Comparing the Excited State Dynamics of CH 2 OO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation.

Ab initio molecular dynamics studies of CH 2 OO molecules following excitation to the minimum-energy geometry of the strongly absorbing S 2 ( 1 ππ*) state reveal a much richer range of behaviors than just the prompt O-O bond fission, with unity quantum yield and retention of overall planarity, identified in previous vertical excitation studies from the ground (S 0 ) state. Trajectories propagated for 100 fs from the minimum-energy region of the S 2 state show a high surface hopping (nonadiabatic coupling) probability between the near-degenerate S 2 and S 1 ( 1 nπ*) states at geometries close to the S 2 minimum, which enables population transfer to the optically dark S 1 state. Greater than 80% of the excited population undergoes O-O bond fission on the S 2 or S 1 potential energy surfaces (PESs) within the analysis period, mostly from nonplanar geometries wherein the CH 2 moiety is twisted relative to the COO plane. Trajectory analysis also reveals recurrences in the O-O stretch coordinate, consistent with the resonance structure observed at the red end of the parent S 2 -S 0 absorption spectrum, and a small propensity for out-of-plane motion after nonadiabatic coupling to the S 1 PES that enables access to a conical intersection between the S 1 and S 0 states and cyclization to dioxirane products.