An MC-SCF study of the S1 and S2 photochemical reactions of benzene

An MC-SCF/4-31G characterization of the possible photochemical pathways of S 1 and S2 benzene is documented. A complete mechanistic scheme is presented through the characterization of minima and transition states on S0, S1, and S2. The full characterization of Born-Oppenheimer violation regions, where the products of the diabatic processes relax to lower electronic states, is also performed. On the S0 surface the reversion of Dewar benzene to benzene is shown to occur via a concerted path along a symmetric coordinate where the bridgehead Dewar benzene bond and the pair of ''quinoid'' double bonds are being synchronously broken. No evidence for an asymmetric path could bc found. The ground-state potential energy surface along the reaction path between benzene and benzvalene has a flat diradicaloid region corresponding to prefulvene. However, prefulvene itself is a transition state. The S1 reaction path from benzene toward prefulvene contains an excited-state minimum with D6h symmetry and a transition state between this minimum and a prefulvenc diradicaloid located on the ground-state surface. The Born-Oppenheimer violation region has been fully characterized by optimizing the conical intersection that occurs between the transition state on S1 and the prefulvene biradicaloid region on S0. The existence of a low-energy diradicaloid minimum on S2 with an immediately adjacent S1/S2 conical intersection at only slightly higher energy has been demonstrated. This suggests that the radiationless decay from S2 is almost completely efficient in contrast to the commonly held view. The different photochemistry of S2 is shown to arise from the fact that the S0/S1 decay that occurs subsequent to passage through the S1/S2 conical intersection occurs at a geometry on the S0/S1 crossing surface where there is a C1-C4 bond similar in length to the bridgehead bond of the S0 transition state between Dewar benzene and benzene. Thus there exists a ground-state reaction path to Dewar benzene from a high-energy region of the S0/S1 crossing surface.