Infra-red investigations of molecular structure. Part V.—The simplest kind of polyatomic molecule

Abstract
The purpose of the first papers of this series was the development of a machinery for the determination of the structure of simple polyatomic molecules. Studies of diatomic molecules were the means by which experience was to be gained for the more ambitious work. So much is now known of the diatomic molecule from the analysis of electronic band spectra that most infrared investigations can only hope to fill in details where, for many examples, the details are already known to a far greater accuracy; and now that confirmation of the essentials of electronic band spectra analysis has been given by the more direct but less precise method of the infra-red and Raman effect, it is clear that further work upon these lines would be redundant. The position is far different for even the simplest of polyatomic molecules. It would be unreasonable to expect from the analysis of their band spectra the same degree of theoretical completeness which has been obtained for the diatomic molecule, but there is a necessity that the analyses should at least not conflict with evidence from slightly different but connected spectroscopic fields. Judged by this last criterion, there have been very few satisfactory analyses of the electronic band spectra of polyatomic molecules. On the other hand, as Mecke has urged more than once, there is a real need for the knowledge which the spectroscopy of these molecules alone can give, particularly if physical ideas of molecular structure are to be introduced into chemistry without a long delay. It is this need which is the first justification of the study of the vibration-rotation spectra of polyatomic molecules. At the worst, such work must give data which will make the analysis of electronic band spectra easier and more certain than it is at the moment. For most molecules, as will be seen later, information should follow from the independent analysis of the vibration-rotation bands, apart from the use as a clue to the electronic bands on the one hand, or the mere assignment of a geometrical arrangement to the nuclei on the other.

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