The 2A1potential energy surface of cyclohexadiene (CHD)/cZc-hexa-1, 3, 5-triene (cZc-HT) has been extensively investigated via ab-initio CAS-SCF and CAS-SCF/MP2 computations. In contrast with previous computational studies, the optimization of the 2A1stationary points has been carried out with no structural constraints. Several important reaction pathways have been fully documented via computation of the intrinsic reaction coordinate (IRC). A complete mechanistic picture of the photochemical ring opening occurring on the 2A1surface after CHD photoexcitation is presented. This picture is consistent with modern and older experimental data. The main 2A1→ lA1radiationless decay channel occurs in the region of a conical intersection located about 1 kcal mol-1above the excited-state ring-opening product cZc-HT*. This finding is in contrast with the generally accepted notion that the 2 A1→ 1A1decay channel corresponds to a 2Ai avoided minimum. Indeed no avoided crossing minimum is found along the computed 2A1ring-opening pathway.
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Celani et al. (1994) studied this question.