Motions of Molecules in Condensed Systems: I. Selection Rules, Relative Intensities, and Orientation Effects for Raman and Infra-Red Spectra

TLDR

Sharpened selection rules have been observed to operate for single crystals containing non‑rotating molecules when these are given special orientations in the light path. The paper presents a method to deduce Raman and infrared selection rules for crystals using only the space‑group designation and unit‑cell population. The method, detailed in an appendix listing point‑symmetry distributions, predicts special orientations and selection rules from space‑group data and crystal external form, enabling users with point‑group knowledge but no space‑group expertise to apply it. The authors find that no selection rules operate in liquids, provide tentative qualitative conclusions on relative intensities of spectral components in condensed systems, and note that their procedures are simpler than prior approaches.

Abstract

A method is described that permits deduction of selection rules for Raman and infra-red spectra of crystals from a knowledge of just the space group designation and the population of the unit cell. When implemented by an appendix to this article showing, in a convenient arrangement, the distribution of point symmetries within space groups the method can be applied by a user familiar with point group manipulation but having no special knowledge of space groups. There are no selection rules operating in the liquid state. Some tentative qualitative conclusions are drawn concerning the relative intensities of certain kinds of components appearing in spectra of condensed systems. Sharpened selection rules are expected, and have been observed by others, to operate for single crystals containing non-rotating molecules when these are given special orientations in the light path. These special orientations and the appropriate selection rules can be predicted from information mentioned above along with inspection of the external form of a crystal. All procedures have a desirable advantage of simplicity over prior ones directed toward the same ends.