Gas-phase Reactions Of Chlorine Atoms Research Articles

Photo-oxidation of Dimethyl Malonate Initiated by Chlorine Atoms in Gas Phase: Kinetics and Mechanism.

The rate coefficient for the gas-phase reaction of chlorine atoms with dimethyl malonate (DMM, CH3OC(O)CH2C(O)OCH3) was determined at 298 K using relative methods giving a value of (3.8 ± 0.4) × 10-12, cm3 molecule-1 s-1). The photo-oxidation mechanism of DMM was also investigated. The main products were identified by infrared spectroscopy, and computational calculations were performed in order to support the experimental data. The results reveal that the photo-oxidation occurs mainly by the abstraction of an H atom from the methyl groups. The CH3OC(O)CH2C(O)OCH2O• radical formed subsequently reacts according to three competitive paths: reaction with molecular oxygen to yield CH3OC(O)CH2C(O)OC(O)H, isomerization-unimolecular decomposition to lead finally to CH3OC(O)C(O)H, CO2, and HC(O)OH, and α-ester rearrangement to form monomethyl malonate and carbon monoxide. The yield of products as a function of oxygen pressure was also determined.

Atmospheric sink of methyl chlorodifluoroacetate and ethyl chlorodifluoroacetate: temperature dependent rate coefficients, product distribution of their reactions with Cl atoms and CF2ClC(O)OH formation

Rate coefficients as a function of temperature and product distribution studies have been performed for the first time for the gas-phase reactions of chlorine atoms with methyl chlorodifluoracetate (k1) and ethyl chlorodifluoroacetate (k2) using the relative rate technique.

Kinetic study of the gas-phase reactions of chlorine atoms with 2-chlorophenol, 2-nitrophenol, and four methyl-2-nitrophenol isomers.

Anthropogenic activities are the main source of nitrophenols and chlorophenols in the atmosphere. Nitro and chlorophenols have a high potential to form ozone and secondary organic aerosol, thus investigations on the major photo oxidation pathways of these compounds are important to assess their contribution to urban air pollution and human health. Presented here are rate coefficients determined at atmospheric pressure and (298 ± 2) K using a relative kinetic method for the reactions of chlorine atoms with 2-chlorophenol (2ClP), 2-nitrophenol (2NP) and four methyl-2-nitrophenol (2-nitrocresol, nM2NP (n = 3,4,5,6)) isomers. The following rate coefficients (in units of cm(3) molecule(-1) s(-1)) have been obtained: (5.9 ± 1.5) × 10(-12) for 2ClP, (6.8 ± 2.3) × 10(-12) for 2NP, and (14.0 ± 4.9) × 10(-11), (4.3 ± 1.5) × 10(-11), (1.94 ± 0.67) × 10(-11) and (2.68 ± 0.75) × 10(-11) for the four methyl-2-nitrophenol isomers 3M2NP, 4M2NP, 5M2NP, and 6M2NP, respectively. This study represents the first kinetic investigation for the reaction of chlorine atoms with all the nitrophenols. In addition, to assist in the interpretation of the results, rate coefficients for the reactions of Cl atoms with the cresol ortho, meta, and para isomers have been determined for the first time. The rate coefficient for the reaction with 2ClP is in good agreement with previous data and the relative reactivity of 2NP, 4M2NP, 5M2NP, and 6M2NP can be rationalized based on known substituent effects. The rate coefficient for 3M2NP is anomalously large; the observation of significant NO2 production in only this reaction suggests that an ipso substitution mechanism is the cause of the enhanced reactivity.

Kinetic study of the gas-phase reactions of hydroxyl radicals and chlorine atoms with cis-3-hexenylformate

Recently, it has been demonstrated that grass and cut grass are important sources of C6 esters’ emissions into the atmosphere. The esters emitted as part of the wound defense mechanisms are responsible for the smell of freshly cut grass and are also highly photochemically reactive. Thus, these grass emissions should be included in models of urban photochemical pollution so that accurate smog predictions can be obtained. In this work, we have carried out a kinetic study of the gas-phase reactions of chlorine atom and hydroxyl radical with cis-3-hexenylformate (HCO2CH2CH2CH=CHC2H5) for the first time. This was performed in a Teflon static reactor at room temperature and atmospheric pressure of nitrogen or air using gas chromatography with flame ionization detection. Kinetic rate coefficients obtained for cis-3-hexenylformate were (in units of cm3 molecule−1 s−1) (2.45 ± 0.30) × 10−10 and (4.61 ± 0.71) × 10−11 for chlorine and hydroxyl radical reactions, respectively. The reaction between the ester and chlorine atoms is very fast, approaching the collision-controlled regime; nevertheless, the rate constant for the reaction with hydroxyl radical is lower. In addition, the rate coefficients for the reactions of cis-3-hexenylformate with different atmospheric reactive species were estimated through a Quantitative Structure–Activity Relationship model in which High Occupied Molecular Orbital energy of the formate is used as a theoretical molecular descriptor. Tropospheric lifetimes, based on the homogeneous sinks of the cis-3-hexenylformate, are estimated from the kinetic data obtained in the present work. The relatively short atmospheric lifetimes of the ester indicate that the compound will be oxidized near its anthropogenic origin source.

Theoretical investigation on mechanism and kinetics of the Cl-initiated hydrogen abstraction reactions of ethyl trifluoroacetate at 298 K.

Theoretical investigations were carried out on the gas-phase reactions of CF3C(O)OCH2CH3, ethyl trifluoroacetate (ETFA) with Cl atoms by means of modern density functional theory methods. The optimized geometries, frequencies and minimum energy path were obtained with the hybrid density functional model MPWB1K using the 6-31+G(d,p) basis set. The single point energy calculations were refined further using the G2(MP2) method. Two conformers relatively close in energy were identified for ETFA; both are likely to be important in the temperature range of our study. The existence of transition states on the corresponding potential energy surface was ascertained by performing intrinsic reaction coordinate calculations. The rate constant at 298 K calculated theoretically using canonical transition state theory was found to be in good agreement with experimentally measured values. Our calculations suggest that H abstraction from the -CH2 group is kinetically and thermodynamically more favorable than abstraction from the -CH3 group. The atmospheric lifetime of ETFA with Cl atoms was determined to be 1.98 years. To the best of our knowledge, this work represents the first determination of the rate coefficients for the gas-phase reaction of chlorine atoms in ETFA.

Kinetics of the gas-phase reactions of chlorine atoms with naphthalene, acenaphthene, and acenaphthylene.

Reactions of polycyclic aromatic hydrocarbons (PAHs) with chlorine atoms may occur in specific areas such as coastal regions and the marine boundary layer. In this work, rate constants for the gas-phase reactions of naphthalene, acenaphthene, and acenaphthylene with chlorine atoms have been measured using the relative rate technique. Experiments were performed at room temperature (293 ± 2 K) and atmospheric pressure in an atmospheric simulation chamber using a proton-transfer reaction mass spectrometer (PTR-MS) to monitor the concentrations of PAHs and the reference compounds (acetone, methanol, 1,3,5-trimethylbenzene, and isoprene) as a function of time. The rate constants obtained in this work were (in units of cm(3) molecule(-1) s(-1)) (4.22 ± 0.46) × 10(-12), (3.01 ± 0.82) × 10(-10), and (4.69 ± 0.82) × 10(-10) for naphthalene, acenaphthene, and acenaphthylene, respectively. These are the first measurements of the rate constants for gas-phase reactions of Cl atoms with acenaphthene and acenaphthylene. The rate constant determined in this study for the reaction of naphthalene with Cl atoms is not in agreement with the only other previously reported value in the literature. The results are used to assess the potential role of chlorine atom reactions in the atmospheric oxidation of PAHs.

Rate Coefficients for the Gas-Phase Reaction of Chlorine Atoms with a Series of Methoxylated Aromatic Compounds

The reaction of a series of oxygenated aromatics (two methoxybenzene and six methoxyphenol isomers) with chlorine atoms has been studied in two simulation chambers with volumes of 1080 and 480 L at the University of Wuppertal. Experiments were performed at 295 ± 2 K and a total pressure of synthetic air of 1 bar using the relative kinetic method with in situ Fourier transform infrared spectroscopy for chemical analysis. The following rate coefficients (in units of cubic centimeter per molecule per second) were determined: (1.07 ± 0.24) × 10(-10) for methoxybenzene, (1.20 ± 0.24) × 10(-10) for 1-methoxy-2-methylbenzene, (2.97 ± 0.66) × 10(-10) for 2-methoxyphenol (guaiacol), (2.99 ± 0.62) × 10(-10) for 3-methoxyphenol, (2.86 ± 0.58) × 10(-10) for 4-methoxyphenol, (3.35 ± 0.68) × 10(-10) for 2-methoxy-4-methylphenol, (4.73 ± 1.06) × 10(-10) for 2,3-dimethoxyphenol, and (2.71 ± 0.61) × 10(-10) for 2,6-dimethoxyphenol (syringol). To the best of our knowledge, this work represents the first determination of the rate coefficients for the gas-phase reaction of the chlorine atoms with the methoxy-aromatic compounds investigated. The reactivity of the methoxylated aromatics toward Cl is compared with that of other substituted aromatic compounds, and the differences in the rate coefficients are interpreted in terms of the type, number, and position of the different substituents on the aromatic ring. The atmospheric implications of the studied reactions are also discussed.

Temperature dependence of the kinetic isotopic effect of the reaction of Cl atoms with C2H5Cl between 298 and 550 K

The kinetics of the gas-phase reactions of chlorine atoms with chloroethane and chloroethane-d5 was studied experimentally. The relative rate method was applied using Cl+CH3Cl as the reference reaction. The overall rate constants for H-abstraction from C2H5Cl (kH), D-abstraction from C2D5Cl (kD), the site-specific rate constants for H-abstraction from α-carbon (kH,α) and β-carbon (kH,β) atoms of C2H5Cl were measured in the temperature range of 298–550K at a total pressure of 100Torr. The derived temperature dependencies of the rate constants are given by kH=(2.41±0.21)×10−11×exp(−316±11/T), kH,α=(1.36±0.09)×10−11×exp(−202±8/T), kH,β=(1.14±0.11)×10−11×exp(−592±16/T) and kD=(1.58±0.14)×10−11×exp(−474±13/T)cm3molecule−1s−1. The values of the kinetic isotope effect, kH/kD were also estimated.

Kinetic study of the reactions of chlorine atoms with fluoromethane and D-fluoromethane in the gas phase

The kinetics of the gas-phase reactions of chlorine atoms with fluoromethane (CH3F) and D-fluoromethane (CD3F) was studied experimentally. The relative rate method was applied using Cl+CH4 as the reference reaction. The rate constants for H-abstraction from CH3F (kH) and D-abstraction from CD3F (kD) were measured in the temperature range of 298–527K and at a total pressure of 100Torr using N2 as a diluent. The temperature dependency of the rate constants are described by the expressions: kH=(1.65±0.26)×10−11×exp(−1150±30/T) and kD=(1.21±0.14)×10−11×exp(−1600±25/T)cm3molecule−1s−1. The kinetic isotope effect, described by the ratio kH/kD, was found to be 6.2±0.4 at room temperature.

Hydrochloroethers in the troposphere: Kinetics with Cl atoms, lifetimes and atmospheric acceptability indices

Abstract Rate constants for the gas-phase reactions of chlorine atoms with 2-chloroethyl ethyl ether ( k 1 ) and bis(2-chloroethyl) ether ( k 2 ) were determined for the first time using a conventional relative rate technique. Experiments were carried out at (298 ± 2) K and atmospheric pressure using synthetic air as bath gas. The decay rates of the hydrochloroethers were measured relative to those of n-pentane and n-heptane. The following rate constants were derived in units of cm 3 molecule −1 s −1 : k 1 = (1.7 ± 0.5) × 10 −10 and k 2 = (1.0 ± 0.3) × 10 −10 . The rate constants obtained are compared with previous literature data to indicate reactivity trends and are used to estimate the global atmospheric lifetimes ( τ global ) of the studied hydrochloroethers. Radiative forcing efficiencies (RE) of the ethers were determined from their integrated IR absorption cross-sections. Using τ global and RE, estimations were made of atmospheric hazard indices of the two chlorinated ethers such as the ozone depletion potential (ODP) and the global warming potential (GWP).

Kinetic study of the reaction of chlorine atoms with dichloromethane and D-dichloromethane in the gas phase

The kinetics of the gas-phase reactions of chlorine atoms with dichloromethane (CH2Cl2) and D-dichloromethane (CD2Cl2) was studied using the relative rate method with Cl+CH3Br as the reference reaction. The rate constants for H-abstraction from CH2Cl2 (kH) and D-abstraction from CD2Cl2 (kD) were measured in the temperature range of 298–527K and at a total pressure of 100Torr using N2 as a diluent. The temperature dependencies of the rate constants (with the 2σ error limits) are described by the expressions: kH=(8.69±0.82)×10−12×exp(−955±20/T) and kD=(6.98±0.91)×10−12×exp(−1285±25/T)cm3molecule−1s−1. The kinetic isotope effect, described by the ratio kH/kD, was found of 3.8±0.2 at room temperature.

Daytime Reactions of 1,8‐Cineole in the Troposphere

Relative rate coefficients for the gas-phase reaction of chlorine atoms (Cl) and hydroxyl radicals (OH) with 1,8-cineole were determined by Fourier-transform infrared (FTIR) spectroscopy between 285 and 313 K at atmospheric pressure. The temperature dependence of both reactions shows simple Arrhenius behaviour which can be represented by the following expressions (in units of cm(3) molecule(-1) s(-1)): k(1,8-cineole+OH)=(6.28 ± 6.53) × 10(-8) exp[(-2549.3 ± 155.7)/T] and k(1,8-cineole+Cl)=(1.35 ± 1.07) × 10(-10) exp[(-151.6 ± 237.7)/T]. Major products of the titled reactions were identified by solid-phase microextraction (SPME) coupled to a GC-MS. Additionally, the first step of the reaction was theoretically studied by ab initio calculations and a reaction mechanism is proposed.

Kinetic study of the reaction of chlorine atoms with bromoethane and D-bromoethane in the gas phase

The kinetics of the gas-phase reactions of chlorine atoms with bromoethane (C2H5Br) and D-bromoethane (C2D5Br) was studied experimentally. The relative rate method was applied using Cl+C2H6 as the reference reaction. The rate constants for H-abstraction from C2H5Br (kH) and D-abstraction from C2D5Br (kD) were measured in the temperature range of 298–527K and at a total pressure of 100Torr using N2 as a diluent. The temperature dependencies of the rate constants are described by the expressions: kH=(2.6±0.3)×10−11×exp(−390±35/T) and kD=(2.8±0.2)×10−11×exp(−750±30/T)cm3molecule−1s−1. The kinetic isotope effect, described by the ratio kH/kD, was found to be 3.2 at room temperature.

Kinetic study of the reaction of chlorine atoms with bromomethane and D-bromomethane in the gas phase

Abstract The kinetics of the gas-phase reactions of chlorine atoms with bromomethane CH 3 Br and D-bromomethane CD 3 Br was studied experimentally. The relative rate method was applied using Cl + CH 4 as the reference reaction. The rate constants for H-abstraction from CH 3 Br ( k H ) and D-abstraction from CD 3 Br ( k D ) were measured in the temperature range of 298–527 K and at a total pressure of 100 Torr. The temperature dependencies of the rate constants are described by the expressions: k H = (1.43 ± 0.22) × 10 −11 × exp(−1040 ± 30/ T ) and k D = (1.36 ± 0.31)×10 −11 × exp(−1595 ± 30/ T ) cm 3 molecule −1 s −1 . The kinetic isotope effect, described by the ratio k H / k D , was found to be (1.07 ± 0.14) × exp(555 ± 20/ T ).

Kinetics and mechanism of the tropospheric reaction of tetrahydropyran with Cl atoms

Relative rate coefficients for the gas-phase reaction of chlorine atoms (Cl) with tetrahydropyran (THP) have been determined by Fourier transform infrared (FTIR) spectroscopy and gas chromatography coupled to a mass spectrometer (GC–MS). The averaged rate coefficient is (2.21 ± 0.32) × 10 −10 cm 3 molecule −1 s −1 (with 2 σ uncertainty) and was obtained using different reference compounds, Cl precursors, and bath gases at 740 ± 5 Torr and 298 ± 2 K. Products of the title reaction were identified and quantified by solid-phase microextraction (SPME) coupled to a GC–MS. Additionally, the first step of the reaction was theoretically studied by ab initio calculations. Reaction mechanism seems to proceed mainly by the H-abstraction from α and γ positions. THP tropospheric lifetime is estimated from the rate coefficient reported here. The atmospheric implications of the Cl reactivity are also discussed.

Kinetics of the reaction of chlorine atoms with isoprene (2-methyl 1,3-butadiene, CH 2[dbnd]C(CH 3)CH [dbnd]CH 2) at 297 ± 2 K

The gas-phase reaction of chlorine atoms with isoprene was studied in 1–20 Torr of N 2 or Ar diluents using pulsed laser photolysis/vacuum UV laser-induced fluorescence and in 1–700 Torr of N 2 diluent using smog chamber relative rate techniques. There was no discernable effect of total pressure or nature of the diluent gas indicating that even in 1 Torr of argon diluent the reaction is near its high pressure limit. A value of k(Cl + isoprene) = (3.44 ± 0.32) × 10 −10 cm 3 molecule −1 s −1 was derived and is recommended for use in atmospheric models. The molar HCl yield in 700 Torr of N 2 diluent was 12.6 ± 3.0%.

Kinetic study of the reaction of chlorine atoms with chloroform in the gas phase

The kinetics of the gas-phase reactions of chlorine atoms with H-chloroform and D-chloroform was studied experimentally. The relative rate method was applied using Cl + CH 3Br as the reference reaction. The rate constants for H-abstraction from CHCl 3 ( k H) and D-abstraction from CDCl 3 ( k D) were measured in the temperature range of 297–527 K and at total pressure of 100 Torr. The derived temperature dependencies of the rate constants are given by k H = (4.8 ± 0.5) × 10 −12 × exp(−1160 ± 30/ T) and k D = (6.4 ± 2.1) × 10 −12 × exp(−1660 ± 110/ T) cm 3 molecule −1 s −1. The kinetic isotope effect described by the ratio k H/ k D was (0.77 ± 0.25) × exp(500 ± 95/ T).

Low-pressure study of the reactions of Cl atoms with acrylic acid and allyl alcohol

The kinetics of the gas-phase reactions of chlorine atoms with acrylic acid (1) and allyl alcohol (2) have been investigated at low pressure (0.5–3Torr) in the temperature range 260–333K using the mass spectrometric discharge-flow method. The temperature dependence of the reaction rate constants at 1Torr have been determined under pseudo-first-order conditions in excess of organic compound over Cl atoms, giving: k1=(1.3±0.9)×10−13exp(1600±200)/Tcm3molecule−1s−1 and k2=(1.1±0.5)×10−11exp(470±135)/Tcm3molecule−1s−1. At room temperature and 1Torr, products of abstraction and addition channels have been detected for reactions (1) and (2). By mass spectrometric quantification, the branching ratio for channels giving HCl, was determined as 0.13±0.05 for reaction (1) and 0.57±0.06 for reaction (2). The results obtained are related to previous studies, and the atmospheric implications are also discussed in relation to the homogeneous sinks of acrylic acid and allyl alcohol.

Gas-phase reaction of chlorine atoms with acrylonitrile. Temperature and pressure dependence

The kinetics and mechanism of the reaction of chlorine atoms with acrylonitrile (CH 2CHCN) (1) have been studied by the mass spectrometric discharge-flow method in the temperature range (260–333) K and at a total pressure between 0.5 and 3 Torr. At 1.0 Torr the following Arrhenius expression for the rate constant has been obtained from the kinetics of Cl radical consumption in excess of CH 2CHCN: k 1=(5.2±1.6)×10 −14exp[(1400±100/ T)] cm 3 molecule −1 s −1. The reaction is found to proceed through an addition mechanism involving a relatively stable adduct. The atmospheric significance of the present results for acrylonitrile is also discussed.

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.