Ab initio investigation of potential-energy surfaces involved in the photophysics of benzene and pyrazine

Potential-energy surfaces of the lowest singlet and triplet excited states of benzene and pyrazine have been calculated using complete-active-space self-consistent-field and multireference configuration interaction (MRCI) techniques. We have focused our attention on the saddle points and surface intersections associated with the reaction path to a biradical form called prefulvene. The barrier heights separating the prefulvenic minimum from the minimum of the planar aromatic form on the ππ* excited singlet surface and on the ground-state surface have been estimated by large-scale MRCI calculations. The conical intersection of the lowest ππ* excited singlet surface with the S₀ surface has been mapped out in two dimensions, the reaction coordinate to prefulvene and the coordinate of maximum coupling perpendicular to it. The relevance of these ab initio potential-energy data for the interpretation of photophysical relaxation pathways in benzene and pyrazine ("channel-three" effect) is discussed.