Rate coefficients for the gas-phase reaction of OH radicals with dimethyl sulfide: temperature and O2partial pressure dependence

Kinetics, product distribution and atmospheric implications of the gas-phase oxidation of allyl sulfides by OH radicals

Relative rate coefficients of the OH radical -initiated oxidation of allyl methyl sulfide (AMS, H₂CCHCH₂SCH₃) and allyl ethyl sulfide (AES, H₂CCHCH₂SCH₂CH₃) have been measured at atmospheric pressure of synthetic air and 298 K: k_AMS= (4.98 ± 1.42) and k_AES= (6.88 ± 1.49) × 10^-11 cm³ molecule^-1 s^-1 by means of in situ FTIR spectroscopy. In addition, the molar yields of the main reaction products of AMS with OH radicals formed in the absence and presence of nitric oxides (NO_X) were determined to be the following: sulfur dioxide (95 ± 12) % and (51 ± 12) % for acrolein (50 ± 9) % and (41 ± 9) %. In the reaction of AES with OH radicals, the following molar yields were obtained: for sulfur dioxide (88 ± 13) % and (56 ± 12) % for acrolein (36 ± 9) % and (41 ± 9) %. The present results suggest that the abstraction at C3 plays an important role in the oxidation mechanism as the addition to the double bond. This work represents the first study of the OH radical interaction with AMS and AES carried out under atmospheric conditions. The atmospheric implications were discussed in terms of the atmospheric residence times of the sulfur-containing compounds studied and the products formed in the presence and absence of NOx. SO₂ formation seems to be the main fate of the gas-phase allyl sulfides oxidation with significant acidifying potentials and short-chain aldehydes production like formaldehyde and acetaldehyde.

Electron Momentum Spectroscopy Study on the Valence Electronic Structure of Dimethyl Sulfide Considering Vibrational Effects

We report an electron momentum spectroscopy study on the valence electronic structure of dimethyl sulfide. The binding energy and electron momentum profiles are measured using a high-sensitivity (e, 2e) apparatus employing a symmetric non-coplanar geometry at an incident energy of 1200 eV plus binding energy. The measurements are compared with the theoretical calculations by density functional theory performed both at equilibrium molecular geometry and by considering vibrational effects through a harmonic analytical quantum mechanical approach. The results demonstrate a significant influence of nuclear vibrational motions on the momentum profiles for valence orbitals of dimethyl sulfide, especially for 5b₂, 1a₂, and 4b₂. A detailed analysis shows that the observed vibrational effects come mainly from vibrational normal modes breaking the mirror symmetry of (CH₃)₂S with respect to a plane perpendicular to the O-S-O plane.

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.

Gas-phase degradation of 2-butanethiol initiated by OH radicals and Cl atoms: kinetics, product yields and mechanism at 298 K and atmospheric pressure

Relative rate coefficients and product distribution of the reaction of 2-butanethiol (2butSH) with OH radicals and Cl atoms were obtained at atmospheric pressure and 298 K. The experiments were performed in a 480 L borosilicate glass photoreactor in synthetic air coupled to a long path "in situ" FTIR spectrometer. The rate coefficients obtained by averaging the values from different experiments were: k _OH = (2.58 ± 0.21) × 10^-11 cm³ per molecule per s and k _Cl = (2.49 ± 0.19) × 10^-10 cm³ per molecule per s. The kinetic values were compared with related alkyl thiols and homologous alkyl alcohols, where it was found that thiols react faster with both oxidants, OH radicals and Cl atoms. SO₂ and 2-butanone were the major products identified for the reactions of 2-butanethiol with OH radicals and Cl atoms. The product yield of the reaction of 2-butanethiol and OH radicals were (81 ± 2)%, and (42 ± 1)% for SO₂ and 2-butanone, respectively. For the reactions of 2-butanethiol with Cl atom, yields of SO₂ and 2-butanone were (59 ± 2)% and (39 ± 2)%, respectively. A degradation mechanism was proposed for the pathways that leads to formation of identified products. The product distribution observed indicated that the H-atom of the S-H group abstraction channel is the main pathway for the reaction of OH radicals and Cl atoms with 2-butanethiol.

Atmospheric chemistry of methyl and ethyl N,N,N',N'-tetramethylphosphorodiamidate and O,S-dimethyl methylphosphonothioate

Rate constants for the reactions of OH radicals with methyl N,N,N',N'-tetramethylphosphorodiamidate [CH3OP(O)[N(CH3)2]2; MTMPDA], ethyl N,N,N',N'-tetramethylphosphorodiamidate [C2H5OP(O)[N(CH3)2]2; ETMPDA], and O,S-dimethyl methylphosphonothioate [CH3OP(O)(CH3)SCH3; OSDMMP] have been measured over the temperature range 281-349 K at atmospheric pressure of air using a relative rate method. The rate expressions obtained were 4.96 × 10(-12) e((1058±71)/T) cm(3) molecule(-1) s(-1) (1.73 × 10(-10) cm(3) molecule(-1) s(-1) at 298 K) for OH + MTMPDA, 4.46 × 10(-12) e((1144±95)/T) cm(3) molecule(-1) s(-1) (2.07 × 10(-10) cm(3) molecule(-1) s(-1) at 298 K) for OH + ETMPDA, and 1.31 × 10(-13) e((1370±229)/T) cm(3) molecule(-1) s(-1) (1.30 × 10(-11) cm(3) molecule(-1) s(-1) at 298 K) for OH + OSDMMP. The rate constant for OH + OSDMMP was independent of O2 content over the range 2.1-71% O2 at 296 ± 2 K. In addition, rate constants for the reactions of NO3 radicals and O3 with MTMPDA, of (1.4 ± 0.1) × 10(-12) cm(3) molecule(-1) s(-1) and <3.5 × 10(-19) cm(3) molecule(-1) s(-1), respectively, were measured at 297 ± 2 K. Products of the OH radical- and, for MTMPDA, NO3 radical-initiated reactions were investigated using gas chromatography and in situ atmospheric pressure ionization mass spectrometry. A product of molecular weight 180 was observed from the OH and NO3 radical-initiated reactions of MTMPDA, and this is attributed to CH3OP(O)[N(CH3)2]N(CH3)CHO. Similarly, a product of molecular weight 194 was observed from the OH + ETMPDA reaction and attributed to C2H5OP(O)[N(CH3)2]N(CH3)CHO. Possible reaction mechanisms are discussed.

Formation pathways of DMSO(2) in the addition channel of the OH-initiated DMS oxidation: A theoretical study

The production of dimethyl sulfoxide (DMSO) and dimethyl sulfone (DMSO(2)) in the dimethyl sulfide (DMS) degradation scheme initiated by the hydroxyl (OH) radical has been shown to be very sensitive to nitrogen oxides (NO(x)) levels. In the present work we have explored the potential energy surfaces corresponding to several reaction pathways which yield DMSO(2) from the CH(3)S(O)(OH)CH(3) adduct [including the formation of CH(3)S(O)(OH)CH(3) from the reaction of DMSO with OH] and the reaction channels that yield DMSO or/and DMSO(2) from the CH(3)S(O(2))(OH)CH(3) adduct are also studied. The formation of the CH(3)S(O(2))(OH)CH(3) adduct from CH(3)S(OH)CH(3) (DMS-OH) and O(2) was analyzed in our previous work. All these pathways due to the presence of NO(x) (NO and NO(2)) and also due to the reactions with O(2), OH and HO(2) are compared with the objective of inferring their kinetic relevance in the laboratory experiments that measure DMSO(2) (and DMSO) formation yields. In particular, our theoretical results clearly show the existence of NO(x)-dependent pathways leading to the formation of DMSO(2), which could explain some of these experimental results in comparison with experimental measurements carried out in NO(x)-free conditions. Our results indicate that the relative importance of the addition channel in the DMS oxidation process can be dependent on the NO(x) content of chamber experiments and of atmospheric conditions.

Kinetic Study on the Reaction of OH Radical with Dimethyl Sulfide in the Absence of Oxygen

A variational transition-state theory calculation for the reaction of OH radical with dimethyl sulfide (DMS) in the absence of oxygen is presented. The potential energy surface was previously studied and the effects of different levels of theory were analyzed. Here we propose a kinetic model for the atmospheric DMS oxidation in the absence of oxygen. For the first time, addition of OH to DMS and CH(3)SOH elimination channels are connected, and the equilibrium approximation in the high-pressure regime is applied to the DMS-OH adduct in the absence of oxygen. Both low- and high-pressure limits are considered to analyze the two different mechanisms of the H-abstraction channel, and two different kinetic approaches are applied to study them. The rate constants for the addition-elimination and H-abstraction routes are compared and the branching ratios are also studied. Tunneling contributions and kinetic isotope effects are analyzed. We conclude, in agreement with experimental observations, that in the absence of oxygen DMS oxidation takes place via H-abstraction with a branching ratio of 1.0 at atmospheric temperatures.

Experimental and Theoretical Studies of the Reaction of the OH Radical with Alkyl Sulfides: 1. Direct Observations of the Formation of the OH−DMS Adduct−Pressure Dependence of the Forward Rate of Addition and Development of a Predictive Expression at Low Temperature

A pulsed laser photolysis-pulsed laser-induced fluorescence (PLP-PLIF) system was employed to study the kinetics and mechanisms of reactions (1) OH + h6-DMS --> products and (2) OH + d6-DMS --> products. We report direct observations of the rate coefficients for the formation and dissociation of the h6-OHDMS and d6-OHDMS adducts over the pressure range 50-650 Torr and between 240 and 245 K, together with measurements of the oxygen dependence of the effective rate coefficients for reactions 1 and 2 under similar conditions. The effective rate coefficients increased as a function of O2 concentration reaching their limiting values in each case. The values of the adduct formation rate, obtained from the O2 dependencies, were in excellent agreement with values determined from direct observation of adduct equilibration in N2. OH regeneration is insignificant. The rate coefficients for the formation of the adduct isotopomers showed slight differences in their falloff behavior and do not approach the high-pressure limit in either case. The equilibrium constants obtained show no dependence on isotopomer and are in good agreement with previous work. A "second-law" analysis of the temperature dependence of the equilibrium constant gives an adduct bond strength (DeltaH degrees =-10.9 +/- 1.0 kcal mol(-1)), also in good agreement with previously reported values. Using the entropy calculated from the ab initio vibrational frequencies, we obtain a "third-law" value for the reaction enthalpy at 240 K, DeltaH(240K) degrees = -10.5 kcal mol(-1) in good agreement with the other approach. The rate coefficient for the reactions of the adducts with O2 was obtained from an analysis of the O2 dependence and was determined to be 6.3 +/- 1.2 x 10(-13) cm3 molecule(-1) s(-1), with no dependence on pressure or isotopomer. The pressure and temperature dependence for all of the elementary processes in the initial steps of the dimethylsulfide (DMS) oxidation mechanism have been characterized in the range 238-245 K, allowing the formulation of an expression which can be used to calculate the effective rate coefficient for reaction 1 at any pressure and oxygen concentration. The expression can calculate the effective rate coefficient for reaction 1 to +/- 40% over the range 220-260 K, with the largest errors at the extremes of this range. Gaussian 03 has been used to calculate the structure of the OH-DMS adduct and its deuterated isotopomer. We find similar bound structures for both isotopomers. The calculated enthalpies of formation of the adducts are lower than the experimentally determined values.