A study of the atmospherically relevant reaction between dimethyl sulphide (DMS) and Cl2 in the absence and presence of water using electronic structure methods

Abstract

The thermodynamics and mechanisms of the atmospherically relevant reaction between dimethyl sulphide (DMS) and molecular chlorine (Cl₂) were investigated in the absence and presence of a single water molecule, using electronic structure methods. Stationary points on the reaction surfaces were located using density functional theory (DFT) with the M06-2X functional and aug-cc-pVTZ (aVTZ) basis sets. Then single point energy calculations were carried out using the UM06-2X/aVTZ optimised stationary point geometries, with aug-cc-pVnZ basis sets (n = T and Q), using the domain-based local pair natural orbitals coupled cluster [DLPNO-UCCSD(T)] method, to give DLPNO-CCSD(T)/CBS//M06-2X/aVTZ relative energies. The reaction can proceed in three ways depending on the initial van der Waals complex formed i.e. via DMS + Cl₂·H₂O, DMS·H₂O + Cl₂, or DMS·Cl₂ + H₂O. It was found that based on computed equilibrium constants for complex formation and estimated initial concentrations of DMS, Cl₂ and H₂O in the atmosphere that [DMS·H₂O] and [Cl₂·H₂O] are likely to be much greater than [DMS·Cl₂] under atmospheric conditions. It was found that both with and without water the reaction can proceed by two pathways (i) formation of the products CH₃SCH₂Cl + HCl + (H₂O) via a covalently bound intermediate (CH₃)₂SCl₂(H₂O) and (ii) formation of the products via a cis-CH₃SClCH₂:HCl (H₂O) intermediate, where (H₂O) applies to the with-water case. Although the pathways and mechanisms are similar in the without- and with-water cases, the relative energies of the transition states are significantly lower and the potential energy diagram is much more complex in the with-water case. However, under tropospheric conditions the overall DMS + Cl₂ rate coefficient is unlikely to be affected by the presence of water as the concentrations of DMS·H₂O and Cl₂·H₂O are estimated to be much lower than the concentrations of DMS, Cl₂ and H₂O. This work extends our earlier study of the reaction of DMS with atomic chlorine (Cl) with and without water (L. Rhyman et al., Phys. Chem. Chem. Phys. 2023, 25, 4780–4793).