We present a theoretical
study of intersystem crossing (ISC) in
acrolein and ketene with the Ehrenfest method that can describe a
superposition of singlet and triplet states. Our simulations illustrate
a new mechanistic effect of ISC, namely, that a superposition of singlets
and triplets yields nonadiabatic dynamics characteristic of that superposition
rather than the constituent state potential energy surfaces. This
effect is particularly significant in ketene, where mixing of singlet
and triplet states along the approach to a singlet/singlet conical
intersection occurs, with the spin–orbit coupling (SOC) remaining
small throughout. In both cases, the effects require many recrossings
of the singlet/triplet state crossing seam, consistent with the textbook
treatment of ISC.