Gas-Phase Reactions of Chlorine Atoms and ClO Radicals with Dimethyl Sulfide. Rate Coefficients and Temperature Dependences

摘要

The results of a discharge flow-mass spectrometric (DF-MS) kinetic study of the reaction between Cl and dimethylsulfide (DMS) (1) over the temperature range 259-364 K at low total pressure between 0.5 and 1 Torr with helium as carrier gas are reported. At room temperature and 1.0 Torr the main products of reaction 1 correspond to an abstraction channel leading to HCl and CH_3SCH_2 with k(1) = (6.9 ± 1.3) * 10~(-11) cm~3 molecule~(-1) s~(-1). The association channel has also been confirmed by mass spectroscopic detection of the adduct CH_3S(Cl)CH_3 with a yield <0.05 under the experimental conditions used. It is now shown that the abstraction channel requires a slight activation energy, k(1) = (2.0 ± 1.2) * 10~(-10) exp[-332 ± 173)/T] cm~3 molecule~(-1) s~(-1). The kinetics and mechanism of the reaction ClO + DMS → products (2) over the temperature range 259 335 K at total pressures between 0.5 and 2 Torr have also been studied by DF-MS. By mass spectroscopic calibration of dimethyl sulfoxide, DMSO, the branching ratio of the channel leading to this product has been measured (0.90 ± 0.49). The rate constant of reaction 1 has been measured under pseudo-first-order conditions in excess of DMS over ClO: k(2) = (1.2 ± 0.7) * 10~(-15) exp[(354 ± 163)/T] cm~3 molecule~(-1) s~(-1) with k(2) = (3.9 ± 1.2) * 10~(-15) cm~3 molecule~(-1) s~(-1) at 298 K. The reaction is postulated to proceed through a channel involving a long-lived intermediate [CH_3S(OCl)CH_3±~* which may decompose back to reactants or to products. Finally, the atmospheric implications through the DMS chemistry of both reactions are discussed.