Ab initio study on the mechanism of the atmospheric reaction OH + O3-->HO2 + O2

The atmospheric reaction (1) OH + O3-->HO2 + O2 was investigated theoretically by using MP2, QCISD, QCISD(T), and CCSD(T) methods with various basis sets. At the highest level of theory, namely, QCISD, the reaction is direct, with only one transition state between reactants and products. However, at the MP2 level, the reaction proceeds through a two-step mechanism and shows two transition states, TS1 and TS2, separated by an intermediate, Int. The different methodologies employed in this paper consistently predict the barrier height of reaction (1) to be within the range 2.16-5.11 kcal mol-1, somewhat higher than the experimental value of 2.0 kcal mol-1.