Reaction of hydroxyl radical with nitric acid: Insights into its mechanism

The rate constant for the reaction of hydroxyl radicals with nitric acid has an unusual pressure and temperature dependence. To explore the mechanism for this reaction, we have measured rate constants for reactions of isotopically substituted species OD + DNO(3), OH + DNO(3), OD + HNO(3), and (18)OH + HNO(3) and the yield of NO(3) product. Deuterium substitution on nitric acid results in more than a 10-fold reduction in the rate constant, removes the pressure dependence (over the observed range of 20-200 Torr in He and SF(6)), and leads to a strongly curved Arrhenius temperature dependence. Deuterium substitution on hydroxyl increases the rate constant slightly but does not change the pressure dependence. There is no evidence for exchange reactions in the isotopically mixed reactions. Absorption measurements of the NO(3) product yield show that the title reaction produces nitrate radical with unit efficiency over all temperatures and pressures studied. We discuss the implications of the measured rare constants, product yields, and lack of isotopic exchange in terms of a mechanism that involves formation of a hydroxyl radical-nitric acid complex and its subsequent reaction to give NO(3) and H(2)O.