Falling Down the Singlet Manifold. A CAS-SCF Mechanistic Study of the Far-UV Photochemistry of Hexa-1,5-dienes

The far-UV photochemistry of hexa-1,5-diene has been characterized using reaction paths obtained from ab-initio CAS-SCF computations. The decay/reaction pathway leading from the S2 potential energy surface to the ground state (S0) [1,2] and [1,3] allyl-shift photoproduct wells has been documented. The S1 and S0 surfaces are entered sequentially through two different (i.e., S2/S1 and S1/S0) radiationless decay channels which have been characterized as conical intersections. This result is consistent with a barrierless concerted mechanism. The structure of the S1 reaction coordinate connecting the two conical intersection points shows that both allyl-shift photoproducts must originate from the same migrating fragment. The proposed mechanism rationalizes the observed far-UV solution photochemistry of a series of variously substituted hexa-1,5-dienes. In particular, it is consistent with the preferential migration of the less substituted allyl fragment regardless of the type of allyl shift considered, the absence of [3,3] and in general [3,2] allyl-shift photoproducts in the reaction mixture, and the retention of the double-bond stereochemistry of the migrating allyl moiety during the reaction. © 1995 American Chemical Society.