The presence of a conical intersection between the S0 and S1, surfaces for ground-state-forbidden photochemical pericyclic reactions is demonstrated by using results from an effective (valence bond) Hamiltonian and MC-SCF computations. The existence of such topological features is an important feature in the mechanism since it permits a fully efficient return to S0 from the S, excited state. An example is presented for the 2 + 2 cycloaddition reaction of two ethylene molecules and the electrocyclic reaction of cir-butadiene.
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Bernardi et al. (1990) studied this question.