Conical intersections in molecular photochemistry - The role of phase change

Conical intersections are now known to be involved in a large variety of photochemical transformations. In this paper, we present a simple method for determining the existence of a conical intersection for a given chemical reaction. The method, based on the phase change theorem, is easy to apply, does not require quantum chemical calculations, and uses ground state surface properties only. The method can be used to analyze all kinds of photochemical reactions, including isomerizations, rearrangements and dissociations. It is shown that if the system passes through a conical intersection on its return to the ground state surface, at least two different products are necessarily formed (in addition, the system may return to the reactant by internal conversion). (C) 2000 Elsevier Science B.V.