Vibrational spectroscopy has both a long and interesting history and an illustrious record of contributions to science. This five-volume compendium is a monumental tribute to the scope and importance of the field.
The subject employs two principal techniques, infrared and Raman spectroscopy, and a number of more specialized ones. Infrared is older than Raman by 128 years. This region of the spectrum was discovered in 1800 by William Herschel, a professional musician who had astronomy as a hobby and became so good at it that he was made Britain's Astronomer Royal. Among the most significant results from infrared studies during the next century or so were the following. (a) The first recognition of infrared characteristic group frequencies. From studies in the photographic infrared, W. deW. Abney and E. R. Festing proposed in 1881 that absorption at certain wavelengths can be attributed to specific chemical groups, and recognized the applicability of infrared absorption to problems in organic chemistry. This was extended by W. H. Julius, who showed in 1892 that the presence of a methyl or methylene group in a molecule gives absorption near 3.45 microns (2900 cm-1 ). (b) Samuel Pierpont Langley's infrared measurements in the early 1880s. Prior to that time the infrared spectrum had been thought to terminate near one micron. Langley refined the bolometer as an infrared detector and devised clever methods for calibrating the wavelength scale as far as 5.3 microns. He showed that solar emission, which goes to zero near one micron, recovers at longer wavelengths after passing beyond the bands of atmospheric water. The infrared spectrum did not terminate after all! Langley also made the first accurate map of the solar spectrum. (c) Experiments, especially by Heinrich Rubens and E. F. Nicols, which extended infrared measurements to much longer wavelengths and closed the gap between infrared and Hertzian (radio) waves. (d) Measurements of the spectral emission of a black body at various temperatures by Otto Lummer and Ernst Pringsheim, against which Max Planck tested his revolutionary quantum theory. (e) The pioneering work of William W. Coblentz which appeared in his 1905 book Investigations of Infra-Red Spectra. This body of work was far ahead of its time, and most of his spectra were not bettered for 40 years. (His infrared spectrum of pyrrole was used in my doctoral thesis in 1941!)
The use of infrared spectroscopy exploded during and shortly after World War II, and the technique has remained an important tool ever since. It is my opinion that the infrared spectrum is the most important physical property available for characterizing molecules because of its combination of uniqueness and broad applicability. Other techniques, especially nuclear magnetic resonance and mass spectroscopy, will do certain things better. However infrared is highly specific, is applicable to any physical state, and can be used for all molecules, both organic and inorganic, except homonuclear diatomics. Hence it is unexcelled for its combination of specificity and versatility.
In spite of this capability, the teaching of infrared spectroscopy in most colleges and universities, and especially the use of characteristic group frequencies in organic chemistry, is now superficial. The subject is viewed as an out-moded fringe topic which anyone can pick up by a little reading. Not so! In academia infrared has largely been replaced by nmr. Nmr certainly merits the attention it receives, but infrared and Raman spectroscopy can often contribute a great deal very simply, and they deserve more emphasis.
Infrared spectroscopy has had remarkable resilience as a useful field of research. Several times during my career I thought that it was time to leave the field for something else. On each occasion advances came along which re-vitalized the subject and offered new opportunities. Modern infrared spectroscopy started in the 1940s and 1950s with the tremendous improvements in instrumentation which put the technique at the heart of chemical research. Myriads of applications were pursued. Later new methods of sample handling were developed, such as the KBr pressed disk technique, attenuated total reflection (ATR), long path gas cells, the diamond anvil cell, and low temperature spectroscopy and matrix isolation. The far infrared region became more accessible. After another pause, Fourier Transform techniques were introduced which revolutionized the field with their vastly improved instrumental performance. Will there be further important advances? I believe so. There is the long-hoped-for continuously tunable infrared laser. If it comes, there will be another quantum jump in the importance of the field. Another useful advance would be fiber optics which are transparent throughout the mid-infrared.
The Raman effect was discovered in 1928. After an impressive initial burst of activity, the field became rather quiescent because of the feebleness of the effect and the serious interference from the fluorescence of many samples. The development of infrared instrumentation during 1940-1960 further relegated Raman spectroscopy to a minor role. However it too has had several re-awakenings. The first was due to the introduction of electronic rather than photographic recording of Raman spectra. The second resulted from the use of lasers as sources, initially done in 1962. The third followed the introduction of a near infrared laser source and Fourier Transform methods by D. B. Chase and Tomas Hirschfeld in 1986. This largely solved the problem of fluorescence. Today it is usually easier to obtain the Raman spectrum of a sample than its infrared spectrum because of much simpler sample preparation --- no alkali halide windows, no restrictions on sample thickness, no need for mulls or KBr pressed disks or ATR devices or diffuse reflection. This reversal in ease of acquisition is astounding to a long-time vibrational spectroscopist. As a result, the use of the Raman effect has also exploded. However Raman spectra are still not as generally useful as infrared ones for several reasons: there are not as many good reference spectra, there is not as much information on characteristic group frequencies, and quantitative analyses are much more difficult because there is no equivalent to the Lambert-Beer Law.
There are many special techniques, and myriads of applications, for both infrared and Raman spectroscopy. In addition there are also other methods for obtaining vibrational frequencies. Two older ones are the analysis of high-resolution electronic spectra of gases (outside the scope of this work), and inelastic neutron scattering. Several newer techniques are mentioned in the first chapter by Prof. Norman Sheppard and in several later chapters.
The fact that five volumes are needed for this Handbook of Vibrational Spectroscopy is ample evidence for the extent and importance of the field. This compendium provides a valuable, detailed survey of vibrational spectroscopy and a useful review of its literature. It is an impressive achievement for which I salute the Editors and Contributors.
Foil A. Miller
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