Simulation of vibrational spectra of isotopic benzenes by an extended molecular mechanics method

Vibrational spectra of benzene and benzene-d₆ in the gas and liquid phase have been simulated by a molecular mechanics method including the calculation of equilibrium structures, thermodynamic quantities, normal frequencies and vibrational transition probabilities. The potential parameters have been estimated by referring to the observed frequencies of benzene, benzene-d₆ and 1,3,5-benzene-d₃ and also to results of ab initio calculations. Four and ten independent parameters are required, respectively, for elucidating the infrared absorption and Raman intensities of these compounds in the gas and liquid phase. The infrared absorption spectrum of benzene-d₁ is reproduced well by using the potential and the intensity parameters estimated for benzene and benzene-d₆. The change of relative band intensities on the phase change has been elucidated in terms of the change of various intensity parameters.