. The general wavenumber regions for various diatomic oscillator groups are shown in Table 2.2, where Z is an atom such as carbon, oxygen, nitrogen, sulfur, and phosphorus. TABLE 2.2 General Wavenumber Regions for Various Simple Diatonic Oscillator Groups Diatomic oscillator Region (cmL1) ZeH 4000e2000 ChC, ChN 2300e2000 C¼O, C¼N, C ¼C 1950e1550 CeO, CeN, CeC 1300e800 CeCl 830e560 12 2. BASIC PRINCIPLES 4. QUANTUM MECHANICAL HARMONIC OSCILLATOR Vibrational spectroscopy relies heavily on the theoretical insight provided by quantum theory. However, given the numerous excellent texts discussing this topic only a very cursory review is presented here. For a more detailed review of the quantum mechanical principles relevant to vibrational spectroscopy the reader is referred elsewhere.2-5 For the classical harmonic oscillation of a diatomic the potential energy (PE) is given by 1 KX2 2 A plot of the potential energy of this diatomic system as a function of the distance, X between the masses, is thus a parabola that is symmetric about the equilibrium internuclear distance, Xe. Here Xe is at the energy minimum and the force constant, K is a measure of the curvature of the potential well near Xe. From quantum mechanics we know that molecules can only exist in quantized energy states. Thus, vibrational energy is not continuously variable but rather can only have certain discrete values. Under certain conditions a molecule can transit from one energy state to another (Dy ¼ 1) which is what is probed by spectroscopy. Figure 2.6 shows the vibrational levels in a potential energy diagram for the quantum mechanical harmonic oscillator. In the case of the harmonic potential these states are equidistant and have energy levels E given by 1 Ei ¼ yi þ hv yi ¼ 0; 1; 2. 2 PE ¼ Here, n is the classical vibrational frequency of the oscillator and y is a quantum number which can have only integer values. This can only change by Dy ¼ 1 in a harmonic oscillator model. The so-called zero point energy occurs when y ¼ 0 where E ¼ ½ hn and this vibrational energy cannot be removed from the molecule. Probability =2 E =1 =0 Xe X E = ( i = 1/2) h c υ o Δυ = ± 1 FIGURE 2.6 Potential energy, E, versus internuclear distance, X, for a diatomic harmonic oscillator. 13 IR ABSORPTION PROCESS Potential energy (E) Anharmonic oscillator Harmonic oscillator Do De i= 0 1/2 h ν Internuclear distance (X) FIGURE 2.7 The potential energy diagram comparison of the anharmonic and the harmonic oscillator. Transitions originate from the y ¼ 0 level, and Do is the energy necessary to break the bond. Figure 2.6 shows the curved potential wells for a harmonic oscillator with the probability functions for the internuclear distance X, within each energy level. These must be expressed as a probability of finding a particle at a given position since by quantum mechanics we cannot be certain of the position of the mass during the vibration (a consequence of Heisenberg’s uncertainty principle). Although we have only considered a harmonic oscillator, a more realistic approach is to introduce anharmonicity. Anharmonicity results if the change in the dipole moment is not linearly proportional to the nuclear displacement coordinate. Figure 2.7 shows the potential energy level diagram for a diatomic harmonic and anharmonic oscillator. Some of the features introduced by an anharmonic oscillator include the following. The anharmonic oscillator provides a more realistic model where the deviation from harmonic oscillation becomes greater as the vibrational quantum number increases. The separation between adjacent levels becomes smaller at higher vibrational levels until finally the dissociation limit is reached. In the case of the harmonic oscillator only transitions to adjacent levels or so-called fundamental transitions are allowed (i.e., 6y ¼ 1) while for the anharmonic oscillator, overtones (6y ¼ 2) and combination bands can also result. Transitions to higher vibrational states are far less probable than the fundamentals and are of much weaker intensity. The energy term corrected for anharmonicity is 1 1 2 hce ne y þ Ey ¼ hne y þ 2 2 where ce ne defines the magnitude of the anharmonicity. 5. IR ABSORPTION PROCESS The typical IR spectrometer broad band source emits all IR frequencies of interest simultaneously where the near-IR region is 14,000e4000 cm1, the mid-IR region is 14 2. BASIC PRINCIPLES 4000e400 cm1, and the far-IR region is 400e10 cm1. Typical of an absorption spectroscopy, the relationship between the intensities of the incident and transmitted IR radiation and the analyte concentration is governed by the LamberteBeer law. The IR spectrum is obtained by plotting the intensity (absorbance or transmittance) versus the wavenumber, which is proportional to the energy difference between the ground and the excited vibrational states. Two important components to the IR absorption process are the radiation frequency and the molecular dipole moment. The interaction of the radiation with molecules can be described in terms of a resonance condition where the specific oscillating radiation frequency matches the natural frequency of a particular normal mode of vibration. In order for energy to be transferred from the IR photon to the molecule via absorption, the molecular vibration must cause a change in the dipole moment of the molecule. This is the familiar selection rule for IR spectroscopy, which requires a change in the dipole moment during the vibration to be IR active. The dipole moment, m, for a molecule is a function of the magnitude of the atomic charges (ei) and their positions (ri) X m ¼ ei r i The dipole moments of uncharged molecules derive from partial charges on the atoms, which can be determined from molecular orbital calculations. As a simple approximation, the partial charges can be estimated by comparison of the electronegativities of the atoms. Homonuclear diatomic molecules such as H2, N2, and O2 have no dipole moment and are IR inactive (but Raman active) while heteronuclear diatomic molecules such as HCl, NO, and CO do have dipole moments and have IR active vibrations. The IR absorption process involves absorption of energy by the molecule if the vibration causes a change in the dipole moment, resulting in a change in the vibrational energy level. Figure 2.8 shows the oscillating electric field of the IR radiation generates forces on the Time + + Forces generated by the photon electric field + + + – – Dipole – – – One photon cycle FIGURE 2.8 The oscillating electric field of the photon generates oscillating, oppositely directed forces on the positive and negative charges of the molecular dipole. The dipole spacing oscillates with the same frequency as the incident photon. 15 THE RAMAN SCATTERING PROCESS molecular dipole where the oscillating electric field drives the oscillation of the molecular dipole moment and alternately increases and decreases the dipole spacing. Here, the electric field is considered to be uniform over the whole molecule since l is much greater than the size of most molecules. In terms of quantum mechanics, the IR absorption is an electric dipole operator mediated transition where the change in the dipole moment, m, with respect to a change in the vibrational amplitude, Q, is greater than zero. vm s0 vQ 0 The measured IR band intensity is proportional to the square of the change in the dipole moment. 6. THE RAMAN SCATTERING PROCESS Light scattering phenomena may be classically described in terms of electromagnetic (EM) radiation produced by oscillating dipoles induced in the molecule by the EM fields of the incident radiation. The light scattered photons include mostly the dominant Rayleigh and the very small amount of Raman scattered light.6 The induced dipole moment occurs as a result of the molecular polarizability a, where the polarizability is the deformability of the electron cloud about the molecule by an external electric field. Figure 2.9 shows the response of a non-polar diatomic placed in an oscillating electric field. Here we represent the static electric field by the plates of a charged capacitor. The negatively charged plate attracts the nuclei, while the positively charged plate attracts the least tightly Electron and proton center in homogeneous diatomic molecule Time Photon electric field – + – = = + – Field turned off FIGURE 2.9 Dipole induced by charged capacitor plates + = + = – Forces on charges induced by the external photon field Induced dipole moment resulting from electron displacement Induced dipole moment of a homonuclear diatomic originating from the oscillating electric field of the incident radiation. The field relative to the proton center displaces the electron center. The charged plates of a capacitor, which induces a dipole moment in the polarizable electron cloud, can represent the electric field. 16 2. BASIC PRINCIPLES Wavelength (nm) 1042 1087 1064 Rayleigh Scattering hc ν Stokes Raman Scattering hc (νL – ν m) Energy Anti-Stokes Raman Scattering hc (νL + νm) 1 1 0 Intensity 0 200 100 0 –100 –200 (νL + νm) FIGURE 2.10 Schematic illustration of Rayleigh scattering as well as Stokes and anti-Stokes Raman scattering. The laser excitation frequency (nL) is represented by the upward arrows and is much higher in energy than the molecular vibrations. The frequency of the scattered photon (downward arrows) is unchanged in Rayleigh scattering but is of either lower or higher frequency in Raman scattering. The dashed lines indicate the “virtual state.” bound outer electrons resulting in an induced dipole moment. This induced dipole moment is an off-resonance interaction mediated by an oscillating electric field. In a typical Raman experiment, a laser is used to irradiate the sample with monochromatic radiation. Laser sources are available for excitation in the UV, visible, and near-IR spectral region (785 and 1064 nm). Thus, if visible excitation is used, the Raman scattered light will also be in the visible region. The Rayleigh and Raman processes are depicted in Fig. 2.10. No energy is lost for the elastically scattered Rayleigh light while the Raman scattered photons lose some energy relative to the exciting energy to the specific vibrational coordinates of the sample. In order for Raman bands to be observed, the molecular vibration must cause a change in the polarizability. Both Rayleigh and Raman are two photon processes involving scattering of incident light ðhcnL Þ, from a “virtual state.” The incident photon is momentarily absorbed by a transition from the ground state into a virtual state and a new photon is created and scattered by a transition from this virtual state. Rayleigh scattering is the most probable event and the scattered intensity is ca. 103 less than that of the original incident radiation. This scattered photon results from a transition from the virtual state back to the ground state and is an elastic scattering of a photon resulting in no change in energy (i.e., occurs at the laser frequency). CLASSICAL DESCRIPTION OF THE RAMAN EFFECT 17 Raman scattering is far less probable than Rayleigh scattering with an observed intensity that is ca. 106 that of the incident light for strong Raman scattering. This scattered photon results from a transition from the virtual state to the first excited state of the molecular vibration. This is described as an inelastic collision between photon and molecule, since the molecule acquires different vibrational energy ðnm Þ and the scattered photon now has different energy and frequency. As shown in Fig. 2.10 two types of Raman scattering exist: Stokes and anti-Stokes. Molecules initially in the ground vibrational state give rise to Stokes Raman scattering hcðnL nm Þwhile molecules initially in vibrational excited state give rise to anti-Stokes Raman scattering, hcðnL þ nm Þ. The intensity ratio of the Stokes relative to the anti-Stokes Raman bands is governed by the absolute temperature of the sample, and the energy difference between the ground and excited vibrational states. At thermal equilibrium Boltzmann’s law describes the ratio of Stokes relative to anti-Stokes Raman lines. The Stokes Raman lines are much more intense than anti-Stokes since at ambient temperature most molecules are found in the ground state. The intensity of the Raman scattered radiation IR is given by: 2 va 4 IR fn Io N vQ where Io is the incident laser intensity, N is the number of scattering molecules in a given state, n is the frequency of the exciting laser, a is the polarizability of the molecules, and Q is the vibrational amplitude. The above expression indicates that the Raman signal has several important parameters for Raman spectroscopy. First, since the signal is concentration dependent, quantitation is possible. Secondly, using shorter wavelength excitation or increasing the laser flux power density can increase the Raman intensity. Lastly, only molecular vibrations which cause a change in polarizability are Raman active. Here the change in the polarizability with respect to a change in the vibrational amplitute, Q, is greater than zero. va s0 vQ The Raman intensity is proportional to the square of the above quantity. 7. CLASSICAL DESCRIPTION OF THE RAMAN EFFECT The most basic description of Raman spectroscopy describes the nature of the interaction of an oscillating electric field using classical arguments.6 Figure 2.11 schematically represents this basic mathematical description of the Raman effect. As discussed above, the electromagnetic field will perturb the charged particles of the molecule resulting in an induced dipole moment: m ¼ aE where a is the polarizability, E is the incident electric field, and m is the induced dipole moment. Both E and a can vary with time. The electric field of the radiation is oscillating 18 2. BASIC PRINCIPLES (a) (c) (b) (d) νo Rayleigh νo + νm νo νm νo ± ν m Raman anti-Stokes νo – νm Raman Stokes FIGURE 2.11 Schematic representing Rayleigh and Raman scattering. In (a) the incident radiation makes the induced dipole moment of the molecule oscillate at the photon frequency. In (b) the molecular vibration can change the polarizability, a, which changes the amplitude of the dipole moment oscillation. The result as shown in (c) is an amplitude modulated dipole moment oscillation. The image (d) shows the components with steady amplitudes which can emit electromagnetic radiation. as a function of time at a frequency n0, which can induce an oscillation of the dipole moment m of the molecule at this same frequency, as shown in Fig. 2.11a. The polarizability a of the molecule has a certain magnitude whose value can vary slightly with time at the much slower molecular vibrational frequency nm, as shown in Fig. 2.11b. The result is seen in Fig. 2.11c, which depicts an amplitude modulation of the dipole moment oscillation of the molecule. This type of modulated wave can be resolved mathematically into three steady amplitude components with frequencies n0, n0 þ nm, and n0 nm as shown in Fig. 2.11d. These dipole moment oscillations of the molecule can emit scattered radiation with these same frequencies called Rayleigh, Raman anti-Stokes, and Raman Stokes frequencies. If a molecular vibration did not cause a variation in the polarizability, then there would be no amplitude modulation of the dipole moment oscillation and there would be no Raman Stokes or anti-Stokes emission. 8. SYMMETRY: IR AND RAMAN ACTIVE VIBRATIONS The symmetry of a molecule, or the lack of it, will define what vibrations are Raman and IR active.5 In general, symmetric or in-phase vibrations and non-polar groups are most easily studied by Raman while asymmetric or out-of-phase vibrations and polar groups are most easily studied by IR. The classification of a molecule by its symmetry enables understanding of the relationship between the molecular structure and the vibrational spectrum. Symmetry elements include planes of symmetry, axes of symmetry, and a center of symmetry. Group Theory is the mathematical discipline, which applies symmetry concepts to vibrational spectroscopy and predicts which vibrations will be IR and Raman active.1,5 The symmetry elements possessed by the molecule allow it to be classified by a point group and vibrational analysis can be applied to individual molecules. A thorough discussion of Group Theory is beyond the scope of this work and interested readers should examine texts dedicated to this topic.7 19 SYMMETRY: IR AND RAMAN ACTIVE VIBRATIONS Totally symmetric vibration Center of symmetry H O H C O O H H C O Raman active Asymmetric vibration O C O IR active FIGURE 2.12 The center of symmetry for H2, CO2, and benzene. The Raman active symmetric stretching vibrations above are symmetric with respect to the center of symmetry. Some IR active asymmetric stretching vibrations are also shown. For small molecules, the IR and Raman activities may often be determined by a simple inspection of the form of the vibrations. For molecules that have a center of symmetry, the rule of mutual exclusion states that no vibration can be active in both the IR and Raman spectra. For such highly symmetrical molecules vibrations which are Raman active are IR inactive and vice versa and some vibrations may be both IR and Raman inactive. Figure 2.12 shows some examples of molecules with this important symmetry element, the center of symmetry. To define a center of symmetry simply start at any atom, go in a straight line through the center and an equal distance beyond to find another, identical atom. In such cases the molecule has no permanent dipole moment. Examples shown below include H2, CO2, and benzene and the rule of mutual exclusion holds. In a molecule with a center of symmetry, vibrations that retain the center of symmetry are IR inactive and may be Raman active. Such vibrations, as shown in Fig. 2.12, generate a change in the polarizability during the vibration but no change in a dipole moment. Conversely, vibrations that do not retain the center of symmetry are Raman inactive, but may be IR active since a change in the dipole moment may occur. For molecules without a center of symmetry, some vibrations can be active in both the IR and Raman spectra. Molecules that do not have a center of symmetry may have other suitable symmetry elements so that some vibrations will be active only in Raman or only in the IR. Good examples of this are the in-phase stretches of inorganic nitrate and sulfate shown in Fig. 2.13. These are Raman active and IR inactive. Here, neither molecule has a center of symmetry but the negative oxygen atoms move radially simultaneously resulting in no dipole moment change. 20 2. BASIC PRINCIPLES Raman active, IR inactive symmetric vibrations (a) (b) O O O N O S O O O Sulfate, in-phase SO4 stretch Nitrate, in-phase NO3 stretch (c) X X X 1,3,5-trisubstituted benzene, 2,4,6 C-radial in-phase stretch FIGURE 2.13 Three different molecules, nitrate, sulfate, and 1,3,5-trisubstituted benzene molecules that do not have a center of symmetry. The in-phase stretching vibrations of all three result in Raman active, but IR inactive vibrations. Another example is the 1,3,5-trisubstituted benzene where the C-Radial in-phase stretch is Raman active and IR inactive. In Figure 2.14 some additional symmetry operations are shown, other than that for a center of symmetry for an XY2 molecule such as water. These include those for a plane of symmetry, a two-fold rotational axis of symmetry, and an identity operation (needed for group theory) which makes no change. If a molecule is symmetrical with respect to a given symmetry element, the symmetry operation will not make any discernible change from the original configuration. As shown in Fig. 2.14, such symmetry operations are equivalent to renumbering the symmetrically related hydrogen (Y) atoms. Figure 2.15 shows the Cartesian displacement vectors (arrows) of the vibrational modes Q1, Q2, and Q3 of the bent triatomic XY2 molecule (such as water), and shows how they are modified by the symmetry operations C2, sv, and s /v. For non-degenerate modes of vibration such as these, the displacement vectors in the first column (the identity column, I) are multiplied by either (þ1) or (1) as shown to give the forms in the other three columns. Multiplication by (þ1) does not change the original form so the resulting form is said to be symmetrical with respect to that symmetry operation. Multiplication by (1) reverses all the vectors of the original form and the resulting form is said to be anti-symmetrical with respect to that symmetry element. As seen in Fig. 2.15, Q1 and Q2 are both totally 21 SYMMETRY: IR AND RAMAN ACTIVE VIBRATIONS (a) (b) After Before 1 1 2 3 1 1 2 3 Identity operation, I (no change) 3 2 3 2 Two-fold axis of rotation, C2 (rotate 180o on axis) (c) (d) 2 3 Plane of symmetry, V 2 3 1 1 1 (reflection in mirror plane) 2 3 Plane of symmetry, 2 3 / (reflection in molecular plane) V FIGURE 2.14 Symmetry operations for an XY2 bent molecule such as water in the equilibrium configuration. I C2 σV σ /V Q1 (1) (1) (1) (1) (1) (1) (1) (1) (1) (–1) (1) (–1) Q2 Q3 FIGURE 2.15 The bent symmetrical XY2 molecule such as H2O performing the three fundamental modes Q1, Q2, and Q3. The vectors in column one (identity I) are transformed by the C2, sv, and s /v, operations into the forms in the remaining columns, where the vectors are like those in column one multiplied by (þ1) symmetrical or (1) antisymmetrical. symmetric modes (i.e., symmetric to all symmetry operations), whereas Q3 is symmetric with respect to the s /v operation but anti-symmetric with respect to the C2 and sv operations. The transformation numbers (þ1 and 1) are used in group theory to characterize the symmetries of non-degenerate vibrational modes. From these symmetries one can deduce that Q1, Q2, and Q3 are all active in both the IR and Raman spectra. In addition, 22 2. BASIC PRINCIPLES the dipole moment change in Q1 and Q2 is parallel to the C2 axis and in Q3 it is perpendicular to the C2 axis and the sv, plane. Doubly degenerate modes occur when two different vibrational modes have the same vibrational frequency as a consequence of symmetry. A simple example is the CeH bending vibration in Cl3CeH molecule where the CeH bond can bend with equal frequency in two mutually perpendicular directions. The treatment of degenerate vibrations is more complex and will not be discussed here. 9. CALCULATING THE VIBRATIONAL SPECTRA OF MOLECULES The basis of much of the current understanding of molecular vibrations and the localized group vibrations that give rise to useful group frequencies observed in the IR and Raman spectra of molecules is based upon extensive historical work calculating vibrational spectra. Historically, normal coordinate analysis first developed by Wilson with a GF matrix method and using empirical molecular force fields has played a vital role in making precise assignments of observed bands. The normal coordinate computation involves calculation of the vibrational frequencies (i.e., eigenvalues) as well as the atomic displacements for each normal mode of vibration. The calculation itself uses structural parameters such as the atomic masses and empirically derived force fields. However, significant limitations exist when using empirical force fields. The tremendous improvements in computational power along with multiple software platforms with graphical user interfaces enables a much greater potential use of ab initio quantum mechanical calculational methods for vibrational analysis. The standard method for calculating the fundamental vibrational frequencies and the normal vibrational coordinates is the Wilson GF matrix method.8 The basic principles of normal coordinate analysis have been covered in detail in classic books on vibrational spectroscopy.1,4,5,8 In the GF matrix approach a matrix, G, which is related to the molecular vibrational kinetic energy is calculated from information about the molecular geometry and atomic masses. Based upon a complete set of force constants a matrix, F, is constructed which is related to the molecular vibrational potential energy. A basis set is selected that is capable of describing all possible internal atomic displacements for the calculation of the G and F matrices. Typically, the molecules will be constructed in Cartesian coordinate space and then transformed to an internal coordinate basis set which consists of changes in bond distances and bond angles. The product matrix GF can then be calculated. The fundamental frequencies and normal coordinates are obtained through the diagonalization of the GF matrix. Here, a transformation matrix L is sought: L1 GFL ¼ L Here, L is a diagonal matrix whose diagonal elements are li’s defined as: li ¼ 4p2 c2 v2 i Where the frequency in cm1 of the ith normal mode is n2i . For the previous equation, it is the matrix L1 which transforms the internal coordinates, R, into the normal coordinates, Q, as: Q ¼ L1 R CALCULATING THE VIBRATIONAL SPECTRA OF MOLECULES 23 In this equation, the column matrix of internal coordinates is R and Q is a column matrix that contains normal coordinates as a linear combination of internal coordinates. In the case of vibrational spectroscopy, the polyatomic molecule is considered to oscillate with a small amplitude about the equilibrium position and the potential energy expression is expanded in a Taylor series and takes the form: ! ! 3N 3N X 3N X vV 1X v2 V dqi þ dqi dqj þ ,,, V ¼ V0 þ vq i 2 i ¼ 1 j ¼ 1 vqi vqj i¼1 e e expressed in internal coordinates, qi,, which are directly connected to the internal bond lengths and angles. The above expression is simplified: 1. The first term Vo ¼ 0 since the vibrational energy is chosen as vibrating atoms about the equilibrium position. 2. At the minimum energy configuration the first derivative is zero by definition. 3. Since the harmonic approximation is used all terms in the Taylor expansion greater than two can be neglected. This leaves only the second order term in the potential energy expression for V. Using Newton’s second law, the above is expressed as: ! 3N X d2 q i vV v2 V ¼ ¼ qj vqi vqi vqj dt2 j¼1 e The above equation of motion is solved to yield a determinant whose eigenvalues (mu2) provides the vibrational frequencies (u). The eigenvectors describe the atomic displacements for each of the vibrational modes characterized by the eigenvalues. These are the normal modes of vibration and the corresponding fundamental frequencies. The force constant fij is defined as the second derivative of the potential energy with respect to the coordinates qi and qj in the equilibrium configuration as: ! v2 V fij ¼ vqi vqj In order to obtain the molecular force field with the force constants given by the above equation, a variety of computational methods are available. In general, the calculations of the vibrational frequencies can be accomplished with either empirical force field method or quantum mechanical methods.9,10 The quantum mechanical method is the most rigorous approach and is typically used for smaller to moderately sized molecules since it is computationally intensive. Since we are examining chemical systems with more than one electron, approximate methods known as ab initio methods utilizing a harmonic oscillator approximation are employed. Because actual molecular vibrations include both harmonic and anharmonic components, a difference is expected between the experimental and calculated vibrational frequencies. Other factors that contribute to the differences between the calculated and experimental frequencies include neglecting electron correlation and the limited size of the basis set. In order to obtain a better match with the experimental frequencies, scaling factors are typically introduced. 24 2. BASIC PRINCIPLES Quantum mechanical ab initio methods and hybrid methods are based upon force constants calculated by Hartree-Fock (HF) and density functional based methods. In general, these methods involve molecular orbital calculations of isolated molecules in a vacuum, such that environmental interactions typically encountered in the liquid and solid state are not taken into account. A full vibrational analysis of small to moderately sized molecules typically takes into account both the vibrational frequencies and intensities to insure reliable assignments of experimentally observed vibrational bands. The ab initio Hartree-Fock (HF) method is an older quantum mechanical based approach.9,10 The HF methodology neglects the mutual interaction (correlation) between electrons which affects the accuracy of the frequency calculations. In general, when using HF calculations with a moderate basis set there will be a difference of ca. 10e15% between the experimental and calculated frequencies and thus a scaling factor of 0.85e0.90. This issue can be resolved somewhat by use of post-HF methods such as configuration interaction (CI), multi-configuration self-consistent field (MCSF), and Møller Plessant perturbation (MP2) methods. Utilizing configuration interaction with a large basis set leads to a scaling factor between 0.92 and 0.96. However, use of these post-HF methods comes with a considerable computational cost that limits the size of the molecule since they scale with the number of electrons to the power of 5e7. The ab initio density functional theory (DFT) based methods have arisen as highly effective computational techniques because they are computationally as efficient as the original HF calculations while taking into account a significant amount of the electron correlation.9,10 The DFT has available a variety of gradient-corrected exchange functions to calculate the density functional force constants. Popular functions include the BLYP and B3LYP. The scaling factors encountered using a large basis set and BLYP or B3LYP often approach 1 (0.96e1.05). Basis set selection is important in minimizing the energy state of the molecule and providing an accurate frequency calculation. Basis sets are Gaussian mathematical functions representative of the atomic orbitals which are linearly combined to describe the molecular orbitals. The simplest basis set is the STO-3G in which the Slater-type orbital (STO) is expanded with three Gaussian-type orbitals (GTO). The more complex split-valence basis sets, 3-21G and 6-31G are more typically used. Here, the 6-31G consists of a core of six GTO’s that are not split and the valence orbitals are split into one basis function constructed from three GTO’s and another that is a single GTO. Because the electron density of a nucleus can be polarized (by other nucleus), a polarization function can also be included. Such functions include the 6-31G* and the 6-31G**. Accurate vibrational analysis requires optimizing the molecular structure and wavefunctions in order to obtain the minimum energy state of the molecule. In practice, this requires selection of a suitable basis set method for the electron correlation. The selection of the basis set and the HF or DFT parameters is important in acquiring acceptable calculated vibrational data necessary to assign experimental IR and Raman spectra. CALCULATING THE VIBRATIONAL SPECTRA OF MOLECULES 25 References 1. Barrow, G. M. Introduction to Molecular Spectroscopy; McGraw-Hill: New York, NY, 1962. 2. Pauling, L.; Wilson, E. B. Introduction to Quantum Mechanics with Applications to Chemistry; McGraw-Hill: New York, NY, 1935. 3. Kauzmann, W. Quantum Chemistry, An Introduction; Academic: New York, NY, 1957. 4. Diem, M. Introduction to Modern Vibrational Spectroscopy; John Wiley: New York, NY, 1993. 5. Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules; D. Van Nostrand Company: New York, NY, 1945. 6. Long, D. A. Raman Spectroscopy; McGraw-Hill: New York, NY, 1977. 7. Cotton, F. A. Chemical Applications of Group Theory; Wiley (Interscience): New York, NY, 1963. 8. Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra; McGraw-Hill: New York, NY, 1955. 9. Handbook of vibrational spectroscopy, vol. 3, Sample Characterization and Spectral Data Processing; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley: 2002. 10. Meier, R. J. Vib. Spectrosc. 2007, 43, 26e37. C H A P T E R 3 Instrumentation and Sampling Methods 1. INSTRUMENTATION Raman scattering and IR absorption are significantly different techniques and require very different instrumentation to measure their spectra. In IR spectroscopy, the image of the IR source through a sample is projected onto a detector whereas in Raman spectroscopy, it is the focused laser beam in the sample that is imaged. In both cases, the emitted light is collected and focused onto a wavelength-sorting device. The monochromators used in dispersive instruments and the interferometers used in Fourier transform instruments are the two basic devices. Historically, IR and Raman spectra were measured with a dispersive instrument. Today, almost all commercially available mid-IR instrumentation are (Fourier Transform) FT-IR spectrometers, which are based upon an interferometer (see below). Raman instruments include both grating-based instruments using multi-channel detectors and interferometer-based spectrometers. 1.1. Dispersive Systems A monochromator consists of an entrance slit, followed by a mirror to insure the light is parallel, a diffraction grating, a focusing mirror, which directs the dispersed radiation to the exit slit, and onto a detector.1, 2 In a scanning type monochromator, a scanning mechanism passes the dispersed radiation over a slit that isolates the frequency range falling on the detector. This type of instrument has limited sensitivity since at any one time, most of the light does not reach the detector. Polychromatic radiation is sorted spatially into monochromatic components using a diffraction grating to bend the radiation by an angle that varies with wavelength. The diffraction grating contains many parallel lines (or grooves) on a reflective planar or on a concave support that are spaced a distance similar to the wavelength of light to be analyzed. Incident radiation approaching the adjacent grooves in-phase is reflected with a path length difference. The path length difference depends upon the groove spacing, the angle of incidence (a), and the angle of reflectance of the radiation (b). 27 28 3. INSTRUMENTATION AND SAMPLING METHODS Incident radiation 1 2 Exiting radiation from adjacent grooves has path length difference FIGURE 3.1 Schematic of a diffraction grating. Wavetrains from two adjacent grooves are displaced by the path length difference. Constructive interference can occur only when the path length difference is equal to the wavelength multiplied by an integer (first, second, third order). The polychromatic radiation will be diffracted at different angles resulting in spatial wavelength discrimination. Figure 3.1 shows a schematic of a diffraction grating with the incident polychromatic radiation and the resultant diffracted light. When the in-phase incident radiation is reflected from the grating, radiation of suitable wavelength is focused onto the exit slit. At the exit slit, the focused radiation will be in-phase for only a selected wavelength and its whole number multiples, which will constructively interfere and pass through the exit slit. Other wavelengths will destructively interfere and will not exit the monochromator. Thus, each of the grooves acts as an individual slit-like source of radiation, diffracting it in various directions. Typically, a selective filter is used to remove the higher-order wavelengths. When the grating is slightly rotated a slightly different wavelength will reach the detector. 1.2. Dispersive Raman Instrumentation Raman instrumentation must be capable of eliminating the overwhelmingly strong Rayleigh scattered radiation while analyzing the weak Raman scattered radiation. A Raman instrument typically consists of a laser excitation source (UV, visible, or near-IR), collection optics, a spectral analyzer (monochromator or interferometer), and a detector.1, 2, 3 The choice of the optics material and the detector type will depend upon the laser excitation wavelength employed. Instrumental design considers how to maximize the two often-conflicting parameters: optical throughput and spectral resolution. The collection optics and the monochromator must be carefully designed to collect as much of the Raman scattered light from the sample and transfer it into the monochromator or interferometer. Until recently, most Raman spectra were recorded using scanning instruments (typically double monochromators) with excitation in the visible region. An example of an array-based simple single grating-based Raman monochromator is depicted in Fig. 3.2. Use of highly sensitive array detectors and high throughput single monochromators with Rayleigh rejection filters have dramatically improved the performance of dispersive Raman systems. The advent of highly efficient Rayleigh line filters to selectively reject the Rayleigh scattered radiation enables the instrument to use only one grating, thereby greatly improving 29 INSTRUMENTATION Slit Source M1 RF G AD M2 FIGURE 3.2 Schematic of a simple array-based high throughput, single monochromator-based Raman instrument. For the source, laser light is scattered at 90 or 180 and collection optics direct light to a Rayleigh filter, RF, to remove the Rayleigh scattered radiation. G is the grating, M1 and M2 are spherical mirrors, and AD is the array detector. the optical throughput. Two commonly used filters include the “holographic notch” and dielectric band filters.3 Use of an array detector results in dramatically improved signal-to-noise ratio (SNR) as a result of the so-called multi-channel advantage. The array detectors (AD) are often photodiode arrays or CCD’s (charge-coupled-devices), where each element (or pixel) records a different spectral band resulting in a multichannel advantage in the measured signal. However, only a limited number of detector elements (typically 256, 512, or 1064 pixels) are present in commercially available detector elements compared to the number of resolvable spectral elements. Thus, to cover the entire spectral range (4000e400 cm1) requires either low-resolution spectrum over the entire spectral range or high resolution over a limited spectral range. One solution to this is to scan the entire spectral range in sections using a multi-channel instrument and adding high resolution spectra together to give the full Raman spectrum. 1.3. Sample Arrangements for Raman Spectroscopy Although the Raman scattered light occurs in all directions, the two most common experimental configurations for collecting Raman scattered radiation typically encountered are 90 and 180 backscattering geometry. Various collection systems have been used in Raman Spectroscopy based upon both reflective and refractive optics. 1, 2, 3Figure 3.3 shows 90 and 180 collection geometries using refractive and reflective optics, respectively. The 180 collection optics are typically used in FT-Raman spectrometers and in Raman microscopes. The 180 collection geometry is the optimum sample arrangement for FT-Raman as a result of narrow band self-absorption in the near-IR region. Raman spectroscopy is well known to be a technique requiring a minimum of sample handling and preparation. Typical Raman accessories include cuvette and tube holders, solids holders, and clamps for irregular solid objects. Often NMR or capillary tubes are used and many times the Raman spectra can be measured directly on the sample in their container. 30 3. INSTRUMENTATION AND SAMPLING METHODS (a) To spectrometer Laser (b) Laser To spectrometer FIGURE 3.3 Two different collection optics for Raman spectroscopy. In (a) a simple 90 collection geometry using refractive optics found in older dispersive instruments. In (b) 180 collection geometry using reflective optics typical of FT-Raman instrumentation. 1.4. Interferometric Spectrometers Both FT-IR and FT-Raman spectroscopy use an interferometer to separate light into individual components.4, 5 The FT-Raman measurements use a Nd:YAG 1064 nm near-IR excitation laser and the Raman scattered light falls in the near-IR region. In the IR spectral region, significant improvements in instrumental performance are realized since the instrument measures all wavelengths of IR light simultaneously without use of an entrance slit. The simultaneous measurement of light results in a multiplex (or Felgett’s) advantage while the latter results in a throughput (or Jacquinot) advantage. The relative weakness of the source (both IR and the Raman scattered light) along with the poor detector sensitivity make the Fourier transform measurements attractive in this spectral region. Figure 3.4 shows the schematic of the Michelson interferometer. Use of an interferometer along with computation using the Fast Fourier Transform enables the generation of IR and Raman spectra from the time-dependent interferogram. The components in a Michelson interferometer include a beam splitter, and a fixed and moving mirror. The collimated light from the source incident on an ideal beamsplitter will be divided into two equal intensity beams where 50% is transmitted to the moving mirror and the other 50% is reflected to the fixed mirror. The light is then reflected off both mirrors back to the beamsplitter where 50% is sent to the detector and the other 50% is lost to the source. As the moving mirror scans a defined distance (Dl), the path difference between the two beams is varied and is called the optical retardation and is two times the distance traveled by the moving mirror (2Dl). The interferometer records interferograms caused by phase-dependent interference of light with varying optical retardation. The principle of operation can be easily described by first considering the source to contain only a single monochromatic wavelength, l. When the position of the moving mirror with 31 INSTRUMENTATION Fixed mirror l Lens Source er itt am Be l sp Movable mirror Lens Detector FIGURE 3.4 Schematic of a Michelson interferometer. The displacement of the moving mirror is indicated by Dl. respect to the beamsplitter is identical to that of the fixed mirror, the optical retardation is zero (zero path difference) and the two beams will combine at the beamsplitter in-phase resulting in constructive interference for the beam passing through to the detector. The detector response will reach a maximum whenever the optical retardation is an integral number of the wavelengths (0, l, 2l.). Similarly, a minimum detector response will result from destructive interference at optical retardation values with intervals of l/2 (l/2, 3l/ 2.). As the mirror scans at constant velocity, a simple sine wave will result as the two beams move in and out of phase. The Fourier transform of the sinusoidal interferogram will give rise to a single band with a characteristic frequency and intensity of the monochromatic source. If a polychromatic source is used, the interferogram will consist of a summation of all the different cosine functions corresponding to all of the wavelengths and of the intensities in the source. Only at zero path difference will all the wavelengths be in-phase. Thus, the resulting interferogram in FT-IR and FT-Raman spectra have a very strong center-burst and rapidly damped intensity in the wings of the interferogram. It is necessary to precisely know the optical path differences in the interferometer and this is accomplished using a heliumeneon laser. A computer is used to perform the fast Fourier transform to generate the spectrum, which can then be further processed. The computer interface in the FT-IR and FT-Raman spectrometers is used for setting measurement parameters, spectral processing, library searching, and for quantitation. 1.4.1. FT-IR SPECTROMETERS In a FT-IR spectrometer, sampling occurs just prior to the detector and its collimating optics. Various sampling techniques employed with IR spectroscopy are discussed below. 32 3. INSTRUMENTATION AND SAMPLING METHODS Commercial FT-IR spectrometers typically operate in a single beam mode requiring sequential measurements of the single beam background and sample spectra, which are then ratioed to provide the final absorbance or %Transmittance IR spectrum. The single beam background spectrum provides a wavelength-dependent throughput of the instrument and is a function of the source emission, detector response, beamsplitter properties, and residual atmospheric absorptions from carbon dioxide and water vapor. 1.4.2. FT-RAMAN SPECTROMETERS Commercial FT-Raman spectrometers use mostly a 1064 nm Nd:YAG laser, notch filters at the laser wavelength to reduce Rayleigh scattered light entrance into the interferometer, high quality interferometers, and sensitive detectors which peak in the near-IR region. For FT-Raman spectrometers, the source depicted in Fig. 3.4 is the Raman scattered light. Presently, by switching only a few optical elements it is now possible to utilize one instrument to collect both the IR and Raman data. A major advantage of near-IR excitation based-FT-Raman spectroscopy is the greatly reduced fluorescence interference encountered for many compounds with visible excitation. This allows the Raman spectra of many compounds to be measured which was previously impossible. This occurs because the near-IR photons normally do not have sufficient energy to access the vibronic states that cause fluorescence. However, fluorescence problems are not completely eliminated for all samples and limitations due to sensitivity and absorption also exist. The y4 dependence shown by the Raman intensity is an intrinsic limitation to the sensitivity using near-IR excitation and can only be addressed by continued improvements in detector sensitivity. The inherent near-IR absorption profile of the sample itself can also pose significant problems. Many compounds have multiple absorption bands due to XeH type bonds in the near-IR spectral region which can attenuate the incident laser and/or the Raman lines. The unequal absorption throughout the Raman spectrum can affect the relative Raman band intensities and is termed as narrow band self-absorption. Thermal decomposition can sometimes occur due to absorption at the laser wavelength. 2. SAMPLING METHODS FOR IR SPECTROSCOPY 2.1. IR Transmitting Materials Infrared spectroscopy can be used to study a wide range of sample types either in bulk or in microscopic amounts over a wide range of temperatures and physical states. Quite often, an IR transmitting material is needed to aid sampling. Since quartz strongly absorbs in the near-IR region, front-silvered or gold-coated reflective optics are often employed rather than refractive lenses. However, transmitting materials are needed for other materials including the beamsplitter. Table 3.1 summarizes some important infrared transmitting materials. For the majority of applications, NaCl or KBr windows are used and ZnSe is frequently used for aqueous or wet samples. The alkali metal halides fuse under pressure to give IR and optically transparent windows, but they are hygroscopic and fog if not treated properly. SAMPLING METHODS FOR IR SPECTROSCOPY TABLE 3.1 33 Selected Infrared Transmitting Material Material Wavenumber range (cmL1) Refractive index Comments NaCl 5000e625 1.52 Common, low cost, hygroscopic KBr 5000e400 1.54 Common, low cost, very hygroscopic BaF2 5000e870 1.45 Water insoluble, easily cracked CaF2 5000e1100 1.40 Water insoluble, good at high pressures KRS-5 5000e275 2.38 Water insoluble, good ATR, deforms, poisonous ZnSe 5000e550 2.41 Water insoluble, good ATR Diamond 4500e2500, 1800e200 2.4 Very hard, inert, diamond Anvil cell also good for ATR 2.2. Sampling Techniques The measured IR spectra are defined by the sample-preparation procedures and the optical spectroscopic techniques employed.6, 7 However, the quality of the spectra often depends upon the spectroscopist’s sample-preparation skills. A wide variety of sampling techniques exists; these are necessary to insure that the optimum spectrum will be measured for the varied physical sample states encountered by the analyst. For any given sample, there will usually be several different sampling techniques that can be used to obtain the spectrum. The available accessory equipment, personal preference of the analyst, or the nature of the particular sample will often dictate the selection of the technique. In general, the IR radiation incident upon a sample can be reflected, absorbed, transmitted, or scattered. Io ¼ Ir þ Ia þ It þ Is Here Io is the intensity of the incident radiation; Ir, Ia, It, and Is are the intensities of the reflected, absorbed, transmitted, and scattered radiation, respectively. Experimentally, any of the above intensity components can be used to measure the IR spectrum of the sample. The extent that light is transmitted, reflected, scattered, or refracted is dependent upon the sample morphology, crystalline state, the angle of the incident light, and the difference in refractive indices of the sample and surrounding matrix. Figure 3.5 depicts the angular dependence of the transmission, refraction, and reflected light. Transmitted light has a 0 angle of incidence while reflectance (which includes specular, external, internal, and diffuse) is observed at larger angles. The critical angle (Qc) beyond which refraction becomes imaginary and all light is reflected is also shown. In general, measurement of acceptable quality FT-IR spectra requires development of requisite sample-preparation skills necessary to measure photometrically accurate spectra. General criteria for suitable IR sample preparations include the following: 1. Suitable band intensities in the spectrum. The strongest band in the spectrum should have intensity in the range of 5e15% transmittance. The resulting preparation must be uniformly thick and homogenously mixed without holes or voids to insure a good quality spectrum. 34 3. INSTRUMENTATION AND SAMPLING METHODS Transmitted Refracted n2 n1 c Total Reflection FIGURE 3.5 The angle of incidence of the light and the refractive index difference help define how much light is transmitted, refracted, and reflected. 2. Baseline. The baseline should be relatively flat. The highest point in the spectrum should lie between 95 and 100% transmittance. 3. Poor atmospheric compensation. Water vapor and carbon dioxide bands should be minimized. Since the sample scan is ratioed against a previously measured background spectral subtraction of water vapor/carbon dioxide, reference spectrum should be used. 4. X-axis scale. Plots should employ a 2X-axis expansion and not a simple linear X-axis. This provides an expansion of the important fingerprint region. Figure 3.6 shows the FT-IR spectrum of starch with two different sample preparations. In the top spectrum (A), the starch was dissolved in water and prepared as a cast film on ZnSe resulting in a good quality, photometrically accurate spectrum. In the case of starch, this is one of the preferred sample-preparation techniques. The Nujol mull preparation of powdered starch provides an unacceptably poor quality FT-IR spectrum and a so-called false spectrum. A broadening of the most intense bands and a strengthening of the weak bands characterize false IR spectra. In general, this is caused by a non-uniform sample film or distribution of sample in a Nujol of KBr matrix. In the case of starch, good quality IR spectrum cannot be obtained by either Nujol mulls or KBr discs. Starch demonstrates that the nature of the particular sample dictates what sample-preparation technique is appropriate. 2.3. Transmission IR Simple transmission IR measurements are commonly made of gas, liquid, and solid-state samples. The most accurate information is obtained if the path length or analyte concentration is adjusted such that the peaks of interest have an absorbance between 0.3 and 0.9. The two simplest types of samples that can be prepared are gas phase samples and neat (low volatility) liquids. Gas phase measurements require only the introduction of the sample into a gas 35 SAMPLING METHODS FOR IR SPECTROSCOPY 851 762 708 934 1240 1447 1410 1364 2928 60 20 1152 1081 1026 40 3380 %Transmittance 80 1643 2152 100 (A): Water cast film/ZnSe 0 100 2156 60 3500 3000 2500 2000 1800 1600 993 930 850 1400 1200 1081 1153 1373 (B): Nujol mull 1457 2853 3291 0 1244 N 20 1000 760 718 N 1643 40 2922 %Transmittance 80 800 600 Wavenumber (cm–1) FIGURE 3.6 The FT-IR spectrum of starch prepared as (A) water cast film on a ZnSe plate and (B) Nujol Mull. The N marks the Nujol bands. cell, which can vary in path length from 10 cm to 8 meters. Highly accurate quantitation of single and multiple components is possible because the path length is well defined and no solid state effects are present (see below). 2.3.1. Liquids and Solutions Liquids and solutions are easily measured by FT-IR spectroscopy. Neat liquids can be easily prepared as a capillary film between two plates. The liquid film should be uniformly thick without holes or voids to insure a good, quantitative IR spectrum. Some common sample-preparation techniques for liquids include the following: 1. Capillary film. Neat liquids can be easily prepared as a capillary film. A drop of liquid sample is simply sandwiched between the cell windows. 2. Demountable cell. Here the technique is identical to the capillary film except a spacer (usually lead or teflon) is used to define the thickness of the liquid film. Disassembly allows access to the windows for easy cleaning or repolishing. The cell thickness is easily changed by selecting a different spacer. For a neat liquid sample, a thickness of about 0.01 mm is suitable. 36 3. INSTRUMENTATION AND SAMPLING METHODS 3. Sealed amalgam cell. This cell is constructed using a lead spacer which is amalgamated with mercury to create a very tight seal. This type of cell provides very accurate path length and is suitable for very volatile liquids. The filling and cleaning of the cell are done through the use of a pair of hypodermic syringe fittings. Fixed path length cells are commercially available in path lengths from 15 mm to 500 mm and can be used to measure the analyte dissolved in a suitable solvent. Both pure liquids and solutions can be measured using the above. Sealed amalgam cells are commonly used for solutions. Additional sample preparations include smears, cast films, and analytes in solution. More viscous liquids can be prepared as a smear on a single window. 2.3.2. Cast Films Samples that have been dissolved in a volatile solvent can be prepared by casting a thin film and allowing the solvent to evaporate. The sample is dissolved into a moderate to highly volatile solvent, and a film is prepared by evaporating the solution directly onto an IR transmitting window. The non-volatile sample remains behind on the plate as a thin film. This sample-preparation technique is widely used, but not limited to, obtaining the IR spectra of polymers Typically, a uniform film can be obtained by spreading the film repeatedly on a window using a spatula or the edge of a capillary pipet. Ideally, the resulting film should be amorphous to eliminate scattering and crystallinity effects. In the case of smaller molecules, evaporation of the solvent can result in formation of a thin crystalline film. It is often important that identical solvents and drying conditions are employed for both the sample and the reference material. This can be important since in many cases, significant differences can be observed in the IR spectrum as a function of the crystalline form. When preparing a cast film, first select a suitable solvent and IR window. The solvent must not react with the sample, but should be volatile and dissolve the sample. For the majority of applications, NaCl or KBr windows are used and ZnSe is frequently used for aqueous solutions. Dissolve the sample in the selected solvent. The concentration is not critical, however, higher concentrations will result in faster sample preparation. Now add a few drops of the solution on the central portion of the infrared window. Gentle heating using a hot plate and a flow of nitrogen can be used to hasten the solvent evaporation. Care should be taken since some materials will decompose even under gentle heat. With more volatile solvents, the solution can be allowed to spread out over the surface of the window and as the solvent evaporates a thin film will remain. However, some solutions will not spread out evenly by themselves and will require manipulation to obtain a thin uniform film (see below). With less volatile solvents (for example water) or solutions that will not spread out evenly on the surface of the window, a flexible spatula or glass pipet should be used to spread a thin film and facilitate the solvent evaporation. For a uniform film, the solution may require a thin film to be stroked on. Examine the resultant film visually to confirm that a thin uniform film free of holes and voids has been prepared. The thickness of the film for FT-IR spectroscopy should be approximately 5 mm. There is only limited control over the thickness of a cast film. In general, the SAMPLING METHODS FOR IR SPECTROSCOPY 37 more concentrated the solution and the more drops placed on the window, the thicker the film. The process of obtaining the correct thickness is simply trial and error. Lastly, confirm from the IR spectrum that all of the solvent has evaporated leaving only the material of interest. The analyst should be familiar with the spectrum of the pure solvent. 2.3.3. SOLID-POWDERED SAMPLES: KBR DISCS AND NUJOL MULLS Solid-powdered samples can be prepared as fine particles dispersed in NUJOL or as a disk in a transparent KBr matrix. A NUJOL mull preparation simply mixes mineral oil and finely ground sample to form a paste, which is then sandwiched, between two IR transmitting windows. A KBr disc uses a mixture of dried KBr powder and finely ground sample. The KBr/sample mixture forms a clear disc when pressed under high pressure using a hydraulic press. Both preparations require that the sample must be sufficiently ground and mixed with the support matrix to insure a good, artifact free IR spectrum. Insufficiently mixed and ground sample particles result in undesirable spectral artifacts. Most obvious is a sloping baseline due to scattering. A distorted band shapes as a result of anomalous dispersion of the refractive index in an absorption band (Christiansen effect). False spectra due to non-uniform sample thickness are characterized by broadening of the bandshape of the strong bands and relative intensification of weak bands. NUJOL MULL A Nujol mull involves mixing of mineral oil and finely ground sample to form a paste, which is then sandwiched, between two IR transmitting windows (typically KBr or NaCl). In general, 30e50 mg of sample is needed for this sample preparation. The sample must be ground to an average particle size of at least 0.5 mm using an agate mortar and pestle. Larger particles approaching the wavelength of light being used will scatter light resulting in a sloping background. A suitably ground sample will be evenly distributed over the mortar and takes on a smooth, glossy appearance. A very small amount of the mulling agent (mineral oil) is added to the mortar, and grinding is continued until the sample is suspended in the mulling agent to form a smooth, creamy paste. A small amount of the mull is transferred to the center of an infrared window. A second window is placed on top of the sample and the two are pressed together. The mull should be spread out into a thin translucent film, free of holes or voids. The representative IR spectra of Nujol and Fluorolube is shown in Fig. 3.7. Nujol has bands characteristic of a long chain hydrocarbon. If detailed information is needed in the CH stretching region and the deformation region, a Fluorolube (perfluorinated hydrocarbon) mull should be used which has no appreciable absorption bands from 4000 to 1350 cm1. If necessary, a split-mull spectrum can be obtained by merging a Nujol and Fluorolube mull spectra. A significant advantages of Nujol mull sample preparation is that no water is introduced into the sample. This can be particularly important when trying to confirm the presence of an OH or NH type species or when analyzing a particularly hydroscopic material. When faced with analyzing particularly toxic solid materials, the analyst can limit potential exposure by simply mulling the sample between the salt plates. 38 3. INSTRUMENTATION AND SAMPLING METHODS 598 50 650 1042 1273 60 902 2955 40 30 511 1377 1463 70 2854 %Transmittance 80 438 90 723 2297 100 3500 3000 2500 2000 1800 1600 1400 1200 970 1126 10 1199 2925 20 1000 800 600 Wavenumber (cm–1) FIGURE 3.7 The FT-IR spectrum of Nujol and Fluorolube. The Nujol spectrum is shown as a solid line and the Fluorolube spectrum is shown as a dotted line. KBR DISC SAMPLE PREPARATION A KBr disc involves mixing of dried KBr powder and finely ground sample. The KBr/ sample mixture forms a clear disc when pressed in a die under high pressure using a hydraulic press. Only 5 mg of sample is needed for this sample preparation. The sample must be ground to an average particle size of at least 0.5 mm using an agate mortar and pestle to minimize a scatter-induced sloping background. The sample concentration varies from 0.1% to 3% (by weight). In general, start with ca. 5 mg of sample and add 500 mg of KBr powder a little at a time and mix well. Transfer the thoroughly mixed KBr preparation into a 13 mm die. Press the disc in a bench top press. With a hydraulic press, anywhere from 2 to 8 tons is sufficient. Solids are more frequently run as KBr discs than as mulls. This preference probably arises because KBr has no absorptions of its own over the entire transmission range and requires far less sample than a mull. However, the KBr disc method does have some disadvantages compared to a mull. KBr is very hygroscopic and some water is almost always introduced in the sample preparation. Figure 3.8 shows a KBr disc prepared without any organic sample. The water introduced from the sample grinding results in bands at ca. 3440, 1630, and 560 cm1, from the OH stretch, OH bend, and OH wag that can complicate interpretation of the spectrum. KBr discs of crystalline materials are also sometimes less reproducible than mulls. The large pressure along with the addition of water can cause changes in hydration state, crystallinity, or polymorphism. In some cases, the sample itself can ion exchange or react with KBr. 2.3.4. Melts Melts are the last commonly used IR sample technique that is particularly useful for polymers. This method involves melting the sample on a salt plate and allowing it to resolidify. This method is not recommended for crystalline materials because of possible orientation 39 SAMPLING METHODS FOR IR SPECTROSCOPY 100 60 1632 3434 %Transmittance 80 70 558 90 50 40 30 20 10 0 3500 3000 2500 2000 1800 1600 1400 1200 1000 Wavenumber (cm–1) 800 600 FIGURE 3.8 The FT-IR spectrum of a KBr disc prepared by mixing KBr powder using a mortar and pestle into a fine powder and preparing a clear disc using a hydraulic press. Characteristic bands from water are observed at 3434, 1632, and ca. 560 cm1. effects. If a sample is known to decompose under heat, this method should not be employed. The technique involves using a hot plate, heat the sample above its melting point, and allowing the sample to become a liquid. Next, using a pipet take one drop of the melt and place it on an IR transmitting window such as KBr. Use the pipet to draw down the liquid into a uniform film and allow it to form a solid. 2.4. Reflection Techniques Two of the more commonly used FT-IR sampling techniques that capitalize on the spectral information that can be obtained from reflection (and refraction) properties are ATR (attenuated total reflection or internal reflection) and DRIFTS (diffuse reflectance FT-IR spectroscopy). 2.4.1. Attenuated Total Reflectance (ATR) ATR is a contact sampling method that involves a crystal with a high refractive index and excellent IR transmitting properties. ATR is one of the more popular sampling techniques used by FT-IR Spectroscopists because it is quick, non-destructive and requires no sample preparation. It is a contact sampling method that involves an IRE (Internal Reflection Element) with a high refractive index and excellent IR transmitting properties. The sampling technique capitalizes on the spectral information that can be obtained from reflection phenomena. The technique is used to measure the IR spectra of surfaces or of material that is too thick or strongly absorbing to be analyzed by more traditional transmission methods. A variety of ATR accessory designs exist and are available from almost all manufacturers of accessories for infrared spectrometry. In the case of ATR, the angle of incidence must be greater than the critical angle so that total internal reflectance occurs. Figure 3.5 depicted the dependence of light transmission, 40 3. INSTRUMENTATION AND SAMPLING METHODS refraction and reflection upon both the material refractive indices and the angle of incidence. However, the reflected light does contain spectral information about the sample at the sampleecrystal interface and the ATR technique capitalizes on this. A single reflection crystal is commonly used in micro-ATR work and a multiple reflectance ATR crystal is often used for bulk samples. An ATR accessory provides an IR spectrum of a sample because the radiation at the reflection point probes the sample with an “evanescent wave.” At a frequency within an absorption band, the reflection will be attenuated while at frequencies well away from an absorption band, all light is reflected. Internal reflectance occurs when the angle of incidence exceeds the “critical” angle. This angle is a function of the real parts of the refractive indices of both the sample and the ATR crystal 1 h2 qC ¼ sin h1 Here, h2 is the refractive index of the sample and h1 is the refractive index of the crystal. Typical crystals such as Ge, ZnSe, or diamond have high refractive indices much higher than that of organics resulting in internal reflections at moderate angles of incidence. Figure 3.9 depicts some of the basic principals of ATR measurements. The refractive index of the sample is a complex with both a real and imaginary component: h ¼ n þ ik The real part of this expression n in the regular refractive index holds when there is no absorption. The imaginary component of the refractive index, ik, holds within an absorption Sample z e–z/dp FIGURE 3.9 dp O1 IRE O2 Diagram of a single reflection ATR crystal depicting the basic principles of the technique. SAMPLING METHODS FOR IR SPECTROSCOPY 41 band resulting in an effect known as anomalous dispersion. The parameter k is directly related to the extinction coefficient found in the LamberteBeer law. The total absorption intensity, A, as a function of q can be expressed as: Z N aðzÞez=dp dz AðqÞ ¼ 0 where z is the depth into the sample and a(z) is the absorption coefficient of the sample as a function of depth. As depicted in Fig. 3.9, the reflected radiation penetrates the sample boundary as a so-called evanescent wave. The electric field amplitude of this evanescent wave exhibits an exponential dependence. The parameter, dp, the effective penetration depth is expressed as: l dp ¼ 1=2 2pnp sin2 q n2sp Where, l equals the wavelength of the radiation, q is the angle of incidence, np is the refractive index of the IRE, and nsp ¼ n2/n1 the ratio of the refractive indices of the sample and IRE. This depth of penetration is defined as the distance from the IRE-sample boundary where the intensity of the evanescent wave decays to 1/e (37%) of its original value. There are a number of important consequences to the above expression. The sampling depth of the ATR method is approximately 2e15 mm, is wavelength dependent, and increases with decreasing wavenumber. Experimentally, varying the angle of incidence of the light and the refractive index of the crystal used can control the sampling depth. Increasing either results in a decreased sampling depth. Careful comparison with classic transmission spectrum also indicates that ATR measurements result in small differences in the peak frequency and bandshape as a result of refractive index effects. ATR requires excellent contact between the sample and the crystal and is therefore an excellent method for liquids or soft, easily deformed solids. Use of micro ATR crystals with a protective ATR coating enable much greater pressure to be used to insure good contact between the ATR crystal and harder materials. Table 3.2 summarizes some of the commonly used ATR crystal materials. 2.4.2. Diffuse Reflectance Diffuse reflectance (DRIFTS, Diffuse Reflectance Infrared Fourier Transform Spectroscopy) is applied to analyze powders and rough surface solids. Typically, the technique is applied to powders since the technique relies upon scattering of radiation within the sample. The incident light on a sample may result in a single reflection from the surface (specular reflectance) or be multiply reflected giving rise to diffusely scattered light over a wide area which is used in DRIFTS measurements. Collection optics in the DRIFTS accessory are designed to reject the specularly reflected radiation and collect as much of the diffuse reflected light as feasible. In general, theories to describe the diffuse reflectance phenomena consider plane-parallel layers within the sample with light scattering in all directions. The observed reflectance 42 3. INSTRUMENTATION AND SAMPLING METHODS TABLE 3.2 Commonly Used ATR)Crystal Materials and Characteristics Material Wavenumber range (cmL1) Refractive index Comments Germanium 5000e850 4.0 Hard and brittle, good ATR material but temperature sensitive. KRS-5 5000e275 2.38 Water insoluble, conventional ATR material. Relatively soft, deforms, poisonous. Reacts with complexing agents. ZnSe 5000e550 2.41 Water insoluble, hard and brittle, good ATR material. Attacked by acids and strong alkalines. Diamond 4500e2500 2.4 Very hard, inert, used in micro-ATR crystals. Often used as a protective film for ATR elements such as ZnSe 3.4 Limited wavelength range for ATR. Material is inert, hard and brittle. 1800e200 Silicon 8300e1500 * ATR is attenuated total reflectance. spectrum using the KubelkaeMunk equation is a ratio of the single beam spectra of the sample diluted by KBr powder against pure powder KBr and is defined as: f ðRN Þ ¼ ð1 RN Þ2 K C ¼ ¼ 2RN s k where K is the absorption coefficient and s is the scattering coefficient of the sample material. The reflectance, RN, indicates the sample is thick enough that no radiation reaches the back surface. The above predicts a linear relationship between K and the maximum value of f(RN) for each peak provided s is constant. The scattering coefficient is dependent on the particle size and packing and must remain constant for reproducible results. Lastly, C is the concentration of the sample and k is related to the particle size and molar absorptivity of the sample, k ¼ s/2.303e. The DRIFTS spectrum can show a strong dependence on the refractive index of the sample, the particle size and size distribution, packing density, and sample homogeneity. For quantitative analysis, the particle size, sample packing, and dilution must be carefully controlled. To obtain the highest quality DRIFTS spectrum, the following precautions are taken. 1. Particle size should be small and uniform. This will result in narrower bandwidths and more accurate relative intensities. The sample/matrix particulates size should be 50 mm or less. 2. Dilute sample in a non-absorbing matrix powder such as KBr or KCl. The finely powdered sample is ~5% relative to the matrix powder. This insures a deeper penetration of the incident light into the sample, which will increase the diffuse scattered light. 3. Homogeneity. The samples should be well mixed. 4. Packing. The sample should be loosely packed in the cup to maximize the IR beam penetration. SAMPLING METHODS FOR IR SPECTROSCOPY 43 For quantitative analysis, the particle size, sample packing, and dilution must be carefully controlled. Despite compliance with the precautions listed above, the ratioed diffuse reflectance spectrum will appear different from that obtained with a classic transmission IR measurement. In general, low intensity bands will be increased relative to intense bands and the strong intensity bands will have broader, rounder peak shapes. A KubelkaeMunk conversion available in most FT-IR software packages can compensate for some of these effects. 2.5. Microscopy Both IR and Raman spectroscopy have been very successfully coupled to microscopy with widespread applications in heterogeneous samples in industrial, forensic, and biological sciences. Vibrational microscopy is used for particle and contaminant analysis, studies of laminar structures, characterization of fibers, and domain structure and chemical compositional mapping. IR microspectroscopy involves coupling a microscope to an infrared (typically FT-IR) spectrometer and provides the capability to obtain spectra of extremely small samples (as small as 10 mm in diameter in certain specialized applications). The technique has widespread applications in the study of heterogeneous samples and is used for particle and contaminant analysis, studies of laminar structures, domain structures, and chemical composition mapping. The microscope system allows one to focus IR radiation onto the sample, to collect and image transmitted or reflected IR radiation from the sample onto a sensitive detector and to view and position the sample. The system includes an optical microscope system for viewing the sample and reflective optics for imaging the IR radiation. The system also includes remote apertures for defining the spatial area of the sample to be examined. Commercial FT-IR microscopes are capable of a variety of sampling techniques including: specular reflectance, diffuse reflectance, micro-ATR, grazing angle, and conventional transmission measurements. Just as when studying materials in bulk, it is the sample itself that determines the most appropriate sampling technique. Several commercial Raman microspectrometers are available with excitation sources that span the UV to the near-IR. The ease of sampling makes Raman spectroscopy an excellent choice for microscopy. However, fluorescence remains a considerable obstacle to application of Raman microscopy to many samples. More recently, IR and Raman imaging have been successfully applied to a variety of heterogeneous systems. 2.5.1. Transmission IR Microscopy The ideal sample for transmission IR microscopy is a thin (5e10 mm), smooth, flat sample that does little to alter the optical path of the IR radiation. This requires the sample to also be large enough (ca. 25 mm) to minimize diffraction of light. Sample-preparation techniques can include: a) slicing and microtoming b) flattening sample with a roller c) squeezing sample between salt plates or a diamond compression cell As with macroscopic IR measurements, sample preparation plays an important role in obtaining quality IR spectra when using an IR microscope. Important experimental 44 3. INSTRUMENTATION AND SAMPLING METHODS parameters include the diffraction light limit, light scattering, and focal shifts due to high refractive index samples. When the sample or aperture size is on the order of the wavelength of light being used, diffraction plays a major role in the measured IR spectrum. Significant distortion of the measured IR spectrum occurs. Diffraction effects can be minimized by the: a) use of widest possible apertures. Keep aperture closed within the sample image but not closed enough to reduce energy throughput significantly. b) use of the same aperture setting in the background and sample scan. c) flattening of sample which will enlarge the sample area. Surface irregularities and fine particles can scatter light. Remedies include adding KBr or KI and applying pressure to obtain a clear plate or adding NUJOL oil to the a roughened surface. Samples or substrates with very high refractive index can give rise to focal shifts. This can be corrected by adjusting the focal system of the microscope by observing either the sharpness of the aperture edge or the energy throughput. At times unwanted internal reflections can also interfere. An example of this is interference fringes that can occur when using a diamond anvil cell. 2.5.2. Reflection IR Microscopy Infrared reflectance measurements are desirable because of the reduced sample preparation required. The IR microscope will be set to reflectance rather than transmittance mode and if necessary, to an additional attachment placed on the IR objective. Commonly used reflectance techniques include specular reflectance and ATR measurements. The most commonly used reflectance technique is ATR. A micro-ATR accessory is employed that attaches to the IR microscope objective. Once contact is established between the sample and the ATR element, the IR spectrum is measured. The resultant spectrum is typical of that obtained using any ATR accessory. Specular reflectance is a less commonly used technique for IR microscopy and involves a front surface mirror-like reflection from the exterior of a material where the angle of incidence is equal to the angle of reflection. The measured spectrum depends on the angle of incidence, the sample index of refraction, absorption properties, and sample surface morphology. The technique requires no sample preparation and is best suited for smooth, highly reflective surfaces and can include samples such as single crystals and thin films on a reflective metal surfaces. Not surprisingly, the resultant specular reflectance IR spectra are very different from typical transmission IR data. The spectra show derivative-like shape within the absorption bands. A KramerseKronig transformation which is often available in FT-IR software is often used to deconvolute the specular reflectance data and provide more absorbancelike spectra. 3. QUANTITATIVE ANALYSIS Any quantitative analyses involve measurement of a test sample and comparison with standards of known concentration. The resulting calibration plot reflects the relationship QUANTITATIVE ANALYSIS 45 between the measured quantity (such as infrared absorbance or the Raman intensity) and the analyte concentration. Provided appropriate standards for calibration of an analytical method selected, vibrational spectroscopy is a valuable potential technique for quantitative analysis.1 Quantitative analysis methods can include both classical and chemometric PLS (principal least squares) analysis. Only classical analysis techniques will be discussed here. For an understanding of chemometric applications to spectroscopy the reader should consult a reference dedicated to this subject.8, 9 Advantages of using vibrational spectroscopy include the following: 1. Wide variety of sample types (gases, liquids, solids) 2. Clearly separated peaks are often found in the IR and Raman spectra 3. Unique bands frequently allow multicomponent analyses of mixtures 4. Typically not much effort is needed to acquire or prepare sample for analysis 5. Sample quantity is in milligram quantity Both the analyte and non-analyte components will be observed by both IR and Raman spectroscopy. Consequently, the main components of the sample must be confirmed prior attempting quantitation. In general, Raman and IR spectroscopy of solids can be performed at concentrations of 1e100%. When strong isolated bands are available, this can be extended to lower concentrations (~0.1 %). IR spectroscopy can be employed to quantitate much lower concentrations (down to ppm levels) by greatly increasing the sample path length for solutions and for gases. 3.1. Relationship of IR and Raman Signal to Analyte Concentration The basis of quantitative analyses in absorption spectroscopy is the LamberteBeer law. log I0 1 ¼ log ¼ A ¼ abC I T where I0 is the incident light, I is light after it has passed through the sample, T is the transmittance, A is the absorbance, a is the absorption coefficient, b is the sample thickness, and C is the sample concentration. The absorption coefficient is a constant specific for the material at a particular wavelength. Where the LamberteBeer law holds, the measured absorbance is a linear function of the concentration (and path length). Both single component and multicomponent analysis is possible. Analytical methods based upon IR and Raman spectroscopy often use either peak heights or peak areas. Commercial instrument software presently allows both peak height and areas to be used when developing analytical methods. The integrated peak areas may provide a more robust calibration if the analyte band changes with increasing concentration or is influenced by molecular interactions. Deviations from the LamberteBeer law resulting in a non-linear relationship between the measured absorbance and the analyte concentration often occur. At the high concentrations typically encountered in IR measurements of solids, the molecular interactions of the sample itself will often cause non-linearity. However, if the calibration takes such non-linearity into account, it will not affect the quantitative accuracy of the analytical method. 46 3. INSTRUMENTATION AND SAMPLING METHODS As discussed in the previous chapter, the intensity of the Raman scattered radiation is given by: 2 va IR fn4 Io N vQ where Io is the incident laser intensity, N is the number of scattering molecules in a given state, n is the frequency of the exciting laser, a is the polarizability of the molecules, and Q is the vibrational amplitude, and the parameter ðva=vQÞis the Raman cross section. Analogous to infrared spectroscopy, the Raman cross section is a constant specific for the material at a particular wavelength. Thus, if the Raman cross section and the incident laser intensity (I0) remain constant, the intensity of the Raman band (IR) is directly proportional to the sample concentration. Once again, at the high concentrations often encountered in Raman measurements interactions of the components in the sample itself often result in non-linearity. To develop a quantitative analysis method for either IR or Raman spectroscopy, well-characterized standards covering a broad range of compositions are required. In the case of the IR spectra of solid-state samples, neither the sample thickness nor the concentration of the sample in the matrix is known. With Raman spectroscopy, the incident laser intensity can be very difficult to precisely control. For this reason, a ratio of an analyte band is often normalized against another component to eliminate these sources of variation.6 The selection of appropriate peaks in the spectrum are chosen as the inputs for the quantitative analysis based on the spectral separation and knowledge of the underlying vibrational modes. 3.2. Quantitative Analysis: Ratio Method A calibration curve defines the relationship between an analytical signal produced by the analyte and its concentration. In the common case of linear calibration, a linear regression will be used to fit the analytical signal y to the concentration x of the analyte in calibration samples. y ¼ a þ bx For determination of an unknown, the calibration equation is inverted. a y x ¼ þ b b A different form of calibration equation is often applicable in spectroscopy. The simplest case involves only two components (C1 þ C2 ¼ 100%) with two isolated analytical bands with absorbances A1 and A2. Here, we assume a linear relationship between the concentration of the analytes and the observed signal with a slope that differs depending upon the analytes response function. For IR spectroscopy, this response will depend upon the molar absorptivities of the bands and for the Raman cross section. This simple case is commonly used when employing the ratio method of IR analysis that relies upon Beer’s law and enables quantitation of components without knowledge of the sample path length. A1 b1 a1 C1 C1 ¼ ¼ k A2 b2 a2 C2 C2 QUANTITATIVE ANALYSIS 47 With simple rearrangement, we then obtain the following: %C1 ¼ 100 1 þ kA2 =A1 In general, the analysis is more complex since the calibration curves are often affected by overlapping bands, additional interfering components, molecular interactions effecting the absorption coefficient and Raman cross sections and in the case of FT-Raman the selfabsorption phenomena discussed below. Thus, the expressions for both IR and Raman are more complex than that suggested by the simple two component case. For IR, we could simply expand the original expression as: A1 ¼ a1 b1 C1 þ a2 b2 C2 For FT-Raman spectroscopy, the expression is more complex and below we add an additional term (1eax) which takes into account near-IR self-absorption of the Raman scattered light. vs 1 eax Þ þ K IR eNIo vU The resultant calibration curves will often be non-linear. Most instrumental software will enable the data to be fit with empirical equations such as quadratic or higher-order polynomial fits. An alternative is the use of a class of semi-empirical equations and linear fractional transformations. The equation form is suitable for the peak ratio method. Unlike quadratic equations, linear fractional transformations are easily inverted, which facilitates the development of calibration equations in a manner similar to the usual practice encountered in the linear calibration case. 3.2.1. Fractional Linear Calibration Equations Fractional linear transformation can be applied to account for typically encountered variation from the simple two-component case discussed above.10 For a mixture of two components A and B, a spectral response (absorbance or Raman intensity) in which there may be interference with the two analytical bands is R1 ¼ k1A CA þ k1B CB R2 ¼ k2A CA þ k2B CB The ratio of concentrations CA/(CA þ CB) (mole fraction) as a function of the ratio of the spectral response R1/(R1 þ R2) has the form of a fractional linear transformation CA aR1 þ bR2 ¼ ðCA þ CB Þ R1 þ dR2 where coefficients a, b, and d are constants which can be determined empirically. In our case, b will often equal zero and the above simplifies to an expression with only two variables: CA aR1 ¼ ðCA þ CB Þ R1 þ dR2 48 3. INSTRUMENTATION AND SAMPLING METHODS The above equation can be employed for the non-linear calibration curves typically encountered in many IR and Raman quantitation procedures. CA R1 1 ¼ ¼ ðCA þ CB Þ R1 þ dR2 1 þ dR2 =R1 In a specialized case where a ¼ 1 and b ¼ 0, the above expression simplifies to the standard expression for the ratio method of a linear system that we started with. 3.3. Example of Quantitation Using Ratio Method of Analysis Below is an example of quantitation of the components in PAMeAETAC water-soluble copolymers using both FT-IR and FT-Raman spectroscopy. In this example, a broad selection of nine well-characterized PAMeAETAC standards are used to develop calibration curves for both IR and FT-Raman spectroscopy. Since the calibration curves are non-linear, a linear fractional transformation equation is used to model the non-linear calibration curves. The structure of poly AMDeAETAC is shown below. poly (AMD—AETAC) CH2 CH C CH2 O CH C NH2 O O n m CH2 CH2 N(CH3)3 Cl Copolymers of acrylamide (AMD) and 2-(acryloyloxy) ethyltrimethylammonium chloride (AETAC) Figure 3.10 shows the FT-IR and FT-Raman spectra of pure PAM, and two PAMeAETAC copolymers with 53 and 90% AETAC(Q9) respectively. These spectra illustrate the many strong characteristic bands that exist for the polymer backbone, primary acrylamide, the ester carbonyl, and the positively charged trimethylammonium chloride groups. Table 3.3 lists assignments of some of the important IR and Raman bands for PAM and AETAC. Figure 3.11 shows the FT-IR and FT-Raman spectra of 47% AMD: 53% AETAC copolymers along with the selected baselines and peak heights used for this analysis. Bands characteristic of both groups are isolated and intense enough to provide quantitative bands for both components. In the IR spectrum, the quantitation method uses the AETAC ester carbonyl and the AMD amide carbonyl at 1734 and 1671 cm1, respectively. In the FT-Raman spectrum, the quantitation method uses the AETAC trimethylammonium chloride band at 719 cm1 and the AMD primary amide NH2 rock band at ca. 1105 cm1. 49 QUANTITATIVE ANALYSIS Figure 3.12 shows the FT-IR calibration plot for AMDeAETEC copolymers using a peak height ratio and plotting the calculated fit using nine calibration standards. The FT-IR calibration plot uses the peak height ratio of the AETAC ester carbonyl relative to the total carbonyl (ester þ amide). The non-linearity observed in the above IR calibration curve is a result of deviations in Beer’s law and overlapping contributions of the two carbonyl bands. The two variable functions chosen to model the calibration curve provides an excellent semi-empirical method to take into account these sources of non-linearity. The mole fraction AETAC with can be calculated from the FT-IR spectrum using the following expression: Mole fraction AETAC ¼ 1:088ðAbs:1734 cm1 Þ ðAbs: 1734 cm1 Þ þ 0:874ðAbs:1670 cm1 Þ (a) 783 1124 PAM 1621 3199 1663 3344 3169 100 80 1166 954 630 1055 1262 1482 1450 1414 1340 1629 1734 20 1671 40 2955 60 3018 %Transmittance 3342 40 20 1177 60 1451 1419 1349 1323 2932 80 2858 100 PolyAMD—AETAC (52.5%) 100 3500 3000 2500 2000 1800 1600 955 1166 1400 877 1057 1341 1256 1453 1390 1485 1733 20 3379 40 1671 60 3013 2960 80 1200 PolyAMD—AETAC (89.6%) 1000 800 600 Wavenumber (cm–1) FIGURE 3.10 The FT-IR and FT-Raman spectra of the following: pure PAM and two PAMeAETAC copolymers (47/53 and 10/90). The FT-IR spectra are shown on the left and the FT-Raman spectra on the right. A cast film sample preparation on ZnSe of all three samples after dissolving the powders in water was used for the FT-IR data. For the FT-Raman data, the samples were in the supplied crystalline form using an NMR tube for sampling. 50 3. INSTRUMENTATION AND SAMPLING METHODS (a) 783 1124 PAM 1621 3199 1663 3344 3169 100 80 1166 954 630 1055 1262 1482 1450 1414 1340 1629 1734 20 1671 40 2955 60 3018 %Transmittance 3342 40 20 1177 60 1451 1419 1349 1323 2932 80 2858 100 PolyAMD—AETAC (52.5%) 100 3500 3000 2500 2000 1800 1600 955 1166 1400 877 1057 1341 1256 1453 1390 1485 1733 20 3379 40 1671 60 3013 2960 80 1200 PolyAMD—AETAC (89.6%) 1000 800 600 Wavenumber (cm–1) FIGURE 3.10 (continued). TABLE 3.3 Some Band Assignments for the Poly AMD-AETEC Samples Group PAM Bands (cmL1) Assignments NH2 3350, 3200 (IR,R) NH2 i.ph. str. and o. ph. str. NH2 1608e1621 (IR,R) NH2 bend NH2 1107 (R) NH2 rock C¼O 1663e1656 (IR,R) C¼O str. CH2eCH 2936 (IR) CH2 o. ph. str. CH2eCH 2925 (R) CH(C¼O) str. CH2eCH 1455 (IR,R) 1324 (IR,R) CH2 bend CeN 1424 (IR,R) Involves CeN str. 51 QUANTITATIVE ANALYSIS TABLE 3.3 Some Band Assignments for the Poly AMD-AETEC Samplesdcont’d Group PAM Bands (cmL1) Assignments C¼O 1608e1621 (IR,R) NH2 bend Ester C¼O 1734 (IR,R) C¼O str. Ester C¼O 1166 (IR) Involves CeO str. CH2eN(CH3)3 3010e3022 (IR,R) CH3 o. ph. str. CH2eN(CH3)3 955 (IR,R) Involves NC4 o. ph. str. þ CH3 rock CH2eN(CH3)3 720 (R) Involves NC4 i. ph. str. Ester C¼O 1734 (IR,R) C¼O str. Peak Ht AETAC C=O 0.8 1671 0.9 1734 AETEC Poly AMD(47%) — AETAC(53%) Peak Ht AMD C=O 1166 0.6 0.3 954 0.4 1262 1482 0.5 1450 1416 Absorbance 0.7 0.2 0.1 719 0.0 30 20 953 1036 1107 1089 1673 5 1620 1732 10 1340 15 1242 Peak Ht AMD NH2 rk 874 846 1450 25 Raman intensity Peak Ht AETAC NC4 i.ph str. 0 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 Wavenumber (cm–1) FIGURE 3.11 The baseline and peak height measurements used for quantitation. The top FT-IR spectrum shows the peak heights used to quantitate the ester and amide carbonyl bands. The bottom FT-Raman spectrum shows the peak heights used to quantitate the amide NH2 rock and the cationic ammonium NC4 in-phase stretch bands. 52 3. INSTRUMENTATION AND SAMPLING METHODS 1.0 A (1733 cm ) [A (1733 cm ) + A (1670 cm )] C=O Absorbance ratio 0.8 1.088 (A 1733 cm ) (A 1733 cm ) + 0.874 (A 1670 cm ) 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 Mole fraction AETAC 0.8 1.0 FIGURE 3.12 FT-IR calibration plot for AMDeAETAC copolymers. The form of the fractional linear transformation used to fit this data is only very slightly non-linear. 1.0 Raman intensity ratio 0.8 0.6 I (719 cm ) 0.4 [I (719 cm ) + I (1105 cm )] 0.2 [(I 719 cm ) +15.96 (I 1105 cm )] 2.39 (IR719 cm ) 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Mole fraction AETAC FIGURE 3.13 The FT-Raman calibration plot for powdered AMDeAETAC copolymers. The form of the fractional linear transformation used to fit this data is quite non-linear. Figure 3.13 shows the FT-Raman calibration plot for AMDeAETEC copolymers using a peak height ratio and plotting the calculated fit for the same nine standards. The form of the fractional linear transformation used to fit this data is quite non-linear. Despite this, the FT-Raman technique provides excellent quantitation of PAMeAETAC copolymers. The mole fraction AETAC with can be calculated from the FT-Raman spectrum using the following expression: 53 QUANTITATIVE ANALYSIS 0.6 Absorbance 0.5 500 1000 2000 3000 4000 0.4 0.3 0.2 0.1 Poly AMD–AETAC (90%) PAM 0 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 Wavelength (nm) FIGURE 3.14 The diffuse reflectance near-IR spectra of pure PAM and poly AMDeAETAC (90%) copolymer. Note the different absorbances at 1152 and 1206 nm where the analytical Raman bands are located. Mole fraction AETAC ¼ 2:39ðIR 719 cm1 Þ ½ðIR 719 cm1 Þ þ 15:96ðIR 1105 cm1 Þ The significant non-linearity in the calibration curve above is due largely to narrow band self-absorption phenomena that are typical of 1064 nm excitation-based Raman spectra.1 Such self-absorption is defined as the absorption of the Raman scattered light by the sample near-IR absorption bands themselves before the light can escape the sample in the NMR tube used in this experiment. Where there is a near-IR absorption band, the Raman band intensity will be decreased. The magnitude of this effect will vary significantly in powdered samples because of the variation in the path lengths the scattered light will follow. In a liquid sample, the magnitude of the Raman band suppression will follow a simple the LamberteBeer law. Figure 3.14 shows the diffuse reflectance near-IR spectra of pure PAM and poly AMDeAETAC (90%) copolymer. The 719 cm1 AETAC band and the 1105 cm1 PAM band both have different near-IR absorbances for these two components. Much of the observed curvature in the correlation plot is a consequence of this selfabsorption process and not molecular environmental induced changes in the Raman cross sections. 54 3. INSTRUMENTATION AND SAMPLING METHODS Chapter 3: Questions 1. Outline the dispersive unit used in an FT-IR spectrometer to separate light into individual components. 2. Summarize the four guidelines for quality FT-IR spectrum. 3. Good sample-preparation skills are necessary to acquire photometrically accurate, high quality IR spectra. How can one differentiate between a good and a false IR spectrum? What causes a false IR spectrum? 4. Describe general regions of the fundamental vibrational spectrum with some characteristic group frequencies. 5. Outline one type of dispersive unit used in a Raman spectrometer to separate light into individual components. 6. Describe components of a Raman Fiber optic Probe. 7. List advantages and disadvantages of Raman spectrometers. Consider wavelength selection. 8. What is the difference between Rayleigh and Raman scattered light? 9. Summarize the advantages and disadvantages to KBr disc and Nujol mull sample preparations. 10. Does Raman or IR spectroscopy differentiate between amorphous and different crystalline materials? 11. Illustrate how light can be transmitted, refracted, and reflected through a crystal. What are the important variables? 12. What samples will provide the best quality IR spectrum using an ATR accessory and why? The worst? 13. How does an ATR measured spectrum differ from a classically measured transmission spectrum? What important parameters can the experimenter control? 14. Define the evanescent wave described by ATR theory. How does it define the sampling depth? 15. Identify three important criteria for selecting suitable ATR crystals. 16. What are the ideal sample characteristics for transmission IR microscopy? 17. List unwanted optical effects encountered in IR microscopy. 18. What is the diffraction light limit and what limits does it impose on IR microscopy? References 1. Infrared and Raman Spectroscopy, Methods and Applications; Schrader, B., Ed.; VCH: New York, NY, 1995. 2. Diem, M. Introduction to Modern Vibrational Spectroscopy; John Wiley: New York, NY, 1993. 3. Analytical Applications of Raman Spectroscopy; Pelletier, M. J., Ed.; Blackwell Science: London, 1999. 4. Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectroscopy; John Wiley: New York, NY, 1986. 5. Chase, D. B.; Rabolt, J. F. Fourier Transform Raman Spectroscopy, From Concept to Experiment; Academic: 1994. 6. Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: New York, NY, 1990. 7. Coleman, P. B. In Practical Sampling Techniques for Infrared Analysis; CRC: Boca Raton, 1993. 8. Beebe, K. R.; Pell, R. J.; Seasholtz, M. B. Chemometrics A Practical Guide; John Wiley: New York, NY, 1998. 9. Martens, H.; Naes, T. Multivariate Calibration; John Wiley: New York, NY, 1989. 10. Peter Fortini, personal communication. C H A P T E R 4 Environmental Dependence of Vibrational Spectra 1. SOLID, LIQUID, GASEOUS STATES Vibrational spectroscopy shows a strong dependence on the environment of the molecule being examined. The IR and Raman spectra are significantly different for molecules in the gaseous, liquid, or solid state. In the gaseous state, the molecules can be considered to be non-interacting and can rotate freely with a characteristic angular momentum. (We are ignoring pressure broadening.) Coupling of the vibrational transition with the rotational degrees of freedom of the molecule occurs resulting in band structure that is highly characteristic of the molecular structure and rotationalevibrational interactions.1,2 This band structure is highly dependent upon the molecular symmetry and falls into four basic types: linear (e.g., diatomics, CO2) spherical top (e.g., CH4, SF6), symmetric top (oblate e.g. CHCl3 prolate e.g., CH3Cl), and asymmetric top (e.g., H2O). Numerous qualitative and quantitative gas phase applications exist using both IR and Raman techniques but will not be covered here. Rotational fine structure can be observed in the vibrationalerotational band cluster if the molecular moment of inertias are sufficiently low as it is typical for simpler molecules. Fine structure occurs because angular momentum is also quantized. For larger molecules, the fine structure is typically unresolved resulting in broad bands. However, the band contour reveals the molecule symmetry and the direction of the dipole moment change during the vibration. Figure 4.1 shows the classical description of the IR gas phase contours of carbon dioxide, a linear type rotator. In Fig. 4.1a, CO2 is bending and rotating about an axis parallel to the change in the dipole moment which is not affected by the rotation. As seen in Fig. 4.1b, this results in a single frequency depicted in Fig. 4.1c. In Fig. 4.1d, this rotation changes the orientation of the dipole moment vector, the vertical component which shown in Fig. 4.1e interacts with the radiation oscillating in the vertical plane. This complex wave function can be resolved trigonometrically into two steady amplitude waves which give rise to two lines in the spectrum at V R and V þ R (see Fig. 4.1f). Although the molecule vibrates at only one frequency, the rotational frequency varies, giving rise to two broad wings on the central band shown without fine structure. The total band cluster is shown in Fig. 4.1g as the sum of Fig. 4.1c and Fig. 4.1f. As shown in Fig. 4.1h and Fig. 4.1j, the CO2 molecule is 55 56 4. ENVIRONMENTAL DEPENDENCE OF VIBRATIONAL SPECTRA Change in dipole moment (a) Time (b) (c) Vibrational (g) Total band Vertical component of dipole moment CO2bending and rotating (d) (h) (e) V+R Dipole moment change is perpendicular to the bond axis (f) Vibration + – Rotation R is variable Change in dipole moment CO2 Out-of-phase stretching and rotating (j) V–R (k) Total band Dipole moment change is parallel to the bond axis FIGURE 4.1 Classical treatment describing the IR gas phase contours of CO2. Although the molecule vibrates at only one frequency, the rotational frequency varies, giving rise to two broad wings on the central band shown without fine structure. In this case the CO2 molecule cannot rotate without rotating the dipole moment vector and the total band contour shown in (k) is a broad doublet with no central peak. stretching out-of-phase. In this case, the CO2 molecule cannot rotate without rotating the dipole moment vector and the total band contour shown in Fig. 4.1k is a broad doublet with no central peak. Figure 4.2 shows the characteristic band clusters for three different vibrational modes of a para-substituted aromatic, an asymmetric top rotator. In larger molecules such as these, the rotational fine structure is typically not observed. The white arrows in the first two inplane vibrations and a white plus sign for the last out-of-plane vibration indicate the dipole moment changes. These vectors are each parallel to one of the axes of rotational moments of inertia as shown in the figure. The rotational moments in the condensed phase state molecules can and do interact and these interactions strongly effect the vibrational spectrum. In the liquid state, molecules may have any orientation relative to the spectrometer. Further, a molecule may be found in several conformations, all of which may have different spectra (e.g., rotational isomers). No rotational fine structure is observed and the resulting overall spectrum may consist of a large number of fairly broad bands. Here, hydrogen bonding can have a dramatic effect on the IR and Raman spectra. Typically, if the molecule is associated, the bands will also be broader than for the non-associated state. 57 HYDROGEN BONDING A o O o o O +o - +- o +o B o o o O o - + - o + o + O O O ~ 1013 cm–1 ~1117 cm–1 ~ 817 cm–1 (a) (b) (c) FIGURE 4.2 Asymmetric top gas phase band contour (para-substituted benzenes). inertia are at a minimum at (a), intermediate at (b), and at a maximum for (c). In the bottom row are typical gas phase IR spectra showing the gas phase contour for a, b, and c+ type bands. The b type bands have no central peak. In a and c type bands, the relative intensities of the central peaks to the band wings vary with changes in the relative a, b, and c moments of inertia. Lastly, in the solid state, the crystallinity (or lack of it) of the molecule can be quite important and the nature of the sample preparation techniques employed can also be important. Figure 4.3 shows the IR spectra of common table sugar (sucrose) prepared as a KBr disc and as a cast film from water and clearly illustrate the differences observed in the IR spectrum of a crystalline and amorphous material. The large number of sharp bands evident in the upper spectrum is characteristic of a crystalline solid. When sugar is prepared as a water cast film, it becomes an amorphous material resulting in broader less distinct bands seen in the lower spectrum. Both spectra have some water present. In general, crystalline solids can be frozen into one or more configurations fixed in a lattice resulting in sharper bands. Often, crystalline splitting of degenerate vibrations can occur and forbidden vibrations may result in weak bands. Vibrational spectroscopy can distinguish between different crystalline forms and quantitatively assay mixtures. This ability has been exploited for various applications. 2. HYDROGEN BONDING Both IR and Raman spectroscopies are very sensitive to the molecular interactions involved in hydrogen bonding.3 The hydrogen bond itself involves an association of a hydrogen atom from an XeH group of a molecule with the Y atom of another group (XeHeY).4,5 The X type atoms involved often have high electronegativities resulting in an 58 4. ENVIRONMENTAL DEPENDENCE OF VIBRATIONAL SPECTRA 583 552 729 683 866 1124 3388 2124 Cast film, amorphous 3500 3000 2500 587 928 1053 3351 20 996 1134 40 1336 1265 1420 60 864 833 1648 80 2932 %Transmittance 100 1069 991 910 1347 2940 40 20 1461 1431 60 1279 1239 2723 80 1628 2476 KBr disc, crystalline solid 3563 %Transmittance 100 2000 1800 1600 1400 1200 1000 800 600 Wavenumber (cm–1) FIGURE 4.3 The FT-IR spectra of common table sugar. The upper spectrum was prepared as a KBr disc while the lower spectrum was prepared as a water cast film on ZnSe. XeH bond which is partially ionic while the HeY bond is suggested to be mostly electrostatic with some weak covalent characteristics. In general, hydrogen bonding to an XeH group results in a decrease in the XeH stretching frequency accompanied by a broadening and intensification of this band. Both OH and NH groups are found to show highly characteristic changes with hydrogen bonding. Figure 4.4 shows the OH stretches for two different phenolic OH stretches. The steric hindrance provided in 2,6-di-tert-butylphenol results in a strong sharp band at 3645 cm1 characteristic of a non-hydrogen bonded OH group. Conversely, the easily accessible OH group of p-creosol results in a broad strong band at 3330 cm1 characteristic of intermolecular hydrogen bonding between different phenolic OH groups. Intermolecular hydrogen bonding in chemical systems are very common. Examples of the condensed phase hydrogen bonded OH and NH stretching frequencies for carboxylic acid dimer and isocyanuric acid are shown below. Carboxylic Acid Dimer O R H O C C O H O R OH Str. ~ 3000 cm–1 59 HYDROGEN BONDING OH (CH3)3C 3024 60 O H O H R R 3083 80 C(CH3)3 OH 3330 40 3645 %Transmittance 100 20 “Free” OH 4000 Hydrogen-bonded OH 3800 3600 3400 CH3 3200 Wavenumber (cm–1) FIGURE 4.4 The solid state IR spectrum of para-creosol and 2,6-di-tert-butylphenol illustrating the effect of hydrogen bonding on the OH stretching frequency. Isocyanuric acid: Note that all of the NH and C¼O groups are involved in hydrogen bonding but only one set is shown for simplicity. O H O N C C N H H N C H O O H C N N C C N O NH Str. ~ 3209, 3080 cm–1 H O The carbonyl C¼O stretching frequency for the carboxylic acid dimer and the isocyanuric acid molecules also have a lower frequency and broader bandwidth as a result of hydrogen bonding. Additional changes in the vibrational spectrum of molecules involved in hydrogen bonding also occur. Bands involving XeH bending will usually increase in wavenumber upon formation of the hydrogen bond. The resultant changes in frequency and bandwidth are not as pronounced as observed in the XeH stretching region. Lastly, new low frequency bands can often be seen involving the stretching and bending of the HeY bond. The OHeO out-of plane hydrogen bend of the carboxylic acid dimer (960e875 cm1) is a classic wellknown example of this. Intramolecular hydrogen bonding can have a dramatic effect on the IR and Raman spectra and typically favors formation of five- and six-membered rings. Conjugated intramolecular hydrogen bonding systems in particular can result in very strong hydrogen bonds. Examples of this include the keto-enol conjugated system which involves conjugated ketone and hydroxy groups. As shown below, a resonance structure can be drawn that places the 60 4. ENVIRONMENTAL DEPENDENCE OF VIBRATIONAL SPECTRA negative charge on the carbonyl oxygen and a positive charge on the oxygen carrying the hydrogen bonding proton. H O C H O O C C CH O C CH Some examples of intramolecularly hydrogen bonded systems are shown below. 2-Hydroxy-4-(methoxy)-benzophenone H O O C OH Str. ~2900 cm–1 OC H3 2-(4,6eDiphenyle1,3,5etriazinee2eyl)e5 alkyloxy phenol N N N H O OH Str. ~ 2900 cm–1 OR 3. FERMI RESONANCE The vibrational spectrum of a molecule typically is considered to have isolated vibrational coordinates resulting fundamental vibrations. For example, carbon dioxide which has four total vibrations (3n 5) has two IR active vibrations, an out-of-phase stretch and a doubly degenerate deformation and a Raman active in-phase stretch. However, under certain conditions the vibrational levels are not well separated and can interact leading to phenomena such as Fermi resonance.6 Such interaction requires suitable symmetry for the two vibrations, an anharmonic nature of the vibration, and a similar energy for the two states (typically within 30 cm1). The vibrations cannot be significantly separated within distinctly different parts of the molecule and they must mechanically interact in order for the two vibrations to mix. In most cases, Fermi resonance involves a fundamental that interacts with a combination or overtone band resulting in two relatively strong bands where only one nearly coincident fundamental is expected. Figure 4.5 shows the idealized vibrational spectrum for a stretching and bending vibration before (Fig. 4.5a) and after Fermi resonance along with the idealized energy level diagram. The idealized Fermi resonance shown involves the overtone of 61 FERMI RESONANCE 1.8 s 1.6 + 2 δ +– x 1.4 s (b) Intensity 1.2 2 δ s 1.0 0.8 0.6 0.4 s 0.2 δ (a) 0.0 3500 3000 2500 2000 1500 1000 Wavenumber (cm–1) FIGURE 4.5 Schematic diagram illustrating Fermi resonance. The idealized vibrational spectrum for a stretching and bending vibration are shown before (a) and after Fermi resonance (b). To the left of the idealized spectra is the energy level diagram. 2360 20 0.20 CO2 o. ph. str. Fermi resonance doublet: CO2 i. ph. str. + CO2 bend overtone 0.15 1278 Raman intensity CO2 bend 669 40 CO2 gas phase Combination band: CO2 i.ph. str. + CO2 o. ph. str. 1385 60 2324 80 3600 100 3728 %Transmittance a bending vibration (nd) with a symmetric stretching vibration (nS) that results in a classic Fermi resonance doublet that straddles the original symmetric band frequency. The Raman spectrum of carbon dioxide shown in Fig. 4.6 provides a classic example of Fermi resonance.6 Only one band for the CO2 in-phase stretch fundamental is expected (1332 cm1) but a classic Fermi resonance doublet is observed at 1385 and 1278 cm1 instead. The two Raman bands are a linear combination of the fundamental in-phase stretch and the CO2 bend overtone. 0.10 2 Χ 669 = 1338 1385 + 1278 = 1331 2 0.05 0.00 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 Wavenumbers (cm–1) FIGURE 4.6 The IR and Raman spectra of carbon dioxide. Note the Fermi resonance doublet at 1385 and 1278 cm1 resulting from interaction of the y ¼ 1 CO2 i.ph. str. and the y ¼ 2 (overtone) of the 669 cm1 CO2 bend. 62 4. ENVIRONMENTAL DEPENDENCE OF VIBRATIONAL SPECTRA TABLE 4.1 Selected Groups that Display Fermi Resonance Bands in their Vibrational Spectrum. Active Assignment IR Raman 1385, 1278 CO2 i. ph. str þ 2x CO2 bend No Yes ~2820, ~2710 CH str. þ 2x CH i.ph. bend Yes Yes ~1300, ~1200 NCO i.ph. str. þ 2x NCO bend Yes Yes ~3000e2650 OH str. þ 2x OH i. pl bend Yes Yes ~3000e2550 OH str. þ 2x C-O str. Yes Yes CH2 ~2920e2890 CH2 str. þ 2 x CH2 bend Yes Yes AryleC(¼O)eCl ~1780, ~1730 C¼O str. þ 2x 880 aryl-C str. Yes Yes Group Bands (cm CO2 L1 ReC(¼0)eH N¼C¼O ReCOOH ) 1 NhCeN¼C(NH2)2 2200, 2170 ChN str. þ (1250 þ925 cm ) Combination band Yes e ReChCeR 2300, 2235 ChC str þ CeChC bend No Yes C¼O 1810, 1780 C¼O str. þ 2x ~900 ring bend Yes e AryleNH2 3355, 3205 NH2 i. ph. str. þ 2x NH2 bend Yes Yes NH4Cl 2825 NH4 i. ph. str. þ 2x NH4 bend Yes Yes e C(¼O)eNHeC 3300 (s), 3100(m) MH str. þ 2x CNH str.-bend Yes Yes Also noteworthy is that the combination band deriving from the bands involving the CO2 inphase str. and o. ph. str. are observed as two doublets in the 3728e3600 cm1 region because of the Raman active CO2 in-phase str. Fermi resonance doublet. In general, Fermi resonance has several effects on the vibrational spectrum. As illustrated by the carbon dioxide case, the resultant bands are shifted from their expected frequency. Furthermore, the intensity is greater than expected for a simple overtone band. Lastly, because the frequencies of the fundamentals themselves are typically environmentally sensitive, changes in solvents or crystalline state can affect the observed Fermi resonance band intensities. Fermi resonance can be found in the IR and Raman spectra of a significant amount of functional groups.3 Table 4.1 summarizes some groups that display Fermi resonance in the vibrational spectrum, with their band frequencies, assignments, and activities. References 1. Barrow, G. M. Introduction to Molecular Spectroscopy; McGraw-Hill: New York, NY, 1962. 2. Infrared and Raman Spectroscopy, Methods and Applications; Schrader, B., Ed.; VCH: New York, NY, 1995. 3. Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: New York, NY, 1990. 4. Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, California, 1960. 5. Bene, J. E.; Jordan, M. J. T. Int. Rev. Phys. Chem. 1990, 18 (1), 119e162. 6. Diem, M. Introduction to Modern Vibrational Spectroscopy; John Wiley: New York, NY, 1993. C H A P T E R 5 Origin of Group Frequencies When a molecule contains a bond or group with a vibrational frequency significantly different from adjacent groups, then the vibrations are localized primarily at this group. Such characteristic IR and Raman bands for particular bond or group are called “group frequencies” and are extremely useful for both qualitative and quantitative analysis. In order to understand the origin of these group frequencies, coupled oscillator systems are presented here and related to actual chemical groups. These model systems are based upon a classical description of different general types of mechanical coupling of bond vibrations.1,2,3 This overview enables us to provide a framework to understand the sensitivity of vibrational spectra to molecular structure. Lastly, some general rules concerning mechanical coupling of vibrations are presented. 1. COUPLED OSCILLATORS The simplest coupled oscillator system to consider is the stretchestretch interaction of a linear triatomic molecule.1,2 Here, only two bonds with two stretching vibrations are present which will mechanically couple. The two-coupled oscillators exert forces on each other when they oscillate, resulting in both in-phase and out-of-phase stretching vibrations (see Fig. 5.1). The out-of-phase stretch is of higher frequency than the in-phase stretch. The key difference between the two vibrations is the displacement of the central atom. Here we label the central atom as Mc and the two equivalent external atoms as M. The out-of-phase (asymmetric) stretch involves movement of all three atoms (see Fig. 5.1a) and the two vectors associated with the central mass reinforce one another (i.e., cooperate), resulting in larger movement of the central atom during the vibration. Characteristic phasing of the out-of-phase stretch is that one bond stretches while the other contracts. We can regard the middle atom as split into two halves moving with the same frequency and phase. Hence we consider the central atom, Mc, as 2 (M/2). Thus, the two oscillators of M e (M/2) vibrate at 180 out-of-phase with each other. The result of this is that the frequency of this type of vibration is strongly dependent on the mass of the central atom. We can extend the classical vibrational frequency definition presented earlier to the out-of-phase stretching vibration by splitting the central mass in half and the simple expression is: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffi 1 2 K pffiffiffi 3 ¼ 1303 nop ¼ 1303 K þ m1 m2 m 63 64 5. ORIGIN OF GROUP FREQUENCIES (a) (b) FIGURE 5.1 The stretching vibrations of a linear triatomic. The in-phase and out-of-phase stretches have been resolved into diatomic Me(N) and Me(M/2) components. The out-of-phase stretch is shown in diagram a and the in-phase stretch in diagram b. where K is the force constant and gives the restoring force for unit displacement from the equilibrium position, and m1 and m2 are the masses for the linear triatomic in Fig. 5.1. For three equivalent masses nop simplifies to the last term in the above equation. In the case of the in-phase (symmetric) stretch, the central atom does not move (see Fig. 5.1b). This is equivalent to the two exterior masses being connected to an infinite mass (N) where both mdN systems oscillate with same frequency and phase. In this case, the two spring force vectors exerted on the central mass oppose one another, resulting in no movement of the central atom during the vibration. Here, the frequency of this vibration is independent of the mass of the central atom in the symmetrical molecule and can be expressed classically as: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffi 1 1 K pffiffiffi 1 nip ¼ 1303 K þ ¼ 1303 m1 N m Comparison of the above simple expressions predicts that the out-of-phase vibrations will be higher in frequency than the in-phase vibration. For a bent triatomic molecule, we find that the frequencies of the in-phase and out-ofphase stretching vibrations are a function of the bond angle, a, between the three atoms.1,2 We again consider the force vectors for the central atom m2 relative to the two exterior atoms, m1 and m3 for the in-phase and out-of-phase stretches. Figure 5.2 (lower left) shows the total restoring force tending to move the central atom along the right-hand bond. The restoring force vectors will by definition be opposite in sign from the Cartesian displacement vectors. For the in-phase mode, the forces act in opposition so there is less restoring force, while for the out-of-phase mode the forces cooperate resulting in more restoring force. The same analysis holds for the left-hand bond. The 65 COUPLED OSCILLATORS Restoring forces for in-phase and out-of-phase vibrations m2 C m2 C X axis m3 S m1 C m3 m1 Vector diagram for m2 atom Cooperation Opposition In-phase KT, ip= K (1 + cos ) Out-of-phase KT, op = K (1 – cos ) FIGURE 5.2 The force diagrams for the central atom in a bent triatomic. The nature of the interaction of the two bond oscillators is a function of the bond angle, a. S and C indicate stretching and contraction of a bond and the bold arrows are the restoring forces. Both the in-phase (contract-contract) and the out-of-phase (stretch-contract) modes are shown. resultant mechanical coupling tends to lower and raise the in-phase and out-of-phase frequencies, respectively relative to that of the m2em3 diatomic model. Taking into account the bond angle dependence, we can calculate the frequency for the inphase and out-of-phase stretching vibrations for bent triatomics. The out-of-phase stretch can be approximately calculated by: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 cosa nop ¼ 1303 K þ m1 m2 where the bending force constant can be assumed to be zero. The in-phase stretch can be approximately calculated by: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 þ cosa nip ¼ 1303 K þ m1 m2 Here, we again neglect the bending force constant, but in this case it does contribute slightly. Figure 5.3 demonstrates the frequency dependence of triatomic in-phase and out-of-phase stretches on the bond angle, a. When a is greater than 90 , then the in-phase stretch is lower in frequency than the out-of-phase stretch, while a is less than 90 , then the in-phase stretch is higher in frequency than the out-of-phase stretch. Lastly, when a ¼ 90 the in-phase and outof-phase stretches are equal because there is no mechanical interaction (vectors are orthogonal). An important extension of the above discussion is to consider the effects of mechanical coupling as a function of chain length.1,2 This is important since it applies to 66 5. ORIGIN OF GROUP FREQUENCIES linear aliphatic systems. Shown in Fig. 5.4 is a plot of vibrational frequencies for coupled CeC stretches of linear aliphatics as a function of increased chain length (i.e., 2,3,4. atoms) showing in-phase and out-of-phase vibrations. Standing waves are used to help describe the vibrations and their relative frequencies. For clarity, a plane (or node) separates the molecular segments, which vibrate out-of-phase with each other. In-phase (symmetric) Out-of-phase (asymmetric) Cooperation: raises frequency Opposition: lowers frequency 180 Bond angle ( ) opp coop 90 opp coop 0 opp coop Wavenumber (cm–1) FIGURE 5.3 The frequency dependence of the in-phase and out-of-phase stretches of a triatomic molecule as a function of the bond angle, a. The arrows shown are displacement vectors. C Wavenumbers (cm–1) 1200 S C S C C S 1100 uncoupled 1000 900 S S S S S 800 1 bond 2 bonds 3 bonds Number of bonds FIGURE 5.4 Plot of vibrations for coupled CeC stretch of linear aliphatics, as a function of increased chain length. 67 COUPLED OSCILLATORS B Wavenembers (cm–1) B B B B 400 N B B O Cooperation 300 O B O 200 Opposition 2 bonds 3 bonds B B 4 bonds Number of bonds FIGURE 5.5 Plot of vibrations for coupled CeC bending vibrations of linear aliphatics, as a function of increased chain length. Note the out-of-phase vibration (opposition) is lower in frequency than the in-phase vibration (cooperation). Bending and open vibrations are indicated with B and O. No change in the bond angle is indicated by N. Usually, the greater the number of nodes for stretching vibrations, the higher is the frequency. Figure 5.4 shows that a connection of several similar oscillators in linear chains results in as many chain frequencies as there are bonds. Thus, similar CeC, CeO, and CeN bonds will interact resulting in prominent IR and Raman bands. The relatively constant mechanical interaction gives results in group frequencies. Fortunately, for longer chains only a few of these vibrations provide important bands. We must also consider bending vibrations. The skeletal single-bond bending vibrations in aliphatic molecules vibrate at much lower frequencies than CeC stretches, CH stretch, or CH bending vibrations and generally fall in the far-IR region. The vibration involves mostly a deformation of the CeCeC bonds and the out-of-phase vibration is lower in frequency than the in-phase vibration. Figure 5.5 plots the vibrations for coupled CeC bending vibrations of linear aliphatics, as a function of increased chain length. 2. Rules of Thumb for Various Oscillator Combinations For oscillator combinations of equal or nearly equal frequency both in-phase and out-ofphase oscillator combinations are involved.1,2 This simplest case considers equal oscillator combinations and includes stretchestretch and bendestretch vibrations. Some examples are shown in Fig. 5.6 and Table 5.1. 68 5. ORIGIN OF GROUP FREQUENCIES CH2 = C = CH2 H 1071 cm–1 CH2 = C = CH2 H H N 3300 cm–1 1980 cm–1 H N 3370 cm–1 R R FIGURE 5.6 Examples of equal oscillator combinations. TABLE 5.1 XY2 Group Examples of XY2 Type Identical Oscillators Out-of-phase stretch In-phase stretch ReNH2 1 3370 cm 3300 cm1 ReCH2eR 2925 cm1 2850 cm1 ReCO 2 1600 cm1 1420 cm1 ReNO2 1550 cm1 1375 cm1 ReSO2eR 1310 cm1 1130 cm1 Examples of some two identical oscillator groups (non-linear M1eM2eM1) shown in Table 5.1 include: two hydrogen atoms on groups such as a methylene or primary amine and two oxygen atoms on groups such as carboxylate, nitro or sulfones. The XY2 group in the above can be considered approximately as an isolated oscillator since it has stretching frequencies significantly higher than the frequencies of the CeC bond oscillators connected to it. As a result only the XY2 group atoms move appreciably during the stretching vibration. Because of this the above groups result in good group frequencies. The out-of-phase stretch is also an important IR group frequency for linear alkane alcohols, ethers, and primary and secondary amines.2 The vibration involves stretches of the CeC, CeO, or CeN bonds, which have very similar force constants. The frequencies shown below support this. CH3 CH3 992 cm1 CH3 OH 1033 cm1 CH3 NH2 1036 cm1 Consequently, stretches involving CeC, CeO, and CeN will strongly couple giving rise to skeletal out-of-phase stretching vibrations in the following general spectral regions: C C Ow1050 cm1 C O Cw1125 cm1 C C Nw1080 cm1 C N Cw1140 cm1 69 COUPLED OSCILLATORS B H B X S Y Amide CNH str./bend 1550 cm–1 S Cooperation: higher frequency H O O X S Y Amide CNH str./open 1250 cm–1 S Opposition: lower frequency FIGURE 5.7 The stretchingebending interaction for the HeXeY group. S stands for stretch, B for bend, and O for open. The approximate band position is also included for the amide group CNH stretch/bend and stretch open vibrations. The CeCeO and CeOeC species in particular give good strong IR group frequencies as a result of a relatively constant interaction with other CeC bonds. Figure 5.7 shows the second type of important equal oscillator vibration, the bend-stretch vibration.2 Mechanical coupling of bending and stretching vibrations can occur if both are of nearly equal frequency. In this case, we consider a simple bent HeXeY oscillator where both X and Y are heavier than the hydrogen. The HeX bending frequency may be nearly identical to that of the XeY stretching frequency, allowing the two oscillators to couple. Two vibrations result involving the XeY stretch and the HeXeY bend which differ only in the relative phases. An example of this is the amide group where X ¼ nitrogen and Y ¼ carbon. The HeX stretching oscillator has much higher frequency than the XeY stretching oscillator and is therefore an isolated vibration (see below for rules for unequal oscillators). As shown above, there are three components for this molecular vibration: the rotation of the XeY bond, the stretching of the XeY bond, and the rotation of the HeX bond. Here, the rotation of the XeY bond has little effect since the vectors involved are orthogonal (i.e., 90 ) with respect to both the XeY stretch and the HeX rotation. However, rotation of the HeX bond results in movement of the X atom either in the same or in opposite direction that it moves during the XeY stretch. The bend/stretch involves cooperation of the X atom vectors 70 5. ORIGIN OF GROUP FREQUENCIES 1. High frequency: only atoms of high frequency component move appreciably. Cl C C C C s C C C N C O C H Os H s s s c C C 2. Low frequency: only the bond length or angle of low frequency component changes appreciably. O C H C CsO C C H s s C N O s C Cl s C FIGURE 5.8 Examples of unequal oscillator combinations. and thus higher frequency while the bend/open involves opposition of the X atom vectors and thus lower frequency. Oscillators with different frequencies have two important approximate rules of thumb: 1. In high frequency vibration of the combination, only atoms of the high frequency component move. 2. In low frequency vibration of the combination, only the bond length or angle of the low frequency component changes. Some simple examples of typical unequal oscillator combinations encountered are shown in Fig. 5.8. Lastly, for systems containing more than two equal oscillators, the resulting vibrations can become much more complex and standing waves can be used to simplify the description of the vibrations.2 An excellent example of this is the stretching vibrations for a benzene ring. Figure 5.9 shows the standing vibrational waves employed to describe the stretching vibrations of six-membered rings. S C S C S C C C S C S 3a C S S S C 1 2a 2b S C S 3b 4 FIGURE 5.9 Standing vibrational waves for stretching vibrations of six-membered rings. 71 COUPLED OSCILLATORS Chapter 5: Questions 1. Using the mathematical expression for a harmonic oscillator, calculate the force constant, K, for the following species: CH 3000 cm1, NH 3400 cm1, OH 3600 cm1. Note that H ¼ 1 amu, C ¼ 12 amu, O ¼ 16 amu, and N ¼ 14 amu. What are the possible sources of the observed variation in the frequencies for the above? 2. The following ethers have bands associated with the in-phase and out-of-phase CeO stretching vibrations: 1100, 810 cm1 1080, 890 cm1 1020, 990 cm1 1220, 890 cm1. Using a scatter plot of frequency versus bond angle, explain the origin of the observed trends. 1100, 810 cm-1 1080, 890 cm-1 O O 1020, 990 cm-1 1220, 890 cm-1 O O 3. Calculate the in-phase and out-of-phase stretching frequencies for six and four membered cyclohexane, cyclopropane, and cyclobutane rings. Here k ¼ 4.2 mdynes/ A and M ¼ 14 amu for the methylene group. The in-phase ring stretch frequency equation is: yip ¼ 1303½kð1 þ cosaÞðM1 þ 1=M2 Þ1=2 ¼ 1303½k=M1=2 2cosa=2 Ring Size a (deg) In-phase stretch (exp) N¼6 109.5 802 N¼5 104 886 N¼4 90 1003 Calculated 4. Calculate the in-phase and out-of-phase stretching frequencies for the following XY2 groups: ReNH2 (6.4 mdynes/A), ReCH2eR (4.9 mdynes/A). 5. Provide a summary of the general types of vibrations encountered. In general, where in the vibrational spectrum will these vibrations be found? 6. The bending vibration for propane is observed at 375 cm1 while for butane it is observed at 427 and 271 cm1. Draw the vibrations and explain the frequency differences. 7. Suggest other possible functional groups beyond those listed in table 5.1 that would fit the XY2 type identical oscillators. 72 5. ORIGIN OF GROUP FREQUENCIES References 1. 2. 3. Infrared and Raman Spectroscopy, Methods and Applications; Schrader, B., Ed.; VCH: New York, NY, 1995. Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: New York, NY, 1990. Barrow, G. M. Introduction to Molecular Spectroscopy; McGraw-Hill: New York, NY, 1962. C H A P T E R 6 IR and Raman Spectra-Structure Correlations: Characteristic Group Frequencies In the following chapter we include generalized Infrared and Raman spectra of selected regions which illustrate the generalized appearance of bands for selected characteristic group frequencies. In each case, the upper spectrum shows the transmission infrared spectra with bands pointing downward and the lower spectrum shows the Raman spectrum with bands pointing up. The vertical axes are expressed in percent infrared transmission and Raman intensity. We also describe the approximate forms of selected vibrations and include tables summarizing important bands and their assignments. 1. XeH STRETCHING GROUP (X[O, S, P, N, SI, B) The set of spectra in Fig. 6.1, illustrates some useful XeH stretching bands. The large differences in XeH stretching frequencies are mostly due to differences in bonding and therefore the force constants. Quite often these bands are more intense in the IR than in the Raman spectra.1-4 As shown in the top two spectra in Fig. 6.1, the OH stretch is particularly weak in the Raman but strong in the IR. The hydrogen-bonded alcoholic OH stretch vibration gives rise to a broad band near 3350 cm1 in the IR spectrum. A more strongly hydrogen-bonded OH in a carboxylic acid dimer has a much broader band centering about 3000 cm1. An exception to this is the SH group of a mercaptan which gives rise to a sharp weak IR band and a strong Raman band at about 2560 cm1. Aliphatic primary amines have a weak NH2 doublet in the IR near 3380 and 3300 cm1 which derive from the out-of-phase and in-phase NH2 stretching vibrations, respectively. The weak shoulder observed near 3200 cm 1 is the Fermi-resonance enhanced overtone of the 1620 cm1 NH2 bend. Typically, the in-phase NH2 stretch is stronger in the Raman spectra than the out-of-phase stretch. In primary amides, O¼CeNH2, bands involving the NH2 stretches are found at 3370 and 3200 cm1, and are typically stronger in the IR than in the Raman spectra. The PH2 group gives rise to a mediumeweak single band in both the IR and Raman spectra near 2280 cm1. 73 74 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES FIGURE 6.1 The generalized IR and Raman spectra of selected XeH stretching bands, where X ¼ oxygen, sulfur, phosphorus, nitrogen, or silicon. Secondary aliphatic amines have a NH stretch band near 3300 cm1 which is quite weak in the IR but stronger in the Raman spectrum. Monosubstituted amides, O¼CeNHe, have a stronger IR band near 3300 cm1 which derives from the NH stretch. An additional weak band near 3100 cm1 is the overtone of the 1550 cm1 CNH stretch/bend. The last group shown is SiH2 species that has a single band at ca. 2130 cm1 which is quite strong in the IR but weak in the Raman. Although not shown in Fig. 6.1, BeOH and BH/BH2 are characterized by moderate to strong IR OH and BH stretching bands but weak Raman bands. The BeOH group has a strong broad band between 3300 and 3200 cm1 from the OH stretching vibration. The BH and BH2 groups have moderate to strong IR bands in the 2630e2350 cm1 region. The BH2 group has both an out-of-phase (2640e2570 cm1) and in-phase (2532e2488 cm1) BH2 stretching bands. Fully annotated IR and Raman spectra of selected carboxylic acids (#24e27), amides (#30e32), alcohols (#40e47), boronic acid (64), and amines (#51e53) are included in the last section for comparison. 2. ALIPHATIC GROUPS The set of spectra in Fig. 6.2 illustrate some selected group frequencies for alkane groups.1-4 The group frequencies are also summarized in Table 6.1. Figure 6.3 illustrates the vibrations of the methylene (CH2)1-4 group. For the non-cyclic (CH2)n chain, the CH2 out-of-phase and the in-phase stretching bands are near 2930 and 2855 cm1, respectively, and the CH2 bend (deformation) band is near 1465 cm1. For the methylene segments (CH2)n in the trans zig-zag form, ALIPHATIC GROUPS 75 FIGURE 6.2 The generalized IR and Raman spectra of linear and branched alkane groups. The spectral components from methyl and methylene groups have been separated in the top two rows of spectra. The bottom row illustrates the sensitivity of IR and Raman spectra to branching in alkane group. the in-phase CH2 twist is observed near 1300 cm1 in the Raman spectrum, and the in-phase (CH2)n rock is found near 723 cm1 in the IR. Figure 6.4 illustrates the vibrations of the CH3 group. For the CH3 group on an alkane carbon, bands near 2960 and 2870 cm1 derive from the out-of-phase and in-phase stretches and the out-of-phase bend results in a band at ca. 1465 in the IR. The in-phase bend of a single CH3 on an alkane carbon results in a band near 1375 cm1 in the IR. The presence of an adjacent group such as a N or O atom can result in a significant frequency shift in the methyl (CH3) in-phase stretch to lower frequency between 2895 and 2815 cm1. When there are two methyl groups on one carbon such as an iso-propyl or a gem-dimethyl group, this splits into two nearly equal intense bands near 1385 and 1365 cm1 in the IR. When there are three methyl groups on one carbon, such as a tertiary-butyl group, there is further splitting into a weak band at ca. 1395 cm1 and a stronger band near 1365 cm1 in the IR spectrum. In addition, for these groups there are strong Raman bands at ca. 800 cm1 which involves the in-phase stretch of the C(C3) group and at ca. 700 cm1 for the in-phase stretch of the C(C4) group. However, although intense, this Raman band will vary considerably in frequency due to strong mechanical coupling with attached CeC, CeO, CeN or CeS groups. Table 6.2 summarizes this variation for a few selected 76 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES TABLE 6.1 Selected Group Frequencies: Aliphatics Intensities1 Group CH3 CH2 CH Assignment 2 Frequency (cmL1) IR R ReCH3 o.ph. str. 2975e2950 vs vs ” i.ph. str. 2885e2860 vs vs ” o.ph. bend 1470e1440 ms ms ” i.ph. bend 1380e1370 m vw ReCH(CH3)2 bend-bend 1385e1380 m vw ” bend-open 1373e1365 m vw R(CH3)3 bend-bend 1395e1385 m vw ” bend-open 1373e1365 ms vw aryl-CH3 i.ph. str. þ 2935e2915 ms ms ” bend overtone 2875e2855 m m ReOeCH3 i.ph. str. 2850e2815 m m ” i.ph. bend 1450e1430 ms m R2NeCH3 i.ph. str 2825e2765 s s O¼CeCH3 i.ph. bend 1380e1365 s w ReCH2eR 3 o. ph. str. 2936e2915 vs vs ” i. ph. str. 2865e2833 vs vs ” Fermi resonance 2920e2890 w m ” bend 1475e1445 ms ms (CH2)>3 i.ph. twist 1305e1295 e m (CH2)>3 i.ph. rock 726e720 m e O¼CeCH2 bend 1445e1405 m m NhCeCH2 bend 1445e1405 m m OeCH2 wag 1390e1340 m m CleCH2 wag 1300e1220 m m R3CH CH bend 1360e1320 w w O¼CeH CH str. þ 2900e2800 m m ” rk overtone 2770e2695 m m ” CH rock 1410e1380 m m s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero o.ph. str. ¼ out-of-phase stretch, i.ph. str. ¼ in-phase stretch, rk ¼ rock 3 Note: In the trans zig-zag segments of the long chain (CH2)>6 group, the Raman active CH2 out-of-phase stretch band is at 2900e2880 cm1 (2883 cm1 in polyethylene). The gauche segments have this at 2936e2916 cm1 in the Raman. 1 2 77 ALIPHATIC GROUPS Nodal plane S O C O O CH2 Out-of-phase stretch O O O O OO O O OO S O O O CH2 Wag O CH2 In-phase stretch O – O + O O O S O O CH2 Twist B O CH2 Bend O O O O O CH2 Rock FIGURE 6.3 Vibrations of the CH2 group. C and S stand for contract and stretch, and B and O stand for bend and open. Movement of atoms are depicted by arrows or by þ and signs if the atom movement is out of the page. Nodal planes are used to help differentiate the vibrations. compounds. Fully annotated IR and Raman spectra of selected aliphatic compounds are included in the last section of fully interpreted spectra for comparison (#1e3). In Fig. 6.5, the relative intensities in the IR and Raman spectra in CH stretching region are illustrated for some selected alkanes. The vibrations involving the methyl and methylene stretches (3000e2800 cm1) give rise to bands that are strong and characteristic in both the IR and Raman spectra but show greater complexity particularly in the Raman spectra due to Fermi-resonance effects.5,6,7 The CH stretching region of the IR and Raman spectra shows additional bands due to Fermi resonance between the methylene CH stretching vibrations and the overtone of the methylene bending (deformation) vibrations (1480e1440 cm1). Bands observed in the 2900 cm1 region as a weak shoulder in the IR spectrum and as more isolated and obvious bands in the Raman spectra derive from Fermi resonance. This occurs in the IR and Raman spectra of aliphatics when: 1. There are at least two almost equivalent neighboring methylene groups. 2. The symmetry and frequency of the methylene CH2 stretch and the CH2 deformation overtone is suitable for Fermi resonance. The above results in multiple Fermi-resonance doublets associated with the methylene group including the broad bands observed in the 2920e2898 cm1 region and results in significantly more complicated Raman spectra of n-alkanes as observed in Fig. 6.5. 78 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES Nodal plane O c O O cO O O s o O O O s o o O O O O o B O O O B CH3 In-phase bend CH3 Out-of-phase bend O o O O B o O O O O O O O O O CH3 Rock O O O O O s CH3 In-phase stretch O o O s CH3 Out-of-phase stretch B s O O O O s O O O O O O B CH3 Torsion FIGURE 6.4 Vibrations of the CH3 group. Nodal planes are used to differentiate between doubly degenerate vibrations. C and S stand for contract and stretch, and B and O stand for bend and open. The conformational equilibrium of n-alkanes due to rotation about the CeC bond of the long chain aliphatic can also affect the IR and Raman spectra in the gas phase or in pressure induced transitions. Thus, for n-Pentane these conformations would include methylene in the all-trans (TT a planar zig-zag form), single gauche (TG), and double gauche (GG) conformers.8 The methylene and methyl stretching bands are sensitive markers of these different conformers. The all trans conformation is typically found in the condensed state under ambient room temperature conditions such as shown in Fig. 6.5. The relative IR intensities of the methyl and methylene bands can be used to estimate the relative concentration of the two species. In the case of the IR spectra in Fig. 6.5, the methyl (CH3) out-of-phase stretch at ca. 2960 cm1 is doubly degenerate and is therefore twice as intense at the methylene (CH2) out-of-phase stretch at 2925 cm1. Using this relationship, the relative amount of methyl and methylene group can be rapidly estimated from the IR spectrum. 79 CONJUGATED ALIPHATICS AND AROMATICS TABLE 6.2 Summary of the Raman In-Phase CeC/CeX Stretching Band for Selected Compounds, where X is Carbon, Nitrogen, Oxygen or Sulfur X-Element CeCeCeX (i.ph.str.) (C)2CeX (i.ph. str.) (C)3CeX (i. ph. str.) Carbon (12) 827 cm1 800 cm1 733 cm1 CH3 CH3 CH3CH2CH2CH3 H3C CH3 CH3 Nitrogen (14) 868 cm1 CH3 H3C CH 802 cm1 748 cm1 CH3 NH2 CH H2N CH3 H3C CH3 Oxygen (16) 822 cm1 819 cm1 CH3 750 cm1 CH3 CH3 OH HO NH2 CH H3C OH CH3 CH3 Sulfur (32) 670 cm1 631 cm1 CH3 SH HS CH CH3 3. CONJUGATED ALIPHATICS AND AROMATICS Selected IR and Raman characteristic frequencies of conjugated aliphatics and aromatics are summarized in Table 6.3.1-4 Included are triple and cumulated double bond species, olefinics, and aromatics. Three sets of generalized IR and Raman spectra also illustrate some of the characteristic bands for these species. Below, we discuss olefinics, triple and cumulated double bonds and aromatics in more detail. 80 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES 100 CH3 CH 80 CH2 CH2 CH3 2852 2930 2925 2853 2875 2853 Cyclohexane CH2 2938 2924 2936 2898 2937 2904 2962 2897 2962 2963 1.0 CH2 2863 2875 2875 2863 2938 2917 2876 2935 2914 CH2 n-Octane CH2 2963 2.0 n-Heptane n-Hexane n-Pentane 3.0 2959 2926 2874 2860 2962 2926 2938 2966 20 2959 2874 2858 60 2856 2873 CH2 2879 %Transmittance CH2 40 Relative intensity CH3 CH3 CH3 CH3 CH3 CH3 CH2 0.0 Wavenumber (cm–1) FIGURE 6.5 The infrared and Raman spectra in the CH stretching region are illustrated for n-pentane, n-hexane, n-heptane, n-octane, and cyclohexane. Additional bands due to Fermi resonance are clearly observed in the Raman spectra. TABLE 6.3 Selected Group Frequencies: Triple Bonds, Cumulated Double Bonds, Conjugated Aliphatics and Aromatics Intensities1 Group Assignment Frequency (cmL1) IR R Triple CeChCeH CH str. 3340e3267 s w Bonds ” ChC str. 2140e2100 w vs ” CH wag 710e578 s,br w CeChCeC ChC str. 2245e2100 e s CH2eChN ChN str. 2260e2240 m vs ” CH2 bend 1440e1405 m m conj-ChN ChN str. 2235e2185 var. s SeChN ChN str. 2170e2135 ms s 81 CONJUGATED ALIPHATICS AND AROMATICS TABLE 6.3 Selected Group Frequencies: Triple Bonds, Cumulated Double Bonds, Conjugated Aliphatics and Aromaticsdcont’d Intensities1 Group Assignment Frequency (cmL1) IR R Cumulated >C¼C¼CH2 CCC o.ph. str. 2000e1900 vs vw Double eN¼C¼O NCO o.ph. str. 2300e2250 vs vw Bonds eN¼C¼S NCS o.ph. str. C¼C Alkane Subst. Aromatics 2200e2000 vs mw C¼C mono, cis, 1,1 2 C¼C str. 1660e1630 m s C¼C trans, tri, tetra 2 C¼C str. 1680e1665 weo s C¼CHeR mono, cis, trans CH str. 3020e2995 m m C¼CH2 mono, 1,1 CH2 o.ph. str. 3090e3075 m m ” CH2 i.ph. str. 3000e2980 m s ” CH2 bend 1420e1400 w m ReCH¼CH2 trans CH i.ph. wag 995e985 s w ” 3 ¼CH2 wag 910e905 s w R2C¼CH2 ¼CH2 wag 900e885 s w ReCH¼CHeR trans ¼CH i.ph. wag 980e965 s e ReCH¼CHeR cis ¼CH i.ph. wag 730e650 ms e ReCH¼CR2 ¼CH wag 840e790 m aryl CH CH str. 3100e3000 mw s ring quadrant str. 1620e1585 var m ” quadrant str. 1590e1565 var m ” semicircle str. 1525e1470 var vw ” semicircle str. 1465e1400 m vw mono, meta, 1,3,5 2,4,6 radial i.ph str. 1010e990 vw vs meta, 1,2,4 & 1,3,5 lone H wag 935e810 m e para and 1,2,4 2 adj. H wag 880e795 s e meta and 1,2,3 3 adj. H wag 825e750 s e ortho and mono 4 & 5 adj. H wag 800e725 s e mono, meta, 1,3,5 ring out-of-plane bend 710e665 s e para ring in-plane bend 650e630 e m mono ring in-plane bend 630e605 w m 3 s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero Conjugation lowers C¼C frequencies 10e50 cm1 (e.g., NhCeCH¼CH2 1607 cm1) 3 Electron donor substituents lower the ¼CH2 wag frequencies (e.g., ReOeCH¼CH2 813 cm1) and electron withdrawers raise them (e.g., NhCeCH¼CH2 960 cm1) 1 2 82 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES 3.1. Alkyl-Substituted Olefinic Groups The set of IR and Raman spectra in Fig. 6.6 illustrate selected group frequencies for the ethylene C¼C groups with alkane group substituents.1-4 Particularly, useful Raman bands for the olefinic group include the ¼CH stretch, in-phase ¼CH2 stretch, and the C¼C stretch. In the IR spectra, the olefinic out-of-plane CH and CH2 wag are quite important. Figure 6.7 illustrates the in-plane olefinic vibrations and Fig. 6.8 summarizes the observed and estimated C¼C stretching frequencies for fifteen different compounds containing an olefinic group. The out-of-plane CH wag vibrations are shown in Fig. 6.9. The in-plane vibrations of the olefin group are depicted in Fig. 6.7. The vinyl and 1,1-disubstituted ethylenes have a ¼CH2 group with ¼CH2 out-of-phase and in-phase stretching bands near 3080 and 2985 cm1, respectively. The ¼CH group in vinyls, cis, trans 1,2-disubstitituted ethylenes, and trisubstituted ethylenes all have a ¼CH stretching band at ca. 3010 cm1. The vinyl, 1,1-disubstituted, and 1,2-cis disubstituted ethylenes all have a C¼C stretch with strong intensity in the Raman and medium intensity in the IR. In the case of 1,2 trans disubstituted, tri-substituted, and tetra-substituted ethylene groups, the C¼C stretch at ca. 1670 cm1 is quite strong in the Raman but weak or absent in the IR. The frequency of the C¼C stretch will vary depending on the nature of the substituent. Conjugation effects tend to be particularly important, usually lowering the C¼C stretching frequency. The selected compounds in Fig. 6.8 vary in the cyclic olefinic structures and most importantly the CeC¼C bond angle. Mechanical coupling of the stretching vibrations of bonds attached to olefinic groups is the critical variable in determining C¼C stretching fequencies in cyclic olefin species.1 The calculated frequencies are from an adapted form of the triatomic model discussed earlier and includes mechanical coupling of the CeC stretch FIGURE 6.6 Generalized IR and Raman spectra of alkyl-substituted olefins. 83 CONJUGATED ALIPHATICS AND AROMATICS O O O O C=C str O O O 1662–1631 cm–1 R–CH=CH-R Trans R2C=CH–R R2C=CR2 1692–1665 cm–1 O O O CH2 Out-of-phase str. 3080 cm–1 CH2 In-phase str. 2990 cm–1 CH Str. 3020 cm–1 CH2 Bend 1415 cm–1 Cis CH rock 1415 cm–1 O O O O O O O O O O O O R–CH=CH2 R2C=CH2 R–CH=CH-R Cis O O FIGURE 6.7 The olefin in-plane vibrations are shown above. Arrows depict the movements of atoms. with the C¼C stretch. The model uses force constants for the C¼C and attached CeC of 8.9 and 4.5 mdynes/A, respectively, neglects the bending force constants, and includes the CeC¼C bond angle. The approximate form of the model describing the total forces on the olefinic group for the external and internal C¼C group are shown on the left side of Fig. 6.8. Despite their simplicity, these calculations are successful in predicting trends in the frequencies in the top two rows. This data demonstrates that little or no change in the C¼C force constant is required to explain the observed frequency shifts when one or both of the olefinic carbon atoms are part of a ring. No calculations are shown for the bottom row, but the experimental data indicates that for ring sizes between four and six, the effects of the indicated changes in both the external and internal CeC¼C bond angles approximately cancel each other, resulting in similar C¼C stretching frequencies. The critical importance that mechanical effects has in determining the C¼C stretching frequencies in cyclic olefin structures has similarly been demonstrated using vibrating ball bearing and coil spring molecular models.9 84 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES F2 H F1 H H C H H C 120 H H C 135 126 H H H C C 150 180 C F2 C H Calc (cm–1) 1639 1661 1696 1756 Obs (cm–1) 1651 1657 1678 1736 H 120 F1 F2 H H 108 H H H H 90 1957 H H C C H 0 60 F2 Calc (cm–1) 1636 1606 1587 1636 1776 Obs (cm–1) 1646 1611 1566 1656 1974 R 126 R 108 R 135 R 1678 1675 R 120 R 120 Obs (cm–1) 1681 150 R 90 H R 60 1883 180 R C C R 0 2250 FIGURE 6.8 The cyclic C¼C stretching frequencies as a function of the CeC¼C bond angles. The calculated frequencies used force constants F1 and F2 values of 8.9 and 4.5 mdynes/A, respectively. The simple model used to calculate the frequencies is summarized on the left-hand side of the figure. + – O O –O +– O O + O O O O Vinyl trans CH wag O O– O + +– O + +O O Vinyl =CH2 wag +O 1,2-Trans-disubst trans CH wag + – O +O O– O –O O O O O+ O + – Vinyl cis CH wag + – +– + +O O O O O 1,1-disubst =CH2 wag O O O O– + O+ 1,2-Cis-disubst Cis CH wag FIGURE 6.9 The out-of-plane CH wag vibrations of the olefinic group are shown above. Movement of atoms are depicted by arrows or by þ and signs. The CH wag bands (see Fig. 6.9) are strong in the IR and highly characteristic of the alkyl substitution on the ethylene C¼C group. These alkyl-substituted olefinic CH wag bands are observed near 990 and 910 cm1 for vinyl groups, near 890 cm1 for 1,1-disubstitution, near 970 cm1 for trans 1,2-disubstitution, and roughly at 680 cm1 for cis 1,2-disubstitution. The nature of the substituent can result in large but predictable frequency shifts of the olefinic CH and CH2 wag. The ¼CH2 wag is sensitive to electronic changes and shows a linear frequency dependence based upon experimentally determined substituent Hammet constant as well as 85 CONJUGATED ALIPHATICS AND AROMATICS 0.8 –COOH –CHO –CN O s–trans –C–CH3 s–cis –CF3 –phenyl O –C–OCH3 O –O–C–CH3 0.2 0.0 – 0.2 –F –CH3 –O–CH3 Cis Gauche – 0.4 800 820 840 = = 0.4 = Hammet constant 0.6 860 880 900 920 940 960 980 Wavenumber (cm–1) FIGURE 6.10 Correlation of the Hammet constant of substituents of mono-substituted ethylene with the CH2 wag frequency. the calculated substituent electronegativity and the electron density on the carbon atom.10,11 Figure 6.10 shows the linear dependence for the mono-substituted ethylene ¼CH2 wag as a function of the intrinsic electronic contribution to the Hammet values.11 These well-defined trends of the frequency dependence of the CH wag vibrations on the substituent properties are also observed in more highly substituted olefinic groups.1 For example, substitution of an electron withdrawing ester group (eCOeOeR) in the XYC ¼CH2 species will raise the frequency of the CH2 wag. Similarly, substitution of an electron donating ether group (eOR) will lower the frequency. The trans CH wag shows a predictable frequency dependence upon the substituent electronegativity. An increase in the substituent electronegativity relative to an aliphatic will lower the CH wag frequency. Fully annotated IR and Raman spectra of selected olefinic compounds are included for comparison in the last section (#4e6, and 31e32). 3.2. Triple and Cumulated Double Bonds Figure 6.11 illustrates some of the characteristic bands for ChC, ChN, and the X¼Y¼Z groups.1-4 The CeChCeH group has a CH stretch band near 3300 cm1 and a hCeH wag band near 630 cm1 which are both strong in the IR and weak in the Raman. The ChC stretch band is found at ca. 2125 cm1, which is weak in the IR but strong in the Raman. The symmetrically substituted ChC stretching vibration is IR inactive but Raman allowed. For the dialkyl acetylenes, the Raman spectra usually has two bands near 2300 and 2230 cm1 resulting from Fermi resonance splitting of the ChC stretch. For the diaryl acetylenes, only a single ChC stretch band is observed in the Raman near 2220 cm1. The ReChN group results in IR and Raman ChN stretch bands near 2240 cm1 which can be lowered by roughly 20 cm1 by conjugation with a suitable substituent. The eSeChN 86 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES FIGURE 6.11 Generalized IR and Raman spectra of selected triple and cumulated double bond compounds. group is characterized by IR and Raman bands near 2150 cm1 which involve the ChN stretch. The eN¼C¼O, eN¼C ¼S, and eN¼C¼N e groups all have very strong IR bands in the 2280e2050 cm1 region resulting from an out-of-phase stretch of the X¼Y¼Z bonds. Fully annotated IR and Raman spectra of two selected nitrile containing compounds as well as an acetylene compound are included in the last section for comparison (#7e9). 3.3. Aromatic Benzene Rings The twenty different normal modes of benzene are shown in Fig. 6.1212. A simple substitution of only one substituent results in a significant increase in the modes of vibration to thirty. Table 6.4 provides the calculated and experimental frequencies along with the unique Wilson numbers and accompanying symmetry for the modes of vibration for benzene.13 As shown in Fig. 6.12, the benzene CeH stretching vibrations are observed in the 3100e3000 cm1 region. In all other vibrations below 3000 cm1 the CeH bond length remains relatively unchanged. Many of the bands observed in the 1600e1000 cm1 region involve in-plane CeH bending vibrations that interact with various ring C¼C vibrations. Out-of-plane CeH bending (i.e., wag) vibrations give rise to bands below ca. 1000 cm1. In general, many of the aromatic ring vibrational modes are insensitive to the substituent groups and can be regarded as good group frequencies for the benzene ring. Some of the vibrations are substituent sensitive and result in mechanical coupling with the substituent group vibrations and can provide important structural information. However, not all of these 87 CONJUGATED ALIPHATICS AND AROMATICS 20a 2 3073 cm-1 3064 cm-1 8a 13 3057 cm-1 1599 cm-1 3 14 1350 cm-1 1309 cm-1 20b 3064 cm-1 7a 7b 3056 cm-1 3056 cm-1 8b 19a 19b 1599 cm-1 1482 cm-1 1482 cm-1 9a 9b 15 1178 cm-1 1146 cm-1 1178 cm-1 + - 18b 18a 12 5 1 + + 1037 cm-1 1037 cm-1 + - - 17b - - - + - - + 17a - + + + 967 cm-1 967 cm-1 1010 cm-1 846 cm-1 6a 11 - 6b 673 cm-1 + + 846 cm-1 707 cm-1 + - - - - + - 16a 16b - - ++ - -+- - + + - - 990 cm-1 - + - + - + - + +-++ 10a 4 10b - + - + + - + - -+-- - 993 cm-1 + 606 cm-1 606 cm-1 398 cm-1 398 cm-1 FIGURE 6.12 Approximate vibrational modes of benzene adapted from (M.A. Palafox, Int. J. Quant. Chem., 77, 661e684, 2000). The electronic structure was determined using density functional method with a 6e31G* basis set. Out-of-plane molecular vibrations are depicted using þ and to indicate out of and into the page, respectively. 88 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES TABLE 6.4 Calculated and Experimental Frequencies for the Normal Ring Modes of Benzene Frequency (cmL1) Wilson no. Symmetry Calc IR (Liquid) IR (Gas) Description 1 a1g 1026 993 993.1 d (CCC) 2 a1g 3218 3073 3073.9 n (CeH) 3 a2g 1375 1350 1350 d (CeH) 4 b2g 719 707 707 g (CCC) 5 b2g 1015 990 990 g (CeH) 6 e2g 618 606 608.1 g (CCC) 7 e2g 3192 3056 3056.7 n (CeH) 8 e2g 1664 1599 1600.9 n (C¼C) 9 e2g 1203 1178 1177.8 d (CeH) 10 e1g 866 846 847 g (CeH) 11 a2u 694 673 673.9 g (CeH) 12 b1u 1013 1010 1010 d (CCC) 13 b1u 3182 3057 3057 n (CeH) 14 b2u 1378 1309 1309.4 n (C¼C) 15 b2u 1178 1146 1149.7 d (CeH) 16 e2u 412 398 398 g (CCC) 17 e2u 978 967 967 g (CeH) 18 e1u 1071 1037 1038.3 d (CeH) 19 e1u 1524 1512 1483.9 n (C¼C) 20 e1u 3208 3064 3064.4 n (CeH) The data is from (International J of Quantum Chemistry, 77, 661e684, 2000) and includes calculated frequencies used density functional method with a 6e31G* basis set and experimental values vibrations are of use since many are either weak or absent in either the IR and Raman spectra. The vibrational modes of the aromatic benzene ring that are sensitive to the mass and electronic properties of ring substituents include the following: 1. 1300e1050 cm1 region, involving in-plane movement of the ring carbons and the substituents. 2. 850e620 cm1 region, involving the CH wag vibrations (intense in IR but weak in Raman) and ring out-of-plane vibrations. Some of the characteristic group frequencies for mono- and di-substituted benzenes are illustrated in the generalized IR and Raman spectra depicted in Fig. 6.13.1-4,14 Many different types of aromatic ring vibrations are illustrated which demonstrate how IR and CONJUGATED ALIPHATICS AND AROMATICS 89 FIGURE 6.13 Generalized IR and Raman spectra of mono- and di-substituted benzenes. The complex ring vibrations can be greatly reduced using a standing wave description. This technique allows easy visualization of the form of the ring vibrations. Raman spectra can be used to characterize aromatic substitution types. Useful group frequencies include the following: • • • • • CH stretch bands Ring stretch bands Ring bend bands CH rock and CH wag bands Aromatic summation bands Figure 6.14 shows the form of the 12 elementary vibrations for a six-membered ring with equal bond lengths, masses, and no substituents. The vibrational standing waves for these vibrations are also shown.1,4 The top row illustrates the six ring stretching vibrations and the bottom row the six bending vibrations for this simple six-membered ring. The dashed lines show the nodal lines used to define the sextant , quadrant, semicircle, or whole ring vibrations. These lines separate molecular segments which vibrate out-of-phase with each other. The doubly degenerate ring modes are labeled with subscripts “a” and “b”. Bands originating from ring vibrations in the IR and Raman spectra shown in Fig. 6.13 are annotated using the above description. The bottom row indicates the accompanying motion of the attached hydrogens in benzene which mechanically couple with the above ring modes. The aryl CH stretch bands are observed in the 3100e3000 cm1 region. The IR spectrum usually has several bands here while the Raman usually has only one. The ring quadrant stretch vibration has two components observed in the IR and Raman spectra near 1600 90 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES S C S S C C S C S C C S S C C S S C S 2b 1 4 3a 3b 2a OBO B O B BO B O B O O B – + – + – + 6 5a 5b 8 – – + – – + – – + 7a 7b + Modes 1 & 6 mix in mono, meta- and 1,3,5 substituted ring _ H H H H H +H C C C C C + C + 1-6 Attached hydrogen motions in benzenes Above ~3000 Below ~3000 CH stretch Carbon radial 1+6 Above ~1250 Below ~1250 CH bend Carbon tangential Above ~710 Below ~710 CH wag Carbon out-of-plane FIGURE 6.14 The twelve elementary ring vibrations for a six-membered ring with equal bond lengths, masses, and no substituents are illustrated with vibrational standing waves. The elementary stretching vibrations are shown in the top row and the in-plane and out-of-plane bending vibrations are shown in the second row. For rings of suitable symmetry, mixing of modes 1 and 6 results in vibrations shown in row three, which provide very strong Raman bands. The fourth row indicates the attached hydrogen motions in benzene which couple with the ring modes. Dotted lines are used to indicate nodal lines. and 1580 cm1. Identical para substituents provide an exception, where no quadrant stretching bands are seen in the IR spectrum since they are forbidden by symmetry. The ring semicircle stretch has two bands in the IR near 1500 and 1450 cm1 except in the para-substituted benzene where they are observed near 1515 and 1415 cm1 in the IR. In mono and meta isomers, there is a very strong Raman band near 1000 cm1. This ring vibration involves movement of the 2,4,6 carbons radially in-phase and is a result of mixing of the whole ring stretch with the sextant in-plane ring bend. In the mono- and meta- substituted benzenes, there is a strong IR band near 690 cm1 called the sextant out-of-plane deformation or ring pucker, where the 2,4,6 and 1,3,5 sets of carbons move oppositely out-of-plane. 3.3.1. Aryl CH Wag The infrared spectrum in the region from ca. 900 to 600 cm1 have intense bands that are highly characteristic of the number of hydrogen atoms present on the aromatic ring. The number of IR bands and their frequencies of vibrations involving the CH wag are significantly determined by the number of adjacent hydrogen atoms present on the benzene ring. To a lesser extent, there is some dependence of the CH wag frequency on the electron 91 CONJUGATED ALIPHATICS AND AROMATICS TABLE 6.5 Selected IR Bands Useful for Substitution Determination of Aromatic Rings in the 950e680 cm1 Spectral Region Substituent st No. of Subst Pattern 1 CH Wag 2nd CH Wag Ring sextant out plane def 1 Mono 780e720 5 Adj CH wag NA 710e690 2 Ortho (1,2) 820e720 4 Adj CH wag NA inactive 2 Meta (1,3) 950e880 lone H wag 870e730 2 Adj H wag 740e690 ring pucker ring pucker 2 Para (1,4) 860e790 2 Adj H wag NA inactive 3 1,2,3 805e760 3 adj H wag NA 720e680 ring pucker 3 1,2,4 900e870 lone H wag 800e760 720e680 ring pucker 3 1,3,5 865e810 lone H wag NA 730e680 ring pucker 4 1,2,3,4 860e800 2 adj H wag NA inactive 4 1,2,3,5 850e840 lone H wag NA 720e680 4 1,2,4,5, 870e860 lone H wag NA inactive 6 1,2,3,4,5,6 NA NA inactive 2 Adj H wag ring pucker donating or withdrawing characteristics of the substituent which effects the total electron density of the aromatic ring carbon atoms. Table 6.5 summarizes some of the more important and intense IR bands in this region. Complications in this spectral region can occur due to mechanical coupling of the CH wag with the bending vibrations of substituents such as eNO2, carboxylic acids, and acid salts.1-4 Some of the less intense CH wag bands are not included in table 6.5 despite the fact that they are found in a relatively small frequency range. The mono-substituted vibrational mode shown in Fig. 6.15 is found at ca. 900 cm1 and is an example of a weaker band that remains useful but is not included in Table 6.4. The range of this band is between 940 and 860 cm1 and the frequency varies as a function of the substituent. Although this band can be used as a group frequency for structural determination, it is of greater use to demonstrate the linear frequency dependence of the band upon the calculated carbon electron density.15 The frequency of the mono-substituted benzene ring CH wag (900 cm1) shows a welldefined linear dependence on the experimentally determined Hammet values and the calculated electron density of the ring carbon atoms. The linear frequency dependence of this band upon the calculated electron density of the benzene ring para carbon atom for monosubstituted benzene ring is shown in Fig. 6.16. In many cases, the band positions for other aromatic CH wag vibrational modes listed in Table 6.5 can be estimated using an empirical expressions and a table of the mono-substituted aromatic band values (w900 cm1 mode). Detailed examples of this type of analysis can be found in the literature (N.B. Colthup, Applied Spectrosc., 30, 589, 1976).1,4,15 Fully annotated IR and Raman spectra of selected aromatic compounds are presented in the last section for comparison and include #10e15, 27, 45, 57, 64e69, 72, and 77. 92 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES + - - + + Approximate vibrational mode of the 900 cm1 band CH wag for a mono-substituted benzene ring. The solid circle represents the substituent. FIGURE 6.15 6.03 6.02 –OCH3 O –OH –NMe2 = –NH–C–CH3 –NH2 –F –C(CH3)3 6.01 –CH3 6.00 5.99 –CH2OH –NH3+ –C=CH2 –CCH –C–OCH3 O = –C–H O 880 890 –C–CH3 & –CF3 900 910 920 O = Mono-substituted aryl CH wag 870 –NO 2 = 5.96 860 O = O –C–NH2 5.98 5.97 –CN = Calculated electron density (para carbon atom) 6.04 –C–Cl 930 940 Wavenumber (cm–1) FIGURE 6.16 Correlation of the benzene ring para carbon electron density as a function of substituent with the (w900 cm1 mode) CH2 wag frequency. Data adapted from N.B. Colthup, Appl. Spectrosc., 30, 589, 1976. 3.4. Fused Ring Aromatics Polynuclear fused ring aromatics such as naphthalenes, anthracenes, and phenanthrenes have characteristic IR and Raman bands in the same general regions as benzene derivatives.1-4 The aryl-CH wag vibrations of these polynuclear aromatic compounds provide strong IR bands that provide important substitution information. Characteristic IR and Raman bands involving ring stretching vibrations are typically observed in the 1630e1500 cm1 region. In general, alkyl-substituted naphthalenes have a doublet near 1600 cm1, and additional bands in the 1520e1505 cm1 and 1400e1390 cm1. In the Raman spectra, a very strong characteristic band is observed between 1390 and 1370 cm1 and another strong band between 1030 and 1010 cm1. Some of the characteristic CH wag IR bands of naphthalenes are summarized in Table 6.6. 93 CONJUGATED ALIPHATICS AND AROMATICS TABLE 6.6 Selected IR bands Useful for Substitution Determination of Naphthalene Rings in the 900e700 cm1 Spectral Region No. of Subst Pattern 1st (cm 1 1 (mono) 1 2 Substituent ) CH Wag 2nd (cmL1) CH Wag 3rd (cmL1) 810e775 3 Adj CH wag 780e760 4 adj H wag NA 2 (mono) 875e823 lone H wag 825e800 2 adj H wag 760e735 1,2 (di) 835e800 2 adj H wag 800e726 4 adj H wag NA L1 CH Wag 4 adj H wag 3.5. Heterocyclic Aromatic Six-Membered Ring Compounds Six-membered heterocyclic aromatic compounds are benzene-like rings where one or more of the ring carbons are replaced with nitrogen atoms. Important examples of these compounds include pyridine which involves replacement with one nitrogen atom and triazine which involves substitution with three nitrogen atoms. Because, the six-membered heterocyclic rings typically have an aromatic double bonds in the ring analogous to benzenes, the resultant IR and Raman spectra have many of the same type of group frequencies. However, substitution of an OH or SH group in the ortho or para position relative to the ring nitrogen atom typically results in tautomerization and the formation of a C¼O or C¼S bond, respectively.1,3,4 Such tautomerization also results in characteristic changes in the bands associated with the ring vibrations. Structural examples of these type of tautomerization is shown in Fig. 6.17 for 2-hydroxypyridine and cyanuric acid. FIGURE 6.17 The resultant keto form resulting from tautomerization of 2-hydroxypyridine and Cyanuric Acid. The keto tautomer of 2-hydroxypyridine results in a carbonyl band between ca. 1650 and 1680 cm1. The keto tautomer of cyanuric acid results in a carbonyl band cluster between 1700 and 1780 cm1. 94 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES 3.5.1. Pyridines The IR and Raman spectra of pyridine compounds have characteristic bands due to the CH stretch, ring stretching and deformation, and substituent informative CH wag vibrations. A fully annotated IR and Raman spectrum of pyridine (#15) is included in the last section for comparison. Analogous to benzene, the CH stretching bands of pyridine compounds is found in the 3100e3000 cm1 region. The quadrant ring stretch vibrations typically give rise to moderate to strong IR and Raman bands in the 1620e1555 cm1 region. The ring semi-circle stretch also results in characteristic IR and Raman bands in the 1500e1410 cm1 region. The Raman spectra of pyridine compounds is typically dominated by a very strong band in the 1050e980 cm1 region that involves the 2,4,6 carbon radial stretch. The quadrant ring in-plane bend also results in a moderate Raman band in the 850e750 cm1 region which is correlated with the substituent position. Similarly, the strong adjacent CH wag observed in the IR in the 850e740 cm1 region is correlated with the pyridine substitution. Some of the characteristic bands for pyridine compounds are listed in Tables 6.7 and 6.8.1-4 Protonation of the pyridine nitrogen to form the pyridinium salt results in multiple bands from the NþeH stretch between 3300 and 1900 cm1 that have moderate IR intensity but are weak in the Raman spectrum. The multiple band structure derives from Fermi resonance. Potentially useful Raman bands with variable intensities include the NH in-plane bend at ca. 1250e1240 cm1 and the NH wag at 940e880 cm1. The N-oxides of pyridines have a strong IR band in the 1300e1200 cm1 region involving the NeO stretch. This band is moderate to weak in the Raman spectra. IR and Raman bands involving the NeO stretch and the ring in-plane bend occurs between 880 and 830 cm1. 3.5.2. Triazines and Melamines The triazine ring in s-triazine, alkyl- and aryl-substituted triazines, and melamine compounds have characteristic IR and Raman bands due to ring stretch and deformation vibrations.1-4,16,17 The quadrant ring stretching vibrations result in strong IR and moderate Raman bands in the 1600e1500 cm1 region. The semi-circle ring stretching vibrations result in multiple strong IR and weak Raman bands in the 1450e1350 cm1 region. Particularly, intense Raman bands of triazine compounds include the N-Radial in-phase stretch in the 1000e980 cm1 region and the quadrant in-plane bend in the 690e660 cm1 region. Lastly, in the IR there is a moderately intense band in the 860e775 cm1 region that involves the ring sextant out-of-plane deformation. Melamine consists of a triazine ring with substitution of an amino group on all three ring carbon atoms. The amino group has multiple IR and Raman bands between 3500 and 3100 cm1 involving the NH2 stretch and a doublet between 1650 and 1620 cm1 from the NH2 deformation. The added complexity is due to differences in hydrogen bonding in the solid crystalline state. Measurements of melamine in DMSO solution results in the expected doublet from the NH2 out-of-phase and in-phase stretch. The triazine ring itself exhibits the expected IR and Raman bands at 1550 cm1 deriving from the quadrant ring stretch and 1470e1430 cm1 from the semi-circle ring stretch. The unique characteristic of Raman bands include the strong band at 984 cm1 from 95 CONJUGATED ALIPHATICS AND AROMATICS TABLE 6.7 Selected In-plane Stretching and Bending Vibrations for Pyridine and Mono-Subsituted Pyridine Intensities1 Pyridine Assignment Region (cmL1) IR R Pyridine Aryl CH str. 3100e3000 m m Quadrant str. 1615e1570 s m Semi-circle str 1490e1440 s mw 2,4,6 carbon radial str. 1035e1025 m vs Ring breath/str. 995e985 m s Quadrant in-plane bend 660e600 e m Quadrant str. 1620e1570 s m Quadrant str. 1580e1560 s m Semi-circle str. 1480e1450 s m Semi-circle str. 1440e1415 s w CH in-plane rk 1050e1040 m s 2,4,6 carbon radial str. 1000e985 m vs Quadrant in-plane bend 850e800 w ms Quadrant str. 1595e1570 m ms Quadrant str. 1585e1560 s m Semi-circle str. 1480e1465 s m Semi-circle str. 1430e1410 s w 2,4,6 carbon radial str. 1030e1010 m vs Quadrant in-plane bend 805e750 w m Quadrant str. 1605e1565 m ms Quadrant str. 1570e1555 m m Semi-circle str. 1500e1480 m m Semi-circle str. 1420e1410 s w 2,4,6 carbon radial str. 1000e985 m vs Quadrant in-plane bend 805e785 w ms 2-Mono- substituted 3-Mono- substituted 4-Mono- substituted s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, var. ¼ variable, e ¼ zero 1 the N-Radial in-phase stretch and the quadrant in-plane ring bend at 675 cm1. A sharp, moderate IR band is observed between 825 and 800 cm1 which involves the traizine ring sextant out-of-plane deformation. This vibration involves an out-of-plane movement of all three carbon atoms and the three nitrogen atoms in opposite directions. 96 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES TABLE 6.8 Selected Out-of-plane CH wag and Out-of-plane Sextant Deformation Vibrations for Pyridine and Mono-Subsituted Pyridine Intensities1 Pyridine Assignment Region (cm Pyridine 5 adjacent H wag IR R 760e74 s e out-of-plane sextant def. 710e700 s e 2-Mono- substituted 4 adjacent H wag 780e740 s e 3-Mono- substituted 3 adjacent H wag 820e770 s e out-of-plane sextant def 730e690 m-s w 2 adjacent H wag 850e790 s e 4-Mono- substituted ) L1 s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, var. ¼ variable, e ¼ zero 1 Tautomerizing the triazine ring by making one of the double bonds external to the ring results in the iso form in which there are fewer than three double bonds in the ring. As discussed above, substitution of the carbon atom with an OH or SH results in formation of the iso form of the triazine ring. In addition to the formation of a C¼O or C¼S bond, the IR active band that derives from the sextant out-of-plane bend will be found in the 795e750 cm1 region. Thus, the sextant out-of-plane bend vibration found near 800 cm1 can be used to discriminate between normal triazine rings and the iso form of the triazine ring. A fully annotated IR and Raman spectrum of ammeline (#34) and trimethyl isocyanurate (#35) are included in the last section for comparison. 3.5.3. Fiveemembered Ring Heterocyclic Compounds Pyrroles, furans, and thiophenes are five-membered ring aromatics containing two conjugated double bonds and a single nitrogen, oxygen or sulfur atom in the ring respectively. Additional compounds exist in which an additional carbon atom (position 2 or 3) are replaced with nitrogen or oxygen atoms. The double bonds are somewhat delocalized in these five-membered rings and the ring vibrations can be described using a standing wave description. Figure 6.18 depicts selected approximate ring vibrations for a five-membered ring containing one heteroatom.1 However, the ring carbon atoms are not identical and changing the substituent can significantly affect the IR and Raman spectra. Table 6.9 lists some of the characteristic ring stretching vibrations for pyrroles, furans, and thiophene compounds.1-4,18 The quadrant and semi-circle ring stretching vibrations typically mix with the CH rock vibrations. The quadrant stretching vibration consists primarily of the C¼C stretch while the first semi-circle stretch consists mainly of the CeXeC in-phase stretch with some CeC contraction. The second semi-circle stretch consists mainly of the CeXeC out-of-phase stretch. Important five-membered ring heteroaromatic vibrations not depicted in Fig. 6.18 include the ring CH stretch, the eCH¼CHe ring double bond cis CH wag and NH stretch in pyrrole type compounds. The NH stretch of pyrroles are typically observed between 3450 and 97 CONJUGATED ALIPHATICS AND AROMATICS Contr Str Str Contr Contr Str Str Str Contr In Out Str Str In Contr Out Down – In In Out Up Up + + + + – Up Down Down – – Down + Up Out FIGURE 6.18 Approximate ring vibrations for a five-membered ring containing one heteroatom (solid circle) and two double bonds. The quadrant stretch predominantly involves the double bond stretching vibration. TABLE 6.9 Selected In-plane Stretching Vibrations for Selected Pyrroles, Furans and Thiophenes Intensities1 Pyridine Assignment Region (cmL1) IR R Pyrrole NH str. 3500e3000 s mew ¼CH str. 3135, 3103 m s Quadrant str. þ CH rk 1530 s e Quadrant str. þ CH rk 1468 m s Semi-circle str þ CH rk 1418 m e Semi-circle str. þ CH rk 1380 s m Ring in-phase str. 1143 m vs Quadrant str. 1560e1540 wem m Quadrant str. 1510e1490 m vs Semi-circle str. 1390e1380 sem s 1-subst pyrrole (Continued) 98 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES TABLE 6.9 Selected In-plane Stretching Vibrations for Selected Pyrroles, Furans and Thiophenesdcont’d Intensities1 Pyridine Assignment Region (cm 2-subst pyrrole Quadrant str. 3-subst pyrrole Furan 2-subst furan Thiophene 2-subst thiophene ) IR R 1570e1545 wem m Quadrant str. 1475e1460 m vs Semi-circle str. 1420e1400 sem s Quadrant str. 1570e1560 wem m Quadrant str. 1490e1480 m vs Semi-circle str. 1430e1420 sem s ¼CH str. 3156, 3121, 3092 m sem Quadrant str. þ CH rk 1590 s w Quadrant str. þ CH rk 1483 s vs Semi-circle str þ CH rk 1378 s s Ring in-phase str. 1138 e vs Quadrant str. 1560e1540 wem m Quadrant str. 1510e1490 m vs Semi-circle str. 1390e1380 sem s ¼CH str. 3110, 2998 m sem Quadrant str. þ CH rk 1588 s e Quadrant str. þ CH rk 1408 s vs Semi-circle str þ CH rk 1357 s s Ring in-phase str. 1031 e vs Quad bend 832 s vs Quadrant str. 1540e1514 wem m Quadrant str. 1455e1430 m vs Semi-circle str. 1361e1345 sem s L1 3200 cm1. However, for five-membered rings with more than one nitrogen atom in the ring such as imidazoles, a strongly hydrogen-bonded NHeN ¼ complex occurs resulting in a broad complex set of bands in the 3000e2600 cm1 observed in both the IR and Raman spectra. Pyrroles and furans which have nitrogen or oxygen atom in the ring, respectively, have moderate to strong IR and Raman bands involving the ¼CH stretch between 3180 and 3090 cm1. In the case of thiophenes which has a sulfur atom in the ring, the ¼CH stretching vibration is somewhat lower in frequency at 3120e3060 cm1. In general, the ¼CH stretching vibrations of five-membered ring heterocycles are slightly higher than CARBONYL GROUPS 99 observed for their six-membered ring analogues. Five-membered ring heterocyclic compounds with a unsubstituted eCH ¼CHe group are characterized by a strong IR band in the 800e700 cm1 region which derives from the in-phase cis CH wag. 4. CARBONYL GROUPS The generalized IR and Raman spectra of the carbonyl region are shown in Fig. 6.19 and the carbonyl group frequencies are summarized in Table 6.10.1-4 The C¼O stretching vibration responsible for these bands typically give rise to strong IR bands and weak Raman bands. In addition to the carbonyl band itself, much useful information can be obtained from the bands of the attached groups. The column of spectra on the left side shows examples of unconjugated carbonyl species including ketones (near 1715 cm1), aldehydes (near 1730 cm1), esters (near 1740 cm1), and acid chlorides (near 1800 cm1). The central column of spectra shows examples of carboxylic acid, carboxylic acid salt, anhydride, and cyclic anhydride while the third column of spectra shows examples of various amides. 4.1. Review of Selected Carbonyl Species Carboxylic acid dimers are characterized by an IR band near 1700 cm1, and a Raman band near 1660 cm1 which derive from the out-of-phase and the in-phase C¼O stretch in the dimer, respectively. Weak, characteristic bands from the CeOH bend are also observed FIGURE 6.19 1400 cm1. The generalized IR and Raman spectra of selected carbonyl compounds between 1900 and 100 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES TABLE 6.10 Selected Group Frequencies: Carbonyls Intensities1 Group C¼O Assignment Frequency (cmL1) IR R ReCOeH C ¼O str. 1740e1720 s m conj-COeH C ¼O str. 1710e1685 s w ReCOeR C ¼O str. 1725e1705 s m conj-COeR C ¼O str. 1700e1670 s m conj-CO-conj C ¼O str. 1680e1640 s m HeCOeOeR C ¼O str. 1725e1720 s m ReCOeOeR C ¼O str. 1750e1735 s m Conj-COeOeR C ¼O str. 1735e1715 s m g lactones C ¼O str. 1795e1760 s m ReCOeOH dimer C ¼O o.ph. str. 1720e1680 s e ” C ¼O i.ph. str. 1670e1630 e m ReCOeN C ¼O str. 1695e1630 s m ReCOeCl C ¼O str. 1810e1775 s m ReCOeOeCOeR C ¼O i.ph. str. 1825e1815 s m ” C ¼O o.ph. str. 1755e1745 ms m Cyclic anhydride C ¼O i.ph. str. 1870e1845 m s ”” C ¼O o.ph. str. 1800e1775 s mw ReCO 2 CO2 o.ph. str. 1650e1540 s w ” CO2 i.ph. str. 1450e1360 ms s s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero 1 near 1420 cm1. Carboxylic acid salts give rise to two bands involving the out-of-phase and in-phase CO2 stretching vibrations. The out-of-phase stretch band is observed near 1570 cm1 and is typically quite strong in the IR but weak in Raman spectrum. The inphase stretch band is observed near 1415 cm1 and is typically strongest in the Raman spectrum. Anhydrides are characterized by two bands which involve the stretching of both C¼O groups in- and out-of-phase with respect to each other. Unconjugated, non-cyclic anhydrides result in bands near 1820 cm1 and 1750 cm1, where the higher frequency, C¼O in-phase stretch is more intense than the out-of-phase stretch for both the IR and the Raman spectra. Five-membered ring cyclic anhydrides give rise to bands near 1850 cm1 and 1780 cm1, where the lower frequency out-of-phase stretch band is strongest in the IR spectrum, while the higher frequency in-phase band is strongest in the Raman spectrum. 101 CARBONYL GROUPS This significant difference between the IR intensity of the in-phase and out-of-phase C¼O stretch for the non-cyclic anhydrides and the cyclic anhydrides is directly related to the relative orientation of the two carbonyl groups. An integrated IR absorbance ratio of the two carbonyl bands can be used to predict the dihedral angle of the CeO bonds.1 Amides are characterized by strong IR bands from the C¼O stretch near 1670 cm1. The Raman bands for the amide carbonyl stretch are typically quite weak. The primary amide group (O¼CeNH2) also has bands near 1610 cm1 involving the NH2 bend as well as near 1410 cm1 involving the CeN stretch. The non-cyclic secondary amide group (O¼CeNHeC) not only has a C¼O stretch at 1670 cm1, but another strong band in the IR spectrum near 1550 cm1 involving the CNH bend and the CeN stretch. In contrast, the cyclic secondary amide (O¼CeNHeC) group does not have a 1550 cm1 band. Amides with no NH have only the C¼O stretch in this region. Fully annotated IR and Raman spectra of selected carbonyl compounds are included in Chapter 8 for comparison (see #16e29 for ketones, esters and anhydrides, 30e39 for amides, ureas, and related compounds). 4.2. Factors that Effect Carbonyl Frequencies The carbonyl stretching frequencies are dependent upon mass, mechanical, and force constant effects. The mass effect is typically the least important factor in understanding carbonyl frequency dependence upon structure. This arises because the carbonyl stretching vibration involves mostly a change in the C¼O bond length with only a slight movement of the attached atoms (bending). This is an excellent example of a high frequency C¼O oscillator attached to a low frequency CeX oscillator (see unequal oscillators discussed earlier). The effect of changing mass on the carbonyl frequency when replacing a carbon atom with a heavier atom is quite slight due to the mass effect alone. The mass effect is more important when decreasing the substituent mass from carbon (12) to hydrogen (1) and should decrease the C¼O frequency about 17 cm1 when comparing a ketone to an aldehyde. However, even in this simple example, the higher C¼O frequency of the aldehyde is counter to that expected by the mass effect, and demonstrates the importance of changes in the force constants. O O C CH2 C CH2 CH2 H The above shows the approximate carbonyl stretching vibration for a ketone and an aldehyde. The very slight movements of the carbon atoms which results in changes in the carbonyl carbon bond angle are not shown. The aldehyde CH bend and C¼O stretch are similar in energy and therefore couple more strongly than either the CeC stretch or the CeC bend. The characteristic frequencies found in strained ring carbonyls is an excellent example of the importance of mechanical effects in carbonyl compounds and is analogous to that observed in cyclic olefin structures. Once again, the bond angle is the critical variable in 102 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES determining the carbonyl stretching frequency. Shown below are the structures, important bond angles, as well as the observed and calculated frequencies based upon a simple mechanical model for the carbonyl stretching frequencies of four cyclic carbonyl compounds. 120° O O 126° C C O O 135° C 150° C Calc (cm–1) 1714 1737 1774 1836 –1) 1715 1740 1782 1822 Obs (cm The cyclic C¼O stretching frequencies as a function of the CeC¼O bond angles is shown above. The same mechanical triatomic model is used for the above calculations as was used for the cyclic olefin structures earlier, where FCeC and FC¼O are 4.5 and 11.1 mdynes/A, respectively. The good agreement between the calculated and observed frequencies indicates that most of the shifts derive from mechanical effects and not from changes in force constants. Force constant effects on carbonyl stretching frequencies include mesomeric, inductive, and field effects.1-4 Carbonyl inductive effects derive from the polar nature of the carbonyl C¼O bond. As shown below, the more the oxygen atom can attract electrons, the weaker the carbonyl C¼O force constant becomes. O O C C O O O R C R R C Cl Cl C Cl 1715 cm–1 1800 cm–1 1828 cm–1 The polarity of the carbonyl bond is a consequence of the greater electronegativity of the oxygen relative to the carbon atom. Electronegative substituents on the carbon atom will compete with the oxygen for electrons and thereby raise the carbonyl frequency. Substitution with strongly electron-withdrawing chlorine atoms provides a classic example of this. Field effects are another example of inductive effects in the carbonyl group. In this case, the presence of an electron rich chlorine spatially near to the oxygen atom favors the non-polar C¼O group and raises the frequency. Thus, IR can discriminate between the cis and gauche rotational isomers in a-chlorocarbonyl compounds (CleCH2eC(¼O)eX). Mesomeric and conjugation effects can be understood by examining resonance structures. Mesomeric effects derive from substituting the carbonyl group with lone pair electron containing heteroatoms such as chlorine, oxygen, or nitrogen. 103 CeO AND CeN STRETCHES O O C Cl C Cl 1800 cm–1 O O C O CH2 C O CH2 O O C NH2 C NH2 1740 cm–1 1660 cm–1 As shown above, the non-bonding electrons rearrange, resulting in donation of electrons to the oxygen atom and thereby weakening the carbonyl C¼O bond. The mesomeric effect is not important for acid chlorides since the resonance structure placing a positive charge on the chlorine atom is not favored. Conversely, the mesomeric effect is the dominant determinant of the carbonyl frequency in amides. Conjugation of a carbonyl with either a vinyl or a phenyl group typically lowers the carbonyl frequency by 20e30 cm1. This is also a result of electron rearrangement, analogous to the mesomeric effect discussed above and can easily be understood by use of resonance structures. Shown below are the approximate frequencies for an unconjugated ketone, single, and double conjugated ketone. O CH2 C CH2 1715 cm–1 O CH2 C CH CH2 1685 cm–1 O CH2 CH C CH CH2 1655 cm–1 Lastly, interaction effects are key in understanding the source of two bands in anhydrides and carboxylic acid salts; these have their origin in both mechanical interactions and electron redistribution. Anhydrides have two carbonyl groups that vibrate in- and out-of-phase to each other resulting in two bands in the IR and Raman spectra. Comparison of the generalized IR spectra of cyclic and non-cyclic anhydrides shows that the relative intensity easily enables rapid identification. Similarly, carboxylic acid salt have two bond-and-a-half CeO bonds which vibrate in- and out-of-phase to give two bands (see Fig. 6.19). The different relative intensities for these two vibrations derive from the total dipole moment change. The electron redistribution that occurs during the in- and out-of-phase vibrations for these compounds is termed as an interaction force constant and also contributes to the observed frequencies. 5. CeO AND CeN STRETCHES The IR and Raman spectra in Fig. 6.20 illustrate some group frequencies for vibrations involving CeO or CeN stretches interacting with connected CeC vibrations.1-4 Table 6.11 104 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES FIGURE 6.20 The generalized IR and Raman spectra of selected CeO and CeN stretching bands. TABLE 6.11 Selected Group Frequencies: Alcohols and Ethers Intensities1 Group OH Assignment Frequency (cmL1) IR R ReOH..O OH str. 3400e3200 sbr vw O¼CeOH dimer OH str. 3200e2600 sbr e ” OH wag 960e875 m e O OH str. 2800e2100 s br vw O OH str. 3100e2200 sbr vw CH2eOeCH2 COC o.ph. str 1270e1060 s w “ COC i.ph. str. 1140e800 m s C¼CeOeCH2 COC o.ph. str. 1225e1200 s w AreOeCH2 AreO str. 1310e1210 s m ” OeCH2 str. 1050e1010 m m CH2eOH CeO str. 1090e1000 s mw “ CeO str. 900e800 mw s .. PeOH .. SeOH CeO 105 N¼O AND OTHER NITROGEN CONTAINING COMPOUNDS TABLE 6.11 Selected Group Frequencies: Alcohols and Ethersdcont’d Intensities1 Group R2CHeOH Assignment Frequency (cm CeO str. 1150e1075 ) L1 IR R m mw “ CeO str. 900e800 mw s R3CeOH CeO str. 1210e1100 s mw “ CeO str. 800e750 mw s AreOH CeO str. 1260e1180 s w O¼CeOeC CeO str. 1300e1140 s w O¼CeOH CeO str. 1300e1200 s w Epoxy ring i.ph. str. 1270e1245 m s ” ring o.ph. str. 935e880 s m ” ring o.ph. str. 880e830 s m s ¼ strong, m¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero 1 summarizes some of the important group frequencies for alcohols and ethers. Typically, vibrations involving these groups tend to give band clusters rather than single bands. The bands involving the CeO and CeN stretches of alcohols and amines show a systematic frequency dependence on their substitution (i.e., primary, secondary.). Primary alcohols and amines have bands at approximately 1050 cm1 and near 1075 cm1 respectively. Secondary alcohols have bands at roughly 1100 cm1, while aliphatic secondary amines have IR bands near 1135 cm1. Tertiary alcohols usually have IR bands near 1200 cm1. Aliphatic ethers have characteristic IR bands near 1120 cm1 while alkylearyl ethers have two sets of bands near 1250 cm1 involving aryl-O stretch and 1040 cm1 involving the OeCH2 stretch. Phenols have IR bands near 1240 cm1 and aromatic-NH2 aniline species have IR bands roughly at 1280 cm1 involving aryl-O and aryl-N stretches, respectively. Fully annotated IR and Raman spectra of selected alcohol and ethers are included in Chapter 8 (alcohols #40e47 and ethers 48e50) for comparison. 6. N[O AND OTHER NITROGEN CONTAINING COMPOUNDS The generalized IR and Raman spectra illustrating group frequencies for some N¼O type compounds are shown in Fig. 6.21.1-4 Table 6.12 summarizes the selected group frequencies for selected nitrogen containing compounds. As shown in Fig. 6.21 and in Table 6.12, NOx containing compounds provide strong IR and Raman strong bands that have excellent group frequencies. The unconjugated nitro group (CeNO2) are illustrated in the top spectra in Fig. 6.21 and have NO2 stretching bands near 1560 and 1375 cm1. The out-of-phase stretch at w1560 cm1 106 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES FIGURE 6.21 The generalized IR and Raman spectra of important bands for nitroalkanes, organic nitrates, and organic nitrites. TABLE 6.12 Selected Group Frequencies: Nitrogen Compounds Intensities1 Group Assignment Frequency (cmL1) IR R CH2eNH2 NH2 o.ph. str. 3500e3300 m vw “ NH2 i.ph. str. 3400e3200 m m “ NH2 bend 1630e1590 m vw “ NH2 wag 900e600 sbr w CH2eNHeCH2 NH str. 3450e3250 vw w “ CeNeC o.ph. str. 1150e1125 m mw AreNH2 CeN str. 1380e1260 sbr m .. X CeNHþ 3 NH3 str. 3200e2700 s vw ” NH3 o.ph. bend 1625e1560 mw vw ” NH3 i.ph. bend 1550e1505 w vw 107 N¼O AND OTHER NITROGEN CONTAINING COMPOUNDS TABLE 6.12 Selected Group Frequencies: Nitrogen Compoundsdcont’d Intensities1 Group Assignment Frequency (cm C2NHþ.. X 2 NH2 str. 3000e2700 L1 ) IR R sbr w ” NH2 bend 1620e1560 mw w C3NHþ..X NH str. 2700e2300 s w O ¼CeNH2 NH2 o.ph. str. 3475e3350 s w ” NH2 i.ph. str. 3385e3180 s w ” NH2 bend 1650e1620 ms w O ¼CeNHeC NH str. 3320e3270 ms w ” CNH str. bend 1570e1515 ms w O ¼CeNHeC cyclic NH str. 3300e3100 ms w >C ¼N C ¼N str 1690e1630 mw ms >C ¼NeOH OH str. 3300e3150 s w ” NeO str. 1000e900 s s CH2eNO2 NO2 o.ph. str. 1600e1530 s mw ” NO2 i.ph. str. 1380e1310 s vs AreNO2 NO2 o.ph. str. 1555e1485 s e ” NO2 i.ph. str. 1357e1318 s vs CeOeNO2 NO2 o.ph. str. 1640e1620 s mw ” NO2 i. ph. str. 1285e1220 s s ” NeO str. 870e840 s s CeOeN ¼O s-trans N ¼O str. 1681e1640 s s ” s-cis N ¼O str. 1625e1600 s s s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero 1 is more intense in the IR spectrum while the in-phase stretch at w1375 cm1 is more intense in the Raman spectrum. In nitroalkanes, the CeN stretch gives rise to bands in the 915e 865 cm1 region that are intense in the Raman and mediumeweak in the IR spectra. These bands are sensitive to the rotational isomer present in the CH2eCH2eNO2 group. When the CeC group is trans to the CNO2 species the CeN band is at w900 cm1 and when the gauche orientation is present, it is at w 880 cm1. The covalent nitrate group (ReOeNO 2 ) shown in the middle spectra in Fig. 6.21 has bands near 1620 and 1275 cm1 involving the NO2 stretch. The out-of-phase stretch at w1620 cm1 is more intense in the IR spectrum while the in-phase stretch at w1275 cm1 is more intense in the Raman spectrum. The NeO stretch also gives rise to a prominent band near 870 cm1. 108 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES The covalent nitrite group (ReOeN¼O) is characterized by N¼O stretching bands between 1600 and 1640 cm1 and a NeO stretch near 800 cm1. The N¼O bands are sensitive to the rotation isomer present in the CH2eOeNO group. The N¼O stretch bands are typically observed near 1640 and 1600 cm1 for the s-trans and s-cis isomers, respectively. Here s-cis indicates the single (OeC) bond is cis to the N¼O bond and s-trans indicates the single (OeC) bond is trans to the N¼O bond. The fully annotated IR and Raman spectra of selected nitrogen containing compounds including amines (#51e56), C¼N type groups (#57e59), N¼O type groups (60e62), and an azo group (63) are found in Chapter 8 for comparison. 7. C-HALOGEN AND CeS CONTAINING COMPOUNDS The generalized IR and Raman spectra illustrating group frequencies for the stretching vibrations of CeCl, CeBr, CeSeC, and CeSeSeC groups is shown in Fig. 6.22.1-4 Table 6.13 shows some selected group frequencies of halogenated compounds. All of these have rotational isomers where the atom that is trans to the halogen or sulfur is either FIGURE 6.22 The generalized IR and Raman spectra for selected vibrations involving the CeCl, CeBr, CeSeC, and CeSeSeC groups. 109 C-HALOGEN AND CeS CONTAINING COMPOUNDS TABLE 6.13 Selected Group Frequencies: Halogen Compounds Intensities1 Group Assignment Frequency (cmL1) IR R CF2 and CF3 CF str. 1350e1120 vs mw AreF CF str. 1270e1100 s m CeCl CeCl str. 830e560 s s CeCH2eCH2eCl trans CeCl str. 730e720 s s “ gauche CeCl str. 660e650 s s CH2eCl CH2 wag 1300e1240 s s CeBr CeBr str. 700e515 s vs CeCH2eCH2eBr trans CeBr str. 650e640 s vs “ gauche CeBr str. 565e560 s vs CH2eBr CH2 wag 1250e1220 s m s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero. 1 a carbon or a hydrogen atom. Both the IR and Raman spectra have isolated bands CeS and C-Halogen stretching bands which are due to the rotational isomers. For chlorinated aliphatics such as CeCH2eCH2eCl, the bands involving the CeCl stretch appear near 730 and 660 cm1. The aliphatic sulfide group (eCH2eCH2eSeCH2eCH2) is quite similar with CeS stretching bands near 720 and 640 cm1. For brominated aliphatics such as CeCH2eCH2eBr, the bands involving the CeBr stretch appear at much lower frequency near 635 and 560 cm1. The CeF stretch of organic fluorine compounds results in strong IR bands but weak to moderately intense Raman bands. Monofluorinated compounds have a strong IR and a weak to moderate Raman band between 1100 and 1000 cm1. Difluorinated compounds (eCFe2) have two very strong IR bands between 1250 and 1050 cm1 involving the CF2 out-of-phase stretch. These bands are weak to moderate in the Raman spectrum. Trifluorinated compounds (eCF3) have multiple strong bands between 1350 and 1050 cm1 involving the CF3 out-of-phase stretch. Both the IR and Raman have some characteristic bands such as the CH2 wags and the disulfide SeS stretch, which are not sensitive to rotational isomers. The disulfide group (ReSeSeR) is characterized by a very strong Raman band near 500 cm1 that involves the SeS stretch. In addition, the CH2eCl group has a CH2 wag band cluster near 1285 cm1, whereas the CH2eBr and CH2eS groups have these bands near 1240 cm1. In aryl-halides, the CeX stretching vibration couples with the aryl ring vibrations resulting in strong IR and moderate Raman bands. Fluorine-substituted aromatics (ArdF) have a moderately strong IR band between 1270 and 1100 cm1 involving the aryl ring CeF stretch. This same band is found between 1100 and 1030 cm1 for the aryl-Cl species and between 1075 and 1025 cm1 for aryl bromo species. These band regions can be further defined from high to lower frequency for para, meta, and ortho substitution of the Cl and 110 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES Br aryl halides. Iodobenzenes (para) have a characteristic aryl-I band between 1061 and 1057 cm1. The fully annotated IR and Raman spectra of selected chlorine, bromine, and fluorine containing compounds are included in Chapter 8 for comparison (#62e69, 99). 8. S[O, P[O, BeO/BeN AND SIeO COMPOUNDS The generalized IR and Raman spectra illustrating group frequencies for some S¼O, SO2, P¼O, and SieO type compounds are shown in Fig. 6.23. Table 6.14 summarizes the selected group frequencies for the chosen S¼O, P¼O, BeO, and SieO containing compounds.1-4 Compounds containing S¼O generally have strong characteristic IR and Raman bands. The characteristic IR and Raman bands of a sulfoxide (R2SO), sulfone (R2SO2), and a dialkyl sulfate (ROeSO2eOR) are shown in the first column of spectra in Fig. 6.23. The frequency of these bands can be correlated with the substituent electronegativity. In general, electronegative substituents tend to raise the S¼O frequency since they favor the S¼O resonance structure. As illustrated in the top left spectra, the intensity of the S¼O stretch near 1050 cm1 is more intense in the IR than the Raman spectrum. For both sulfones and dialkyl sulfates, the SO2 group results in two bands due to the in- and out-of-phase stretches. The higher frequency out-of-phase SO2 stretch is stronger in the IR and the in-phase SO2 stretch is more intense in the Raman spectra. Table 6.14 also summarizes FIGURE 6.23 The generalized IR and Raman spectra illustrating group frequencies for some S ¼O, P ¼O, and SieO type compounds. 111 S¼O, P¼O, BeO/BeN AND SIeO COMPOUNDS important bands for sulfonamides (ReSO2eN), sulfonates (ReSO2eOR), sulfonyl chlorides, sulfonic acids (anhydrous: ReSO2eOH), and sulfonic acid salts (ReSO3 ). The characteristic IR and Raman bands of a phosphine oxide (R3P ¼O) and phosphorus ester ((ReO)3P ¼O) are shown in the second column of spectra in Fig. 6.23. The P¼O stretching frequency depicted have been shown to correlate with the sum of the substituent TABLE 6.14 Selected Group Frequencies: Sulfur, Phosphorus, Silicon, and Boron Compounds Intensities1 Group Sulfur Cpds Phosphorus Cpds Silicon Cpds Assignment Frequency (cmL1) IR R SH SH str. 2590e2540 w s CeSO2eC SO2 o.ph. str. 1340e1290 s w “ SO2 i.ph. str. 1165e1120 s s CeSO2eN SO2 o.ph. str. 1380e1310 s m “ SO2 i.ph. str. 1180e1140 s s CeSO2eOeC SO2 o.ph. str. 1375e1335 s ms “ SO2 i.ph. str. 1195e1165 s s CeSO2eCl SO2 o.ph. str. 1390e1361 s w “ SO2 i.ph. str. 1181e1168 s s CeSO2eOH anhydrous OH str. 3100e2200 sbr w ““ SO2 o.ph. str. 1352e1342 s w ““ SO2 i.ph. str. 1165e1150 s s þ CeSO 3 H3O (hydrate) OH str. 2800e2100 sbr e ““ SO3 o.ph. str. 1230e1120 s e ““ SO3 i.ph. str. 1085e1025 m e C2S ¼O S ¼O str. 1065e1030 s w PH PH str. 2450e2270 ms mw P ¼O P ¼O str. 1320e1140 s m PeOH OH str. 2800e2100 sbr. vw PeOeCH2 PeOeC str. 1050e970 s mw PeOeAr OeAr str. 1240e1160 s mw “ PeO str. 995e855 s w SiH SieH str. 2250e2100 vs m SieO SieO str. 1100e1000 vs w SieCH3 CH3 i.ph. bend 1270e1250 vs w 2 (Continued) 112 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES TABLE 6.14 Selected Group Frequencies: Sulfur, Phosphorus, Silicon, and Boron Compoundsdcont’d Intensities1 Group Boron Cpds Assignment Frequency (cm IR R BeH str 2640e2350 s mw BH (complete octet) BeHeB (Bridge) BeH str 2400e2200 s mw BeHeB str. 2220e1540 s mw BeOH OH str 3300e3200 s w BeO BeO str 1380e1310 s var BeN BeN str. 1465e1330 s var B-Phenyl CeBeC/ring str. 1440e1430 s BH ) L1 s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero Electronegative substituents raise the P ¼O frequency (example: R3P ¼O w1160 cm1, (ReO)3P ¼O w 1270 cm1) 1 2 electronegativities. In general, the IR spectrum provides a better selection of strong characteristic bands. Table 6.14 summarizes selected important group frequencies for phosphorus compounds including the PH stretch, the OH stretch of the PeOH group, and the PeOeC stretch of the PeOeCH2 group. The PeO-phenyl group gives rise to two bands involving the phenoxy CeO stretch and the PeO stretch. The characteristic IR and Raman bands of a siloxane polymer is shown in the bottom right hands corner in Fig. 6.21. In general, IR spectroscopy provides a much better selection of strong, characteristic group frequency bands of silicon compounds. For silicones, the SiCH3 group has a good IR in-phase CH3 bending band near 1265 cm1 and the SieO stretch has a strong, broad IR band in the 1100e1000 cm1 region. Table 6.14 also summarizes selected important group frequencies for boron containing compounds. The BeO stretch results in a strong characteristic IR band between 1380 and 1310 for both organic and inorganic compounds containing the BeO species. Compounds containing BeN species also result in strong characteristic IR bands between 1465 and 1330 cm1. Little information on the characteristic Raman bands BeO and BeN containing species are available in the literature. The interpreted IR and Raman spectra for sulfur containing compounds (70e76, 83, 84), phosphorus containing compounds (77e80, 88, 89), silicon containing compounds (81, 93, 108), and boron containing compounds (64, 90) are included in Chapter 8 for comparison. 9. INORGANICS Both IR and Raman spectroscopy provides important structural information on a multitude of ionic inorganic compounds.19 Both techniques are sensitive to the crystalline form of the inorganic species. Table 6.15 summarizes selected IR and Raman bands for a few common inorganic compounds. The interpreted IR and Raman spectra of several selected inorganic compounds (#82e94) are included in Chapter 8 for comparison. 113 INORGANICS TABLE 6.15 Selected Group Frequencies: Common Inorganic Compounds Inorganics IR bands (cmL1) Raman bands (cmL1) NH4 þ 3100 s, 1410 s 3100 w, 1410 w 2170 vs, 1300 m, 1210 m 2170 m, 1300 s, 1260 s 2060 vs 2060 s 2100 m 2080 s 1450 vs, 880 m, 710 w 1065 s 1650 m, 1320 vs 1270 m, 1030 s 1390 vs, 830 m, 720 w 1040 s 1270 vs, 820 w 1320 s 1130 vs, 620 m 980 s 1240 vs, 1040 m, 870 m 1040 s, 870 m 980 vs 980 s NCO NCS CN CO3 HCO3 NO3 NO2 SO4 2 HSO4 SO3 3 1030 vs, 570 m 940 s (eSiO 2 )x 1100 vs, 470 m e TiO2 660 vs, 540 vs e PO4 s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, e ¼ zero. 114 6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES Chapter 6: Problems 1. Explain the frequency dependence observed for CH3 in-phase bending band in the following series: BeCH3 ¼ 1310 cm1, CeCH3 ¼ 1380 cm1, NeCH3 ¼ 1410 cm1, OeCH3 ¼ 1445 cm1, and FeCH3 ¼ 1475 cm1. 2. Explain the observed splitting in the CH3 in-phase bend IR band in isopropyl or gem-dimethyl groups and t-butyl groups. 3. Explain the observed frequency dependence of the C¼CH2 wag vibration: (NhC)2eC¼CH2 ¼ 985 cm1, NhCeCH ¼CH2 ¼ 960 cm1, ReC¼CH2 ¼ 910 cm1, ReOeC¼CH2 ¼ 813 cm1. Hint draw possible resonance structures and consider mesomeric electron donation to the ¼CH2 carbon. How would you expect the IR intensity to vary? 4. Trans-substituted olefins (ReC¼CeR) show the following systematic increase in the observed CH wag. An isolated trans alkene (T) has a CH wag at 965 cm-1, a conjugated transetrans diene (TT) at 986 cm1, the transetransetrans alkene (TTT) at 994 cm1, and the transetransetransetrans alkene (TTTT) at 997 cm1. This is a result of vibrational interaction with conjugated diene groups. Using diagrams of the CH wag explain this progression. Hint: mechanical interaction (see N.B. Colthup, Appl Spectrosc., 25, 368, 1971). 5. The P¼O, S¼O, and SO2 stretching frequencies are relatively unaffected by conjugation or strained ring. How does the geometry relate to this? The inductive effect has a significant impact on the P¼O, S¼O, and SO2 stretching frequencies. Where the inductive effect is: X3P¼O 4 X3PþeO How could changing the substituents change the frequency (i.e., what simple parameter would explain the frequency shifts)? 6. Draw the five expected ring stretching modes for a five-membered ring. Use standing wave to help. Use your vibrations to assign IR bands observed for the following ring stretch vibrations: Rings Region 1 (IR:var, R:var) Region 2 (IR:m, R:vs) Region 3 (IR:ms, R:s) 1-Substituted pyrrole 2-Substituted pyrrole 2-Substituted Furans 2-Substituted Thiophenes 1560e1540 1570e1545 1605e1560 1535e1514 1510e1490 1475e1460 1515e1460 1454e1430 1390e1380 1420e1400 1400e1370 1361e1347 7. The aromatic sextant stretch typically is IR and Raman forbidden and is therefore not observed. This band has been assigned for certain Cl-substituted aromatics to occur between 1330 and 1305 cm1 which is below that observed for either the quadrant or semi-circle ring stretches. Draw the two different extremes of the sextant ring stretch. The lower frequency is due to an interaction force INORGANICS 115 constant. Explain this concept as it relates to the observation for the aromatic sextant stretch. 8. How does the interaction force constant explain the observed differences between 2,4-pentanedione (CH3eCOeCH2eCOeCH3 at 1725 and 1707 cm1) and an anhydride (CH3eCOeOeCOeCH3 at 1833 and 1764 cm1). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: New York, NY, 1990. Bellamy, L. J.; The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: New York, NY, 1975; vol. 1. Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic: San Diego, 1991. Socrates, G. Infrared Characteristic Group Frequencies, 2nd ed.; John Wiley: New York, NY, 1994. Jordanov, B.; Tsankov, D.; Korte, E. H. J. Mol. Struct. 2003, 651e653, 101e107. Atamas, N. A.; Yaremko, A. M.; Seeger, T.; Leipertz, A.; Bienko, A.; Latajka, Z.; Ratajczak, H.; Barnes, A. J. J. Mol. Struct. 2004, 708, 189e195. Amorim da Costa, A. M.; Marques, M. P. M.; Batista de Carvalho, LA. E. Vib. Spectrsoc. 2002, 29, 61e67. Balabin, R. M. J. Phys. Chem. A 2009, 113, 1012e1019. Colthup, N. B. J. Chem. Educ. 1961, 38 (8), 394e396. Colthup, N. B.; Orloff, M. K. Spectrochim. Acta, Part A 1971, 27A, 1299. Determined by the author using Hammet values from Domingo, L. R.; Perez, P.; Contreras, R. J. Org. Chem. 2003, 68, 6060e6062. Tasumi, M.; Urano, T.; Nakata, M. J. Mol. Struct. 1986, 146, 383e396. Palafox, M. A. Int. J. Quantum Chem. 2000, 77, 661e684. Versanyi, G. Vibrational Spectra of Benzene Derivatives; Academic: NY, 1969. Colthup, N. B. Appl. Spectrosc. 1976, 30, 589. Larkin, P. J.; Makowski, M. P.; Colthup, N. B. Spectrochim. Acta A 1999, 55, 1011e1020. Larkin, P. J.; Makowski, M. P.; Colthup, N. B.; Flood, L. Vibr. Spectrosc. 1998, 17, 53e72. El-Azhary, A. A.; Hilal, R. H. Spectrochim. Acta, Part A 1997, 53, 1365e1373. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part A: Theory and Applications in Inorganic Chemistry, Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry, 5th ed.; John Wiley: New York, NY, 1997. C H A P T E R 7 General Outline and Strategies for IR and Raman Spectral Interpretation Spectroscopists must develop an analytical toolbox suitable for their needs. This includes the ability to measure a suitable spectrum, understand the basic chemistry of the compound, and systematically interpret the spectrum. In this chapter, the IR and Raman spectrum will be discussed briefly in terms of spectral regions rather than functional groups. Both Raman and FT-IR spectroscopies provide important molecular structural information. However, the successful application of both techniques relies heavily on the skill of the spectroscopist. There are common skills used for solving structural problems using vibrational spectroscopy. In general, tools and skills that can be called upon include the following components: • Interpersonal and communication skillsdDefine the problem and get background information on the sample. • InstrumentationdSelect most appropriate technique and sample preparation. • Data analysis techniquesdExperience is important in selecting the most appropriate tool for your application. • Spectral interpretation skills. • Spectral database tools (spectral libraries, interpretation software). • Ab initio-based normal mode analysis (ultimate structural verification). 1. TOOLS OF THE TRADE IR and Raman spectroscopies are used to study a very wide range of sample types and may be examined either in bulk or in microscopic amounts over a wide range of temperatures and physical states (e.g., gases, liquids, latexes, powders, films, fibers, or as a surface or embedded layer). Because IR and Raman spectroscopies have a very broad range of applications, it is important to be knowledgeable about the advantages and limitations of various available techniques in order to select the most appropriate technique for the problem facing the spectroscopist. Often a broad-based knowledge of spectroscopy is required including an understanding of the instrument, the data analysis as well as spectral interpretation skills. 117 118 7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION Basic instrumental techniques for bulk analysis include mid-IR (most commonly FT-IR) as well as Raman spectroscopy. Raman spectroscopy has various laser excitation sources and instrument designs (such as FT-Raman or grating-based instruments). When examining smaller samples micro-analysis techniques must be employed such as microscopy (IR and Raman, imaging, and confocal), and micro-ATR for IR. Once a suitable sampling technique has been selected to provide the spectrum (or spectra) needed, a data analysis technique to provide information needed to solve the problem is required. For identification and structural verification purposes, this can include spectral databases and interpretation. Chemometric data analysis may be employed for developing quantitative models or analysis of large spectral data sets such as those encountered in imaging techniques. 723 394 357 310 776 720 902 840 1138 1081 1046 996 935 878 1123 1070 1298 891 1063 1130 1296 548 688 CH rk Trans chain C–C–C i. ph.str 375 Trans chain C–C–C o. ph. str 671 893 twist 1175 1129 1099 1063 1296 720 941 891 1099 1348 1295 1229 1188 1472 1431 3 O CH3—(CH2)14—C—OH C–O str. CH 1462 1438 1422 1703 2955 1302 1466 1451 1408 1463 1473 2638 CO o. ph. str CH chain i. ph. rk . CO i. ph. str . CH chain i. ph. twist 720 1113 1302 1421 CH bend 4 [CH3—(CH2)14—CO2—]2 Ca++ CH chain C–C–C i. ph.str CH chain C–C–C o. ph. str . 891 CH i. ph. str . 2724 2610 CH o. ph. str CH3—(CH2)15—OH Trans chain C–C–C o. ph str. . Dimer O… H…O o. pl. wag 1176 1130 1103 1063 0.40 Ra I CH o. ph. str CH chain i. ph. rock crystal splitting Trans chain C–C– C i. ph str . 1370 1296 20 H O bend 1542 1470 60 C–CC–O o. ph. str . . CH chain wag H O OH. str 1569 IR %T C=O i. ph. str . in dimer CH bend 1461 1441 0.0 100 Trans chain CH o. ph. str . C=O o. ph. str . in dimer 1634 0.4 CH i. ph. str 1622 1576 Ra I CH o. ph. str . 2846 2850 0.8 CH o. ph.str 2918 20 2959 2924 60 3429 IR %T C–C–C o. ph. str CH wag OH str . 1 CH i. ph. rk . CH i. ph. bend C–O–H bend CH o. ph. bend CH i. ph. str . 2724 0.00 100 CH i. ph. twist 1459 1437 1369 CH o. ph. str . CH o. ph. str str. 2846 2849 2725 2882 2676 Ra 0.16 I 0.08 . 2881 20 0.24 CH bend + CH o. ph. bend CH , CH str . 2733 2963 60 2918 2956 OH str 3303 IR %T 2846 2850 0.0 100 2881 2917 2957 0.1 CH o.ph. str 2875 2859 2959 2937 60 20 Ra I 1379 100 IR %T 0.00 3500 3000 2500 2000 1800 1600 Wavenumber 1400 (cm ) 1200 1000 800 600 400 FIGURE 7.1 The FT-IR and FT-Raman spectra of n-heptane, 1-hexadecanol, palmitic acid, and calcium palmitic acid salt. IR SAMPLE PREPARATION ISSUES 119 Success will often depend on your selection of the most suitable technique for the problem at hand. As a rule, mid-IR provides a better general workhorse technique than Raman spectroscopy because it contains a greater selection of bands characteristic of functional groups. This is illustrated in the Fig. 7.1 of the IR and Raman spectra for several organic species. The Raman spectra have characteristic bands for the aliphatic backbone, but do not have strong bands associated with important functional groups of the alcohol, carboxylic acid, and carboxylic acid salt. IR has strong characteristic bands for the aliphatic methyl and methylene groups as well as the alcohol, carboxylic acid, and carboxylic acid salt groups. The Raman spectrum of each of the four species is very similar with bands characteristic of the methyl, methylene, and skeletal carbon backbone. In the four compounds selected (n-heptane, 1-hexadecanol, palmitic acid, and calcium palmitic acid salt), the Raman spectrum does not provide strong bands for the hydroxyl, carbonyl, or carboxylic acid. Moderate-to-strong Raman bands can occur for carbonyl groups that are more polarizable (e.g., amide carbonyls or the very strong carbonyl observed in isocyanurate). There are functional groups for which Raman is the preferred method while some functional groups provide useful bands in both types of spectra. In general, IR provides a broader representation of characteristic bands for a greater variety of functional groups. The Raman spectra have characteristic bands for the aliphatic backbone, but do not have strong bands associated with important functional groups of the alcohol, carboxylic acid, and carboxylic acid salt. 2. IR SAMPLE PREPARATION ISSUES The quality of the information that can be derived from a FT-IR spectrum is directly related to the quality of the spectrum itself. Therefore selection of the appropriate sample technique and the resultant spectrum quality is critical. Operators must develop requisite sample-preparation skills necessary to measure photometrically accurate spectra. In the case of neat liquids, the liquid film should be uniformly thick (ca. 5e8 mm) without holes or voids to insure a good, quantitative IR spectrum. Similarly, films (cast or pressed) should also be uniformly thick without holes or voids. Lastly, solid-powdered samples can be prepared as a uniform dispersion in either a Nujol or a KBr matrix. In both cases, the sample powder must be sufficiently grounded and mixed with the support matrix to insure a good, artifact-free IR spectrum. A wide variety of IR sample preparation techniques exist because there is not a universal sampling technique that will provide high-quality IR spectra for all samples. The nature of the particular sample will often dictate the selection of the technique. As an example, the IR spectra of potato starch measured using a Nujol mull, KBr disc, water cast film, and ATR sampling are shown in Fig. 7.2. Starch provides an excellent example of the importance of selecting an appropriate IR sample-preparation technique suitable for the sample of interest. Both the Nujol mull and KBr disc sample preparations are inappropriate for starch and result in poor-quality IR spectra. The Nujol mull provides an extremely false spectrum and results in very limited spectral information that could be used for identification and interpretation. The KBr disc 120 7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION Water cast film/ZnSe 1152 1081 1025 2929 3382 573 1457 1373 1156 3066 Nujol mull 1246 1645 N 984 932 858 764 719 N 20 1364 60 40 3500 3000 2500 2000 1800 1600 1400 1200 1000 Wavenumber 1150 1077 764 709 800 600 ATR 3500 3000 2500 2000 995 928 1239 853 KBrdisc 576 525 3433 20 1159 1082 1019 2926 40 1416 1372 1645 60 928 856 761 1636 3279 80 2928 2085 (d) 1456 1416 1337 1244 (b) 100 Transmittance 932 852 2154 Transmittance 80 762 708 2149 (c) 1642 (a) 100 1800 1600 1400 1200 1000 800 600 Wavenumber FIGURE 7.2 IR Spectra of potato starch: Comparison of (a) Nujol mull (N ¼ Nujol signal), (b) KBr disc, (c) water cast film on ZnSe sample preparation, and (d) ATR sampling. spectrum, while false, is a significant improvement over the Nujol mull preparation and could be used for identification. Dissolving the starch in water and preparing a cast film on ZnSe results in a photometrically accurate IR spectrum. The resultant IR spectrum is of an amorphous film. Lastly, using ATR, an excellent IR spectrum is obtained of the starch powder. Note that the ATR technique results in changes in peak position, shape, and also results, as is typical of ATR, in a wavelength dependence of the band intensities. Five different general criteria should be kept in mind for suitable IR sample preparation for both samples and library reference spectrum. 1. Suitable band intensities in the spectrum. The strongest band in the spectrum should have intensity in the range of 5e15% transmittance. The resulting preparation must be uniformly thick and homogenously mixed without holes or voids to insure a good quality spectrum. 2. Baseline. The baseline should relatively be flat. The highest point in the spectrum should lie between 95 and 100% transmittance. 3. Atmospheric compensation. Water vapor and carbon dioxide bands should be minimized. Since the sample scan is ratioed against a previously measured background, spectral subtraction of water vapor/carbon dioxide, reference spectrum should be used. OVERVIEW OF SPECTRAL INTERPRETATION 121 4. Spurious bands. Any band that does not belong to the sample is spurious and must be accounted for or better yet eliminated. Use of an IR window contaminated with a previous sample is a very common source of spurious bands. 5. X-axis scale. Plots should employ a 2X-axis expansion and not a simple linear X-axis. This provides an expansion of the important fingerprint region. Some spectral databases such as the Aldrich collection utilize a 4X-axis expansion. 2.1. Select Suitable Sampling and Know the Limitations The sample and problem at hand will determine if a bulk or microscopy measurement is used. The sampling by Raman spectroscopy is straightforward with fluorescence interference representing the main limitation. In the case of IR spectroscopy, sample preparation choices must be made to produce the most photometrically accurate spectrum. Sample preparation for bulk analysis includes KBr disc, Nujol mull, ATR, DRIFTS, and cast films. For a KBr disc preparation, the entire spectrum is available, but KBr is hydroscopic. Water will be introduced in the KBr disc preparation and this will affect the ability to interpret the OH/NH stretching region. For a Nujol mull sample preparation, there will be interferences from the mulling oil aliphatic groups. ATR can be considered a more universal sampling choice but the IR band intensities exhibit a wavelength dependence. Some band shape distortions will occur. Less commonly used techniques include DRIFTS where the analyst must be concerned about the photometric accuracy of the spectrum. Lastly, a solvent cast films can be used. Issues with this technique include preparing a uniform film, eliminating much or all of the solvent, and the resultant crystalline state of the material. 3. OVERVIEW OF SPECTRAL INTERPRETATION Spectral interpretation requires a systematic, knowledge-based approach for optimal success. Both the sample IR and Raman spectrum and any reference spectrum must be of good quality. Before beginning to interpret the IR and Raman spectrum, the analyst should get a suitable amount of background information. Spectral interpretation is a critical skill that is learned with practice and experience. Resources to use include electronic and paper libraries, empirical characteristic frequencies for chemical functional groups as well as outside vendor and interior libraries. The quality of the available reference library is critical. The IR and Raman libraries ideally should capture knowledge of the spectroscopy of the band assignments of molecules included in the database in order to facilitate interpretation of other molecules. 3.1. Hierarchy of Quality for Spectral Reference Libraries Spectral reference libraries have a hierarchy of quality. The usefulness of a spectral library is defined in part by how accurate it is and how much known information is captured in it. For example, the resolution of both the sample and reference libraries should match. In the case of IR spectrum, the sample preparation should always be indicated. In the case of Raman spectral reference libraries, a non-resonant excitation frequency should be employed and the final processed spectrum should include a background intensity correction. 122 7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION The following is a general hierarchy of spectral library quality: • Spectrum incorrectly identified with wrong structure. (This is the worst case scenario.) An example of this can include an IR spectrum dominated by water vapor and carbon dioxide. • A false IR spectrum with trade name only (useless). • A photometrically accurate IR or Raman spectrum with trade name only (almost useless). • A photometrically accurate IR or Raman spectrum with structure (Best case scenario of commercial libraries). • A fully interpreted photometrically accurate IR and Raman spectrum with structure (Best case scenario of in-house libraries). 3.2. Computer-Based Libraries and Software Tools Computer-based systems that provide guidance in spectral identification or group frequency analyses are a useful tool, but have limitations that must be understood. The usefulness of a spectral library is defined in part by how accurate it is and how much known information is captured in it. The spectral library must contain structures similar to the unknown in order to be useful. The critical decision to be made is whether the measured spectrum can be considered consistent with the reference spectrum. A common first step for structural identification is to employ a spectral library search. A fundamental limitation is that libraries contain no knowledge of the chemistry of the system and are limited by the size and scope of spectral libraries. Libraries are most useful if the compound of interest, with suitable sample preparation/presentation, exists in the database. A reference spectrum and sample of the same species, but different crystallinity, in the library will negatively affect the accuracy of the identification. In general, a number of automatic search methodologies exist that can be used to compare the spectrum of an unknown sample against many different spectra in the library. These include a simple peak matching to discrimination analysis using Euclidean distance, wavelength correlation (or other algorithms). A search simply reports the best-known match from the spectral library. The unknown may be from an entirely different class of materials. Another approach is to use an empirically based, computer-assisted structural interpretation to aid in a group frequency analysis of the IR and Raman spectrum. The analyst should also use their background knowledge of the chemistry of the sample to help screen such results. One approach involves software that incorporates numerous correlation tables organized by functional group fragments. The analysis of spectrum results in suggested fragments with highlights of important bands for that fragment. These fragments can be combined to provide a hypothetical structure for confirmation. Some software allows the addition of functional group band assignments by the analyst. This can enable the software to focus on the type of molecules commonly encountered in the particular lab. Approximate intensities are often included and include the frequency range. This is a more flexible analysis tool than the “brute force” spectral library approach. The most rigorous approach to structural verification involves using ab initio determined structure with normal mode analysis. The calculations are based on an isolated molecule and result in the theoretical IR and Raman band frequencies, relative intensities, and corresponding vibrations. Interactions encountered in the condensed phase such as hydrogen bonding OVERVIEW OF SPECTRAL INTERPRETATION 123 and various environments from different crystalline structures are not taken into account. Therefore, the computational results are most appropriate for gas-phase spectra. The size of the molecule limits this approach because of the computationally intensive analysis employed. Analysis of the Cartesian displacement vectors describing the vibrations from the normal mode analysis can provide important insight into group frequencies and mechanical interactions of the vibrations in the molecule. This approach provides the most rigorous structural verification approach. 3.3. Define the Problem that Needs to be Solved The first step involves defining the problem. Ideally, this should be done before the spectrum of a sample is measured. A complete unknown is rare. Get all the background chemistry information available so that you have a good sample history. Obviously, the analyst should be clear if the sample is research or product related. If this is a research sample, obtain the reaction scheme. Ask the submitter what are the expected products and impurities and if this is an intermediate or final product. If the sample is product related then the analyst should determine the product area/general area of chemistry. Examples can include paper industry related, polymer additives, polymers, and pharmaceutical API’s. IR and Raman spectroscopy are commonly employed for contaminant analysis. In this case, find out what the source matrix is and if this is believed to be a contaminant. In some instances, an identification of extracted materials is needed. In this case, determine the matrix from which the material was extracted. What solvent was used? What filters were used? The analyst must know what spectral signals might be expected from these possible contaminants. Questions to be asked to the submitter can include the following: Where did the sample come from exactly? Was there any sample manipulation prior to analysis? Has there been similar work done in the past? What other techniques are being used in the analysis? 3.4. Examine the IR and Raman Spectrum The first step of interpreting the IR and Raman spectrum is to identify any common impurities that could be present. Some common impurities include silicone oil, water, plasticizers such as phthalates, flow aids such as Mg stearate and commonly encountered polymers (e.g., PE, Nylon, Teflon), and long-chain aliphatics. Look for possible solvents that may have been introduced by any extraction procedures and not fully removed from an isolated reaction product. The analyst should also look for inorganic sulfates, sulfites, or nitrates that may be present as a result of treatment of a reaction mixture with sulfuric acid or nitric acid. Next, determine if the spectrum represents single component or multiple components. Almost all organic and many inorganic species have IR and Raman bands. A complex spectrum can represent either a complex molecule with a wide range of functional groups or multiple components. Multiple component identification is challenging and is a skill that improves with practice. If you have assigned multiple species, review if that is consistent with the chemical background. If possible estimate relative amounts of components present. 124 7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION Be aware of orientation/crystallinity effects since vibrational spectroscopy has excellent environmental sensitivity. This can result in very different IR and Raman spectra for amorphous versus crystalline states. Most of the observed spectral differences are a result of changes in hydrogen-bonding environments. Because of this the analyst should try to ensure that the reference spectra followed the same sample-preparation method and has the same crystalline state as the unknown. Some sample preparations (e.g., cast films) can result in a different crystalline state from the sample of interest. The last step for structural identification and verification is to assign all main peaks in the IR and Raman spectrum. A standard systematic way to begin analyzing the IR/Raman spectrum is to begin at the high wavenumber end of the spectrum (4000 cm1) and look for the presence and absence of characteristic absorptions as you move to lower wavenumbers. The most common and distinctive bands can then be organized into several spectral regions. Whenever possible, assignments should utilize confirming bands. If possible, measuring a full spectral region (4000e400 cm1) is important because, for example, the confirming bands for many inorganic species occurs below 650 cm1 (vide infra). When possible use the basic knowledge about the sample and the chemistry associated with it. Check for group frequencies associated with suspected structural components. Computer-based spectral libraries and structural interpretation software may be valuable tools. Remember to keep in mind the limitations and spectral contributions from any sample-preparation techniques used. Try to assign the most intense bands first. In most cases, you will not be able to fully and accurately assign group frequencies to all of the bands in the spectrum. 4. INTERPRETATION GUIDELINES AND MAJOR SPECTRAeSTRUCTURE CORRELATIONS This section is to be used as a general guide to interpret IR and Raman spectra systematically. As a first check, examine the overall spectral appearance. If the spectrum is very simple with only a few vibrational bands then focus on a low molecular weight organic or inorganic species. Look at inorganic salts such as sulfate, carbonate, or nitrate or simple common organic solvents and water. For organics and hydrocarbons look for bands in the IR and Raman spectra in the region between 3200 and 2700 cm1, because these are excellent group frequencies associated with CeH-type stretches. After a quick evaluation of the spectral appearance and determining, if hydrocarbons are present, begin to interpret the vibrational spectrum. Start from 4000 cm1 and work systematically to lower wavenumbers. The presence and absence of functional groups needs to be noted. 4.1. Hydroxy (OH), Amino (NH), and Acetylene (hCH) Groups: 3700e3100 cmL1 Bands in this region are typically due to various OH and NH stretching vibrations. Hydrogen bonding will have a large effect on the amino and hydroxyl stretching vibrations. The OH stretch is medium to strong in intensity in the IR spectrum but is generally weak in the Raman spectrum. IR spectroscopy will be excellent for NH stretching vibrations. Raman INTERPRETATION GUIDELINES AND MAJOR SPECTRAeSTRUCTURE CORRELATIONS 125 spectroscopy will be poor for OH and acetylene CH stretching vibrations but moderate for NH stretching vibrations. In the condensed phase, the hydroxyl OH stretching band is generally intermolecularly hydrogen bonded with a broad band appearing between 3550 and 3230 cm1. In some cases due to crystalline structure or steric hindrance, the hydroxyl group is not hydrogen bonded and resultant sharp and moderately strong free OH stretching bands are observed between 3670 and 3580 cm1. The OH stretches for water alcohols and phenols are generally observed at 3400 cm1 but can be found between 3700 and 3300 cm1. The observed OH stretch depends upon the strength of the hydrogen bond. Acidic protons for the hydroxyl species of carboxylic acid dimers and phosphoric acids result in very broad bands centered at ca. 3000 cm1. IR bands for hydrogen-bonded amino groups are generally not as broad as those observed for hydrogen-bonded hydroxyl groups. Primary amine (NH) groups result in a single weak band between 3500 and 3300 cm1 while a secondary amine (NH2) group results in a weak doublet between 3550 and 3250 cm1 with a spacing of about 70 cm1. A secondary amide NH results in a stronger band between 3470 and 3200 cm1 while a primary amide (NH2) group results in a strong doublet between 3550 and 3150 cm1 with a spacing of about 100e200 cm1. The acetylene hCH group has a moderate-to-strong sharp IR band between 3340 and 3280 cm1. The band is quite weak in the Raman spectrum. Lastly, weak overtones from carbonyl bands can occur between 3500 and 3400 cme1 region. Typically, the overtone band frequencies will be ca. 20 cm1 less than twice the carbonyl stretching band wavenumber and the intensity will be significantly less intense than the carbonyl stretching band. 4.2. Unsaturated Aryl and Olefinic Groups: 3200e2980 cmL1 Both IR and Raman spectroscopy provide moderately strong bands for these groups. If sharp bands are observed between 3000 and 3200 cm1 then it is likely an unsaturated group is present. Aromatics have multiple weak bands from CH stretch vibrations between 3100 and 3000 cm1. Most olefins have characteristic CH stretching bands in the 3100e2980 cm1 region. Observation of isolated bands at 3010 cm1 and 3040 cm1 most likely indicates a simple olefinic group. Pyroles and furans will typically have bands in the 3180 and 3090 cm1 region. Three-membered rings such as cyclopropyl and epoxy rings will also have bands in 3100e2990 cm1 and 3060e2990 cm1 regions, respectively. 4.3. Aliphatic Groups: 3000e2700 cmL1 Well-defined, strong characteristic IR and Raman bands for CeH stretching vibrations characteristic of aliphatics are typically observed. Both methyl and methylene groups (CH3, CH2) result in doublets at slightly different frequencies. Methyl CH stretching bands typically have bands between 2975e 2950 cm1 and 2885e2865 cm1. Methylene CH stretching bands typically have bands between 2940e2915 cm1 and 2870e2840 cm1. The relative intensities of the bands can be used to estimate the relative amount of methyl to methylene groups. Heteroatoms adjacent to these groups result in characteristic group frequencies: 126 7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION The methoxy group has one band at ca. 2835 cm1, methyl and methylene groups next to the nitrogen atom in tertiary amines result in bands at ca. 2800 cm1, and aldehydes result in two peaks near 2730 cm1 (due to Fermi resonance). 4.4. Acidic Protons: 3100e2400 cmL1 Broad bands in this region are typically due to acidic and strongly hydrogen-bonded hydrogen atoms. In general, IR is a much better technique than Raman spectroscopy for these groups. Hydrogen bonding is an important determinant in the frequencies for these groups. The main OH stretching band for carboxylic acid dimers is found at ca. 3000 cm1 with additional overtone/combination bands at ca. 2650 cm1 and 2550 cm1. Primary, secondary, and tertiary amine salts absorb in this region and have distinctive band structure. The NeHþ stretch for these groups are characterized by moderate-to-strong IR and Raman bands. Primary amine salts for the eNHþ 3 have an absorption band(s) between 3350 and þ , >NH , and C¼NHþ have bands between 2700 and 3100 cm1. Amines with > NHþ 2 1 2250 cm . Phosphoric acids have broad absorption bands in this region. 4.5. SH, BH, PH, and SiH: 2600e2100 cmL1 Both IR and Raman can be very useful for these species. The SH stretch for mercaptans and thiophenols occurs in the region 2590e2540 cm1 and is generally quite strong in the Raman spectrum, but weak in the IR. BH stretches occur in the region 2630e2350 cm1 and are moderate to strong in the IR spectrum, but are generally weak in the Raman spectrum. PH stretches occur in the region 2440e2275 cm1 and are moderately intense in the IR spectrum and moderate to weak in the Raman spectrum. SiH stretches occur in the region 2250e2100 cm1 and are strong in both the IR and Raman spectrum. 4.6. Triple Bonds and Cumulated Double Bonds: 2300e1900 cmL1 Both IR and Raman can be very useful for these species. The IR and Raman intensities for the XhY and X¼Y¼Z stretching vibrations are variable. Refer to chapter 6 (section 3.2) to use the table and idealized spectra (Figure 6.11 and table 6.3). Contributions from atmospheric carbon dioxide generate a false signal in this region of the IR spectrum. 4.7. Carbonyl-Containing Species: 1900e1550 cmL1 IR is excellent for carbonyl species while Raman is quite variable. The carbonyl C¼O stretching vibration results in strong characteristic IR bands. Raman bands for this vibration are typically moderate to weak with some structures resulting in a strong C¼O stretch. This band is easily identified in the IR spectrum because of its intensity and its lack of interference from most other group frequencies. Carbonyl groups are present in many different compounds such as ketones, carboxylic acids, aldehydes esters, amides, acid anhydrides, acid halides, lactams, lactones, urethanes, and carbamates. Bands above 1775 cm-1 are often due to carbonyl compounds such as anhydrides (doublet), acid halide, or strained ring carbonyl (such as lactones and carbonates in five-membered rings). Bands between INTERPRETATION GUIDELINES AND MAJOR SPECTRAeSTRUCTURE CORRELATIONS 127 1750 and 1700 cm1 fall in the middle of the carbonyl range. Carbonyl compounds in this region can include aldehydes, esters, carbamates, ketones, and carboxylic acids. Bands below 1700 cm1 fall in the low end of the carbonyl range. Such carbonyl compounds can include amides, ureas, and carboxylic acid salts. Since conjugation can lower carbonyl frequencies, this range will also include conjugated aldehydes, ketones, esters, and carboxylic acids. In general, bands below 1600 cm1 are due to the out-of-phase stretch of carboxylic acid salts. The remaining carbonyl signals are due to C¼O stretches and occur above 1600 cm1. Fermi resonance sometimes gives rise to two bands in the C¼O stretching region even though only one C¼O bond is present. See Chapter 2 for an explanation of this effect. 4.8. Olefinic (C[C), Imino (C[N), and Azo (N[N) Compounds: 1690e1400 cmL1 The stretching vibrations of these groups are characterized by strong Raman band which makes Raman an excellent technique for these types of compounds. The intensities of the IR absorptions are variable. The C¼C stretching vibrations result in variable IR intensities. Most olefin C¼C stretching bands occur between 1680 and 1600 cm1. In IR this results in a narrow, weak absorption band. Conjugation with another double bond lowers the frequency and often increases the IR band intensity. Imino (C¼N) stretching vibrations result in strong Raman and moderately strong IR bands. Most C¼N stretching vibrations occur between 1690 and 1630 cm1. Azo (N¼N) stretching vibrations give rise to strong, well-defined Raman bands, but a very poor quality IR bands. Most Azo N¼N stretching bands occur between 1580 and 1400 cm1. The nature of the compound is very important in analyzing spectra of azo species. Symmetrically substituted trans azo compounds give rise to strong Raman bands. Although the signal is forbidden in the IR, non-symmetrically substituted trans azo compounds can give rise to strong IR signals. 4.9. Organic Nitrates (N[O): 1660e1450 cme1 These species include nitro (ReNO2), organic nitrates (ReNO2), organic nitrites (ReOeN¼O), and nitroso groups. The N¼O stretching vibrations result in strong IR bands and generally moderate Raman bands. Aliphatic and aromatic nitro compounds have two IR strong bands between 1580e1475 cme1 and 1390e1320 cme1. Organic nitrites have two strong IR bands between 1660e1615 cme1 and 1300e1270 cme1. Only the lower band is observed by Raman spectroscopy. Inorganic nitrate salts (NO 3 ) provide a broad band assoe1 (IR strong, Raman moderate). ciated with the out-of-phase NO 3 between 1410 and 1350 cm Organic nitrites provide strong IR and Raman bands with generally two bands resulting because of rotational isomers. The cis form exhibits a band in the region 1625e1610 cme1 while the trans form exhibits a band in the region 1680e1650 cme1. Nitroso groups are characterized by strong IR and Raman bands. In the solid and liquid state, nitroso groups exist as dimers in either the cis or trans state. The aliphatic cis form results in two bands: 1425e1330 cme1 and 1345e1320 cme1. The trans form results in a single band in the region 1290e1175 cme1. The aromatic cis form results in two bands: 1400e1390 cme1 and 1410 cme1. The trans form results in a single band in the region 1300e1250 cme1. 128 7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION 4.10. Amine NH Deformation Vibrations for Amines, Amine Salts, and Amide Compounds: 1660e1500 cme1 NH deformation bands result in moderate IR bands and weak Raman bands. Primary amine NH2 in-phase deformation occurs in the region 1660e1590 cme1. Amine salts result e1 in a NHþ while a NHþ 2 in-phase deformation band found between 1620 and 1560 cm 3 out-of-phase deformation results in two sets of bands between 1635e1585 cme1 and 1585e1560 cme1. In secondary carbamates and mono-substituted amides, the CNH group results in a strong IR band between 1570 and 1510 cme1. Note that water also has a band near 1640 cme1 from the OH deformation. 4.11. Aromatic and Hetero-aromatic Rings: 1620e1420 cme1 Sharp bands in this region involve the aromatic ring quadrant and semi-circle stretching vibrations. The IR intensities for these bands are variable in intensity. The Raman intensities are moderate for the quadrant stretch and quite weak for the semi-circle stretching bands. Typically two sets of bands are observed in this region due to the quadrant and semi-circle ring stretch vibrations. These may occur as single bands or as a multiple component envelop of bands. In general, the quadrant stretch will result in bands near 1600 and 1580 cme1 while the semi-circle stretch will result in bands near 1500 and 1460 cme1 The relative intensity of the IR bands will vary with the aromatic substituent pattern and type of substituent. Sixmembered ring heteroaromatics resulting from substituting an aromatic carbon atom with a nitrogen atom result in similar characteristic ring-stretching bands between 1600 and 1500 cme1. These include single nitrogen-substituted aromatic molecules such as pyridines, quinolines, and isoquinolines, two nitrogen-substituted aromatic molecules such as pyrimidines and quinazolines, and three nitrogen-substituted molecules such as triazine species. Simple aromatic compounds will also generate a series of weak IR absorption bands between 2000 and 1700 cme1 which derive from multiple combination bands. 4.12. Methyl and Methylene Deformation Vibrations: 1500e1250 cme1 These alkane deformation vibrations results in moderate IR bands and weak to moderate Raman bands. When these groups have identical substituents then the CH2 “scissors” deformation and CH3 out-of-phase deformation are found in the same frequency region, ca. 1480e1430 cme1. When the CH2 and CH3 are on hydrocarbons, this band is near 1460 cme1. When the CH2 (methylene) is near an unsaturated group, the deformation band is found near 1440 cme1. If the methylene is adjacent to chlorine, bromine, iodine, sulfur, phosphorus, nitrile, nitro, or carbonyl, this band occurs between 1450 and 1405 cme1. The CH3 in-phase deformation is a useful moderately intense IR band, but provides only a weak Raman band. The CH3 in-phase (“umbrella”) deformation is strongly dependent upon the electronegativity of the adjacent atom. The more electronegative the atom, the higher the frequency with a range of 1470e1250 cme1. Examples include OeCH3 at ca. 1450 cme1, CeCH3 at ca. 1450 cme1, and SieCH3 at ca. 1265 cme1. In aliphatic groups, steric hindrance to the vibrations due to the presence of two or three adjacent methyl INTERPRETATION GUIDELINES AND MAJOR SPECTRAeSTRUCTURE CORRELATIONS 129 groups can result in highly characteristic bands. Here CeCH3 results in a single band at 1378 cme1, while the isopropyl C(CH3)2 has an equal intensity doublet at 1385 and 1368 cme1, and the t-butyl C(CH3)3 has a non-equally intense doublet at 1395 and 1368 cme1. 4.13. Carbonate, Nitrate, Ammonium, and BeO Type Compounds: 1480e1310 cme1 Inorganic carbonate, nitrate, and ammonium ions provide a broad, intense IR band near 1400 cme1 but only a weak Raman band. IR bands result from the out-of-phase XY3 stretch for carbonate- and nitrate-containing compounds. Strong sharp Raman bands in the 1040e1100 cme1 region result from the in-phase XY3 stretch. The ammonium ion deformae1 region. tion band is due to a NHþ 4 bend and is found in the 1480e1390 cm The BeO stretch provides a strong characteristic IR band. Inorganic boric acid has a strong broad band near 1410 cme1 due to the BO3 out-of-phase stretch. Organic borates [B(RO)3], boronates [PhB(RO)2], metaborates (cyclic six-membered BeO ring), boronic acid esters, and anhydrides are all characterized by strong IR bands in the region 1390e1310 cme1 involving the BeO stretch. 4.14. Organic and Inorganic SOx Type and Thiocarbonyl (C[S) Compounds: 1400e900 cme1 In general, both IR and Raman provide strong characteristic bands associated with SOx and C¼S stretching vibrations. Inorganic SOx type compounds include sulfites (SO2e 3 ), sulfates (SO2e 4 ) as well as more complex SXOY type compounds (thiosulfate, pyrosulfite, dithionate). Sulfites have a strong broad IR band and a strong doublet Raman band between 1010 and 900 cme1 involving the SO3 out-of-phase and in-phase stretching vibrations. Sulfates are characterized by a very strong IR band between 1130 and1080 cme1 involving e1 the SOe2 4 out-of-phase stretch and a strong Raman band between 1065 and 955 cm e2 involving the SO4 in-phase stretch. SXOY type compounds are characterized by strong IR and Raman bands between 1250 and 1000 cme1 involving complex S¼O stretching vibrations. Organic SOX type compounds include >S¼O, >SO2 as well as sulfonic acids (eSO3H), and þ sulfonic acid salts (eSOe 3 M ) (See chapter 6, section 8). In general, the stretching frequencies involving the S¼O group of these types of compound show a dependence on the electronegativity of the substituents. Sulfoxides (R2>S¼O) are characterized by a strong IR band and a moderate-to-weak Raman band between 1070 and 1030 cme1 from the S¼O stretch. Similarly, sulfinic acids (eS(¼O)eOH) are characterized by a very strong IR and moderate-tostrong Raman band in the region 1090e990 cme1 due to the S¼O stretch. The S¼O stretch is shifted to higher frequencies for sulfinic acid esters (RdS(¼O) dOeR) at 1140e1125 cme1 (strong IR; moderate intensity Raman) and dialkyl sulfites ((RO)2S¼O) in the region 1220e1170 cme1 (strong IR; strong, polarized Raman). Sulfones (Rd SO2dR) are characterized by two vibrations, an out-of-phase and in-phase stretch of the SO2 group. In most cases, strong IR bands result for both in-phase and out-of-phase vibrations while a strong Raman band only occurs for the in-phase SO2 stretch. Sulfones (>SO2) have strong characteristic bands at 1360e1290 cme1 and 1200e1120 cme1. Sulfanamides (RdSO2dNh) 130 7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION have strong bands at 1360e1315 cme1 and 1180e1140 cme1. The higher frequencies are due to the influence of the more electronegative nitrogen atom. Sulfonyl halides (dSO2dX) have strong bands at 1385e1360 cme1 and 1190e1160 cme1. The out-of-phase SO2 stretch results in a moderate-to-strong Raman band. Covalent sulfonates (RdSO2dOR) and organic sulfates (eOdSO2dOe) have bands at 1420e1330 cme1 and 1200e1145 cme1. Organic sulfate salts (ROdSO2dOe Mþ) are characterized by a strong IR band in the region 1315e1210 cme1 and a strong Raman band in the region 1140e1050 cme1. þ Organic sulfonic acids (dSO3H) and salts (dSOe 3 M ) are characterized by strong IR and Raman bands involving the SO2 and SO3 stretching vibrations. Sulfonic acids (dSO3H), if not dried, are easily ionizable with small amounts of water resulting in a hydrated form (dSOe 3 H3Oþ). Anhydrous sulfonic acids (ReSO2dOH) have strong IR and Raman bands at 1355e1340 cme1 and 1200e1100 cme1 from the SO2 out-of-phase and in-phase stretches, þ e1 and respectively. Hydrated sulfonic acids (RSOe 3 H3O ) have bands at 1230e1120 cm e1 1120e1025 cm from the SO3 out-of-phase and in-phase stretches, respectively. Both SO3 vibrations result in strong IR bands. However, a moderate-to-strong Raman band is observed þ only for the SO3 in-phase stretch. Sulfonic acid salts (dSOe 3 M ) have bands at e1 e1 1250e1140 cm and 1070e1030 cm . The out-of-phase SO3 stretch results in very strong IR bands but weak Raman bands. However, the in-phase SO3 stretch results in strong IR and Raman bands. Thiocarbonyl compounds (>C¼S) are generally characterized by a strong IR and Raman band between 1250 and 1200 cme1 that involve the C¼S stretch. 4.15. P[O-Containing Compounds: 1350e1080 cme1 The P¼O stretching vibration for organic phosphorus compounds results in a strong IR band between 1350 and 1140 cme1 while the Raman band intensity is moderate to weak. The frequency of the P¼O stretch shows a strong dependence on the substituent electronegativity and hydrogen bonding effects. Inorganic POx type compounds include dibasic e 3e phosphate (HPO2e 4 ), monobasic phosphate (H2PO4 ), phosphate (PO4 ). The phosphate outof-phase stretch has strong IR and Raman bands between 1180 and 1000 cme1. The dibasic e1 from phosphate (HOdPO2e 3 ) has strong IR and Raman bands between 1150 and 1000 cm e the PO3 out-of-phase stretch. The monobasic phosphate ((HO)2 PO2 ) has strong IR and Raman bands between 1200 and 950 cme1 from the PO2 out-of-phase stretch. The P¼O band can appear as a doublet, possibly due to Fermi resonance or, in some cases, rotational isomerism. 4.16. Fluorinated Alkane Groups: 1350e1000 cme1 The CeF stretch of organic fluorine compounds results in strong IR bands but weak to moderately intense Raman bands. Monofluorinated compounds have a strong IR and a weak to moderate Raman band between 1100 and 1000 cme1. Difluorinated compounds (dCF2d) have two very strong IR bands between 1250 and 1050 cme1 involving the CF2 out-of-phase stretch. These bands are weak to moderate in the Raman spectrum. Trifluorinated compounds (dCF3) have multiple strong bands between 1350 and 1050 cme1 involving the CF3 out-of-phase stretch. Fluorine substituted aromatics (ArdF) have a moderately strong IR band between 1270 and 1100 cme1 involving the aryl ring CeF stretch. INTERPRETATION GUIDELINES AND MAJOR SPECTRAeSTRUCTURE CORRELATIONS 131 4.17. CeO Stretching Vibrations: 1300e750 cme1 Bands in the region 1300e750 cme1 are associated with the CeO stretch of species such as alcohols, ethers, esters, carboxylic acids, and anhydrides. Ethers are characterized by CdOdC out-of-phase and in-phase stretches. In general, the out-of-phase CeOeC stretch results in strong IR bands in the range 1270e1060 cme1 but weak Raman bands. The in-phase CeOeC stretch results in strong Raman bands and moderate IR bands at 1140e800 cme1. Alcohols are characterized by CeO type stretch vibrations in the region 1200e750 cme1. Primary alcohols are characterized by a strong IR band between 1090 and 1000 cme1 and a strong Raman band between 900 and 800 cme1. Secondary alcohols are characterized by multiple strong IR bands between 1150 and 1075 cme1 and a strong Raman band between 900 and 800 cme1. Tertiary alcohols are characterized by multiple strong IR bands between 1210 and 1100 cme1 and a strong Raman band between 800 and 750 cme1. Carbonyl compounds such as esters, carboxylic acids, and anhydrides include a CeO group that is often used to confirm the functional group. The CeO stretch of carboxylic acids is characterized by moderately intense IR bands in the range 1380e1210 cme1 with the corresponding Raman bands having highly variable intensity. Similarly, esters result in strong IR bands between 1300 and 1100 cme1 and corresponding Raman bands of variable intensity. Anhydrides are characterized by strong IR and Raman bands between 1310 and 980 cme1 involving the CeOeC stretching vibrations. 4.18. SieO and PeO Containing Compounds: 1280e830 cme1 The SieO group is characterized by strong IR bands and variable Raman bands. Compounds containing PeOeC group are characterized by bands in the range 1090e920 cme1 which involve the out-of-phase stretching vibration. These characteristic bands are typically strong in the IR and moderate to weak in the Raman spectrum. The PeOeP and PeOH groups are characterized by strong IR bands but only moderate-toweak Raman bands. The PeOeP group is characterized by a strong IR band between 1025 and 870 cme1 involving the PeOeP out-of-phase stretch. The PeOH group is characterized by bands involving the PeO stretch between 1000 and 900 cme1 with strong IR intensity, but moderate-to-weak Raman intensity. The siloxane SieOeSi and SieOH groups are characterized by strong IR bands at 1090e1010 cme1 and 955e830 cme1, respectively, involving the SieO stretch. Raman spectra of siloxanes do not provide useful bands in this region. Alkoxysilanes, SieOeR, are characterized by an out-of-phase stretch (1110e1000 cme1) which results in a strong IR band and an in-phase stretch (1070e990 cme1) which results in moderate-to-strong intensity IR and Raman bands. 4.19. CH Wag of Olefinic and Acetylenic Compounds: 1000e600 cme1 The CH wag vibration provides important medium-to-strong IR bands for characterizing alkenes and acetylenes. These bands are typically weak in the Raman spectrum. The position of these bands is dependent on the properties (electron withdrawing) of the substituents. 132 7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION Vinyl groups, eCH¼CH2, have strong IR bands between 1000e940 cme1 and 960e810 cme1 (995e980 cme1 and 915e905 cme1, respectively, for hydrocarbons). Vinylidene groups, >C¼CH2, exhibit strong IR bands between 985 and 700 cme1 (895e885 cme1 for hydrocarbons). Trans-di-substituted ethylenes, eCH¼CHe, have strong IR bands between 980 and 890 cme1 (1000e955 cme1 for hydrocarbons). Vibrational interaction with conjugated diene groups results in a systematic increase in the observed CH wag. An isolated trans alkene (T) has a CH wag at 965 cme1, a conjugated transetrans diene (TT) at 986 cme1, the transetranse trans alkene (TTT) at 994 cme1, and the transetransetransetrans alkene (TTTT) at 997 cme1. Cis-di-substituted ethylenes have strong IR bands between 800 and 600 cme1 (730e650 cme1 for hydrocarbons). Tri-substituted alkenes, >CH¼CHe, absorb between 850 and 790 cme1. The CH wag of acetylenic, ChCH, have moderate-to-strong IR bands between 700 and 578 cme1. 4.20. Aromatic In-plane 2,4,6 Radial Carbon In-phase Stretch: 1290e990 cme1 Very strong Raman bands are observed from aromatic ring vibrations involving the 2,4,6 radial carbon in-phase stretch. A very strong Raman band between 1010 and 990 cme1 is observed for mono-, 1,3 di-, and 1,3,5 tri-substituted benzenes. Only a weak to moderately intense sharp IR band is observed. 4.21. Aromatic CH Wag: 900e700 cme1 The aromatic ring CH wag vibrations results in strong IR absorption bands but only weak Raman bands. The frequencies of the aromatic CH wag are largely dependent upon the number of adjacent hydrogen atoms since the adjacent CH wag vibrations are mechanically coupled to one another. For this reason, the IR bands from the in-phase CH wag vibrations are quite useful in distinguishing aromatic ring substitutions. Mono-substituted benzenes have a band at ca. 750 cme1 involving the five adjacent CH wag (overall region between 820 and 728 cme1). Ortho di-substituted benzenes also have a band at ca. 750 cme1 involving the four adjacent CH wag (overall region between 790 and 728 cme1). The three adjacent CH wag vibrations of meta- or 1,2,3 tri-substituted benzenes have a band at ca. 782 cme1 (overall region between 825 and 750 cme1). The two adjacent CH wag vibrations of para, 1,2,4 tri-substituted, or 1,2,3,4 substituted benzenes have a band at ca. 817 cme1 (overall region between 880 and 795 cme1). The lone CH wag vibration (meta, or 1,2,4,. or 1,3,5,. or 1,2,3,5,. or 1,2,4,5,. or penta) substituted benzenes have a band at ca. 860 cme1 (overall region between 935 and 810 cme1). The aryl CH adjacent classification can also be extended to pyridines and napthalenes. 4.22. Halogen-Carbon Stretch: 850e480 cme1 Strong IR and Raman bands result for the halo-alkanes involving the CeX stretching vibrations (X¼F, Cl, Br, I). Fluoroalkanes are summarized above. The CeCl stretching INTERPRETATION GUIDELINES AND MAJOR SPECTRAeSTRUCTURE CORRELATIONS 133 vibrations occur between 860 and 505 cme1. Compounds with more than one chlorine atom will have multiple bands deriving from the in-phase and out-of-phase CeClx (X ¼ 2,3,4) which may also couple with other groups. The CeBr stretching vibrations occur between 680 and 485 cme1. Compounds with more than one bromine atom will have multiple bands deriving from the in-phase and out-of-phase CeBr2 which may also couple with other groups. The CeI stretching vibrations occur between 610 and 485 cme1. 4.23. OH, NH, NH2 Wag: 900e500 cme1 In the condensed state, moderately intense (but broad) IR bands are found between 900 and 500 cme1 due to the XeH wag of hydrogen-bonded water, amines, amides, and alcohols. Hydrogen bonding significantly influences the frequency for these characteristic IR bands. No significant Raman bands from these vibrations are observed. Alcohols exhibit bands that are generally centered around 650 cme1. Water absorbs below 800 cme1. Primary amines have bands between 900 and 770 cme1 and secondary amines between 750 and 680 cme1. Amides have bands between 750 and 550 cme1. 4.24. Metal Oxides: 800e200 cme1 Metal oxides of various types have strong IR and Raman bands from the metal-O-metal group. C H A P T E R 8 Illustrated IR and Raman Spectra Demonstrating Important Functional Groups The IR and Raman spectra of each compound are presented in a stacked format with the spectrum number given in the upper right-hand corner of each IR spectrum. The sample preparation and type of crystal used for the IR measurement is listed in parenthesis after the compound name in the spectral index below. The IR measurements were made using a BioRad FTS-575C spectrometer. All Raman measurements were made using 180o backscattering on a BioRad FT-Raman spectrometer (575C with Raman III accessory) and background corrected using a KBr white light reflectance measurement. A melting point or NMR tube was used for the Raman measurements. A single bounce diamond ATR accessory was used for the IR measurements of the polymers. The assignment shown on all of the spectra are consistent with that discussed in the text. Page numbers of spectra are shown on the right hand side. 1. ALIPHATIC 1. Heptane, [IR: capillary film/ZnSe], 2. Cyclohexane, [IR: capillary film/ZnSe], 3. Isooctane (2,2,4-Trimentyl pentane), [IR: capillary film/KBr], 140 140 140 2. C[C DOUBLE BONDS 4. 1-Hexene, [IR: capillary film/KBr], 5. cis-3-Heptene, [IR: capillary film/KBr], 6. trans-3-Heptene, [IR: capillary film/KBr], 141 141 141 3. TRIPLE BONDS 7. Acetonitrile, [IR: capillary film/KBr], 8. Acrylonitrile, [IR: capillary film/KBr], 9. 1-Octyne, [IR: capillary film/KBr], 142 142 142 135 136 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 4. AROMATIC RINGS 10. Toluene, [IR: capillary film/ZnSe], 11. o-Xylene, [IR: capillary film/KRS5], 12. m-Xylene, [IR: capillary film/KRS5], 13. p-Xylene, [IR: capillary film/KBr], 14. m-Tetramethyl xylene diisocyanate, [IR: smear/KBr], 15. Pyridine, [IR: capillary film/KRS5], 143 143 143 144 144 144 5. KETONES, ESTERS, AND ANHYDRIDES 16. Acetone, [IR: capillary film/KBr], 17. n-Butyl acetate, [IR: capillary film/KBr], 18. Dimethyl carbonate, [IR: capillary film/KBr], 19. Dimethyl malonate, [IR: capillary film/KBr], 20. Dimethyl terephthalate, [IR: KBr disc], 21. Butyric anhydride, [IR: capillary film/KBr], 22. Maleic anhydride, (Furan-2,5-dione) [IR: KBr disc], 23. 4-Hydroxy benzaldehyde, [IR: KBr disc], 24. Glycolic acid, (2-Hydroxyacetic acid) [IR: KBr disc], 25. Palmitic acid, [IR: KBr disc], 26. Oleic acid, (Z-Heptadec-9-enoic acid): [IR: capillary film/KBr], 27. Benzoic acid, [IR: KBr disc], 28. Stearic acid, calcium salt, [IR: KBr disc], 29. Glycine (Zwitterion), (2-Aminoacetic acid) [IR: KBr disc], 145 145 145 146 146 146 147 147 147 148 148 148 149 149 6. AMIDES, UREAS, AND RELATED COMPOUNDS 30. Acetamide, [IR: methanol cast film/KRS5], 31. Acrylamide, [IR: KBr disc], 32. N-butyl methyl acrylamide, [IR: smear/KRS5], 33. 1,3-Dimethyl urea, [IR: KBr disc], 34. Ammeline, (4,6-Diamino-1,3,5-triazin-2(1H)-one) [IR: KBr disc], 35. Trimethyl isocyanurate, (1,3,5-Trimethyl-1,3,5-triazinane-2,4,6-trione) [IR: KBr disc], 36. 2-Imidazolidone, (Imidazolidin-2-one) [IR: methylene chloride cast film/KRS5], 37. Dimethyl formamide, [IR: capillary film/ZnSe], 38. Ethyl carbamate, [IR: methanol cast film/KRS5], 39. Acetohydroxamic acid, (N-Hydroxyacetamide) [IR: KBr disc], 149 150 150 150 151 151 151 152 152 152 COMPOUND INDEX 137 7. ALCOHOLS 40. Methanol, [IR: capillary film/KBr], 41. Isopropyl alcohol, (Propan-2-ol) [IR: capillary film/KBr], 42. t-Butyl alcohol, (2-Methylpropan-2-ol) [IR: capillary film/KBr], 43. 1-Butanol, [IR: smear/KRS5], 44. 1-Hexadecanol, [IR: methylene chloride cast film/ZnSe], 45. Phenol, [IR: melt/KRS5], 46. 2,6-Di-t-butyl phenol, [IR: methylene chloride cast film/ZnSe], 47. Diethylene glycol, (2,2’-oxydiethanol) [IR: smear/ZnSe], 153 153 153 154 154 154 155 155 8. ETHERS 48. n-Dibutyl ether, (1-Butoxypentane) [IR: capillary film/ZnSe], 49. Tetrahydrofuran, [IR: capillary film/ZnSe], 50. Epichlorohydrin, (2-(chloromethyl)oxirane) [IR: capillary film/ZnSe], 155 156 156 9. AMINES AND AMINE SALTS 51. Isobutyl amine, (2-Methylpropan-1-amine) [IR: capillary film/ZnSe], 52. Aniline, [IR: capillary film/KRS5], 53. n-Dibutyl amine, (n-Butylpentan-1-amine) [IR: capillary film/ZnSe], 54. n-Tributyl amine, (N,N-Dibutylpentan-1-amine) [IR: capillary film/ZnSe], 55. Trimethyl amine HCl, [IR: KBr disc], 56. Tetramethyl ammonium chloride, [IR: KBr disc], 156 157 157 157 158 158 10. C[N COMPOUNDS 57. Benzamidine HCl, [IR: KBr disc], 58. Acetamide oxime, (Z)-N’-Hydroxyacetimidamide [IR: KBr disc], 59. Pipenidindione dioxime, (2E,6E)-Piperidine-2,6-dione dioxime [IR: KBr disc], 158 159 159 11. N[O COMPOUNDS 60. Isoamyl nitrate, [IR: capillary film/KBr], 61. 2-nitropropane, [IR: capillary film/KBr], 62. Isopropyl nitrate, [IR: capillary film/KBr], 159 160 160 138 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 12. AZO COMPOUND 63. Azo-tert-butane, (E)-1,2-di-tert-butyldiazene [IR: capillary film/KBr], 160 13. BORONIC ACID COMPOUND 64. meta-tolyl boronic acid, [IR: Nujol mull/KBr], 161 14. CHLORINE, BROMINE, AND FLUORINE COMPOUNDS 65. 1-Chlorooctane, [IR: capillary film/KBr], 66. 1-Bromohexane, [IR: capillary film/KBr], 67. 3-Fluorotoluene, (1-Fluoro-3-methylbenzene) [IR: capillary film/KBr], 68. 3-Chlorotoluene, (1-Chloro-3-methylbenzene) [IR: capillary film/KBr], 69. 3-Bromotoluene, (1-Bromo-3-methylbenzene) [IR: capillary film/KBr], 161 161 162 162 162 15. SULFUR COMPOUNDS 70. Dimethyl sulfoxide, (Methylsulfinylmethane) [IR: capillary film/KBr], 71. Methane sulfonic acid, sodium salt, [IR: Nujol/Fluorolube mixed mull on KBr], 72. para-Toluene sulfonic acid hydrate, [IR: KBr disc], 73. Dodecyl sulfate, sodium salt, [IR: Nujol/Fluorolube mixed mull on KBr], 74. Sodium isopropyl xanthate, [IR: KBr disc], 75. Zinc dimethyl dithiocarbamate, [IR: KBr disc], 76. 2-Imidazolidinethione, (Imidazolidine-2-thione) [IR: KBr disc], 163 163 163 164 164 164 165 16. PHOSPHORUS COMPOUNDS 77. Triphenyl phosphine, [IR: KBr disc], 78. Tributyl phosphite, [IR: capillary film/KBr], 79. n-Dibutyl phosphite, [IR: capillary film/KBr], 80. Sodium di-isobutyl dithiophosphate, [IR: water cast film/KRS5], 165 165 166 166 17. SILOXANE COMPOUNDS 81. Siloxane polymer (silicone grease), [IR: smear/KBr], 166 COMPOUND INDEX 139 18. INORGANIC COMPOUNDS 82. Ammonium chloride, [IR: KBr disc], 83. Sodium sulfate, [IR: KBr disc], 84. Sodium sulfite, [IR: KBr disc], 85. Sodium carbonate, [IR: KBr disc], 86. Sodium bicarbonate, [IR: KBr disc], 87. Potassium nitrate, [IR: KBr disc], 88. Tribasic sodium phosphate, [IR: KBr disc], 89. Dibasic sodium phosphate, [IR: KBr disc], 90. Boric acid, [IR: KBr disc], 91. Sodium perchlorate, [IR: KBr disc], 92. Sodium molybdate, [IR: KBr disc], 93. Silica gel, [IR: KBr disc], 94. Talc, [IR: ATR], 167 167 167 168 168 168 169 169 169 170 170 170 171 19. POLYMERS AND BIOPOLYMERS 95. Isotactic polyethylene, [IR: ATR], 96. Polybutadiene, [IR: ATR], 97. Polyvinyl alcohol (PVA), [IR: ATR], 98. Polyethylene oxide, [IR: ATR], 99. Polytetrafluoroethylene (Teflon), [IR: ATR], 100. Poly acrylic acid, [IR: ATR], 101. Polyvinyl pyrrolidone, [IR: ATR], 102. Nylon 6,6, [IR: ATR], 103. Cellulose, [IR: ATR], 104. Starch, [IR: ATR], 105. Sodium carboxymethyl cellulose, [IR: ATR], 106. Hydroxypropylmethyl cellulose, [IR: ATR], 107. Ethyl cellulose, [IR: ATR], 108. Siloxane polymer, [IR: cap film/KBr], 109. Poly acrylamide (PAM), [IR: water cast film/ZnSe], 110. Poly-2-(acryloyloxy)-Ethyltrimethyl Ammonium Chloride (AETAC), [IR: water cast film/ZnSe], 171 171 172 172 172 173 173 173 174 174 174 175 175 175 176 176 140 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 100 1379 B B B B 394 357 310 B CH 3 776 1451 (CH2) 5 C-C-C o. ph. str 902 840 2859 Ra I CH 3 CH2 i. ph. twist 1302 2959 2875 CH2 bend + CH3 o. ph. bend CH2, CH3 str. CH2 i. ph. rk. CH3 i. ph. bend 2733 0.1 CH3 o.ph. str 2963 Heptane 20 2937 40 1138 1081 1046 60 1466 IR %T 1 723 80 0.0 100 2 904 862 1039 1257 60 2660 2793 80 IR %T 40 1029 Ring i. ph. str. 427 1157 1445 0.4 1267 CH2 bend 2665 Ra I CH2 i.ph. str. 2938 2853 CH2 o.ph. str. 1348 0.8 802 2853 2925 Cyclohexane 1450 S 20 0.0 isopropyl 980 C C C C i. ph. str. 303 513 C 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumbers(cm–1) 1200 1000 CH3 C C C 827 929 1248 1169 1098 1353 1452 C CH3 i. ph. str. 2715 0.1 CH3 C 2873 2908 2957 0.2 CH3 o. ph. bend. CH3 CH CH2 746 CH3 i. ph. str. 3 CH3 CH3 i. ph. bend. t-butyl CH3 o. ph. str. 0.3 1366 2902 2956 Isooctane 20 Ra I 1469 1393 2871 60 40 1248 80 IR %T 1169 100 800 600 400 141 C¼C DOUBLE BONDS 4 553 739 993 H CH 2 910 C CH(CH 2)3CH 3 311 629 1105 1058 915 875 820 360 CH2 chain twist 1-Hexene H H 1219 1642 632 1103 1380 1459 1642 2870 2735 Ra I 1443 1417 =CH2 o. ph. str. 3081 0.1 2877 =CH str. C H C C C H =CH2 CH2 bend bend C=C str. H H CH3 i. ph. bend 1297 3078 CH3 o. ph. str. 2 x 910 2961 20 =CH2 o. ph. str. 3000 IR 60 %T 40 1822 80 1297 100 0.0 100 H C C C2H5 711 870 968 1069 312 709 486 389 852 897 1069 1020 970 1450 2731 1269 CH2 bend + CH3 o. ph. bend cis-3-Heptene 1460 1378 1655 2932 2874 C C C 3H 7 1655 =CH str. Ra 0.1 I CH3 i. ph. str. H H C=C str. 3009 0.2 CH3 i. ph. str. H cis o. pl. wag CH3 i. ph. bend =CH rk CH2 o. ph. str. 2876 CH3 o. ph. str. 2963 3007 20 =CH str. 2936 IR 60 %T 40 1304 5 80 0.0 772 1050 1291 C H trans o. pl. wag C2H5 H C C 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumbers(cm–1) 800 600 400 326 1000 H 467 1067 1030 1200 908 866 808 1248 C3H7 trans-3-Heptene C 966 1460 1441 1379 2931 2875 2730 2997 Ra 0.1 I 2965 C=C str. HC=CH i. ph. str. 6 H CH3 i. ph. bend CH2 bend + CH3 o. ph. bend 1441 CH3 o. ph. str. 1670 0.2 CH3 o. ph. str. 2876 20 HC=CH o. ph. str. 2936 2963 80 IR 60 %T 40 3027 100 142 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 750 918 1039 1444 1376 2293 2 X 380 bend N C C-C str, C-C-N bend 380 920 CH3 i. ph. bend. CH3 1443 3715 3003 2884 CH3 o. ph. str. C≡N str. 7 CH3 rk CH3 o. ph. bend. 2253 0.3 0.2 Ra I 0.1 2254 CH3 i. ph. str. 2293 20 CH3 o. ph. str. 2943 Acetonitrile 40 918 + 1376 2733 60 918 + 2254 3165 80 IR %T 3003 2945 100 0.0 870 805 =CH2 bend C C C H cis CH wag H H H C 687 1094 H =CH2 wag + trans CH wag H H C C H H2C C C H C N 241 1610 0.3 1287 1655 1609 C=C str. 2230 Acrylonitrile 2 x 969 C≡N str. 8 969 =CH2 str. =CH str. 1413 40 20 1939 2229 60 2990 IR %T 3119 3071 80 2280 100 568 689 1094 872 C-C str. 1287 3120 0.1 2991 0.2 Ra I 1414 3035 i. pl. bend 0.0 ≡C-H str. 527 885 1113 725 9 CH3 rk 629 ≡C-CH 2 bend CH2 i. ph. str. CH2 o. ph. str. ≡C-H wag CH2 bend + CH3 o. ph. bend 339 8 2x ≡C-H wag 2119 3314 20 2958 2934 2861 C≡C str. 1467 14601433 1380 1327 60 40 1-Octyne 1687 2120 80 IR %T 1242 CH3 i. ph. bend 100 292 632 885 843 810 1114 1077 1014 1439 1327 1304 2 2851 2731 4 3314 Ra I 2930 6 0 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 1000 800 600 400 143 AROMATIC RINGS 10 896 464 1005 695 729 O 523 1032 219 787 O 623 2 x 1381 CH3 Toluene 1080 1030 1379 1461 Aryl CH3 i. ph. bend 1606 2983 2921 Aryl CH str. 1605 1858 1803 1943 3056 0.6 Ra 0.4 I 0.2 O Ring Quad. Str. Aryl CH3 i. ph. str. + 2x 1461 cm-1 1211 1158 0.8 CH3 o. ph. bend 5 adj Aryl CH i. ph. wag Ring Semi. Str. 1438 1381 Aryl CH str. 2737 20 Ring CH wag summation bands 3028 40 CH3 i. ph. bend 1496 60 2921 3086 80 IR %T 2872 2735 2 x 1379 1735 100 0.0 100 o-Xylene 505 435 Aryl CH i. pl. bend 986 506 257 583 1053 1223 CH3 CH3 1158 1291 2 x 1385 1450 1385 3045 2920 0.2 582 Aryl CH3 i. ph. bend 4 adj aromatic CH wag 735 743 CH3 o. ph. bend Ring semi-circle CH3 o. ph. bend Ra 0.4 I 932 1120 1053 1020 985 1467 1496 1383 1606 Ring Quad. str. CH3 i. ph. str. + overtone 1223 1291 1788 CH3 o. ph. str. 2940 1608 1583 0.6 Aryl CH str. CH3 i. ph. str. + overtone 2733 0.8 3017 20 2878 3063 40 2 x 1383 1900 60 2731 3105 IR %T 1942 11 80 0.0 100 227 278 0.0 3500 3000 2500 2000 1800 Wavenumbers(cm–1) 1200 m-Xylene 515 538 726 1095 1035 1251 1400 1171 1379 1600 432 691 769 O i. ph. bend CH3 876 905 1001 3 3 adj CH wag O 516 CH3 1095 1460 1492 1612 2864 Ring Ring Quad. str. semi-circle Aryl CH Aryl CH3 i. ph. str. + 2x 1460 cm-1 1451 Ra 0.3 I Lone aryl CH wag 1612 1593 Aryl CH str. 3051 2919 2865 2733 0.5 3017 2921 20 1040 1250 1377 2x 1377 40 1170 1772 1653 60 1932 2732 IR %T 1854 12 80 1000 800 400 600 144 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 1043 1220 1119 1379 1455 483 795 460 830 1206 O 812 Aryl CH3 i. ph bend 646 388 0.2 1630 Ring Quad. str 3055 3014 2922 2866 2735 0.4 Ra I 1517 Aryl CH3 i. ph. str. + 2x 1455 CH3 O 1184 0.6 2 adj. H wag Ring semi-circle str. 1448 1380 1314 Aryl CH str. CH3 CH3 o. ph. bend 1619 1580 p-Xylene 20 13 313 3020 2923 40 1891 60 2x 1379 2869 IR %T 3095 80 1793 2733 100 0.0 1003 500 633 580 797 706 1086 1157 1273 343 795 604 749 705 N N Ring Quad. i. pl. bend 654 606 1219 1149 1486 1575 0.2 2957 3147 0.4 406 885 992 993 1031 N 5 adj. H wag 3059 0.6 Ring semi-circle str. 1032 Ring Quad. str 1068 1217 1148 1439 1.0 0.8 0.0 3500 3000 2500 2000 1800 262 NCO 565 685 O 931 1489 1483 1582 Aryl CH str. Ra I NCO O 15 1634 1873 1923 20 Pyridine 1232 2259 3054 3027 3079 60 40 1988 3002 H2O 3410 IR %T 3 adj. H wag H2O 3660 80 1423 1368 1606 2879 0.0 100 NCO i. ph. str 1173 1117 1088 0.1 Aryl lone CH wag CH3 i. ph bend 1276 0.2 3071 2987 2941 2868 2769 2724 Ra I Ring Quad. str NCO o. ph. str. CH3 str. 1463 1443 1416 0.3 Ring semi-circle str. 1608 Aryl CH str. CH3 o. ph. str. 2256 20 NCO Comb. 2259 + 1416 2936 40 3058 60 2983 IR %T 929 863 14 80 3655 m-Tetramethyl xylene diisocyanate 100 1600 1400 –1 Wavenumber(cm ) 1200 1000 800 600 400 145 KETONES, ESTERS AND ANHYDRIDES 16 530 Acetone 390 899 1067 1222 1429 1358 2698 531 C-C-C i. ph. str. C CH3 C=O i. pl. rk. 788 CH3 o. ph. bend 1710 3005 2848 0.2 903 1093 1283 1716 C=O str. 0.4 Ra I O CH3 CH3 i. ph. bend 2924 CH3 o. ph. str. C-C-C o. ph. str. 1222 CH3 i. ph. str. 20 0.6 1422 2 x 1716 (C=O) 1363 40 530 + 1222 1748 60 2925 IR %T 3005 80 3414 100 0.0 634 607 739 951 1066 1244 1367 C 1032 1464 C O O C=O o. pl. wag O C=O i. pl. rk. O-CH2 str. O CH 3 C O nC 4H9 480 430 308 635 808 1124 1065 1033 0.04 Chain i. ph. CH2 twist 1303 1739 0.08 920 881 839 C-C-O o. ph. str. C=O str. 2739 Ra I CH3 (CH2) o. ph. str. 1452 0.12 CH3 (CH2) i. ph. bend CH3 (C=O) i. ph. bend 17 O n-Butyl acetate 2940 20 1387 2 x 1743 (C=O) 2962 40 2876 60 2877 IR %T 1743 80 3463 100 0.00 O 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 1000 800 692 600 400 256 860 367 C-O-C-O-C i. ph. str. 517 916 O C O CH 3 970 CH 3 0.0 3500 Dimethyl carbonate 915 970 795 1095 1214 1277 (O) CH3 o. ph. bend 1754 18 O-C-O o. ph. str. 1212 1163 1119 2964 0.2 C=O str. 2888 2848 3035 0.4 Ra I 1757 (O) CH3 str. 20 1435 CH3 i. ph. bend 40 1455 60 1458 IR %T 1605 80 2857 3009 2963 100 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 0.04 1026 1343 1278 1206 1154 1740 580 851 785 687 O C OCH3 382 308 574 688 (O)CH3 o. ph. bend 787 2851 3000 Ra 0.08 I 1751 0.12 O CH3O C CH2 CH2 (C=O) bend C=O o. ph str. C=O i. ph str. involves O-C str. 916 847 (O) CH3 o. ph. str. 19 1022 994 951 0.16 1757 CH3 i. ph. str. + 2x bend 20 1439 (O)CH3 i. ph. bend 40 1460 1415 60 1414 2850 3006 3473 2959 IR %T 2958 Dimethyl malonate 80 954 involves C-C-O str. 100 1190 1157 146 0.00 Semicircle str. 498 814 O O 708 635 o 272 354 843 1108 961 C CH3 o 798 2845 C O 2 adj. Aromatic CH wag + C=O wag 1450 1282 C-O-C str. O 730 1108 1192 1016 955 876 1408 1387 C-C-O o, ph. str. Ring Quad str. 2961 3086 3042 0.4 1280 C=O str. Aryl CH str. 0.8 Ra I 1726 1720 (O) CH3 str. 1435 (O) CH3 i. ph. bend 20 1.2 1535 1961 1905 1504 2 x 1720 CH3 1610 40 3017 2960 2844 60 3422 IR %T 20 = Dimethyl terephthalate 80 2102 100 0.0 COOH impurity 563 750 792 873 930 C H 3 (C H 2 ) 3 1030 1051 C O C (C H 2 ) 3 C H 3 353 293 460 570 C-O str. 749 1413 1296 1240 1140 1117 O O 901 869 829 0.02 CH2(C=O) bend 1099 1043 2746 Ra 0.04 I 1812 1750 0.06 CH3 i. ph. bend 1453 1415 C=O C=O o. ph. str. i. ph. str. 2880 0.08 CH2 bend + CH3 o. ph. bend 1301 1819 20 21 1712 2970 40 1750 2940 CH3 o. ph. str. 2918 60 2941 Butyric anhydride IR %T 2880 80 1462 100 0.00 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 1000 800 600 400 147 KETONES, ESTERS AND ANHYDRIDES 697 896 839 559 H O CC wag C-O-C i. ph. str. 410 549 1240 271 872 1594 O H 643 1061 C-C-O o. ph. str. O 768 1787 638 1108 1060 1242 1268 1460 1434 1290 1399 1633 1842 C=O o. ph. str 1857 3122 3060 1.0 C=C str. = =CH str. 2.0 Ra I 1783 C=O i. ph. str 1634 20 1856 40 1591 1933 3188 3123 60 3593 IR %T 22 H2O 3435 80 Maleic anhydride 100 0.0 H 604 C OH Para Quad. i. pl. bend 338 266 224 416 453 837 793 646 604 862 o 4-Hydroxy benzaldehyde 507 708 640 789 835 859 1113 O o 1164 1286 1218 1160 1388 1314 1516 1666 1453 Ar. Ar-O str. Semi circle str. 23 Ar. 2 adj. H wag 1015 1.0 2881 2791 2.0 3169 3066 Ra I C=O str. 1586 3.0 Ar. Quad str. Ar CH str. 1452 1389 1315 1286 1219 OH str. AldehydeCH str. + 2 x bend 1598 20 1665 3171 40 1906 60 2789 2969 2880 2831 80 IR %T OH wag AldCH bend 100 0.0 889 533 617 679 530 493 677 1200 O CH 2 O-C-C-O i. ph. str. 1086 1231 1400 885 935 999 1088 1250 1429 1360 1713 C-C-O o. ph. str. 1726 3281 0.1 2975 2947 CH2 str. Ra I CH2 bend H H O O-C-C-O o. ph. str. = C=O str. (acid OH bonded to alcO) 0.2 1362 1432 2565 2647 2764 3294 2938 40 20 1560 (CH2)OH str. 1730 60 (O=C)OH str. 2238 IR %T 24 OH wag 80 0.0 3500 3000 2500 2000 1800 1600 Wavenumber(cm–1) 1000 800 600 400 C O Glycolic acid 100 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 0.0 100 548 688 375 671 893 1099 1129 1063 1296 Trans chain C-C-C o. ph. str. 1711 723 938 1285 1247 1462 1413 2674 CH2 twist CH 3(CH 2) 7 C Cis CH wag (CH2 ) 7 C OH C H 226 C 724 857 1656 H 1119 1083 1022 972 2855 Dimer CH2 rk O…H…O o. pl. wag 1303 1267 0.1 C=C str. 2726 3007 0.2 C O C=O o. ph. str. in dimer 0.3 H H C-O str. CH2 bend 1441 0.4 CH2 chain i. ph. str. 2854 CH2 o. ph. str. 0.5 Ra I C OH 26 3005 =CH str. 2898 2926 Oleic acid 20 O CH3 (CH2)1 4 OH str. 60 40 CH2 rk Trans chain C-C-C i. ph. str. 80 IR %T 720 941 891 1472 1431 1348 1295 1229 1188 2882 2849 CH2 twist 1175 0.2 C=O i. ph. str. in dimer 1634 0.4 Dimer O…H…O o. pl. wag C-O str. CH2 bend 2725 2959 2924 0.6 Ra I C=O o. ph. str. in dimer 2846 0.8 Trans chain CH2 o. ph. str. CH2 i. ph. str. 1703 2955 20 CH3 o. ph. str. CH2 o. ph. str. 2918 Palmitic acid 40 2676 60 1099 OH str. 80 IR %T 25 CH2 wag 100 1462 1438 1422 148 0.0 Aryl CH str. 550 805 707 665 1180 1126 1292 1326 O C OH o OH C-O str. i. pl. o bend 618 421 796 o 1028 1180 1132 1.0 1290 C=O i. ph. str. in dimer 1443 2.0 1635 1603 3.0 3073 Ra I Ring quad. str. Dimer O…H…O o. pl. wag 27 1002 C=O o. ph. str. in dimer 5.0 4.0 1687 3071 20 3007 2834 2674 2559 Benzoic acid 40 1582 60 1454 1424 1496 80 IR %T 5 adj Ar-H wag + C=O wag 934 Ring semi. str. OH str. + combination bands 1027 100 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 1000 800 600 400 149 AMIDES, UREAS AND RELATED COMPOUNDS CH2 chain wag 720 1113 1302 1421 CH2 chain i. ph. twist 1569 CH2 chain C-C-C i. ph. str. CH2 chain C-C-C o. ph. str. 959 937 891 CH2 bend Ca 2 Stearic acid 1622 1576 CH2 i. ph. str. 0.20 0.10 CH3(CH2)16CO2 CO2 o. ph. str. 2724 2610 Ra I CH2 chain o. ph. str. CO2 i. ph. str. 1130 1103 1063 0.30 CH2 chain i. ph. rk. 1176 0.40 1542 1470 2957 CH2 o. ph. str. 2850 20 2917 CH3 o. ph. str. 2881 40 H2O bend 2846 60 3429 80 IR %T 28 H2O str. 1461 1441 1370 1296 100 0.00 100 29 NH3+ str. 503 359 495 602 696 0.0 100 1006 458 C-C-N i. ph. str. 0.00 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 1000 800 600 400 Acetamide 587 O=C-N bend 583 878 CH3 NH2 i. ph. rock bend 1151 C-N str. 467 876 826 1048 1007 1154 1354 C-C=O bend 1396 1466 2934 1679 1639 1591 3353 CH3 str. NH2 bend NH2 wag = C=O str. NH2 i. ph. str. Ra 0.20 I 1647 3167 3355 C NH2 NH2 o. ph. str. 0.30 0.10 CH3 3159 0.40 O 1405 1356 40 20 1744 60 2820 IR %T 30 CH3 o. ph. bend 80 Glycine (Zwitterion) 608 698 911 893 1032 893 CH2 CO2 N-C-C i. ph. str. 1139 1109 1035 1668 1631 1568 1515 1440 1410 NH3+ str. 2615 3143 0.2 3007 2972 2904 0.4 NH3 1325 CO2 NH3 i.ph. i.ph. str. bend 0.6 Ra I 1333 1444 1112 2122 CH2 str. 1412 20 CO2 o.ph. str. + NH3 o. ph. bend 1611 3169 3008 2970 2895 40 CH2 bend 2614 60 3383 IR %T + summation bands 1522 80 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS H 702 665 841 810 1049 C CH2 CH NH2 306 845 1145 =CH2 wag 1053 C-C-N o. ph. str. 505 1136 1281 1352 1429 1433 1284 1636 =CH rk O C=O wag 616 0.2 C=C str. =CH2 i. ph. str. 31 NH2 wag 1681 0.4 =CH str. 3334 3162 3104 3033 3011 Ra I =CH2 o. ph. str. wag NH2 rk NH2 bend =CH2 bend C=O str. 0.6 1613 2 x 961 2 x 1429 1673 20 3354 Acrylamide 40 1589 60 C C 1922 IR %T NH2 str. o. ph. i. ph. 3186 3101 3039 2813 2737 80 C-C-N H o. ph. str. 988 961 100 960 150 0.0 2X 1542 C-N-H str-bend 705 620 871 808 986 959 1087 1233 1311 C-O-C o. ph. str. CH2 bend 32 H H C C H =CH2 wag H C C wag H CH 2 CH O C O(C 4H 9) N H 949 873 833 1633 1541 1628 1668 C=C str. 1119 1064 =CH2 o. ph. str. 0.04 2935 =CH str. 1453 1411 Ra I 0.08 NH str. 1670 0.12 C=O str. 3104 3030 2938 2876 20 2960 3297 40 2872 60 NH wag 1459 1404 3066 IR %T 3310 N-butyl methyl acrylamide 80 1301 1237 100 0.00 C=O str. 775 675 1175 509 C N H H H N C N (N) CH3 i. ph. bend NH bend + C-N str. 1624 1552 1473 1423 2811 2945 2904 0.4 3346 0.8 Ra I N H N-C-N i. ph. str. 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 CH 3 240 NH str. O CH 3 511 1.2 1269 1043 CH3 o. ph. str. 932 20 NH wag 33 1178 CH3 i. ph. str. 3343 40 1420 60 1543 1,3-Dimethyl urea IR %T 1627 1591 80 932 2 x 2 x 1543 1591 3176 3050 2947 2902 2812 100 1000 800 600 400 151 AMIDES, UREAS AND RELATED COMPOUNDS 417 389 235 580 O N 864 653 N 428 N 555 N H Ammeline O N NH 2 698 994 1175 1278 685 635 582 869 793 994 1169 1283 1412 1447 1617 N O N N 1445 3186 3059 2878 0.2 3459 Ra I 1707 1668 0.4 N H 2N N Ring quad str. + NH2 bend C=O str. H-bonded NH2 str. 1548 “free” N-H str. 1514 0.6 Ring N i. ph. radial N 1719 20 34 3185 3089 3464 40 O 1684 2865 60 …. 1615 80 IR %T N-H NH2 str. 2661 100 0.0 Trimethyl isocyanurate 483 750 424 1056 1136 940 484 700 752 0.2 1685 3007 0.4 940 2965 0.6 Ra I CH3 Ring C-N str. i. ph. bend + CH3 o. ph. bend C=O i. ph. str. 1136 CH3 i. ph. str. 1387 0.8 579 C=O o. ph. str. O CH3 1477 N CH3 1460 N 1678 O N 1751 20 H 3C 35 1391 O 40 1279 60 1277 2967 IR %T 3429 80 2543 100 0.0 CH2 bend 1275 N-C-N o. ph. str. C N 1204 1400 1200 511 710 1379 1518 1488 1659 2905 2964 0.8 3304 N O C N 5 membered ring i. ph. str. NH bend + C-N str. 0.4 H 498 N 698 C N H H 770 N 934 C=O str. 1.2 Ra I 1105 1038 991 1510 1451 1675 CH2 N-H str. o. ph. str. H H 1109 1043 999 1.6 CH2 i. ph. str. 3308 20 2902 40 1576 3005 60 2957 IR %T 36 NH wag 80 0.0 3500 3000 2500 2000 1800 1600 Wavenumber(cm–1) 1000 800 400 600 H 2-Imidazolidone 100 152 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 37 658 865 1093 659 867 405 355 319 522 CH 3 998 226 397 O CH 2 513 679 C 858 O NH 2 674 626 857 792 996 38 O-CH2 str 1128 1080 1275 1460 1443 NH2 rk 1077 1131 1342 1487 1427 1382 1616 1701 1690 1625 3206 3407 0.20 (O ) C O str C-N str. NH2 bend N C bend NH2 wag 1926 2916 2986 C=O str. 2902 NH2 NH2 o. ph. i. ph. str. str. 2983 0.30 CH3 o. ph. str. 2936 3337 3214 20 3422 Ethyl carbamate 40 O 1093 1662 CH2 CH3 bend 60 0.10 1441 1408 CH (=O) str. 80 Ra I NC3 i. ph. str. C=O str. 0.00 100 IR %T N C H CH3 (N)CH3 bend 1500 2931 Ra 0.08 I 0.04 O CH3 CH i. pl bend 0.16 0.12 1257 1503 CH3 i. ph. str. CH3 o. ph. str. 20 1439 1388 2 X 1676 40 1484 60 1676 IR %T 2861 Dimethyl formamide 80 2930 2860 3336 100 0.00 100 OH wag 444 641 CH3 O C N 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) H 349 309 273 225 449 588 0.00 3500 OH 647 993 964 590 752 964 993 1090 1378 1086 C=O str. 39 C-N-O str. 1346 1522 1623 CH3 i. ph. str. 1040 1192 1321 0.10 CH3 i. ph. bend CH3 o. ph. CNH bend str/bend 1620 1566 0.20 Ra I 2937 2870 2944 CH3 o. ph. str. NH str. 2996 0.30 3207 20 3040 40 1450 60 1438 1390 IR %T OH str. 3202 Acetohydroxamic acid 80 1200 1000 800 600 400 153 ALCOHOLS 100 40 664 1115 3366 0.00 100 C-O str. 1152 1103 1458 OH str. OH 438 CH3 bend CH3 OH-CH bend 41 80 818 655 H wag O CH 3 CH 953 1159 1128 OH CH 3 820 C-C-O o. ph. str. + CH3 rk C 491 430 953 C O C i. ph. str. 1132 1343 2722 1454 2976 3376 0.2 2924 0.4 Ra I CH3 i. ph. bend CH3 o. ph. str. 2883 OH str. CH3 o. ph. bend CH3 i. ph. str. 2972 3347 20 2933 40 1466 1409 1380 1310 60 2885 IR %T Methanol 1030 1036 CH3 rk wag Isopropyl alcohol 2945 2833 bend 0.08 0.04 O O CH3 i. ph. str. 2945 3350 C CH3 o. ph. str. 0.12 Ra I H H 40 20 1421 60 2837 IR %T 1450 80 C C O i. ph. str. 348 C 0.00 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 1000 800 600 400 t-Butyl alcohol 642 749 1023 1237 752 CH 3 C 474 0.10 42 C OH C3-C-O str. 916 2714 0.30 Ra I 0.20 CH3 CH3 o. ph. bend CH3 i. ph. str. H O wag CH 3 914 2972 CH3 OH str. o. ph. str. 1455 0.40 CH3 i. ph. bend 2977 2920 0.50 3368 20 1200 CH3 o. ph. bend 40 1209 60 1472 1383 1365 2882 80 IR %T 1659 2567 0.0 100 154 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 100 656 1300 1254 1114 1070 1030 964 901 880 CH3 (CH 2)3 487 811 828 C-C-C-O i. ph. str. wag OH 398 355 738 953 1043 H C-C-O o. ph. str. (CH2)n i. ph. twist 1451 CH2 bend, CH3 o. ph. bend 847 1215 1113 1070 2872 2877 2959 2938 OH str. O CH3 i. ph. str. CH2 i. ph. str. 2737 0.08 Ra I 0.04 CH2 o. ph. str. 3231 0.12 3331 20 2934 40 1-Butanol 1379 1340 60 1462 IR %T 515 43 80 0.00 1473 1463 CH2 o. ph. bend 878 935 996 891 618 534 503 691 811 O 800 536 1000 620 814 245 754 1001 O 1026 1169 885 1024 1070 1169 1238 1370 1499 1501 4 45 wag OH Ar. 5 adj CH wag O Aryl Ar. C-O str. Semicircle str 1598 3060 8 1474 1597 Ar. Ring Quad str. Aryl CH str. 1254 12 3048 3258 Phenol 20 H 980 1773 1709 1846 2051 60 O 40 Ra I 1063 1130 1459 1437 1369 2724 OH str. (CH2 )15 OH Trans chain C-C-C o. ph str. C-O-H bend 1934 IR %T CH2 chain i. ph. rock crystal splitting Trans chain C-C-C i. ph str. 1296 CH2 i. ph. str. 0.00 100 80 C-CC-O o. ph. str. CH3 0.16 Ra I 0.08 44 720 C-O-H bend 1123 1070 2638 CH2 o. ph. str. 2846 2850 0.24 CH3 o. ph. str. 2918 20 2881 1-Hexadecanol 40 2956 60 3303 IR %T 1298 OH str. 80 1408 100 0 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 600 400 155 ALCOHOLS AND ETHERS 46 C(CH 3) 3 623 Aryl-O str. 291 326 447 808 747 528 845 932 1116 1096 1020 1272 1231 1198 2,6-Di-t-butyl phenol 588 747 (CH3 )3C T-Butyl CH3 i. ph. bend 1471 449 880 843 795 1094 1171 1231 OH CH3 o. ph. bend 1586 3 adj H wag 1143 1391 1362 1481 1315 1645 1798 1584 1426 2914 2959 3644 Aryl CH str. 3639 4 Ring semicircle. str. CH3 o. ph. str. Free OH str. 8 Ra I Ring Quad. str. 3081 2959 2914 12 2874 60 IR %T 40 20 1695 3083 80 1854 1912 100 0 896 922 1062 649 812 OH Diethylene glycol (CH2)2 354 896 824 529 O C-C-O(H) o. ph. str. 1119 1081 1059 (CH2)2 1241 HO 1279 1463 C-O-C o. ph. str. 2704 0.2 CH2 bend 1236 1456 1423 2930 0.4 3349 Ra I CH2 o. ph. str. CH2 i. ph. str. 2875 0.6 2936 3370 20 2875 40 1131 H2O (impurity) O bend 1354 OH str 60 1657 IR %T 47 H C 80 392 100 0.0 600 400 291 839 879 960 1117 1058 1300 1484 1449 2797 2735 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumber (cm–1) 1200 1000 800 n-Dibutyl ether 839 962 1041 1234 1302 1375 739 O nC4H9 CH2 i. ph. twist 0.8 Ra I 0.4 nC4H9 C-O-C o. ph. str. 1124 2864 CH2 bend + CH3 o. ph. bend 2872 2935 2913 CH2 o. ph. str. CH2 i. ph. str. CH3 i. ph. bend 2938 1.2 CH3 o. ph. str. 2960 60 IR %T 40 1464 2796 80 20 48 2734 100 156 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 577 656 787 1183 1290 O 40 912 1071 CH2 i. ph. str. CH2 bend. 1232 1070 1029 1486 2719 2657 0.1 1450 2962 0.2 5 membered ring i. ph. str. 914 CH2 o. ph. str. 0.3 C-O-C o. ph. str. 2861 2975 20 Ra I 49 2875 Tetrahydrofuran 60 IR %T 1365 80 1461 2682 1971 100 0.0 100 50 724 CH CH2 C-Cl str. 374 CH2 723 761 855 962 927 Cl Ring 223 516 844 962 443 Ring o. ph. str. i. ph. str. 1206 1137 1092 1398 1479 1433 3066 0.1 Ring CH2 wag 2965 3007 Ra I CH2-Cl bend CH2-Cl o. ph. str. 0.2 O CH2-(Cl) wag 1255 Ring CH2 bend Ring CH2 i. ph. str. 1193 1137 1092 1481 2926 2964 Ring CH2 o. ph. str. 1433 1398 1267 20 2927 Epichlorohydrin 40 3063 60 3003 IR %T 1518 80 0.0 0.04 CH3 i. ph. bend C-N str. CH2 bend + CH3 o. ph. bend NH2 CH3 CH CH2 NH2 wag C 0.00 3500 3000 2500 2000 1800 1600 1400 Wavenumber (cm–1) 1200 1000 800 600 400 280 i. ph. str. 483 C 953 1173 1132 1063 1338 C C 362 798 CH3 1458 2719 778 840 1065 940 51 1279 1616 CH3 i. ph. str. 1469 1388 2872 NH2 bend 2870 2956 3329 0.12 Ra I 0.08 2931 NH2 i. ph. str. 3382 Isobutyl amine 20 NH2 o. ph. str. 2957 60 40 3377 3300 80 IR %T 2737 2 x 1618 100 157 AMINES AND AMINE SALTS 2 x 1621 1840 501 Aniline 998 753 691 881 1027 993 1175 1277 757 390 533 622 Ar-N str. 816 1029 1602 1467 O 1280 0.2 O 1174 1156 0.4 5 adj. H wag O Ring SemiRing Quad. str. circle str. 3356 3209 Ra I NH2 bend 1604 0.6 Aryl NH2 o. ph. NH CH str. 2 str. i. ph. str. 3053 0.8 1499 20 NH 2 1622 3433 40 52 NH2 wag 3034 3214 3071 60 3356 IR %T 1929 80 235 100 0.0 735 Chain CH2 twist NH C4 H9 278 C4 H9 n-Dibutyl amine 961 1302 1234 1132 1462 2870 2810 NH wag C-N-C o. ph. str. 447 0.4 CH2 bend + CH3 o. ph. bend 53 1058 1027 896 878 834 0.8 3327 Ra I CH2 i. ph. str. 1132 1.2 CH2 o. ph. str. NH str. CH2 (N) i. ph. str. 1298 1.6 2875 2929 20 CH3 i. ph. bend 2810 2735 CH3 o. ph. str. 2934 40 2958 60 2964 IR %T 1445 80 1378 3287 100 0.0 100 C 4 H9 C 4 H9 267 903 881 828 947 0.00 3500 3000 2500 2000 n-Tributyl amine 901 1086 940 1251 1180 1304 1377 732 N Chain CH2 twist 1301 1464 C 4 H9 C-N str. 1179 1109 1050 0.05 CH2 bend + CH3 o. ph. bend CH2 i. ph. str. 1380 0.10 CH3 i. ph. bend 1445 2960 2934 Ra 0.15 I CH2 (N) i. ph. str. 1645 CH2 o. ph. str. 0.20 2933 CH3 o. ph. str. 2874 20 2957 40 2863 2798 60 54 1690 2680 80 2799 2734 IR %T 1800 1600 1400 Wavenumber (cm–1) 1200 1000 800 600 400 158 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 55 466 818 1167 1067 1371 989 NC3 i. ph. str. CH3 CH3 N H Cl 819 CH3 CH3 rk 1071 468 410 990 (N) CH3 i. ph. bend 1243 0.1 NC3 o. ph. str. 1464 1435 1404 2875 2817 Ra I 1260 1414 1485 2480 NH str. (+ summation bands) 3012 (N) CH3 o. ph. str. (N) CH3 o. ph. bend 2599 2471 0.2 (N) CH3 i. ph. str. 2701 20 2947 Trimethyl amine HCl 40 2629 3012 2960 IR 60 %T 1732 3419 80 2139 2054 100 0.0 457 1406 457 NC4 i. ph. str. 387 1287 1454 1401 2 x 1401 Cl 758 951 CH3 rk 948 (N) CH3 o. ph. bend N CH3 CH3 1181 0. 4 (N) CH3 i. ph. bend 1492 (N) CH3 i. ph. str. 2878 2787 Ra 0. 8 I CH3 1481 1.2 1071 1298 1753 1832 2347 2573 2469 2919 3094 2952 (N) CH3 o. ph. str. 56 CH3 1529 20 3018 IR 60 %T 40 3018 2950 80 3478 3397 Tetramethyl ammonium chloride 100 0.0 522 691 610 927 1028 839 Ring o. pl. sextant bend 1003 617 692 394 765 1607 57 O 1168 1097 1032 NH2 O NCN i. ph. str. 1531 1483 3178 3065 0.4 Cl C 1368 1296 1605 NH2 Aryl CH str. Ra 1.2 I 0.8 5 adj. H wag Ring Semicircle str. Ring Quad str. 1.6 NH2 o. pl. wag 1175 1110 1807 2361 1526 1482 1446 3236 20 NCN o. ph. str. 3063 40 2 x 700 wag 2758 60 3420 Benzamidine HCl IR %T NH2 str. 1676 80 785 100 0.0 3500 3000 2500 2000 1800 1600 1400 1200 Wavenumber(cm–1) 1000 800 600 400 100 O-H….N str. NH str. 2 x 1644 O-H….N str. CH2 o. ph. str 2 x 1604 CH3 o. ph. str CH CH3 i. ph. str. CH2 i. ph. str CH3 CH3 2294 CH3 i. ph. str C¼N AND N¼O COMPOUNDS C=N str. C=N str. (C 2H 4) O 2000 N O N=O str. NH CH3 2 bend o. ph. bend s-cis s-trans 1647 1600 CH2 bend C-N str. CH3 i. ph. bend C-N-C o. ph. str. CH3 i. ph. bend CH2 bend + CH3 o. ph. bend 1400 Wavenumber (cm–1) 1800 = CH3 o. ph. str. NH2 NH2 o. ph. i. ph. str. str. 3163 80 IR 60 %T 40 20 0.3 0.1 Ra 0.2 I 0.0 100 80 20 0.6 0.4 0.2 0.0 100 80 60 40 20 1.6 1.2 0.8 2500 1200 N-O str C C C i. ph. str. 800 475 CH 3 NH/OH wag C-C-N i. ph. str. NH/OH wag HO N N-O str. N-O str. C 1000 600 H N C 58 NH 2 N OH 413 59 N OH 400 60 250 IR 60 %T 40 Ra I IR %T Ra I 0.4 0.0 3000 352 159 Acetamide oxime Isoamyl nitrate 297 265 Pipenidindione dioxime 337 416 358 303 409 411 423 3492 3500 603 511 511 461 513 513 654 692 921 902 917 620 603 620 700 645 592 600 982 809 800 833 1045 1063 894 821 777 708 651 589 782 763 1102 1048 1021 1016 983 1184 1171 1136 974 942 874 957 911 1387 1371 1117 1139 1365 1399 1421 1362 1452 1053 1289 1252 1255 1184 1237 1169 1432 1468 1657 1676 1587 1604 1483 1427 1465 1430 1386 1331 1337 1308 2449 1593 1650 1646 1605 1821 2770 2766 2943 2886 2886 1665 1670 1644 3076 2793 2941 2983 2965 2926 2934 3397 3396 3088 2960 2963 2726 2875 2874 2726 3372 3490 3367 3266 3181 160 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 721 524 621 851 C CH N NO 2 CH 3 851 i. ph. str. 620 953 902 318 525 NO2 i. ph. str. + CH3 bend 1102 1551 0.1 NO2 o. ph. str. 1448 1400 2997 0.2 1179 1552 2950 0.3 Ra I 1102 C Isopropyl CH str + NO2 str. combination 61 CH 3 C 1179 20 1360 1306 40 1305 1359 60 2874 2735 2-nitropropane IR %T 1468 80 1400 2996 2945 2900 100 0.0 706 853 CH O NO2 880 NO2 i. ph. str. C C O 333 852 561 CH 3 459 908 1102 1279 562 760 1037 1183 1144 N i. ph. str. 706 257 C 409 1378 CH 3 941 908 0.4 62 N-O str. 1183 1146 1101 0.8 CH3 o. ph. bend 1624 2877 2742 1.2 NO2 o. ph. str. 2929 1.6 1277 2948 2.0 CH3 i. ph. bend 1626 Isopropyl CH str + NO2 str. combination 1351 20 1450 40 Ra I 1470 1390 2994 60 2996 Isopropyl nitrate IR %T 2551 80 2944 2887 3229 100 0.0 CH3 str. 544 263 364 594 503 715 928 842 1400 1027 1600 1240 1207 1450 1366 2707 t-Butyl CH3 i. ph. bend 1572 N=N str. 553 Involves (C)3 C-N o. ph. str. CH3 o. ph. bend 5 2874 Ra I Involves C-N=N bend + C-C str. 796 N (H3C)3C 1241 1211 2931 10 C(CH3)3 N Involves N-C(C)3 i. p. str. 1362 2867 20 1477 1453 1385 40 2974 IR %T 2976 2923 Azo-tert-butane 60 63 715 80 868 1026 100 0 3500 3000 2500 2000 1800 Wavenumber(cm–1) 1200 1000 800 600 400 161 BORONIC ACID, CHLORINE AND BROMINE COMPOUNDS 727 226 1001 Involves B-OH bend 505 584 381 335 528 702 665 3060 3036 2918 2865 827 801 O 1583 OH 1202 1168 1089 1045 1605 B Aryl CH3 str. Involves O-B-O str. meta-tolyl boronic acid (3-Methylphenylboronic acid) 492 652 592 708 917 1042 1224 1199 1181 1121 1311 3 adj H wag O OH 10 3206 64 H3C Aryl CH str. Ra I Aryl Ring Quad Str. 1348 OH str. 1419 20 1414 1379 1346 1310 40 2924 2854 3266 1456 60 1604 1582 3032 Nujol 80 IR %T 841 788 765 B-OH wag 100 0 65 Cl C C-Cl str. H trans to Cl 457 730 246 H 1-Chlorooctane 654 725 C CH2 i. ph. twist 1304 Cl 656 1306 1462 2857 CH2 i. ph. str. H C-Cl str. C trans to Cl 893 846 2857 CH2 bend + CH3 o. ph. bend (Cl) CH2 wag CH3 i. ph. bend 1170 1119 1080 1039 0. 2 CH2 (Cl) o. ph. str. CH3 2731 0. 4 (CH 2) 7 1445 0. 6 2928 CH2 o. ph. str. 2874 0. 8 CH3 o. ph. str. 2958 2936 20 2958 40 Ra I Cl 60 2996 IR %T 1377 80 915 100 0. 0 66 726 646 564 852 1047 1204 565 Br H 801 754 1000 800 485 962 1111 1067 646 CH2 i. ph. twist C-Br str H trans to Br 0.00 3500 3000 2500 2000 1800 1600 1400 Wavenumber (cm–1) 1200 1-Bromohexane C-Br str C trans to Br C CH2 bend + CH3 o. ph. bend CH2 o. ph. str. C 600 400 249 1279 1241 953 H Br 380 CH2 i. ph. str. CH2 i. ph. rk (Br) CH2 wag 1306 0.04 1465 2859 3011 Ra I 0.08 CH3 i. ph. bend 1440 0.12 CH3 o. ph. str. CH 3 2875 0.16 (C H 2) 5 2732 20 CH2 (Br) o. ph. str. Br 2930 40 2959 60 2963 2936 IR %T 3003 80 1379 100 162 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 528 728 682 925 444 1004 887 857 1079 777 729 1004 1251 528 245 3 adj H wag O 514 1265 1142 Lone H wag O 557 1620 1596 10 Aryl CH3 i. ph. bd. 1265 1251 1160 1142 1079 20 67 Involves Aryl Ring Ar-F str. Quad Str. Ring semi-circle Aryl CH3 str. Ra I 1382 1518 F 1489 H3C Aryl CH str. 1490 1446 1381 20 3080 3061 2924 2870 3-Fluorotoluene IR 60 %T 40 1620 1590 80 1460 3040 2924 2867 100 0 434 682 236 521 O 416 684 1217 1384 1078 856 773 3 adj H wag Involves Ar-Cl str. 856 1602 1577 388 O 2924 2862 10 Ring semi-circle Aryl Ring Quad Str. Aryl CH3 str. Ra 20 I Aryl CH3 i. ph. bend 1217 1165 1099 1079 Aryl CH str. Cl Lone H wag 999 H3C 1477 1603 1577 20 3064 3-Chlorotoluene 40 1164 1100 1042 998 68 1452 1383 IR 80 %T 60 1862 3063 2924 2861 1936 100 0 100 30 Ring semi-circle Aryl Ring Quad Str. Aryl CH str. Ra 20 I Aryl CH3 str. Involves Ar-Br str. 308 667 3 adj H wag O 834 1200 800 522 1212 1165 1093 1071 1028 1380 1450 1601 1570 2922 2861 3061 O 10 429 869 1040 995 Lone H wag 681 Aryl CH3 i. ph. bend 667 Br 834 770 H3C 1071 20 996 1601 1569 40 1212 1165 1452 1381 1936 69 1473 IR 60 %T 3-Bromotoluene 2922 2859 3061 80 0 3500 3000 2500 2000 1800 1600 1400 Wavenumber (cm–1) 1000 600 400 163 SULFUR COMPOUNDS 100 670 335 307 384 699 984 564 535 790 908 SO3 bend. 353 577 544 968 1204 1170 0.2 1062 SO3 o. ph. str. 1453 1412 3021 0.4 C-S str. 799 (S) CH3 o. ph. bend SO3 i. ph. str. 1052 (S) CH3 o. ph. str. 1184 (S) CH3 i. ph. bend (S) CH3 i. ph. str. 71 1078 Na 2940 0.6 SO3 1249 1220 20 Ra I CH 3 40 1337 60 1435 3018 2940 IR %T 1415 2258 100 80 Dimethyl sulfoxide 700 669 900 955 1053 CH 3 Methane sulfonic acid, sodium salt 0.0 S 1421 2998 CH 3 C-S-C o. ph. str. S=O str. 1045 Ra 0.4 I 0.2 (S) CH3 o. ph. bend O (S) CH3 rock 954 (S) CH3 o. ph. str. C-S-C i. ph. str. (S) CH3 i. ph. bend (S) CH3 i. ph. str. 2914 0.6 H2O OH str. 2819 20 1408 H2O bend 40 1311 1659 2094 1996 70 1437 60 2336 2995 2911 2822 IR %T 3442 80 CH3 o. ph. bend (Sulfonic acid Hydrate) 493 566 801 270 234 553 396 635 O 703 680 1000 677 816 706 846 1200 SO3bend O 818 1007 1181 1124 1037 1045 1014 H3 O Aryl 2 adj. H wag SO3i. ph. str. + p-aryl ring mode 1215 1188 SO3 1379 1308 CH 3 1454 3066 2986 0.1 Aryl-S 1599 0.3 Ra I 0.2 Ring Quad. str. (aryl) CH3 i. ph. str. 2932 Aryl CH str. 1401 Ring semi-circle str. 20 0.4 1455 1597 1495 1659 1850 2243 40 2670 60 2930 IR %T 72 SO3o. ph. str. 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumber (cm–1) 800 600 400 316 H3O+ str. 80 1126 100 para-Toluene sulfonic acid hydrate 0.0 164 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 422 407 920 992 836 763 722 635 592 637 599 575 1250 1221 1150 1085 1086 1064 S-O-C i. ph. str. 891 838 O SO3 Na SO3 bend C-C-C chain o. ph. str. 993 CH3 (CH2 )11 C-C-C chain i. ph. str. 1371 0.1 2729 2954 1454 1437 0.2 Ra I 1379 1468 CH2 i. ph. twist SO3 i. ph. str. 73 CH2 i. ph. rk. S-O-C o. ph. str. 1130 CH2 o. ph. str. 0.3 CH2 i. ph. str. 1297 CH3 o. ph. str. CH3 i. ph. bend CH2 bend + CH3 o. ph. SO3 bend o. ph. str. 2848 20 H2O bend 2955 40 H2O str. 2918 2850 60 2881 IR %T 3458 Dodecyl sulfate, sodium salt 80 1658 100 0.0 CH bend 812 C-O-C, CCC i. pl. bend 393 364 292 465 664 815 907 S 507 449 608 905 1089 1037 1140 1189 CS2 i. ph. str. 943 xH2O 2731 74 C-C-C O-CC2 o. ph. str. i. ph. str. C-O-C CS2 o. ph. str. o. ph. str. S CH O C - Na 1320 1463 1442 1373 CH3 o. ph. bend CH3 i. ph. bend 1627 CH3 2873 0.2 H2O str. 2978 2936 0.4 1630 1609 CH3 0.6 Ra I 2107 2976 H2O bend CH3 o. ph. str. H2O wag hindered rotation CH3 rk 1345 1327 1189 1168 1140 1104 1052 0.8 H2O 1459 1448 20 3503 40 3203 60 3381 IR %T 3349 Sodium isopropyl xanthate 80 2929 2872 H2O str. 2358 100 567 445 1049 1149 571 974 CS2 i. ph. str. C-N str. + CH3 bend 223 371 299 885 2 965 Zn S 1146 CH3 1245 S N C 1512 1445 1391 2929 0.1 CH3 2859 2800 3013 0.2 438 CS2 o. ph. str. 0.3 979 0.4 Ra I 75 669 1625 1935 2091 CH3 o. ph. str. 1244 H2O str. 1392 20 1439 40 H2O bend 1522 60 IR %T 2928 2857 80 3435 Zinc dimethyl dithiocarbamate 0.0 100 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumber (cm–1) 1200 1000 800 600 400 165 SULFUR AND PHOSPHORUS COMPOUNDS 2-Imidazolidinethione 592 681 920 509 N H 510 N 5 membered ring i. ph. str. 330 2966 2894 C 665 NH bend + CN str. str. 3285 3245 N2C=S str N C N Ra I 0.8 0.4 H S 923 H CH2 NH str. o. ph. 1109 1050 1001 979 1.2 1047 1002 1372 1.6 H 1520 1483 1467 1378 1310 1284 1212 3249 20 1521 CH2 i. ph. str. 1500 1465 40 1310 1277 1207 2885 60 1714 2574 2961 80 IR %T 76 CH2 bend 100 0.0 (CH 2)3C H3 (CH 2)3C H 3 914 1400 1200 252 3000 2500 2000 1800 1600 Wavenumber (cm-1) 213 1000 800 600 400 Tributyl phosphite 964 0.0 3500 272 552 878 774 721 1148 1062 1023 CH2 twist 545 O O PO3 i. ph. str. 833 817 772 718 P 1120 1064 1036 967 906 884 0.4 1264 1464 O 2737 H 3(CH 2)3 PO3 o. ph. str. + P-O-C i. ph. str. P-O-C o. ph. str. + C4O chain str. 1300 1262 1232 Ra 1.2 I 0.8 1450 1.6 CH2 bend + CH3 o. ph. bend 1381 CH2 o. ph. str. CH3 i. ph. bend CH2 i. ph. str. 2875 2.0 CH3 o. ph. str. 2936 2910 20 2936 2874 40 1381 2426 2735 78 60 2960 IR %T 3461 0.0 100 80 Triphenyl phosphine 492 697 500 1571 3 1.0 853 743 O 683 619 P O 1178 1158 1095 1028 Ra I 2.0 O 1584 3049 3.0 1000 Ring Semicircle str. 4.0 5 adj. H wag P-phenyl str. Ring Quad. Str. Aryl CH str. 77 409 1476 1435 40 20 1024 996 1090 Phenyl CH wag summation bands 1178 1308 1281 1815 1768 1667 1582 60 IR %T 1958 1888 3063 3001 80 3443 100 166 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 554 732 901 832 795 1150 1464 1383 1069 1033 P-O-C o. ph. str. + PH wag . 976 1261 (CH2)3 CH3 H 294 395 475 832 775 720 958 907 886 0.4 O P O P PO2 O i. ph. str. O 1150 1121 1063 0.8 CH3 (CH 2)3 CH2 bend + P=O str. CH3 o. ph. CH2 bend twist 1302 1259 1.2 2734 Ra I P-H str. 1453 1.6 1391 2.0 2430 CH3 o. ph. str. O CH3 i. ph. bend CH3 i. ph. str. 2913 2962 40 20 H2O bend 2876 H2O str. 79 1640 2425 60 2875 IR %T 2963 2937 n-Dibutyl phosphite 80 3478 100 80 841 812 686 468 282 417 564 367 319 455 523 830 797 864 702 687 662 81 Si CH3 i. ph def. Si CH3 CH3 2000 1800 Si CH3 rk Si-O-Si o. ph. str. 491 800 1020 1092 CH3 O 710 Si 688 0.4 O 790 0.8 570 910 950 965 926 995 1179 1130 1372 1343 1301 1260 1261 CH3 2966 1.2 867 1.6 1265 2907 0 PS2 o. ph. str. Si CH3 o. ph def. Si CH3 i. ph str. 1413 Si CH3 o. ph str. 2809 20 2963 60 40 PS2 i. ph. str. 994 P-O-C i. ph. str. 1445 1412 1385 H2O bend 1161 1130 1392 1368 1468 1263 P-O-C o. ph. str. 1464 - Na S 1632 O 1721 80 Ra I (CH3)2CH CH2 S P 2906 2851 0.0 100 IR %T O 1604 3374 0.4 (CH3)2CH CH2 2723 1.2 0.8 1621 2725 1.6 1945 Ra I 2963 2901 2.0 CH3 i. ph. bend 2875 2.4 H2O str. CH3 o. ph. str. CH2 bend + CH3 o. ph. bend CH3 i. ph. str. 2960 40 20 H2O bend 2875 3409 80 60 2087 100 IR %T 3114 Siloxane polymer (silicone grease) Sodium di-isobutyl dithiophosphate 0.0 0.0 3500 3000 2500 1600 1400 1200 1000 800 600 400 167 INORGANIC COMPOUNDS 671 1647 H Cl H 1402 NH4 i. ph. str. NH4 bend 1759 1405 2 x 1405 2822 0.4 N 474 3047 0.8 3140 Ra I NH4 o. ph. str. H 2 x 1402 3042 3136 20 1.2 82 H 2806 40 1755 60 2006 IR %T H2O bend H2O str. 2004 80 Ammonium chloride 100 0.0 83 1622 SO4 i. ph. str. 3.0 2.0 SO 4 SO4 o. ph. str. 2 Na 1153 1132 1101 1.0 648 633 621 Ra I SO4 o. ph. bend 994 1130 20 SO4 i. ph. bend 466 451 H2O bend H2O str. 639 40 Sodium sulfate 60 617 IR %T 3434 80 2096 100 0.0 1214 988 1.6 SO3 1200 1000 2 Na SO3 o. ph. bend 639 0.8 SO3 o. ph. str.. SO3 i. ph. bend 498 1.2 SO3 i. ph. str.. 950 2.0 Ra I 970 SO3 i. ph. + o. ph str. 0.4 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 800 600 400 Sodium sulfite 20 2 x 632 84 496 H2O bend 632 + 496 1137 2 x 970 H2O str. 1435 60 40 1622 1931 IR %T 3435 80 496 + 970 632 100 168 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 695 CO3 i. ph. str. 0.5 CO3 o. ph. str. 0.4 Ra 0.3 I 0.2 CO3 o. pl. bend CO3 i. pl. bend 1081 1442 1081 + 702 85 880 1615 1442 + 1081 H2O str. 20 1778 2495 3436 IR 60 %T 40 H2O bend 1429 0.1 702 CO3 2 Na 1769 Sodium carbonate 80 2855 2955 100 0.0 100 86 657 567 87 826 NO3 o. ph. str. 1051 NO3 i. ph. str. 2.0 NO3 o. pl. bend NO3 i. pl. bend 1345 1360 716 NO3 2 K 1.0 226 205 992 1051 + 716 3.0 Ra I 696 1046 1309 1621 1764 3435 1385 + 1051 H2O str. 1385 20 2398 2741 H2O bend 80 IR 60 %T 40 1457 1437 1684 1623 1913 0 100 698 686 660 647 Na O 834 C O CO2 bend O-CO2 o. ph. str. + OH def. 1268 CO2 o. ph. str. O 2535 Ra 60 I 40 H 2906 80 CO2 i. ph. str. 837 1452 100 20 Potassium nitrate 1400 1696 1661 1618 20 1046 1033 1000 1845 2045 1922 2545 3070 IR 60 %T 40 2919 OH str. 3431 Sodium bicarbonate 80 0.0 3500 3000 2500 2000 1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 600 400 169 INORGANIC COMPOUNDS 100 O 3 12 H20 O 0.0 100 590 560 HO OH 1200 600 1165 1400 811 1378 3254 3178 4 2 800 600 0 3500 3000 2500 234 882 BO3 o. ph. str. B 6 Boric acid OH OH str. 500 8 BO3 def. 209 10 Ra I 1194 1449 3217 12 816 BO3 i. ph. str. 20 90 546 40 647 884 60 OH wag + BO3 o. pl. bend 2095 2031 80 IR %T 2362 2262 2631 2516 0 100 547 524 462 1181 1358 1790 2852 2 2402 4 861 O OH wag + PO3 o. pl. bend 505 462 399 2 1131 1066 HO PO3 i. ph. str. 999 Na P 6 Ra I PO3 o. ph. str. O 588 577 P-OH bend 2 O 860 1156 10 1073 20 8 89 1392 1359 1796 40 2454 60 3430 IR %T Involves P-O(H) str. P-OH str + P-OH bend H2O 2853 80 416 Na O 551 H2O 0.8 549 O P 936 948 1.6 PO4 i. ph. str. 1072 1031 1004 993 2.4 3311 Ra I PO4 o. ph. str. 3 Dibasic sodium phosphate 3.2 938 1013 3345 3545 692 40 20 Tribasic sodium phosphate 1423 1384 1566 H2O 60 1664 IR %T 88 H2O 2279 80 2000 1800 1600 Wavenumber (cm–1) 1000 400 170 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 100 MoO4 i. ph. str. 483 445 MoO4 o. ph. str. 60 285 3296 20 324 Na2MoO4 • H2O 40 843 808 834 Ra I 674 639 600 547 904 1402 1560 H2O 80 92 MoO4 o. ph. str. 896 858 899 834 822 20 3305 40 1698 1677 2223 H2O 60 3453 Sodium molybdate 80 1801 0 100 IR %T 657 629 621 NaClO4 • H2O 626 1085 ClO4 o. ph. str. 891 40 20 ClO4 i. pl. bend. ClO4 o. pl. bend. 1149 1098 1089 Ra 80 I 60 953 1135 ClO4 i. ph. str. 120 100 659 941 H2O 20 1108 40 1632 60 3536 3438 Sodium perchlorate 91 H2O 2060 80 IR %T 0 100 468 800 479 1.6 1.2 399 1183 20 969 1872 1970 H2O 1092 40 Si-O-Si o. ph. str. O Ra I 0.4 O 793 • x H2O Si 977 0.8 1000 800 1078 Silica gel IR %T 1636 60 93 H2O 3443 80 x 0.0 3500 3000 2500 2000 1800 1600 Wavenumber (cm–1) 1400 1200 600 400 171 INORGANICS AND POLYMERS 100 60 94 1065 IR %T involves MgO/SiO str. 3675 3661 80 40 Talc 676 665 H2O (hydrate) OH str 944 1051 1015 1.0 3MgO 4SiO2 H2O (Talc) • 792 Ra I • 468 432 362 2.0 291 20 0.0 321 251 529 456 399 842 809 3000 2500 2000 1800 1600 1400 1200 Wavenumber (cm–1) 1000 466 800 600 400 * N Polybutadiene 726 911 964 CHCH2 Cis, Trans 0.0 3500 Isotactic polyethylene 998 973 899 841 809 1037 999 973 899 993 1077 1219 1152 1239 1209 3078 1.0 CH2CH 1011 1668 Ra 2.0 I 96 CH2 def 1438 3.0 1655 3006 4.0 * Trans CH wag 2908 50 Cis CH wag C=C str =CH str. CH2 str 2844 60 * N 1079 70 1304 80 1312 3073 3006 2915 2844 IR %T 1653 1640 0.0 100 90 1167 1455 1460 1436 1360 1330 1.0 CH CH def. 1447 1436 1404 2.0 H2C CH2 def. CH3 o. ph. def CH2 o. ph str. Ra 4.0 I 3.0 * CH3 i. ph. def. CH3 o. ph str. 5.0 CH3 1376 CH3 i. ph str. 80 95 1277 90 2953 2950 2924 2904 2884 2917 2867 2838 2723 2840 IR %T 1359 1304 100 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 916 841 80 620 841 734 0 10 CF2 o. ph.str. * N 3500 3000 2500 2000 1800 1600 1400 Wavenumber (cm–1) 1200 599 575 CF2CF2 1299 * 2 1218 1381 6 387 290 8 0 279 626 637 CF2 def. 1147 20 1202 40 4 231 363 844 99 IR 60 %T Ra I 483 421 856 915 961 947 858 1141 1281 1295 0.0 100 582 536 * N 1237 2940 OCH2CH2 Involves C-O-C i. ph. str. Involves C-O-C o. ph. str. 1479 1.8 1023 1087 1148 1094 1028 1240 CH2 Def. 929 2885 1093 1059 20 1062 CH2 str. 98 1145 2880 40 * Polytetrafluoroethylene (Teflon) 1142 1444 60 1467 2946 2807 2695 * N 1417 1360 1341 1279 1241 3386 0.0 100 CH OH 1607 CH2 2720 * 1.0 Involves C-O str. 1367 2.0 Ra I IR %T 80 1238 1419 1370 1322 CH2 Def. 2909 OH str. 1.0 Polyethylene oxide 1568 1710 2937 CH str. CH2 o.ph.str. 80 CH def. 2906 IR 90 %T Ra I 97 OH def. 3280 Polyvinyl alcohol (PVA) 98 1447 1396 172 1000 800 600 400 IR %T 3450 3429 3296 3077 2932 2859 3069 2977 2949 2882 2925 2938 OH str. CH2 o. ph. str. * 2650 CH2 HO O N O CH 2000 * N N 1228 1167 1182 1102 1073 1017 H N 648 606 554 1231 1167 1105 1004 933 895 844 912 904 819 795 733 646 607 Poly acrylic acid 173 Polyvinyl pyrrolidone 316 100 364 607 506 101 * N 102 H N 400 (CH2)6 N-C=O bend C O 600 NH wag (CH2)4 756 1452 1412 1377 1339 1318 Involves C-O str. C O 800 Nylon 6,6 1456 1372 1315 898 852 1697 CH2 def. * 1000 601 728 1284 1270 1492 1460 934 936 687 1370 1347 1311 1231 1421 1107 1076 1023 1043 952 1426 C=O str. 1274 1200 1180 1141 1494 1456 CH def 1200 1128 1063 1473 1464 1416 1371 1673 1609 1655 CH2 def. 1298 1235 CH2 Involves def. CNH str/open 1400 1382 C=O str. 1535 1441 CNH str/bend 1600 C=O str. 1800 Wavenumber (cm–1) 1550 1475 1671 1631 POLYMERS AND BIOPOLYMERS C CH2 CH CH2 i. ph. str. 2500 1637 100 80 CH2 o. ph. str. 3032 2936 90 70 60 5.0 4.0 1.0 90 0.0 100 80 70 Ra 3.0 I 2.0 IR %T 60 50 40 3.0 1.0 0.0 100 NH str. 2732 90 80 70 3.0 2.0 1.0 3000 2867 2919 Ra 2.0 I IR %T Ra I 0.0 3500 3304 174 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS CH def 2730 479 992 OH N 578 765 717 * O 1262 1211 3395 OH Starch 1380 1339 O O HO 1459 2908 OH 864 O O HO 939 4.0 Ra I 3.0 441 Involves Ring in-phase and C-O str of Semi-circle str. COH/C-O-C OH 361 308 OH str. CH2 def 1.0 927 856 CH def 5.0 2.0 761 706 1241 OH def 60 Starch 1337 1639 H2O 1148 1077 70 104 1410 3309 80 2927 90 50 457 437 Involves C-O str. of COH/C-O-C 0.0 100 IR %T 519 491 1474 N OH Cellulose 568 O O HO 996 970 897 * 1149 1379 1339 1294 HO O O OH 3381 1.0 OH 2895 Ra 3.0 I 2.0 898 Ring semi-circle str. OH 4.0 252 5.0 1120 1095 1054 1030 OH def 379 CH2 def OH str. 1123 1085 1054 Cellulose 40 663 H2O 60 50 103 1427 1369 1315 1281 1202 1160 1105 70 2939 2894 80 3334 3274 IR %T 1642 Involves C-O str. of COH/C-O-C 90 0.0 CH2CO2 Sodium Carboxyl Methyl Cellulose 3000 2500 2000 699 Involves C-O str. 0.0 3500 579 N OR R = H or 700 Ra I 1.0 916 1418 1374 1328 1263 * O * 1451 2900 RO O Ring semi-circle str. 477 430 356 302 CO2 i.ph.str. OR 105 904 1115 1100 1048 1021 1413 1590 CO2 o.ph.str. OH str. 1611 80 2.0 1322 1266 Involves C-O str. of COH/C-O-C 2878 3223 IR %T 90 3392 Sodium carboxymethyl cellulose 98 1800 1600 1400 Wavenumber (cm–1) 1200 1000 800 600 400 175 POLYMERS AND BIOPOLYMERS 100 458 701 1267 1201 946 891 854 1455 1369 1.0 Hydroxypropylmethyl cellulose 1051 Involves C-O str. of COH/C-O-C OH 2837 Ring in-phase and Semi-circle str. 290 CH def CH3 1030 2935 2893 1100 N CH2CH 3438 Ra I R = H or 1152 1122 OR CH3 or 60 2.0 CH2 def O * OH str. 1191 * O 70 1453 H2O OR RO 944 80 1373 1313 2967 2899 2836 3451 IR %T 1636 106 90 CH2 def N 438 388 882 823 699 1458 1486 2815 2716 251 Involves C-O str. of COH/C-O-C CH2CH3 915 OR Ring in-phase and Semi-circle str. 1115 2932 2876 1101 1054 * O * R = H or Ra 1.0 I 917 882 817 1484 1444 OR RO O 2975 2.0 107 Ethyl cellulose CH2 (O) i.ph. str. 1375 1310 1280 1203 H2O 1159 CH3 o.ph. str. of OCH2CH3 OH str. 1403 1371 1354 1270 IR 90 %T 80 1636 2974 2928 2871 3471 0.0 100 0.0 Si-O-Si o. ph. str. Si CH3 * 3113 Siloxane polymer N 1411 2.0 O 643 Si CH3 709 O 790 * 867 3.0 Si CH3 rk CH3 1263 2965 CH3 1.0 842 Si CH3 i. ph. def. Ra 4.0 I 492 2906 1261 CH3 str. o. ph. str. 754 687 1413 Si CH3 o. ph. def. o. ph. str. 20 1089 1029 40 108 800 2962 1593 2904 80 IR %T 60 5.0 1942 100 0.0 3500 3000 2500 2000 1400 1800 1600 1200 Wavenumber (cm–1) 1000 800 600 400 8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 100 109 783 1177 1124 483 639 843 774 999 1325 1456 1621 Amide C=O str. CH3 (N+) o. ph. bend 953 1270 NH2 str. 1341 1730 1673 Ester C=O str. NC4 o.ph. str. + CH3 rk 1452 1733 CH3 (N+) o.ph. str. NC4 i.ph. str. 454 1485 CH2 o. ph. str. 720 O 1671 3013 2960 3379 NH2 str. + H2O OH str. 877 H 2N 955 O 874 849 O 110 C CH 3 0.20 Ra I 0.10 1108 1451 1419 1349 1323 C 2830 0.30 O 1 H3 C 60 20 H 2N Involves Ester C-O str. CH 1057 N CH 2 1166 C 2H 4 9 1144 1084 1039 H3 C CH 1453 1390 1341 1256 CH 2 Cl 80 40 CH C C-C-N i. ph. str. 1210 3359 NH2 bend 100 IR %T CH 2 C-N str. 0.00 3022 2970 2931 Poly-2-(acryloyloxy)-Ethyltrimethyl Ammonium Chloride (AETAC) 0.10 Ra I 0.05 C=O str. NH2 bend NH2 wag NH2 rock CH2 bend CH rk 1424 NH2 i. ph. str. 0.15 1663 NH2 o. ph. str. CH (C=O) str. 1658 1610 0.20 CH2 o. ph. str. 2925 20 3199 3344 40 3208 Poly acrylamide (PAM) 60 IR %T 2858 2932 2858 80 379 176 0.00 4000 3400 3000 2500 2000 1800 1600 1400 Wavenumber(cm-1) 1200 1000 800 600 400 200 C H A P T E R 9 Unknown IR and Raman Spectra On the following pages, 44 unknown spectra are presented without labels or interpretation to help test and develop the readers knowledge and understanding of functional group analysis and spectral band assignment. The spectra are found on pages 178 to 199. The sample preparation used to acquire the spectra is provided and where appropriate hints are included to help guide the interpretation. For interpretation and an explanation of the key features useful to help identify each compound an answer key is included at the end of the unknown spectra starting on page 200. 177 178 9. UNKNOWN IR AND RAMAN SPECTRA 903 862 1257 80 2793 2660 2612 3154 100 Neat/cap film 60 1450 IR %T 40 20 801 2917 2852 1 5.0 4.0 1157 1266 1347 2665 1.0 1444 2.0 426 384 3.0 1028 2938 2852 Ra I 0.0 811 984 919 1168 80 2720 3182 100 1468 40 2957 2903 2871 20 2 2873 0 809 8.0 2959 6.0 469 0.0 3500 3000 2500 2000 1800 1600 1400 Wavenumber(cm–1) 1200 1000 800 600 400 249 420 956 921 870 1036 1158 2.0 1346 1320 1252 2719 1464 1452 Ra I 4.0 306 IR %T Neat/cap film 1386 1367 2840 60 10 5 3500 3000 2500 2000 1800 Wavenumber (cm–1) 0 1600 1400 1200 1000 800 413 100 600 302 545 722 938 531 768 855 400 259 418 376 495 693 887 1059 1019 971 1211 1160 886 1446 1649 1374 565 1057 1220 1299 1782 2728 522 433 60 220 796 816 1020 1111 1202 1293 1429 1413 1651 2970 3075 2938 2853 100 503 655 644 847 925 1111 1268 1476 1461 1393 1363 1646 1788 6.0 1269 1218 1194 20 1516 60 2728 2985 2913 Ra I 4.0 1897 2.0 3077 20 1463 1449 1378 40 2905 2869 80 3057 3094 3024 80 1618 1575 2963 40 2923 2876 2907 2779 2713 IR %T 3065 IR %T 3016 2963 UNKNOWN IR AND RAMAN SPECTRA 179 Neat/cap film 3 0.0 Neat/cap film 4 15 Ra I 1.0 3500 3000 2500 2.0 2000 1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 600 379 529 1386 1343 558 912 872 840 767 741 1096 1049 1261 1232 1627 1581 0.0 400 252 478 437 409 860 567 511 1076 1021 978 1163 1464 1428 1623 701 1376 771 790 408 1268 1214 1165 566 732 700 855 1076 1020 977 533 1398 1509 1463 1439 1597 1800 1745 1690 1924 100 559 766 841 1427 40 870 4.0 1464 60 916 3.0 2294 0 15 1097 1049 5.0 2250 40 1329 1262 1232 20 2750 60 3098 3068 3048 2968 2944 2861 80 1453 1431 Ra I 2250 IR %T 2880 80 2841 2751 5 3012 3059 2909 2860 Ra I 2973 2931 2941 2881 IR %T 3635 3544 180 9. UNKNOWN IR AND RAMAN SPECTRA Neat/cap film 20 5 10 100 0 Neat/cap film 6 1.0 3500 3000 2500 2000 1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 0.0 600 400 276 398 0.0 100 289 513 480 888 854 806 755 686 1143 1094 1033 2235 374 Ra 1.5 I 355 488 660 738 952 901 847 1073 1046 1030 1011992 1114 1449 1436 1381 1332 1260 2301 804 739 887 1181 1150 1072 1032 1464 1456 1436 1380 1339 1279 2053 2472 2738 2669 3177 3456 100 454 515 Ra 2.0 I 749 2.8 828 40 1114 1072 1030 966 902 880 810 1.0 2874 1.8 2964 2920 2935 2873 IR 60 %T 1298 80 1433 1379 1339 1295 1252 1216 IR 60 %T 1466 2738 0.5 2962 20 2960 2937 2908 2876 29342874 40 1253 1222 1450 20 3332 80 2736 3330 UNKNOWN IR AND RAMAN SPECTRA 181 Neat/cap film 7 Neat/cap film 8 182 100 80 60 IR %T 40 20 3.0 1.0 0.0 100 80 60 40 20 28 20 Ra 2.0 I IR %T Ra I 10 0 3500 3548 3327 3326 3000 3169 30643088 3030 2931 2873 3059 3006 2938 3004 2959 2930 2873 3003 2733 2635 2500 2427 2415 2000 1800 1600 1400 1200 Wavenumber (cm–1) 9. UNKNOWN IR AND RAMAN SPECTRA 2736 2937 2913 2875 1952 1876 1810 1751 1671 1672 1607 1586 1602 1461 1454 1496 1458 1438 1379 1457 1440 1369 1303 1209 1177 1157 1000 1030 1301 1289 1233 1209 1003 1021 1094 1043 1011 1080 1038 912 800 815 799 752 780 735 698 409 738 648 641 533 508 468 420 9 10 375 337 Neat/cap film 400 461 Neat/cap film 600 595 485 1003 1131 916 890 892 859 816 621 595 969 1091 1043 1347 1300 1232 183 UNKNOWN IR AND RAMAN SPECTRA 60 H2O 3433 IR %T 809 80 KBr disc 1467 H2O 1630 100 474 40 1111 20 0 2.0 11 Ra I 1.0 0.0 100 864 80 702 687 662 2906 1445 1412 1385 Smear 800 12 1.8 491 2907 1261 20 1092 40 1020 IR %T 2963 60 688 790 867 2809 1413 0.5 1265 2966 1.0 3114 Ra I 710 1.5 0.0 3500 3000 2500 2000 1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 600 400 1.0 3500 3000 2500 2000 1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 488 435 853 1015 1128 1217 1305 Ra I 835 874 1121 40 1255 889 614 1083 1048 1290 1367 1455 1977 2 288 1071 1029 1227 1366 1332 1488 1451 2719 4 2963 2876 914 1070 2975 2860 20 1444 80 2754 2694 40 1399 60 2962 2916 911 658 1241 1181 1365 1460 1967 2682 80 2719 IR %T 2855 Ra I 2967 2855 IR %T 2889 184 9. UNKNOWN IR AND RAMAN SPECTRA 100 Neat/cap film 60 13 8 6 100 0 Neat/cap film 20 14 3.0 2.0 0.0 600 400 0.4 0.2 3500 3000 2500 2000 1800 1.0 0.6 Wavenumber (cm–1) 1600 1400 1200 436 378 711 897 1000 100 800 600 261 212 443 614 553 758 785 Ra I 656 871 1105 1303 1248 1174 1154 1038 1022 996 1248 755 692 1041 1601 40 0.0 400 279 508 60 1050 1026 1003 987 70 712 1087 80 1337 1316 1250 1205 1160 20 898 1.2 1118 IR %T 1422 1498 Neat/cap film 1456 1601 1588 3072 20 1379 1339 1260 1642 1794 1733 3007 2944 2837 3172 1077 784 1173 1303 553 511 883 1336 1701 1840 2030 2548 2489 3092 3063 3003 2956 2836 1468 1442 1588 60 1470 0.8 1604 Ra I 2895 90 2914 2899 2857 80 3296 3331 IR %T 3416 UNKNOWN IR AND RAMAN SPECTRA 185 100 15 10 0 ATR of paper: ID Components 16 1.4 1.0 3500 3000 2500 2000 1800 Wavenumber (cm–1) 1600 1400 1200 1000 800 478 431 411 340 3.0 1258 831 60 942 883 1131 1113 1044 1004 477 431 794 765 733 1258 1465 1432 1410 1380 1483 386 846 1010 1202 1159 1296 1380 1451 2.0 988 1664 Nujol 831 795 765 734 2.0 883 Neat/Cap film Nujol 1167 1132 1111 1043 1007 942 20 2256 1456 1626 1716 1662 1838 2636 2720 921 772 722 538 517 851 1306 1245 1209 1156 1097 1071 1378 1362 60 1303 40 1484 80 2736 2607 Nujol mull 1451 1412 Ra I 3277 3187 3024 3000 20 2925 2934 2875 2876 2997 2916 2867 80 2739 0 2998 IR %T 3046 Ra I 40 2963 IR %T 3049 186 9. UNKNOWN IR AND RAMAN SPECTRA 100 17 1.0 100 0.0 18 Hint: Ether 0.0 600 400 3500 3000 2500 2000 Wavenumber (cm–1) 1800 1600 1400 1200 1000 577 Ra I 440 478 1155 1080 1017 538 896 1071 1156 1261 800 600 253 415 689 2.0 438 575 528 861 766 709 930 1243 Ra I 357 309 1126 940 865 1049 1264 1607 1591 672 764 880 1000 1604 1153 1089 1065 997 1320 1297 1243 1489 1474 1441 838 1203 1587 20 766 709 2.0 1461 1420 1367 60 1641 80 2081 3339 3262 1538 1703 1924 2794 2674 3064 60 1381 1338 40 2930 3078 3063 80 1460 20 2910 IR %T 40 3421 IR %T 3423 UNKNOWN IR AND RAMAN SPECTRA 187 100 ATR 19 4.0 100 0.0 KBr disc 20 4.0 0.0 400 3500 3000 2500 2000 1800 Wavenumber (cm–1) 1600 1400 1200 1000 626 5 655 650 622 462 923 1046 1012 2.0 926 80 800 474 100 600 323 590 873 818 772 724 960 1113 1057 1169 1302 1453 1415 1716 2875 1168 1358 29212960 2935 1466 1412 60 1011 1444 H2O 1452 4.0 2876 Neat/Cap film 1714 2736 20 3003 2934 2854 40 1415 1343 1580 60 2929 IR %T 3011 IR %T 589 540 475 723 896 994 1111 1259 1231 3628 3417 80 3433 188 9. UNKNOWN IR AND RAMAN SPECTRA 100 21 Ra I 0.0 KBr disc 40 20 22 Ra I 10 0 400 2.0 3500 3000 2500 339 6.0 2000 1800 Wavenumber (cm–1) 1600 1400 1200 1000 800 415 Ra I 4.0 631 20 892 857 Neat, cap film 1247 630 929 893 861 1084 1060 1159 1368 273 211 1066 1045 981 1321 1265 1445 1428 353 726 678 655 2969 2916 1265 20 953 60 1462 1451 1384 2915 1451 1437 2872 1061 970 955 1375 2835 654 784 758 1320 80 1161 1126 1061 40 2556 4.0 2928 6.0 2875 2835 2730 40 2573 2.0 2969 8.0 2809 2722 60 1449 1385 1316 1247 2721 IR %T 2925 2866 Neat, cap film 2963 80 3204 Ra I 2927 2866 2912 IR %T 2968 UNKNOWN IR AND RAMAN SPECTRA 189 100 23 Note: compound contains S 100 0.0 600 24 Note: compound contains S 0.0 400 3500 3000 2500 2000 1800 Wavenumber (cm–1) 1600 1400 1200 0 1000 800 391 349 10 282 1071 810 700 679 1309 60 649 751 1134 1182 40 977 1004 Neat/Cap film 1280 911 831 2 1456 514 245 528 Note: compound contains F 297 557 1079 1160 1265 1251 1490 1445 1381 777 728 1004 1142 1265 1489 Neat/Cap film 682 925 1251 1619 1590 20 1180 1137 1101 1071 Ra I 8 1619 1595 444 887 1079 1041 1004 1743 1930 3040 2956 2923 2866 528 728 857 1459 40 1353 20 1354 IR %T 60 1627 4 3079 3061 2967 2923 2869 80 1628 1607 80 3086 Ra I 3090 IR %T 3047 190 9. UNKNOWN IR AND RAMAN SPECTRA 100 25 10 6 100 0 26 Note: compound contains F 600 400 3500 3000 2500 2000 1800 Wavenumber (cm–1) 1600 1400 1200 1000 800 431 800 600 280 545 Note: compound contains Cl 458 597 885 Neat/Cap film 381 596 544 859 800 1075 1204 1147 457 430 720 1434 815 751 1025 1195 1163 338 271 530 665 617 1072 1433 1372 1595 1007 1217 1194 1765 750 692 926 1163 1371 1493 1594 1743 1431 3071 3044 2939 664 594 530 499 815 892 1070 1026 1013 60 885 1076 10 1204 20 1149 40 1369 60 1264 Ra I 1764 Neat/Cap film 1575 1547 IR %T 1767 3071 40 1575 1548 1796 80 2939 5 1797 1765 3184 Ra I 2530 3082 IR %T 3146 3083 80 3470 100 3550 UNKNOWN IR AND RAMAN SPECTRA 191 H2O 20 27 10 0 100 28 0 400 1.0 3500 2.0 3000 2500 2000 1800 Wavenumber (cm–1) 1600 1400 0.0 1000 800 600 351 243 1056 1058 535 477 607 1246 725 662 795 100 446 534 1200 612 1213 5.0 1201 1187 Note: Compound contains S and is a monohydrate 798 Nujol 893 20 1377 1309 1272 1450 1601 1272 237 400 652 960 894 859 834 1164 1124 1384 1580 1.0 1022 3.0 1727 2.0 1625 1040 1123 1073 1287 1274 1729 3071 3439 705 771 959 652 1600 1580 1488 1040 1381 743 1463 2873 2861 60 1304 1290 1195 1178 1116 60 1461 80 2233 2096 2734 3.0 2898 2960 2937 2931 2875 Neat/Cap film 1459 1420 1441 Ra I 40 2855 Ra I 3073 20 2731 40 2929 2889 IR %T 2974 IR %T 3544 3484 80 3475 192 9. UNKNOWN IR AND RAMAN SPECTRA 100 29 0.0 Nujol mull Nujol 30 4.0 400 2 3500 3000 2500 4 2000 1800 1600 1400 Wavenumber (cm–1) 1200 1000 800 288 232 649 600 440 362 842 405 542 996 622 1296 727 558 839 776 916 1225 1460 1445 1382 1341 580 905 983 1153 1380 1358 1280 845 773 753 1055 1040 463 1445 397 1457 2922 2876 3367 3291 3193 1045 975 1181 1150 1377 1356 1611 921 861 809 769 1461 60 772 6 914 1025 KBr disc 1021 20 1150 1112 1051 40 1119 1074 60 1148 80 1272 1345 Ra I 0.8 2936 2962 1.2 2735 2963 Neat cap film 2877 20 1460 1403 1371 IR %T 40 1630 0.4 2945 2914 2892 2797 2695 2542 3314 Ra I 3378 IR %T 2960 2946 2892 3410 3315 80 3398 3285 UNKNOWN IR AND RAMAN SPECTRA 193 100 31 0.0 100 32 8 0 400 3500 3000 2500 2000 1800 1600 1400 Wavenumber (cm–1) 1200 1000 695 ATR 800 410 3.0 1002 754 60 622 794 761 1032 1183 1156 906 1181 1154 1070 1028 631 389 1033 1152 1296 100 0 40 34 Ra I 2.0 0.0 600 400 224 322 813 775 934 949 1198 285 566 470 1450 866 770 580 888 1027 619 811 935 1603 1759 3067 1307 1482 1396 1363 1231 1246 1214 1152 1120 1432 Nujol mull 1372 1604 1590 1472 1451 1379 40 1331 Impurity 1493 1452 1601 1727 2915 60 1452 1.0 2715 2968 6 1603 IR %T 4 3070 3001 80 1584 80 3060 3026 2923 2851 2 3054 Ra I 2907 2854 20 3626 IR %T 3626 194 9. UNKNOWN IR AND RAMAN SPECTRA 100 33 8 1.0 3500 3000 2500 2000 1800 Wavenumber (cm–1) 1600 1400 1200 1000 881 800 731 837 600 281 621 Smear 1032 20 1082 1004 40 1218 H2O 728 705 1457 781 932 883 621 1008 1084 1218 1419 1378 403 896 879 834 1059 1135 1299 280 2963 2937 2874 2958 1444 2928 2873 1131 1464 2809 735 1206 1337 1302 961 898 1096 1378 60 1159 60 1302 80 1466 1488 1.0 1709 2.0 2811 2734 Ra I 1442 3002 Neat cap film 2854 3.0 2924 2955 40 1605 1586 2732 2925 2891 2852 Ra I 3061 IR %T 3287 80 3062 20 3326 IR %T 3378 UNKNOWN IR AND RAMAN SPECTRA 195 100 35 0.0 100 36 2.0 Note: Quaternary amine salt 0.0 400 3500 3000 2 2500 2000 1800 1600 Wavenumber (cm–1) 1400 1200 1000 800 6 250 538 722 697 755 458 862 996 600 297 466 777 1003 1298 1452 936 384 891 1076 1152 1373 620 4.0 292 Note: Contains P 1003 H2O 685 618 1030 1312 1485 1651 1654 1556 1296 1440 1373 602 1152 3086 2976 2936 2877 465 721 1041 1003 936 80 392 4 1180 20 1191 1121 40 1439 60 1096 KBr disc 1624 1591 2.0 2879 2935 Neat cap film 1593 Ra I 2976 3290 40 1575 Ra I 2741 3289 20 3056 2991 60 2963 80 3060 IR %T 3145 IR %T 3435 196 9. UNKNOWN IR AND RAMAN SPECTRA 100 37 0.0 100 H2O 38 0 400 2.0 3500 3000 2500 2000 1800 Wavenumber (cm–1) 1600 1400 1200 1000 800 237 768 715 815 Neat cap film 832 100 666 1073 1029 741 692 968 600 439 532 670 336 825 805 752 1003 1167 1222 1091 1284 0.0 1297 Note: Contains Boron 1149 1120 1070 1031 962 904 80 1260 1231 1150 1118 60 1591 959 1555 3081 3042 2926 511 877 835 772 750 695 626 1223 1171 1129 1071 1615 1425 1359 1310 1283 KBr disc 1299 6.0 1417 0.4 1616 1589 1557 1483 1444 1378 1.6 1485 1431 1466 20 1337 0.8 1450 40 2910 2874 2926 1.2 2909 20 2959 2935 1926 2145 80 1262 Ra I 3279 60 1483 2736 2937 2875 IR %T 2963 Ra I 40 3076 3048 2963 IR %T 3416 UNKNOWN IR AND RAMAN SPECTRA 197 100 39 40 Note: Contains Boron 4.0 0.0 400 3500 3000 2500 2000 4 1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 600 429 707 656 346 856 559 636 750 1118 420 223 661 619 800 1181 1160 1341 1314 1367 1603 Ra I 10 1330 1574 2925 703 996 1442 1368 1381 1350 1309 Nujol Mull 1119 1367 1477 1408 3057 20 1337 60 1635 3144 2994 689 1603 2854 580 1494 3078 3025 1088 1026 760 1180 1456 60 961 8 1573 KBr disc 1738 1726 80 2920 2852 2752 40 1710 1660 Ra I 40 3175 IR %T 1801 20 3331 IR %T 3317 80 3162 198 9. UNKNOWN IR AND RAMAN SPECTRA 100 41 Note: Purchased as Phenyl Boronic Acid with an HPLC assay of 97% 0 100 42 12 0 400 4 3500 3000 2500 2000 1800 Wavenumber (cm–1) 1600 1400 1200 8 1000 800 600 388 378 583 676 617 584 463 814 767 732 1027 1195 758 927 348 998 1089 1193 1569 1509 1447 1384 2921 284 1265 1236 6 780 984 1650 1632 1550 1467 1437 60 2199 2865 2747 661 626 827 1383 1232 762 926 669 1302 1087 995 978 1304 1265 1535 1824 1579 2486 2745 2646 3091 3029 1473 1444 Nujol mull 1443 16 3120 2981 40 1556 Ra I 4 1659 20 2684 IR %T 40 2925 2853 2 3130 20 3123 Ra I 3470 3419 3334 IR %T 3469 3419 3330 UNKNOWN IR AND RAMAN SPECTRA 199 100 80 60 43 8 Note: Nitrogen 5 member ring aromatic heterocycle 0 100 KBr disc 80 44 Note: Nitrogen 6 member ring aromatic heterocycle 12 0 400 200 9. UNKNOWN IR AND RAMAN SPECTRA 1. Cyclohexane: 2930, 2855 CH2 (note no 2960 CH3), 1452 CH2 (note no 1378 CH3), (note no 723 CH2 chain). 2. 2,4-dimethyl pentane: 2965 CH3 o.ph. str., 2940e2850 CH3 and CH2 str., 1387, 1369 C(CH3)2 i.ph def, (note no 723 CH2 chain). CH3 CH3 CH3CHCH2CHCH3 3. 2,5-dimethyl-1,5-hexadiene: 3080, 2980 ¼CH2 str., 2950-2860 CH3 and CH2 str., 1780 2x890, 1650 C¼C str., 1450 CH2 and CH3 def, 890 R2C ¼CH2 CH wag. CH3 H3C H2C C C CH2 CH2 H2C 4. 4-tert-butyltoluene: Aliphatic group: 2965, 2876 cm1 CH3 str. 1470e1460 cm1 CH3, 1392, 1363 cm1 C(CH3)3, 1375 cm1 CH3. Aromatic group: 3100e3000 cm1 aromatic CH str., 1517, 1407 cm1 para aromatic ring, 817 cm1 2 adj aryl H. H3C C(CH3)3 5. 1-methyl naphthalene: 3100e3000 arom CH, 2975e2860 CH3, 1597, 1510, 1398 aromatic ring, 1462, 1378 CH3, 790 3 adj aryl H, 772 4 adj aryl H. CH3 UNKNOWN IR AND RAMAN SPECTRA 201 6. n-Butyronitrile: 2970 CH3, 2250 ChN str., 1460 CH3, CH2 def, 1425 CH2(eChN) def 1375 CH3 i.ph. def. H2C H3C C N CH2 7. 2-Hexyne: Aliphatic group 2964/2873 cm1 CH3 str., 2935, 2920 cm1 CH2 str., 1464e 1450 cm1 CH3/CH2 def, 1380 CH3 i. ph. def., 1339 CH2 wag. Acetylene group, ChC str in Raman at 2301 and 2235 cm1 (Fermi resonance), 375 cm1 CeChC bend (Raman). H2C H3C C C CH3 CH2 8. n-Butanol: Aliphatic group: 2960, 2874 cm1CH3 str., 2935 CH2 str., 1470e1450 CH3, CH2 def, 1378 CH3 i.ph def. Hydroxy group (primary alcohol): 3300 cm1 OH str., 1420 broad OH bend, 1070e1025 involves CeO str of CH2eOH, w660 cm1 (broad) OH wag. H2C HO H2C CH3 CH2 9. Trans-2-Hexen-1-ol: Aliphatic group: 2959, 2873 cm1CH3 str., 1458 CH3, CH2 def, 1379 CH3 i.ph def. Olefinic group: 3004 cm1 CH str., 1672 cm1 C ¼C str., 1300 trans CH rk, 969 cm1 (IR) trans CH wag. Hydroxy group (primary alcohol): 3327 cm1 OH str., 1091, 1043, 1091 cm1 involves CeO str., w650 cm1 (broad) OH wag. H2 C H3C C H2 H C CH2OH C H 10. Benzyl alcohol: 3330 OH, 3100e3000 arom CH, 2925, 2870 CH2, 1600, 1493 arom ring, 1452 CH2/arom ring, 1420 broad OH, 1015 CH2eOH, 735 5 adj H, 696 arom ring. OH CH2 202 9. UNKNOWN IR AND RAMAN SPECTRA 11. Silicon dioxide: SiO2 1130e1000 IR strong (involves OeSieO o. ph. str.). Note:featureless Raman spectrum. 12. Dow Corning vacuum grease, dimethyl siloxane polymer: 2966e2963 CH3 o. ph. str., 2907e2906 CH3 i. ph. str., w1412 CH3 o. ph. def, 1261 CH3 i. ph. def, 1090e1020 IR involves SieOeSi str. CH3 * O CH3 Si O Si CH3 CH3 * N 13. Tetrahydrofuran: Aliphatic 2975e2860 suggests only methylene. Two bands at 1070 and 911 cm1 suggest ether (CeOeC out-of-ph. and in-ph. str., respectively). O 14. 1,4-Dioxane: Aliphatic 2955e2850 CH2, 1450e1442 CH2, 1362 O CH2 wag, 1120 cm1 involves CeOeC o. ph. str. 900e830 cm1 involves CeOeC in-phs. str. O O 15. Anisole: 3100e3000 aromatic CH, 3000e2838 CH3(O), 1602, 1588, 1498 cm1 aromatic ring, 1468, 1452, 1440 cm1 CH3(O) þ aromatic ring, 996 cm1 (Raman) 2,4,6 C Radial i.ph.str., 1247, 1040 arom-OeCH3, 752 cm1 5 adjacent aryl H, 690 cm1 aromatic ring. CH3 O 16. Paper: Mostly cellulose and carbonate, some water, with a trace of organic sizing: carbonyl and olefinic. 17. Trans, trans-2,4-Hexadien-1-ol: Olefinic group, 3024, 3003 cm1 CH str., 1672 cm1 C ¼C str., 1300 cm1 trans CH rk, 988 cm1 (IR) trans CH wag (note higher frequency due to adjacent trans, trans olefin). Hydroxy (primary alcohol) group. 3277, 3187 cm1 OH str., 1097, 1071, w1000 (shoulder) cm1 involves CeO str. UNKNOWN IR AND RAMAN SPECTRA 203 OH 18. 1,2-Epoxypentane: Aliphatic CH str with epoxy CH2 o. ph. str at ca. 3050 cm1 and CH2 def. at 1483 cm1. Epoxy CeOeC in-ph str. At 1258 cm1 (both IR and Raman) and CeOeC out-of-ph str at 831 cm1. Aliphatic peak intensity of methylene (2935 cm1) to methyl (2875 cm1) consistent with ca. 2 methylene to 1 methyl group. CH2CH2CH3 O 19. Meta (3) chlorophenol: 3360 cm1 OH str., 3100e3000 aryl CH str (note no aliphatic (CH2, CH3) 3000e2800 cm1, 1605, 1592, 1478, 1448 cm1 aryl ring, 1000 cm1 (Raman) 2,4,6 C Radial i.ph.str., 1245e1190 cm1 involves aryl-OH str, 852 lone H, 772 3 adj H, 680 aryl ring. Cl HO 20. Dextrin: Polysaccaride/carbohydrate: 3421 cm1 OH str., 1338 cm1 OH def., 1155, 1080, 1017 cm1 involves CeO str. of CeOH/CeOeC polysaccharide ring system. Axial connection similar to starch result in characteristic IR bands 930, 861, 766, and 709 involving the ring in-phase and semi-circle str. The cluster of Raman bands centered around the intense 478 cm1 band also is diagnostic of the axial type connection. Presence of water is evident from bands at 2081 (combination) and 1641 cm1 (def.). Biopolymeric aliphatic: CH2 and CH str bands between 2930 and 2820 cm1 and CH2 def at 1460 cm1. HO O O * OH n * OH 204 9. UNKNOWN IR AND RAMAN SPECTRA 21. 2-Hexanone: Ketone C ¼O str 1716 cm1and C ¼O overtone at 3417 cm1. 1412 cm1 from CH2e(C ¼O) and 1358e1380 cm1 from CH3 i.ph def. Aliphatic group: CH2 and CH3 stretches at 2935, 2875 , and 2960 cm1, respectively. Peak intensity of methylene (2935 cm1) to methyl (2875 cm1) consistent with ca. 3 methylene to 2 methyl. No evidence of branching in Raman spectra. O 22. Sodium acetate: Simple IR and Raman spectra. Dominant IR bands at 1580 and 1444 cm1 indicate a carboxylate salt (CO2 out-of-phase and in-phase stretch, respectively). O O Na 23. Ethyl methyl sulfide: Aliphatic group. The CH2eS has bands at 2928 cm1 from the CH2 o.ph.str., 2872e875 cm1 from the CH2 i.ph.str., 1265 cm1 (IR) from the CH2 wag and 1437e1428 cm1 from the CH2 def. The CH3eS has bands at 2969 cm1 from the CH3 o.ph.str., 2915 cm1 from the CH3 i.ph.str., the 1375 cm1 is from the CH3 i.ph.def. and bands between 1070 and 950 cm1 involve the CH3 rock þ CeC str. Bands involving the CeSeC str are observed between 780 and 650 cm1. H3C CH2CH3 S 24. 2-Propanethiol: SH group: SH str results in characteristic band at 2550e2580 cm1. Aliphatic group: The isopropyl group has characteristic bands at 2963 cm1 (CH3 o.ph.str), 2866 cm1 (CH3 i.ph.str), 1462e1451 cm1 (CH3 o. ph. def.), and in the IR spectrum at 1384, 1368 cm1 (CH3 i.ph. def). The CHeS group has bands at 2912e2930 cm1 (CH str.) and 1247 cm1 (CH wag). The isopropyl CeC/CeS str results in a strong Raman band at 631 cm1 involving the in-phase (C)2CeS str. H3C CH H3C SH UNKNOWN IR AND RAMAN SPECTRA 205 25. 3-Fluorotoluene (1-fluoro-3-methylbenzene): Aliphatic (CH3) 2967e2866 cm1 methyl CH3 str., 1459 cm1 CH3 o. ph. def, 1381 cm1 CH3 i. ph. def. Aromatic group: 3100e3000 cm1 aryl CH str., 1619e1590 cm1 aryl ring quadrant str. (IR/Raman), 1489 cm1 (IR) aryl ring semi-circle str., 1004 cm1 (Raman) 2,4,6 C Radial i.ph.str., 857 cm1 lone H wag, 777 cm1 (IR) 3 adj H wag, 682 cm1 (IR) ring pucker. Aryl-F results in IR/Raman bands at 1265, 1251 cm1 involving the Aryl-F str. F CH3 26. 1,3-Bis(trifluoromethyl)benzene: Note no aliphatic observed. Aromatic group: 3100e3000 cm1 aryl CH str., 1628e1607 cm1 aryl ring quadrant str. (IR/Raman), 1004 cm1 (Raman) 2,4,6 C Radial i.ph.str., 911 cm1 lone H wag, 810 cm1 (IR) 3 adj H wag, 700 cm1 (IR) ring pucker. CF3 group: 1354, 1309, 1280 involves CF3 o. ph. str., 1181, 1134 cm1 involves CF3 i. ph. str. F3C CF3 27. Phenyl acetate: Aromatic group: 3100e3000 cm1 aryl CH str., 1594 cm1 aryl ring quadrant str. (IR/Raman), 1493 cm1 (IR) aryl ring semi-circle str 1007 cm1 (Raman) 2,4,6 C Radial i.ph.str., 750 cm1 (IR/Raman) 5 adjacent H wag, 692 cm1 aryl ring pucker. Acetate group: 1765e1740 cm1 C ¼O str., 1431 cm1 (O)CH3 o. ph. def., 1371 cm1 (O)CH3 i. ph. def., 1217e1163 cm1 involves feOeCeC o. ph. str., 815 cm1 involves feOeCeC i. ph. str.. 2939 cm1 CH3 o. ph. st. O C H3C O 206 9. UNKNOWN IR AND RAMAN SPECTRA 28. 2,4,6-Trichlorobenzoyl chloride: Aromatic group: 3146e3082 cm1 aryl CH str., 1575e1547 cm1 aryl ring quadrant str. (IR/Raman), 1434 cm1 (IR) aryl ring semi-circle str. The band at 800 cm1 derives from the lone H wag and the band at 720 cm1 involves the aryl ring pucker. The band at 1075 cm1 (IR/Raman) involves the aryl-Cl str. Aromatic acid chloride group: 1796 cm1 C ¼O str. and the weaker band at 1767 cm1 involves the overtone of the 885 cm1 that is Fermi resonance enhanced with the carbonyl. The 885 cm1 and 1204 cm1 involves the (aryl)CeC(¼O) str. Cl O Cl Cl Cl 29. Dioctyl phthalate: Aliphatic group: CH2 stretches at 2931, 2873 cm1 and CH3 stretches at 2960 and 2861 cm1, respectively. Peak intensity of methylene (2931 cm1) to methyl (2960 cm1) consistent with ca. 4 methylene to 2 methyl. Methylene and methyl deformation bands are observed at 1463 and 1381 cm1. Phthalate Carbonyl Group: C ¼O str (IR/Raman) at 1727 cm1, band cluster between 1290 and 1270 cm1 involves CeCeO o. ph. str. and the band at 1040 cm1 involves the CeCeO i. ph. str. The band at 1123 involves the OeC str of the aliphatic group. Aromatic group: 3100e3000 cm1 aryl CH str., 1601e1580 cm1 aryl ring quadrant str. (IR/Raman), 1488 cm1 (IR) aryl ring semi-circle str. The band at 743 cm1 derives from the 4 adj H wag. O O O O UNKNOWN IR AND RAMAN SPECTRA 207 30. 1-Hexane sulfonic acid sodium salt (sodium 1-hexanesulfonate): Note sample was prepared as a nujol mull which interferes with IR aliphatic bands. Raman aliphatic bands indicates both methyl and methylene groups. No aromatic observed. The water results in bands in the IR spectrum at 3544 and 3484 cm1 from the OH str., 1625 cm1 from the OH def., a combination bands at 2233 and 2096 cm1 and the broad OH bend at ca. 622 cm1. Sulfonic acid salt group: Strong IR bands from 1213 to1187 cm1 derive from the SO3 o. ph. str. The strong IR/Raman band at ca. 1058 cm1 derives from the SO3 i. ph. str. The band at ca. 795 cm1 (IR/Raman) involves the CeS str. O H3 C (H2C)5 S O Na O 31. 3-Amino pentane: Aliphatic group: CH3 stretches at 2962, 2876 cm1 and CH2 stretch at 2922 cm1, respectively. The peak intensity of methylene (2922 cm1) to methyl (22962 cm1) is consistent with ca. 1 methylene to 1 methyl. Aliphatic bands from methyl and methylene deformation vibrations are observed at 1461, 1377 and 1356 cm1. Primary Amine group: The NH2 stretching vibrations result in a doublet between 3380 and 3290 cm1 with a shoulder at 3193 cm1 which derives from an overtone (Fermi-resonance enhanced) of the 1611 cm1 NH2 deformation band. The band cluster from 921 to 769 cm1 derives from the NH2 wag and twist vibrations. Bands involving the CeN stretch occur at w1150 and w1045 cm1. NH2 32. a-D-Glucose: Water: The band at 1630 cm1 indicates some water is present in the KBr disc preparation. Aliphatic group: CH2 and CH str vibrations are observed between 2945 and 2892 cm1. The CH2 deformation has a strong characteristic band at 160 cm1. Hydroxyl group: OH stretch at 3410, 3315 cm1. Note different hydrogen bonding environments. OH def at ca. 1345 cm1. Bands between 1150 and 996 cm1 involve the CeO str of primary and secondary alcohols. Primary alcohols (eCH2eOH) are mostly the lower frequency 1025e1000 cm1 band cluster and the secondary alcohols (>CHeOH) are mostly the 1150, 1112 cm1 bands. The cyclic ether has bands at w 1120e1110 involving the CeOeC o. ph. str. and at w840 cm1 from the CeOeC i. ph.str. H OH H O HO H HO H H OH OH 208 9. UNKNOWN IR AND RAMAN SPECTRA 33. Butylated hydroxy toluene: Note Nujol mull aliphatic interference. Aliphatic group: 2968, w2876 cm1 CH3 str. 1472e1450 cm1 CH3 o.ph. def., t-butyl 1396, 1363 cm1 C(CH3)3 i. ph. def., 1375 cm1 CH3 i. ph. def. Aromatic group: 3100e3000 cm1 aromatic CH str., 1604e1590 cm1 (IR/R) quadrant ring str., 1482, 1432 cm1 (IR) semi-circle str., 866 cm1 (IR) aryl lone H wag. Phenol hydroxyl group: 3626 (IR/R) OH str., 1246e1198 cm1 (IR/R) involves aryl-O str. CH3 OH CH3 H3C CH3 H3C CH3 CH3 34. Polystyrene: Aliphatic group: 2923e2854 cm1 CH2 str. 1452 CH2 def. Aromatic group: 3100e3000 cm1 aromatic CH, 1603, 1584, 1493 cm1 aromatic ring quad and semi-circle str., 1002 cm1 (Raman) 2,4,6 C Radial i. ph.str., 754 cm1 (IR) 5 adjacent aryl H, 695 cm1 (IR) aromatic ring pucker. * H2 C CH * 35. Di-n-butyl amine: Aliphatic group: 2964/2958 cm1 and 2873 cm1 CH3 str., 2928 cm1 (IR) CH2 o. ph. str., Note multiple bands in Raman from trans and gauche methylene groups. 1298 cm1 (Raman) CH2 twist of chain. 1464/1445 cm1 involves methyl and methylene deformation. 1378 cm1 CH3 i. ph. def. Secondary amine group: 3287 cm1 NH str., 2809 cm1 CH2 (N) i. ph. str., 1131 cm1 CeNeC o. ph. str., 735 cm1 NH wag. (CH2)3CH3 HN (CH2)3CH3 UNKNOWN IR AND RAMAN SPECTRA 209 36. Benzalkonium chloride: Aromatic group: 3100e3000 aromatic CH, w1600, 1488 cm1 aromatic ring quad and semi-circle str., 1002 cm1 (Raman) 2,4,6 C Radial i. ph.str., 728 cm1 (IR) 5 adjacent aryl H, 705 cm1 (IR) aromatic ring pucker. Quaternary amine salt: (Nþ) CH3 has CH3 o. ph. str. at 3002 cm1, the CH3 i. ph. str. at 2852 cm1, the CH3 o. ph. bend/def at 1488 cm1. The NC4 i. ph. str. is expected between 800 and 950 cm1 and may possibly be assigned to the band at 837 cm1. Cl N C7H15 37. n-Ethyl acetamide: Aliphatic group: 2976, 2877 cm1 CH3 str., 2936 CH2 str., 1452 cm1 CH3/CH2 def., 1373 CH3 i. ph. def. Amide group: 3290 cm1 NH str., 1654 cm1 C ¼O str., 1556 cm1 CNH str./bend, 3086 cm1 overtone of 1556 cm1 band (2 1556), 1298 cm1 CNH str./open, 721 cm1 NH wag. H N O C CH2CH3 CH3 38. Triphenyl phosphine oxide: Aromatic group: 3100e3000 aromatic CH str., 1593, 1575, 1485/ 1439 cm1 aromatic ring quad and semi-circle str., 1121 cm1 involves P-aryl str., 1003 cm1 (Raman) 2,4,6 C Radial i.ph.str., 755 cm1 (IR) 5 adjacent aryl H, 697 cm1 (IR) aromatic ring pucker. Phosphine oxide (P ¼O) group: 1191/1180 cm1 (IR/Raman) P ¼O str. P O 210 9. UNKNOWN IR AND RAMAN SPECTRA 39. 4-Methyl-pyridine-3-boronic acid: Boronic acid group: 3420e3200 cm1 (broad) OH str., 1359 cm1 involves BeO str., 1071 cm1 BeOH def., 772 (B)OH wag. Pyridine group: 3080e3040 cm1 aryl CH str., 1615, 1591, 1555 aryl quadrant str., 1480e1425 cm1 involves aryl semi-circle str., 1091 cm1 involves 2,4,6 C Radial i.ph.str., 1003 cm1 involves the whole ring breath (str). 877 cm1 lone H wag, 835 cm1 2 adj H wag. CH3 OH B OH N 40. Tributyl borate: Aliphatic group: 2959 and 2874 cm1 CH3 str., 2935 CH2 o. ph. str., 1485 cm1 (O)CH2 def., 1466e1458 cm1 CH3, CH2 def. Borate ester group: 1337e1297 cm1 involves the B(O)3 o. ph. str., 832, 815 cm1 (Raman) most likely involves the B(O)3 i. ph. str., while the 666 cm1 band (IR) is from the B(O)3 out-of-plane deformation. 1407 cm1 involves the OCH2 wag þ BeO str. Bands at 1073 and 1029 cm1 involve mostly the CeO str. C4H9 O B C4H9 O C4H9 O 41. Phenyl boroxole: Phenyl boronic acid is a labile compound that in the solid state readily forms a Boron ether. The Boron ether when put in an acidic medium readily hydrolyzes to form the phenyl boronic acid. Raman spectrum indicates no aliphatic group is present (IR Nujol mull preparation interferes). No bands associated with the BeOH group are observed including the (B)OH str (w3250 cm1), the BeOH bend (w1000 cm1), and the BeOH wag (w800 cm1) (see interpreted IR/Raman spectra # 64 of m-tolylboronic acid). Aromatic group: 3100e3000 cm1 aromatic CH str., 1603, 1574, 1494/1442 cm1 aromatic ring quad and semi-circle str., 1180 cm1 involves B-aryl str., 996 cm1 (Raman) 2,4,6 C Radial i.ph.str., 760 cm1 (IR) 5 adjacent aryl H, 703 cm1 (IR) aromatic ring pucker. BeO linkage: Strong characteristic IR bands involving the BeO str. are observed between 1381 and 1310 cm1 in both the IR (strong) and Raman (moderate). 211 UNKNOWN IR AND RAMAN SPECTRA B OH 3 O O B B B OH + H2O O 42. Azodicarbonamide: NH2 group: NH2 o. ph. and i. ph. str at 3331 and 3175 cm1, respectively (IR and Raman). NH2 bend/def at 1635 cm1 (IR). NH2 rk (Raman and IR) at 1118 cm1 and the NH2 wag at 750 cm1 (IR) with an overtone 2 NH2 wag at 1477 cm1. Amide C ¼O group: C ¼O str. in IR at 1738/1726 cm1. The IR and Raman bands at 1367, 1337, and 1300 cm1 involve the CeN str. Azo N ¼N group: Strong Raman band at 1573 cm1 involves the N ¼N str. O C NH2 N H2N N C O 43. 4-Methyl-imidazole: NeH group: This is characterized by complex hydrogen bonding involving NeH$$$N ¼ due to p-type hydrogen bonding of the unsaturated N atom in the heterocyclic ring. Thus a broad envelope of NH str vibrations are observed from 3400 to2700 cm1 in the IR spectrum. The NH wag occurs at 926 cm1 and the broad band at 1824 cm1 is due to the Fermi-resonance enhanced overtone of the NH wag. Imadazole ring: Ring ¼CH str. bands at 3120, 3091, and 3029 cm1. The out-of-phase C ¼C/C ¼N str. (Quadrant str.) results in bands between 1580 and 1509 cm1 and the in-phase C ¼C/C¼N str. (semi-circle str.) results in bands at w1440 cm1. The band cluster at 1304, 1265 cm1 may involve the imidazole ring in-phase breathing mode. Methyl group: Bands at 2921 and 2865 involve the CH3 str. and the band at 1384 cm1 involves the CH3 i. ph. def. N N H 212 9. UNKNOWN IR AND RAMAN SPECTRA 44. Melamine: Amino group: 3500e3100 cm1 involves NH2 str. Note differences in hydrogen bonding (i.e., strongly hydrogen-bonded versus weakly hydrogen-bonded NH). 1650e1620 cm1 doublet from NH2 deformation. w582 cm1 possibly NH2 wag. Triazine group: 1550 cm1 quadrant ring str., 1470e1430 cm1 semi-circle ring str., 984 cm1 (Raman) N-Radial i. ph. str., 811 cm1 ring pucker (sextant out-of-plane bend), 675 cm1 quadrant in-plane ring bend. H2N NH2 N N N NH2 A P P E N D I X IR Correlation Charts Infrared group frequency correlation charts 1700 3400 3300 3200 3100 3000 2900 2800 2700 1600 1500 1400 1300 CH3, CH2, CH, bend CH3, CH2, CH, stretch C≡C–H R–CH3 Arom-H –CH(CH3)2 C=CH 2 –C(CH3)3 C=CH R–CH2–R R–CH3 R–O–CH3 Arom-CH3 R–CH2–R O=C–CH2– R–O–CH3 R2–N–CH3 N≡C–CH2– O=C–H >C=CH2 O=C–H 3600 3400 3200 3000 2800 2600 2400 2200 O=C–CH3 Si–CH3 X-H stretch R–OH …..O O=C–OH ……O S–OH ….. O P–OH ….. O R–NH2 R–NH–R R–NH3+ Cl− R2–NH2+ Cl− R3–NH+ Cl− O=C–NH2 O=C–NH–C O=C–NH–C cyclic S–H P–H Si–H 213 1200 214 2400 IR CORRELATION CHARTS 2300 2200 2100 2000 1900 1900 1800 1800 1700 C≡C C≡N, X=Y=Z str. 1600 1500 1400 C=O str. Cyclic anhydride CH2–C≡N R–CO–O–CO–R R–CO–Cl Conj C≡N S–C≡N C–C≡C–H R–CO–O–R CONH–CO–O–R –N=C=O H–CO–O–R R–CO–H –N=C=S >C=C=CH2 Conj-CO–H R–CO–R Conj-CO–R Conj-CO-Conj R–CO–OH dimer R–CO–N R–CO2− 1700 1600 1500 C=C s tr. 1400 1000 900 800 700 Olef CH wag C=C R–CH=CH2 R2C=CH2 C=C R–CH=CH–R trans C=C R–CH=CH–R trans RCH=CR2 Ar Ring str. Ring Ar-H wag Ring mono, 1,3 or 1,3 5 Lone Ar-H 2 adj Ar-H 3 adj Ar-H 4 & 5 adj Ar-H 600 1300 215 IR CORRELATION CHARTS 1400 130 1100 120 1000 900 800 1500 1400 1300 C–O str. 1200 1100 1000 SO, PO, SiO str. O=C–OH C–SO2–C O=C–O–C C–SO2–N Ar-OH C–SO2–O–C R C–OH C–SO2–Cl 3 R2CH–OH C–SO2–OH anhydride C–SO − H O+ –CH2 –OH 3 3 C–SO3 − Na+ Epoxy Ar-O–CH2 C2S=O C=C–O–CH2 P=O –CH2–O–CH2 P–O–CH2 Si–O 1700 1600 900 1500 1400 1300 1200 1100 1000 900 NH bend, CN str., NO str. C–NH2 C–NH2 CH2–NH–CH2 Ar-NH2 C–NH3 + C2 –NH2 + O=C–NH2 O=C–NH–C C=N C=N–O–C CH2–NO2 Ar-NO2 C–O–NO2 C–O–N=O 800 700 800 Index A A. See Absorption intensity a. See Angle of incidence Absorbed radiation, 33 Absorption intensity (A), in attenuated total reflectance, 41 Acetamide, 136, 149f Acetamideoxime, 137, 159f Acetohydroxamic acid, 136, 152f Acetone, 136, 145f Acetonitrile, 135, 142f Acetylene group, spectra-structure correlations of, 124e125 Acetylenic compounds, spectra-structure correlations of, 131e132 Acid halides, spectra-structure correlations of, 126e127 Acidic protons, spectra-structure correlations of, 126 Acrylamide, 136, 150f Acrylonitrile, 135, 142f AD. See Array detector AETAC. See Poly-2-(acryloyloxy)-Ethyltrimethyl Ammonium Chloride Alcohols group frequencies of, 104e105t IR and Raman spectra of, 136e137, 153f, 154f, 155f spectra-structure correlations of, 103e105, 104e105t, 104f Aldehydes spectra-structure correlations of, 126 stretching vibration for, 101 Aliphatic groups IR and Raman spectra of, 135, 140f spectra-structure correlations of, 74e78, 75f, 76t, 77f, 78f, 79t, 80f, 125e126 conjugated, 79e99, 80e81t Aliphatic primary amines, IR and Raman spectra of, 73, 74f Aliphatic secondary amines, IR and Raman spectra of, 73, 74f Alkane groups, spectra-structure correlations of, 74, 75f, 76t fluorinated, 130 Alkoxysilanes, spectra-structure correlations of, 131 AMD band assignments for, 48, 50e51t diffuse reflectance of, 53, 53f FT-IR calibration plot of, 49, 52f FT-Raman calibration plot of, 52, 52f quantitative analysis of, 48e53 spectra of, 48, 49e50f Amide compounds IR and Raman spectra of, 136, 149f, 150f, 151f, 152f spectra-structure correlations of, 101, 128 Amines IR and Raman spectra of, 137, 156f, 157f, 158f spectra-structure correlations of, 126 deformation vibrations, 128 Amine salts IR and Raman spectra of, 137, 156f, 157f, 158f spectra-structure correlations of, 128 Amino (NH) group correlation chart for, 215 environmental dependence of vibrational spectra and, 58e59 IR and Raman spectra of, 74, 74f spectra-structure correlations of, 124e125, 133 3-Amino pentane, 193f, 207 Ammeline, 136, 151f Ammonium, spectra-structure correlations of, 129 Ammonium chloride, 138, 167f Amplitude in classical harmonic oscillator, 10, 10f dipole moment modulation by, 18, 18f vibrational, dipole moment and, 15 Analyte concentration, IR and Raman signal and, 45e46 Angle of incidence (a) for attenuated total reflectance, 39e40, 40f in dispersive systems, 27, 28f refractive index and, 33, 34f triatomic molecule stretches and, 65, 66f Angle of radiation (b), in dispersive systems, 27, 28f Anharmonic oscillator, potential energy of, 13, 13f Anhydrides IR and Raman spectra of, 136, 145f, 146f, 147f, 148f, 149f spectra-structure correlations of, 100e101, 126e127 217 218 INDEX Aniline, 137, 157f Anisole, 185f, 202 Anti-Stokes Raman scattering, 16f, 17 classical description of, 17e18, 18f Aromatic rings correlation chart for, 214 IR and Raman spectra of, 136, 143f, 144f spectra-structure correlations of, 128, 132 benzene, 86e91, 87f, 88r, 89f, 90f conjugated, 79e99, 80e81t fused, 92, 93t heterocyclic, 93e99, 93f Array detector (AD), signal-to-noise ratio and, 29 Aryl CH wag, 90e91, 91t, 92f Aryl groups, spectra-structure correlations of, 125 Asymmetric vibrations, IR spectroscopy for, 1 Atmospheric compensation, for sample preparation, 34, 120 Atomic masses in diatomic oscillator, 11 in GF matrix method, 22 ATR. See Attenuated total reflectance Attenuated total reflectance (ATR), 39e41, 40f, 42t with microscopy, 44 of starch, 119e120, 120f Azo compounds IR and Raman spectra of, 137, 160f spectra-structure correlations of, 127 Azodicarbonamide, 198f, 211 Azo-tert-butane, 137, 160f B b. See Angle of radiation B3LYP function, 24 BaF2, in IR transmission, 32, 33t Band assignments, for PAM-AETAC samples, 48, 50e51t Band intensities, for sample preparation, 33, 120 Band shape, in IR and Raman vibrational bands, 2 Baseline, for sample preparation, 34, 130 3-21G Basis set, 24 6-31G Basis set, 24 Basis set, in vibrational spectra assessment, 24 Beam splitter, in Michelson interferometer, 30e31, 31f Bending vibration. See Deformation vibrations Bent triatomic molecule, as coupled oscillators, 64e65, 65f, 66f Benzalkonium chloride, 195f, 209 Benzamidine HCl, 137, 158f Benzene. See also 1,3,5-Trisubstituted benzene Cartesian coordinate displacements for, 9, 9f center of symmetry of, 19, 19f elementary ring vibrations of, 89, 90f mono- and di-substituted, 88e89, 89f normal ring modes of, 86, 87f, 88t spectra-structure correlations of, 86e91, 87f, 88t, 89f, 90f stretching vibration of, 70, 70f Benzoic acid, 136, 148f Benzyl alcohol, 182f, 202 BH stretch, spectra-structure correlations of, 126 Bipolymers, IR and Raman spectra of, 139, 171f, 172f, 173f, 174f, 175f, 176f 1,3-Bis(trifluoromethyl)benzene, 190f, 205 BLYP function, 24 BeN type compounds group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112 BeO type compounds group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112, 129 Boltzmann’s law, in Raman scattering, 17 Bond angle, stretching frequency and, 101e102 Boric acid, 139, 169f Boronic acid compound, IR and Raman spectra of, 138, 161f 3-Bromo toluene, 138, 162f 1-Bromohexane, 138, 161f 1-Butanol, 137, 154f n-Butanol, 181f, 201 n-Butyl acetate, 136, 145f t-Butyl alcohol, 137, 153f N-Butyl methyl acrylamide, 136, 150f Butylated hydroxy toluene, 194f, 208 Butyric anhydride, 136, 146f n-Butyronitrile, 180f, 201 C CaF2, in IR transmission, 32, 33t Calcium palmitic acid salt, IR spectroscopy v. Raman spectroscopy, 118f, 119 Calculating, vibrational spectra, 22e24 Capillary film, 35 Carbon dioxide (CO2) center of symmetry of, 19, 19f Fermi resonance and, 60e62, 61f, 62t fundamental vibrations for, 8, 9f IR gas phase contours of, 55, 56f Carbonates, spectra-structure correlations of, 126e127, 129 Carbonyl groups environmental dependence of vibrational spectra and, 59 group frequencies for, 100t 219 INDEX factors that effect, 99f, 101e103 IR and Raman spectra of, 99f review of, 99e101 spectra-structure correlations of, 99e103, 99f, 100t, 125, 126e127 Carboxylic acid dimers hydrogen bonding in, 58 IR and Raman spectra of, 73, 74f spectra-structure correlations of, 99e100 Cartesian coordinate displacements for harmonic oscillator, 9, 9f for XY2 molecules, 20e22, 21f Cast films preparation of, 36e37 of starch, 119e120, 120f CeBr group frequencies of, 108, 109t IR and Raman spectra of, 108, 108f, 138, 161f, 162f spectra-structure correlations of, 108e110, 108f, 109t CeCl group frequencies of, 108, 109t IR and Raman spectra of, 108, 108f, 138, 161f, 162f spectra-structure correlations of, 108e110, 108f, 109t wavenumber regions for, 11, 11t Cellulose, 139, 174f Center of symmetry, spectroscopy and, 19, 19f CH bend, correlation chart for, 213 CH stretch, correlation chart for, 4, 4f, 213 CH2. See Methylene CH3. See Methyl Chain length, vibrational frequency and, 66e67, 66f Chemometric data analysis, 118 3-Chloro toluene, 138, 162f 1-Chloro-octane, 138, 161f CI. See Configuration interaction method Classic harmonic oscillator, 10e11, 10f, 11t potential energy of, 12 C]N compounds, IR and Raman spectra of, 137, 158f, 159f CeN stretching vibrations correlation chart for, 215 group frequencies of, 104e105t IR and Raman spectra of, 103, 104f spectra-structure correlations of, 103e105, 104e105t, 104f CeO stretching vibrations correlation chart for, 215 group frequencies of, 104e105t IR and Raman spectra of, 103, 104f spectra-structure correlations of, 103e105, 104e105t, 104f, 131 C]O stretching vibrations, correlation chart for, 214 CO2. See Carbon dioxide Combination bands, anharmonic oscillators and, 13 Communication skills, for spectra interpretation, 117 Computer-based libraries, for spectral interpretation, 122e123 Configuration interaction (CI) method, 24 Conjugation effects, in carbonyl groups, 102e103 Coordinate analysis, with GF matrix method, 22 Correlation charts for CH3, CH2, and CH, 4, 4f development of, 4 group frequency, 213e215 Coupled oscillators bent triatomic molecule, 64e65, 65f, 66f equal combinations, 67e68, 68f, 68t group frequencies of, 63e67, 64f, 65f, 66f, 67f linear aliphatic systems, 65e67, 66f, 67f linear molecules, 63e64, 64f rules of thumb for, 67e70, 68f, 68t, 69f, 70f unequal combinations, 70, 70f Crystallinity, in IR spectra, 57, 58f CeSeC compounds IR and Raman spectra of, 108, 108f spectra-structure correlations of, 108e110, 108f, 109t CeSeSeC compounds IR and Raman spectra of, 108, 108f spectra-structure correlations of, 108e110, 108f, 109t Cyanuric acid, spectra-structure correlations of, 93, 93f Cyclohexane, 135, 140f, 178f, 200 n-Cyclohexane, IR and Raman spectra for, 80f Cyclopropyl rings, spectra-structure correlations of, 125 D Data analysis techniques, for spectra interpretation, 117 Deformation vibrations. See also X-H deformation of amines, spectra-structure correlations of, 128 of CO2, 8, 9f Fermi resonance and, 60e61, 61f of H2O, 8, 9f of linear aliphatic systems, 67, 67f of methyl groups, spectra-structure correlations of, 128e129 of methylene groups, spectra-structure correlations of, 128e129 Degenerate vibrations, 22 Degrees of freedom calculating, 8 molecular motion and, 8e9, 9f 220 INDEX Demountable cell, 35 Density functional theory (DFT) methods, 24 Dextrin, 187f, 203 DFT. See Density functional theory methods Dialkyl acetylenes, spectra-structure correlations of, 85 Dialkyl sulfates, spectra-structure correlations of, 110e111, 110f, 111e112t Diamond, for IR transmission, 32, 33t Diatomic molecule Cartesian coordinate displacements for, 9, 9f as classic harmonic oscillator, 10e11, 10f classical vibrational frequency for, 10 dipole moments of, 14e15, 14f potential energy of, 12, 12f wavenumber regions for, 11, 11t Dibasic phosphate, spectra-structure correlations of, 130 Dibasic sodium phosphate, 139, 169f n-Dibutyl amine, 137, 157f n-Dibutyl ether, 137, 155f n-Dibutyl phosphite, 138, 166f Diethylene glycol, 137, 155f Diffraction grating, in dispersive systems, 27e28, 28f Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), 41e43 Dimethyl carbonate, 136, 145f Dimethyl formamide, 136, 152f Dimethyl malonate, 136, 146f 2,4-Dimethyl pentane, 178f, 200 Dimethyl siloxane polymer, 183f, 202 Dimethyl sulfoxide, 138, 163f Dimethyl terephthalate, 136, 146f 1,3-Dimethyl urea, 136, 150f 2,5-Dimethyl-1,5-hexadiene, 179f, 200 Di-n-butyl amine, 195f, 208e209 Dioctyl phthalate, 192f, 206 1,4-Dioxane, 184f, 202 2-(4,6eDiphenyle1,3,5etriazinee2eyl)e5 alkyloxy phenol, hydrogen bonding in, 60 Dipole moment amplitude modulation of, 18, 18f in IR absorption, 14e15, 14f in Raman scattering, 15e16, 15f vibrational amplitude and, 15 Dispersive Raman instrumentation, 28e29, 29f Dispersive systems, 27e28, 28f Dissociation limit, anharmonic oscillator and, 13 2,6-Di-t-butyl phenol, 137, 155f Dodecyl sulfate, 138, 164f Double bond correlation chart for, 214 cumulated, spectra-structure correlations of, 80e81t, 85e86, 86f, 126 force constant for, 11, 11t IR and Raman spectra of, 135, 141f wavenumber regions for, 11, 11t Double bond stretch, vibrational spectrum for, 3, 3f dp. See Effective penetration depth DRIFTS. See Diffuse Reflectance Infrared Fourier Transform Spectroscopy E Eel. See Electronic energy Effective penetration depth (dp), in attenuated total reflectance, 41 Electric fields dipole moments and, 14e15, 14f in electromagnetic radiation, 7, 7f in Raman scattering, 15e18, 15f, 18f Electromagnetic radiation, 7e8, 7f, 8f absorption of, 7e8, 8f parameters of, 7, 7f in Raman scattering, 15, 17e18, 18f Electronic energy (Eel), 8 Empirical characteristic frequencies, for vibrational spectroscopy interpretation, 2e3, 3f Empirical force fields in coordinate analysis, 22 in GF matrix method, 23 for molecular force field, 23 Energy. See also Potential energy anharmonic oscillator and, 13 photon absorption and, 7e8, 8f Environmental dependence, of vibrational spectra, 55e62 Fermi resonance, 60e62, 61f, 62t hydrogen bonding, 57e60, 59f solid, liquid, gaseous states, 55e57, 56f, 57f, 58f Epichlorohydrin, 137, 156f Epoxy rings, spectra-structure correlations of, 125 1,2-Epoxypentane, 186f, 203 Erot. See Rotational energy Esters, IR and Raman spectra of, 136, 145f, 146f, 147f, 148f, 149f Ethers group frequencies of, 104e105t IR and Raman spectra of, 137, 155f, 156f spectra-structure correlations of, 103e105, 104e105t, 104f n-Ethyl acetamide, 196f, 209 Ethyl carbamate, 136, 152f Ethyl cellulose, 139, 175f Ethyl methyl sulfide, 189f, 204 Ethylenes, spectra-structure correlations of, 132 Evib. See Vibrational energy 221 INDEX F Far-infrared (Far-IR) spectroscopy, frequencies of, 13e14 Far-IR spectroscopy. See Far-infrared spectroscopy Fermi resonance idealized, 60e61, 61f selected groups and, 62t vibrational spectra and, 60e62, 61f Fluorinated alkane groups, spectra-structure correlations of, 130 Fluorine compounds, IR and Raman spectra of, 138, 161f, 162f 3-Fluoro toluene, 138, 162f Fluorolube, FT-IR spectrum of, 38f 3-Fluorotoluene, 190f, 205 Force constant (K) for bond types, 11, 11t in classical harmonic oscillator, 10 in GF matrix method, 23 stretching frequencies and, 102 Fractional linear calibration equations, 47e48 Frequency of electromagnetic radiation, 7, 7f in IR absorption, 14 in IR and Raman vibrational bands, 2 of photons, 7, 7f vibrational, classical, 10e11, 10f FT-IR calibration plot for AMD-AETAC copolymers, 49, 52f of PAM-AETAC samples, 49, 52f FT-IR spectroscopy interferometer for, 30e32, 31f of PAM-AETAC samples, 48, 49e50f ratio method for, 46e47 FT-Raman calibration plot for AMD-AETAC copolymers, 52, 52f of PAM-AETAC samples, 52, 52f FT-Raman spectroscopy interferometer for, 30e32, 31f of PAM-AETAC samples, 48, 49e50f ratio method for, 47 Fundamental transitions, harmonic and anharmonic oscillators and, 13 Fundamental vibrational spectrum, 3, 3f Furans in-plane vibrations of, 96, 97e98t Furans, spectra-structure correlations of, 96e99, 97e98t, 97f, 125 G Gaseous state, environmental dependence of vibrational spectra in, 55e57, 56f, 57f Gaussian-type orbitals (GTO), in STO-3G basis set, 24 GF matrix method, coordinate analysis with, 22e23 a-D-Glucose, 193f, 207e208 Glycine, 136, 149f Glycolic acid, 136, 147f Groove spacing, in dispersive systems, 27, 28f Group frequencies of alcohols and ethers, 104e105t of boron compounds, 110, 111e112t of carbonyl groups, 100t factors that effect, 99f, 101e103 correlation charts of, 213e215 coupled oscillators, 63e67, 64f, 65f, 66f, 67f of halogen compounds, 108, 109t of inorganic compounds, 112, 113t of nitrogen containing compounds, 105, 106e107t origin of, 63e71 oscillator combination rules of thumb, 67e70, 68f, 68t, 69f, 70f of phosphorus compounds, 110, 111e112t of silicon compounds, 110, 111e112t of sulfur compounds, 110, 111e112t X-H stretching group, 73e74, 74f Group Theory symmetry and, 18 for vibrational spectroscopy interpretation, 2e3, 3f GTO. See Gaussian-type orbitals H H2. See Hydrogen H2O. See Water Halogen-carbon stretch, spectra-structure correlations of, 132e133 Hammet constant, methylene group correlation with, 84e85, 85f Harmonic oscillator Cartesian coordinate displacements for, 9, 9f classic, 10e11, 10f, 11t potential energy of, 12, 12f, 13f quantum mechanical, 12e13, 12f, 13f Hartree-Fock (HF) method, 24 Heisenberg’s uncertainty principle, harmonic oscillator and, 12f, 13 Heptane, 135, 140f n-Heptane IR and Raman spectra for, 80f IR spectroscopy v. Raman spectroscopy, 118f, 119 cis-3-Heptene, 135, 141f trans-3-Heptene, 135, 141f Hetero-aromatic rings, spectra-structure correlations of, 128 222 INDEX Heterocyclic aromatic five-membered rings, spectrastructure correlations of, 96e99, 97e98t, 97f Heterocyclic aromatic six-membered rings, spectrastructure correlations of, 93e99, 93f pyridines, 94, 95t, 96t triazines and melamines, 94e96 Heteronuclear diatomic molecule, dipole moments of, 14 1-Hexadecanol IR and Raman spectra of, 137, 154f IR spectroscopy v. Raman spectroscopy, 118f, 119 n-Hexane, IR and Raman spectra for, 80f 1-Hexane sulfonic acid sodium salt, 192f, 207 2-Hexanone, 188f, 204 1-Hexene, 135, 141f 2-Hexyne, 181f, 201 HF. See Hartree-Fock method Homonuclear diatomic molecule, dipole moments of, 14 Hydrogen (H2), center of symmetry of, 19, 19f Hydrogen bonding in acidic proton vibrational frequencies, 126 in amino and hydroxy stretching vibrations, 124 environmental dependence of vibrational spectra and, 57e60, 59f intramolecular, 59e60 4-Hydroxy benzaldehyde, 136, 147f Hydroxy (OH) group environmental dependence of vibrational spectra and, 58, 60f IR and Raman spectra of, 73, 74f spectra-structure correlations of, 124e125, 133 2-Hydroxy-4-(methoxy)-benzophenone, hydrogen bonding in, 60 Hydroxypropylmethyl cellulose, 139, 175f 2-Hydroxypyridine, spectra-structure correlations of, 93, 93f I Identification, spectral databases and interpretation for, 118 2-Imidazolidinethione, 138, 165f 2-Imidazolidone, 136, 151f Imino compounds, spectra-structure correlations of, 127 Impurities, identifying, 123 Infrared (IR) absorption process, 13e15, 14f dipole moment in, 14e15, 14f frequencies in, 13e14 Infrared (IR) gas phase contours, of CO2, 55, 56f Infrared (IR) spectroscopy. See also FT-IR spectroscopy; Mid-infrared spectroscopy; Near-infrared spectroscopy analyte concentration and, 45e46 application of, 1 center of symmetry and, 19, 19f dispersive systems for, 27e28, 28f historical perspective on, 3e4 instrument development for, 4 interpretation of, 2, 117e133 Raman spectroscopy v., 1e2, 2t, 118f, 119 symmetry and, 18 tools for, 117e119, 118f transmission, 34e39 Infrared (IR) transmitting materials, 32, 33t Inorganic compounds IR and Raman spectra of, 138e139, 167f, 168f, 169f, 170f, 171f spectra-structure correlations of, 112, 113t In-phase stretch of bent triatomic molecule, 64e65, 65f, 66f of CO2, 8, 9f Fermi resonance and, 61, 61f of H2O, 8, 9f of linear molecules, 63e64, 64f In-plane vibrations of olefinic groups, 82, 83f of pyridines, 94, 95t of pyrroles, furans, and thiophenes, 96, 97e98t Instrumentation, 27e32 dispersive Raman instrumentation, 28e29, 29f dispersive systems, 27e28, 28f interferometric spectrometers, 30e32, 31f Raman spectroscopy sample arrangements, 29, 30f for spectra interpretation, 117 Intensity in IR and Raman vibrational bands, 2 of Raman scattering, 17, 46 wavenumber v., 14 Interaction effects, in carbonyl groups, 103 Interferometric spectrometers, 30e32, 31f Internal reflectance, in attenuated total reflectance, 40, 40f Internal reflection element (IRE), in attenuated total reflectance, 39 Interpersonal skills, for spectra interpretation, 117 Intramolecular hydrogen bonding, 59e60 IR absorption process. See Infrared absorption process IR gas phase contours. See Infrared gas phase contours IR spectroscopy. See Infrared spectroscopy IR transmitting materials. See Infrared transmitting materials IRE. See Internal reflection element Isoamyl nitrate, 137, 159f Isobutyl amine, 137, 156f Isocyanuric acid, hydrogen bonding in, 59 223 INDEX Isooctane, 135, 140f Isopropyl alcohol, 136, 153f Isopropyl nitrate, 137, 160f Isoquinolines, spectra-structure correlations of, 128 Isotactic polyethylene, 139, 171f K K. See Force constant KBr disc preparation of, 38, 39f KBr windows, in IR transmission, 32, 33t Ketones, stretching vibration for, 101 KRS-5, in IR transmission, 32, 33t Kubelka-Munk equation, in DRIFTS, 41e43 L Lactones, spectra-structure correlations of, 126e127 Lambert-Beer law in absorption spectroscopy, 14 in attenuated total reflectance, 41 in quantitative analysis, 45e46 Linear molecules in-phase stretch of, 63e64, 64f molecular motions for, 8, 9f out-of-phase stretch of, 63e64, 64f Liquids environmental dependence of vibrational spectra in, 55e57 preparation for, 35e36 M Magnetic fields, in electromagnetic radiation, 7 Maleic anhydride, 136, 147f MCSF. See Multi-configuration self-consistent field method Melamines, 199f, 212 spectra-structure correlations of, 94e96 Melts, preparation of, 38e39 Mesomeric effects, in carbonyl groups, 102e103 Meta (3) chlorophenol, 187f, 203 Metal oxides, spectra-structure correlations of, 133 Meta-tolyl boronic acid, 138, 161f Methane sulfonic acid, 138, 163f Methanol, 136, 153f Methoxy group, spectra-structure correlations of, 126 Methyl (CH3) bend, correlation chart for, 213 Methyl (CH3) groups deformation vibrations of, spectra-structure correlations of, 128e129 spectra-structure correlations of, 75, 76t, 78f, 125e126 1-Methyl naphthalene, 180f, 200 Methyl (CH3) stretch, correlation chart for, 4, 4f, 213 Methylene (CH2) bend, correlation chart for, 213 Methylene (CH2) groups deformation vibrations of, spectra-structure correlations of, 128e129 Hammet constant correlation with, 84e85, 85f spectra-structure correlations of, 74e75, 76t, 77f, 125e126 Methylene (CH2) stretch, correlation chart for, 4, 4f, 213 4-Methyl-imidazole, 199f, 211e212 4-Methyl-pyridine-3-boronic acid, 197f, 210 Michelson interferometer, 30e31, 31f Microscopy, 43e44 reflection IR, 44 transmission IR, 43e44 Mid-infrared (Mid-IR) spectroscopy application of, 1 frequencies of, 13e14 Raman spectroscopy v., 1e2, 2t Mid-IR. See Mid-infrared spectroscopy Mirror, in Michelson interferometer, 30e31, 31f Molecular force field, 23 Molecular geometry, in GF matrix method, 22 Molecular motion degrees of freedom and, 8e9, 9f of linear molecules, 8, 9f of non-linear molecules, 8, 9f Molecular vibrational kinetic energy, in GF matrix method, 22 Møller Plessant perturbation (MP2) method, 24 Monobasic phosphate, spectra-structure correlations of, 130 MP2. See Møller Plessant perturbation method Multi-configuration self-consistent field (MCSF) method, 24 N NaCl windows, in IR transmission, 32, 33t Napthalenes, spectra-structure correlations of, 92, 93t Near-infrared (Near-IR) spectroscopy application of, 1 frequencies of, 13e14 Raman spectroscopy v., 1e2, 2t Near-IR. See Near-infrared spectroscopy Newton’s second law, with GF matrix method, 23 NH group. See Amino group NH2 group IR and Raman spectra of, 73, 74f spectra-structure correlations of, 133 Nitrates in-phase stretching of, 19, 20f organic, spectra-structure correlations of, 127 spectra-structure correlations of, 129 Nitrites, organic, spectra-structure correlations of, 127 224 INDEX Nitrogen containing compounds group frequencies of, 105, 106e107t IR and Raman spectra of, 105, 106f spectra-structure correlations of, 105e108, 106e107t, 106f 2-Nitropropane, 137, 160f Nitroso groups, spectra-structure correlations of, 127 N]O compounds group frequencies of, 105, 106e107t IR and Raman spectra of, 105, 106f, 137, 159f, 160f spectra-structure correlations of, 105e108, 106e107t, 106f N-O stretching vibrations, correlation chart for, 215 Non-linear molecules, molecular motions for, 8, 9f Non-polar groups, Raman spectroscopy for, 1 Nujol Mull FT-IR spectrum with, 34, 35f, 38f preparation of, 37, 38f of starch, 119e120, 120f Nylon, 6, 6, 139, 173f O n-Octane, IR and Raman spectra for, 80f 1-Octyne, 135, 142f OH group. See Hydroxy group Olefinic compounds correlation chart for, 214 spectra-structure correlations of, 127, 131e132 Olefinic groups in-plane vibrations of, 82, 83f out-of-plane vibrations of, 84, 84f Olefinic groups, spectra-structure correlations of, 82e85, 82f, 83f, 84f, 85f, 125 Oleic acid, 136, 148f Out-of-phase stretch of bent triatomic molecule, 64e65, 65f, 66f of CO2, 8, 9f of H2O, 8, 9f of linear aliphatic systems, 66e67, 66f of linear groups, 68e69, 69f of linear molecules, 63e64, 64f Out-of-plane vibrations of olefinic groups, 84, 84f of pyridines, 94, 96t Overtones, anharmonic oscillators and, 13 P Palmitic acid, IR spectroscopy v. Raman spectroscopy, 118f, 119 Palmitic acid, 136, 148f PAM. See Poly acrylamide PAM-AETAC band assignments for, 48, 50e51t diffuse reflectance of, 53, 53f FT-IR calibration plot of, 49, 52f FT-Raman calibration plot of, 52, 52f quantitative analysis of, 48e53 spectra of, 48, 49e50f Paper, 185f, 203 PE. See Potential energy n-Pentane, IR and Raman spectra for, 80f PH stretch, spectra-structure correlations of, 126 Phenol, 137, 154f Phenyl acetate, 191f, 205e206 Phenyl boronic acid, 198f, 210e211 Phosphate, spectra-structure correlations of, 130 Phosphine oxide. See also P]O Phosphine oxide, spectra-structure correlations of, 110f, 111e112t Phosphoric acids, spectra-structure correlations of, 126 Phosphorus compounds, IR and Raman spectra of, 138, 165f, 166f Phosphorus ester, spectra-structure correlations of, 110f, 111e112t Photon energy, parameters of, 7 Photon frequency, parameters of, 7, 7f Photons, 7 absorption of, 7e8, 8f parameters of, 7, 7f in Raman scattering, 16e17, 16f Pipenidindione dioxime, 137, 159f P]O compounds correlation chart for, 215 group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112, 130 PeO containing compounds, spectra-structure correlations of, 131 Polar groups, IR spectroscopy for, 1 Polarizability, in Raman scattering, 15e16, 16f, 18, 18f Poly acrylamide (PAM) band assignments for, 48, 50e51t diffuse reflectance of, 53, 53f FT-IR calibration plot of, 49, 52f FT-Raman calibration plot of, 52, 52f IR and Raman spectra of, 139, 176f quantitative analysis of, 48e53 spectra of, 48, 49e50f Poly acrylic acid, 139, 173f Poly-2-(acryloyloxy)-Ethyltrimethyl Ammonium Chloride (AETAC) band assignments for, 48, 50e51t diffuse reflectance of, 53, 53f FT-IR calibration plot of, 49, 52f FT-Raman calibration plot of, 52, 52f 225 INDEX IR and Raman spectra of, 139, 176f quantitative analysis of, 48e53 spectra of, 48, 49e50f Polybutadiene, 139, 171f Polyethylene oxide, 139, 172f Polymers, IR and Raman spectra of, 139, 171f, 172f, 173f, 174f, 175f, 176f Polystyrene, 194f, 208 Polytetrafluoroethylene, 139, 172f Polyvinyl alcohol, 139, 172f Polyvinyl pyrrolidone, 139, 173f Potassium nitrate, 139, 168f Potential energy (PE) of anharmonic oscillator, 13, 13f of harmonic oscillator, 12, 12f, 13f Primary amine salts, spectra-structure correlations of, 126 2-Propanethiol, 189f, 204e205 Pyridines in-plane vibrations of, 94, 95t IR and Raman spectra of, 136, 144f out-of-plane vibrations of, 94, 96t spectra-structure correlations of, 94, 95t, 96t, 128 Pyrimidines, spectra-structure correlations of, 128 Pyrroles in-plane vibrations of, 96, 97e98t Pyrroles, spectra-structure correlations of, 96e99, 97e98t, 97f, 125 Q Quantitative analysis, 44e53 analyte concentration and, 45e46 examples of, 48e53 of PAM-AETAC samples, 48e53 ratio method, 46e48 Quantum mechanical harmonic oscillator, 12e13, 13f potential energy of, 12, 12f Quantum mechanical method, for molecular force field, 23 Quantum mechanics, vibrational energy in, 12 Quinazolines, spectra-structure correlations of, 128 Quinolines, spectra-structure correlations of, 128 R Raman scattering center of symmetry and, 19, 19f classical description of, 17e18, 18f intensity of, 17, 46 process of, 15e17, 15f, 16f Raman spectroscopy. See also FT-Raman spectroscopy analyte concentration and, 45e46 application of, 1 dispersive Raman instrumentation for, 28e29, 29f historical perspective on, 3e4 interpretation of, 2, 117e133 IR spectroscopy v., 1e2, 2t, 118f, 119 sample arrangements for, 29, 30f symmetry and, 18 tools for, 117e119, 118f Ratio method, 46e48 fractional linear calibration equations for, 47e48 Rayleigh light filters, 28e29, 29f Rayleigh scattering classical description of, 17e18, 18f dispersive Raman instrumentation and, 28e29, 29f oscillating dipoles in, 15 process of, 16, 16f Reflected radiation, 33 Reflection IR, 39e43 ATR, 39e41, 40f, 42t DRIFTS, 41e43 microscopy with, 44 Refractive index angle of incidence and, 33, 34f for attenuated total reflectance, 40e41, 40f Ring vibrations, of benzene, 89, 90f Rotational energy (Erot), 8 3n-5 Rule-of-thumb, 8, 9f 3n-6 Rule-of-thumb, 8, 9f S Sample preparation, 33e34, 34f, 35f for attenuated total reflectance, 39 of cast films, 36e37 issues with, 119e121, 120f for liquids, 35e36 of melts, 38e39 of solid-powdered samples, 37e38, 38f, 39f Sampling methods, 32e44 IR transmitting materials, 32, 33t microscopy, 43e44 reflection IR, 39e43 ATR, 39e41, 40f, 42t DRIFTS, 41e43 techniques for, 33e34, 34f, 35f, 119e121, 120f transmission IR, 34e39 cast films, 36e37 liquids and solutions, 35e36 melts, 38e39 solid-powdered samples, 37e38, 38f, 39f Scattered radiation, 33 Sealed amalgam cell, 36 SH stretch, spectra-structure correlations of, 126 Signal-to-noise ratio (SNR), array detector and, 29 SiH stretch, spectra-structure correlations of, 126 Silica gel, 139, 170f 226 INDEX Silicon dioxide, 183f, 202 Siloxane compounds, IR and Raman spectra of, 138, 166f Siloxane polymer, 138e139, 166f, 175f Single bond force constant for, 11, 11t wavenumber regions for, 11, 11t Single bond stretch, vibrational spectrum for, 3, 3f SieO containing compounds correlation chart for, 215 group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112, 131 Six-membered rings, stretching waves for, 70, 70f Slater-type orbital (STO), in STO-3G basis set, 24 SNR. See Signal-to-noise ratio S]O compounds correlation chart for, 215 group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112 Sodium acetate, 188f, 204 Sodium bicarbonate, 139, 168f Sodium carbonate, 139, 168f Sodium carboxymethyl cellulose, 139, 174f Sodium di-isobutyl dithiophosphate, 138, 166f Sodium isopropyl xanthate, 138, 164f Sodium molybdate, 139, 170f Sodium perchlorate, 139, 170f Sodium sulfate, 138, 167f Sodium sulfite, 138, 167f Software tools, for spectral interpretation, 122e123 Solid state, environmental dependence of vibrational spectra in, 55e57, 58f Solid-powdered samples, preparation of, 37e38, 38f, 39f Solutions, preparation for, 35e36 Solvents, identifying, 123 Sox compounds, spectra-structure correlations of, 129e130 Spectral databases, for identification and structural verification, 118 Spectral database tools computer-based, 122e123 for spectra interpretation, 117 Spectral interpretation, 121e124 computer-based libraries and software tools for, 122e123 examine the spectrum, 123e124 problem definition, 123 spectral reference library hierarchy of quality, 121e122 Spectral reference libraries computer-based, 122e123 hierarchy of quality of, 121e122 Specular reflectance, with microscopy, 44 Spurious bands, sample preparation and, 121 Starch IR and Raman spectra of, 34, 35f, 139, 174f sample preparation of, 119, 120f Stearic acid, 136, 149f STO. See Slater-type orbital STO-3G basis set, 24 Stokes Raman scattering, 16f, 17 classical description of, 17e18, 18f Stretch. See Double bond stretch; In-phase stretch; Out-of-phase stretch; Single bond stretch; Triple bond stretch Stretching frequencies bond angle and, 101e102 force constant and, 102 Stretching vibration for aldehydes, 101 of carbonyl groups, 101e103 CeO, spectra-structure correlations of, 131 Fermi resonance and, 60e61, 61f hydrogen bonding in, 124e125 for ketone, 101 of six-membered rings, 70, 70f Stretch-stretch interaction, group frequencies of, 63e64, 64f Structural verification, spectral databases and interpretation for, 118 Structure correlations, with spectra, 124e133 800-200 cm 1, 133 850-480 cm 1, 132e133 900-500 cm 1, 133 900-700 cm 1, 132 1000-600 cm 1, 131e132 1280-830 cm 1, 131 1300-750 cm 1, 131 1350-1000 cm 1, 130 1350-1080 cm 1, 130 1400-900 cm 1, 129e130 1480-1310 cm 1, 129 1500-1250 cm 1, 128e129 1620-1420 cm 1, 128 1660-1450 cm 1, 127 1660-1500 cm 1, 128 1690-1400 cm 1, 127 1900-1550 cm 1, 126e127 2300-1900 cm 1, 126 2600-2100 cm 1, 126 3000-2700 cm 1, 125e126 3100-2400 cm 1, 126 3200-2980 cm 1, 125 3700-3100 cm 1, 124e125 227 INDEX Sulfanamides, spectra-structure correlations of, 129e130 Sulfates in-phase stretching of, 19, 20f spectra-structure correlations of, 129e130 Sulfinic acids, spectra-structure correlations of, 129e130 Sulfonamides, spectra-structure correlations of, 110f, 111e112t Sulfonates, spectra-structure correlations of, 110f, 111e112t Sulfones, spectra-structure correlations of, 110e111, 110f, 111e112t, 129e130 Sulfonic acids, spectra-structure correlations of, 110f, 111e112t, 129e130 Sulfonic acid salts, spectra-structure correlations of, 110f, 111e112t, 129e130 Sulfonyl chlorides, spectra-structure correlations of, 110f, 111e112t Sulfonyl halides, spectra-structure correlations of, 129e130 Sulfoxide. See also S]O Sulfoxides, spectra-structure correlations of, 110e111, 110f, 111e112t, 129e130 Sulfur compounds, IR and Raman spectra of, 138, 163f, 164f, 165f Symmetric vibrations, Raman spectroscopy for, 1 Symmetry center of, 18e19, 19f Group Theory and, 18 of in-phase stretch of linear molecules, 64, 64f IR and Raman active vibrations, 18e22, 19f, 20f, 21f of XY2 bent molecule, 20, 20f T Talc, 139, 171f Taylor series, for GF matrix method, 23 4-Tert-butyltoluene, 179f, 200 Tetrahydrofuran, 137, 156f, 184f, 202 Tetramethyl ammonium chloride, 137, 158f m-Tetramethyl xylene diisocyanate, 136, 144f Thiocarbonyl compounds, spectra-structure correlations of, 129e130 Thiophenes in-plane vibrations of, 96, 97e98t Thiophenes, spectra-structure correlations of, 96e99, 97e98t, 97f Toluene, 136, 143f para-Toluene sulfonic acid hydrate, 138, 163f Trans, trans-2,4-hexadien-1-ol, 186f, 203 Trans-2-hexen-1-ol, 182f, 201 Transmission IR, 34e39 of cast films, 36e37 of liquids and solutions, 35e36 of melts, 38e39 microscopy with, 43e44 of solid-powdered samples, 37e38, 38f, 39f Transmitted radiation, 33 Triazines, spectra-structure correlations of, 94e96 Triazine species, spectra-structure correlations of, 128 Tribasic sodium phosphate, 139, 169f n-Tributyl amine, 137, 157f Tributyl borate, 197f, 210 Tributyl phosphite, 138, 165f 2,4,6-Trichlorobenzoyl chloride, 191f, 206 Trimethyl amine, 137, 158f Trimethyl isocyanurate, 136, 151f Triphenyl phosphine, 138, 165f Triphenyl phosphine oxide, 196f, 209e210 Triple bond correlation chart for, 214 force constant for, 11, 11t IR and Raman spectra of, 135, 142f spectra-structure correlations of, 80e81t, 85e86, 86f, 126 wavenumber regions for, 11, 11t Triple bond stretch, vibrational spectrum for, 3, 3f 1,3,5-Trisubstituted benzene, in-phase stretching of, 20, 20f U Ureas, IR and Raman spectra of, 136, 149f, 150f, 151f, 152f V Vibrating mechanical molecular models, 3 Vibrational amplitude, dipole moment and, 15 Vibrational energy (Evib), 8 in quantum mechanics, 12 Vibrational frequency for bent triatomic molecule, 64e65, 65f, 66f classical, 10e11, 10f for linear aliphatic systems, 66e67, 66f, 67f for linear molecules, 63e64, 64f Vibrational quantum number, anharmonic oscillator and, 13 Vibrational spectroscopy advantages of, 45 application of, 1 calculating, 22e24 environmental dependence of, 55e62 Fermi resonance, 60e62, 61f, 62t hydrogen bonding, 57e60, 59f solid, liquid, gaseous states, 55e57, 56f, 57f, 58f interpretation of, 2 techniques for, 1 Vibrational spectrum, 3, 3f Vinyl groups, spectra-structure correlations of, 132 228 INDEX W Water (H2O), fundamental vibrations for, 8, 9f Wavelength, of electromagnetic radiation, 7, 7f Wavenumbers in classical harmonic oscillator, 10e11 of electromagnetic radiation, 7, 7f intensity v., 14 Wavenumber regions, for diatomic oscillator groups, 11, 11t X X-axis scale, for sample preparation, 34, 121 X-H deformation, vibrational spectrum for, 3, 3f X-H stretch correlation chart for, 213 vibrational spectrum for, 3, 3f X-H stretching group, IR and Raman spectra of, 73e74, 74f wavenumber regions for, 11, 11t XY2 bent molecule Cartesian coordinate displacements, 20e22, 21f symmetry of, 20, 20f XY2 type identical oscillators, 68, 68f, 68t Xylenes, 136, 143f Z Zero point energy, 12 Zinc dimethyl dithiocarbamate, 138, 164f ZnSe FT-IR spectrum with, 34, 35f in IR transmission, 32, 33t
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