Perspectives in Modern Chemical Spectroscopy

Chemistry is primarily concerned with the structure and transformation of matter at the molecular level. Over the past few decades the rate of progress in evaluating the results of chemical reactions has been increasingly determined by the availability of a wide range of physical methods. With major help from these techniques, whole new frontier areas of inorganic, organic and biological chemistry have been opened up with great efficiency.

The principal techniques of vibrational spectroscopy, infrared absorption and Raman scattering have been practised for many years. They are general methods, giving qualitative and quantitative information directly on solids, liquids and gases, and on both the physical and chemical nature of the samples.

Infrared spectroscopy has long been recognised as a powerful technique for obtaining structural information about molecules and for analysis. The traditional limitation of the technique was that samples had to be presented in a manner suitable for transmission of an infrared light beam, so that highly scattering or opaque samples were unsuitable. Thus a very wide range of scientifically and commercially important materials were precluded from study. Whilst transmission methods are still important today, and indeed are used by many spectroscopists as reference methods, newer techniques have become available which have widened the range of possible applications considerably. In this chapter three of these new methods: diffuse reflectance, attenuated total reflectance and photoacoustics are considered in addition to transmission methods. Together these methods represent those most widely used by spectroscopists and can, between them, produce spectra from samples in almost any physical form.

Of the wide variety of spectroscopic techniques available to the modern spectroscopist, electronic absorption spectroscopy has perhaps the longest history, effectively tracing its origins back to the original work of Bunsen and Kirchoff on the spectroscopy of atomic species in flames. Consequently, the technique was responsible for the rapid expansion of the periodic table in the mid to late nineteenth century and, more importantly, for the development and testing of theories of atomic and molecular electronic structure in the early twentieth century. As a technique with a long history, it would be expected that electronic absorption spectroscopy would be widely used. This is true. However, we must contrast the use of electronic absorption spectroscopy with that of infrared and NMR techniques. While the latter are widely used in a qualitative sense, the former finds most use in quantitative measurements. This contrast is discussed briefly below. First, however, some basic theory needs to be considered.

Luminescence spectroscopy provides one of the most sensitive and selective methods of analysis for many inorganic and organic compounds. The techniques for measuring fluorescence, phosphorescence, chemiluminescence and bio-luminescence spectra have become highly developed since the mid-1950s. Measurements of the wavelength distributions of luminescence provide important information on the nature and energy of the emitting species. Measurements of the variation of luminescence intensity as a function of time can assist not only the identification of excited electronic states but also the determination of the efficiency with which the luminescence process occurs. The two main aims of this chapter are to provide:

NMR arises because of the magnetic moment associated with the non-zero spin quantum number of certain nuclei. Fortunately the occurrence of non-zero spin quantum numbers is common in the periodic table, and thus NMR can be observed in isotopes of most elements. Examples are given in Table 1. The magnetic moment, μ, associated with the quantum number, |I|, responds to a macroscopic magnetic field by orientating in it. 2|I| + 1 orientations are allowed. Each orientation corresponds to one energy level. Hydrogen (I = ½) has two energy levels, which correspond to orientations parallel (low energy) and anti-parallel to the magnetic field. Only one transition is observed therefore. Deuterium on the other hand has I = 1 so that three levels and two transitions occur; in practice however these two transitions may be degenerate so that only one resonance line is observed.

The high-resolution NMR spectroscopy of isotropic liquids or solutions has provided valuable structural information for many years (see P. Belton, Chapter 6). Considerable effort has been applied to provide more and more sensitive equipment, and more and more sophisticated analytical computer software and pulse programs. However, there are materials which are not liquids and will not dissolve in suitable solvents making this information unavailable. The past fifteen years has seen the advancement of an NMR technique which has allowed a great deal of structural information to be obtained from solid materials.

To understand the concepts of laser action, we first need to appreciate the nature of the stimulated emission process on which it is based. Molecules in excited states generally have very short decay lifetimes (often between 10⁻⁷s and 10⁻⁹s) and by releasing energy they rapidly undergo relaxation processes. In this way, they undergo transitions to more stable states of lower energy; there are many different mechanisms for the release of energy, some of which are radiative, in the sense that light is emitted, and some of which are non-radiative. However, although chemical distinctions can be made between different types of radiative decay such as fluorescence and phosphorescence (see Chap. 5), the essential physics is the same — photons are emitted which match the energy difference between the initially excited state and the final state involved in the transition. Since this kind of photon emission occurs without any external stimulus, it is referred to as spontaneous emission.

There are four — somewhat inelegant — questions to be asked about any spectroscopy: What is it? How does it occur? How do you measure it? What can you learn from it? Inevitably, the answers to these questions are inter-linked but I will try to separate them as I deal with each in turn.

Circular dichroism (Δε) is defined as the difference between the molar absorption coefficients for left (ε_L) and right (ε_R) circularly polarised light. A circular dichroism (CD) spectrum, consisting of a plot of Δε against wavelength or frequency, can therefore be observed only within absorption bands. CD generally arises from electronic transitions, although recently CD within vibrational bands has been measured. The two forms of CD spectroscopy to be discussed here are natural and magnetically induced.

Mass spectrometry evolved out of researches into particle physics in the late 19th century. J.J. Thomson built the parabola mass spectrograph, generally considered to be the first mass spectrometer worthy of the name, in the first decade of the twentieth century. He went on to recognise negative ions, isotopes, “metastable” ions and ion-molecule reactions, and also predicted the analytical possibilities of the technique [1,2]. Aston built on this work and constructed instruments which helped to establish the presence of isotopes in most elements in the periodic table, at the same time measuring the “accurate masses” of many simple nuclei. [3]. Between 1940 and the early 1950s, the number of mass spectrometers throughout the world increased, according to A.O. Nier, a pioneer in the field, from “probably fewer than a dozen…[to] many hundreds” [4]. Many early applications of mass spectrometry were confined to petroleum chemistry (with a few biological, nuclear and geological applications). However, by the late 1950s the potential of the technique in general organic chemistry had been realised. A crucial development was the combination of gas chromatography and mass spectrometry (GC/MS), which took place in the mid 1960s. This produced a very rapid expansion in the application of mass spectrometry to chemical and biochemical studies. A further development was the routine use of computers to supervise the acquisition and processing of data, which GC/MS instruments produce in enormous amounts. The 1960s also saw the introduction of chemical ionisation (CI) followed in the 1970s and early 1980s by desorption chemical ionisation (DCI), field desorption (FD) and fast atom bombardment (FAB). These techniques helped to extend the range of applications of mass spectrometry, yielding information about the molecular weights and structures of thermally unstable and involatile compunds. Recent innovations include: coupled liquid chromatography/mass spectrometry, tandem mass spectrometry, supercritical fluid chromatography/mass spectrometry, laser desorption, and the development of a diverse range of instruments, from small benchtop GC/MS systems to large, multiple-analyser devices. This arsenal of technology is capable of tackling such a wide variety of problems that today virtually all types of material can be analysed with the aid of mass spectrometry.

An unpaired electron can be aligned by an applied magnetic field so that its spin is precessing about the field. The direction of spin precession can be either clock- or anticlockwise corresponding to the two energy states of the electron (Fig. 1). Irradiation with an electromagnetic beam of the correct frequency can induce transitions of the unpaired electron from one spin direction to the other. The transition is induced by the oscillating magnetic field component of the electromagnetic radiation. Thus the transition is a magnetic dipole process and consequently the selection rule for the transition is that the spin component along the direction of the applied field, given by M_s, must change by ± 1 unit. The energy equation for resonancee is the well-known expression ${\rm h}v_0=g\mu_{\rm B}{\rm B}_0$ where g is the splitting factor, μ_B the Bohr magneton and B₀ the applied field. For a magnetic induction field of 0.33 T and a g-factor of 2.0 the microwave energy, hv₀, is 0.3 cm⁻¹. A resonance spectrum is obtained by placing the sample in a microwave cavity tuned to the frequency of the microwave radiation. An external magnetic field is applied and swept until the resonance condition is found.