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

Abstract

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=5.82×10 cm s and k=4.75×10 cm s , 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 0.29 kcal mol for energy differences between stable isomers.

Heterogeneous degradation of pesticides by OH radicals in the atmosphere: Influence of humidity and particle type on the kinetics

“…The chemical structure of pesticides seems to have a significant influence on their reactivity since the two pesticides of the pyrethroid chemical family (deltamethrin and permethrin) were degraded by OH radicals whereas the two pesticides of the triazole chemical family (difenoconazole and tetraconazole) were not (or very slowly) degraded. Indeed, the triazole reactivity toward OH radicals in the gas phase (i.e., an OH-addition onto carbons atoms of the triazole cycle) was considered as a very slow reaction at ambient temperature (Derbel et al, 2018), which might explain our results. The number of pesticides studied in each of these two families is too limited to come to a conclusion, nevertheless, a similar behavior might be extended to other compounds of these families of pesticides.…”

Heterogeneous atmospheric degradation of current-use pesticides by nitrate radicals

“…For instance, triazole reactivity toward OH radicals in the gas phase is considered as a very slow reaction at ambient temperature (Derbel et al, 2018), which might explain the nondegradation of tetraconazole and difenoconazole with OH radicals, however, tetraconazole is the fastest degraded pesticide by NO 3 radicals, showing that the probability of reactions are different between OH and NO 3 radicals.…”