Criegee intermediates (CR2OO) are important active intermediates in the atmosphere. They are widely involved in the single and bimolecular reactions in the atmosphere and produce a variety of metastable species (such as OH, HCHO and CO2). Therefore, CR2OO plays an important role in the atmospheric cycle and environmental pollution control. The reaction involving Criegee intermediates has always been one of the hotspots of experimental and theoretical research. Because Criegee intermediates exist in the troposphere and are exposed to sufficient sunlight during the day, their photophysical and photochemical processes are of great significance. At present, the photochemical reaction of Criegee intermediates (such as O-O bond or C-O bond breakage) has been reported experimentally and theoretically. Previous theoretical studies have found that the simplest Criegee intermediate (CH2OO) can break the O-O bond through two spin-blocked reaction channels (H2CO X1A1 + O 1D and H2CO a3A + O 3P) after being stimulated to B1A'.

Criegee intermediate can be isomerized to diethylene oxide in the ground state (the latter can be used as an oxidant to participate in subsequent reactions), but the process of its excited state isomerization has not been reported so far. we used multi-configuration electronic structure calculation and surface hopping molecular dynamics to find a possible photoisomerization pathway of the simplest Criegee intermediate, which was participated by A1A dark state and isomerized into dioxirane. A simple Criegee intermediate molecule in A1A state obtained by internal conversion from B1A to A1A or by selective excitation can be transferred back to the ground state without radiation via potential energy surface crossing (A/X1A-MECI) between A/X1A states driven by an out-of-plane rotation mode, thus producing a more stable diethylene oxide. Dynamic calculations show that the lifetime of the excited state of A1A is 138 fs, while the time of formation of diethylene oxide is about 254 fs. The discovery of A1A X1A photoisomerization pathway not only enriches the mechanism of Criegee intermediates, but also provides theoretical guidance for the study of their state-selective chemistry.

The first author of this paper is Li Yazhen, a Master studuent of 2015. The corresponding authors are Professor Liu Fengyi and Professor Wang Wenliang. The research was supported by the National Natural Science Foundation of China (21473107, 21473108, 21636006) and the special fund for basic scientific research of Central Universities (GK201502002).

Link:

Li, Yazhen; Gong, Qianqian; Yue, Ling; Wang, Wenliang*; Liu, Fengyi*.
Photochemistry of the Simplest Criegee Intermediate, CH2OO: Photoisomerization Channel toward Dioxirane Revealed by CASPT2 Calculations and Trajectory Surface-Hopping Dynamics 
J. Phys. Chem. Lett. 2018, 9, 978–981.