Theory of Infrared Spectroscopy

Abstract This article is devoted to the fundamentals of infrared spectroscopy. While it is well known that infrared spectroscopy is an extremely important analytical technique, its theoretical basis is not easily assimilated by most analytical chemists. It must be admitted that a certain understanding of abstract concepts, such as those encountered in quantum mechanics, is required. However, with all apologies to the reader, it is the objective here to present the essentials of these ideas, as needed in analytical applications of infrared spectroscopy. It should be noted that most of these principles apply equally to Raman spectroscopy, a subject which is described in other articles. It is the Born–Oppenheimer approximation that provides the theoretical basis for the analysis of infrared spectra. The rotational and vibrational degrees of freedom of a molecule are specified to allow the spectra of molecules in the gas phase to be interpreted. It is shown that the logical extension of these arguments can be employed in the analysis of the spectra of liquids and crystalline solids. In later sections, an effort has been made to explain how the basic ideas developed earlier can be applied to the interpretation of the infrared spectra of hydrogen‐bonded molecules, polymers and amorphous systems. A brief description of the notion of characteristic group frequencies, as routinely employed in organic chemistry, has been included to allow the practicing chemist to appreciate the possibilities – as well as the limitations – of this most important approach to the subject. To advance his appreciation of the advantages of infrared spectroscopy in analytical chemistry, the reader should refer to the various applications, as listed at the end of this article.