Reaction path of a sub-200 fs photochemical electrocyclic reaction

Ab initio multistate second-order perturbation theory (MS-CASPT2) calculati ons are used to map the reaction path for the ultrafast photochemical elect rocyclic ring-opening of cyclohexa-1,3-diene (CHD): This path is characteri zed by evolution along a complex reaction coordinate extending over two bar rierless excited state potential energy surfaces and ultimately leading to deactivation through a S-1/S-0 conical intersection. The observed excited-s tate dynamics involve three sequential phases with lifetimes (traveling tim es) of 10, 43, and 77 fs, respectively. In this work we associate each phas e to the evolution of the CHD molecular structure along a different mode. I n 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 ultima tely 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 po ints spanning the low-lying part of the 2A(1) energy surface. Points (a) an d (b) are validated by discussing the results of previously reported and ne w femtosecond time-resolved spectroscopic data on CHD and on the two dialky l derivatives alpha -terpinene and alpha -phellandrene. An interpretation i n terms of driving forces is also given.