AN MC-SCF STUDY OF STYRENE SINGLET-STATE PHOTOISOMERIZATION

The decay processes involved in the photochemical double bond isomeriz ation of styrene are documented by means of MC-SCF computations. Possi ble intersystem crossing (ISC) and internal conversion (IC) pathways h ave been studied by geometry optimization of the lowest points on the potential energy surface crossings and computation of the spin-orbit c oupling constants. The isomerization of beta-methylstyrene (1-phenylpr opene) proceeds (Lewis, F. D.; Bassani, D. M. J. Am. Chem. Sec. 1993, 115, 7523-7524) via temperature-independent and temperature dependent pathways in solution. The temperature-independent isomerization proces s is consistent with a reaction path that begins with ISC at an S-1/T- 2 crossing which occurs at the planar S-1 minimum. The lowest-energy S -1/S-0 crossing minimum (conical intersection) is benzene-like, and wi ll not lead to isomerization. Rather, the second temperature-dependent isomerization mechanism also begins with ISC either at the twisted S- 1 minimum (also an S-1/T-2 crossing) or at the planar S-1 minimum afte r adiabatic cis-trans isomerization on S-1 has occurred. Decay from T- 2 to S-0 takes place via a T-2/T-1 conical intersection, followed by o ne of two different T-1/S-) crossing points: the expected twisted T-1 minimum, or a higher-energy benzene-like structure. Because of the lar ge energy gap, S-1--> S-0 IC at the twisted S-1 minimum is unlikely to take place as previously suggested.