Comparing the Excited State Dynamics of CH2OO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation

Ab initio molecular dynamics studies of CH2OO molecules following excitation to the minimum-energy geometry of the strongly absorbing S-2 ((1)pi pi*) 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)n pi*) 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 CH2 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.

Automated summary

Ab initio molecular dynamics studies of CH2OO molecules following excitation to the S-2 ((1)pi pi*) state reveal diverse behaviors, including high surface hopping probability, O-O bond fission, and twisted geometries.