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

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>i.e. via</i> 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) <i>via</i> a covalently bound intermediate (CH₃)₂SCl₂(H₂O) and (ii) formation of the products <i>via</i> a <i>cis</i>-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 <i>et al.</i>, <i>Phys. Chem. Chem. Phys.</i> 2023, <b>25</b>, 4780-4793).