Ab initio study of ultrafast photochemical ring-opening reaction of 1,3-cyclohexadiene

The mechanism of ultrafast photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD), which has been inferred based on CASPT2//CASSCF(6,6) calculations of potential energy surfaces (PESs) by Garavelli et. al. (J. Phys. Chem. A, 105, 4458-4469 (2001)), has not been fully understood. The unsolved problem is that within the framework of the CASPT2//CASSCF(6,6) method the S-1/S-0 conical intersection (CI) is located too far from a pericyclic minimum of the S-1 state (S(1)pmin) for the photochemical ring-opening reaction of CHD to proceed to produce cZc-1,3,5-hexatriene (cZc-HT) on a ultrafast time scale of around 200 fs. In this study, it has been shown that the PESs obtained by the CASPT2//CASPT2 calculations give a reasonable explanation to why the photochemical ring-opening reation of CHD proceeds on the ultrafast time scale. The most important and remarkable feature of the CASPT2//CASPT2 PESs is that both energetic and structural distances between the S(1)pmin and the S-1/S-0 CI, both of which have structures largely breaking from C-2 symmetry, are small enough for a nonadiabatic transition to occur in the vicinity of the S(1)pmin.