Ab initio investigations of the potential energy surfaces of the XO + HO2 reaction (X = chlorine or bromine)

Abstract

The potential energy surfaces of the title reactions have been studied computationally using coupled cluster techniques. Many true minimum and transition state structures have been located, which connect the reactants with various product channels. The potential surfaces for the bromine reactions are found to be very similar to those of the chlorine analogues. On the singlet surface it is found that XO and HO₂ come together to form chain HOOOX, which is 64 kJ mol⁻¹ (Cl) and 72 kJ mol⁻¹ (Br) more stable than the reactants. The energy barrier for HOOOX dissociation to either HOX + O₂ (¹Δ) or HX + O₃ is calculated to be very high (at least 90 kJ mol⁻¹ for both the chlorine and bromine surfaces). By contrast, reaction along the triplet surface to form HOX + O₂ (³Σ) is found to be energetically facile, with a small (ca. 10 kJ mol⁻¹) negative activation barrier with respect to the reactants. The title reactions are therefore predicted to proceed along the triplet surface, implying that the branching ratio into the minor product channels (HX + O₃) is essentially zero. Extensive comparisons are drawn between the present studies and previous experimental and theoretical work. The present calculations predict a slight negative temperature dependence for the overall rate coefficient of the reactions along the triplet surface, in agreement with the current experimental and theoretical consensus. The atmospheric implications of the present studies are discussed.