Vibrational Selection Rules for Dimethylacetylene

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Abstract
The molecular symmetry group of dimethylacetylene was defined by Longuet-Higgins, and its double group was introduced by Hougen. In this paper we determine the form of the normal coordinates of dimethylacetylene and classify them in the double group. Using the exact normal coordinates it is not always possible to obtain selection rules on purely vibrational transitions. However, by approximating the potential function we can obtain ``approximate normal coordinates'' and, thereby, derive ``approximate vibrational selection rules.'' These selection rules prove useful in interpreting the infrared and Raman spectra of dimethylacetylene and other ethanelike molecules that have a low torsional barrier. One important result is that we do not generally expect there to be any coincidences between infrared and Raman fundamental bands.

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