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Thermal decomposition of 2-phenylethanol: A computational study on mechanism
Chemical Physics Letters 556 (2013) 29–34
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Thermal decomposition of 2-phenylethanol: A computational study on mechanism Yasuyuki Sakai a, Hiromitsu Ando a, Tatsuo Oguchi b, Yoshinori Murakami c,⇑ a
Department of Mechanical Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan Department of Environmental and Life Sciences, Toyohashi University of Technology, 1-1 Hibarigaoka, Tenpaku-cho, Toyohashi 441-8580, Japan c Department of General Science, Hachinohe National College of Technology, 16-1 Uwanotai, Tamonoki, Hachinohe, Aomori 039-1192, Japan b
a r t i c l e
i n f o
Article history: Received 28 September 2012 In final form 20 November 2012 Available online 7 December 2012
a b s t r a c t Quantum mechanical calculations for the thermal decomposition of 2-phenylethanol have been performed using the CBS-QB3 method. Based on the potential energy surfaces at the CBS-QB3 level of theory, the preferred reaction channel for the thermal decomposition of 2-phenylethanol was the six-membered cyclic rearrangement reaction and the dehydration reaction to form styrene and H2O. Further quantum chemical calculations of the subsequent reactions followed by the six-membered cyclic rearrange reaction of 2-phenylethanol were carried out and it was revealed that the barrier height for the ring opening reaction was the lowest among all of the other subsequent reactions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermal decomposition of alcohol has been long-time attracted attention due to the importance to combustion chemistry [1,2]. Although there are lots of experimental and theoretical investigations on the thermal decomposition of aliphatic alcohol [3–5], little is known for the mechanism of the thermal decomposition of alcohol containing unsaturated hydrocarbons or aromatic hydrocarbons. The pioneering work for the thermal decomposition of alcohol containing aromatic hydrocarbons was carried out by Taylor [6]. He investigated the final products of the thermal decomposition of 2-phenylethanol and found that toluene was generated by the thermal decomposition of 2-phenylethanol as the first-order thermal decomposition product. He also indicated that styrene was formed by the dehydration reactions as shown below.
ðIÞ
ðIIÞ
⇑ Corresponding author. E-mail address: [email protected] (Y. Murakami). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.11.050
Later Chuchani et al. [7] performed the final product analysis of the pyrolysis of 2-phenylethanol in a static flow reactor and found that the pyrolysis of 2-phenylethanol at around 750 K yielded primarily styrene with a small amount of toluene formation. Based on these experimental observations they proposed that styrene was formed by the dehydration reaction (Scheme II), while toluene was formed by the rearrangement reaction of 2-phenylethanol via the six-membered cyclic transition state structures (Scheme I), which occurred frequently in the thermal decomposition of bhydroxyolefin in the gas phase [8]. The theoretical calculations of the transition state structures for such cyclic rearrangement reactions of 2-phenylethanol were carried out using various types of theories by Mora et al. [9], but the complete dissociation channels for the thermal decomposition of 2-phenylethanol have not been clearly understood. In the present work we have aimed to search the complete reaction pathways for the thermal decomposition of 2-phenylethanol using the CBS-QB3 method and to compare with the experimental observations carried out by Taylor [6] and Chuchani et al. [7]. 2. Calculation methods All quantum chemical calculations were performed using the GAUSSIAN 03 software packages [10]. Geometry optimization for the reactants, products, various intermediates, and transition states in the present reaction systems were carried out by the complete basis set CBS-QB3 method [11,12]. The CBS-QB3 procedures include geometry optimization and frequency calculations at the B3LYP level of theory with the CBSB7 basis set, followed by CCSD(T)/6-31+G(d0 ). MP4SDQ/CBSB4, and MP2/CBSB3 single point calculations. The total CBS-QB3 energy is obtained by calculating the sum of the MP2/6-311G(3d2f, 2df, 2p) energy extrapolated to
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Y. Sakai et al. / Chemical Physics Letters 556 (2013) 29–34
the complete basis set limit and the MP4(SDQ), CCSD(T), zero-point energy and empirical corrections. To confirm whether the calculated transition states connected the desired reactants and products, the intrinsic reaction coordinate (IRC) procedures were also performed for some transition state structures in the present study. According to the previous investigations on the predictions of the rate constants for the reactions of cycloalkylperoxy radicals using the transition state theory combined with the CBS-QB3 method [13], the predicted rate constants agreed well with the experimental data as well as the previous calculations available in the literature. Thus it is anticipated that the present calculations will also give the same level of accuracies toward the potential energy surfaces for the thermal decomposition reaction of 2-phenylethanol.
3. Results and discussions The schematic pictures of the molecular structures for the intermediates and products involved in the reaction of 2-phenylethanol at the B3LYP/CBSB7 are shown in Figure 1. Also, Figure 2 is the schematic figures for the optimized geometries of the transition states at the B3LYP/CBSB7 level of theory. Finally, the relative energies for various species formed by the thermal decomposition of 2-phenylethanol are calculated at the B3LYP/CBSB7 and CBSQB3 levels of theories as tabulated in Table 1. The calculated relative energies reported previously by other authors are also tabulated in Table 1 for comparison.
3.1. Thermal decompositions of 2-phenylethanol For the thermal decomposition of 2-phenylethanol there are eight molecular elimination reaction channels
2-Phenylethanol ! INT1 þ H2 O ! INT2 þ H2
ð1Þ ð2Þ
! INT3 þ H2
ð3Þ
! INT4 þ CH2 O ! INT5 þ CH2 O
ð4Þ ð5Þ
! INT6 þ C2 H4
ð6Þ
! INT7 þ CH2 CHOH
ð7Þ
! INT4 þ trans-HCOH
ð8Þ
and five simple bond fission barrierless reactions forming two radical species.
2-Phenylethanol ! INT8 þ CH2 CH2 OH
ð9Þ
! INT9 þ OH
ð10Þ
! INT10 þ CH2 OH ! INT11 þ H
ð11Þ ð12Þ
! INT12 þ H
ð13Þ
! INT13 þ H
ð14Þ
The potential energy surfaces for these reaction pathways are presented in Figure 3. Firstly, the reaction (1) is the dehydration reaction, which was reported as one of the major channels for
Figure 1. Schematic figures of the intermediate species formed by the thermal decomposition of 2-phenylethanol.
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Y. Sakai et al. / Chemical Physics Letters 556 (2013) 29–34
TS1
TS2
TS4
TS7
TS10
TS5
TS8
TS11
TS3
TS6
TS9
TS12
Figure 2. Schematic pictures for the transition state structures for the thermal decomposition of 2-phenylethanol and o-isotoluene (INT5) optimized at B3LYP/CBSB7 level of theory.
the thermal decomposition of 2-phenylethanol by Taylor [6] and Chuchani et al. [7]. The barrier height for the reaction (1) was 64.3 kcal mol1, which is the second lowest among all of the decomposition channels for the title reaction. The barrier height for the dehydration reactions (1) previously calculated by Mora et al. [9] using at the level of MPW1PW91/6-31++G(d, p) theory was reported as 64.5 kcal/mol, which was in good agreement with the present calculated results of 64.3 kcal/mol using the CBS-QB3 method. The reaction pathways for the reactions (1)–(4) and (8) are analogous to those for the thermal decomposition of ethanol, which were previously calculated by Park et al. [4]. Table 2 tabulated the comparison of the barrier heights of the transition states for the reaction pathways (1)–(4) and (8) of 2-phenylethanol in the
present study and for the corresponding reaction pathways of ethanol calculated by Park et al. [4] and Sivaramakrishnan et al. [5]. Table 2 also tabulated the barrier heights of the analogous reactions (1)–(4) and (8) for the thermal decomposition of ethanol with the CBS-QB3 method calculated in the present study. As shown in Table 2, most of the barrier heights of the transition states for the thermal decomposition reactions of 2-phenylethanol became lower than the barrier heights for the corresponding reactions of ethanol. These results suggest that the substitution of H-atom to an aromatic ring in C2H5OH has an effect to reduce the barrier heights of the transition states for the thermal decomposition reactions. The reaction (5) is the rearrangement reaction of 2-phenylethanol via the six-membered cyclic transition state structures as was
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Y. Sakai et al. / Chemical Physics Letters 556 (2013) 29–34
Table 1 Calculated total energies (in hartree) and relative energies (kcal mol1) for various species using B3LYP/CBSB7 and CBS-QB3 levels. (a) Thermal decomposition of 2-phenylethanol and (b) Thermal decomposition of o-isotoluene (INT5).
a b
Species
E[B3LYP/CBSB7]
DE (kcal mol1)
E[CBS-QB3]
DE (kcal mol1)
(a) 2-Phenylethanol INT1 + H2O TS1 INT2 + H2 TS2 INT3 + H2 TS3 INT4 + CH2O TS4 INT5 + CH2O TS5 INT6 + C2H4 TS6 INT7 + CH2CHOH TS7 INT4 + trans-HCOH TS8 INT8 + CH2CH2OH INT9 + OH INT10 + CH2OH INT11 + H INT12 + H INT13 + H
386.1925 386.1747 386.0896 386.1599 386.0500 386.1482 386.0240 386.1724 386.0516 386.1179 386.1035 386.1658 386.0524 386.1653 386.0240 386.0882 386.0645 386.0323 386.0411 386.0795 386.0238 386.0354 386.0432
0.0 11.2 64.5 20.4 89.4 27.8 105.7 12.6 88.4 46.8 55.8 16.7 87.9 17.1 105.7 65.5 80.3 100.5 95.0 70.9 105.9 98.6 93.7
385.3865 385.3764 385.2840 385.3634 385.2499 385.3514 385.2200 385.3646 385.2488 385.3117 385.2944 385.3558 385.2448 385.3538 385.2176 385.2803 385.2561 385.2169 385.2441 385.2662 385.2212 385.2359 385.2448
0.0 6.3 64.3 14.5 85.8 22.1 104.5 13.7 86.4 46.9 57.8 19.3 88.9 20.5 106.0 66.7 81.8 106.4 89.4 75.5 103.7 94.5 88.
(b) INT5 INT4 TS9 INT10 + H INT14 TS10 INT15 TS11 INT16 TS12
271.5816 271.6361 271.4821 271.4861 271.5243 271.5052 271.4972 271.4943 271.5877 271.4919
0.0 34.2 62.4 59.9 36.0 47.9 52.9 54.7 3.9 56.3
270.9676 271.0204 270.8739 270.8779 270.9064 270.8899 270.8931 270.8843 270.9737 270.8824
0.0 33.2 58.8 56.3 38.4 48.7 46.7 52.2 3.9 53.5
Reference
64.5a
57.9a
31.6b 59.8b 58.3b
46.8b 52.7b 3.9b 54.0b
The MPW1PW91/6-31++G(d, p) lenvel from Ref. [9]. The G3B3 method from Ref. [14].
Figure 3. The potential energy surfaces for the thermal decomposition reactions of 2-phenylethanol calculated at the CBS-QB3 level of theory, respectively. The parentheses are the values (kcal mol1) of the CBS-QB3 method calculated in the present study.
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Y. Sakai et al. / Chemical Physics Letters 556 (2013) 29–34
Table 2 Relative energy (kcal mol1) of the transition state (TS) structure for each product channel given in the parenthesis at the CBS-QB3, G2M(RCC2) and QCISD(T)/CBS level of theories. Species
2-Phenylethanol
Species
C2H5OH
CBS-QB3 2-Phenylethanol TS1 TS2 TS3 TS4 TS8
CH3CH2OH TS(C2H4 + H2O) TS(CH3CHO + H2) TS(CH2CHOH + H2) TS(CH4 + CH2O) TS(CH4 + CHOH)
G2 M(RCC2)
QCISD(T)/CBS
0.0 67.3 86.0 109.4 87.9 85.0
0.0a 66.6a 86.0a 106.3a 99.7a 84.3a
0.0b 66.0b 85.5b – 89.8b 84.6b
Data taken from Ref. [4]. Data taken from Ref. [5].
illustrated in Scheme I. The barrier height for this reaction was 57.8 kcal mol1 and thus it was the lowest among all of the decomposition reactions for 2-phenylethanol. The barrier height for the six-membered cyclic rearrangement reaction (5) calculated by Mora et al. [9] varied between 45 to 60 kcal mol1 depending to on the levels of theories used, the present activation barrier height of 57.8 kcal mol1 was within the range of their calculated values and was in good agreements with their result of 57.9 kcal mol1 calculated at the level of MPW1PW91/6-31++G(d, p) theory. The reactions (6) and (7) are the reactions that were caused by the interactions between the side chain and aromatic ring structures of 2-phenylethanol. Those reactions have not been previously reported, but the barrier heights for the reactions (6) and (7) were 88.9 and 106.0 kcal mol1, respectively, and thus they are far above the lowest and the second lowest barrier heights for the reactions (1) and (5). The barrier heights for the simple bond fission reactions (9)–(14) are between 75 and 107 kcal mol1. They are also still higher than the barrier heights for the reactions (1) and (5). From these calculations it can be concluded that the reaction pathways (5) and (1) are the lowest and the second lowest energy channel, respectively, and therefore it is likely that the most important pathways for the thermal decomposition of 2-phenylethanol are also the reactions (5) and (1). This is consistent with the previous experimental conclusions by Taylor [6] and Chuchani et al. [7]. 3.2. Subsequent reactions after the six-membered cyclic rearrangements of 2-phenylethanol For the thermal decomposition of 2-phenylethanol, it was confirmed that the reaction pathway that had the lowest barrier height was the reaction channel of the six-membered cyclic rearrangement reaction (5) forming CH2O and the corresponding product, o-isotoluene (INT5). Chuchani et al. [7] proposed that o-isotoluene (INT5) formed after the six-membered cyclic rearrangement reaction (5) further to isomerized to become toluene (Reaction (I) given in the introduction) based on the observation that toluene was formed after the thermal decomposition of 2phenylethanol. Previously Zhang et al. [14] performed a theoretical calculation for the subsequent isomerization and decomposition reaction of o-isotoluene (INT5) using the G3B3 method. In the present study we have also attempted to search for the subsequent decomposition pathways forming o-isotoluene (INT5) using the CBS-QB3 method. Here, further decomposition pathways of toluene after the isomerization reaction from o-isotoluene (INT5) have not been calculated in the present study because there have already been a lot of theoretical and experimental investigations so far [15,16] and it seems to be relatively negligible importance at the experimental conditions of Taylor [6] and Chuchani et al. [7] with relatively lower temperature regions (T < 1000 K). The
reaction scheme for the subsequent reaction of o-isotoluene (INT5) calculated in the present work is as follows,
INT5 ! INT4
ð15Þ
! INT10 þ H
ð16Þ
! INT14 ! INT15
ð17Þ ð18Þ
INT15 ! INT16
ð19Þ
Figure 4 summarized the potential energy diagram for the subsequent reaction pathways of o-isotoluene (INT5) at the CBSQB3 level of theory. The relative energies for the intermediates, the products and the transition states relative to the reactants calculated by Zhang et al. [14] were also given in parenthesis of Figure 4. As shown in the parenthesis of Figure 4, the calculated relative energies at the CBS-QB3 method and the G3B3 method are in good agreement with each other. The barrier height for the isomerization reaction (15) to form toluene (INT4), which Taylor [6] and Chuchani et al. [7] were
80
TS11 (52.2) [52.7]
60
Energy ( kcal / mol)
a b
0.0 64.3 85.8 104.5 86.4 81.8
CBS-QB3
TS10 (48.7) INT15 (46.7) [46.8]
40
TS9 (58.8) TS12 [59.8] (53.5) [54.0]
INT10 + H (56.3) [58.3] INT14 (38.4)
20
0
INT5 (0) [0]
INT16 (-3.9) [-3.9]
-20
-40
INT4 (-33.2) [-31.6]
Figure 4. The potential energy surfaces for the thermal decomposition reactions of o-isotoluene (INT5) produced the thermal decomposition of 2-phenylethanol calculated at the CBS-QB3 level of theory, respectively. The parenthesis above and below are the values (kcal mol1) of the CBS-QB3 method calculated in the present study and the values (kcal mol1) of the G3B3 method in Ref. [14], respectively.
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Y. Sakai et al. / Chemical Physics Letters 556 (2013) 29–34
previously regarded as the major reaction pathway for the isomerization of INT5, was 58.8 kcal mol1 and it is highest among the other energy reaction paths. The heat of reaction for the simple bond fission barrierless reaction (16) to form benzyl (INT10) and H-atom was 56.3 kcal mol1, which was comparable to the value of the barrier height (58.8 kcal mol1) for the isomerization reaction (15) to form toluene (INT4). The reaction channel that had lowest activation barrier height was the ring opening reaction pathway (17). The activation barrier height was 48.7 kcal mol1. The activation barrier height for the H-atom migration reaction (18) was 52.2 kcal mol1 and this reaction pathway was also supposed to be one of the preferred reaction channels. Once H-atom migrated to m-isotoluene (INT15) and then subsequent H-atom migration to p-isotoluene (INT16) could undergo with relatively lower activation barrier height (53.5 kcal mol1). Thus the isomerization reaction pathway (15), which Taylor [6] and Chuchani et al. [7] were previously regarded as the major reaction pathway for the isomerization of o-isotoluene (INT5), was not the lowest energy reaction path, but the ring opening reaction pathway (17) and the H-atom migration reactions (18) and (19) were more preferred reaction channels than the isomerization reaction (15) to form toluene (INT4). The main discrepancy with experiment by Taylor [6] and Chuchani et al. [7] was the fact that both of them observed toluene and styrene in the experiments, but on the other hand o-isotoluene (INT5) was found to be the dominant product from the present quantum chemical calculations. Supposing that the ring opening product (INT14) have no further decomposition channels and thus o-isotoluene (INT5) and the ring opening product (INT14) are reversible with a very fast rate, then the equilibrium will favor o-isotoluene (INT5) at the experimental temperature ranges of Taylor [6] and Chuchani et al. [7]. The high-temperature equilibrium between o-isotoluene (INT5) and the ring opening product (INT14) under the combustion environment were supported with the fact that the barrier height of TS10 relative to the total energy of INT14 was very small (10.3 kcal mol1). Once the o-isotoluene (INT5) was abundant under the combustion environment, benzyl radical (INT10) was formed by the H-atom loss reaction (16) from the o-isotoluene (INT5) and then finally toluene was quickly converted from benzyl radicals by abstracting H-atom from other molecules. If the above scheme is correct, H atoms are abundant under the experimental conditions of Taylor [6] and Chuchani et al. [7]. Then the following reaction scheme will proceed and therefore styrene will also probably be formed as one of the major final products.
H þ 2-phenylethanol ! PhCHCH2 OH þ H2 PhCHCH2 OH ! PhCHCH2 þ OH
ð20Þ ð21Þ
where Ph and PhCHCH2 represent phenyl (C6H5) group and styrene, respectively. The reasons why toluene and styrene were formed as the major products for the thermal decomposition of 2-phenylethanol were not yet clearly understood at this moment. Further investigations such as constructing the chemical models for the thermal
decomposition of 2-phenylethanol including the side reactions such as reactions (20) and (21) using the rate constants predicted by the quantum chemical calculations are needed to clarify the reasons. 4. Conclusions The potential energy surfaces for the thermal decomposition of 2-phenylethanol were obtained using the CBS-QB3 method. The most energetically preferred reaction pathway was the six-membered cyclic rearrangement reaction (5), although the dehydration reaction (1) had the second lowest barrier height among the other thermal decomposition reaction pathways. The subsequent decomposition reaction of o-isotoluene (INT5) formed by the sixmembered cyclic rearrangement reaction (5) was also investigated by calculating the barrier height for each dissociative reaction channel using the CBS-QB3 method. It was found that the barrier height for the ring opening reaction pathway (17) was the lowest, while the reaction pathway (15) forming toluene (INT4), which was previously believed to the main subsequent reaction pathway by Taylor [6] and Chuchani et al. [7], was highest. Plausible mechanism of toluene and styrene formation by the thermal decomposition of 2-phenylethanol was proposed. Acknowledgments The authors gratefully acknowledged for the financial support by Grand-in-Aid for Scientific Research (KAKENHI B, 23360091) from the Ministry of Education, Culture, Sports, Science and Technology. References [1] A. Frassoldati, A. Cuoci, T. Faravelli, U. Niemann, E. Ranzi, R. Seiser, K. Seshadri, Combust. Flame 157 (2010) 2. [2] R. Grana, A. Frassoldati, T. Faravelli, U. Niemann, E. Ranzi, R. Seiser, R. Cattolica, K. Seshadri, Combust. Flame 157 (2010) 2137. [3] N.M. Marinov, Int. J. Chem. Kinet. 31 (1999) 183. [4] J. Park, R.S. Zhu, M.C. Lin, J. Chem. Phys. 117 (2002) 3224. [5] R. Sivaramakrishnan, M.-C. Su, J.V. Michael, S.J. Klippenstein, L.B. Harding, B. Ruscic, J. Phys. Chem. A 114 (2010) 9425. [6] R. Taylor, J. Chem. Soc. Perkin. Trans. 2 (1988) 183. [7] G. Chuchani, A. Rotinov, R.M. Dominguez, Int. J. Chem. Kinet. 31 (1999) 401. [8] J. Quijano, J. David, C. Sanchez, E. Rincon, D. Guerra, L.A. Leon, R. Notario, J.L. Abboud, J. Mol. Struct. (Theochem.) 580 (2002) 201. [9] J.R. Mora, J. Lezama, J.M. Albornoz, A. Hernandez, T. Cordova, G. Chuchani, J. Phys. Org. Chem. 22 (2009) 1198. [10] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, et al., Gaussian 03, Revision C.02, Gaussian Inc., Pittsburgh, PA, 2003. [11] J.A. Montgomery, M.J. Frisch, J.W. Ochterski, G.A. Petersson, J. Chem. Phys. 110 (1999) 2822. [12] J.A. Montgomery, M.J. Frisch, J.W. Ochterski, G.A. Petersson, J. Chem. Phys. 112 (2000) 6532. [13] B. Sirjean, P.A. Glaude, M.F. Ruiz-Lopez, R. Fournet, J. Phys. Chem. A 113 (2009) 6924. [14] T. Zhang, L. Zhang, X. Hong, K. Zhang, F. Qi, C.K. Law, T. Ye, P. Zhao, Y. Chen, Combust. Flame 156 (2009) 2071. [15] S.J. Klippenstein, L.B. Harding, Y. Georgievskii, Proc. Combust. Inst. 31 (2007) 221. [16] M. Derudi, D. Polino, C. Cavallotti, Phys. Chem. Chem. Phys. 13 (2011) 21308.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Thermal decomposition of 2-phenylethanol: A computational study on mechanism Yasuyuki Sakai a, Hiromitsu Ando a, Tatsuo Oguchi b, Yoshinori Murakami c,⇑ a
Department of Mechanical Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan Department of Environmental and Life Sciences, Toyohashi University of Technology, 1-1 Hibarigaoka, Tenpaku-cho, Toyohashi 441-8580, Japan c Department of General Science, Hachinohe National College of Technology, 16-1 Uwanotai, Tamonoki, Hachinohe, Aomori 039-1192, Japan b
a r t i c l e
i n f o
Article history: Received 28 September 2012 In final form 20 November 2012 Available online 7 December 2012
a b s t r a c t Quantum mechanical calculations for the thermal decomposition of 2-phenylethanol have been performed using the CBS-QB3 method. Based on the potential energy surfaces at the CBS-QB3 level of theory, the preferred reaction channel for the thermal decomposition of 2-phenylethanol was the six-membered cyclic rearrangement reaction and the dehydration reaction to form styrene and H2O. Further quantum chemical calculations of the subsequent reactions followed by the six-membered cyclic rearrange reaction of 2-phenylethanol were carried out and it was revealed that the barrier height for the ring opening reaction was the lowest among all of the other subsequent reactions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermal decomposition of alcohol has been long-time attracted attention due to the importance to combustion chemistry [1,2]. Although there are lots of experimental and theoretical investigations on the thermal decomposition of aliphatic alcohol [3–5], little is known for the mechanism of the thermal decomposition of alcohol containing unsaturated hydrocarbons or aromatic hydrocarbons. The pioneering work for the thermal decomposition of alcohol containing aromatic hydrocarbons was carried out by Taylor [6]. He investigated the final products of the thermal decomposition of 2-phenylethanol and found that toluene was generated by the thermal decomposition of 2-phenylethanol as the first-order thermal decomposition product. He also indicated that styrene was formed by the dehydration reactions as shown below.
ðIÞ
ðIIÞ
⇑ Corresponding author. E-mail address: [email protected] (Y. Murakami). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.11.050
Later Chuchani et al. [7] performed the final product analysis of the pyrolysis of 2-phenylethanol in a static flow reactor and found that the pyrolysis of 2-phenylethanol at around 750 K yielded primarily styrene with a small amount of toluene formation. Based on these experimental observations they proposed that styrene was formed by the dehydration reaction (Scheme II), while toluene was formed by the rearrangement reaction of 2-phenylethanol via the six-membered cyclic transition state structures (Scheme I), which occurred frequently in the thermal decomposition of bhydroxyolefin in the gas phase [8]. The theoretical calculations of the transition state structures for such cyclic rearrangement reactions of 2-phenylethanol were carried out using various types of theories by Mora et al. [9], but the complete dissociation channels for the thermal decomposition of 2-phenylethanol have not been clearly understood. In the present work we have aimed to search the complete reaction pathways for the thermal decomposition of 2-phenylethanol using the CBS-QB3 method and to compare with the experimental observations carried out by Taylor [6] and Chuchani et al. [7]. 2. Calculation methods All quantum chemical calculations were performed using the GAUSSIAN 03 software packages [10]. Geometry optimization for the reactants, products, various intermediates, and transition states in the present reaction systems were carried out by the complete basis set CBS-QB3 method [11,12]. The CBS-QB3 procedures include geometry optimization and frequency calculations at the B3LYP level of theory with the CBSB7 basis set, followed by CCSD(T)/6-31+G(d0 ). MP4SDQ/CBSB4, and MP2/CBSB3 single point calculations. The total CBS-QB3 energy is obtained by calculating the sum of the MP2/6-311G(3d2f, 2df, 2p) energy extrapolated to
30
Y. Sakai et al. / Chemical Physics Letters 556 (2013) 29–34
the complete basis set limit and the MP4(SDQ), CCSD(T), zero-point energy and empirical corrections. To confirm whether the calculated transition states connected the desired reactants and products, the intrinsic reaction coordinate (IRC) procedures were also performed for some transition state structures in the present study. According to the previous investigations on the predictions of the rate constants for the reactions of cycloalkylperoxy radicals using the transition state theory combined with the CBS-QB3 method [13], the predicted rate constants agreed well with the experimental data as well as the previous calculations available in the literature. Thus it is anticipated that the present calculations will also give the same level of accuracies toward the potential energy surfaces for the thermal decomposition reaction of 2-phenylethanol.
3. Results and discussions The schematic pictures of the molecular structures for the intermediates and products involved in the reaction of 2-phenylethanol at the B3LYP/CBSB7 are shown in Figure 1. Also, Figure 2 is the schematic figures for the optimized geometries of the transition states at the B3LYP/CBSB7 level of theory. Finally, the relative energies for various species formed by the thermal decomposition of 2-phenylethanol are calculated at the B3LYP/CBSB7 and CBSQB3 levels of theories as tabulated in Table 1. The calculated relative energies reported previously by other authors are also tabulated in Table 1 for comparison.
3.1. Thermal decompositions of 2-phenylethanol For the thermal decomposition of 2-phenylethanol there are eight molecular elimination reaction channels
2-Phenylethanol ! INT1 þ H2 O ! INT2 þ H2
ð1Þ ð2Þ
! INT3 þ H2
ð3Þ
! INT4 þ CH2 O ! INT5 þ CH2 O
ð4Þ ð5Þ
! INT6 þ C2 H4
ð6Þ
! INT7 þ CH2 CHOH
ð7Þ
! INT4 þ trans-HCOH
ð8Þ
and five simple bond fission barrierless reactions forming two radical species.
2-Phenylethanol ! INT8 þ CH2 CH2 OH
ð9Þ
! INT9 þ OH
ð10Þ
! INT10 þ CH2 OH ! INT11 þ H
ð11Þ ð12Þ
! INT12 þ H
ð13Þ
! INT13 þ H
ð14Þ
The potential energy surfaces for these reaction pathways are presented in Figure 3. Firstly, the reaction (1) is the dehydration reaction, which was reported as one of the major channels for
Figure 1. Schematic figures of the intermediate species formed by the thermal decomposition of 2-phenylethanol.
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Y. Sakai et al. / Chemical Physics Letters 556 (2013) 29–34
TS1
TS2
TS4
TS7
TS10
TS5
TS8
TS11
TS3
TS6
TS9
TS12
Figure 2. Schematic pictures for the transition state structures for the thermal decomposition of 2-phenylethanol and o-isotoluene (INT5) optimized at B3LYP/CBSB7 level of theory.
the thermal decomposition of 2-phenylethanol by Taylor [6] and Chuchani et al. [7]. The barrier height for the reaction (1) was 64.3 kcal mol1, which is the second lowest among all of the decomposition channels for the title reaction. The barrier height for the dehydration reactions (1) previously calculated by Mora et al. [9] using at the level of MPW1PW91/6-31++G(d, p) theory was reported as 64.5 kcal/mol, which was in good agreement with the present calculated results of 64.3 kcal/mol using the CBS-QB3 method. The reaction pathways for the reactions (1)–(4) and (8) are analogous to those for the thermal decomposition of ethanol, which were previously calculated by Park et al. [4]. Table 2 tabulated the comparison of the barrier heights of the transition states for the reaction pathways (1)–(4) and (8) of 2-phenylethanol in the
present study and for the corresponding reaction pathways of ethanol calculated by Park et al. [4] and Sivaramakrishnan et al. [5]. Table 2 also tabulated the barrier heights of the analogous reactions (1)–(4) and (8) for the thermal decomposition of ethanol with the CBS-QB3 method calculated in the present study. As shown in Table 2, most of the barrier heights of the transition states for the thermal decomposition reactions of 2-phenylethanol became lower than the barrier heights for the corresponding reactions of ethanol. These results suggest that the substitution of H-atom to an aromatic ring in C2H5OH has an effect to reduce the barrier heights of the transition states for the thermal decomposition reactions. The reaction (5) is the rearrangement reaction of 2-phenylethanol via the six-membered cyclic transition state structures as was
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Table 1 Calculated total energies (in hartree) and relative energies (kcal mol1) for various species using B3LYP/CBSB7 and CBS-QB3 levels. (a) Thermal decomposition of 2-phenylethanol and (b) Thermal decomposition of o-isotoluene (INT5).
a b
Species
E[B3LYP/CBSB7]
DE (kcal mol1)
E[CBS-QB3]
DE (kcal mol1)
(a) 2-Phenylethanol INT1 + H2O TS1 INT2 + H2 TS2 INT3 + H2 TS3 INT4 + CH2O TS4 INT5 + CH2O TS5 INT6 + C2H4 TS6 INT7 + CH2CHOH TS7 INT4 + trans-HCOH TS8 INT8 + CH2CH2OH INT9 + OH INT10 + CH2OH INT11 + H INT12 + H INT13 + H
386.1925 386.1747 386.0896 386.1599 386.0500 386.1482 386.0240 386.1724 386.0516 386.1179 386.1035 386.1658 386.0524 386.1653 386.0240 386.0882 386.0645 386.0323 386.0411 386.0795 386.0238 386.0354 386.0432
0.0 11.2 64.5 20.4 89.4 27.8 105.7 12.6 88.4 46.8 55.8 16.7 87.9 17.1 105.7 65.5 80.3 100.5 95.0 70.9 105.9 98.6 93.7
385.3865 385.3764 385.2840 385.3634 385.2499 385.3514 385.2200 385.3646 385.2488 385.3117 385.2944 385.3558 385.2448 385.3538 385.2176 385.2803 385.2561 385.2169 385.2441 385.2662 385.2212 385.2359 385.2448
0.0 6.3 64.3 14.5 85.8 22.1 104.5 13.7 86.4 46.9 57.8 19.3 88.9 20.5 106.0 66.7 81.8 106.4 89.4 75.5 103.7 94.5 88.
(b) INT5 INT4 TS9 INT10 + H INT14 TS10 INT15 TS11 INT16 TS12
271.5816 271.6361 271.4821 271.4861 271.5243 271.5052 271.4972 271.4943 271.5877 271.4919
0.0 34.2 62.4 59.9 36.0 47.9 52.9 54.7 3.9 56.3
270.9676 271.0204 270.8739 270.8779 270.9064 270.8899 270.8931 270.8843 270.9737 270.8824
0.0 33.2 58.8 56.3 38.4 48.7 46.7 52.2 3.9 53.5
Reference
64.5a
57.9a
31.6b 59.8b 58.3b
46.8b 52.7b 3.9b 54.0b
The MPW1PW91/6-31++G(d, p) lenvel from Ref. [9]. The G3B3 method from Ref. [14].
Figure 3. The potential energy surfaces for the thermal decomposition reactions of 2-phenylethanol calculated at the CBS-QB3 level of theory, respectively. The parentheses are the values (kcal mol1) of the CBS-QB3 method calculated in the present study.
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Table 2 Relative energy (kcal mol1) of the transition state (TS) structure for each product channel given in the parenthesis at the CBS-QB3, G2M(RCC2) and QCISD(T)/CBS level of theories. Species
2-Phenylethanol
Species
C2H5OH
CBS-QB3 2-Phenylethanol TS1 TS2 TS3 TS4 TS8
CH3CH2OH TS(C2H4 + H2O) TS(CH3CHO + H2) TS(CH2CHOH + H2) TS(CH4 + CH2O) TS(CH4 + CHOH)
G2 M(RCC2)
QCISD(T)/CBS
0.0 67.3 86.0 109.4 87.9 85.0
0.0a 66.6a 86.0a 106.3a 99.7a 84.3a
0.0b 66.0b 85.5b – 89.8b 84.6b
Data taken from Ref. [4]. Data taken from Ref. [5].
illustrated in Scheme I. The barrier height for this reaction was 57.8 kcal mol1 and thus it was the lowest among all of the decomposition reactions for 2-phenylethanol. The barrier height for the six-membered cyclic rearrangement reaction (5) calculated by Mora et al. [9] varied between 45 to 60 kcal mol1 depending to on the levels of theories used, the present activation barrier height of 57.8 kcal mol1 was within the range of their calculated values and was in good agreements with their result of 57.9 kcal mol1 calculated at the level of MPW1PW91/6-31++G(d, p) theory. The reactions (6) and (7) are the reactions that were caused by the interactions between the side chain and aromatic ring structures of 2-phenylethanol. Those reactions have not been previously reported, but the barrier heights for the reactions (6) and (7) were 88.9 and 106.0 kcal mol1, respectively, and thus they are far above the lowest and the second lowest barrier heights for the reactions (1) and (5). The barrier heights for the simple bond fission reactions (9)–(14) are between 75 and 107 kcal mol1. They are also still higher than the barrier heights for the reactions (1) and (5). From these calculations it can be concluded that the reaction pathways (5) and (1) are the lowest and the second lowest energy channel, respectively, and therefore it is likely that the most important pathways for the thermal decomposition of 2-phenylethanol are also the reactions (5) and (1). This is consistent with the previous experimental conclusions by Taylor [6] and Chuchani et al. [7]. 3.2. Subsequent reactions after the six-membered cyclic rearrangements of 2-phenylethanol For the thermal decomposition of 2-phenylethanol, it was confirmed that the reaction pathway that had the lowest barrier height was the reaction channel of the six-membered cyclic rearrangement reaction (5) forming CH2O and the corresponding product, o-isotoluene (INT5). Chuchani et al. [7] proposed that o-isotoluene (INT5) formed after the six-membered cyclic rearrangement reaction (5) further to isomerized to become toluene (Reaction (I) given in the introduction) based on the observation that toluene was formed after the thermal decomposition of 2phenylethanol. Previously Zhang et al. [14] performed a theoretical calculation for the subsequent isomerization and decomposition reaction of o-isotoluene (INT5) using the G3B3 method. In the present study we have also attempted to search for the subsequent decomposition pathways forming o-isotoluene (INT5) using the CBS-QB3 method. Here, further decomposition pathways of toluene after the isomerization reaction from o-isotoluene (INT5) have not been calculated in the present study because there have already been a lot of theoretical and experimental investigations so far [15,16] and it seems to be relatively negligible importance at the experimental conditions of Taylor [6] and Chuchani et al. [7] with relatively lower temperature regions (T < 1000 K). The
reaction scheme for the subsequent reaction of o-isotoluene (INT5) calculated in the present work is as follows,
INT5 ! INT4
ð15Þ
! INT10 þ H
ð16Þ
! INT14 ! INT15
ð17Þ ð18Þ
INT15 ! INT16
ð19Þ
Figure 4 summarized the potential energy diagram for the subsequent reaction pathways of o-isotoluene (INT5) at the CBSQB3 level of theory. The relative energies for the intermediates, the products and the transition states relative to the reactants calculated by Zhang et al. [14] were also given in parenthesis of Figure 4. As shown in the parenthesis of Figure 4, the calculated relative energies at the CBS-QB3 method and the G3B3 method are in good agreement with each other. The barrier height for the isomerization reaction (15) to form toluene (INT4), which Taylor [6] and Chuchani et al. [7] were
80
TS11 (52.2) [52.7]
60
Energy ( kcal / mol)
a b
0.0 64.3 85.8 104.5 86.4 81.8
CBS-QB3
TS10 (48.7) INT15 (46.7) [46.8]
40
TS9 (58.8) TS12 [59.8] (53.5) [54.0]
INT10 + H (56.3) [58.3] INT14 (38.4)
20
0
INT5 (0) [0]
INT16 (-3.9) [-3.9]
-20
-40
INT4 (-33.2) [-31.6]
Figure 4. The potential energy surfaces for the thermal decomposition reactions of o-isotoluene (INT5) produced the thermal decomposition of 2-phenylethanol calculated at the CBS-QB3 level of theory, respectively. The parenthesis above and below are the values (kcal mol1) of the CBS-QB3 method calculated in the present study and the values (kcal mol1) of the G3B3 method in Ref. [14], respectively.
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previously regarded as the major reaction pathway for the isomerization of INT5, was 58.8 kcal mol1 and it is highest among the other energy reaction paths. The heat of reaction for the simple bond fission barrierless reaction (16) to form benzyl (INT10) and H-atom was 56.3 kcal mol1, which was comparable to the value of the barrier height (58.8 kcal mol1) for the isomerization reaction (15) to form toluene (INT4). The reaction channel that had lowest activation barrier height was the ring opening reaction pathway (17). The activation barrier height was 48.7 kcal mol1. The activation barrier height for the H-atom migration reaction (18) was 52.2 kcal mol1 and this reaction pathway was also supposed to be one of the preferred reaction channels. Once H-atom migrated to m-isotoluene (INT15) and then subsequent H-atom migration to p-isotoluene (INT16) could undergo with relatively lower activation barrier height (53.5 kcal mol1). Thus the isomerization reaction pathway (15), which Taylor [6] and Chuchani et al. [7] were previously regarded as the major reaction pathway for the isomerization of o-isotoluene (INT5), was not the lowest energy reaction path, but the ring opening reaction pathway (17) and the H-atom migration reactions (18) and (19) were more preferred reaction channels than the isomerization reaction (15) to form toluene (INT4). The main discrepancy with experiment by Taylor [6] and Chuchani et al. [7] was the fact that both of them observed toluene and styrene in the experiments, but on the other hand o-isotoluene (INT5) was found to be the dominant product from the present quantum chemical calculations. Supposing that the ring opening product (INT14) have no further decomposition channels and thus o-isotoluene (INT5) and the ring opening product (INT14) are reversible with a very fast rate, then the equilibrium will favor o-isotoluene (INT5) at the experimental temperature ranges of Taylor [6] and Chuchani et al. [7]. The high-temperature equilibrium between o-isotoluene (INT5) and the ring opening product (INT14) under the combustion environment were supported with the fact that the barrier height of TS10 relative to the total energy of INT14 was very small (10.3 kcal mol1). Once the o-isotoluene (INT5) was abundant under the combustion environment, benzyl radical (INT10) was formed by the H-atom loss reaction (16) from the o-isotoluene (INT5) and then finally toluene was quickly converted from benzyl radicals by abstracting H-atom from other molecules. If the above scheme is correct, H atoms are abundant under the experimental conditions of Taylor [6] and Chuchani et al. [7]. Then the following reaction scheme will proceed and therefore styrene will also probably be formed as one of the major final products.
H þ 2-phenylethanol ! PhCHCH2 OH þ H2 PhCHCH2 OH ! PhCHCH2 þ OH
ð20Þ ð21Þ
where Ph and PhCHCH2 represent phenyl (C6H5) group and styrene, respectively. The reasons why toluene and styrene were formed as the major products for the thermal decomposition of 2-phenylethanol were not yet clearly understood at this moment. Further investigations such as constructing the chemical models for the thermal
decomposition of 2-phenylethanol including the side reactions such as reactions (20) and (21) using the rate constants predicted by the quantum chemical calculations are needed to clarify the reasons. 4. Conclusions The potential energy surfaces for the thermal decomposition of 2-phenylethanol were obtained using the CBS-QB3 method. The most energetically preferred reaction pathway was the six-membered cyclic rearrangement reaction (5), although the dehydration reaction (1) had the second lowest barrier height among the other thermal decomposition reaction pathways. The subsequent decomposition reaction of o-isotoluene (INT5) formed by the sixmembered cyclic rearrangement reaction (5) was also investigated by calculating the barrier height for each dissociative reaction channel using the CBS-QB3 method. It was found that the barrier height for the ring opening reaction pathway (17) was the lowest, while the reaction pathway (15) forming toluene (INT4), which was previously believed to the main subsequent reaction pathway by Taylor [6] and Chuchani et al. [7], was highest. Plausible mechanism of toluene and styrene formation by the thermal decomposition of 2-phenylethanol was proposed. Acknowledgments The authors gratefully acknowledged for the financial support by Grand-in-Aid for Scientific Research (KAKENHI B, 23360091) from the Ministry of Education, Culture, Sports, Science and Technology. References [1] A. Frassoldati, A. Cuoci, T. Faravelli, U. Niemann, E. Ranzi, R. Seiser, K. Seshadri, Combust. Flame 157 (2010) 2. [2] R. Grana, A. Frassoldati, T. Faravelli, U. Niemann, E. Ranzi, R. Seiser, R. Cattolica, K. Seshadri, Combust. Flame 157 (2010) 2137. [3] N.M. Marinov, Int. J. Chem. Kinet. 31 (1999) 183. [4] J. Park, R.S. Zhu, M.C. Lin, J. Chem. Phys. 117 (2002) 3224. [5] R. Sivaramakrishnan, M.-C. Su, J.V. Michael, S.J. Klippenstein, L.B. Harding, B. Ruscic, J. Phys. Chem. A 114 (2010) 9425. [6] R. Taylor, J. Chem. Soc. Perkin. Trans. 2 (1988) 183. [7] G. Chuchani, A. Rotinov, R.M. Dominguez, Int. J. Chem. Kinet. 31 (1999) 401. [8] J. Quijano, J. David, C. Sanchez, E. Rincon, D. Guerra, L.A. Leon, R. Notario, J.L. Abboud, J. Mol. Struct. (Theochem.) 580 (2002) 201. [9] J.R. Mora, J. Lezama, J.M. Albornoz, A. Hernandez, T. Cordova, G. Chuchani, J. Phys. Org. Chem. 22 (2009) 1198. [10] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, et al., Gaussian 03, Revision C.02, Gaussian Inc., Pittsburgh, PA, 2003. [11] J.A. Montgomery, M.J. Frisch, J.W. Ochterski, G.A. Petersson, J. Chem. Phys. 110 (1999) 2822. [12] J.A. Montgomery, M.J. Frisch, J.W. Ochterski, G.A. Petersson, J. Chem. Phys. 112 (2000) 6532. [13] B. Sirjean, P.A. Glaude, M.F. Ruiz-Lopez, R. Fournet, J. Phys. Chem. A 113 (2009) 6924. [14] T. Zhang, L. Zhang, X. Hong, K. Zhang, F. Qi, C.K. Law, T. Ye, P. Zhao, Y. Chen, Combust. Flame 156 (2009) 2071. [15] S.J. Klippenstein, L.B. Harding, Y. Georgievskii, Proc. Combust. Inst. 31 (2007) 221. [16] M. Derudi, D. Polino, C. Cavallotti, Phys. Chem. Chem. Phys. 13 (2011) 21308.