Near Ultraviolet Spectrum of Crystalline Hexamethyl Benzene and the Structure of the Hexamethyl Benzene Molecule

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Abstract
The absorption and fluorescence spectra of single oriented crystals of hexamethyl benzene were photographed at high resolution with polarized light at 20°K in the 2800 A region. The analysis shows that the 0–0 transition is observed with much greater intensity in light polarized perpendicular to the plane of the benzene ring than in light polarized in this plane. In the perpendicular component the vibrational structure of a weak allowed transition appears together with the 0–0 band. The in‐plane spectrum is a forbidden transition, similar to the benzene 2600 A absorption. A detailed calculation shows that the perturbation by the crystal lattice cannot be the cause of the observed behavior of the 0–0 band in polarized light. It is concluded, therefore, that this behavior must be caused by the lowering of the molecular symmetry by the methyl groups. The probable symmetry of the molecule is S6. This symmetry approximately fixes the positions of the hydrogen atoms and determines the shape of the methyl rolling potential.