Absolute Rate Constants for the Reactions of Cl Atoms with a Series of Esters

Atmospheric Degradation Mechanism of Isoamyl Acetate Initiated by OH Radicals and Cl Atoms Revealed by Quantum Chemical Calculations and Kinetic Modeling

Isoamyl acetate is one of the volatile organic compound class molecules relevant to agricultural and industrial applications. With the growing interest in isoamyl acetate applications in industry, the atmospheric fate of isoamyl acetate must be considered. Reaction mechanisms, potential energy profiles, and rate constants of isoamyl acetate reaction with atmospheric relevant oxidant OH radicals and Cl atoms have been obtained from the quantum chemical calculations and kinetic modeling. The geometry optimizations were conducted using M06-2X/6-311++G(3df,3pd) followed by single point-energy calculations at the DLPNO-CCSD(T) method with an extrapolated complete basis set. The rate constants were calculated by solving the master equation. A hydrogen-abstraction reaction dominates the first step of isoamyl acetate degradation, while the addition-substitution reaction plays a small role in the degradation products. The kinetic study was conducted to evaluate the rate constants within a temperature range of 200-400 K. The total rate constants for the isoamyl acetate degradation reactions initiated by the OH radical and Cl atom were determined to be 6.96 × 10-12 and 1.27 × 10-10 cm3 molecule-1 s-1, respectively, under standard temperature and pressure conditions. The product degradation mechanism, ozone formation potential, and atmospheric impacts were discussed.

Modeling-Experiment-Theory Analysis of Reactions Initiated from Cl + Methyl Formate

Methyl formate (MF; CH3OCHO) is the smallest representative of esters, which are common components of biodiesel. The present study characterizes the thermal dissociation kinetics of the radicals formed by H atom abstraction from MF─CH3OCO and CH2OCHO─through a combination of modeling, experiment, and theory. For the experimental effort, excimer laser photolysis of Cl2 was used as a source of Cl atoms to initiate reactions with MF in the gas phase. Time-resolved species profiles of MF, Cl2, HCl, CO2, CH3, CH3Cl, CH2O, and CH2ClOCHO were measured and quantified using photoionization mass spectrometry at temperatures of 400-750 K and 10 Torr. The experimental data were simulated using a kinetic model, which was informed by ab initio-based theoretical kinetics calculations and included chlorine chemistry and secondary reactions of radical decomposition products. We calculated the rate coefficients for the H-abstraction reactions Cl + MF → HCl + CH3OCO (R1a) and Cl + MF → HCl + CH2OCHO (R1b): k1a,theory = 6.71 × 10-15·T1.14·exp(-606/T) cm3/molecule·s; k1b,theory = 4.67 × 10-18·T2.21·exp(-245/T) cm3/molecule·s over T = 200-2000 K. Electronic structure calculations indicate that the barriers to CH3OCO and CH2OCHO dissociation are 13.7 and 31.6 kcal/mol and lead to CH3 + CO2 (R3) and CH2O + HCO (R5), respectively. The master equation-based theoretical rate coefficients are k3,theory (P = ∞) = 2.94 × 109·T1.21·exp(-6209/T) s-1 and k5,theory (P = ∞) = 8.45 × 108·T1.39·exp(-15132/T) s-1 over T = 300-1500 K. The calculated branching fractions into R1a and R1b and the rate coefficient for R5 were validated by modeling of the experimental species time profiles and found to be in excellent agreement with theory. Additionally, we found that the bimolecular reactions CH2OCHO + Cl, CH2OCHO + Cl2, and CH3 + Cl2 were critical to accurately model the experimental data and constrain the kinetics of MF-radicals. Inclusion of the kinetic parameters determined in this study showed a significant impact on combustion simulations of larger methyl esters, which are considered as biodiesel surrogates.

Atmospheric degradation of 3-ethoxy-1-propanol by reactions with Cl, OH and NO3

An experimental kinetic and mechanistic study of the reactions of 3-ethoxy-1-propanol (CH₃CH₂OCH₂CH₂CH₂OH) with Cl atoms and OH and NO₃ radicals has been carried out at room temperature and atmospheric pressure. FTIR (Fourier Transform Infrared Spectroscopy) and GC-MS (Gas Chromatography/Mass Spectrometry) were used as detection techniques. The rate coefficients were measured with a relative method (units cm³ molecule^-1 s^-1): (3.46 ± 0.22) × 10^-10, (3.48 ± 0.19) × 10^-11 and (1.08 ± 0.07) × 10^-14 for Cl, OH and NO₃ reactions, respectively. Qualitative and quantitative products analysis was carried out and formaldehyde, ethyl formate, ethyl 3-hydroxypropanoate and nitrated compounds were positively identified. A reaction mechanism has been proposed which involves attack by the oxidant at the methylene group in the α-position to an oxygen atom of the ether or alcohol groups, followed by the subsequent reactions of the resulting radicals. The tropospheric reactivity of 3-ethoxy-1-propanol (3E1P) has been compared with the reactivity of other hydroxy ethers to extend our knowledge of this type of compound. The atmospheric implications for 3E1P have been established by estimating parameters such as lifetimes, global warming potential (GWP) and the Photochemical Ozone Creation Potential (POCP_E). According to the calculated tropospheric lifetimes, the dominant loss process of 3E1P is its daytime reaction with the OH radical and this has an impact on a local scale.

Cl Atoms and OH Radicals Initiated Kinetic and Mechanistic Study on the Degradation of Propyl Butanoate under Tropospheric Conditions

The reactivity of various OVOCs (mainly esters) in the troposphere leads to the generation of various organics, which in turn leads to an increase in the cloud acidity of the Earth's atmosphere. Hence, it becomes necessary to understand the mechanistic aspects of the reaction of these molecules with dominant atmospheric agents. In this study, the tropospheric degradation of one such ester, propyl butanoate (PB; CH₃CH₂CH₂COOCH₂CH₂CH₃) was studied with OH radicals and Cl atoms at the CCSD(T)//M06-2x/6-311+G(2d,2p) and CCSD(T)//BHandHLYP/6-311+G(2d,2p) level of theories over the studied temperature range of 200-400 K. The Arrhenius expressions obtained using the CVT/SCT/ISPE method were calculated as k_PB + Cl (200-400 K) = 1.3 × 10^-14 T^1.3 exp[1335/T] cm³ molecule^-1 s^-1 and k_PB + OH (200-400 K) = 1.8 × 10^-26 T^4.6 exp[4469/T] cm³ molecule^-1 s^-1. The obtained kinetics was also well validated against the SAR (structure-activity relationship)-based rate coefficients. The most prominent H-abstraction reaction channels were investigated for the PB + OH/Cl reaction. The abstraction of H atoms attached to the carbon atom present in the β-position to the ester (-C(O)O-) functionality was found to go via the lowest energy activation barriers for the reaction of PB toward both OH radicals and Cl atoms. The product degradation channels were also elucidated in an O₂/NO_x-rich environment. Moreover, to gauge the impact of the emitted PB on the troposphere, atmospheric lifetimes, radiative efficiencies, global warming potentials, and photochemical ozone creation potentials were also calculated and are included in the manuscript.

Cl-initiated photo-oxidation reactions of methyl propionate in atmospheric condition

Cl-initiated photo-oxidation reaction of methyl propionate was investigated experimentally using relative rate method. Gas chromatography/mass spectrometry (GC-MS) and GC/infrared spectroscopy (GC-IR) were used as analytical tools to follow the concentrations of reactants and products during reaction. The gas-phase kinetics of methyl propionate with Cl atoms was measured over the temperature range of 263-363 K at 760 Torr in N₂ atmosphere using C₂H₆ and C₂H₄ as reference compounds. The temperature-dependent rate coefficient for the reaction of methyl propionate with Cl atom was obtained as k(T) = [(3.25 ± 1.23) × 10^-16] T² exp [- (33 ± 4) / T] cm³ molecule^-1 s^-1. Theoretical calculations were also performed at CCSD(T)/cc-pVDZ//B3LYP/6-31G(d,p) level of theory, and the rate coefficients for H abstraction reactions were evaluated using canonical variational transition state theory (CVT/SCT) with interpolated single point energy (ISPE) method over the temperature range of 200-400 K. The rate coefficients over the studied temperature range yielded the Arrhenius expression k(T) = (7.22 × 10^-16) T^1.5 exp (466 / T) cm³ molecule^-1 s^-1. The reaction mechanism based on product analysis, thermochemistry, branching ratios, atmospheric implications, degradation pathways, and cumulative lifetime of methyl propionate is also presented in this manuscript. Graphical abstract ᅟ.

Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl, and NO3: Reaction Kinetics and Mechanism

Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O₃, OH, Cl, and NO₃) have been investigated in a Teflon reactor at 298 ± 3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm³ molecule^-1 s^-1) are (1.8 ± 0.3) × 10^-18, (3.1 ± 0.7) × 10^-11, (2.5 ± 0.5) × 10^-10, and (1.1 ± 0.4) × 10^-14, for reactions with O₃, OH, Cl, and NO_3, respectively. While results for reactions with O₃, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO₃ is reported for the first time in this study. On the basis of determined rate constants, the tropospheric lifetimes of AA are τ_O3 = 9 days, τ_OH = 5 h, τ_Cl = 5 days, τ_NO3 = 2 days. On the basis of the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, during both daytime and nighttime.

Study of low temperature chlorine atom initiated oxidation of methyl and ethyl butyrate using synchrotron photoionization TOF-mass spectrometry

The initial oxidation products of methyl butyrate (MB) and ethyl butyrate (EB) are studied using a time- and energy-resolved photoionization mass spectrometer.

Photolysis of n-Propyl Formate in the Presence of O2 and NO2: Peroxy Formyl Propyl Nitrate CH3CH2CH2OC(O)OONO2 Synthesis and Characterization

The photo-oxidation of n-propyl formate (initiated by chlorine atoms) was studied in the presence of NO2, and the products were identified. The Cl atom attack to the molecule occurs in four sites, leading to the formation of formic acid, carbon dioxide, dicarbonylic products, nitrates, peroxy propionyl nitrate (CH3CH2C(O)OONO2, PPN), and a new peroxynitrate, peroxy formyl propyl nitrate (CH3CH2CH2OC(O)OONO2, PFPN). To characterize bulk quantities of the PFPN, its synthesis was carried out by the photolysis of mixtures of CH3CH2CH2OC(O)H, NO2, Cl2, and O2. After purification, its infrared spectrum and thermal stability were determined. The main infrared absorption bands and their corresponding cross sections are 796, 1219, 1302, 1741, and 1831 cm(-1) (1.16, 3.11, 0.88, 2.42, and 1.34 × 10(-18) cm(2) molec(-1), respectively). Thermal decomposition was studied as a function of pressure from 6.0 to 1000 mbar at 298 K, and the activation energy was determined between 293 and 304 K at total pressures of 9.0 and 1000 mbar (Ea = 98 ± 3 and 110 ± 2 kJ/mol, respectively). The atmospheric thermal lifetimes were obtained from kinetic parameters.

Reactions of Cl atoms with alkyl esters: kinetic, mechanism and atmospheric implications

Rate coefficients have been measured for the reaction of Cl atoms with a series of alkyl esters at 298 ± 2 K and atmospheric pressure in a large volume photoreactor using the relative kinetic technique. The kinetic data have been used in conjunction with other literature studies on the reactions of Cl atoms with esters to revise the existing values for ester substituent factors in a structure activity relationship (SAR) for Cl reactions. Product studies are reported for the reactions of Cl atoms with isopropyl ethanoate and methyl-2-methyl-propanoate under NO x -free conditions. These studies highlight the types of products that can be expected when oxidation occurs at R groups on the acyl (-C(O)OR) and oxy (RC(O)O-) sides of the ester functionality where R is a straight or branched chain alkyl entity. Possible atmospheric repercussions of the atmospheric chemistry of esters are considered.

Quantum chemical study on the atmospheric photooxidation of ethyl acetate

The mechanism for OH radical initiated atmospheric photoxidation reaction of ethyl acetate was carried out by using the density functional theory method. Geometries have been optimized at the B3LYP level with a standard 6-31G(d,p) basis set. The single-point energy calculations have been performed at the MP2/6-31G(d), MP2/6-311++G(d,p), and CCSD(T)/6-31G(d) levels, respectively. All of the possible degradation channels involved in the oxidation of ethyl acetate by OH radicals have been presented and discussed. Among the five possible hydrogen abstraction pathways of the reaction of ethyl acetate with OH radicals, the hydrogen abstractions from the C1–H3 and C2–H5 bonds are the dominant reaction pathways due to the low potential barriers and strong exothermicity. The β-ester rearrangement of IM6 is energetically favorable but is not expected to be important. The α-ester rearrangement reaction and O2 direct abstraction from IM17 are the more favorable pathways and are strongly competitive. In addition, the α-ester rearrangement reaction is confirmed to be a one-step process. Acetic acid, formic acetic anhydride, acetoxyacetaldehyde, and acetic anhydride are the main products for the reaction of ethyl acetate with OH radicals.

Kinetics and mechanism for chlorine-initiated atmospheric oxidation of ethyl formate

The chlorine-initiated reaction mechanism of ethyl formate in the atmosphere was investigated using the density functional theory method. The geometry parameters and frequencies of all of the stationary points were calculated at the B3LYP/ 6-31G(d,p) level. The single-point energy calculations were carried out at different levels, including MP2/6-31G(d), MP2/6-311++G(d,p), and CCSD(T)/6-31G(d). A detailed oxidation mechanism is provided and discussed. Present results show that α-ester rearrangement reaction and the O2 direct abstraction from IM6 (HC(O)OCH(O)CH3) are the more favorable pathway and are competitive. The 1,4-H shift isomerization of IM6 proved to be feasible under general atmospheric conditions. The decomposition of IM18 (CH3CH2OC(O)O) is favorable both thermodynamically and kinetically. Canonical variational transition theory with small-curvature tunneling correction was employed to predict the rate constants. The overall rate constant of ethyl formate at 298 K is 8.63 × 10−12 cm3 molecule−1 s−1. The Arrhenius equations of rate constants at the temperature range of 200–380 K were fitted.

Gas phase reactions of unsaturated esters with Cl atoms

BACKGROUND, AIM, AND SCOPE

Acrylate and methacrylate esters are alpha,beta-unsaturated esters that contain vinyl groups directly attached to the carbonyl carbon (CH(2)=CHCOO- and CH(2)=CCH(3)COO-, respectively) and are widely used in the polymer plastic and resin production. Rate coefficients for Cl reactions for most of the unsaturated esters have not been previously determined, and a good understanding is needed of all the atmospheric oxidation processes of these compounds in order to determine lifetimes in the atmosphere and to evaluate the impact of these reactions on the formation of photo-oxidants and therefore on health and environment.

MATERIALS AND METHODS

The relative rate technique has been used to obtain rate coefficients for the reactions between the Cl atom and a series of unsaturated esters. The experiments have been carried out in a static Teflon reactor at room temperature and atmospheric pressure (N(2) as bath gas) using gas chromatography with flame ionization detection as detection system.

RESULTS

The following rate coefficients are obtained (in cubic meter per molecule per second): methyl acrylate + Cl = 1.71 +/- 0.13 x 10(-10); methyl methacrylate + Cl = 2.30 +/- 0.18 x 10(-10); ethyl acrylate + Cl = 1.82 +/- 0.13 x 10(-10); ethyl methacrylate + Cl = 2.71 +/- 0.21 x 10(-10); butyl acrylate + Cl = 2.94 +/- 0.23 x 10(-10); butyl methacrylate + Cl = 3.83 +/- 0.30 x 10(-10); methyl 3-methyl acrylate + Cl = 2.21 +/- 0.17 x 10(-10); and methyl 3,3-dimethyl acrylate + Cl = 3.58 +/- 0.28 x 10(-10).

DISCUSSION

Rate coefficients calculated for Cl reactions are around one order of magnitude higher than OH ones. The effect in the reactivity of increased substitution at the carbon-carbon double bond is analyzed and also the effect of the identity of the alkyl group R in the -C(O)OR. Atmospheric lifetimes of the compounds against the attack by the major oxidants are estimated and the atmospheric implications are discussed.

CONCLUSIONS

The dominant atmospheric loss process for acrylate esters is clearly their daytime reaction with the hydroxyl radical. However, in coastal areas and in the marine boundary layer and in some industrial zones, Cl-atom-initiated degradation of the unsaturated esters considered here can be a significant if not dominant homogeneous loss process.

RECOMMENDATIONS AND PERSPECTIVES

Product analysis should be necessary in order to evaluate the real environmental impact of these reactions. OH and ozone reactions of most of the considered compounds have already been studied and products determined, but kinetic and products information for NO(3) radical reactions is especially scarce.

The Cl-initiated oxidation of CH(3)C(O)OCH=CH (2), CH (3)C(O)OCH (2)CH=CH (2), and CH (2)=CHC(O)O(CH (2)) (3)CH (3) in the troposphere

BACKGROUND, AIM, AND SCOPE

Unsaturated esters are emitted to the atmosphere from biogenic and anthropogenic sources, including those from the polymer industry. Little information exists concerning the atmospheric degradation of unsaturated esters, which are mainly initiated by OH radicals. Limited information is available on the degradation of alkenes by Cl atoms and almost no data exists for the reactions of unsaturated esters with Cl atoms. This data is necessary to assess the impact of such reactions in maritime environments where, under circumstances, OH radical- and Cl atom-initiated oxidation of the compounds can be important. Rate coefficients for the reactions of chlorine atoms with vinyl acetate, allyl acetate, and n-butyl acrylate have been determined at 298 +/- 3 K and atmospheric pressure. The kinetic data have been used in combination with that for structurally similar compounds to infer the kinetic contributions from the possible reaction channels to the overall reaction rate.

MATERIALS AND METHODS

The decay of the organics was followed using in situ Fourier transform infrared spectroscopy and the rate coefficients were determined using a relative kinetic method and different hydrocarbon reference compounds.

RESULTS

The following room temperature rate coefficients (in cm(3) molecule(-1) s(-1)) were obtained: k (1) (Cl + CH(3)C(O)OCH=CH(2)) = (2.68 +/- 0.91) x 10(-10), k (2) (Cl + CH(3)C(O)OCH(2)CH=CH(2)) = (1.30 +/- 0.45) x 10(-10), and k (3) (Cl + CH(2)=CHC(O)O(CH(2))(3)CH(3)) = (2.50 +/- 0.78) x 10(-10), where the uncertainties are a combination of the 2sigma statistical errors from linear regression analyses and a contribution to cover uncertainties in the rate coefficients of the reference hydrocarbons.

DISCUSSION

This is the first kinetic study of the title reactions under atmospheric conditions. The kinetic data were analyzed in terms of reactivity trends and used to estimate the atmospheric lifetimes of the esters and assess their potential importance in the marine atmosphere.

CONCLUSIONS

Although reaction with OH radicals is the major atmospheric sink for the unsaturated esters studied, reaction with Cl atoms can compete in the early morning hours in coastal areas where the Cl concentration can reach peak values as high as 1 x 10(5) atoms cm(-3). The calculated residence times show that the chemistry of unsaturated esters will impact air quality locally near their emission sources.

RECOMMENDATIONS AND PERSPECTIVES

The reactions need to be studied over the range of temperatures and pressures generally encountered in the marine atmosphere. In addition, product studies should also be performed as a function of temperature since this will allow degradation mechanisms to be derived, which are representative for the wide range of conditions occurring in marine environments. Inclusion of the kinetic and product data in tropospheric numerical models will allow an assessment of potential environmental impacts of the esters for different marine pollution scenarios.

Atmospheric Chemistry of a Model Biodiesel Fuel, CH3C(O)O(CH2)2OC(O)CH3: Kinetics, Mechanisms, and Products of Cl Atom and OH Radical Initiated Oxidation in the Presence and Absence of NOx

Relative rate techniques were used to study the kinetics of the reactions of Cl atoms and OH radicals with ethylene glycol diacetate, CH3C(O)O(CH2)2OC(O)CH3, in 700 Torr of N2/O2 diluent at 296 K. The rate constants measured were k(Cl + CH3C(O)O(CH2)2OC(O)CH3) = (5.7 +/- 1.1) x 10(-12) and k(OH + CH3C(O)O(CH2)2OC(O)CH3) = (2.36 +/- 0.34) x 10(-12) cm3 molecule-1 s-1. Product studies of the Cl atom initiated oxidation of ethylene glycol diacetate in the absence of NO in 700 Torr of O2/N2 diluent at 296 K show the primary products to be CH3C(O)OC(O)CH2OC(O)CH3, CH3C(O)OC(O)H, and CH3C(O)OH. Product studies of the Cl atom initiated oxidation of ethylene glycol diacetate in the presence of NO in 700 Torr of O2/N2 diluent at 296 K show the primary products to be CH3C(O)OC(O)H and CH3C(O)OH. The CH3C(O)OCH2O* radical is formed during the Cl atom initiated oxidation of ethylene glycol diacetate, and two loss mechanisms were identified: reaction with O2 to give CH3C(O)OC(O)H and alpha-ester rearrangement to give CH3C(O)OH and HC(O) radicals. The reaction of CH3C(O)OCH2O2* with NO gives chemically activated CH3C(O)OCH2O* radicals which are more likely to undergo decomposition via the alpha-ester rearrangement than CH3C(O)OCH2O* radicals produced in the peroxy radical self-reaction.

Atmospheric Chemistry of CF3CH2CH2OH: Kinetics, Mechanisms and Products of Cl Atom and OH Radical Initiated Oxidation in the Presence and Absence of NOX

Relative rate techniques were used to study the kinetics of the reactions of Cl atoms and OH radicals with CF(3)CH(2)C(O)H and CF(3)CH(2)CH(2)OH in 700 Torr of N(2) or air diluent at 296 +/- 2 K. The rate constants determined were k(Cl+CF(3)CH(2)C(O)H) = (1.81 +/- 0.27) x 10(-11), k(OH+CF(3)CH(2)C(O)H) = (2.57 +/- 0.44) x 10(-12), k(Cl+CF(3)CH(2)CH(2)OH) = (1.59 +/- 0.20) x 10(-11), and k(OH+CF(3)CH(2)CH(2)OH) = (6.91 +/- 0.91) x 10(-13) cm(3) molecule(-1) s(-1). Product studies of the chlorine initiated oxidation of CF(3)CH(2)CH(2)OH in the absence of NO show the sole primary product to be CF(3)CH(2)C(O)H. Product studies of the chlorine initiated oxidation of CF(3)CH(2)CH(2)OH in the presence of NO show the primary products to be CF(3)CH(2)C(O)H (81%), HC(O)OH (10%), and CF(3)C(O)H. Product studies of the chlorine initiated oxidation of CF(3)CH(2)C(O)H in the absence of NO show the primary products to be CF(3)C(O)H (76%), CF(3)CH(2)C(O)OH (14%), and CF(3)CH(2)C(O)OOH (< or =10%). As part of this work, an upper limit of k(O(3)+CF(3)CH(2)CH(2)OH) < 2 x 10(-21) cm(3) molecule(-1) s(-1) was established. Results are discussed with respect to the atmospheric chemistry of fluorinated alcohols.