A theoretical study on the mechanism of internal conversion of S1 benzene

The mechanism of internal conversion of S1 benzene has been studied theoretically. The MCSCF and MCSCF-CI methods were employed to calculate the potential energy surfaces of ground and excited states. It was found that the S1 potential surface crosses with the ground state surface near the saddle point of the benzene–prefulvene isomerization reaction. A simple theoretical model, based on the reaction path concept and the calculated potential surface characteristics, was used to calculate the rate of internal conversion as a function of excess vibrational energy of S1 benzene. The results obtained were that: (a) the internal conversion occurs through the tunneling mechanism at a low excess energy range, E<3700 cm−1; and that (b) both the surface hopping transition and the reaction to prefulvene are accessible at a high energy range, E>3700 cm−1. A new theoretical explanation for the channel three decay was proposed on the basis of the results of present calculations.