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

molecular dynamics studies of CHOO molecules following excitation to the minimum-energy geometry of the strongly absorbing S (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) state. Trajectories propagated for 100 fs from the minimum-energy region of the S state show a high surface hopping (nonadiabatic coupling) probability between the near-degenerate S and S (nπ*) states at geometries close to the S minimum, which enables population transfer to the optically dark S state. Greater than 80% of the excited population undergoes O-O bond fission on the S or S potential energy surfaces (PESs) within the analysis period, mostly from nonplanar geometries wherein the CH 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-S absorption spectrum, and a small propensity for out-of-plane motion after nonadiabatic coupling to the S PES that enables access to a conical intersection between the S and S states and cyclization to dioxirane products.