Infrared Spectroscopy: Fundamentals and Applications / Edition 1

by Barbara H. Stuart

Overview

Provides an introduction to those needing to use infrared spectroscopy for the first time, explaining the fundamental aspects of this technique, how to obtain a spectrum and how to analyse infrared data covering a wide range of applications.

• Includes instrumental and sampling techniques
• Covers biological and industrial applications
• Includes suitable questions and problems in each chapter to assist in the analysis and interpretation of representative infrared spectra

Part of the ANTS (Analytical Techniques in the Sciences) Series.

Product Details

ISBN-13:	9780470854280
Publisher:	Wiley
Publication date:	07/30/2004
Series:	Analytical Techniques in the Sciences (AnTs) , #14
Pages:	248
Product dimensions:	6.02(w) x 9.15(h) x 0.56(d)

About the Author

Barbara Stuart, B.Sc. (Sydney), M.Sc. (Sydney), Ph.D. (London), D.I.C., MRACI, MRSC, Cchem
After graduating with a B.Sc. degree from the University of Sydney in Australia, Barbara Stuart then worked as a tutor at this university. She also carried out research in the field of biophysical chemistry in the Department of Physical Chemistry and graduated with an M.Sc. in 1990. The author then moved to the UK to carry out doctoral studies in polymer engineering within the Department of Chemical Engineering and Chemical Technology at Imperial College (University of London). After obtaining her Ph.D. in 1993, she took up a position as a Lecturer in Physical Chemistry at the University of Greenwich in South East London. Barbara returned to Australia in 1995, joining the staff at the University of Technology, Sydney, where she is currently a Senior Lecturer in the Department of Chemistry, Materials and Forensic Science. She is presently conducting research in the fields of polymer spectroscopy, materials conservation and forensic science. Barbara is the author of three other books published by John Wiley and Sons, Ltd, namely Modern Infrared Spectroscopy and Biological Applications of Infrared Spectroscopy, both in the ACOL series of open learning texts, and Polymer Analysis in this current AnTS series of texts.

Read an Excerpt

Infrared Spectroscopy

Fundamentals and Applications

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By Barbara H. Stuart

John Wiley & Sons

Copyright © 2004 John Wiley & Sons, Ltd
All right reserved.
ISBN: 0-470-85428-6

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Chapter One

Introduction

Learning Objectives

To understand the origin of electromagnetic radiation.

To determine the frequency, wavelength, wavenumber and energy change associated with an infrared transition.

To appreciate the factors governing the intensity of bands in an infrared spectrum.

To predict the number of fundamental modes of vibration of a molecule.

To understand the influences of force constants and reduced masses on the frequency of band vibrations.

To appreciate the different possible modes of vibration.

To recognize the factors that complicate the interpretation of infrared spectra.

Infrared spectroscopy is certainly one of the most important analytical techniques available to today's scientists. One of the great advantages of infrared spectroscopy is that virtually any sample in virtually any state may be studied. Liquids, solutions, pastes, powders, films, fibres, gases and surfaces can all be examined with a judicious choice of sampling technique. As a consequence of the improved instrumentation, a variety of new sensitive techniques have now been developed in order to examine formerly intractable samples.

Infrared spectrometers have beencommercially available since the 1940s. At that time, the instruments relied on prisms to act as dispersive elements, but by the mid 1950s, diffraction gratings had been introduced into dispersive machines. The most significant advances in infrared spectroscopy, however, have come about as a result of the introduction of Fourier-transform spectrometers. This type of instrument employs an interferometer and exploits the well-established mathematical process of Fourier-transformation. Fourier-transform infrared (FTIR) spectroscopy has dramatically improved the quality of infrared spectra and minimized the time required to obtain data. In addition, with constant improvements to computers, infrared spectroscopy has made further great strides.

Infrared spectroscopy is a technique based on the vibrations of the atoms of a molecule. An infrared spectrum is commonly obtained by passing infrared radiation through a sample and determining what fraction of the incident radiation is absorbed at a particular energy. The energy at which any peak in an absorption spectrum appears corresponds to the frequency of a vibration of a part of a sample molecule. In this introductory chapter, the basic ideas and definitions associated with infrared spectroscopy will be described. The vibrations of molecules will be looked at here, as these are crucial to the interpretation of infrared spectra.

Once this chapter has been completed, some idea about the information to be gained from infrared spectroscopy should have been gained. The following chapter will aid in an understanding of how an infrared spectrometer produces a spectrum. After working through that chapter, it should be possible to record a spectrum and in order to do this a decision on an appropriate sampling technique needs to be made. The sampling procedure depends very much on the type of sample to be examined, for instance, whether it is a solid, liquid or gas. Chapter 2 also outlines the various sampling techniques that are commonly available. Once the spectrum has been recorded, the information it can provide needs to be extracted. Chapter 3, on spectrum interpretation, will assist in the understanding of the information to be gained from an infrared spectrum. As infrared spectroscopy is now used in such a wide variety of scientific fields, some of the many applications of the technique are examined in Chapters 4 to 8. These chapters should provide guidance as to how to approach a particular analytical problem in a specific field. The applications have been divided into separate chapters on organic and inorganic molecules, polymers, biological applications and industrial applications. This book is, of course, not meant to provide a comprehensive review of the use of infrared spectroscopy in each of these fields. However, an overview of the approaches taken in these areas is provided, along with appropriate references to the literature available in each of these disciplines.

1.1 Electromagnetic Radiation

The visible part of the electromagnetic spectrum is, by definition, radiation visible to the human eye. Other detection systems reveal radiation beyond the visible regions of the spectrum and these are classified as radiowave, microwave, infrared, ultraviolet, X-ray and [gamma]-ray. These regions are illustrated in Figure 1.1, together with the processes involved in the interaction of the radiation of these regions with matter. The electromagnetic spectrum and the varied interactions between these radiations and many forms of matter can be considered in terms of either classical or quantum theories.

The nature of the various radiations shown in Figure 1.1 have been interpreted by Maxwell's classical theory of electro- and magneto-dynamics - hence, the term electromagnetic radiation. According to this theory, radiation is considered as two mutually perpendicular electric and magnetic fields, oscillating in single planes at right angles to each other. These fields are in phase and are being propagated as a sine wave, as shown in Figure 1.2. The magnitudes of the electric and magnetic vectors are represented by E and B, respectively.

A significant discovery made about electromagnetic radiation was that the velocity of propagation in a vacuum was constant for all regions of the spectrum. This is known as the velocity of light, c, and has the value 2.997 925 × [10.sup.8] m [s.sup.-1]. If one complete wave travelling a fixed distance each cycle is visualized, it may be observed that the velocity of this wave is the product of the wavelength, [lambda] (the distance between adjacent peaks), and the frequency, v (the number of cycles per second). Therefore:

(1.1) c = [lambda]v

The presentation of spectral regions may be in terms of wavelength as metres or sub-multiples of a metre. The following units are commonly encountered in spectroscopy:

1 Å = [10.sup.-10] m 1 nm = [10.sup.-9] m 1 µm = [10.sup.-6] m

Another unit which is widely used in infrared spectroscopy is the wavenumber, [bar.v], in [cm.sup.-1]. This is the number of waves in a length of one centimetre and is given by the following relationship:

(1.2) [bar.v] = 1/[lambda] = v/c

This unit has the advantage of being linear with energy.

During the 19th Century, a number of experimental observations were made which were not consistent with the classical view that matter could interact with energy in a continuous form. Work by Einstein, Planck and Bohr indicated that in many ways electromagnetic radiation could be regarded as a stream of particles (or quanta) for which the energy, E, is given by the Bohr equation, as follows:

(1.3) E = hv

where h is the Planck constant (h = 6.626 × [10.sup.-34] J s) and v is equivalent to the classical frequency.

Processes of change, including those of vibration and rotation associated with infrared spectroscopy, can be represented in terms of quantized discrete energy levels [E.sub.0], [E.sub.1], [E.sub.2], etc., as shown in Figure 1.3. Each atom or molecule in a system must exist in one or other of these levels. In a large assembly of molecules, there will be a distribution of all atoms or molecules among these various energy levels. The latter are a function of an integer (the quantum number) and a parameter associated with the particular atomic or molecular process associated with that state. Whenever a molecule interacts with radiation, a quantum of energy (or photon) is either emitted or absorbed. In each case, the energy of the quantum of radiation must exactly fit the energy gap [E.sub.1] - [E.sub.0] or [E.sub.2] - [E.sub.1], etc. The energy of the quantum is related to the frequency by the following:

(1.4) [DELTA]E = hv

Hence, the frequency of emission or absorption of radiation for a transition between the energy states [E.sub.0] and [E.sub.1] is given by:

(1.5) v = ([E.sub.1] - [E.sub.0])/h

Associated with the uptake of energy of quantized absorption is some deactivation mechanism whereby the atom or molecule returns to its original state. Associated with the loss of energy by emission of a quantum of energy or photon is some prior excitation mechanism. Both of these associated mechanisms are represented by the dotted lines in Figure 1.3.

1.2 Infrared Absorptions

For a molecule to show infrared absorptions it must possess a specific feature, i.e. an electric dipole moment of the molecule must change during the vibration. This is the selection rule for infrared spectroscopy. Figure 1.4 illustrates an example of an 'infrared-active' molecule, a heteronuclear diatomic molecule. The dipole moment of such a molecule changes as the bond expands and contracts. By comparison, an example of an 'infrared-inactive' molecule is a homonuclear diatomic molecule because its dipole moment remains zero no matter how long the bond.

An understanding of molecular symmetry and group theory is important when initially assigning infrared bands. A detailed description of such theory is beyond the scope of this book, but symmetry and group theory are discussed in detail in other texts. Fortunately, it is not necessary to work from first principles each time a new infrared spectrum is obtained.

Infrared absorptions are not infinitely narrow and there are several factors that contribute to the broadening. For gases, the Doppler effect, in which radiation is shifted in frequency when the radiation source is moving towards or away from the observer, is a factor. There is also the broadening of bands due to the collisions between molecules. Another source of line broadening is the finite lifetime of the states involved in the transition. From quantum mechanics, when the Schrödinger equation is solved for a system which is changing with time, the energy states of the system do not have precisely defined energies and this leads to lifetime broadening. There is a relationship between the lifetime of an excited state and the bandwidth of the absorption band associated with the transition to the excited state, and this is a consequence of the Heisenberg Uncertainty Principle. This relationship demonstrates that the shorter the lifetime of a state, then the less well defined is its energy.

1.3 Normal Modes of Vibration

The interactions of infrared radiation with matter may be understood in terms of changes in molecular dipoles associated with vibrations and rotations. In order to begin with a basic model, a molecule can be looked upon as a system of masses joined by bonds with spring-like properties. Taking first the simple case of diatomic molecules, such molecules have three degrees of translational freedom and two degrees of rotational freedom. The atoms in the molecules can also move relative to one other, that is, bond lengths can vary or one atom can move out of its present plane. This is a description of stretching and bending movements that are collectively referred to as vibrations. For a diatomic molecule, only one vibration that corresponds to the stretching and compression of the bond is possible. This accounts for one degree of vibrational freedom.

Polyatomic molecules containing many (N) atoms will have 3N degrees of freedom. Looking first at the case of molecules containing three atoms, two groups of triatomic molecules may be distinguished, i.e. linear and non-linear. Two simple examples of linear and non-linear triatomics are represented by C[O.sub.2] and [H.sub.2]O, respectively (illustrated in Figure 1.5). Both C[O.sub.2] and [H.sub.2]O have three degrees of translational freedom. Water has three degrees of rotational freedom, but the linear molecule carbon dioxide has only two since no detectable energy is involved in rotation around the O=C=O axis. Subtracting these from 3N, there are 3N-5 degrees of freedom for C[O.sub.2] (or any linear molecule) and 3N-6 for water (or any non-linear molecule). N in both examples is three, and so C[O.sub.2] has four vibrational modes and water has three. The degrees of freedom for polyatomic molecules are summarized in Table 1.1.

Whereas a diatomic molecule has only one mode of vibration which corresponds to a stretching motion, a non-linear B-A-B type triatomic molecule has three modes, two of which correspond to stretching motions, with the remainder corresponding to a bending motion. A linear type triatomic has four modes, two of which have the same frequency, and are said to be degenerate.

Two other concepts are also used to explain the frequency of vibrational modes. These are the stiffness of the bond and the masses of the atoms at each end of the bond. The stiffness of the bond can be characterized by a proportionality constant termed the force constant, k (derived from Hooke's law). The reduced mass, µ, provides a useful way of simplifying our calculations by combining the individual atomic masses, and may be expressed as follows:

(1.6) (1/µ) = (1/[m.sub.1]) + (1/[m.sub.2])

where [m.sub.1] and [m.sub.2] are the masses of the atoms at the ends of the bond. A practical alternative way of expressing the reduced mass is:

(1.7) µ = [m.sub.1][m.sub.2]/([m.sub.1] + [m.sub.2])

The equation relating the force constant, the reduced mass and the frequency of absorption is:

(1.8) v = (1/2[pi])[square root of](k/µ)

This equation may be modified so that direct use of the wavenumber values for bond vibrational frequencies can be made, namely:

(1.9) [bar.v] = (1/2[pi]c)[square root of](k/µ)

where c is the speed of light.

A molecule can only absorb radiation when the incoming infrared radiation is of the same frequency as one of the fundamental modes of vibration of the molecule. This means that the vibrational motion of a small part of the molecule is increased while the rest of the molecule is left unaffected.

Vibrations can involve either a change in bond length (stretching) or bond angle (bending) (Figure 1.6). Some bonds can stretch in-phase (symmetrical stretching) or out-of-phase (asymmetric stretching), as shown in Figure 1.7.

Continues...

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Table of Contents

Series Preface.

Preface.

Acronyms, Abbreviations and Symbols.

About the Author.

1. Introduction.

1.1 Electromagnetic Radiation.

1.2 Infrared Absorptions.

1.3 Normal Modes of Vibration.

1.4 Complicating Factors.

1.4.1 Overtone and Combination Bands.

1.4.2 Fermi Resonance.

1.4.3 Coupling.

1.4.4 Vibration–Rotation Bands.

References.

2. Experimental Methods.

2.1 Introduction.

2.2 Dispersive Infrared Spectrometers.

2.3 Fourier-Transform Infrared Spectrometers.

2.3.1 Michelson Interferometers.

2.3.2 Sources and Detectors.

2.3.3 Fourier-Transformation.

2.3.4 Moving Mirrors.

2.3.5 Signal-Averaging.

2.3.6 Advantages.

2.3.7 Computers.

2.3.8 Spectra.

2.4 Transmission Methods.

2.4.1 Liquids and Solutions.

2.4.2 Solids.

2.4.3 Gases.

2.4.4 Pathlength Calibration.

2.5 Reflectance Methods.

2.5.1 Attenuated Total Reflectance Spectroscopy.

2.5.2 Specular Reflectance Spectroscopy.

2.5.3 Diffuse Reflectance Spectroscopy.

2.5.4 Photoacoustic Spectroscopy.

2.6 Microsampling Methods.

2.7 Chromatography–Infrared Spectroscopy.

2.8 Thermal Analysis–Infrared Spectroscopy.

2.9 Other Techniques.

References.

3. Spectral Analysis.

3.1 Introduction.

3.2 Group Frequencies.

3.2.1 Mid-Infrared Region.

3.2.2 Near-Infrared Region.

3.2.3 Far-Infrared Region.

3.3 Identification.

3.4 Hydrogen Bonding.

3.5 Spectrum Manipulation.

3.5.1 Baseline Correction.

3.5.2 Smoothing.

3.5.3 Difference Spectra.

3.5.4 Derivatives.

3.5.5 Deconvolution.

3.5.6 Curve-Fitting.

3.6 Concentration.

3.7 Simple Quantitative Analysis.

3.7.1 Analysis of Liquid Samples.

3.7.2 Analysis of Solid Samples.

3.8 Multi-Component Analysis.

3.9 Calibration Methods.

References.

4. Organic Molecules.

4.1 Introduction.

4.2 Aliphatic Hydrocarbons.

4.3 Aromatic Compounds.

4.4 Oxygen-Containing Compounds.

4.4.1 Alcohols and Phenols.

4.4.2 Ethers.

4.4.3 Aldehydes and Ketones.

4.4.4 Esters.

4.4.5 Carboxylic Acids and Anhydrides.

4.5 Nitrogen-Containing Compounds.

4.5.1 Amines.

4.5.2 Amides.

4.6 Halogen-Containing Compounds.

4.7 Heterocyclic Compounds.

4.8 Boron Compounds.

4.9 Silicon Compounds.

4.10 Phosphorus Compounds.

4.11 Sulfur Compounds.

4.12 Near-Infrared Spectra.

4.13 Identification.

References.

5. Inorganic Molecules.

5.1 Introduction.

5.2 General Considerations.

5.3 Normal Modes of Vibration.

5.4 Coordination Compounds.

5.5 Isomerism.

5.6 Metal Carbonyls.

5.7 Organometallic Compounds.

5.8 Minerals.

References.

6. Polymers.

6.1 Introduction.

6.2 Identification.

6.3 Polymerization.

6.4 Structure.

6.5 Surfaces.

6.6 Degradation.

References.

7. Biological Applications.

7.1 Introduction.

7.2 Lipids.

7.3 Proteins and Peptides.

7.4 Nucleic Acids.

7.5 Disease Diagnosis.

7.6 Microbial Cells.

7.7 Plants.

7.8 Clinical Chemistry.

References.

8. Industrial and Environmental Applications.

8.1 Introduction.

8.2 Pharmaceutical Applications.

8.3 Food Science.

8.4 Agricultural Applications.

8.5 Pulp and Paper Industries.

8.6 Paint Industry.

8.7 Environmental Applications.

References.

Responses to Self-Assessment Questions.

Bibliography.

Glossary of Terms.

SI Units and Physical Constants.

Periodic Table.

Index.