ClO Reaction Research Articles

Reactions of α-Nucleophiles with Alkyl Chlorides: Competition between SN2 and E2 Mechanisms and the Gas-Phase α-Effect

Reaction rate constants and deuterium kinetic isotope effects for the reactions of BrO(-) with RCl (R = methyl, ethyl, isopropyl, and tert-butyl) were measured using a tandem flowing afterglow-selected ion flow tube instrument. These results provide qualitative insight into the competition between two classical organic mechanisms, nucleophilic substitution (S(N)2) and base-induced elimination (E2). As the extent of substitution in the neutral reactants increases, the kinetic isotope effects become increasingly more normal, consistent with the gradual onset of the E2 channel. These results are in excellent agreement with previously reported trends for the analogous reactions of ClO(-) with RCl. [Villano et al. J. Am. Chem. Soc. 2006, 128, 728.] However, the reactions of BrO(-) and ClO(-) with methyl chloride, ethyl chloride, and isopropyl chloride were found to occur by an additional reaction pathway, which has not previously been reported. This reaction likely proceeds initially through a traditional S(N)2 transition state, followed by an elimination step in the S(N)2 product ion-dipole complex. Furthermore, the controversial alpha-nucleophilic character of these two anions and of the HO(2)(-) anion is examined. No enhanced reactivity is displayed. These results suggest that the alpha-effect is not due to an intrinsic property of the anion but instead due to a solvent effect.

HOCl chemistry in the Antarctic Stratospheric Vortex 2002, as observed with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)

Abstract. In the 2002 Antarctic polar vortex enhanced HOCl mixing ratios were detected by the Michelson Interferometer for Passive Atmospheric Sounding both at altitudes of around 35 km (1000 K potential temperature), where HOCl abundances are ruled by gas phase chemistry and at around 18–24 km (475–625 K), which belongs to the altitude domain where heterogeneous chlorine chemistry is relevant. At altitudes of 33 to 40 km polar vortex HOCl mixing ratios were found to be around 0.14 ppbv as long as the polar vortex was intact, centered at the pole, and thus received relatively little sunlight. This is the altitude region where in midlatitudinal and tropic atmospheres peak HOCl mixing ratios significantly above 0.2 ppbv (in terms of daily mean values) are observed. After deformation and displacement of the polar vortex in the course of a major warming, ClO-rich vortex air was more exposed to sunlight, where enhanced HOx abundances led to largely increased HOCl mixing ratios (up to 0.3 ppbv), exceeding typical midlatitudinal and tropical amounts significantly. The HOCl increase was preceded by an increase of ClO. Model runs could reproduce these measurements only when the Stimpfle et al. (1979) rate constant for the reaction ClO+HO2→HOCl+O2 was used but not with the current JPL recommendation. At an altitude of 24 km, HOCl mixing ratios of up to 0.15 ppbv were detected. This HOCl enhancement, which is already visible in 18 September data, is attributed to heterogeneous chemistry, which is in agreement with observations of polar stratospheric clouds. The measurements were compared to a model run where no polar stratospheric clouds appeared during the observation period. The fact that HOCl still was produced in the model run suggests that a significant part of HOCl was generated from ClO rather than directly via heterogeneous reaction. Excess ClO, lower ClONO2 and earlier loss of HOCl in the measurements are attributed to ongoing heterogeneous chemistry which is not reproduced by the model. On 11 October, polar vortex mean daytime mixing ratios were only 0.03 ppbv.

Chlorine dioxide reaction with selected amino acids in water

Chlorine dioxide is a hypochlorite alternative disinfectant agent. In this context, we have determined the products formed in the reaction of ClO 2 with selected amino acids as model compounds that can be present in natural waters. The reaction of tryptophane, histidine and tyrosine (10 ppm each) with ClO 2 were studied at molar ratios ranging from 0.25 to 4 in the presence or absence of oxygen. It was found that in the absence of oxygen adding substoichiometric amounts of ClO 2 creates products that are structurally similar to the starting amino acids. Through a series of cascade reactions the initial product distribution gradually evolves toward simple, small carbon chain products that are far from the starting amino acid. The reaction product distribution revealed that chlorine dioxide can attack the electron-rich aromatic moieties as well as the nitrogen atom lone electron pair. Our study is relevant to gain knowledge on the reaction mechanism of ClO 2 with ubiquitous amino acids present in natural waters.

The photochemical decomposition of gaseous OClO at 436 nm: Dependence of products distribution on total pressure

A systematic experimental study of the photochemical decomposition of 5–80 Torr gaseous OClO at 293 and 303 K has been performed. The photolysis was also investigated in the presence of the added inert gases He, Ar, N 2, O 2, CO 2 and SF 6 at pressures ranging from 10 to 600 Torr. The final products Cl 2O 4, Cl 2O 6, Cl 2 and O 2 are formed. The amount of Cl 2O 4 and Cl 2O 6 was determined by IR and UV spectrophotometry, respectively. Both products reach a limit of about 30% of the total yield. Only very small amounts of ClClO 2 have been detected. The quantum yields for Cl 2O 4 and Cl 2O 6 formation satisfy the condition Φ(Cl 2O 4) + Φ(Cl 2O 6) ≅ 1 throughout. In the absence of an inert gas Φ(Cl 2O 4) decreases from a value close to 1 to about 0.5 as OClO pressure is decreased, indicating the formation of Cl 2O 4 by the reaction ClO 3 + ClO → Cl 2O 4. For about 5 Torr OClO mean pressure and in the presence of an inert gas, Φ(Cl 2O 4) increases as total pressure is increased reaching a value close to 1 for 300 Torr of N 2 or 50 Torr of SF 6. At the higher inert gas pressures Φ(Cl 2O 4) slightly decreases when OClO is increased. This result is attributed to the interplay established between reactive and collisional deactivation process in the excited radical intermediate Cl 2O 5 * generated in the process ClO 3 * + OClO → Cl 2O 5 * → ClO 4 + ClO. The generally accepted reaction for the formation of Cl 2O 6, namely 2ClO 3 → Cl 2O 6, can be ruled out and a complicated mechanism is needed to account for the experimental observations.

Observed and modeled HOCl profiles in the midlatitude stratosphere: Implication for ozone loss

Vertical profiles of stratospheric HOCl calculated with a diurnal steady‐state photochemical model that uses currently recommended reaction rates and photolysis cross sections underestimate observed profiles of HOCl obtained by two balloon‐borne instruments, FIRS‐2 (a far‐infrared emission spectrometer) and MkIV (a mid‐infrared, solar absorption spectrometer). Considerable uncertainty (a factor of two) persists in laboratory measurements of the rate constant (k1) for the reaction ClO + HO2 → HOCl + O2. Agreement between modeled and measured HOCl can be attained using a value of k1 from Stimpfle et al. (1979) that is about a factor‐of‐two faster than the currently recommended rate constant. Comparison of modeled and measured HOCl suggests that models using the currently recommended value for k1 may underestimate the role of the HOCl catalytic cycle for ozone depletion, important in the midlatitude lower stratosphere.

Electrochemical study of hydrophilic ion transfers across cholesterol modified water–nitrobenzene interface by means of thin film electrodes

Electrochemically driven ion transfer reactions of ClO 4 - , NO 3 - , SCN −, and K + across a cholesterol modified water–nitrobenzene (W–NB) interface have been studied by means of square-wave and cyclic voltammetry at thin organic film modified electrodes. It has been found that cholesterol exhibits a profound accelerating effect toward the kinetics of ClO 4 - , NO 3 - , SCN − transfers, whereas a retarding kinetic effect was observed for K +. Besides, the cholesterol layer alters significantly the mechanism of ion transfer, prompting adsorption of all studied ions as a consequence of specific interactions between cholesterol molecules and the transferring ion.

Ab Initio/Density Functional Theory and Multichannel RRKM Study for the ClO + CH2O Reaction

The potential energy surface for the CH(2)O + ClO reaction was calculated at the QCISD(T)/6-311G(2d,2p)//B3LYP/6-311G(d,p) level of theory. The rate constants for the lower barrier reaction channels producing HOCl + HCO, H atom, OCH(2)OCl, cis-HC(O)OCl and trans-HC(O)OCl have been calculated by TST and multichannel RRKM theory. Over the temperature range of 200-2000 K, the overall rate constants were k(200-2000K) = 1.19 x 10(-13)T(0.79) exp(-3000.00/T). At 250 K, the calculated overall rate constant was 5.80 x 10(-17) cm(3) molecule(-1) s(-1), which was in good agreement with the experimental upper limit data. The calculated results demonstrated that the formation of HOCl + HCO was the dominant reaction channel and was exothermic by 9.7 kcal/mol with a barrier of 5.0 kcal/mol. When it retrograded to the reactants CH(2)O + ClO, an energy barrier of 14.7 kcal/mol is required. Furthermore, when HOCl decomposed into H + ClO, the energy required was 93.3 kcal/mol. These results suggest that the decomposition in both the forward and backward directions for HOCl would be difficult in the ground electronic state.

Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory

The competition between bimolecular nucleophilc substitution and base-induced elimination is investigated through kinetic isotope effect measurements for gas-phase reactions of RCl + ClO- (R = methyl, ethyl, isopropyl, and tert-butyl) utilizing a FA-SIFT instrument. The overall reaction rate constants and the kinetic isotope effect for the reaction of C2H5Cl + ClO- are compared to computational results. [Hu, W. P.; Truhlar, D. G. J. Am. Chem. Soc. 1996, 118, 860.] Experimental results show that as the degree of substitution in the neutral reactant increases the E2 channel becomes dominant. The systematic change in the overall kinetic isotope effects indicates that, for the reaction of ClO- with C2H5Cl, both the SN2 and E2 pathways do occur, as predicted by computation; however the experimental reaction rate constants and KIE deviate strongly from the computational result.

A Systematic Computational Study of the Reactions of HO2 with RO2: The HO2 + CH2ClO2, CHCl2O2, and CCl3O2 Reactions

Potential energy surfaces for the reactions of HO(2) with CH(2)ClO(2), CHCl(2)O(2), and CCl(3)O(2) have been calculated using coupled cluster theory and density functional theory (B3LYP). It is revealed that all the reactions take place on both singlet and triplet surfaces. Potential wells exist in the entrance channels for both surfaces. The reaction mechanism on the triplet surface is simple, including hydrogen abstraction and S(N)2-type displacement. The reaction mechanism on the singlet surface is more complicated. Interestingly, the corresponding transition states prefer to be 4-, 5-, or 7-member-ring structures. For the HO(2) + CH(2)ClO(2) reaction, there are two major product channels, viz., the formation of CH(2)ClOOH + O(2) via hydrogen abstraction on the triplet surface and the formation of CHClO + OH + HO(2) via a 5-member-ring transition state. Meanwhile, two O(3)-forming channels, namely, CH(2)O + HCl + O(3) and CH(2)ClOH + O(3) might be competitive at elevated temperatures. The HO(2) + CHCl(2)O(2) reaction has a mechanism similar to that of the HO(2) + CH(2)ClO(2) reaction. For the HO(2) + CCl(3)O(2) reaction, the formation of CCl(3)O(2)H + O(2) is the dominant channel. The Cl-substitution effect on the geometries, barriers, and heats of reaction is discussed. In addition, the unimolecular decomposition of the excited ROOH (e.g., CH(2)ClOOH, CHCl(2)OOH, and CCl(3)OOH) molecules has been investigated. The implication of the present mechanisms in atmospheric chemistry is discussed in comparison with the experimental measurements.

Nighttime OClO in the winter Arctic vortex

We show that a nighttime profile of OClO in the Arctic vortex during the winter of 2000 is overestimated, by nearly a factor of 2, using an isentropic trajectory model constrained by observed profiles of ClOx (ClO + 2 × ClOOCl) and BrO. Calculated abundances of nighttime OClO are shown to be sensitive to the abundance of BrOx (BrO + BrCl), details of the air parcel history during the most recent sunrise/sunset transitions, and the BrCl yield from the reaction BrO + ClO. Many uncertainties are considered, and the discrepancy between measured and modeled nighttime OClO appears to be robust. This discrepancy suggests that production of OClO occurs more slowly than implied by standard photochemistry. If the yield of BrCl from the reaction of BrO + ClO is increased from 7% (JPL 2002 value) to 11% (near the upper limit of the uncertainty), good agreement is found between measured and modeled nighttime OClO. This study highlights the importance of accurate knowledge of BrO + ClO reaction kinetics as well as air parcel trajectories for proper interpretation of nighttime OClO. These factors have a considerably smaller impact on the interpretation of OClO observations obtained during twilight (90° ≤ SZA ≤ 92°), when photolytic processes are still active.

Reaction Mechanisms and Topological Studies of Electron Density on the Reaction of ClO and ClO Radical

Reaction Mechanisms and Topological Studies of Electron Density on the Reaction of ClO and ClO Radical

Ab initio Studies of ClOx Reactions: Prediction of the Rate Constants of ClO+NO2 for the Forward and Reverse Processes

The potential-energy surface for the reaction of ClO with NO2 has been constructed at the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) level of theory. Six ClNO3 isomers are located; these are ClONO2, pc-ClOONO, pt-ClOONO, OClNO2, pt-OClONO, pc-OClONO, with predicted energies relative to the reactants of -25.6, -0.5, 1.0, 1.9, 12.2 and 13.6 kcal mol-1, and heats of formation at 0 K of 7.8, 32.9, 34.4, 35.5, 45.6 and 47.0 kcal mol-1, respectively. Isomerizations among them are also discussed. The rate constants for the low-energy pathways have been computed by statistical theory calculations. For the association reaction producing exclusively ClONO2, the predicted low- and high-pressure-limit rate constants in N2 for the temperature range of 200-600 K can be represented by: (N2)=3.19 x 10-17 T-5.54 exp(-384 K/T) cm6 molecule-2 s-1 and =3.33 x 10-7 T-1.48 exp(-18 K/T) cm3 molecule-1 s-1. The predicted low- and high-pressure-limit rate constants for the decomposition of ClONO2 in N2 at 200-600 K can be expressed, respectively, by =6.08 x 1013 T-6.54 exp(-13813 K/T) cm3 molecule-1 s-1 and =4.59 x 1023 T-2.43 exp(-13437 K/T) s-1. The predicted values compare satisfactorily with available experimental data. The reverse Cl+NO3 reaction was found to be independent of the pressure, giving exclusively ClO+NO2; the predicted rate constant can be expressed as k(Cl+NO3)=1.19 x 10-9 T-0.60 exp(58 K/T) cm3 molecule-1 s-1..

Oxidation of chlorine(III) by hypobromous acid: kinetics and mechanism.

The kinetics and mechanism of the chlorine(III)-HOBr reaction were studied by the stopped-flow method under acidic conditions, pH 1.0-3.0, in 1.0 M NaClO(4) and at 25.0 degrees C. The overall redox process occurs in two consecutive steps via the formation of the BrClO(2) intermediate. The electron transfer reactions are coupled with bromine hydrolysis, the formation of the tribromide ion, and the protolytic equilibrium of chlorine(III). On the basis of simultaneous evaluation of the kinetic traces, the following rate constants were obtained for the redox steps: HClO(2) + HOBr right harpoon over left harpoon BrClO(2) + H(2)O, k(3) = (3.34 +/- 0.02) x 10(4) M(-1) s(-1), k(-3) = (3.5 +/- 1.3) x 10(3) s(-1); BrClO(2) + ClO(2)(-)<==>2ClO(2) + Br(-), k(4) = (2.9 +/- 1.0) x 10(7) M(-1) s(-1). The second step was practically irreversible under the conditions applied, and the value of k(-4) could not be determined. The equilibrium constant for the formation of BrClO(2), K(3) = 9.5 M(-1), was calculated from the kinetic results, and it was confirmed that this species is a very powerful oxidant. The redox potential was also estimated for the BrClO(2) + e(-) = Br(-) + ClO(2) reaction: epsilon(0) approximately 1.70 V.

Potential-dependent electrochemiluminescence of luminol in alkaline solution at a gold electrode

The behavior of luminol electrochemiluminescence (ECL) at a polycrystalline gold electrode was studied under conventional cyclic voltammetric (CV) conditions. At least six ECL peaks were observed at 0.28 (ECL-1), 0.56 (ECL-2), 0.95 (ECL-3), 1.37 (ECL-4), −0.43 (ECL-5) and 1.00 (ECL-6, a broad wave after the reverse scan from +1.66) V (vs. SCE), respectively, on the curve of ECL intensity versus the potential. These ECL peaks were found to depend on the presence of O 2 and N 2, the pH of the solution, KCl concentration, scan rate, and potential scan ranges. The emitter of all ECL peaks was identified as 3-aminophthalate by analyzing the CL spectra. It is believed that ECL-1 at 0.28 V was correlated to luminol radicals produced by the electro-oxidation of luminol anion and ECL-2 at 0.56 V was caused by the reaction of luminol radical anions with gold oxide formed on the electrode surface. ECL-1 and ECL-2 could be strongly enhanced by O 2 and O 2 − . ECL-3 at 0.95 V was likely to be due to the reaction of luminol radical anions with O 2 oxidized by OH −. ECL-4 at 1.37 V suggested that OH − was electro-oxidized to HO 2 − at this potential and then to O 2 − , which reacted with luminol radical anions to produce light emission. ECL-5 at −0.43 V seems to be due to the reaction of luminol with ClO − electrogenerated at higher positive potential and HO 2 − electrogenerated at negative potential. ECL-6 was attributed to the reaction of luminol radical anions and ClO − electrogenerated at higher positive potential. The results indicated that luminol ECL can be readily initiated by various oxygen-containing species electrogenerated at different potentials, leading to multi-channel light emissions. Furthermore, the present work also reveals that ECL-2 is a predominant ECL reaction route at a gold electrode with higher potential scan rates under CV conditions.

Nucleophile assistance of electron-transfer reactions between nitrogen dioxide and chlorine dioxide concurrent with the nitrogen dioxide disproportionation.

The reaction of chlorine dioxide with excess NO(2)(-) to form ClO(2)(-) and NO(3)(-) in the presence of a large concentration of ClO(2)(-) is followed via stopped-flow spectroscopy. Concentrations are set to establish a preequilibrium among ClO(2), NO(2)(-), ClO(2)(-), and an intermediate, NO(2). Studies are conducted at pH 12.0 to avoid complications due to the ClO(2)(-)/NO(2)(-) reaction. These conditions enable the kinetic study of the ClO(2) reaction with nitrogen dioxide as well as the NO(2) disproportionation reaction. The rate of the NO(2)/ClO(2) electron-transfer reaction is accelerated by different nucleophiles (NO(2)(-) > Br(-) > OH(-) > CO(3)(2-) > PO(4)(3-) > ClO(2)(-) > H(2)O). The third-order rate constants for the nucleophile-assisted reactions between NO(2) and ClO(2) (k(Nu), M(-2) s(-1)) at 25.0 degrees C vary from 4.4 x 10(6) for NO(2-) to 2.0 x 10(3) when H(2)O is the nucleophile. The nucleophile is found to associate with NO(2) and not with ClO(2) in the rate-determining step to give NuNO(2)(+) + ClO(2)(-). The concurrent NO(2) disproportionation reaction exhibits no nucleophilic effect and has a rate constant of 4.8 x 10(7) M(-1) s(-1). The ClO(2)/NO(2)/nucleophile reaction is another example of a system that exhibits general nucleophilic acceleration of electron transfer. This system also represents an alternative way to study the rate of NO(2) disproportionation.

Ab initio studies of ClOx reactions. IX. Combination and disproportionation reactions of ClO and s-ClO3 radicals

The mechanism for the reaction ClO+ClO3 on both singlet and triplet state potential surfaces has been investigated with the modified Gaussian-2 method based on the B3LYP/6-311+G(3df ) optimized stationary-point geometries. The result shows that the barrierless association reaction producing ClOClO3 and two lower barrier O-atom abstraction reactions take place primarily on the singlet state potential surface; they are energetically more favorable than those occurring on the triplet state surface. Rate constants calculated by variational transition state and Rice–Ramsperger–Kassel–Marcus theories suggest that the major products are ClOClO3 at low temperatures (&amp;lt;700 K) and OClO+ClOO at high temperatures. The following rate constants are recommended for atmospheric chemistry and combustion applications: k1∞(ClOClO3)=1.43×10−10 T0.094exp(−82/T) cm3 molecule−1 s−1 (200–3000 K) and k10(ClOClO3)=0.14 T−10.19exp(−1597/T) cm6 molecule−2 s−1 (200–800 K) for He as the third-body. In addition, a potential high-temperature reverse reaction involving Cl and ClO4 has been predicted to yield exclusively the ClO and ClO3 products; its rate constant is predicted to be: k4(ClO+ClO3)=8.05×10−11 T0.158 exp(−49/T) cm3 molecule−1 s−1 (200–3000 K). The heats of formation at 0 K for s-ClO3, ClO4, and ClOClO3 have been predicted to be 46.7, 59.4, and 38.3 kcal/mol with about 1 kcal/mol uncertainty using the new heat of formation of OClO, 24.1±0.1 kcal/mol, based on the most recent bond dissociation energy of O–ClO reported in the literature.

Ab initio studies of ClOx reactions. VII. Isomers of Cl2O3 and their roles in the ClO+OClO reaction

The mechanism for the reaction ClO+OClO has been investigated by ab initio molecular orbital and transition state theory calculations. Nine stable isomers of Cl2O3 (including optical isomers) are located at the B3LYP/6-311+G(3df ) level. The transition states between pairs of isomers are explored and the stability of the isomers and their dissociation mechanisms are discussed. The relative stability predicted by the modified Gaussian-2 (G2M) method at the G2M//B3LYP/6-311+G(3df ) level is ClOCl(O)O&amp;gt;ClOOOCl(C2)&amp;gt;ClOOOCl(Cs)&amp;gt;ClClO3(C3v)&amp;gt;cyc-ClOOCl(O)&amp;gt;ClOOClO. The formation of ClOCl(O)O is dominant at low temperatures, taking place barrierlessly with the second and third order rate constant, k1∞=3.0×10−10 cm3 molecule−1 s−1 and k10=1.1×10−17 T−5.5 exp(−398/T) cm6 molecule−2 s−1 in the temperature range of 200–400 K for N2 as the third-body. Over 500 K, formation of ClOO+ClO becomes dominant and ClOCl+1O2 is also competitive. Their overall rate constants can be given by kClOO=1.0×10−22 T2.8 exp(−78/T) and kClOCl=9.6×10−22 T2.4 exp(−1670/T)cm3 molecule−1 s−1, respectively, in the temperature range of 500–2500 K.

Formation of aldehydes upon ClO 2 disinfection

Carbonyl compounds are considered to be the most common ozonation by-products. Apart from well-known chlorites and chlorates, chlorine dioxide (ClO 2) also generates organic by-products. The goal of this paper is an investigation into the influence of disinfection with ClO 2 on the aldehydes formation. Three types of waters were disinfected with different doses of ClO 2 at the various pH values. The results of size exclusion chromatography show that the reaction of ClO 2 with humic fraction of natural organic matter proceeds mainly on an aromatic part of the molecules. The results obtained indicate that the level of carbonyls concentration can significantly increase with the time of ClO 2 reaction with carbonyl precursors in treated water. There is no noticeable correlation between quantity of aldehydes and pH value of disinfected water. The range of “productivity” of aldehydes in water treated with chlorine dioxide is similar to the range estimated for the carbonyls formed upon ozonation.

Reaction ClO + ClO → products: Modeling and parameterization for use in atmospheric models

AbstractThe interaction of two ClO radicals is important in the Antarctic stratosphere. Current models assume a single ClOOCl product and use the parameters suggested in the NASA/JPL or IUPAC compilations to describe the rate constant as a function of temperature and pressure (in air). It has been noted that these parameters suggest an inordinately high energy transfer efficiency and perhaps a somewhat high negative temperature dependence at the high pressure limit. Recent potential energy surface calculations reveal the possibility of the formation of ClOClO. In this study RRKM/master equation modeling of this surface was performed. Rational input parameters confirm that the formation of ClOClO in 10–15% yields may be important, but they do not conform with the shape and temperature dependence of the data. The simplest best representation of the extant data remains the single channel NASA/JPL format with parameters k0 (cm6 molecules−2 s−1) = 1.6 × 10−32 (T/300)−4.5 and k (cm3 molecules−1 s−1) = 2.0 × 10−12 (T/300)−2.4. However, the data can be represented as the sum of the two pathways as well. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 206–211, 2003

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

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.