Role of water clusters in the reaction of the simplest Criegee intermediate CH₂OO with water vapour

Abstract

A comprehensive theoretical study on the role of water clusters ((H₂O) _n , n = 3–5) in the reaction of CH₂OO with water vapour is done at the CCSD(T)//M06-2X/6-311 + G(2df,2p) level of theory. Our simulation results show that the contribution of entropic effect is significantly increased with the increase in the number of water molecules involved. The main products of CH₂OO reaction with water vapour are α-hydroxymethyl hydroperoxide and its isomers. Contrary to the reaction of CH₂OO with unimolecular water (10.91 kcal mol⁻¹), the barriers of reactions with bi- and trimolecular water are further reduced to 5.25 and 4.75 kcal mol⁻¹, respectively. Among these three reactions, the predominant pathway is the reaction of CH₂OO with bimolecular water owing to its lower barrier and the high concentration of (H₂O)₂ ([(H₂O)₂] ≈ 2.47 × 10¹⁴ molecules cm⁻³). The presence of water clusters exhibits a dramatic catalytic effect in the CH₂OO reaction with bimolecular water as it lowers the energy barrier by 4.0 kcal mol⁻¹. The relative reaction rates, which are determined at temperatures from 273 to 323 K and 85 % relative humidity, reveal that the most favourable pathway for the atmospheric removal of CH₂OO is indeed via its reaction with bimolecular water.