Infrared Intensities and Molecular Structures of Inorganic and Coordination Compounds

Abstract

Abstract The observed infrared absorption band of a vibrating molecule may be characterized by several parameters, including the position, i.e., frequency, the intensity of the absorption areas, and the shapes o r band half-width. Whereas the frequency of a band depends upon the geometrical distribution of masses undergoing oscillation and the forces holding the nuclei together, the intensity is governed mainly by the charge redistribution within the bond under specific vibrational transition. During recent years the intensities of infrared bands have attracted considerable interest in the field of vibrational spectroscopy of polyatomic molecules. In earlier articles [1,2] we discussed the vibrational band intensities pertaining to some typical classes of hydrocarbons, and the present article is devoted to the infrared intensities. of inorganic and coordination compounds. A careful literature survey reveals that progress in measuring the absolute infrared intensities of inorganic and coordination compounds has been relatively slow and scanty in comparison to organic molecules. However, it has been realized that the intensities of infrared bands, although more difficult to measure accurately than frequencies, possess greater intrinsic value as far as molecular properties are concerned and play an important role in understanding the nature of molecular vibrations and chemical groups and in deriving information regarding the polar properties of bonds. In addition, the quantities derived from experimentally measured IR intensities, e. g., charges on atoms in molecules, have recently achieved widespread attention which in turn could be used to supplement the results of nuclear quadrupole resonance spectroscopy (NQR) and electron scanning chemical analysis (ESCA). The significance as well as necessity of intensity measurements have been stressed on many occasions, particularly after the appearance of two main papers, one by Thompson [3] and the other by Coulson [4]. Since then many reviews [5–10], reports [11–20], and monographs [21–25] by internationally established authorities have dealt with the intensity problem. Methods to compute theoretical infrared intensities have also been published [26–34]. More recently, Overend [35] offered a plea for the routine measurements of integrated vibrational band intensities. Therefore, it can be said that the last two decades have seen a growth of both theoretical understandings and techniques of experimental measurements of the intensities of molecules and ions. Hence, the IR intensity spectroscopy of inorganic and coordination compounds warrants a systematic discussion and criticism of the subject.