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 HO2 with CH2ClO2, CHCl2O2, and CCl3O2 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 HO2 + CH2ClO2 reaction, there are two major product channels, viz., the formation of CH2ClOOH + O-2 via hydrogen abstraction on the triplet surface and the formation of CHClO + OH + HO2 via a 5-member-ring transition state. Meanwhile, two O-3-forming channels, namely, CH2O + HCl + O-3 and CH2ClOH + O-3 might be competitive at elevated temperatures. The HO2 + CHCl2O2 reaction has a mechanism similar to that of the HO2 + CH2ClO2 reaction. For the HO2 + CCl3O2 reaction, the formation of CCl3O2H + 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., CH2ClOOH, CHCl2OOH, and CCl3OOH) molecules has been investigated. The implication of the present mechanisms in atmospheric chemistry is discussed in comparison with the experimental measurements.