The reactions of SO3 with HO2 radical and H2O•••HO2 radical complex. Theoretical study on the atmospheric formation of HSO5 and H2SO4

The influence of a single water molecule on the gas-phase reactivity of the HO2 radical has been investigated by studying the reactions of SO3 with the HO2 radical and with the H2O center dot center dot center dot H radical complex. The naked reaction leads to the formation of the HSO5 radical, with a computed binding energy of 13.81 kcal mol(-1). The reaction with the H2O center dot center dot center dot HO2 radical complex can give two different products, namely (a) HSO5 + H2O, which has a binding energy that is computed to be 4.76 kcal mol(-1) more stable than the SO3 + H2O center dot center dot center dot HO2 reactants (Delta(E + ZPE) at 0K) and an estimated branching ratio of about 34% at 298K and (b) sulfuric acid and the hydroperoxyl radical, which is computed to be 10.51 kcal mol(-1) below the energy of the reactants (Delta(E + ZPE) at 0K), with an estimated branching ratio of about 66% at 298K. The fact that one of the products is H2SO4 may have relevance in the chemistry of the atmosphere. Interestingly, the water molecule acts as a catalyst, [as it occurs in (a)] or as a reactant [as it occurs in (b)]. For a sake of completeness we have also calculated the anharmonic vibrational frequencies for HO2, HSO5, the HSO5 center dot center dot center dot H2O hydrogen bonded complex, H2SO4, and two H2SO4 center dot center dot center dot H2O complexes, in order to help with the possible experimental identification of some of these species.