Vibrational Exciton Structure in Crystals of Isotopic Benzenes

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Abstract
New high‐resolution infrared spectra of crystalline isotopic benzenes C6H6, C6H5D, p‐, m‐C6H4D2, sym‐C6H3D3, and C6D6 are presented. Using these data in conjunction with isotopic mixed‐crystal data and the polarized single‐crystal spectra (C6H6) of Zwerdling and Halford, the exciton band structures and coupling constants for nondegenerate vibrations of C6H6 and C6D6 are obtained. One vibration of sym‐C6H3D3 has also been analyzed. It is assumed that when the band shapes and splittings are nearly identical, the polarizations can be carried over from the C6H6 crystal to C6D6 and sym‐C6H3D3 crystals. It is not possible to analyze completely the degenerate bands of these isotopes since experimental data do not yield values of all the independent matrix elements appearing in the expression for the energy of the exciton components. In the isotopic modifications of lower symmetry, exciton structure is not easily interpreted due to an orientational effect, crystal induced Fermi resonance, and the inevitable presence of isotopic impurities.

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