WHAT HAPPENS DURING THE PICOSECOND LIFETIME OF 2A(1) CYCLOHEXA-1,3-DIENE - A CAS-SCF STUDY OF THE CYCLOHEXADIENE HEXATRIENE PHOTOCHEMICAL INTERCONVERSION

The 2A(1) potential energy surface of cyclohexadiene (CHD)/cZc-hexa-1, 3,5-triene (cZc-HT) has been extensively investigated via ab-initio CA S-SCF and CAS-SCF/MP2 computations. In contrast with previous computat ional studies, the optimization of the 2A(1) stationary points has bee n carried out with no structural constraints. Several important reacti on pathways have been fully documented via computation of the intrinsi c reaction coordinate (IRC). A complete mechanistic picture of the pho tochemical ring opening occurring on the 2A(1) surface after CHD photo excitation is presented. This picture is consistent with modern and ol der experimental data. The main 2A(1) --> 1A(1) radiationless decay ch annel occurs in the region of a conical intersection located about 1 k cal mol(-1) above the excited-state ring-opening product cZc-HT. This finding is in contrast with the generally accepted notion that the 2A (1) --> 1A(1) decay channel corresponds to a 2A(1) avoided minimum. In deed no avoided crossing minimum is found along the computed 2A(1) rin g-opening pathway.