Low-Temperature Phase Transition of Crystalline Hexamethylbenzene. II

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Abstract
The absorption spectrum of the ab plane of crystalline hexamethylbenzene in polarized light has been studied. This spectrum is forbidden and isotropic below the λ‐point transition at 110°K and allowed and very anisotropic above this temperature. It is proposed that these results support the conclusions presented in an earlier paper based on the spectrum of the bc plane. A crystal field calculation of the intensity expected to be induced by this perturbation is shown to support also the conclusion that the molecule is slightly distorted in the high temperature crystal modification. A mechanism for the phase transition is proposed and is based on the assumption that the methyl groups begin to rotate about the C–C bond at 110°K.

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