Reaction Mechanism of CH + C3H6: A ... - American Chemical Society

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Reaction Mechanism of CH + C3H6: A Theoretical Study Yan Li, Hui-ling Liu, Zhong-Jun Zhou, Xu-ri Huang,* and Chia-chung Sun State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, Changchun 130023, People’s Republic of China ReceiVed: March 5, 2010; ReVised Manuscript ReceiVed: July 7, 2010

A detailed theoretical study is performed at the B3LYP/6-311G(d,p) and G3B3 (single-point) levels as an attempt to explore the reaction mechanism of CH with C3H6. It is shown that the barrierless association of CH with C3H6 forms two energy-rich isomers CH3-cCHCHCH2 (1), and CH2CH2CHCH2 (4). Isomers 1 and 4 are predicted to undergo subsequent isomerization and dissociation steps leading to ten dissociation products P1 (CH3-cCHCHCH + H), P2 (CH3-cCCHCH2 + H), P3 (cCHCHCH2 + CH3), P4 (CH3CHCCH2 + H), P5 (cis-CH2CHCHCH2 + H), P6 (trans-CH2CHCHCH2 + H), P7 (C2H4 + C2H3), P8 (CH3CCH + CH3), P9 (CH3CCCH3 + H) and P12 (CH2CCH2 + CH3), which are thermodynamically and kinetically possible. Among these products, P5, P6, and P7 may be the most favorable products with comparable yields; P1, P2, and P3 may be the much less competitive products, followed by the almost negligible P4, P8, P9, and P12. Since the isomers and transition states involved in the CH + C3H6 reaction all lie lower than the reactant, the title reaction is expected to be fast, which is consistent with the measured large rate constant in experiment. The present study may lead us to a deep understanding of the CH radical chemistry. 1. Introduction The methylidyne radical, CH plays an important role in various fields including combustion chemistry,1,2 interstellar chemistry,3,4 and planetary atmosphere chemistry.5 It has been detected in the interstellar medium,6–8 comets,9 stellar atmospheres,10,11 and flames.12 Furthermore, the CH radical is one of the most reactive species since the C atom of CH possesses a singly occupied and a vacant nonbonding molecular orbital. Up to now, a large number of experimental and theoretical investigations have been reported on the spectroscopic properties13–19 and formation enthalpy20 of CH as well as its reactions, such as those with O2,21 N2,22–25 CH,26 NH3,27 H2O,27 HF,27 H2S,28 CH4,28–30 C2H6,30–32 C3H8,30 C4H10,30 C5H12,30,31 C2H4,31,33,34 C3H6,29,34 C4H8,34 C2H2,31,33–38 CH3CCH,29,34 and CH2CCH2.29,33,34 Both CH and C3H639 have been found in the interstellar medium, and the reaction between them may play a potential role in synthesizing new interstellar molecules. Thus, the reaction of CH with C3H6 attracts our great interest. However, to the best of our knowledge, only two experimental studies have been reported in recent years. In 2005, Daugey et al.29 examined the title reaction by using a supersonic flow reactor combined with pulsed laser photolysis (PLP) and laser-induced fluorescence (LIF) technique. The obtained rate constant is k ) (3.86-4.58) × 10-10cm3 molecule-1 s-1 over the temperature range of 77-170 K. In 2008, Loison et al.34 investigated the same reaction at room temperature, in a low-pressure fast-flow reactor. The obtained rate constant at 300 K is k ) (4.2 ( 0.8) × 10 cm3 molecule-1 s-1. In view of the large rate constant obtained by Daugey et al. and Loison et al., the title reaction may take place very fast and may provide a way to synthesize a long carbon chain. Furthermore, based on the measured hydrogen branching ratios of 0.78 ( 0.10, Loison et al. suggested that H-elimination channel is the major process for the title reaction. However, it is difficult to discuss the * To whom correspondence should be addressed. E-mail: xurihuang@ gmail.com.

mechanism of the title reaction without detailed potential energy surface investigation. Therefore, in the present paper, we performed a detailed theoretical calculation on the reaction of CH with C3H6. The present study aims to provide elaborated isomerization and dissociation pathways and thereby to deeply understand the mechanism of the title reaction. 2. Theoretical Methods All the calculations are performed using the Gaussian 03 program package.40 The geometries and harmonic frequencies of the reactant, products, isomers and transition states are calculated at the B3LYP/6-311G(d,p) level. Connections of the transition states between designated isomers are confirmed by intrinsic reaction coordinate (IRC) calculations at the same level of theory. The single-point energy calculations are performed at the G3B3 level using the B3LYP/6-311G(d,p)-optimizedgeometries and scaled B3LYP/6-311G(d,p)-zero-point energies. The G3B3 method comprises a series of high-level single-point energy calculations and spin-orbital correction, thus it can provide accurate energetics.41,42 The average absolute deviation from experiment for the G3B3 energies for 299 test chemicals is reported to be 0.99 kcal/mol. Our choice of B3LYP is motivated by its good performance for the geometry optimizations and prediction of vibrational frequencies for isomers and transition states based on many previous studies.43–45 Furthermore, B3LYP is able to suppress effectively the problem of spin contaminant. Moreover, for the current reaction system, the 6-311G(d,p) basis set is a balance choice in consideration of computational efficiency and accuracy. 3. Results and Discussion The optimized structures of reactant and products are shown in Figure 1, while the optimized structures of isomers and transition states are shown in Figures 2 and 3., respectively. The symbol TSm/n is used to denote the transition state connecting isomers m and n. All products are labeled with the

10.1021/jp102029w  2010 American Chemical Society Published on Web 08/12/2010

Reaction Mechanism of CH + C3H6

J. Phys. Chem. A, Vol. 114, No. 35, 2010 9497

Figure 1. The optimized structures of the reactant and products at the B3LYP/6-311G(d,p) level. Distances are given in angstroms and angles in degrees.

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Figure 2. The optimized structures of the isomers at the B3LYP/6-311G(d,p) level. Distances are given in angstroms and angles in degrees.

letter P and a subscript n (n is the Arabic number) as shown from P1 to P13. For convenient discussion, the energy of R (CH + C3H6) is set as zero for reference. Unless otherwise specified, the G3B3//B3LYP/6-311G(d,p) relative energies are used

throughout. The energetics of reactant, products, isomers, and transition states are listed in Table 1. The vibrational frequencies (cm-1) and moments of inertia (au) of the reactant, some important products, isomers, and transition states are listed in


Figure 3.


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Figure 3. The optimized structures of the transition states at the B3LYP/6-311G(d,p) leve. Distances are given in angstroms and angles in degrees.

Table 2. Furthermore, by means of the interrelation among the reactant, isomers, transition states and products as well as their relative energies, the schematic potential energy surface (PES) of the CH + C3H6 reaction at the G3B3//B3LYP/6-311G(d,p) level is displayed in Figure 4. Furthermore, for understanding the reaction mechanism easily, a simple frame graph is shown in Scheme 1. It should be pointed out that the value behind every species is its relative energy with respect to R (CH + C3H6), and the value on every arrowhead is the energy barrier, both given in kcal/mol. 3.1. Initial Association. The initial association of the CH radical with C3H6 molecule may have two patterns: (i) carbenoid addition to the CdC bond of C3H6 to form the three-membered cyclic isomer CH3-cCHCHCH2 1 [1a (-95.0), 1b (-94.9)]. The isomeric pair 1a and 1b can be easily converted to each other with the small barrier 1.5 (1a f 1b) and 1.4 (1b f 1a). (ii) Carbenoid insertion into the C-H σ-bond of -CH3 radical in C3H6 to generate CH2CH2CHCH2 4a (-109.6) via a weakly bound complex CH · · · CH3CHCH2 2 (-16.9). Isomer 4a can easily convert to 4b with a small barrier of 1.3. With the large heat released from the initial steps, isomers 1 and 4 can take various changes. In the following part, we mainly discuss the evolution pathways of 1 and 4. 3.2. Isomerization and Dissociation Pathways. As shown in Figure 4, four kinds of conversion pathways are located for CH3-cCHCHCH2 1 (1a, 1b) (-95.0, -94.9), that is, (i) H-elimination to produce P1(CH3-cCHCHCH + H) (-47.6), and P2 (CH3-cCCHCH2 + H) (-51.0); (ii) CH3-extrusion to form P3 (cCHCHCH2 + CH3) (-57.1); (iii) ring-opening to form

CH3CHCHCH2 3 (3a, 3b) (-125.9, -125.3); (iv) and (v) H-shift accompanied by ring-opening to form CH3CH(C)CH3 8 (-48.9) and CH3C(CH2)CH2 9 (-125.6), respectively. The corresponding transition states for these five channels are TS1a/P1 (-45.4), TS1b/P1 (-46.3), TS1b/P2 (-47.5), TS1b/P3 (-51.2), TS1a/ 3a (-75.1), TS1b/3b (-73.8), TS1a/8 (-47.8), and TS1a/9 (-26.4). Clearly, channel (v) with much higher transition state TS1a/9 (-26.4) is of no interest. Further changes of 9 are not considered. Channels (i) and (ii), which are associated with direct H- or CH3-extrusion, have only one step, whereas channels (iii) and (iv) have two or more steps. In the following part, we focus on the pathways associated with the isomers formed in channels (iii) and (iv). The low-lying isomer CH3CHCHCH2 3 (3a, 3b) (-125.9, 125.3) formed in channel (iii) has four evolution pathways (i) 1,2-H-shift to give rise to CH2CH2CHCH2 4 (4a, 4b) (-109.6, 109.0); (ii) C-C bond rupture to form product P4(CH3CHCCH2 + H) (-70.3); (iii) 3,4-H-shift to form CH3CHCCH3 5 (5a, 5b) (-105.4, -104.6); (iv) 2,3-H-shift to generate CH3CH2CCH2 7 (-103.3) and (v) 2,4-H-shift to form CH3CH2CHCH (6a, 6b, 6c, 6d) (-99.8, -99.4, -99.9, -99.3). By comparison, we find that the transition states TS3a/6a (-20.9) and TS3b/6c (-29.9) in channel (v) lie much higher than those in channels (i), (ii), (iii), and (iv), that is, TS3a/4a (-76.7) or TS3b/4b (-77.6) in channel (i), TS3a/P4 (-66.6) or TS3b/P4 (-65.8) in channel (ii); TS3a/5a (-61.0) or TS3b/ 5b (-60.5) in channel (iii) and TS3b/7 (-50.8) in channel (iv). Thus, channel (v) should be of no interest. Furthermore, isomer 4 (4a, 4b) can undergo either internal H-elimination to generate

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P5(cis-CH2CHCHCH2 + H) (-79.1) and P6(trans-CH2CHCHCH2 + H) (-82.0) or internal C-C bond dissociation to produce P7(C2H4 + C2H3) (-76.9). For isomer 5 (5a, 5b), two pathways are located, (i) CH3-extrusion to form P8(CH3CCH + CH3); (ii) H-extrusion to produce P4(CH3CHCCH2 + H) and P7(CH3CCCH3 + H). Isomer 7 can take CH3-or H-extrusion to produce P12(CH2CCH2 + CH3) (-80.8), and P4, respectively. Starting from CH3CH(C)CH3 8 (-48.9) formed in channel (iv), two kinds of pathways are identified: (i) isomerizes to 5 (5a, 5b); and (ii) dissociate to P10 (CCHCH3 + CH3) (-34.1). The much higher relative energies of TS8/P10 (-20.0) makes channel (ii) have little contribution to final fragmentation. As a result, the most relevant pathways from isomers 1 and 4 can be depicted as:

Path (5) R f (1a, 1b) f (3a, 3b) f (4a, 4b) f P5

Path (1) R f (1a, 1b) f P1

Path (10) R f (1a, 1b) f (3a, 3b) f (5a, 5b) f P4

Path (2) R f (1a, 1b) f P2

Path (11) R f (1a, 1b) f (3a, 3b) f 7 f P12

Path (3) R f (1a, 1b) f P3

Path (12) R f (1a, 1b) f (3a, 3b) f 7 f P4

Path (4) R f (1a, 1b) f (3a, 3b) f P4

Path (13) R f (1a, 1b) f 8 f (5a, 5b) f P8

Path (6) R f (1a, 1b) f (3a, 3b) f (4a, 4b) f P6 Path (7) R f (1a, 1b) f (3a, 3b) f (4a, 4b) f P7 Path (8) R f (1a, 1b) f (3a, 3b) f (5a, 5b) f P8 Path (9) R f (1a, 1b) f (3a, 3b) f (5a, 5b) f P9

TABLE 1: Total (a.u.) and Relative Energies (in parentheses) (kcal/mol) of Reactant, Products, Isomers, and Transition Stats at the G3B3//B3LYP/6-311G(d,p) Level species

G3B3

reactant P1 (CH3-cCHCHCH + H) P2 (CH3-cCCHCH2 + H) P3 (cCHCHCH2 + CH3) P4(CH3CHCCH2 + H) P5 (cis-CH2CHCHCH2 + H) P6 (trans-CH2CHCHCH2 + H) P7 (C2H4 + C2H3) P8 (CH3CCH + CH3) P9 (CH3CCCH3 + H) P10 (CCHCH3 + CH3) P11 (CH3CH2CCH + H) P12 (CH2CCH2 + CH3) P13 (C2H2 + C2H5) 1a 1b 2 3a 3b 4a 4b 5a 5b 6a 6b 6c 6d 7 8 9 10 TS6d/P13 TS7/P4 TS7/P12 TS8/P10 TS9/10 TS9/P12 TS10/P8

-156.2227533 -156.2986587 -156.3040683 -156.3138258 -156.3348347 -156.3488465 -156.3535066 -156.3452913 -156.3526976 -156.3410224 -156.2770182 -156.3326618 -156.3514395 -156.3446648 -156.3742044 -156.3739340 -156.2496953 -156.4234207 -156.4224289 -156.3973615 -156.3965132 -156.3907267 -156.3894167 -156.3817906 -156.3811589 -156.3818793 -156.3810639 -156.3873572 -156.3006618 -156.422868 -156.3866829 -156.3315036 -156.3289269 -156.3365691 -156.2546664 -156.3243933 -156.3344885 -156.3347598

(0.0) (-47.6) (-51.0) (-57.1) (-70.3) (-79.1) (-82.0) (-76.9) (-81.5) (-74.2) (-34.1) (-69.0) (-80.8) (-76.5) (-95.0) (-94.9) (-16.9) (-125.9) (-125.3) (-109.6) (-109.0) (-105.4) (-104.6) (-99.8) (-99.4) (-99.9) (-99.3) (-103.3) (-48.9) (-125.6) (-102.9) (-68.2) (-66.6) (-71.4) (-20.0) (-63.8) (-70.1) (-70.3)

species

G3B3

TS1a/1b TS1a/3a TS1a/8 TS1a/9 TS1a/P1 TS1b/3b TS1b/P1 TS1b/P2 TS1b/P3 TS2/4a TS3a/3b TS3a/4a TS3a/5a TS3a/6a TS3a/P4 TS3b/4b TS3b/5b TS3b/6c TS3b/7 TS3b/P4 TS4a/4b TS4a/P6 TS4a/P7 TS4b/P5 TS4b/P7 TS5a/5b TS5a/8 TS5a/P9 TS5b/P4 TS5b/P8 TS6a/6b TS6a/6c TS6b/6d TS6b/P13 TS6c/6d TS6c/7 TS6c/P11 TS6d/7

-156.3718232 -156.3424619 -156.2988486 -156.2648686 -156.2951308 -156.3403047 -156.2965623 -156.2985057 -156.3044095 -156.2464471 -156.4010517 -156.3450391 -156.3200219 -156.2561250 -156.3288682 -156.3464606 -156.3191471 -156.2704121 -156.3037054 -156.3276518 -156.3953728 -156.3440011 -156.3383994 -156.3418448 -156.3392 -156.3814653 -156.2872395 -156.3345132 -156.3314489 -156.3374777 -156.3754288 -156.3792470 -156.3781428 -156.3306277 -156.3757962 -156.3056336 -156.3253373 -156.3103768

(-93.5) (-75.1) (-47.8) (-26.4) (-45.4) (-73.8) (-46.3) (-47.5) (-51.2) (-14.9) (-111.9) (-76.7) (-61.0) (-20.9) (-66.6) (-77.6) (-60.5) (-29.9) (-50.8) (-65.8) (-108.3) (-76.1) (-72.6) (-74.7) (-73.1) (-99.6) (-40.5) (-70.1) (-68.2) (-72.0) (-95.8) (-98.2) (-97.5) (-67.7) (-96.0) (-52.0) (-64.4) (-55.0)



TABLE 2: Vibrational Frequencies and Moment of Inertia of Reactant, Some Important Products, Isomers, and Transition States at the B3LYP/6-311G(d,p) Level of Theory species

frequencies (cm-1)

moment of inertia (au)

C 3H 6

38.4

194.5

221.8

CH CH3-cCHCHCH

0.0 88.7

4.2 280.0

4.2 309.8

CH3-cCCHCH2

87.3

283.9

348.0

cCHCHCH2

59.5

82.6

130.1

CH3 cis-CH2CHCHCH2

6.3 83.0

6.3 322.7

12.6 396.8

trans-CH2CHCHCH2

42.5

407.6

450.1

C2H4

12.3

59.7

71.9

C2H3 1a

7.6 106.0

55.3 279.7

62.9 314.5

1b

104.2

279.8

319.2

2

149.2

365.6

491.3

4a

73.1

416.6

427.6

4b

108.7

315.9

406.8

TS1a/P1

111.0

295.4

321.4

TS1b/P1

104.0

306.4

333.9

TS1b/P2

104.3

302.1

348.4

TS1b/P3

122.0

387.5

423.1

TS2/4a

144.8

376.4

479.1

TS4a/P6

59.8

424.0

455.9

TS4a/P7

72.3

584.6

632.4

TS4b/P5

102.8

332.9

410.9

TS4b/P7

114.8

473.6

563.8

205, 425, 591, 923, 941, 948, 1029, 1070, 1189, 1327, 1408, 1449, 1481, 1495, 1713, 3012, 3056, 3092, 3120, 3127, 3208 2804 187, 365, 415, 620, 778, 834, 874, 952, 1008, 1038, 1093, 1118, 1222, 1397, 1408, 1491, 1497, 1719, 3009, 3030, 3062, 3086, 3235, 3281 156, 297, 323, 675, 737, 941, 975, 987, 1051, 1063, 1081, 1120, 1190, 1406, 1477, 1482, 1523, 1862, 3015, 3018, 3066, 3074, 3096, 3266 607, 787, 871, 923, 1015, 1032, 1069, 1110, 1154, 1521, 1733, 3032, 3095, 3247, 3294 505, 1403, 1403, 3104, 3283, 3283 162, 276, 473, 620, 757, 884, 943, 945, 1019, 1033, 1069, 1105, 1312, 1344, 1438, 1464, 1674, 1695, 3119, 3130, 3153, 3140, 3218, 3220 174, 300, 519, 540, 781, 899, 935, 936, 1001, 1004, 1058, 1227, 1315, 1320, 1415, 1473, 1653, 1706, 3123, 3232, 3135, 3136, 3219, 3220 835, 973, 974, 1066, 1239, 1380, 1472, 1692, 3122, 3137, 3193, 3221 711, 819, 922, 1045, 1392, 1650, 3037, 3132, 3237 206, 324, 359, 612, 773, 819, 881, 900, 937, 1034, 1073, 1092, 1122, 1170, 1246, 1382, 1411, 1471, 1491, 1500, 3016, 3073, 3074, 3075, 3092, 3145, 3203 222, 323, 368, 594, 750, 813, 864, 920, 1006, 1042, 1075, 1086, 1137, 1161, 1242, 1384, 1410, 1470, 1492, 1501, 3020, 3071, 3073, 3079, 3096, 3141, 3203 71, 127, 166, 237, 431, 521, 658, 840, 928, 964, 993, 1013, 1096, 1107, 1268, 1321, 1443, 1451, 1688, 1710, 1832, 2907, 3048, 3111, 3125, 3137, 3223 98, 127, 326, 420, 493, 659, 796, 898, 945, 1029, 1047, 1073, 1115, 1240, 1319, 1336, 1445, 1457, 1463, 1703, 2938, 3013, 3121, 3129, 3136, 3207, 3241 150, 153, 281, 489, 539, 591, 824, 877, 946, 1027, 1033, 1053, 1127, 1226, 1328, 1360, 1436, 1454, 1467, 1706, 2904, 3005, 3122, 3136, 3142, 3215, 3244 363i, 177, 205, 269, 377, 436, 637, 775, 829, 862, 951, 1002, 1039, 1093, 1121, 1222, 1392, 1406, 1488, 1497, 1668, 3012, 3023, 3067, 3091, 3241, 3283 289i, 144, 182, 222, 369, 422, 632, 779, 835, 864, 951, 1012, 1036, 1094, 1119, 1220, 1394, 1408, 1491, 1497, 1678, 3012, 3044, 3066, 3088, 3238, 3282 518i, 181, 252, 342, 362, 408, 677, 716, 939, 959, 1001, 1041, 1069, 1086, 1105, 1183, 1406, 1476, 1487, 1511, 1788, 3020, 3027, 3081, 3095, 3109, 3262 388i, 74, 159, 199, 426, 440, 591, 704, 791, 825, 917, 1004, 1036, 1066, 1089, 1153, 1411, 1415, 1497, 1603, 3029, 3089, 3093, 3250, 3259, 3262, 3286 105i, 114, 173, 242, 423, 537, 614, 826, 936, 960, 993, 1012, 1078, 1128, 1253, 1326, 1446, 1450, 1587, 1702, 1824, 2935, 3044, 3120, 3138, 3153, 3223 729i, 155, 296, 353, 424, 515, 550, 753, 891, 901, 941, 1000, 1007, 1056, 1218, 1296, 1319, 1405, 1470, 1596, 1693, 3150, 3136, 3140, 3144, 3221, 3230 284i, 12, 143, 231, 237, 342, 780, 828, 876, 888, 921, 933, 1023, 1081, 1238, 1321, 1396, 1468, 1594, 1644, 3050, 3129, 3134, 3143, 3205, 3216, 3233 603i, 140, 277, 349, 379, 497, 611, 737, 878, 918, 947, 1016, 1030, 1069, 1102, 1298, 1337, 1433, 1461, 1619, 1686, 3132, 3135, 3141, 3149, 3221, 3230 275i, 50, 131, 252, 255, 370, 775, 829, 867, 892, 920, 936, 1024, 1081, 1237, 1327, 1388, 1466, 1600, 1650, 3059, 3129, 3140, 3143, 3200, 3204, 3231

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Figure 4. The schematic potential energy surface (PES) of the CH + C3H6 reaction. Erel are the relative energies (kcal/mol).

Path (14) R f (1a, 1b) f 8 f (5a, 5b) f P9 Path (15) R f (1a, 1b) f 8 f (5a, 5b) f P4

Path (2) R f (1a, 1b) f P2 Path (3) R f (1a, 1b) f P3 Path (4) R f (1a, 1b) f (3a, 3b) f P4

Path (16) R f 2 f (4a, 4b) f P5 Path (17) R f 2 f (4a, 4b) f P6 Path (18) R f 2 f (4a, 4b) f P7 As shown in scheme 1, for 1 f 5 conversion, the energy barriers in paths (8) and (9) are much higher than those in paths (13) and (14), and thus paths (8) and (9) are the optimal channels to form P8 and P9, respectively. Furthermore, paths (16-18) are simpler than paths (5-7), and they should be the optimal channels to form P4, P5 and P12. Moreover, the energy barriers in path (4) are lower than those in paths (10), (12), and (15); therefore, path (4) should be the optimal channel to generate P13. 4. Reaction Mechanism In this article, we have obtained 10 dissociation products P1-9 and P12. For simplifying the discussion, the most feasible formation pathways of these 10 products are listed again:

Path (1) R f (1a, 1b) f P1

Path (11) R f (1a, 1b) f (3a, 3b) f 7 f P12 Path (13) R f (1a, 1b) f 8 f (5a, 5b) f P8 Path (14) R f (1a, 1b) f 8 f (5a, 5b) f P9 Path (16) R f 2 f (4a, 4b) f P5 Path (17) R f 2 f (4a, 4b) f P6 Path (18) R f 2 f (4a, 4b) f P7 We can easily find that, except for paths (16-18), all the other paths are starting from 1. First, we will compare the feasibility of the paths starting from isomer 1. Obviously, paths (1-3) involve simple addition-elimination process, whereas paths (4), (11), (13), and (14) proceed via more complicated processes. For example, only one barrier needs to surmounted in paths (1-3), that is 49.6 (1a f P1) or 48.6 (1b f P1) kcal/mol in path (1), 47.4 (1b f P2) kcal/mol in path (2), and 43.7 (1b f P3) kcal/mol in path (3), while two barriers need to climbed in path (4), which are 19.9 (1a f



SCHEME 1: Simple Frame Diagram for the Reaction CH + C3H6

3a) and 59.3 (3a f P4) kcal/mol [or 21.1(1b f 3b) and 58.5 (3b f P4) kcal/mol] and three barriers must be climbed in paths (11), (13) and (14), that is, 19.9 (1a f 3a) or 21.1 (1b f 3b), 74.5 (3b f 7), and 31.9 (7 f P12) kcal/mol in path (11); 47.2 (1a f 8), 8.4 (8 f 5a), and 32.6 (5b f P8) kcal/ mol in path (13); and 47.2 (1a f 8), 8.4 (8 f 5a), and 35.3 (5a f P9) kcal/mol in path (14). Therefore, paths (4), (11), (13), and (14) cannot be competitive with paths (1-3). Formation of P4, P8, P9, and P12 is quite unlikely. Paths (1-3) may have comparable contribution to the final product because the energy barriers involved inpaths (1-3) are very close. Finally, let us discuss the feasibility of the pathways starting from isomer 2. Because the energy barriers of 34.3 (4b f P5) kcal/mol in path (16), 33.5 (4a f P6) kcal/mol in path (17) and 37.0 (4a f P7) kcal/mol or 35.9 (4b f P7) kcal/mol in path (18) are much lower than those in paths (1-3), paths (16-18) should be more competitive than paths (1-3). Considering the barriers involved inpaths (16-18) are very close, these three paths may compete with each other. For CH + C3H6 reaction, we predict that a total of 10 dissociation products may be observed. P5, P6, and P7 may be the most favorable products with comparable yields. P1, P2, and P3 may be the second feasible products, and P4, P8, P9 and P12 may be the least possible products with almost negligible yields. Of course, for some competitive processes, it is desirable to perform kinetic calculations, while such studies are beyond the scope of the present paper. On the other hand, we provide the vibrational frequencies (cm-1)

and moment of inertia (au) of the critical species of some reaction channels to assist future kinetic studies, for which the present study provides an important starting point. 5. Comparison with Experiment It is worthwhile making a comparison of our theoretical results with previous experimental findings. Based on our theoretical calculations, all the isomers and transition states lie lower than the reactant R (CH + C3H6), the title reaction is expected to be fast. This is in accord with the previously measured large rate constant, that is, k ) (3.86-4.58) × 10-10 cm3 molecule-1 s-1 (over the temperature range of 77-170 K) and k ) (4.2 ( 0.8) × 10-10 cm3 molecule-1 s-1 (300 K) obtained by Daugey et al.29 and Loison et al.,34 respectively. In addition, formation of the chainlike isomers CH2CH2CHCH2, CH3C(CH2)CH2, CH3CHCHCH2, and the cyclic isomer cycloC4H7 predicted by Loison et al.34 is indeed identified in our calculations (denoted as isomers 4, 9, 3, and 1). Furthermore, the predicted products H + CH2CHCHCH2 and CH3 + CH2CCH2 by Loison et al.34 are also located as P5 and P6, and P12, respectively. However, based on our calculations, P12(CH3 + CH2CCH2) may have negligible contribution to the final product, while Loison et al. suggested that CH3 + CH2CCH2 is a minor product with branching ratio of 0.22 ( 0.10. In fact, Loison et al. stated that “The branching ratios for CH reactions with C3H6 should be used with care as they are highly speculative.”

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On the other hand, we note that a detailed theoretical investigation of C4H7 radical had been carried out by Miller et al.46 Generally, Miller et al’s results are consistent with our calculations. The discrepancy lies in that only linear C4H7 radical and chainlike products are considered in Miller et al’ study, whereas in our results, we obtained cyclic isomer 1 (1a,1b) as well as three cyclic products denoted as P1 (CH3cCHCHCH + H), P2 (CH3-cCCHCH2 + H), and P3 (cCHCHCH2 + CH3). In view of these aspects, further investigations for the CH + C3H6 reaction are still desirable. To the best of our knowledge, this is the first theoretical study on the reaction of CH with C3H6, our calculation results may shed some light on the title reaction. 6. Conclusion A detailed doublet potential energy surface for the reaction of CH with C3H6 is explored theoretically at the B3LYP/6311G(d,p) and G3B3 (single-point) levels. Various possible reaction pathways are probed. It is shown that 10 dissociation products P1-9 and P12 are thermodynamically and kinetically feasible. Among these products, P5, P6, and P7 may be the most competitive products with comparable quantities, whereas P1, P2, and P3 may be the less feasible products, followed by the almost negligible products P4, P8, P9 and P12. Our calculated results are consistent with previous experimental observations and may provide some insight into the gas-phase reaction between CH and C3H6. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No20773048). References and Notes (1) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287. (2) Miller, J. A.; Kee, R. J.; Westbrook, C. Annu. ReV. Phys. Chem. 1990, 41, 345. (3) Amin, M. Y.; EI Nawawy, M. S. Earth, Moon, Planets 1997, 75, 25. (4) Brownsword, R. A.; Sims, I. R.; Smith, I. W. M.; Stewart, D. W. A.; Canosa, A.; Rowe, B. R. Astrophys. J. 1997, 485, 195. (5) Canosa, A.; Sims, I. R.; Travers, D.; Smith, I. W. M.; Rowe, B. R. Astron. Astrophys. 1997, 323, 644. (6) Turner, B. E.; Zuckerman, B. Astrophys. J. 1974, 187, L59. (7) Rydbeck, O. E. H.; Ellder, J.; Irvine, W. M. Nature 1973, 246, 466. (8) Lien, D. J. Astrophys. J. 1984, 284, 578. (9) Arpigny, C. Annu. ReV. Astron. Astrophys. 1965, 3, 351. (10) Ridgway, S. T.; Carbon, D. F.; Hall, D. N. B.; Jewell, J. Astrophys. J. Suppl. 1984, 54, 177. (11) Lambert, D. L.; Gustafsson, B.; Eriksson, K.; Hinkle, K. H. Astrophys. J. Suppl. 1986, 62, 373. (12) Bleekrode, R.; Nieuwpoort, W. C. J. Chem. Phys. 1965, 43, 3680. (13) Zachwieja, M. J. Mol. Spectrosc. 1995, 170, 285. (14) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure, Constantsof Diatomic Molecules; Van Nostrand Reinhold: New York, 1979; Vol. 4. (15) Herzberg, G.; Johns, J. W. C. Astrophys. J. 1969, 158, 399. (16) Bernath, P. F.; Brazier, C. R.; Olsen, T.; Hailey, R.; Fernando, W. T. M. L.; Woods, C.; Hardwick, J. L. J. Mol. Spectrosc. 1991, 147, 16. (17) Bernath, P. F. J. Chem. Phys. 1987, 86, 4838. (18) Luque, J.; Berg, P. A.; Jeffries, J. B.; Smith, G. P.; Crosley, D. R.; Scherer, J. J. Appl. Phys. B: Laser Opt. 2004, 78, 93.

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Reaction Mechanism of CH + C3H6: A ... - American Chemical Society

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