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...