Ab initio and direct dynamics study on the hydrogen abstraction reaction C2H3 + CH3CHO

Computational and Theoretical Chemistry 1075 (2016) 63–69

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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc

Ab initio and direct dynamics study on the hydrogen abstraction reaction C2H3 + CH3CHO Chaoxu Chen, Jinou Song ⇑, Chonglin Song, Gang Lv, Zhijun Li ⇑ State Key Laboratory of Engines, Tianjin University, Tianjin, China

a r t i c l e

i n f o

Article history: Received 27 April 2015 Received in revised form 13 November 2015 Accepted 18 November 2015 Available online 28 November 2015 Keywords: Vinyl radical Acetaldehyde Rate constant Conventional transition state theory Tunneling effect

a b s t r a c t The mechanism for the hydrogen abstraction reaction C2H3 + CH3CHO has been investigated by the CCSD (T)/cc-pVTZ method based on the geometries of stationary points optimized at the B3LYP/6-311++G(d,p) level of theory. Two abstraction channels have been identified for the production of C2H4 + CH2CHO and C2H4 + CH3CO. The potential barrier heights of the corresponding transition states TSR/P1 and TSR/P2 were predicted to be 9.47 and 5.95 kcal/mol at the CCSD(T)//B3LYP level of theory, respectively. The rate constants and branching ratios for the two H-abstraction channels were calculated using conventional transition state theory with Eckart tunneling correction at the temperature range 300–2500 K. The predicted rate constants have been compared with available literature data. Both the forward and reverse rate constants have positive temperature dependence and the tunneling effect is only important at low temperatures. The branching ratio calculation shows that the channel producing C2H4 + CH3CO remains predominant throughout the entire studied temperature range. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction

bond energy and bond order (BEBO) method over the temperature range of 480–520 K and gave a rate constants expression of

Ethanol is an important alternative fuel and widely used as a fuel component to improve motor vehicle fuel properties around the world [1–3]. Chemical kinetic and flame studies of ethanol were conducted to understand their oxidation and co-oxidation with hydrocarbon fuel [4–6]. The role of radical pool connecting different fuels is confirmed in these works, but no new reaction channels were explored between the intermediate species from ethanol and hydrocarbon fuel pyrolysis. Ethanol can reduce engine exhaust emissions of unburned hydrocarbons and carbon monoxide but increase the emission of acetaldehyde (CH3CHO) [7–9]. CH3CHO is also one of the major intermediate species appearing during the combustion of ethanol [4,10]. Up to now, lots of theoretical and experimental investigations have been performed on the acetaldehyde reaction with H, D, O, Cl, NH2, CH3, O2, etc. [11–15]. The vinyl radical (C2H3) is a key intermediate in the combustion reactions of large hydrocarbon molecules, and can react with atoms, radicals and molecules such as H2, C2H2, CH3,CO, O2, NO, O, and HCHO [16–31]. Despite their importance in combustion, the reaction between CH3CHO and C2H3 has not been comprehensively studied to the best of our knowledge. Only Scherzer et al. [32] in 1987 investigated the H-abstraction reaction C2H3 + CH3CHO ? C2H4 + CH3CO using the

k ¼ 1:35  1013 eð15382=RTÞ cm3 molecule s1 . Theoretically, there should be two hydrogen abstraction channels when C2H3 radical attacks at two different sites of the CH3CHO molecule:

⇑ Corresponding authors. E-mail addresses: [email protected] (J. Song), [email protected] (Z. Li). http://dx.doi.org/10.1016/j.comptc.2015.11.018 2210-271X/Ó 2015 Elsevier B.V. All rights reserved.

1

C2 H3 þ CH3 CHO ! C2 H4 þ CH2 CHO

ð1Þ

! C2 H4 þ CH3 CO

ð2Þ

This paper carried out a detailed mechanistic study on the C2H3 + CH3CHO reaction. Since the products of the title reaction are characterized by different reactivities and are expected to have different fates in the combustion system, it is of primary importance to establish reliable values for the branching ratios. As the experiments were not able to firmly evaluate the branching ratios for the product formation so far, theoretical studies seem to be necessary. In this paper, ab initio calculations were performed for all reactants, products, and transition states. The rate constants for these reaction channels were obtained by applying transition state theory along with appropriate quantum-mechanical tunneling corrections, and the dominant channel was determined. 2. Computational methods The geometries of the reactants, products and transition states of the title reaction were optimized with the density functional theory at the B3LYP/6-311++G(d,p) level of theory [33–36].

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Harmonic frequency calculations were carried out to identify the obtained structures for local minima (with all real frequencies) and transition state (with one and only one imaginary frequency) and to obtain the zero-point energy (ZPE) corrections. Intrinsic reaction coordinate (IRC) calculations [37,38] were performed at the same level to confirm that every transition state is connected to the designated reactants and products correctly. The QCISD (T)/6-311++G(d,p) and CCSD(T)/cc-pVTZ [39–41] single-point calculations were also implemented based on the optimized geometries at the B3LYP/6-311++G(d,p) level of theory (denoted as QCISD(T)//B3LYP and CCSD(T)//B3LYP, respectively) to improve the accuracy of the energetics parameters. For each reaction channel, the minimum energy path (MEP) was calculated in mass-weighted Cartesian coordinates with a gradient step of 0.1 (amu)1/2 bohr at the B3LYP/6-311++G(d,p) level of theory, and the potential energy curve was further refined at the CCSD(T)// B3LYP level. Gaussian 09 program package [42] was employed for these electronic structure calculations. The rate constants for the two hydrogen abstraction channels were calculated over the temperature range 300–2500 K by conventional transition state theory (TST) [43] with the KiSThelp2014 program package [44], and the Eckart tunneling correction (TST/Eck) was also included [45]. 3. Results and discussion 3.1. Potential energy surface and reaction mechanism For the C2H3 + CH3CHO hydrogen abstraction reaction system, the two production channels, as alluded to in the introduction, have been considered. At the B3LYP/6-311++G(d,p) level of theory, the optimized geometries of the reactants, transition states, and products are shown in Fig. 1 along with the available experimental data [46]. The deviations of all the bond angle and bond length are less than 2.4° and 0.01 Å, respectively, which shows a great agreement between the calculated parameters and the experimental values. As can be seen from Fig. 1, in channel (1) where the vinyl radical attacks the H-atom of the methyl in CH3CHO forming C2H4 and CH2CHO, the breaking C–H bond at the TSR/P1 is elongated by 14.1% and the forming C–H bond is 35.9% longer than that at the C2H4 product. In channel (2) where the H-atom is abstracted from the aldehyde group in the CH3CHO to form C2H4 and CH3CO, the breaking C–H bond at the TSR/P2 is elongated by 11.1% and the forming C–H bond is 44.06% longer than that at C2H4. These structural parameters indicate that both transition states of the two channels are more reactant-like, and that both channels proceed via an early transition state, as expected for the exothermic reaction, in accordance with the postulate of Hammond [47]. Table 1 lists the harmonic vibrational frequencies and zero-point energies (ZPE) of all the stationary points at the B3LYP/6-311++G(d,p) level of theory along with the available experimental data [46]. It can be found that the theoretical results are consistently greater than the corresponding experimental values, but the deviations of the calculated frequencies compared with the experimental data are generally within 7%, indicating that a great agreement has been achieved. The character of each transition state is confirmed by normal mode analysis, which yields one and only one imaginary frequency (i.e., 1223 cm1 for the TSR/P1, 879 cm1 for the TSR/P2) whose eigenvector corresponds to the direction of each reaction. Tables 2 and 3 list the theoretical reaction energetics parameters including the reaction energies, the reaction enthalpies, the forward and the reverse classical potential barrier heights of channels (1) and (2) at the B3LYP/6-311++G(d,p), QCISD(T)//B3LYP and CCSD

(T)//B3LYP levels of theory, along with the available experimental data [46,48]. The reaction enthalpy of channel (1) at B3LYP/6-311 ++G(d,p) level of theory is predicted to be 15.37 kcal/mol with a deviation of 0.77 kcal/mol to the experimental value, but after the energy is refined at QCISD(T)//B3LYP and CCSD(T)//B3LYP levels of theory, the results become 13.95 kcal/mol and 14.45 kcal/mol with the deviations decreasing to 0.65 kcal/mol and 0.15 kcal/mol, respectively. For channel (2), the calculated reaction enthalpy at CCSD(T)//B3LYP level of theory (19.70 kcal/mol) is slightly smaller than that at B3LYP/6-311++G(d,p) level (19.94 kcal/mol) and QCISD(T)//B3LYP level (20.09 kcal/mol) which is the closest to the experimental value with a deviation of 0.51 kcal/mol (the deviations at B3LYP/6-311++G(d,p) and CCSD(T)//B3LYP levels of theory are 0.66 kcal/mol and 0.9 kcal/mol, respectively). Theoretical results at three levels of theory all agree well with the experimental values, and the CCSD(T)//B3LYP method can consistently provide reliable energy results. To determine whether the single reference CCSD(T) method is able to properly describe the energetics of the title reaction, the T1 diagnostic tests [49] have been performed for transition states. A diagnostic value of 0.03 was obtained for both TSR/P1 and TSR/P2, slightly larger than threshold value of 0.02. So the multi-reference methods CASPT2(7e,7o)/CBS, CASPT2(9e,9o)/CBS and MRCI(7e,7o)/CBS [50–52] were employed to recompute the energy barriers, which turn out to be about 1–2 kcal/mol different from the CCSD(T) results (Table 4). The leading coefficients calculated with CASPT2(7e,7o)/CBS, CASPT2(9e,9o)/CBS and MRCI (7e,7o)/CBS are about 0.95, 0.95 and 0.88, respectively, which are close to 1. These indicate that the multi-reference character of this system is quite mild. It has been reported that the CCSD(T) method is of modest use for the systems with mild multi-reference character [53] and good accuracies were achieved on the similar systems of the C6H5 + CH3CHO and OH + CH3CHO reactions with single reference treatment [54,55]. In light of these factors, the single reference method CCSD(T) is justified in this work. Table 5 lists the forward potential energy barriers calculated with QCISD(T) and CCSD(T) methods and cc-pVDZ, 6-311++G(d,p) and cc-pVTZ basis sets based on the optimized geometries at B3LYP/6311++G(d,p) level of theory. For the CCSD(T) method, the cc-pVDZ basis set with 105 basis functions gives the highest barriers for both channels. With a larger basis set 6-311++G(d,p) (159 basis functions), the energy barriers decrease about 1.0 kcal/mol for channel (1) and 0.5 kcal/mol for channel (2). When the basis set gets even larger (for cc-pVTZ with 248 basis functions), the barriers for both channels decrease further, but only slightly. The discrepancies between the values computed with 6-311++G(d,p) and with cc-pVTZ are insignificant. Similar results were obtained with the QCISD(T) method. In light of these, the cc-pVTZ basis set is adequate for the C2H3 + CH3CHO reaction. Therefore, the CCSD(T)/cc-pVTZ method was chosen to refine the potential energy profiles in this work. Fig. 2 shows the potential energy surface (PES) of the C2H3 + CH3CHO reaction at the CCSD(T)//B3LYP and QCISD(T)//B3LYP levels of theory. For convenience, the total energy of the reactants was set as zero, and the two hydrogen abstraction channels mentioned in the introduction were confirmed. The calculated forward potential barriers at the CCSD(T)//B3LYP level of theory are 9.47 and 5.95 kcal/mol for TSR/P1 and TSR/P2, respectively. The negative heats of reaction, 14.25 and 19.90 kcal/mol for channels (1) and (2) respectively, show that both channels are predicted to be exothermic, which agrees with the two channels’ early transition states as discussed above. It can be found that the QCISD(T)//B3LYP method provides nearly same barrier height for channel (1), and a little smaller one for channel (2). According to the low barrier heights, both channels could be kinetically favorable.

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Fig. 1. Optimized geometries of the reactants, transition states and products at the B3LYP/6-311++G(d,p) level of theory (bond lengths in angstroms (Å) and angles in degrees (°)). The values in the parentheses are the corresponding experimental values [46].

3.2. Rate constant calculations In order to obtain more accurate energetics information, the potential energy curves calculated at B3LYP/6-311++G(d,p) were further refined at the CCSD(T)/cc-pVTZ level of theory. The classical potential energy ðV MEP Þ and the vibrational adiabatic ground-state   potential energy V Ga curves of channels (1) and (2) as a function of the intrinsic reaction coordinate (s) at the CCSD(T)//B3LYP level of theory are plotted in Figs. 3 and 4, respectively. The vibrational adiabatic ground-state potential energy is defined as V Ga ðsÞ ¼ V MEP ðsÞ þ ZPEðsÞ, and the ZPE is obtained at B3LYP/6-311++G(d,p) level. The classical potential energy of the transition state (i.e., V MEP ðs ¼ 0Þ) was set as zero. Fig. 3 shows that the V MEP and V Ga curves are similar in shape and the position of the maximum value does not shift, indicating that the variational effect for channel (1) would be negligible in evaluating the rate constants. Similar results can be obtained from Fig. 4 for channel (2). In view of the small variational effect, the conventional transition state theory along with the Eckart tunneling correction is employed to predict the forward and reverse rate constants of both channels. Figs. 5 and 6 show the forward rate constants of channels (1) and (2) respectively. It is reported that C2H3 is similar to CH3 in

some reactions of H-abstraction [46], and Tsang [56] suggested the same rate expressions for the reaction C2H3 + CH3OH and the reaction CH3 + CH3OH, so the experimental rate constants for reaction CH3 + CH3CHO ? C2H4 + CH2CHO between 1000 and 1700 K obtained by Yasunaga et al. [12] and recommended expression for the reaction CH3 + CH3CHO ? C2H4 + CH3CO from the available kinetic mechanisms [57,58] are presented in Figs. 5 and 6 as well, respectively. The theoretical values of channel (2) obtained by Scherzer et al. [32] is also presented in Fig. 6, and it agree well with the rate constant of channel (2) calculated by TST/Eck. It can be found that the rate constants of both channels have the same order of magnitude as ones of the similar hydrogen abstraction reaction CH3 + CH3CHO, indicating the same importance of title reaction in the combustion progress. The forward TST/Eck rate constants of the channels (1) and (2) within 300 – 2500 K were fitted by the three-parameter expression as follows: 1

s1 ðchannel 1Þ

ð3Þ

1

s1 ðchannel 2Þ

ð4Þ

k ¼ 3:304  1025 T 3:96 eð25990=RTÞ cm3 molecule

k ¼ 2:579  1023 T 3:62 eð16810=RTÞ cm3 molecule

Figs. 7 and 8 show the reverse rate constants of channel (1) and (2), respectively. It can be found that both the predicted forward and reverse rate constants of two channels have positive

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Table 1 Theoretical and experimental harmonic vibrational frequencies (cm1) and ZPE (kcal/mol) of all the stationary points at the B3LYP/6-311++G(d,p) level of theory. Species

Frequencies

ZPE

C2H3 Expt CH3CHO

3238, 3136, 3039, 1644, 1390, 1041, 923, 816,706 3141, 2953, 2912, 1360, 895, 857, 674 3136, 3075, 3021, 2871, 1808, 1469, 1460, 1420, 1377, 1133, 1128, 886, 776, 510, 153 3014, 2964, 2923, 2716, 1743, 1433, 1431, 1395, 1352, 1114, 1102, 867, 764, 509, 150 3227, 3198, 3141, 3126, 1684, 1471, 1377, 1237, 1056, 975, 972, 833 3105, 3086, 3026, 2989, 1623, 1444, 1342, 1217, 1023, 949, 940, 826 3253, 3138, 2945, 1545, 1470, 1394, 1157, 978, 975, 759, 507, 444 2828, 1528, 1486, 1376, 1143, 957, 703, 500, 404 3114, 3108, 3016, 1925, 1457, 1453, 1357, 1049, 956, 854, 469, 110 1875, 1420 3196, 3163, 3159, 3077, 3076, 2885, 1753, 1650, 1452, 1412, 1404, 1384, 1323, 1155, 1131, 1101, 950, 942, 886, 875, 841, 632, 560, 471, 324, 160, 147, 69, 24, 1223i 3174, 3139, 3126, 3091, 3062, 3026, 1842, 1643, 1464, 1462, 1457, 1404, 1371, 1207, 1183, 1100, 947, 902, 899, 880, 860, 617, 423, 324, 204, 173, 114, 72, 16, 879i

22.78

Expt C2H4 Expt CH2CHO Expt CH3CO Expt TS R/P1

TS R/P2

34.63

31.88

26.54

Table 5 The effect of basis set on forward potential energy barriers. Methods

Energy barriers (kcal/mol)

QCISD(T)/cc-pVDZ QCISD(T)/6-311++G(d,p) QCISD(T)/cc-pVTZ CCSD(T)/cc-pVDZ CCSD(T)/6-311++G(d,p) CCSD(T)/cc-pVTZ

20

Channel 1

Channel 2

10.51 9.48 9.34 10.63 9.62 9.47

6.26 5.74 5.71 6.47 5.99 5.95

ΔE(kcal/mol)

26.97

TSR/P1 9.47(9.48)

10 56.04

TSR/P2 5.95(5.74) R

0 56.01

-10 P1 -14.25(-13.76) P2

-20 Table 2 The reaction energetic parameters (kcal/mol) of channel (1) at various levels of theory.

a b c

Method

DE

DH298.15K

b V– f

B3LYP/6-311++G(d,p) QCISD(T)//B3LYP CCSD(T)//B3LYP Exptl [46,48]

15.18 14.66 14.25

15.37 13.95 14.45 14.6 ± 1.20

6.17 9.48 9.47

a

c V– f

21.35 24.14 23.72

-19.90(-20.30)

Fig. 2. Schematic potential energy surface for the C2H3 + CH3CHO reaction computed at CCSD(T)//B3LYP level of theory, the relative energies at QCISD(T)//B3LYP also given in parentheses.

60

Reaction energy with zero-point energy correction. Reaction energy with forward classical barrier height. Reaction energy with reverse classical barrier height.

50

VGa

a b c

Method

DE a

DH298.15K

b V– f

c V– f

B3LYP/6-311++G(d,p) QCISD(T)//B3LYP CCSD(T)//B3LYP Exptl [46,48]

20.16 20.30 19.90

19.94 20.09 19.70 20.6 ± 1.30

2.46 5.74 5.95

22.62 26.04 25.85

0 -10

VMEP

-3

-2

-1

0

s ((amu)

Table 4 The comparison of energy barriers calculated with single reference method and multi-reference method.

CASPT2(7e,7o)/CBS CASPT2(9e,9o)/CBS MRCI(7e,7o)/CBS CCSD(T)/cc-pVTZ

30

-20

Reaction energy with zero-point energy correction. Reaction energy with forward classical barrier height. Reaction energy with reverse classical barrier height.

Methods

E (kcal/mol)

40 Table 3 The reaction energetic parameters (kcal/mol) of channel (2) at various levels of theory.

Energy barriers (kcal/mol) Channel 1

Channel 2

8.9 7.3 9.8 9.47

6.6 5.2 6.9 5.95

temperature dependence in the calculated temperature range, and the TST/Eck rate constants are larger than the TST ones at low temperatures, but they gradually merge as the temperature increases.

1/2

1

2

3

bohr)

Fig. 3. The classical potential   energy ðV MEP Þ and the vibrational adiabatic groundstate potential energy V Ga curves of channel (1) as a function of the reaction coordinate s ((amu)1/2 bohr) at the CCSD(T)//B3LYP level of theory.

The ratios of TST/Eck to TST forward rate constants are 5.65 and 2.22 at 300 K, 1.75 and 1.32 at 500 K, and 1.07 and 1.04 at 1450 K for channel (1) and (2), respectively, implying that the tunneling effect is important only in the low temperature region. Furthermore, the tunneling effect in the reverse reaction is smaller than in the forward one. For the two individual channels discussed above, the branching ratios have been evaluated and the result is plotted in Fig. 9. It is evident from this figure that channel (2) forming C2H4 + CH3CO is predominant and channel (1) producing C2H4 + CH2CHO remains noncompetitive in the entire temperature range.

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-12

60

VGa

40

E (kcal/mol)

log[k(cm3 molecule -1 s-1 )]

50

30 0 -10

VMEP

-20 -30

-3

-2

-1

0

1

2

k TST k TST/Eck

-16

-20

-24

-28

-32

3

0.0

s ((amu)1/2 bohr)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000/T(K-1 )

Fig. 4. The classical potential energy ðV MEP Þ and the vibrational adiabatic ground  state potential energy V Ga curves of channel (2) as a function of the reaction

Fig. 7. Calculated reverse rate constants of channel (1) at the CCSD(T)//B3LYP level of theory.

coordinate s ((amu)1/2 bohr) at the CCSD(T)//B3LYP level of theory.

-11 -12

-12

TST

log[k(cm3molecule-1s-1)]

-14 -15 -16 -17 -18 -19 -20 -21 0.0

0.5

1.0

1.5

2.0

2.5

3.0

k TST k TST/Eck

-16

log[k(cm3 molecule -1 s-1 )]

k kTST/Eck Ref.12

-13

-20

-24

-28

-32

3.5

0.0

-1

0.5

1000/T(K )

1.0

1.5

2.0

2.5

3.0

3.5

1000/T(K-1 )

Fig. 5. Comparison of the calculated forward rate constants of channel (1) at the CCSD(T)//B3LYP level of theory with the literature data.

Fig. 8. Calculated reverse rate constants of channel (2) at the CCSD(T)//B3LYP level of theory.

-10

-12

kTST

1.0

kTST/Eck Ref.32

0.8

Ref.50 Ref.51

-13 -14 -15 -16

Branching ratio

log[k(cm3molecule-1s-1)]

-11

Channel 2

0.6

0.4

0.2

Channel 1

-17 -18

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000/T(K-1) Fig. 6. Comparison of the calculated forward rate constants of channel (2) at the CCSD(T)//B3LYP level of theory with the literature data.

0

500

1000

1500

2000

2500

T (K) Fig. 9. Branching ratios for the hydrogen abstraction reaction C2H3 + CH3CHO ? products.

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4. Conclusions An ab initio and direct dynamics study of the hydrogen abstraction reaction C2H3 + CH3CHO has been presented at the CCSD(T)// B3LYP level of theory. Two abstraction channels producing CH2CHO and CH3CO are confirmed and their barriers predicted to be 9.47 and 5.95 kcal/mol, respectively. This indicates that both channels could be kinetically favorable. The forward and reverse rate constants for the two H-abstraction channels were evaluated using conventional transition state theory with Eckart tunneling correction at the temperature range 300–2500 K. The predicted forward rate constants have been compared with the available literature data. Both the forward and reverse rate constants have positive temperature dependence and the tunneling effect is important only in low temperature region. The branching ratio calculation shows that the channel producing C2H4 + CH3CO is dominant, and the channel producing C2H4 + CH2CHO remains small and noncompetitive throughout the calculated temperature range. Acknowledgments This work was supported by the National Natural Science Foundation of China (51276126, 51576139, and 51276128) and the National Key Basic Research and Development Program of China (2013CB228502). The authors gratefully acknowledge the assistance of Professor Xiaoqing You and Dr. Tanjin He at Tsinghua University for performing multi-reference calculations. References [1] M.K. Balki, C. Sayin, M. Canakci, The effect of different alcohol fuels on the performance, emission and combustion characteristics of a gasoline engine, Fuel 115 (2014) 901–906. [2] A.C. Hansen, Q. Zhang, P.W. Lyne, Ethanol–diesel fuel blends – a review, Bioresour. Technol. 96 (2005) 277–285. [3] W.D. Hsieh, R.H. Chen, T.L. Wu, T.H. Lin, Engine performance and pollutant emission of an SI engine using ethanol–gasoline blended fuels, Atmos. Environ. 36 (2002) 403–410. [4] N. Leplat, P. Dagaut, C. Togbé, J. Vandooren, Numerical and experimental study of ethanol combustion and oxidation in laminar premixed flames and in jetstirred reactor, Combust. Flame 158 (2011) 705–725. [5] P. Dagaut, C. Togbé, Experimental and modeling study of the kinetics of oxidation of ethanol-n-heptane mixtures in a jet-stirred reactor, Fuel 89 (2010) 280–286. [6] P. Dagaut, C. Togbé, Experimental and modeling study of the kinetics of oxidation of ethanol–gasoline surrogate mixtures (E85 surrogate) in a jetstirred reactor, Energy Fuel 22 (2008) 3499–3505. [7] L.A. Graham, S.L. Belisle, C.L. Baas, Emissions from light duty gasoline vehicles operating on low blend ethanol gasoline and E85, Atmos. Environ. 42 (2008) 4498–4516. [8] C.S. Cheung, Y. Di, Z. Huang, Experimental investigation of regulated and unregulated emissions from a diesel engine fueled with ultralow-sulfur diesel fuel blended with ethanol and dodecanol, Atmos. Environ. 42 (2008) 8843–8851. [9] C.L. Song, W.M. Zhang, Y.Q. Pei, G.L. Fan, G.P. Xu, Comparative effects of MTBE and ethanol additions into gasoline on exhaust emissions, Atmos. Environ. 40 (2006) 1957–1970. [10] J. Li, A. Kazakov, F.L. Dryer, Ethanol pyrolysis experiments in a variable pressure flow reactor, Int. J. Chem. Kinet. 33 (2001) 859–867. [11] R. Sivaramakrishnan, J. Michael, S. Klippenstein, Direct observation of roaming radicals in the thermal decomposition of acetaldehyde, J. Phys. Chem. A 114 (2009) 755–764. [12] K. Yasunaga, S. Kubo, H. Hoshikawa, T. Kamesawa, Y. Hidaka, Shock-tube and modeling study of acetaldehyde pyrolysis and oxidation, Int. J. Chem. Kinet. 40 (2008) 73–102. [13] D. Baulch, C. Cobos, R. Cox, P. Frank, G. Hayman, T. Just, J. Kerr, T. Murrells, M. Pilling, J. Troe, Evaluated kinetic data for combustion modeling, Supplement I, J. Phys. Chem. Ref. Data 23 (1994) 847–848. [14] W. Payne, D. Nava, F. Nesbitt, L. Stief, Rate constant for the reaction of atomic chlorine with acetaldehyde from 210 to 343 K, J. Phys. Chem. 94 (1990) 7190–7193. [15] J. Ehbrecht, W. Hack, P. Rouveirolles, Hydrogen abstraction reactions by NH2  from hydrocarbons in the gas phase, Ber. Bunsen-Ges. Phys. (X2B1)-radicals Chem. 91 (1987) 700–708. [16] L.K. Huynh, A. Violi, Thermal decomposition of methyl butanoate: ab initio study of a biodiesel fuel surrogate, J. Org. Chem. 73 (2008) 94–101.

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