Reaction path of a sub-200 fs photochemical electrocyclic reaction

Ab initio multistate second-order perturbation theory (MS-CASPT2) calculations are used to map the reaction path for the ultrafast photochemical electrocyclic ring-opening of cyclohexa-1,3-diene (CHD): This path is characterized by evolution along a complex reaction coordinate extending over two barrierless excited state potential energy surfaces and ultimately leading to deactivation through a S-1/S-0 conical intersection. The observed excited-state dynamics involve three sequential phases with lifetimes (traveling times) of 10, 43, and 77 fs, respectively. In this work we associate each phase to the evolution of the CHD molecular structure along a different mode. In particular, we show that (a) the decay of CHD from its spectroscopic (1B(2)) state to a lower lying dark (2A(1)) excited state involves motion along a highly curved coordinate corresponding to a mixture of a bond expansion and symmetry breaking skeletal bending, (b) the evolution pn the 2A(1) (S-1) state and the find 2A(1)--> 1A(1) (i.e., S-1-->S-0) decay involve a large amplitude displacement along the same asymmetric bending mode which ultimately leads to a S-1/S-0 conical intersection, and (c) the application of a novel strategy for mapping the multidimensional S-1/S-0 intersection space indicates that the ultrashort 77 fs Lifetime of the 2A(1) excited state is due to the existence of an extensive set of S-1/S-0 conical intersection points spanning the low-lying part of the 2A(1) energy surface. Points (a) and (b) are validated by discussing the results of previously reported and new femtosecond time-resolved spectroscopic data on CHD and on the two dialkyl derivatives alpha -terpinene and alpha -phellandrene. An interpretation in terms of driving forces is also given.