Geometry of the Lowest Triplet State of Benzene

The phosphorescence, fluorescence, and absorption spectra of seven isotopic benzenes in a C6D6 host crystal were obtained at 4.2°K. The isotopes having less than D3h symmetry show splittings in their phosphorescence and singlet absorption spectra. Expected splittings in the fluorescence spectrum have not yet been observed because of the greater difficulty in obtaining the required high‐resolution fluorescence spectra. The splittings are interpreted in terms of a distortion, which may arise either from extrinsic or intrinsic perturbations, in the 1B1u and 3B1u states of crystalline benzene. This distortion causes conformers of isotopic benzenes with less than trigonal symmetry to have different zero‐point energies and leads to the observed line multiplicity. The vibronic structure of the phosphorescence and fluorescence emissions implies that the magnitude of the distortion is small.Benzene molecules in the low‐symmetry crystalline field are not expected to retain a hexagonal shape. However, the splittings...

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