Fast Peroxy Radical Isomerization and OH Recycling in the Reaction of OH Radicals with Dimethyl Sulfide

Dimethyl sulfide (DMS), produced by marine organisms, represents the most abundant, biogenic sulfur emission into the Earth's atmosphere. The gas-phase degradation of DMS is mainly initiated by the reaction with the OH radical forming first CH₃SCH₂O₂ radicals from the dominant H-abstraction channel. It is experimentally shown that these peroxy radicals undergo a two-step isomerization process finally forming a product consistent with the formula HOOCH₂SCHO. The isomerization process is accompanied by OH recycling. The rate-limiting first isomerization step, CH₃SCH₂O₂ → CH₂SCH₂OOH, followed by O₂ addition, proceeds with k = (0.23 ± 0.12) s^-1 at 295 ± 2 K. Competing bimolecular CH₃SCH₂O₂ reactions with NO, HO₂, or RO₂ radicals are less important for trace-gas conditions over the oceans. Results of atmospheric chemistry simulations demonstrate the predominance (≥95%) of CH₃SCH₂O₂ isomerization. The rapid peroxy radical isomerization, not yet considered in models, substantially changes the understanding of DMS's degradation processes in the atmosphere.

Reaction mechanism and kinetics of the atmospheric oxidation of 1,4-thioxane by NO3 — A theoretical study

Volatile organic compounds (VOCs) are emitted as pollutants into the atmosphere from many natural and artificial sources. The oxidation of VOCs by atmospheric species plays a key role in the degradation of VOCs. In the present investigation, the atmospheric degradation of a cyclic organosulfur compound, 1,4-thioxane, by an NO3• radical is studied. Pathways for the reaction of 1,4-thioxane with the NO3• radical were modeled through electronic structure calculations using density functional theory methods B3LYP, M06-2X, and MP2 with the 6–31G(d,p) basis set. The NO3•-initiated reaction of 1,4-thioxane was found to proceed in three ways: by single-hydrogen atom abstraction, by direct transfer of the O atom of NO3• to the S atom moiety of 1,4-thioxane, or by two-hydrogen atom transfer reactions leading to the formation of a peroxy radical intermediate, which further undergoes secondary reactions with other atmospheric species. Structures, energies, and vibrational frequencies obtained from M06-2X/6–31G(d,p) electronic structure calculations were subsequently used to perform canonical variational transition-state theory calculations to determine the rate constants over the temperature range of 278–350 K and to study the lifetime of 1,4-thioxane in the atmosphere. The rate constant calculated for the reaction of 1,4-thioxane with the NO3• radical is in good agreement with the available experimental data.

Reaction Mechanisms and Kinetics for the Oxidations of Dimethyl Sulfide, Dimethyl Disulfide, and Methyl Mercaptan by the Nitrate Radical

This study examines the initial oxidation routes of the three major reduced sulfur compounds (CH(3)SH, CH(3)SCH(3), and CH(3)SSCH(3)) by the nitrate radical using density functional and ab initio methods. Stationary points along each reaction pathway are examined using different levels of theory and basis sets to ensure the convergence of the results. Kinetics calculations follow on the determined reaction pathways to obtain the rate constants. This study shows that sulfur compounds exhibit a general trend of hydrogen abstraction following the formation of an initial sulfur-nitrate complex. The results are in agreement with experimental work on CH(3)SCH(3) and CH(3)SH, while refuting a proposal of several previous studies that oxygen addition is the dominant oxidation pathway in the case of CH(3)SSCH(3). The rate constants obtained from kinetics calculations are consistent with experimental findings and exhibit negative temperature dependence. Overall, this study confirms the importance of nitrate in the oxidation of reduced sulfur compounds in the atmosphere.

Temperature and Pressure Dependence of the Rate Constants of the Reaction of NO3 Radical with CH3SCH3

The reaction of nitrate radical with dimethyl sulfide was studied with cavity ring-down spectroscopy in 20-200 Torr of N2 diluent in the temperature range of 283-318 K. The rate constant for this reaction, k(1), is found to be temperature dependent and pressure independent: k1 = 4.5(-2.8)(+4.0) x 10(-13) exp[(310 +/- 220)/T] cm3 molecule(-1) s(-1). The uncertainties are two standard deviations from regression analyses. The present rate constants are in good agreement with those reported by Daykin and Wine (Int. J. Chem. Kinet. 1990, 22, 1083) and may be used in the atmospheric model calculation. Theoretical calculations were carried out to verify the existence of an intermediate complex.

Rate Constants of the Reaction of NO3 with CH3I Measured with Use of Cavity Ring-Down Spectroscopy

We have applied cavity ring-down spectroscopy to a kinetic study of the reaction of NO3 with CH3I in 20-200 Torr of N2 diluent at 298 K. The rate constant of the reaction of NO3 + CH3I was determined to be (4.1 +/- 0.2) x 10(-13) cm3 molecule(-1) s(-1) in 100 Torr of N2 diluent at 298 K and is pressure-independent. This reaction may significantly contribute to the formation of reactive iodine compounds in the atmosphere.