Photochemistry of the Simplest Criegee Intermediate, CH₂OO: Photoisomerization Channel toward Dioxirane Revealed by CASPT2 Calculations and Trajectory Surface-Hopping Dynamics

Abstract

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 AA → XA photoisomerization channel of the simple Criegee intermediate (CHOO) that leads to a cyclic dioxirane. The excited molecules on the AA state, which can have either originated from the BA state via BA → AA internal conversion or formed by state-selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular A/XA 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 AA state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the AA → XA photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates.

Photodissociation Dynamics of CH2OO on Multiple Potential Energy Surfaces: Experiment and Theory

“…Instead, we have used CASSCF with the same basis set as the CASPT2 PE profiles, which has been utilized for TSH studies of Criegee intermediates in the past. 50 However, the lack of dynamic correlation in the CASSCF method will manifest in less accurate TSH simulations, which may impact the resultant dynamics and branching ratios. As evident from the CASPT2 and CASSCF PE profiles in Figure 1, the relative CASSCF/ CASPT2 energies of the product asymptotes are different (e.g., relative to the Franck−Condon excitation region), although the vertically excited electronic states correlate to the same asymptotic limits as in CASPT2.…”

“…Given the large active space and number of states, it is computationally expensive to perform the TSH simulations using the preferred CASPT2 method. Instead, we have used CASSCF with the same basis set as the CASPT2 PE profiles, which has been utilized for TSH studies of Criegee intermediates in the past . However, the lack of dynamic correlation in the CASSCF method will manifest in less accurate TSH simulations, which may impact the resultant dynamics and branching ratios.…”

Atmospheric Chemistry of Enols: The Formation Mechanisms of Formic and Peroxyformic Acids in Ozonolysis of Vinyl Alcohol

“…Meanwhile, the produced CH 2 OO and anti -HOCHOO with high internal energy could either be stabilized to sCI by collision or isomerize promptly. The sCI can still undergo some unimolecular and bimolecular reactions (particularly with water dimer), forming dioxiranes, vinyl-hydroperoxides,o and the OH radicals. − Such reactions contribute greatly to the formation of SOA in the atmosphere, which has been investigated in many experimental and theoretical studies. − …”

Theoretical reinvestigation of the ozonolysis mechanism of allyl alcohol

“…The photodissociation and photoisomerization reactions of AA-CIs may be competitive with their bimolecular reactions at high altitudes, similar to the simplest CI (CH 2 OO). [47,48] On the other hand, AA-CIs could react quickly with other atmospheric acids at relatively low altitudes, particularly with the carboxylic acids and the nitric acids. It has been experimentally shown that the reactions of the C1 and C2 CIs with nitric, formic, and acetic acids approach the collision limit.…”