Competition between concerted and stepwise dynamics in the triplet di-π-methane rearrangement.

The molecular dynamics of the triplet-state Zimmerman di-π-methane rearrangement of dibenzobarrelene were computed with B3LYP and M06-2X density functionals. All productive quasiclassical trajectories involve sequential formation and cleavage of C-C bonds and an intermediate with lifetimes ranging from 13 to 1160 fs. Both dynamically concerted and stepwise trajectories are found. The average lifetime of this intermediate is significantly shorter than predicted by either transition-state theory or the Rice-Ramsperger-Kassel-Marcus model, thus indicating the non-statistical nature of the reaction mechanism.