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

Abstract

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 CH3SCH2 with k(1) = (6.9 ± 1.3) × 10-11 cm3 molecule-1 s-1. The association channel has also been confirmed by mass spectroscopic detection of the adduct CH3S(Cl)CH3 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] cm3 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 2 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] cm3 molecule-1 s-1 with k(2) = (3.9 ± 1.2) × 10-15 cm3 molecule-1 s-1 at 298 K. The reaction is postulated to proceed through a channel involving a long-lived intermediate [CH3S(OCl)CH3]* which may decompose back to reactants or to products. Finally, the atmospheric implications through the DMS chemistry of both reactions are discussed.

A kinetics and mechanistic study of the atmospherically relevant reaction between molecular chlorine and dimethyl sulfide (DMS)

“…The low pressure limit value is (6.9 ± 1.3) x 10 -11 cm 3 molecule -1 s -1 (12,13). Reaction (2) proceeds by two channels, (2a) the hydrogen abstraction channel, which is pressure independent, and (2b) the addition channel which is pressure dependent.…”

“…A modelling study conducted in this work found that, shortly after sunrise, the oxidation rate of DMS by Cl, produced by photolysis of Cl 2 , could under favourable conditions, be an order of magnitude higher than the rate of oxidation by OH. The rate constant for the reaction between Cl atoms and DMS at 298 K and 1 torr pressure has been measured as (6.9 ± 1.3) x 10 -11 cm 3 molecule -1 s -1 using discharge-flow mass spectrometry (12), and at 3 torr pressure and at 297 K it has been measured as (1.8 ± 0.3) x10 -10 cm 3 molecule -1 s -1 using time-resolved resonance detection of Cl (13). A more recent study using cavity ring-down spectroscopy (14) determined the rate constant at atmospheric pressure as (3.6 ± 0.2) x10 -10 cm 3 molecule -1 s -1 and a low-pressure limit rate constant consistent with that found in references (12) and (13).…”

“…The rate constant for the reaction between Cl atoms and DMS at 298 K and 1 torr pressure has been measured as (6.9 ± 1.3) x 10 -11 cm 3 molecule -1 s -1 using discharge-flow mass spectrometry (12), and at 3 torr pressure and at 297 K it has been measured as (1.8 ± 0.3) x10 -10 cm 3 molecule -1 s -1 using time-resolved resonance detection of Cl (13). A more recent study using cavity ring-down spectroscopy (14) determined the rate constant at atmospheric pressure as (3.6 ± 0.2) x10 -10 cm 3 molecule -1 s -1 and a low-pressure limit rate constant consistent with that found in references (12) and (13). It has been found that the Cl + DMS reaction, reaction (2), proceeds via two routes, a direct stripping channel (2a) that proceeds without a barrier to give CH 3 SCH 2 and HCl and an addition channel (2b) to give (CH 3 ) 2 SCl (13-16).…”

Gas‐phase reaction of Cl with dimethyl sulfide: Temperature and oxygen partial pressure dependence of the rate coefficient

“…In a recent low-pressure DF-MS study by Díaz-de-Mera et al [10], a rate coefficient for Cl with DMS of (0.69 ± 0.13) × 10 −10 cm 3 molecule…”

“…There have been several laboratory kinetic studies of the Cl atom reaction with DMS at low pressure using FP-RF (flash photolysis-resonance fluorescence; 3 Torr N 2 ) [9] and DF-MS (discharge flow-mass spectrometry; 0.5-1 Torr He) [10] techniques and at high total pressure using FP-RF (60 Torr N 2 ) [9] and the relative kinetic (740 Torr N 2 /760 Torr air) [11,12] techniques. In the early extensive FP-RF study of Stickel et al [9] a temperature and pressure dependence was established.…”