The role of conical intersections and excited state reaction paths in photochemical pericyclic reactions

n this paper, we discuss the photochemical mechanisms of the following three prototype pericyclic reactions: 1. the electrocyclic ring closure of cis-butadiene; 2. the photochemical dimerization of two ethylenes; 3. the photochemical sigmatropic shifts in but-1-ene. These photochemical reactivity problems have been investigated using an ab initio CAS-SCF approach, followed in some cases by CAS-PT2 calculations. This study shows that the photochemical mechanisms of these classes of reactions are controlled in each case by a conical intersection between the photochemically relevant excited state and the ground state. In these reactions such conical intersections provide a very efficient means of radiationless deactivation or chemical transformation of the reacting system. © 1997 Elsevier Science S.A.