Tropospheric Reactions of Triazoles with Hydroxyl Radicals: Hydroxyl Addition is Faster than Hydrogen Abstraction

We present the results of a systematic investigation, at the BHandHLYP/AVTZ density functional (DFT) level, of the tautomeric equilibria of 1,2,3- and 1,2,4-triazoles and their reactions with hydroxyl radicals in the gas phase. A total of twenty-six chemical reactions has been studied, and thermodynamical data and rate constants are reported. The reactions can be classified in two categories: hydrogen abstraction and OH addition. Nine of these reactions are favourable at room temperature. It was found that OH addition proceeds more rapidly than hydrogen abstraction, by several orders of magnitude. For the most stable tautomers, which presumably dominate in the gaz-phase, the fastest reactions are OH addition to 2H-1,2,3-triazole and site-specific OH addition to carbon atom 5 of 1H-1,2,4-triazole. In absolute values, however, the rate constants are rather small, k=5.82x10(-20)cm(3)s(-1) and k=4.75x10(-18)cm(3)s(-1), respectively, at room temperature. Therefore, under the conditions of the troposphere, triazoles cannot be eliminated by reactions with OH. The accuracy of the computational approach has been demonstrated by studying the tautomeric equilibria of 1,2,3- and 1,2,4-triazoles in the gas phase. The computed equilibrium constants are in excellent agreement with those derived from spectroscopic observations. Additional assessment of the quality of the computed data was made by comparison with the results from high-level CCSD(T)-F12/AVTZ ab initio calculations for three exemplary structures. This comparison demonstrates the high accuracy of our DFT results of 0.29kcalmol(-1) for energy differences between stable isomers.