10.1021/acs.jpclett.8b00023.s001 Yazhen Li Qianqian Gong Ling Yue Wenliang Wang Fengyi Liu Photochemistry of the Simplest Criegee Intermediate, CH<sub>2</sub>OO: Photoisomerization Channel toward Dioxirane Revealed by CASPT2 Calculations and Trajectory Surface-Hopping Dynamics 2018 American Chemical Society Simplest Criegee Intermediate B 1 dynamic CASSCF trajectory surface-hopping CH 2 OO multireference CASPT 2 photochemistry Criegee intermediates MECI CASPT 2 Calculations dioxirane Trajectory Surface-Hopping Dynamics photoisomerization channel 2018-02-08 00:00:00 article https://acs.figshare.com/articles/Photochemistry_of_the_Simplest_Criegee_Intermediate_CH_sub_2_sub_OO_Photoisomerization_Channel_toward_Dioxirane_Revealed_by_CASPT2_Calculations_and_Trajectory_Surface-Hopping_Dynamics/5877381 The photochemistry of Criegee intermediates plays a significant role in atmospheric chemistry, but it is relatively less explored compared with their thermal reactions. Using multireference CASPT2 electronic structure calculations and CASSCF trajectory surface-hopping molecular dynamics, we have revealed a dark-state-involved <i>A</i><sup>1</sup>A → <i>X</i><sup>1</sup>A photoisomerization channel of the simple Criegee intermediate (CH<sub>2</sub>OO) that leads to a cyclic dioxirane. The excited molecules on the <i>A</i><sup>1</sup>A state, which can have either originated from the <i>B</i><sup>1</sup>A state via <i>B</i><sup>1</sup>A → <i>A</i><sup>1</sup>A internal conversion or formed by state-selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular <i>A</i>/<i>X</i><sup>1</sup>A minimal-energy crossing point (MECI) then radiationless decay to the ground state with an average time constant of ∼138 fs, finally forming dioxirane at ∼254 fs. The dynamics starting from the <i>A</i><sup>1</sup>A state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the <i>A</i><sup>1</sup>A → <i>X</i><sup>1</sup>A photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates.