An MC-SCF Study of Styrene Singlet-State Photoisomerization

The decay processes involved in the photochemical double bond isomerization of styrene are documented by means of MC-SCF computations. Possible intersystem crossing (ISC) and internal conversion (IC) pathways have been studied by geometry optimization of the lowest points on the potential energy surface crossings and computation of the spin–orbit coupling constants. The isomerization of β-methylstyrene (1-phenylpropene) proceeds (Lewis, F. D.; Bassani, D. M. J. Am. Chem. Soc. 1993, 115, 7523—7524) via temperature-independent and temperature-dependent pathways in solution. The temperature-independent isomerization process is consistent with a reaction path that begins with ISC at an S1/T2 crossing which occurs at the planar S1 minimum. The lowest-energy S1/S0 crossing minimum (conical intersection) is benzene-like, and will not lead to isomerization. Rather, the second temperature-dependent isomerization mechanism also begins with ISC either at the twisted Si minimum (also an S1/T2 crossing) or at the planar S1 minimum after adiabatic cis–trans isomerization on S1 has occurred. Decay from T2 to S0 takes place via a T2/T1 conical intersection, followed by one of two different T1/S0 crossing points: the expected twisted T1 minimum, or a higher-energy benzene-like structure. Because of the large energy gap, S1 → S0 IC at the twisted S1 minimum is unlikely to take place as previously suggested. © 1995, American Chemical Society. All rights reserved.