Ab initio Study of Ultrafast Photochemical Ring-Opening Reaction of 1,3-Cyclohexadiene

摘要

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_Ipmin) for the photochemical ring-opening reaction of CHD to proceed to produce cZc-l,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 Si pmin.