Introduction, Basic Theory, and Principles

1 Introduction

The vibrational spectroscopies, infrared and Raman, are techniques that are widely used in industry. They provide information on the chemical structures and physical characteristics of materials; they are used for the identification of substances by 'fingerprinting'; and they are used to provide quantitative or semi-quantitative information on products and processes. Samples may be examined in bulk or microscopic amounts over a wide range of temperatures, from very hot to very cold, in a whole range of physical states, e.g. as vapours, liquids, latexes, powders, films, and fibres, or as a surface or an embedded layer. The techniques have a very broad range of applications and provide solutions to a host of interesting, commercially important, and challenging analytical problems. They are used to analyse and characterise feedstocks, catalysts, by-products, end and formulated products, processed and fabricated materials, and in deformulation (reverse engineering) studies of competitors' products.

In the Research Laboratory, vibrational spectroscopies are frequently used for reaction following, or for giving chemical group information on new compounds. They are amongst the few techniques which can assist molecular interaction studies such as hydrogen bonding, and provide molecular orientation information for surface studies. In the Process Development and Works environments, quantitative information for process monitoring and product quality assurance/control (QA/QC) can be very important; an increasing development here is in the use of multicomponent analysis of infrared and Raman data, employing regressional analysis and other chemometric treatments. Simple 'fingerprinting' techniques are used extensively in identifying raw materials, but more often in characterising formulated products. The latter is an important QA technique, and a common first step in dealing with customer problems in support of Technical Service/Marketing Departments. Industrially, Raman has been a much less used technique than infrared spectroscopy, largely due to problems associated with colour and fluorescence. However, with recent advances in instrument technology, coupled with the ability to use Raman to effectively examine aqueous solutions and samples inside glass containers, there is a rapid increase in industrial applications of the Raman technique.

Infrared (IR) spectroscopy and Raman spectroscopy are both vibrational techniques; the former is concerned essentially with the absorption of radiation, the latter with scattered radiation. The two techniques are complementary. Their spectra may be considered as being recorded essentially over the same spectral range; both give rise to bands in similar positions originating from the same chemical group. Generally, vibrations which have large changes of dipole moment, e.g. the stretching of a carbonyl group (vC=O), gives rise to strong infrared bands and much weaker Raman bands, whilst for vibrations from groups which cause large changes in polarisability, e.g. the symmetrical unsaturated group (vC=C), the reverse is true, i.e. the band within the Raman spectrum will be relatively much stronger, and weak or even absent in its infrared counterpart. (Here, v is the notation used to describe a fundamental stretching vibration frequency.)

Although the theory and basic principles of each technique and interpretation of their spectra have similarities, important differences need to be highlighted. However, it is not our intention in this chapter to give an in-depth theoretical approach to the interpretation of vibrational spectra. Basic principles will be set out to enable an initial assessment of spectra and to help avoid a few pitfalls. In infrared spectroscopy, a spectrum is recorded of the absorption of energy from photons by the vibrations of molecular bonds, as the irradiating frequency/wavelength is varied. Raman spectroscopy records the spectrum of light scattered by the molecule when excited by a monochromatic beam of radiation. This latter record contains radiation at the exciting line frequency and bands shifted by amounts equal to the frequency of the molecular vibrations. The strength and shape of the bands within a spectrum are dependent on the chemical and physical state of the molecules, the sample preparation method, the accessory used to mount or contain the prepared sample, and the operating conditions of the spectrometer. Spectra may also be presented in different ways after manipulation by computer software packages. Interpretation of a spectrum requires knowledge of all these factors, which may be affecting the spectrum. This may appear to be a daunting task, but by observing and remembering a few basic principles everything else becomes essentially a variation on a basic theme. The black art element sometimes ascribed to vibrational spectroscopy is then much diminished. In attempting interpretation, in an industrial context, a too focused theoretical assignment of each individual band can lead to loss of sight of the overall picture. Simple practical and supplementary information may be ignored and answers are sometimes generated which common sense should tell us are impossible, e.g. a black powder cannot be acetone, but may be 'wet' with the solvent! (True: we have been presented with interpretation examples like this.) In this chapter we will deal with the simple theory, describe the factors affecting interpretation, and then attempt to set out a step-by-step approach to interpretation, which should be sufficient to establish a working knowledge and a practical approach to problem-solving. Once this is established, the interested, or insatiable, should delve further with the help of the bibliography at the end of the chapter.

2 Simple Theory

We will start with the presumption that the spectrum obtained is meant to solve a problem, and therefore contains some information of value. (All correctly acquired vibrational spectra contain some information, even if it is negative information.) Obtaining a spectrum for its own sake is not the fundamental purpose, in most industrial situations. As vibrational spectroscopy employs some apparently strange units, we will start by determining its position in the electromagnetic spectrum.

The so-called mid-infrared range, as the wavelength (λ) range is usually referred to, is approximately 2.5-25 microns (µm), which is equivalent approximately to 4000–400 cm-1. The latter units are wavenumbers ([??]) or reciprocal centimetres (cm-1), not frequency (v). (The equivalent frequency values are 120 THz to 12 THz, (1.2 × 1014 Hz to 12 × 1012 Hz); in terms of photon energy this corresponds to ~0.5 eV to ~0.05 eV.) Wavenumbers is the commonly used notation; strictly speaking Raman shifts should be referred to as delta cm-1 (cm-1). Early infrared spectrophotometers recorded spectra which were linear in wavelength; (and a few aficionados may still be found who employ these units, particularly if they are entrenched in long-established, hard-copy reference libraries of spectra). However, the majority of information of interest to organic chemists occurs in the 1800–600 cm-1 (~5.5–15 µm) range. Linear wavenumber scales make band positions generally easier to resolve and discuss; 1645 ('sixteen forty-five') and 1650 ('sixteen fifty') are significantly easier numbers to remember and debate than 6.055 and 6.095. A linear wavelength scale compresses the broad vOH and vNH bands, in the 2.5–4 µm region, compared to a linear wavenumber (4000–2500 cm-1) plot, whilst the bands below 10 µm (1000 cm-1) are relatively more spread out. Notwithstanding, a key basic point to remember at this stage is that we are dealing with infrared wavelengths of the order of 1–15 microns. This becomes particularly important and relevant when discussing particle size effects, penetration depths, spectral artefacts, and microscopy in later chapters.

3 Molecular Vibrations

Molecules vibrate when struck by a photon. If the frequency (v) and hence the energy (E) of the photon matches a fundamental vibration of the molecule and causes resonance, the molecule absorbs energy from the photon, see Figure 1, (cf. tuning forks and tables, sopranos and broken glass); h is Planck's constant; E = hv = hc/λ. = hc[bar.v], where c represents the speed of light in vacuo. This interaction results in an infrared absorption band. If the photon is not absorbed, but inelastically scattered, this scattered radiation contains some information about the molecular vibrations. This information appears generally as very weak bands compared to the intensity of the elastically scattered radiation, with wavenumber shifts from that of the irradiating wavelength. These shifts have values similar to the positions of related infrared absorption bands and the phenomenon is known as Raman scattering. Initially we will concentrate on the infrared interactions.

A linear triatomic molecule, such as CO2, has three fundamental modes of vibration, a symmetric stretch, a bending or deformation mode, and an antisymmetric stretch. These are labelled as v1, v2 and v3 respectively. A bent molecule, such as H2O, has four normal modes of vibration, two of which (orthogonal bending motions) are degenerate (i.e. of identical frequency), see Figure 2. (Deformation modes are more commonly denoted by the symbol, δ).

In Figure 2 these vibrations are depicted as 'linear spring and ball-like' models, whereas, in a three-dimensional sense, the molecules can be viewed as electron clouds with sausage- and kidney-like shapes. Each molecule is now shown, in Figure 3, as a cloud of charges. As the molecule vibrates, changes in dipole moments may occur. Only those vibrations of a molecule which exhibit a strong change of dipole moment have strong infrared absorption bands. For example, carbonyl groups, [??]C=O, which are both asymmetric and ionic in nature have strong bands in the infrared spectrum. Symmetrical groups such as unsaturated -C=C-, and the dithio, -S-S-, are weak infrared absorbers, but have strong Raman bands. Although overall the dipole moment may not change in the stretching of symmetrical molecular moieties, the electron cloud may deform. This change of polarisability leads to the Raman band, as explained in the next section. It can be seen from Figures 2 and 3 that the symmetrical stretch of a CO2 molecule leads to no overall change in dipole moment, and hence this vibration is infrared inactive. (Vibrations of diatomic molecules, such as N2 and O2 are infrared inactive, but give rise to Raman bands.)

4 Selection Rules

In energy level terms, the transition from the ground state to the next energy level by the absorption of one quantum of light is a fundamental vibration. This is the mechanism that causes the appearance of absorption bands in the IR spectrum, when a sample is exposed to polychromatic light. If a sample is exposed to monochromatic light, which is not absorbed, additional frequencies may be observed in the scattered light. The latter can be considered as three simple cases, see Table 1.

A molecule is instantaneously excited to a virtual (unstable) state by a photon and immediately drops back to its original vibrational energy level. Overall no energy change occurs and the scattered photon maintains the original frequency. This elastic collision is the source of the intense 'Rayleigh' line (exciting line) in the scattered radiation spectrum. If the molecule drops back from the excited (virtual) state not to the ground state but to the first energy level, the photon energy is reduced. This appears as a 'Stokes' line. A few molecules exist already in an excited state at the first energy level. These may, by a similar process, be excited to an unstable energy level, but fall back to the ground state. The energy of the photon increases, which causes the appearance of an 'anti-Stokes' line. These are comparatively much weaker, progressively so with increasing wavenumber shifts, as the majority of molecules are in the ground state at room temperature. These effects were first observed experimentally, in 1929, by C.V. Raman and K.S. Krishnan. Since then the phenomenon has been referred to as Raman spectroscopy.

At the beginning of this section molecular vibrations were seen to be due to stretching, bending, and scissors-like motions, with strong dipole moment changes causing strong IR bands, and weak Raman bands, while symmetrical polarisability changes are the source of strong Raman bands with weak IR bands. Figure 4 illustrates the complementary information from the simple molecule, carbon disulphide.

Predicting the principal IR absorption bands for small molecules is relatively simple. The number of normal vibrations (B), which is related to the vibrational degrees of freedom, can be calculated from the formula:

B = 3N - 6, or, for a linear molecule, B = 3N - 5,

where N is the number of atoms in the molecule. For large polyatomic molecules, the number of bands becomes very large, and will also include overtone (harmonics) and combination bands. The spectrum would thus be a plethora of peaks and impossible to interpret, except for the fact that fortunately many of these bands overlap, and what we see at room temperature are broad envelopes with recognisable positions and shapes. Bands in a spectrum arise from the absorption of energy or radiation scattering, caused by chemical groups of two or more atoms, i.e. not individual atoms vibrating. Band (vibration) positions and shapes will also be influenced by the overall shape of the molecule, molecular conformations and orientations, and molecular packing (crystallinity, density). Much time can be spent in assigning these 'fingerprint' bands to the bending, stretching, or scissors modes, but this does not necessarily help in the majority of first attempts to identify materials from their vibrational spectra. For the molecule and its spectrum shown in Figure 5, for example, all the vOH groups give characteristic strong bands in the 3500–2000 cm-1 region; a carbonyl band at approximately 1700 cm-1 is consistent with a carboxylic acid; and sharp bands in the 3100, 1600 and 800–700 cm-1 regions are attributable to the aromatic ring. A band at 950 cm-1 is present due to carboxylic acids forming dimers. Many of the bands below 1500 cm-1 appear in the spectrum arising from the total molecule vibrations i.e. the 'fingerprint' bands. The Raman spectrum appears somewhat simpler as the strong bands in the IR spectrum due to hydrogen bonding and asymmetry are much reduced. The strong bands in the Raman spectrum are largely due to the mono-substituted aromatic group. The band at 2900 Δcm-1, due to the CH2 group, is hidden somewhat by the strong vOH bands in the IR spectrum, but can be clearly seen in the Raman spectrum.

5 Practical Interpretation of Spectra

The phrase 'interpretation of vibrational spectra' can be employed in many different ways. The spectrum of a molecule can be the subject of a detailed mathematical interpretation which might take several years. At the other extreme, a cursory five second look at a spectrum will, based on familiarity, produce the interpretation, 'Yes, that is ethanol'. In this chapter we are concerned with the practitioner who wishes to solve a problem by using vibrational spectroscopy to identify a substance or to provide structural information. We do not have time or space for a full, rigorous, mathematical approach. Instead, we will concentrate on assisting the reader to attain a practical and pragmatic, if not theoretically polished, result to the interpretation of a vibrational spectrum. This is achieved by being aware of the features which can lead the unwary to an erroneous result.

The positions of bands from specific groups, and the overall shape of the spectrum are dependent on the chemical and physical environment of the sample being examined. Whether the molecules are in a gaseous, liquid, solid, or polymeric form will affect their ability and degrees of freedom to vibrate. In general, vapours and crystalline solids have sharper spectra, whilst liquids and polymers have broader spectra. Temperature, pressure, and polymorphism will also most likely affect the physical forms, and hence the spectra. Chemical groups and total molecular shape may also respond to hydrogen bonding and pH changes. Tables 2(a) to 2(d) give an indication of where the more common groups have bands in the mid-infrared region of the spectrum. The bars indicate likely positions for guidance and are not absolute. The intensity of the bars give very approximate indications of relative band strengths. Band shape, strength, splittings, and associated group correlations must also be taken into account.