Theoretical studies of the HO+O⇔HO2⇔H+O2 reaction. II. Classical trajectory calculations on an ab initio potential for temperatures between 300 and 5000 K

A comparably simple new analytical expression of the potential energy surface for the HO+O⇔HO2⇔H+O2 reaction system is designed on the basis of previous high precision ab initio calculations along the minimum energy path of the HO2→H+O2 and HO2→HO+O dissociations. Thermal rate constants for the reaction HO+O→H+O2 are determined by extensive classical trajectory calculations. The results depend on the policy to solve the zeropoint energy problem. We show that, with the chosen policy, there are nearly equal amounts of statistical and nonstatistical backdissociations HO+O←HO2 following HO+O→HO2; however, backdissociations become important only at temperatures above about 500 K. Below 500 K, the reaction is completely capture-controlled. Below 300 K, classical trajectory treatments become inadequate, because quantum effects then are so important that only the quantum statistical adiabatic channel model gives reliable results. For the reaction HO+O→H+O2 and the range 300–5000 K, a rate constant of k/10−11 cm3 molecule−1 s−1=0.026(T/1000 K)1.47+1.92(1000 K/T)0.46 is obtained from the trajectory calculations. Converting experimental results for the reaction H+O2→HO+O to the reverse reaction on the basis of the revised enthalpy of formation of OH, agreement between experiment and theory within better than 20% is obtained between 300 and 5000 K.