Ab initio study on the complex forming reaction of OH and H₂O in the gas phase

The estimation of the concentration of hydroxyl radical (OH) in the atmosphere is essential to build atmospheric models and to understand the mechanisms of the reactions involved in OH. Although water vapor is one of the most abundant species in the troposphere, only a few studies have been performed for the reaction of OH and water vapor. Here I demonstrate an ab initio study on the complex forming reation of OH with H₂O in the gas phase performed based on density functional theory to calculate the reaction rate and the energy states of the reactant and the OH-H₂O complex. The structure of the complex, which belongs to the Cs point group, was optimized at global minima. The transition state was not found at the B3LYP and MP2 levels of theory. Rate constants of the forward and the reverse reactions were calculated as 1.1×10^-16 cm³ molecule^-1 s^-1 and 5.3×10⁹ s^-1, respectively. The extremely slow rates of complex forming reaction and the resulting hydrogen atom exchange reaction of OH and H₂O, which are consistent with experimentally determined values, imply a negligible possibility of a change in OH reactivity through the title reaction.