EXCITED-STATE REACTION PATHWAYS FOR S-CIS BUTA-1,3-DIENE

The topology and energetics of the potential energy surfaces associated with the 2A1 and 1B2 valence excited states of s-cis butadiene have been investigated via ab initio quantum chemical computations at a level of theory which includes dynamic correlation effects and extended basis sets. The results support a photochemical ring-closure mechanism involving 1B2 and 1A1 reaction/ relaxation pathways that are disrotatory. The reaction path on the 2A1 surface begins at a 1B 2/2A1 conical intersection and the ground state photoproducts are produced via radiationless decay at a second 2A 1/1A1 conical intersection which has been documented in a previous publication. A local Cs equilibrium structure on 1B 2 potential energy surface has been optimized using the complete active space-self-consistent field and configuration interaction singles methods. The 1B2/2A1 conical intersection is located near this Cs equilibrium structure and offers a rationalization of the experimentally observed femtosecond lifetime of this state. The observed preferential disrotatory stereochemistry appears to be simply determined by a difference in the energy barriers located along the 2A1 disrotatory and conrotatory paths. This finding is in contrast with the generally accepted notion that the stereochemistry is determined by a different rate of internal conversion at a "disrotatory" and "conrotatory" avoided crossing minima. Indeed, no avoided crossing can be located along the 2A 1 paths. © 1995 American Institute of Physics.