ASYMMETRICAL REACTION-PATH OF PHOTOCYCLIZATION 1,3-BUTADIENE INTO CYCLOBUTENE

Using the semiempirical method MINDO/3, an asymmetrical reaction path of the photocyclization of 1,3-butadiene to cyclobutene was located on the potential-energy surface (PES) of the first excited singlet state (S1). Through a "funnel" on this path, the system passes non-adiabatically onto the PES of the ground state (S0) where it relaxes into a topomer of cyclobutene, as predicted by the Woodward-Hoffmann rules. The reasons for the absence of the photo-opening reaction of unsubstituted cyclobutene are discussed.