Ab initio studies of ClO _x reactions. 3. Kinetics and mechanism for the OH + OClO reaction

The mechanism for the OH + OClO reaction on the singlet and triplet surfaces and its rate constants for formation of various products have been investigated by means of ab initio molecular orbital theory and variational RRKM theory calculations. The geometric parameters of stationary points were optimized at the B3LYP level of theory with the 6-311G(d,p) and 6-311+G(3df,2p) basis sets, and the potential energy surfaces were evaluated at the G2M(CC2)//B3LYP/6-311+G(3df,2p) level of theory. Three main product channels, all located on the singlet PES, have been identified: (1) HOO + ClO, (2) HOCl + ¹O₂, and (3) HOClO₂, the association product. The predicted results show that the rate constants for channels 1 and 2 are pressure-independent up to 1000 arm and that for channel 3 is strongly pressure dependent. Below 1000 K, all rate constants were found to vary negatively with temperature. The individual and total rate constants in the temperature range from 200 to 1000 K at 1 Torr He pressure can be represented by k₁(T) = 1.22 × 10^-22T^2.75 exp(1682/T), k₂(T) = 5.47 × 10^-20T^2.07 exp(2064/7), k₃(T) = 1.37 × 10⁴T^-6.61 exp(-536/7) (200-500 K) and 4.99 × 10⁵⁴T^-22.36 exp(-9807/T) (500-1000 K), and k_tot(T) = 1.78 × 10^-20T^2.25 exp(2100/T) in units of cm³ molecule^-1 s^-1. The predicted rate constant, with the HOCl + ¹O₂ as the major products in the 300-500 K range, agrees well with available experimental data obtained at 1 Torr He pressure. The high- and low-pressure limits of k₃ can be effectively given by k₃ ^∞(T) = 3.24 × 10^-11T^0.28 exp(-18/T) cm³ molecule^-1 s^-1 in 200-2500 K and k₃ ^°(7) = 1.28 × 10^-13T^-6.36 exp(-635/7) for 200-800 K, 7.37 × 10⁸⁴T^-36.02 exp(-22134/T) for 800-1000 K, and 2.91 × 10^-13T^-8.42 exp(11500/T) for 1000-2500 K in units of cm⁶ molecule^-2 s^-1 with N₂ as the third body.