Ab Initio Studies of ClOx Reactions: VI. Theoretical Prediction of Total Rate Constant and Product Branching Probabilities for the HO2 + ClO Reaction

Abstract

The reaction of HO2 with ClO has been investigated by ab initio molecular orbital and variational transition state theory calculations. The geometric parameters of the reaction system HO2 + ClO were optimized at the B3LYP and BH&HLYP levels of theory with the basis set 6-311+G(3df,2p). Both singlet and triplet potential energy surfaces were predicted by the modified Gaussian 2 (G2M) method. On the singlet surface, the reaction forms two HOOOCl isomers lying below the reactants by 20 kcal/mol. Their stabilization contributes significantly to the observed overall HO2 + ClO rate constant. The predicted high- and low-pressure association rate constants for the 150−600 K range can be represented by k∞ = 9.04 × 10-17 T1.22 exp(897/T) cm3 molecule-1 s-1 and k0 = 9.33 × 10-24 T-3.45 exp(472/T) cm6 molecule-2 s-1 for N2 as the third-body. Dissociation of these excited HOOOCl intermediates produces HO + ClOO, HCl + 1O3, HOCl + 1O2, and HO + OClO as minor products via multistep mechanisms. On the triplet surface, fo...