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Experimental and Computational Investigation on the Gas Phase Reaction of Ethyl Formate with Cl Atoms M. Balaganesh, Manas Ranjan Dash, and B. Rajakumar* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India S Supporting Information *

ABSTRACT: The rate coefficient for the gas-phase reaction of Cl atoms with ethyl formate was measured over the temperature range of 268−343 K using relative rate methods, with ethyl chloride as a reference compound. The temperature dependent relative rate coefficients for the ethyl formate + Cl reaction were measured, and the modified Arrhenius expression kethyl formate(268−343) = (2.54 ± 0.57) × 10−23 T4.1 exp {−(981 ± 102)/T} cm3 molecule−1 s−1 was obtained with 2σ error limits. The room temperature rate coefficient for the title reaction is (9.84 ± 0.79) × 10−12 cm3 molecule−1 s−1, which is in good agreement with reported values. To complement the experimental measurement, computational methods were used to calculate the rate coefficient for the ethyl formate + Cl reaction atoms using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) and the CCSD (T)/cc-pVDZ//M062X/6-31+g(d,p) level of theory. The temperature dependent Arrhenius expression was obtained to be 2.97 × 10−18 T2.4 exp[−(390/T)] cm3 molecule−1 s−1 over the temperature range of 200−400 K. The thermodynamic parameters and branching ratio were calculated. Also, the atmospheric lifetime and global warming potentials (GWPs) were calculated for ethyl formate.

1. INTRODUCTION Esters are the main category of oxygenated volatile organic compounds used in food flavorings and in perfumes. They are present in fruits and are emitted to the atmosphere naturally. They are also produced in the atmosphere as an oxidation product of ethers. Thus, the atmospheric oxidation of these species has the potential to contribute to air quality on regional and global scales. The present study focuses on ethyl formate, which is the simplest model of ethyl esters and is also considered as a biodiesel. Though oxidation of ethyl formate by OH radical is the major sink, oxidation by Cl atoms cannot be ruled out, because of its high reactivity and its sufficient concentration in the marine boundary layer. Cl + C2H5OC(O)H → products

formate with Cl atoms using the PLP/VUV-LIF technique at 298 K and 7 Torr in Ar as buffer gas, and they reported the rate coefficient to be (9.50 ± 0.3) × 10−12 cm3 molecule−1 s−1. Though there are four experimental studies available in the literature which are based on the relative rate technique and absolute measurements, no temperature dependent study is available to date, to the best of our knowledge. In the present study, we report the rate coefficients measured using the relative rate technique over the temperature range of 268−343 K for the first time. We also report the rate coefficients, thermodynamic parameters, and branching ratios for the reaction over the temperature range of 200−400 K, calculated using canonical variational transition state theory with small curvature tunneling (CVT/SCT) in combination with DFT methods. In addition, the atmospheric lifetimes and global warming potentials of the test molecule are reported in this paper.

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There are four experimental studies available using absolute methods and relative rate techniques for the title reaction. Notario et al.1 studied the reaction of ethyl formate with Cl atoms using the PLP-RF technique at 298 K and 15−60 Torr in He as buffer gas, and they reported the rate coefficient to be (1.34 ± 0.15) × 10−11 cm3 molecule−1 s−1. Sellevåg and Nielsen2 studied the reaction of ethyl formate with Cl atoms using the relative rate technique at 298 K and 760 Torr in N2 as buffer gas, and they reported the rate coefficient to be (1.10 ± 0.07) × 10−11 cm3 molecule−1 s−1. Wallington et al.3 studied the reaction of ethyl formate with Cl atoms using the PLP-RF technique at 298 K and 700 Torr in N2 as buffer gas, and they reported the rate coefficient to be (9.57 ± 1.3) × 10−12 cm3 molecule−1 s−1. Recently, Ide et al.4 studied the reaction of ethyl © 2014 American Chemical Society

2. METHODOLOGIES 2.1. Experimental Section. The experiments were carried out in a ∼1250 cm3 volume double-walled Pyrex reaction chamber. UV fused silica broadband precision windows (Thorslabs) with 2 in. diameter were used to allow the ultraviolet radiation. The temperature in the reaction cell was maintained by circulating a cooled/heated fluid. The temperature inside the reaction chamber was calibrated using a K-type thermocouple within the Received: March 25, 2014 Revised: June 16, 2014 Published: June 19, 2014 5272

dx.doi.org/10.1021/jp502963w | J. Phys. Chem. A 2014, 118, 5272−5278

The Journal of Physical Chemistry A

Article

Table 1. Relative Rate Measurements of Cl Atom Reaction with Ethyl Formate over the Temperature Range of 268−343 K and at 740 Torr in N2, Using Ethyl Chloride as a Reference Compound T (K) 268 283 298

313 328

343

a

(kethyl formate/kethyl chloride) ± 2σ (kethyl formate/kethyl chloride)ave ± 2σ 1.26 1.28 1.28 1.20 1.17 1.16 1.24 1.22 1.22 1.23 1.14 1.25 1.25 1.25 1.16

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.05 0.15 0.06 0.05 0.06 0.03 0.05 0.05 0.04 0.07 0.05 0.08 0.06 0.08 0.05

(k ± 2σ) × 1012a (cm3 molecule−1 s−1)

1.27 ± 0.02

9.35 ± 0.15

1.24 ± 0.11

9.70 ± 0.86

1.19 ± 0.09

9.84 ± 0.74

1.22 ± 0.01

10.6 ± 0.09

1.21 ± 0.12

11.0 ± 1.09

1.22 ± 0.10

11.6 ± 0.95

lit. k × 1012 (cm3 molecule−1 s−1) at 298 K

(13.4 ± 1.5)1 (11.0 ± 0.7)2 (9.6 ± 1.3)3 (9.5 ± 0.3)4

Uncertainties associated with the rate coefficients of reference compound were not considered.

the reactant and reference compound, and also no other products were observed, which confirmed the absence of any loss due to direct photolysis. Chemicals. Ethyl formate (purity 99%, Sigma-Aldrich), ethyl chloride (purity 99%, Sigma-Aldrich), oxalyl chloride (purity 98%, SPECTROCHEM), nitrogen (99.995%), zero air (98%), and oxygen (98%). Ethyl formate and oxalyl chloride were subjected to repeated freeze−pump−thaw cycles before use. 2.2. Computational. The geometries of the reactants, transition states (TSs), and products of all possible abstraction channels were optimized at the Minnesota 2006 density functional with 54% Hartree−Fock exchange (M062X) with the Pople basis set 6-31+G(d,p).7−9 Harmonic vibrational frequencies for all the stationary points located on the potential energy surface were obtained using the same level of theory. The performance of the M062X functional is tested against a diverse database by Truhlar et al.,7 and it is concluded that this functional is good for main-group thermochemistry, for kinetics, and also for the prediction of noncovalent interactions. The error which is predicted by the M06-2X functional is lesser than that predicted by the B98 functional, which in turn is lesser than that predicted by the B3LYP functional.7,8 All the reactants and products were recognized with zero imaginary frequency, and TSs were recognized with one imaginary frequency. Intrinsic reaction coordinate10 (IRC) calculations were performed to verify that the designated transition states connect the reactants and products. To improve the accuracy of the calculated reaction energy, the single-point energy calculations were carried out for all the stationary points at coupled-cluster with single and double and perturbative triple excitations (CCSD(T)) combined with the correlation-consistent polarized valence double-ζ basis set (cc-pVDZ) level of theory. The theoretical rate coefficients were calculated using canonical variational transition state theory11−13 (CVT) with smallcurvature tunneling14,15 (SCT) by employing the POLYRATE 2008 program16 and GAUSSRATE 2009A17 for the title reactions. All the electronic structure calculations were carried out using Gaussian 09,18 and normal modes of stationary points are viewed in GaussView.19

uncertainty of 2 K. Chlorine atoms were generated by photolysis of oxalyl choride ((COCl)2) at 254 nm, using two UV lamps (SANKYO DENKI G8T5, 8 W). (COCl)2 + hυ (≈254 nm) → 2Cl + 2CO

The typical (COCl)2 concentration used in the experiments is 6.65 × 1017 molecules cm−3, which would lead to the formation of approximately 1010−1011 Cl atoms in the reaction cell. The detailed experimental procedure and set up were described elsewhere in our recently reported articles.5,6 The title reactions were investigated over the temperature range of 268−343 K with pressure at 740 Torr of N2. The reaction chamber was filled with 10 Torr of ethyl formate, 20 Torr of ethyl chloride, which was used as reference compound, and 100 Torr of oxalyl chloride, which served as a precursor for chlorine atoms. The reaction mixtures were photolyzed for 10−15 min. The concentrations of the ethyl formate and reference compounds were monitored using a gas chromatograph (Agilent Technologies 6890N) after each photolysis step. The kinetic data were obtained from the measurement of the simultaneous loss of reactant and reference compound, using the standard expression ⎛ [reactant]0 ⎞ ⎛ k reactant ⎞ ⎛ [reference]0 ⎞ ln⎜ ⎟ ⎟=⎜ ⎟ ln⎜ ⎝ [reactant]t ⎠ ⎝ k reference ⎠ ⎝ [reference]t ⎠

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where [reactant]0 and [reference]0 are the initial concentrations of reactant and reference compounds; [reactant]t and [reference]t are the corresponding concentrations at time t. To verify that the depletion in concentration of reactant and reference compounds occurs only by reaction with Cl atoms, we loaded reactant, reference compounds, (COCl)2, and nitrogen in the reaction chamber and allowed mixing for about 6 h in the absence of light, and the concentrations of each compound were monitored by GC at different time intervals. There was no significant decrease in concentration of any molecules, indicating the absence of dark reactions as well as wall losses. To ensure that there is no loss due to the direct photolysis of reactant and reference, the reactant, reference compound, and nitrogen were loaded in the reaction chamber and were photolyzed for 30 min in the absence of (COCl)2. As expected, we did not observe any significant decrease in the concentration of 5273



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3. RESULTS AND DISCUSSION 3.1. Experimental Section. The temperature dependence of the reaction of Cl atoms with ethyl formate was investigated over the temperature range of 268−343 K, using ethyl chloride as a reference compound. This work represents the first kinetic study of the temperature dependence of the reactions of Cl atom with ethyl formate. The rate coefficient ratios (kethyl formate/kethyl chloride) were measured at 268, 283, 298, 313, 328, and 343 K, and the obtained rate coefficients are given in Table 1. Every experiment was repeated at least twice to ensure the reproducibility. For each experiment, slope and errors were obtained from the linear least-squares fitting of the data. The typical relative rate plot of the decrease in the concentration of ethyl formate and reference compounds (ethyl chloride) due to their reaction with Cl atoms is shown in Figure 1. As expected, Figure 2. Geometries of the reactants, transition states, and products optimized at the M062X/6-31+G (d,p) level of theory. White represents hydrogen, gray represents carbon, red represents oxygen, and green represents chlorine atoms in the structures. The bond lengths (Å) given on the structures are obtained at the M062X/631+G (d,p) level of theory.

energies predicted at the CCSD(T)/cc-pVDZ//M06-2X/631+G(d,p) + ZPE (M06-2X/6-31+G(d,p)) level of theory is shown in Figure 3. The vibrational frequencies and structural

Figure 1. Plot of the relative decrease in the concentration of ethyl formate due to its reaction with Cl atom at 298 K in N2 with C2H5Cl used as reference compound.

a straight line with a zero intercept was obtained. The quoted errors are two least-squares standard deviations and include uncertainties associated with the rate coefficients of the reference compound. Wine and Semmes20 studied the rate coefficients of Cl atoms with ethyl chloride over the temperature range of 257−426 K employing a time-resolved resonance fluorescence spectroscopic technique. We have used the rate expression kethyl chloride = (2.34 ± 0.42) × 10−11 exp [−(310 ± 56)/T] cm3 molecule−1 s−1 derived by Wine and Semmes. The Arrhenius plot of rate coefficients measured for the title reaction is shown in Figure 4. A positive temperature dependence on rate coefficients was observed for this reaction within the experimental uncertainties. The three parameter fit of our measured rate coefficients between 268 and 343 K resulted in an Arrhenius expression: kethyl formate(268−343) = [(2.54 ± 0.57) × 10−23]T4.1 exp{−(981 + 102)/T} cm3 molecule−1 s−1 with 2σ error limits. As seen in Table 1, the mean value of the slopes is kethyl formate/kethyl chloride = 1.19 ± 0.05 at 298 K. From this slope, using kethyl chloride = (8.27 ± 0.4) × 10−12 cm3 molecule−1 s−1at 298 K, the rate coefficient of ethyl formate + Cl reaction was calculated to be (9.84 ± 0.78) × 10−12 cm3 molecule−1 s−1 at 298 K, which is in very good agreement with the literature values. The quoted errors are two least-squares standard deviations and do not include uncertainties associated with the rate coefficients of the reference compound. 3.2. Computational. 3.2.1. Electronic Structures and Energetics. The optimized geometries of all the stationary points along the potential energy surface are shown in Figure 2. The energy level diagram for the title reaction based on the

Figure 3. Energy level diagram for the ethyl formate + Cl reaction obtained at the CCSD(T)/cc-pVDZ//M062X/6-31+G(d,p) + ZPE(M06-2X/6-31+G(d,p)) level of theory. The energies are given in the units of kcal/mol.

parameters of various species involved in the title reactions obtained at the M06-2X/6-31+G(d,p) level of theory are given in the Supporting Information, SI (Tables S-I to S-II). The zero point corrected relative energies were used for ease of discussion throughout the manuscript, unless otherwise stated. Reaction of Cl atoms with ethyl formate undergoes abstraction of hydrogen atoms from three different sites, namely −CH3, −CH2−, and HCOO− groups. Since ethyl formate possesses a Cs point group, the two hydrogens on the −CH2− group are identical. The two hydrogens which are cis to the −O− atom are also identical. Therefore, we found four independent possible hydrogen abstraction pathways for the title reaction. First one corresponds to the hydrogen abstraction from the HCOO− group, which leads to the formation of product P1 through the 5274



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by Truhlar et al. In the present study, single-point energies evaluated at stationary points are used to correct the lower-level reaction path. The minimum energy pathway is obtained using direct dynamics for a small range of the reaction path with the mass scaled reaction coordinate ‘s’ from −1.0 Å to 1.0 Å by using the Page−McIver integrator with a step size of 0.1 Å. A Hessian matrix was calculated every step. Also the harmonic frequencies were scaled22 by 0.979 along the reaction path. All vibrations are treated harmonically except for the lowest vibrational modes. The internal rotation modes are treated using the hindered rotor approximation, whereas the remaining vibrations are treated harmonically. The generalized rate coefficient can be minimized by varying the transition state dividing surface along the reaction coordinate to get the canonical variational transition state rate coefficient using the following equations

transition state TS1. In TS1, the breaking C−H bond and forming H−Cl bond lengths are increased by about 27% and 15% when compared to the C−H bond in ethyl formate and the H−Cl bond in HCl, respectively. This pathway is the low barrier pathway (0.87 kcal/mol), and the relative energy of the product P1 is −1.69 kcal/mol. The second one corresponds to the hydrogen abstraction from the −CH2− group, which leads to the formation of product P2 through the transition state TS2. In TS2, the breaking C−H bond and forming H−Cl bond lengths are increased by about 24% and 17% when compared to the C−H bond in ethyl formate and the H−Cl bond in HCl, respectively. This pathway is the lowest barrier or barrierless (0.32 kcal/mol) among other pathways and is forming stable product P2 (−1.36 kcal/mol). The third one corresponds to the hydrogen (cis to the −O− atom) abstraction from the −CH3− group, which leads to the formation of product P3 through the transition state TS3. This pathway leads to the product P3, whose relative energy is 3.21 kcal/mol, and the barrier energy for the pathway is 5.24 kcal/mol. The fourth one corresponds to the hydrogen (trans to the −O− atom) abstraction from the −CH3− group, which leads to the formation of product P4 through the transition state TS4. This pathway requires about 6.18 kcal/mol to cross the barrier, leading to the product P4 (4.04 kcal/mol) above the reactant’s energy. In both the transition states TS3 and TS4, the breaking C−H bond and forming H−Cl bond lengths are increased by about 37% and 10% when compared to the C−H bond in ethyl formate and the H−Cl bond in HCl, respectively. The changes in standard enthalpy and standard Gibb’s free energy for the abstraction pathways were calculated to predict the spontaneity and feasibility. The hydrogen abstractions from three different carbon sites (HCOO−, −CH2−, and −CH3) are spontaneous, but the order of spontaneity is −CH2− > HCOO− > −CH3. The hydrogen abstraction from the HCOO- pathway is less endothermic (0.85 kcal/mol), the hydrogen abstraction from the −CH2− pathway is less exothermic (−1.12 kcal/mol), and the hydrogen abstraction from the −CH3 pathway is endothermic (∼4 kcal/mol). The change in standard enthalpy and standard Gibb’s free energy for these three different sites are given in Table 2.

kGT(T , s) = σ

kCVT(T ) = min kGT(T , s) = kGT[T , sCVT(T )] s

where kGT and kCVT are the rate coefficients of generalized and canonical variational transition state theories, respectively, σ is the reaction path degeneracy, kB is Boltzmann’s constant, h is Planck’s constant, T is temperature in Kelvin, and sCVT is the reaction coordinate (s) at which the canonical variational transition state dividing surface was found. QGT and ΦR are the partition functions of a generalized TS at “s” and reactants, respectively. VMEP(s) is the potential energy of generalized TS at “s”. The canonical variational transition state is located by maximizing the free energy of activation with respect to “s”. The tunneling corrected rate coefficients were obtained by multiplying the CVT rate constant by a temperature dependent transmission coefficient κCVT/SCT(T). kCVT/SCT(T ) = κ CVT/SCT(T )kCVT(T )

The two spin−orbit23 states 2p3/2 (lowest) and 2p1/2 of Cl having degeneracies of 4 and 2, respectively, and separated by 882.3515 cm−1 were included in the electronic partition function calculations. The temperature dependent rate coefficients were calculated based on the above explained CVT/SCT/ISPE methodologies and computational methods over the temperature range of 200−400 K for the title reaction. The rate coefficients are compared with the available literature values in the studied temperature range and are given in Table 3. All reported rate coefficients are compared with the current study and are shown in Figure 4. The temperature dependent rate coefficients follow a non-Arrhenius behavior. Therefore, a three parameter fit was used to obtain the Arrhenius expression, and it is k(200−400 K) = (2.97 × 10−18)T2.4 exp[−(390/T)] cm3 molecule−1 s−1. The computed rate coefficient at 298 K is 9.53 × 10−12 cm3 molecule−1 s−1, which is in very good agreement with all the available reported rate coefficients reported by Notario et al.1 [(1.34 ± 0.15) × 10−11 cm3 molecule−1 s−1 at 298 K and 15−60 Torr in He as buffer gas, measured using the PLP-RF technique]; Sellevåg and Nielsen2 [(1.10 ± 0.07) × 10−11 cm3 molecule−1 s−1 at 298 K and 760 Torr in N2 as buffer gas, measured using the relative rate technique]; Wallington et al.3 [(9.57 ± 1.3) × 10−12 cm3 molecule−1 s−1 at 298 K and 700 Torr in N2 as buffer gas, measured using PLP-RF technique]; and Ide et al.4 [(9.50 ± 0.3) × 10−12 cm3 molecule−1 s−1 at 298 K and 7 Torr in Ar as buffer gas, measured using PLP/VUV-LIF

Table 2. Change in Standard Enthalpy (kcal/mol) and Gibb’s Free Energy (kcal/mol) for the Different Sites of Ethyl Formate + Cl Reaction at 298 K Based on the M062X/ 6-31+G (d, p) Level of Theory Different site P1 P2 P3 P4

(HCOO−) (−CH2−) (−CH3) (−CH3)

ΔHo (kcal/mol)

ΔGo (kcal/mol)

0.85 −1.12 4.02 4.49

−12.88 −15.08 −10.19 −9.54

⎛ − V (s ) ⎞ kBT ⎛ QGT(T , s) ⎞ ⎜ ⎟ exp⎜ MEP ⎟ R h ⎝ Φ (T ) ⎠ ⎝ kBT ⎠

3.2.2. Rate Coefficients. The dual level direct dynamic approach (CCSD(T)/cc-pVDZ//M06-2X/6-31+g(d,p)) was used to study the initial step in the reactions of Cl atoms with ethyl formate. In this methodology, the reaction path information, including geometries, first derivatives, and frequencies of stationary points along the reaction path of the initial step, was obtained at lower level M06-2X/6-31+g(d,p). To improve the accuracy of barrier heights and reaction energies, we employed higher-level calculations at CCSD(T)/cc-pVDZ. The rate coefficients are calculated by variational transition state theory (VTST) with the interpolated single-point energies21 (ISPE) methods, developed 5275



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Table 3. Rate Coefficients (cm3 molecule−1 s−1) for Ethyl Formate + Cl Reaction over the Temperature Range of 200−400 K Obtained at the CCSD(T)/cc-pVDZ//M062X/ 6-31+G(d,p) Level of Theory Rate Coefficient (cm3 molecule−1 s−1) T (K) 200 225 250 275 298 325 350 375 400 a

TST 6.46 6.89 7.44 8.06 8.73 9.62 1.05 1.15 1.26

× × × × × × × × ×

10−12 10−12 10−12 10−12 10−12 10−12 10−11 10−11 10−11

CVT/SCT 6.14 6.57 7.12 7.74 8.40 9.25 1.01 1.11 1.21

× × × × × × × × ×

10−12 10−12 10−12 10−12 10−12 10−12 10−11 10−11 10−11

Present exptl and lit. values

((9.84 ((1.34 ((1.10 ((9.57 ((9.50

± ± ± ± ±

0.78) 0.40) 0.40) 0.31) 0.40)

× × × × ×

10−12)a 10−11)1 10−11)2 10−12)3 10−12)4

Figure 5. Calculated branching ratios of kH, kCH2, and kCH3 to the total rate coefficient for the ethyl formate + Cl reaction over the temperature range of 200−400 K.

Experimentally measured rate coefficient in the present study.

increase in temperature, whereas the abstraction of hydrogen from the −CH3 group is a minor pathway, but observed to be increasing gradually with the temperature. The percentage contributions of hydrogen abstraction from −CH2−, HCOO−, and −CH3 groups to the total rate coefficient are 67, 33, and 0.1%, respectively. The complete temperature dependent branching ratios are given in Table 4. Table 4. Branching Ratio of Different Sites for Ethyl Formate + Cl Reactions over the Temperature Range of 200−400 K Obtained at the CCSD(T)/cc-pVDZ//M062X/ 6-31+G(d,p) Level of Theory Branching ratio (%)

Figure 4. Arrhenius plot of the rate coefficient data obtained for the ethyl formate + Cl reaction over the temperature range of 200−400 K. The temperature dependent rate coefficient obtained using the experimentally measured rate coefficients is k(268−343 K) = [(2.54 ± 0.57) × 10−23]T4.1 exp{−(981 ± 102)/T} cm3 molecule−1 s−1, and that obtained with the computed rate coefficients it is k(200−400 K) = (2.97 × 10−18)T2.4 exp[−(390/T)] cm3 molecule−1 s−1.

technique]. Also, the computed rate coefficient is in very good agreement with the experimentally measured rate coefficient at 298 K, in the present investigation. The calculated activation energy is −0.77 kcal/mol, which is 2.5 times lower than the experimentally measured value (−1.95 kcal/mol) in this study. 3.2.3. Branching Ratios. The contribution of each pathway to the global rate coefficient is a very useful parameter, to find out major and minor pathways. In the case of the title reaction, three different pathways are possible and are given below. HCOOCH 2CH3 + Cl → HCl + COOCH 2CH3

T (K)

H

CH2

CH3

200 225 250 275 296 298 325 350 375 400

27.30 29.15 30.61 31.97 32.97 32.97 34.02 34.88 35.63 36.29

72.69 70.83 69.35 67.96 66.93 66.92 65.79 64.84 63.97 63.18

0.01 0.02 0.04 0.07 0.10 0.11 0.19 0.28 0.40 0.54

4. ATMOSPHERIC IMPLICATIONS Atmospheric degradation for ethyl formate mainly happens via its reaction with tropospheric oxidants such as OH and NO3 radicals, O3, and possibly Cl atoms. The atmospheric lifetimes (τ) of ethyl formate due to its reaction with Cl atoms were estimated using the equation 1 τ= k Cl[Cl]

(kH)

HCOOCH 2CH3 + Cl → HCl + HCOOCHCH3 (kCH2)

HCOOCH 2CH3 + Cl → HCl + HCOOCH 2CH 2

where kCl is the rate coefficient of the ethyl formate with Cl atom reaction at 277 K and [Cl] is the concentration of Cl atoms. We have considered two different chlorine concentrations such as the global average concentration (1.0 × 103 atom cm−3)24,25 and the concentration (1.3 × 105 atom cm−3)26 at the marine boundary layer. The atmospheric lifetimes of this compound at ambient conditions are estimated to be 1276 and 1310 days, based on the obtained experimental and theoretical rate coefficients, respectively, whereas the atmospheric lifetimes of ethyl formate based on marine boundary conditions are

(kCH3)

The branching ratios of the −CH3, −CH2−, and HCOO− groups over the temperature range of 200−400 K are shown in Figure 5. It is clear from the figure that the abstraction of hydrogen from the −CH2− group is more dominant over the studied temperature range but decreasing gradually with the increase in temperature. Similarly, the abstraction of hydrogen from the HCOO− group is more dominant over the studied temperature range but found to be increasing gradually with the 5276



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ACKNOWLEDGMENTS The authors thank Professor D. G. Truhlar for providing the POLYRATE 2008 and GAUSSRATE 2009A programs. Also, the authors thank the High Performance Computing Service and Mr. V. Ravichandran for providing computer resources. M.B. thanks G. Srinivasulu for the scientific discussions.

9.7 days and 10 days, based on the obtained experimental and theoretical rate coefficient values, respectively. Though the lifetime of this compound is very low at marine boundary conditions, it is long-lived at ambient conditions. So, the contribution of this compound to GWP will be significant at ambient condition. The GWPs of the test molecule were calculated at different time horizons, and the procedures for calculation of GWPs27−29 are described elsewhere in our previous article9 and references therein. The computed lifetimes and GWP for the different time horizons (20, 200, and 500 years) are given in Table 5. When the globally averaged Cl concentrations are

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[Cl] atoms/cm3

Exptl

Theor

2.5 × 103 1.3 × 105

1276 9.8

1310 10.1

a

GWPs (yrs) Total Radiative Forcing (w/m2 ppbv) 0.0869

20

100

500

786 6

241 1.84

75 0.58

The experimentally estimated life time of this study is given in parentheses.

taken into account, the GWP of ethyl formate is significantly large (∼785) at 20 years of time horizon. However, it is just negligible when the concentrations at the marine boundary layer are taken into account.

5. CONCLUSIONS The rate coefficients for the gas-phase reaction of Cl atoms with ethyl formate were measured over the temperature range of 268−343 K using relative rate methods, with ethyl chloride as a reference compound. A nonlinear Arrhenius behavior was observed over the studied temperature range. To complement the experimental measurement, computational methods were used to calculate the rate coefficient for the reaction of Cl atoms with ethyl formate using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) and the CCSD(T)/cc-pVDZ//M062X/6-31+g(d, p) level of theory. Similar to the experimental findings, a nonlinear behavior was observed over the studied temperature range. The pathway, that is, hydrogen abstraction form −CH2−, is exothermic, whereas the pathways such as hydrogen abstraction from HCOO− and −CH3− are endothermic. The hydrogen abstraction from −CH2− pathway is more spontaneous than the hydrogen abstraction from HCOO− pathway, which in turn is more spontaneous than the hydrogen abstraction from −CH3− pathway. The lifetime of this compound is very low at marine boundary conditions, but it is long-lived in ambient conditions. The contribution of this compound to GWP will be significant in ambient conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Vibrational frequencies and structural parameters of various species involved in the title reactions obtained at the M06-2X/ 6-31+G(d,p) level of theory. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Table 5. Computed Atmospheric Lifetimes at 277 K and Global Warming Potentials (GWPs) of Ethyl Formate Computed for the Time Horizons 20, 100, and 500 yearsa Life time (days) at 277 K

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Notes

The authors declare no competing financial interest. 5277



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