Mj. Bearpark et al., AN MC-SCF STUDY OF STYRENE SINGLET-STATE PHOTOISOMERIZATION, Journal of the American Chemical Society, 117(26), 1995, pp. 6944-6953
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.