Thermal Decomposition of Ethanol. 4. Ab Initio Chemical Kinetics for

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Thermal Decomposition of Ethanol. 4. Ab Initio Chemical Kinetics for Reactions of H Atoms with CH3CH2O and CH3CHOH Radicals Z. F. Xu, Kun Xu,† and M. C. Lin* Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050, Republic of China ABSTRACT: The potential energy surfaces of H-atom reactions with CH3CH2O and CH3CHOH, two major radicals in the decomposition and oxidation of ethanol, have been studied at the CCSD(T)/6-311þG(3df,2p) level of theory with geometric optimization carried out at the BH&HLYP/6-311þG(3df,2p) level. The direct hydrogen abstraction channels and the indirect association/decomposition channels from the chemically activated ethanol molecule have been considered for both reactions. The rate constants for both reactions have been calculated at 1003000 K and 104 Torr to 103 atm Ar pressure by microcanonical VTST/RRKM theory with master equation solution for all accessible product channels. The results show that the major product channel of the CH3CH2O þ H reaction is CH3 þ CH2OH under atmospheric pressure conditions. Only at high pressure and low temperature, the rate constant for CH3CH2OH formation by collisonal deactivation becomes dominant. For CH3CHOH þ H, there are three major product channels; at high temperatures, CH3þCH2OH production predominates at low pressures (P < 100 Torr), while the formation of CH3CH2OH by collisional deactivation becomes competitive at high pressures and low temperatures (T < 500 K). At high temperatures, the direct hydrogen abstraction reaction producing CH2CHOH þ H2 becomes dominant. Rate constants for all accessible product channels in both systems have been predicted and tabulated for modeling applications. The predicted value for CH3CHOH þ H at 295 K and 1 Torr pressure agrees closely with available experimental data. For practical modeling applications, the rate constants for the thermal unimolecular decomposition of ethanol giving key accessible products have been predicted; those for the two major product channels taking place by dehydration and CC breaking agree closely with available literature data.

1. INTRODUCTION Ethanol is currently one of the most important renewable energy sources.1 Both ethoxy (CH3CH2O) and 1-hydroxyethyl (CH3CHOH) radicals are major intermediates in the thermal decomposition and oxidation of ethanol which play an important role in the chain-propagation step;2 their decomposition reactions produce primarily H atoms which are the key chain carrier in the thermal decomposition process. The reactions of these radicals with H atoms generate small, more reactive radicals such as CH3 and OH,3 resulting in chain-branching. In the literature, there are only two experimental studies on the kinetics of CH3CHOH with H, published by Bartels et al.3 in 1982 and Edelbuttel-Einhaus et al.4 in 1992. The former studied this reaction at about 295 K in an ArHe buffer gas at 0.12 Torr using a Laval nozzle reactor with quantitive product analysis by mass spectrometry with molecular beam sampling and synchronous ion counting techniques. The rate constant was determined to be 8.30 1011 cm3 molecule1s1 under their experimental condition. The latter performed a kinetic measurement and reported the rate constant, (3.32 ( 1.66) 1011 cm3molecule1s1, at 295 K and 1 Torr (ArHe) pressure using an isothermal discharge flow reactor r 2011 American Chemical Society

and molecular beam sampling mass spectrometry with laserenhanced multiphoton ionization (REMPI) and electron impact ionization. In related works, the thermal unimolecular decomposition of ethanol has been studied by some authors510 theoretically and experimentally. Most of these studies considered the C2H4 þ H2O, CH3 þ CH2OH, and CH3CH2 þ OH product channels as major decomposition reactions of ethanol; they have been discussed in detail. Higher energy decomposition channels producing CH3CHOH and CH3CH2O radicals, which cannot be readily measured, have not been addressed. We expect the coexistence of the two radicals with H atoms in the thermal decomposition of ethanol. The interaction of H with these radicals producing small, more reactive radicals as aforementioned may lead to important chain-branching in the decomposition of ethanol particularly under high-temperature, highpressure conditions. Our goal is to elucidate the detailed mechanisms involved in the title reactions and to provide the Received: November 5, 2010 Revised: February 22, 2011 Published: March 29, 2011 3509

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absolute rate constants for the key product channels for hightemperature simulation applications. The computed rate constant for CH3CHOH þ H will be compared with the available experimental data cited above.

2. COMPUTATIONAL METHODS For both CH3CH2O þ H and CH3CHOH þ H reactions, the geometric parameters of the reactants, products, transition states (TSs), and intermediates are optimized at the BH&HLYP/ 6-311þG(3df,2p) level of theory11 with use of the Gaussian 03 program.11 All of the stationary points are identified for local minima or transition states by vibrational analysis. Unscaled vibrational frequencies were employed for calculation of zeropoint energy (ZPE) corrections, the characterization of stationary points, and rate constants. For a more accurate evaluation of the energetic parameters, single-point energy calculations of the stationary points are carried out by the CCSD(T)/6-311þ G(3df,2p) method12 based on the optimized geometries at the BH&HLYP/6-311þG(3df,2p) level. To confirm the reliability of the BH&HLYP method for optimization, several critical reaction channels have been reoptimized by the CCSD method, as discussed in the next section. Rate constant calculations were carried out with the VARIFLEX program13 based on the microcanonical Rice RamspergerKasselMarcus (RRKM) theory and variational transition-state theory (VTST).1419 The vibrational-state densities were calculated by the direct state counting approach with the BeyerSwinehart algorithm. The component rates were evaluated at the E/J-resolved level, and the pressure dependence was treated by one-dimensional master equation using the Boltzmann probability of the complex for the J-distribution. For a barrierless association or loose transition-state decomposition process, the potential energy path is approximated with the fitted Morse potential, V(R) = De{1 exp[β(R Re)]}2, along the reaction coordinate in conjunction with the other two potentials, which are corresponding to the “conserved” and “transitional” degrees of freedom orthogonal to the reaction coordinate, as will be discussed later. Here, De is the binding energy excluding zero-point vibrational energy for an association reaction, R is the reaction coordinate (i.e., the distance between the two bonding atoms), and Re is the equilibrium value of R of the stable intermediate structure. The potential for the conserved degrees of freedom corresponds to the normal-mode vibrations in the separated fragments and is assumed to be the same as in the fragments. The potential for the transitional degrees of freedom is described in term of internal angles with sinusoidal functions.20 The coefficient in the potential expression can be determined by the appropriate force constant matrix (Fij(R)) at the potential minimum, assuming that Fij(R) decays exponentially with the bond distance: Fij ðRÞ ¼ Fij ðR0 Þ exp½ ηðR R0 Þ Here, R is the bond distance along with the reaction coordinate; R0 is the bond distance at the equalibrium structure; and η is a decay parameter with R increasing. We estimated the decay parameters in the vicinity of variational transition states. For the CH3CH2OH f CH3CH2O þ H process, there are two internal angles corresponding to the decay parameters η( — C2O3H4) = 1.0835 Å1 and η( — C1C2O3H4) = 0.3362 Å1. For the

Figure 1. Geometric parameters of the stationary points (length in angstroms and angle in degrees) optimized at the BH&HLYP/ 6-311þG(3df,2p) level of theory. The numbers in parentheses are predicted at the CCSD/6-311þG(3df,2p) level of theory .

CH3CH2OH f CH3CHOH þ H process, the decay parameters of its two internal angles are η( — C1C2H5) = 1.0089 Å1 and η( — O3C1C2H5) = 0.9419 Å1. The CH3CH2OH f CH3 þ CH2OH process has five internal angles, and their decay parameters are η( — C1C2O3) = 1.2174 Å1, η( — C1C2O3H4) = 0.0129 Å1, η( — C2C1H7) = 1.5821 Å1, η( — O3C2C1H7) = 2.9633 Å1, and η( — H9C1H7C2) = 1.4271 Å1. For the CH3CH2OH f CH3CH2 þ OH process, the decay parameters of its four internal angles are η( — C1C2O3) = 1.8785 Å1, η( — H7C1C2O3) = 0.9246 Å1, η( — H4O3C2) = 1.5943 Å1, and η( — H4O3C2C1) = 2.1084 Å1. For the CH3CH2OH f CH2CH2OH þ H process, the decay parameters of its two internal angles are η( — H9C1C2) = 0.7416 Å1 and η( — H9C1C2O3) = 0.7427 Å1. An exponential down model with ÆΔEædown = 400 cm1 is employed for the energy-transfer coefficients of Ar buffer gas; the collision rate of CH3CH2OH with the diluent buffer gas is taken to be the Lennard-Jones (L-J) collision rate. The L-J parameters of Ar (ε/kB = 134.2 K and σ = 3.54 Å) and CH3CH2OH (ε/kB = 450.2 K and σ = 4.317 Å) are taken from the literature.21 Eckart tunneling coefficients22 are used to correct the rate constants for the hydrogen migration quantum effects at low temperatures. 3510

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Figure 2. Schematic diagrams of the potential energy surfaces predicted at the CCSD(T)/6-311þG(3df,2p)//BH&HLYP/6-311þG(3df,2p) level of theory with zero-point vibrational energy corrections: (A) CH3CH2O þ H and (B) CH3CHOH þ H. [E(CH3CH2O) = 154.113 4354 au; E(CH3CHOH) = 154.129 074 au and E(H) = 0.499810 au; E(CH3CH2OH) = 154.788 61 au]. The numbers in parentheses are predicted at the CCSD(T)/6-311þG(3df,2p)//CCSD/6-311þG(3df,2p) level of theory with ZPE correction. [E(CH3CH2O) = 154.113875 au; E(CH3CHOH) = 154.129 6977 au and E(H) = 0.499 810 au; E(CH3CH2OH) = 154.789 264 au].

3. RESULTS AND DISCUSSION 3.1. Potential Energy Surfaces. The main geometric parameters and the relative energies are shown in Figures 1 and 2, respectively. The harmonic vibrational frequencies and moments of inertia for all of the stationary points computed at the BH&HLYP/6-311þG(3df,2p) level are summarized in Table 1 for kinetics calculation. The predicted stationary point energies relative to ethanol are listed in Table 2 with previous available theoretical and experimental data for comparison. From Table 2, we can find that the predicted energies with CCSD(T)//BH&HLYP are almost the same as those with CCSD(T)//CCSD and the maximum deviations in comparison with the experimental data are less than 2 kcal/mol. The G3B3//B3LYP and G2M//B3LYP results appear to be somewhat larger than ours with a maximum difference of 10 kcal/mol except CH 2CH2 þ H2 O, CH3 CH þ H2 O, and TS1. Most recently Sivaramakrishnan and co-workers6 performed a study for the decomposition of ethanol

experimentally and theoretically. Their results computed by QCISD(T)/CBS and ATcT show a maximum deviation from ours by about 2 kcal/mol, suggesting that our predicted potential energy surface at the CCSD(T)/BH&HLYP level is reasonable and reliable (with less than 2% error in dissociation energies which is basically within the experimental error range). In Figure 2A, it can be seen that the possible reaction pathways of CH3CH2O þ H include one direct hydrogen abstraction channel and one barrierless association channel followed by several decomposition steps from CH3CH2OH: CH3 CH2 O þ H f CH3 CHO þ H2 fCH3 CH2 OH f decomposition products ðaÞ For the CH3CHOH þ H reaction, two direct hydrogen abstraction channels and one barrierless association followed by the same decomposition paths as in the former reaction are 3511



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Table 1. Vibrational Frequencies and Moments of Inertia for the Reactants, Intermediates, Transition States, and Products Optimized at the BH&HLYP/6-311þG(3df,2p) Level of Theorya

CH3CH2O

frequencies (cm1)

Ii (au)

species

45.1, 185.5, 208.8

111, 270, 446, 893, 926, 1115, 1142, 1300, 1420, 1458, 1471, 1538, 1549, 3023, 3041, 3114, 3180, 3190

CH3CHOH 38.2, 188.4, 214.5 CH3CH2OH 50.1, 190.0, 218.0

185, 360, 423, 585, 962, 1057, 1094, 1255, 1324, 1446, 1504, 1519, 1542, 3048, 3131, 3184, 3223, 4029 245, 284, 432, 846, 937, 1076, 1156, 1227, 1310, 1351, 1458, 1510, 1535, 1553, 1584, 3072, 3098,

CH2OH

9.0, 59.1, 67.6

414, 573, 1092, 1261, 1401, 1541, 3217, 3359, 4028 529, 1456, 1456, 3192, 3373, 3373

3116, 3184, 3188, 4028 CH3

6.1, 6.1, 12.3

CH3CH2

17.0, 78.3, 84.4

115, 491, 837, 1017, 1094, 1238, 1455, 1519, 1535, 1536, 3044, 3125, 3167, 3226, 3328

OH

3.1, 3.1, 0.0

3896

LM

58.2, 491.2, 538.6

39, 53, 66, 175, 238, 375, 638, 807, 835, 1116, 1213, 1383, 1504, 1514, 1703, 3041, 3082, 3119, 3187, 3784, 4058

TS1

65.2, 193.4, 230.8 i2163, 325, 407, 559, 625, 762, 853, 866, 1078, 1198, 1267, 1281, 1437, 1516, 1581, 1688, 3208, 3231, (65.5, 192.8, 229.5) 3289, 3324, 3938 (i2181, 344, 402, 571, 625, 762, 828, 840, 1084, 1171, 1237, 1250, 1436, 1479, 1545, 1673,

TS2

55.6, 243.1, 276.2

3150, 3164, 3236, 3259, 3821) i1164, 225, 255, 426, 471, 757, 888, 954, 1117, 1173, 1305, 1434, 1494, 1553, 1558, 2255, 3070,

(57.5, 243.2, 277.6)

3125, 3165, 3192, 3992 (i1068, 228, 249, 409, 447, 738, 870, 939, 1081, 1139, 1282, 1404, 1471, 1524, 1555, 2292, 3021, 3078, 3090, 3142, 3880)

a

TS3

54.6, 251.0, 280.5

i1249, 132, 281, 450, 660, 774, 899, 1098, 1183, 1207, 1259, 1397, 1481, 1498, 1537, 2351, 3131, 3208, 3241, 3288, 3996

TS4

49.9, 192.0, 211.3

i2196, 260, 455, 651, 862, 933, 1011, 1083, 1194, 1309, 1423, 1449, 1486, 1524, 1540, 2062, 2283, 3053, 3114, 3183, 3220

TS5 TS6

54.5, 216.0, 247.2 49.0, 189.8, 222.4

i2146, 35, 505, 513, 621, 814, 946, 1108, 1236, 1280, 1316, 1414, 1504, 1515, 1577, 2168, 3050, 3107, 3111, 3215, 3252 i1949, 322, 497, 550, 622, 980, 1014, 1047, 1116, 1240, 1268, 1405, 1440, 1488, 1578, 1641, 1925, 3078, 3292, 3311, 3968

TS7

86.8, 195.7, 240.5

i64, 80, 128, 236, 437, 652, 918, 958, 1114, 1130, 1292, 1421, 1437, 1461, 1537, 1549, 2892, 3052, 3114, 3180, 3192

TS8

40.3, 227.4, 252.2

i1066, 74, 190, 316, 441, 573, 724, 954, 1012, 1082, 1145, 1272, 1442, 1495, 1518, 1536, 2490, 3061, 3131, 3176, 3192

TS9

66.1, 218.0, 252.1

i322, 77, 130, 193, 222, 347, 448, 963, 1035, 1101, 1263, 1323, 1445, 1500, 1521, 1531, 2902, 3080, 3185, 3282, 4031

The numbers in parentheses are predicted at the CCSD/6-311þG(3df, 2p) level of theory.

Table 2. Stationary Point Energies (kcal/mol) Relative to C2H5OH and Comparison with Literature and Experimental Data CCSD(T)//BH&HLYPa

CCSD(T)//CCSDb

G3B3//B3LYPc

G2M//B3LYPd

QCISD(T)/CBSe

ATcTf

exptg

C2H5OH CH3 þ CH2OH

0.0 83.1

0.0 83.3

0.0 84.6

0.0 87.5

0.0 85.6

0.0 85.33 ( 0.11

0.0 85.3 ( 0.4

CH3CH2 þ OH

90.3

90.6

91.5

94.8

92.6

92.05 ( 0.10

92.1 ( 0.4

CH3CHOH þ H

91.2

91.4

93.4

93.7

93.46 ( 0.14

91.5 ( 1.1

CH2CH2OH þ H

98.5

100.3

100.7

100.46 ( 0.14

98.0 ( 2.1

CH3CH2O þ H

99.8

100.7

103.5

104.0

103.93 ( 0.11

101.0 ( 2.1

CH2CH2 þ H2O

10.6

10.3

9.4

9.7

9.33 ( 0.06

9.4 ( 0.3

CH3CH þ H2O

84.0

84.5

84.3

80.3

84.4

84.26 ( 0.27

LM CH4 þ CHOH

80.3 61.3

78.7 62.4

81.3 65.8

81.4 63.0

62.68 ( 0.12

CH3CHO þ H2

13.0

14.1

14.7

15.1

14.79 ( 0.08

14.8 ( 0.3

9.1

10.1

9.1

11.0

10.75 ( 0.06

10.9 ( 0.3

66.6

66.6

66.0

CH4 þ CH2O TS1

67.0

66.4

TS2

81.2

80.7

82.9

81.6

TS3

82.7

84.3

84.6

TS4

86.3

86.0

85.5

TS5 TS6

88.2 106.7

99.7 106.3

89.8

a

This work at the CCSD(T)/6-311þG(3df,2p)//BH&HLYP/6-311þG(3df,2p) level. b This work at the CCSD(T)/6-311þG(3df,2p)//CCSD/6311þG(3df,2p) level. c Reference 10 at the G3B3//B3LYP/6-31G(d) level. d Reference 8 at the G2M//B3LYP/6-311G(d,p) level. e Reference 6 at the QCISD(T)/CBS level. f Reference 6 g Experiemntal values calculated from heats of formation cited from ref 23: ΔfH0(CH3CH2O) = 2.5 ( 2 kcal/ mol from ΔfH298(CH3CH2O) = 6 ( 2 kcal/mol);24 ΔfH0(CH2O) = 25.1 ( 0.1 kcal/mol, ΔfH0(H) = 51.6 kcal/mol, ΔfH0(CH2OH) = 2.5 ( 0.2 kcal/mol, ΔfH0(CH3CH2OH) = 51.9 ( 0.1 kcal/mol, ΔfH0(OH) = 8.9 ( 0.1 kcal/mol, ΔfH0(CH3) = 35.9 ( 0.1 kcal/mol, ΔfH0(CH4) = 15.9 ( 0.1 kcal/mol, ΔfH0(H2O) = 57.1 ( 0.1 kcal/mol, and ΔfH0(CH3CHO) = 37.1 ( 0.2 kcal/mol from ΔfH298(CH3CHO) = 39.6 ( 0.2 kcal/mol);25 ΔfH0(C2H5) = 31.2 ( 0.2 kcal/mol from ΔfH298(CH3CH2) = 28.7 ( 0.2 kcal/mol);26 ΔfH0(CH3CHOH) = 11.3 ( 3.0 kcal/mol from ΔfH298(CH3CHOH) = 14.5 ( 3.0 kcal/mol);27 ΔfH0(CH3CHOH) = 12.0 ( 1.0 kcal/mol and ΔfHo0 (CH2CH2OH) = 5.5 ( 2.0 kcal/ mol);28 ΔfHo0 (CH2CH2) = 14.6 ( 0.1 kcal/mol). 3512



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considered: CH3 CHOH þ H f CH3 CHO þ H2

f CH2 CHOH þ H2 fCH3 CH2 OH f decomposition products ðbÞ as depicted in Figure 2B. Obviously, parts A and B of Figure 2 are essentially the same except that the relative energies of all stationary points in Figure 2B are shifted upward by 8.6 kcal/ mol, which is the difference in the heats of formation of CH3CH2O and CH3CHOH. Reaction a: CH3CH2O þ H. The first reaction channel of this reaction system refers to the barrierless association process from CH3CH2O þ H to CH3CH2OH with loose transition states. It is a strongly exothermic process and the enthalpy change for the association reaction is predicted to be 99.8 kcal/mol, which is in good agreement with the value of 101.0 ( 2.1 kcal/mol, evaluated from the experimental heats of formation.23,24 From Figure 2A, one can see that there are 10 subchannels for further decomposition reactions following the CH3CH2OH formation, including four loose transition state processes and six tight transition state ones. Except CH3CHOH þ H as a potential product pair from the decomposition of CH3CH2OH, others were discussed in the previous paper published by Park et al. 8 in 2002. In the present paper, we will only have a brief discussion on them on the basis of the CCSD(T)/6-311þ(3df,2p)//BH&HLYP/6-311þ(3df,2p) energies. Four loose transition state subchannels occur by direct bond breakings from CH3CH2OH to the products CH3 þ CH2OH, CH3CH2 þ OH, CH3CHOH þ H, and CH2CH2OH þ H with the breaking bonds of CC, CO, CRH, and CβH, respectively. These four product pairs are predicted to be lower in energy than the CH3CH2O þ H reactants by 16.7, 9.5, 8.6, and 1.3 kcal/mol, respectively. From the experimental heats of formation,2327 the heats of reaction for formation of the same product pairs given above are, respectively, 15.7 ( 2.2, 9.0 ( 2.3, 8.8 ( 4.0, and 3.0 ( 4.0 kcal/mol, which agree very well with the theoretical values. Of the tight TS decomposition subchannels, the formation of CH2CH2 þ H2O has the lowest transition state (TS1) with the energy of 32.8 kcal/mol relative to that of the reactants. TS1 is a four-membered ring transition state with an H atom migrating from the methyl to the hydroxyl group. The second one occurs via a transition state (TS2) and a hydrogen-bonded complex (LM) to form the CH3CH þ H2O products. TS2 and LM lie below the reactants by 18.6 and 19.5 kcal/mol, respectively. TS2 is a three-membered ring transition state with the CRO bond breaking and an R-H atom migrating from the CR to the O atom. The third, fourth, and fifth subchannels connect the product pairs, CHOH þ CH4, CH2CHO þ H2, and CH2O þ CH4, with TS3, TS4, and TS5, located at 17.1, 13.5, and 11.6 kcal/ mol, respectively, relative to the reactants. TS3 is a threemembered ring transition state, while both TS4 and TS5 have four-membered ring structures. The sixth decomposition subchannel has the highest transiton state (TS6) with a fourmembered ring transition state lying above the reactants by 6.9 kcal/mol. Comparing with the previous study by Park et al.8 optimized at the B3LYP/6-311G(d,p) level theory, the geometric parameters of these transition states are very close. In

Figure 3. Potential energy curves of the loose transiton-sate processes calculated at the CASPT2(8,8)/6-311þG(3df,2p)//CAS(8,8)/ 6-311þG(3df,2p) level of theory: v1, CH3CH2OH f CH3CH2O þ H; v2, CH3CH2OH f CH3CHOH þ H; v3, CH3CH2OH f CH2OH þ CH3; v4, CH3CH2OH f CH3CH2 þ OH.

addition, more reliable calculations have been carried out for the loose transiton state processes and the two important product channels via TS1 and TS2 at the CCSD(T)/6-311þ G(3df,2p)//CCSD/6-311þG(3df,2p) level of theory, as shown in Figure 1 for the geometric parameters and Figure 2 for the relative energies. These results agree closely with those predicted at the CCSD(T)/6-311þG(3df,2p)//BH&HLYP/6311þG(3df,2p) level. Additionally, on the basisof the experimental heats of formation from the literature,23,24,28 the three heats of reaction for CH3CH2O þ H f CH2CH2 þ H2O, CH3CH2O þ H f CH3CHO þ H2, and CH3CH2O þ H f CH2O þ CH4, evaluated to be 91.6 ( 2.1, 86.2 ( 2.2, and 90.1 ( 2.1 kcal/mol, respectively, are in close agreement with the theoretical values computed at the CCSD(T)/ 6-311þG(3df,2p)//BH&HLYP/6-311þG(3df,2p) level shown in Figure 2A, 89.2, 86.2, and 90.7 kcal/mol, respectively. The discussion reflects the reliability of the computational method employed for the present reaction systems. Besides the CH3CH2O þ H association channel, the direct hydrogen abstraction reaction can take place via the transiton state TS7 with the H radical attacking one of the H atoms of the CH2 group. At TS7, the distance of HH is 2.053 Å and the CH breaking bond elongates to 1.102 Å. The imaginary frequency is predicted to be only i64 cm1 and the forward barrier is 1.0 kcal/mol. It implies that the potential energy surface in the vicinity of TS7 is quite flat and the structure of TS7 is reactant-like. For the sake of kinetic calculations, the five loose transition state processes CH3 CH2 OH f CH3 CH2 O þ H f f f f

CH3 CHOH þ H CH3 þ CH2 OH CH3 CH2 þ OH CH2 CH2 OH þ H

ðc1Þ ðc2Þ ðc3Þ ðc4Þ ðc5Þ

have been optimized by the multiconfigurational self-consistent field theory (CASSCF)29,30 with eight active electons and eight active orbitals with the 6-311þG(3df,2p) basis set. By manually 3513


The Journal of Physical Chemistry A Scheme 1. Processes a

Scheme 2. Processes b

varying the lengths of the breaking bonds from CH3CH2OH, we can obtain the minimum energy paths for the loose transition states of the dissociation processes. To get more reliable potentials, the single-point energies are corrected by the second-order multireference perturbation theory (CASPT2)31 based on the CASSCF geometries. Both CASSCF and CASPT2 are performed by the Molpro code.32 The potential curves are shown in Figure 3. They can be fitted into the Morse functions for the dissociation reactions, Vc1 ðRO H Þ ¼ 110:0f1 exp½ 2:973ðR 1:03Þg2 Vc2 ðRC H Þ ¼ 100:3f1 exp½ 1:870ðR 1:11Þg2 Vc3 ðRC C Þ ¼ 91:6f1 exp½ 2:338ðR 1:59Þg2 Vc4 ðRC O Þ ¼ 98:3f1 exp½ 2:075ðR 1:45Þg2 Vc5 ðRC H Þ ¼ 108:6f1 exp½ 1:936ðR 1:11Þg2 in units of kilocalories per mole; they are employed for variational transition state rate constant calculations. Reaction b: CH3CHOH þ H. Figure 2B illustrates the potential energy surface of this reaction system. As aforementioned, Figure 2B is essentially the same as Figure 2A, except that the reference energy has been shifted downward by 8.6 kcal/mol and the direct hydrogen abstraction channels are different. The barrierless association process from CH3CHOH þ H to CH3CH2OH generates 91.2 kcal/mol of internal energy. The minimum energy path of this association process is shown in Figure 3. The subsequent reaction steps for the decomposition of the internally excited CH3CH2OH are no longer repeated here. The two hydrogen abstraction channels considered in the CH3CHOH þ H system are shown in Figure 2B. In the first

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channel, the H atom abstracts the H from the hydroxyl group via transition state, TS8, at which the HO breaking bond and the HH forming bond are 1.400 and 1.006 Å, respectively. The imaginary frequency of TS8 is i1066 cm1, and its potential energy is 3.2 kcal/mol above that of the reactants. The second hydrogen abstraction channel goes via TS9 with a significantly lower barrier of 0.8 kcal/mol. The H atom abstracts one of three H atoms from the methyl group. The imaginary frequency of TS9 is i322 cm1, and its Cβ-H breaking bond and the HH forming bond are 1.905 and 1.098 Å, respectively. The latter process is apparently favored over the former because of the lower energy barrier, looser TS structure, and greater statistical factor as well. 3.2. Rate Constant Calculations. On the basis of the potential energy surfaces discussed above, the various product channels of the H atom reactions with CH3CH2O and CH3CHOH can be mechanistically presented in Schemes 1 and 2 for the association/decomposition and direct abstraction processes.In both systems, the direct abstraction reactions a10 (Scheme 1) and b10 and b11 (Scheme 2) are pressure-independent, whereas the association/decomposition reactions (a2a9 (Scheme 1) and b2b9 (Scheme 2)) compete with each other as well as with the collisional deactivation processes (a1 (Scheme 1) and b1 (Scheme 2)), which give rise to the pressure dependency of all product channels deriving from the decomposition of the chemically activated CH3CH2OH*. The extent of pressure effect in each decomposition channel depends on the relative magnitude of the sum of all specific decomposition rate constants, ∑ki(Ei(), and the effective deactivation rate constant, aproximated with the L-J collision rate. The largest pressure dependence of ki(T) (i = 29) appears when both quantities are comparable. At the low-pressure limit, the deactivation process (a1 or b1) is too small to compete with the decomposition processes, the rate constants for the association/decomposition product channels ki(T) (i = 29) become pressure-independent; similarly, at the high-pressure limit, both C2H5O1 þ H reactions are also pressure-independent and dominated by the formation of C2H5OH through collisonal deactivation. The values of the highP limit rate constants represent the association processes controlled by their entropy changes during the association reactions. For the above two reaction systems, the transition-state theory is applied to predict the rate constants for the abstraction processes (a10, b10, and b11) and the variational RRKM theory is employed for prediction of the multichannel association/ decomposition rate constants (a1a9 and b1b9) by solving the master equation in the temperature range of 1002000 K and pressure range of 104 Torr to 104 atm. The Morse potentials presented above are used to represent the minimum energy paths of the loose transtion states for the association and decomposition processes. Process a: CH3CH2O þ H f Products. The rate constants for formation of individual products of the CH3CH2O þ H reaction are presented in Figure 4. As seen from the figure, ka1(C2H5OH) for the formation of CH3CH2OH by collisional deactivation is strongly pressure-dependent with a pattern opposite to that of the decomposition processes (a2a9) because of the competitive effect of the stabilization vs decomposition as alluded to above. At lower pressures, ka1(C2H5OH) becomes smaller and less competitive; at pressures over 1 atm, it is approaching the high-pressure limit at T < 500 K. Also, ka1(C2H5OH) exhibits a strong negative temperature dependence due to the decrease in the collisional deactivation rate at higher T’s. All of the rate constants for the decomposition processes exhibit noticeable 3514



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Figure 4. Predicted rate constants for each individual product channel of the CH3CH2O þ H reaction at 1002000 K and 104 Torr to 103 atm.

positive-temperature and negative-pressure dependencies, as shown in Figure 4 (a2a9), reflecting the stabilization vs decomposition competitive effect. These rate constants become negligibly small at high pressures and low temperatures. At low pressures, their values approach the limits which represent

the thermally averaged product of the microscopic (energydependent) association and product-branching ratios. Of these decomposition channels, ka2(CH3þCH2OH) is predominant because of its loose transition state and lower dissociation energy. In this reaction system, the rate constant for the direct hydrogen 3515



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Table 3. Three-Parameter Arrhenius Expressions of Predicted Individual Product Rate Constants (cm3 molecule1 s1) channela

T/K

a2

100500

1 Torr 4.96 1013T0.893 exp(8.7/T)

1 atm

100 atm

2.26 1012T0.701 exp(174/T)

4.25 106T1.05 exp(2582/T)

5002000 a3

100500

2.06 10

16 1.46

2.95 10

23 3.28

1.05 10

12 0.191

T

exp(28.8/T)

9.04 10

16 1.27

1.33 10

22 3.10

5.64 10

11 0.738

T

exp(157/T)

100500

T

exp(321/T)

T

exp(142/T)

a6

100500 5002000 100500

T

exp(70.7/T)

9 0.935

exp(501/T) 2.67 10 T 8.34 1020T1.86 exp(165/T)

T

exp(262/T)

10 0.813

9.95 10 T exp(359/T) 3.66 1019T1.67 exp(9.0/T)

100500

1.51 10

18 1.51

2.74 10

21 1.98

T

exp(114/T)

6.75 10

18 1.32

1.25 10

20 1.78

T

exp(37.8/T)

100500

T

exp(192/T)

T

exp(40.7/T)

100500

b2

5002000 100500

3.01 1022T2.40 exp(249/T) 7.94 1012T0.319 exp(67.8/T)

5002000 b3

7.06 1031T5.26 exp(357/T)

1.32 1021T2.21 exp(90.5/T)

9.25 1032T5.66 exp(399/T)

3.80 1023T4.35 exp(212/T)

1.27 1018T1.42 exp(1531/T) 4.57 1033T7.36 exp(660/T)

1.44 107T0.891 exp(1461/T)

86.3T3.27 exp(5756/T)

100500

1.01 1021T3.11 exp(42.6/T)

2.08 1030T6.26 exp(124/T)

4.66 1042T9.95 exp(661/T)

5002000

4.16 1011T0.272 exp(1699/T)

4.02 109T0.830 exp(2414/T)

2.35T3.22 exp(6428/T) 3.03 1027T4.30 exp(4411/T)

b4

1002000

1.01 1023T2.15 exp(4095/T)

4.95 1023T2.94 exp(4266/T)

b5

100500

1.41 109T0.888 exp(111/T)

5.04 1018T2.15 exp(10.5/T)

1.00 1025T4.31 exp(333/T)

5002000

3.44 106T2.07 exp(342/T)

7.81 103T3.02 exp(1432/T)

4.18 103T4.49 exp(5266/T)

b6

100500 5002000

2.66 1019T1.47 exp(148/T)

1.09 1027T4.50 exp(230/T) 2.30 1018T1.23 exp(340/T)

5.18 1038T7.67 exp(700/T) 4.59 109T1.31 exp(4688/T)

b7

100500

2.50 1017T0.900 exp(85.9/T)

5.83 1026T4.01 exp(191/T)

8.88 1036T6.98 exp(633/T)

5002000 b8

100500

1.43 10

20 1.51

T

exp(189/T)

5002000 b9

100500

2.01 10

22 2.20

T

exp(260/T)

5002000 a

9.25 1029T5.01 exp(328/T)

9.60 1018T1.02 exp(1550/T)

5002000 a9

4.35 102T2.98 exp(3042/T) 7.44 1030T5.31 exp(360/T)

6.52 1015T0.532 exp(1659/T)

5002000 a8

1.17 1022T3.61 exp(170/T)

2.28 1016T0.945 exp(1570/T)

5002000 a7

9.68 1034T6.89 exp(519/T) 4.76 1018T1.85 exp(1836/T)

5002000 a5

2.33 1029T5.92 exp(407/T) 4.34 108T0.984 exp(2883/T)

5002000 a4

5.15 1025T5.02 exp(324/T)

4.10 10

18 1.15

3.66 10

28 4.27

7.42 10

21 1.62

6.02 10

30 4.94

5.56 10

22 2.10

T T T T T

exp(134/T)

1.30 107T1.72 exp(4705/T)

exp(219/T)

2.39 1038T7.38 exp(682/T)

exp(5.4/T)

2.41 1011T0.969 exp(4414/T)

exp(279/T)

8.42 1041T8.30 exp(778/T)

exp(107/T)

3.11 1012T0.568 exp(4487/T)

Channels a2a9, Scheme 1; channels b2b9, Scheme 2.

Figure 5. Predicted branching ratios for the CH3CH2O þ H reaction at 1 Torr, 1 atm, and 100 atm.

abstraction channel, ka10 (CH3CHOþH2), shows a strong positive temperature dependence due to its positive reaction barrier. The products of this channel are the same as those of (a8); however, ka8(CH3CHOþH2 ) is much smaller than ka10(CH3CHOþH2 ) because of the tight transition state, TS4, of (a8) and the small barrier for the direct hydrogen abstraction.

The high- and low-pressure limits of ka1(CH3CH2OH) and the direct abstraction rate constant ka10(CH3CHOþH2) can be expressed by the three-parameter Arrhenius equations: k¥ a1 ðCH3 CH2 OHÞ=ðcm3 molecule1 s1 Þ ¼ 5:11 1013 T 0:894 expð 6:5=TÞ 3516

ð100 2000 KÞ



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Figure 6. Predicted rate constants for each individual product channel of the CH3CHOH þ H reaction at 1002000 K and 104 Torr to 103 atm.

ka10 ðCH3 CHO þ H2 Þ=ðcm3 molecule1 s1 Þ

k0 a1 ðCH3 CH2 OHÞ=ðcm6 molecule2 s1 Þ ¼ 1:69 1028 T 5:39 expð 482=TÞ ð100 500 KÞ ¼ 1:04 104 T 15:55 expð 5590=TÞ ð500 2000 KÞ

¼ 1:24 1014 T 1:15 expð 339=TÞ ð100 2000 KÞ 3517



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Figure 7. Predicted braching ratios for the CH3CHOH þ H reaction at 1 Torr, 1 atm, and 100 atm.

Figure 8. Predicted individual product rate constants and total rate constant (ka,total) for the CH3CHOH þ H reaction in comparison with experimental data: (a) Ref 3; (b) Ref 4.

Table 3 lists the predicted rate constants for other individual product channels at 1 Torr and 1 and 100 atm. Figure 5 displays the branching ratios of the individual product channels of the CH3CH2O þ H system. Only three product channels dominate distinctively. At low pressures (P < 100 atm) and high temperatures (T > 500 K), the formation of CH3 þ CH2OH dominates the reaction, whereas at high pressures and low temperatures, the formation of the stabilization product, C2H5OH, becomes most important. The pressure independent abstraction reaction becomes competitive above 500 K with an increasing significance at higher temperatures. Process b: CH3CHOH þ Hf Products. Figure 6 shows the predicted rate constants for this reaction system at 1002000 K and 104 Torr to 103 atm. The temperature and pressure effects are similar to those discussed above for the isomeric CH3CH2O þ H reaction due to the weaker secondary CH bond strength by 8.6 kcal/mol. The energies of the transition states (TS1TS5) and products relative to the CH3CHOH þ H reactants decrease with concomitantly less excess energies for decomposition reactions b2b9 than those in the CH3CH2O þ H system. Accordingly, the rate constants for (b2b9) exhibit stronger pressure effects as one would expect. Between the two abstraction reactions, b10 and b11, the latter process producing CH2CHOH is much greater because of its significantly smaller reaction barrier, looser TS structure, and greater reaction path degeneracy as discussed in the preceding section.

Figure 9. (A) Predicted high-pressure rate constants and (B) predicted low-pressure rate constants for ethanol decompostion reaction.

The three-parameter Arrhenius equations for the highand low-pressure limits of kb1(CH3CH2OH) and the direct abstraction rate constants kb10(CH3CHOþH2) and kb11(CH2CHOHþH2) can be represented by k¥ b1 ðCH3 CH2 OHÞ=ðcm3 molecule1 s1 Þ ¼ 2:05 1012 T 0:529 expð15:5=TÞ ð100 500 KÞ ¼ 5:99 1011 T 0:060 expð 220=TÞ ð500 2000 KÞ k0 b1 ðCH3 CH2 OHÞ=ðcm6 molecule2 s1 Þ

3518

¼ 1:19 1022 T 5:86 expð 493=TÞ

ð100 500 KÞ

¼ 7:63 108 T 15:72 expð 5396=TÞ

ð500 2000 KÞ


5.93 10139T37.11 exp(56360/T) exp(26330/T) 1.52 10 T exp(19712/T) 2.43 10 T exp(18429/T) 1.37 10 T

9.65 106T1.85 exp(31479/T)

1.04 10139T36.84 exp(57719/T) exp(30195/T)

exp(34618/T)

93 25.25

1.46 10 T exp(37540/T)

82 22.62

3.61 10 T exp(38844/T) 4.24 10 T

80 22.11

21 2.42

1.06 10 T exp(23432/T) 3.31 10 T exp(21848/T)

34 6.46 40 8.30

8.86 10 T

3.89 1030T4.15 exp(54202/T)

1.37 10120T32.01 exp(43410/T) exp(27537/T)

exp(57331/T) 6.63 10 T exp(59735/T)

98 26.48 87 23.94

1.25 10 T exp(57618/T) 7.17 10 T

84 23.35

44 8.44

1.03 10 T exp(22619/T) 6.52 10 T exp(21421/T)

56 11.86 60 13.32

5.00 10 T

3.01 1095T25.47 exp(27384/T)

3.58 1045T8.41 exp(52071/T) exp(55344/T) 3.45 10 T exp(57388/T)

95 25.73 86 23.77

1.77 10 T exp(58124/T) 2.36 10 T

84 23.30

61 13.30

1.47 1086T23.31 exp(21917/T)

72 16.61

6.02 1082T22.61 exp(20151/T)

76 17.91

6.42 1081T22.43 exp(19702/T)

2.40 1026T3.13 exp(49453/T)

2.04 10151T39.93 exp(66327/T) 1.03 1062T13.25 exp(52214/T) 5.92 10 T exp(32886/T) 4.51 1076T17.82 exp(54919/T) 2.91 10 T exp(24860/T) 3.55 1084T20.35 exp(56162/T) exp(22811/T) 1.47 10 T 1.89 1087T21.25 exp(56525/T)

101 27.36

1.02 1040T7.23 exp(52491/T)

89 24.35

2.38 1051T10.67 exp(54958/T)

86 23.58

2.60 1056T12.20 exp(56025/T)

exp(52902/T)

8.44 10215T56.64 exp(110943/T)

1.08 10 T exp(55397/T)

2.73 10136T36.31 exp(56797/T)

2.27 10 T exp(57530/T) 1.95 10 T exp(58478/T)

6.31 10 T

5.53 10108T29.32 exp(38225/T)

10 atm

35 5.70 45 8.71

2 atm 1 atm

49 10.08

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kb10 ðCH3 CHO þ H2 Þ=ðcm3 molecule1 s1 Þ ¼ 2:26 1015 T 1:29 expð 1421=TÞ

ð100 2000 KÞ

kb11 ðCH2 CHOH þ H2 Þ=ðcm3 molecule1 s1 Þ ¼ 8:13 1016 T 1:70 expð 296=TÞ ð100 2000 KÞ The predicted rate constants for other product channels are listed in Table 3. The branching ratios of the individual product channels of the CH3CHOH þ H system are shown in Figure 7. Similar to the isomeric CH3CH2O þ H reaction, only three product channels dominate noticeably—the direct abstraction reaction giving CH2CHOH and the competitive deactivation and decomposition producing C2H5OH and CH3 þ CH2OH, respectively. At low pressures and high temperatures, the formation of CH3 þ CH2OH dominates; conversely at high pressures and low temperatures, the formation of the stabilization product, C2H5OH, is dominant, while the pressure-independent abstraction reaction becomes competitive at higher temperatures. Figure 8 presents the predicted total (ktotal) and individual product rate constants for the CH3CHOH þ H reaction at 1 Torr pressure in comparison with available experimental data. ktotal is very close to kb2(CH3þCH2OH) at T < 1000 K and kb11(CH2CHOHþH2) at T > 1000 K. Other individual product channels are seen to have relatively minor contributions to the total rate constant. Two experimental data, 8.3 1011 and (3.32 ( 1.66) 1011 cm3 molecule1 s1, obtained at 295 K and ∼1 Torr by Bartels et al.3 and Edelbuttel-Einhaus et al.,4 respectively, agree reasonablely with the predicted value, 4.94 1011 cm3 molecule1 s1. Process c1c: Thermal Unimolecular Decomposition of C2H5OH. Six channels of the unimolecular decomposition reaction of ethanol, including reactions c1c5 and CH3 CH2 OH f CH2 CH2 þ H2 O

ðc6Þ

have been considered. Figure 9 shows the high- and low-pressure limits in the temperature range of 5003000 K. The high-P limit rate constants are all with large positive temperature dependences, reflecting their high intrinsic transition state energies. But the low-P limit rate constants become negatively temperature dependent at high temperatures (T > 1500 K) because of the reduced collisional activation efficiency. Reaction c6 is competitive at lower temperatures because of its lower reaction barrier; however, the reactions c3 and c4 become competitive at higher temperatures because of their loose transition states. The rate constants of the H-atom elimination processes (c1, c2, and c5) are relatively very small attributable to their large endothermicities. These high- and low-pressure rate constants have been fitted to three parameters Arrhenius expressions as follows:

18003000

5001800 c6

20003000

5002000 c5

18003000

18003000

5001800 c4

c3

20003000 5001800

5002000 c2

20003000

5002000 c1

T/K

k¥ c1 =s1

channel

Table 4. Three-Parameter Arrhenius Expressions of Predicted Individual Product Rate Constants (s1) for Decomposition of Ethanol

2.66 10101T27.51 exp(33372/T)

24 2.27

100 atm


¼ 2:70 1015 T 0:305 expð 50998=TÞ k c1 =ðcm3 molecule1 s1 Þ

ð500 3000 KÞ

0

¼ 4:64 1064 T 19:76 expð 60865=TÞ ð500 1800 KÞ ¼ 7:05 1067 T 23:49 expð 24877=TÞ ð1800 3000 KÞ k¥ c2 =s1 ¼ 7:54 1016 T 0:275 expð 47326=TÞ 3519

ð500 3000 KÞ



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Figure 10. Predicted braching ratios for ethanol decompostion reaction at (A) 1, (B) 2, (C) 10, and (D) 100 atm.

k0 c2 =ðcm3 molecule1 s1 Þ ¼ 1:28 1073 T 22:47 expð 58817=TÞ ð500 1800 KÞ

k0 c5 =ðcm3 molecule1 s1 Þ ¼ 8:14 1070 T 21:65 expð 61954=TÞ ð500 1800 KÞ

¼ 9:26 1058 T 21:20 expð 19034=TÞ ð1800 3000 KÞ

¼ 4:51 1059 T 21:47 expð 19345=TÞ ð1800 3000 KÞ

k¥ c3 =s1

k¥ c6 =s1

¼ 2:10 1023 T 1:45 expð 44192=TÞ ð500 3000 KÞ k c3 =ðcm3 molecule1 s1 Þ

¼ 2:62 105 T 2:36 expð 31248=TÞ ð500 3000 KÞ k0 c6 =ðcm3 molecule1 s1 Þ ¼ 2:13 1073 T 23:41 expð 43929=TÞ ð500 1500 KÞ

0

¼ 6:45 1051 T 16:29 expð 49158=TÞ

ð500 1500 KÞ

¼ 2:07 1091 T 29:66 expð 38972=TÞ ð1500 3000 KÞ

k¥ c4 =s1 ¼ 2:14 1027 T 2:88 expð 48196=TÞ ð500 3000 KÞ k0 c4 =ðcm3 molecule1 s1 Þ ¼ 1:74 1059 T 18:35 expð 54407=TÞ ð500 1800 KÞ ¼ 1:93 1086 T 27:89 expð 38509=TÞ ð1800 3000 KÞ

k¥ c5 =s1 ¼ 2:01 1017 T 0:149 expð 51243=TÞ ð500 3000 KÞ

¼ 3:88 1054 T 20:15 expð 15965=TÞ ð1500 3000 KÞ The rate constants of the six decomposition channels of ethanol at 1, 2, 10, and 100 atm Ar pressures have been calculated and listed in Table 4 in terms of three-parameter Arrhenius expressions. Figure 10 shows the branching ratios of these decomposition channels for the four specified pressures. Clearly the largest branching ratios belong to those of c6 and c3 at T < 900 K and in the 10002000 K range, respectively, while other decomposition channels have much smaller branching ratios because of their greater dissociation energies. With the temperature increasing over 2000 K, the branching ratios of c3 and c6 decrease to about 0.25 and 0.10, respectively, while those of c1, c2, and c5 rapidly go up to about 0.24, 0.20, and 0.10, respectively. However, the branching ratio of c4 producing the 3520



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be reasonably reliable and are recommended for kinetic modeling on the combustion of this important green fuel system over a wide range of conditions.

Figure 11. Comparison of the predicted constants with those in literature for the decomposition reactions of (A) CH3CH2OH f CH3 þ CH2OH and (B) CH3CH2OH f CH2CH2 þ H2O at infinite high pressure: (a) ref 10; (b) ref 9; (c) ref 5; (d) ref 8.

CH3CH2 þ OH products rises to the maximum of about 0.19 at about 1800 K and declines to about 0.10 at T > 2000 K. This result implies that the dominant decomposition products of CH3CH2OH are CH2CH2þH2O in the lower temperature range and CH3 þ CH2OH in the mediam temperature range, but at T > 2000 K the decomposition reaction gives a mixture of products from those six channels. We have also calculated the rate constant and product branching ratio for reaction c4 on the basis of the experimental dissociaiton energy, 92.1 kcal/mol, which is higher than our predicted value by 1.8 kcal/mol, as shown in Figure 10 by the blue dashed line (labeled as c40 ). At 1500 K, kc4 is about 1.5 times greater than kc40 . However, at T > 2000 K, both values are almost same. In Figure 11 we compare the predicted rate constants for c3 and c6 with available experimental data and theoretically predicted results in the infinite high-pressure limit at 7002500 K. For CH3CH2OH f CH3 þ CH2OH (c3) shown in Figure 11A, our predicted rate constant lies between those of Park et al. predicted at the G2M//B3LYP/6-311G(d,p) level of theory8 and Li and co-workers obtained by the G3B3//B3LYP/6-31G(d, p) method.10 The deviations are within 25%. However, the rate constants recommended by Tsang9 and Marinov5 are larger by 2.61.8 and 2.58.5 times, respectively. For CH3CH2OH f CH2CH2 þ H2O (c6), our predicted rate constant is in good agreement with that of Park et al.8 but is slightly smaller than other authors’ results5,9,10 by a factor of 1.54.3, as shown in Figure 11B. As aforementioned, the predicted rate constants at 1, 2, 10, and 100 atm Ar pressures are listed in Table 4; these and the high- and low-pressure limits presented above are believed to

4. CONCLUSIONS Reactions of CH3CH2O and CH3CHOH with H atoms to form various products by direct H-abstraction and the association/decomposition processes, including collisional deactivation giving CH3CH2OH and decomposition reactions taking place by dehydration, dehydrogenation, and CC, CH, and OH bond-breaking from the excited CH3CH2OH, have been characterized in detail by CCSD(T)/6-311þG(3df,2p)//BH& HLYP/6-311þG(3df,2p) calculations. The results of VTST and/or VRRKM calculations using a multichannel master equation show that both reaction systems are dominated by the two competitive association/decomposition processes producing CH3CH2OH by collisonal deactivation at high pressure and low temperature and CH3 þ CH2OH at low pressure and high temperature, with increasing contributions from the pressureindependent direct H-abstraction reactions at higher temperatures, particularly in the CH3CHOH þ H system which generates CH2CHOH þ H2 as major products under practical combustion conditions. The total rate constant for the CH3CHOH þ H reaction predicted at 295 K and 1 Torr pressure was found to be in close agreement with experimental results. We have also predicted the rate constants for the thermal unimolecular decomposition of CH3CH2OH by dehydration and CC, CH, and OH bond-breaking processes. For the former two low-energy decomposition reactions producing C2H4 þ H2O and CH3 þ CH2OH, respectively, the computed rate constants compare quite reasonably with available data in the literature. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses †

School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907.

’ ACKNOWLEDGMENT We gratefully acknowledge the support from the Basic Energy Sciences, Department of Energy, under Contract No. DE-FG0297-ER14784, for the preliminary study of this work by KX at Emory University as a summer student. Z.F.X. and M.C.L. thank the National Science Council of Taiwan (NSCT) for support for completion of this work at National Chiao Tung University (NCTU). M.C.L. also acknowledges NSCT and TSMC (Taiwan Semiconductor Manufacturing Co.) for support of a distinguished visiting professorship at NCTU, Hsinchu, Taiwan. ’ REFERENCES (1) Marshal, E. Science 1989, 246, 199. (2) Warnatz, J. In Combustion Chemistry; Gardiner, W. C., Ed.; Springer-Verlag: New York, 1984; p197. (3) Bartels, M.; Hoyermann, K.; Sievert, R. Symp. (Int.) Combust., [Proc.] 1982, 19, 61. 3521


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P.; Pfl€uger, K.; Pitzer, R.; Reiher, M.; Schumann, U.; Stoll, H.; Stone, A. J.; Tarroni, R. ; Thorsteinsson, T.; Wang, M.; Wolf, A. MOLPRO, Version 2009.1, a package of ab initio programs; 2009; www.molpro.net.

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Thermal Decomposition of Ethanol. 4. Ab Initio Chemical Kinetics for

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