The effect of molecular distortions on spin-orbit coupling in simple hydrocarbons

Recent work [D.N.S. Parker et al., Chem. Phys. Lett. 469 (2009) 43-49] has found intersystem crossing (ISC) on an ultrafast timescale in electronically excited benzene, a surprise as hydrocarbons generally have small spin-orbit coupling. In this paper, the effect of molecular distortions on spin-orbit coupling (SOC) is calculated for cyclobutadiene and benzene. At equilibrium the SOC in both molecules is negligible, and therefore terms arising from molecular distortions must play a significant role in any fast ISC. We show that out-of-plane C-H bends, which leads to the hybridisation of sigma and pi orbitals, are responsible for the most significant effect. The S-1/S-0 conical intersection is an important feature for understanding the photochemistry of these molecules. We examine the SOC along the vector from the Franck-Condon point to the lowest energy point on the crossing seam and discuss the potential importance of the SOC to the ultrafast dynamics. (C) 2010 Elsevier B.V. All rights reserved.