DO PHOTOCHEMICAL RING-OPENINGS OCCUR IN THE SPECTROSCOPIC STATE - B-1(2) PATHWAYS FOR THE CYCLOHEXADIENE HEXATRIENE PHOTOCHEMICAL INTERCONVERSION/

The potential energy paths (i.e., the minimum energy paths) that contr ol the excited-state evolution of cyclohexadiene (CHD) and cZc-hexatri ene (cZc-HT) from the Franck-Condon (FC) region are documented. These paths indicate that both CHD and cZc-HT undergo a barrierless motion i n the IB? spectroscopic State until decay to the lower lying (2)A(1) s tate occurs via two distinct B-1(2)/(2)A(1) conical intersections. Rem arkably, it is found that both conical intersections correspond to ope n (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 reactio n of cZc-HT is initiated upon decay of (2)A(1) cZc-HT to the ground st ate picoseconds after the initial excitation.