High-level quantum chemical methods for the study of photochemical processes

Multireference configuration interaction calculations have been performed on the excited state energy surfaces of the methyleneimmonium cation using recently developed methods for the computation of analytic gradients and nonadiabatic coupling terms. Excited-state structures and minima on the crossing seam have been determined. It was found that the topology of the methyleneimmonium surfaces is qualitatively different from that of the isoelectronic ethylene. In the former case a conical intersection between the S₁ and ground states is found for the twisting motion around the CN bond, whereas a more complicated motion including pyramidalization and hydrogen-transfer is needed in case of ethylene.