The 1B2u�1A1g transition of benzene and of some deuteriated benzenes: solvent and temperature effects

Abstract

Spectral parameters of ¹B_2uâ†�¹A_1g electronic transition in benzene, [1,3,5-²H₃]benzene, and [²H₆]benzene have been measured in polar and non-polar solvents in the temperature range 77–300°K. Vibrational frequencies active in the excited state are not modified by solvents. Ham bands form in every compound a progression O–O+vω′, where ω′ is about 5% larger than the totally symmetric ‘ ring breathing’ frequency of the excited state. This difference is interpreted on the basis of band overlapping with terms of other progressions active in the vapour spectra.

Intensity measurements lead to the conclusion that the appearance of Ham bands is not correlated with solvent polarizability or polarity. In the present experimental conditions the formation of an addition complex between benzene and solvent is excluded. It is suggested that the main role in activating Ham bands is played by the potential generated by the random distribution of the molecules of the solvent, which perturbs the π-charge distribution of benzene.