Spectroscopic study of the reaction between Br2 and dimethyl sulfide (DMS), and comparison with a parallel study made on Cl2 + DMS: possible atmospheric implications

Abstract

The reaction between molecular bromine and dimethyl sulfide (DMS) has been studied both as a co-condensation reaction in low temperature matrices by infrared (IR) matrix isolation spectroscopy and in the gas-phase at low pressures by UV photoelectron spectroscopy (PES). The co-condensation reaction leads to the formation of the molecular van der Waals adduct DMS–Br₂. This was identified by IR spectroscopy supported by results of electronic structure calculations. Calculation of the minimum energy structures in important regions of the reaction surface and computed IR spectra of these structures, which could be compared with the experimental spectra, allowed the structure of the adduct (C_s) to be determined. The low pressure (ca. 10⁻⁵ mbar) gas-phase reaction was studied by UV-PES, but did not yield any observable products, indicating that a third body is necessary for the adduct to be stabilised. These results are compared with parallel co-condensation and gas-phase reactions between DMS and Cl₂. For this reaction, a similar van der Waals adduct DMS–Cl₂ is observed by IR spectroscopy in the co-condensation reactions, but in the gas-phase, this adduct converts to a covalently bound structure Me₂SCl₂, observed in PES studies, which ultimately decomposes to monochlorodimethylsulfide and HCl. For these DMS + X₂ reactions, computed relative energies of minima and transition states on the potential energy surfaces are presented which provide an interpretation for the products observed from the two reactions studied. The implications of the results obtained to atmospheric chemistry are discussed.