ABINITIO 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 point-s 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 pipi* excited singlet surface and on the ground-state surface have been estimated by large-scale MRCI calculations. The conical intersection of the lowest pipi* 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.