Computational study on the mechanisms and kinetics of the CH₂BrO₂ + ClO reaction in the atmosphere

The singlet and triplet potential energy surfaces for the CH₂BrO₂ + ClO reaction are studied at the CCSD(T)/cc-pVTZ//B3LYP/6-311++G(d,p) level. CH₂BrO₂ is revealed to react with ClO through two kinds of mechanisms on the triplet potential energy surface (PES), namely, S_N2 displacement and H-abstraction, and the production of P3 (CHBrO₂ + HOCl) via H-abstraction is the dominant channel. Addition/elimination and S_N2 displacement mechanisms exist on the singlet PES and are more complicated. The RRKM calculations of the mechanism and product distribution in the CH₂BrO₂ + ClO reaction show that the stabilization of IM1 (CH₂BrOOOBr) is dominant at T ≤ 600 K, while the pathway of producing P1 (CHBrO + HO₂ + Cl) occupies the entire reaction at T > 600 K. The total rate constants are independent of pressure, while the individual rate constants are sensitive to pressure. The lifetime of CH₂BrO₂ in the presence of ClO is estimated to be 20.27 h. Moreover, time-dependent density functional theory (TDDFT) calculations suggest that IM1 (CH₂BrOOOCl), IM2 (CH₂BrOOClO) and IM3 (CH₂BrOClO₂) will photolyze under the sunlight.