Molecular distortions in the triplet state of benzene-polyhalo-alkane complexes

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Abstract
Benzene and polyhalo-alkanes containing weakly acidic protons, form intermolecular complexes in dilute solutions at low temperature. In these complexes the near u.-v. absorption and fluorescence spectra of the benzenes are shifted to higher frequencies, but there is no change in their vibrational structure. The ã1B2u→[graphic omitted]1A1g electronic transition remains symmetry forbidden in the complex and the band origin is very weak. In the phosphorescence spectrum, however, the intermolecular perturbation completely alters the vibrational structure and both the “true” origin and the progression in the totally symmetric breathing frequency ν1 based upon it, become intense. It is suggested that the intermolecular perturbation promotes a pseudo Jahn–Teller distortion of the ring in the ã3B1u state, which lowers its point symmetry to D2h. A Franck–Condon analysis of the band structure is used in an approximate partial reconstruction of the phosphorescence spectral profile and calculations of the degree of distortion are made from it.

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