Reaction mechanism, energetics, and kinetics of the water-assisted thioformaldehyde + ˙OH reaction and the fate of its product radical under tropospheric conditions

Abstract

The reactions of thioformaldehyde (H₂CS) with OH radicals and assisted by a single water molecule have been investigated using high level ab initio quantum chemistry calculations. The H₂CS + ˙OH reaction can in principle proceed through: (1) abstraction, and (2) addition pathways. The barrier height for the addition reaction in the absence of a catalyst was found to be −0.8 kcal mol⁻¹, relative to the separated reactants, which has a ∼1.0 kcal mol⁻¹ lower barrier than the abstraction channel. The H₂CS + ˙OH reaction assisted by a single water molecule reduces the barrier heights significantly for both the addition and abstraction channels, to −5.5 and −6.7 kcal mol⁻¹ respectively, compared to the un-catalyzed H₂CS + ˙OH reaction. These values suggest that water lowers the barriers by ∼6.0 kcal mol⁻¹ for both reaction paths. The rate constants for the H₂CS⋯H₂O + ˙OH and OH⋯H₂O + H₂CS bimolecular reaction channels were calculated using Canonical Variational Transition state theory (CVT) in conjunction with the Small Curvature Tunneling (SCT) method over the atmospherically relevant temperatures between 200 and 400 K. Rate constants for the H₂CS + ˙OH reaction paths for comparison with the H₂CS + ˙OH + H₂O reaction in the same temperature range were also computed. The results suggest that the rate of the H₂CS + ˙OH + H₂O reaction is slower than that of the H₂CS + ˙OH reaction by ∼1–4 orders of magnitude in the temperatures between 200 and 400 K. For example, at 300 K, the rates of the H₂CS + ˙OH + H₂O and H₂CS + ˙OH reactions were found to be 2.2 × 10⁻⁸ s⁻¹ and 6.4 × 10⁻⁶ s⁻¹, respectively, calculated using [OH] = 1.0 × 10⁶ molecules cm⁻³, and [H₂O] = 8.2 × 10¹⁷ molecules cm⁻³ (300 K, RH 100%) atmospheric conditions. Electronic structure calculations on the H₂C(OH)S˙ product in the presence of ³O₂ were also performed. The results show that H₂CS is removed from the atmosphere primarily by reacting with ˙OH and O₂ to form thioformic acid, HO₂, formaldehyde, and SO₂ as the main end products.