Do Photochemical Ring-Openings Occur in the Spectroscopic State? 1B2 Pathways for the Cyclohexadiene/Hexatriene Photochemical Interconversion

The potential energy paths (i.e., the minimum energy paths) that control the excited-state evolution of cyclohexadiene (CHD) and cZc-hexatriene (cZc-HT) from the Franck-Condon (FC) region are documented. These paths indicate that both CHD and cZc-HT undergo a barrierless motion in the 1B2 spectroscopic state until decay to the lower lying 2A1 state occurs via two distinct 1B2/2A1 conical intersections. Remarkably, it is found that both conical intersections correspond to open (i.e., acyclic) molecular structures. On the basis of these results and of the available experimental data, it is concluded that although the photochemical ring-opening of CHD occurs (in femtoseconds) in the spectroscopic state, the associated photochemical ring-closure reaction of cZc-HT is initiated upon decay of 2A1 cZc-HT to the ground state picoseconds after the initial excitation. © 1996 American Chemical Society.