Photochemical reaction pathways of ethylene

Conical intersections are a common feature in most nondiabatic singlet photoreactions. For ethylene, several S1–S0 conical intersections could be located by CASSCF calculations. The energetically lowest one is different from the one determined by Ohmine [J. Chem. Phys. 83, 2348 (1985)]; its geometry suggests that cis–trans isomerization and a [1,2]hydrogen shift occur via the same funnel.