A systematic computational study of the reactions of HO2 with RO2: the HO2 + CH2ClO2, CHCl2O2, and CCl3O2 reactions.

Potential energy surfaces for the reactions of HO(2) with CH(2)ClO(2), CHCl(2)O(2), and CCl(3)O(2) have been calculated using coupled cluster theory and density functional theory (B3LYP). It is revealed that all the reactions take place on both singlet and triplet surfaces. Potential wells exist in the entrance channels for both surfaces. The reaction mechanism on the triplet surface is simple, including hydrogen abstraction and S(N)2-type displacement. The reaction mechanism on the singlet surface is more complicated. Interestingly, the corresponding transition states prefer to be 4-, 5-, or 7-member-ring structures. For the HO(2) + CH(2)ClO(2) reaction, there are two major product channels, viz., the formation of CH(2)ClOOH + O(2) via hydrogen abstraction on the triplet surface and the formation of CHClO + OH + HO(2) via a 5-member-ring transition state. Meanwhile, two O(3)-forming channels, namely, CH(2)O + HCl + O(3) and CH(2)ClOH + O(3) might be competitive at elevated temperatures. The HO(2) + CHCl(2)O(2) reaction has a mechanism similar to that of the HO(2) + CH(2)ClO(2) reaction. For the HO(2) + CCl(3)O(2) reaction, the formation of CCl(3)O(2)H + O(2) is the dominant channel. The Cl-substitution effect on the geometries, barriers, and heats of reaction is discussed. In addition, the unimolecular decomposition of the excited ROOH (e.g., CH(2)ClOOH, CHCl(2)OOH, and CCl(3)OOH) molecules has been investigated. The implication of the present mechanisms in atmospheric chemistry is discussed in comparison with the experimental measurements.