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Theoretical study on the unimolecular decomposition of proline
Computational and Theoretical Chemistry 1018 (2013) 45–49
Contents lists available at SciVerse ScienceDirect
Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Theoretical study on the unimolecular decomposition of proline Saleh Rawadieh a, Ibrahem Altarawneh a, Heba B. Alateyat b, Mohammednoor Altarawneh a,c,⇑ a
Chemical Engineering Department, Al-Hussein Bin Talal University, Ma’an, Jordan Princess Alia University College, Al-Balqa’ Applied University, Jordan c Process Safety and Environmental Protection Research Group, School of Engineering, The University of Newcastle, NSW 2308, Australia b
a r t i c l e
i n f o
Article history: Received 22 April 2013 Received in revised form 26 May 2013 Accepted 27 May 2013 Available online 20 June 2013 Keywords: Proline Amino acids CBS-QB3 RRKM theory
a b s t r a c t As a representative model compound for cyclic amino acids in biomass, initial reactions in the decomposition of proline are thoroughly investigated herein. The weakest bond in the proline molecule is found to be the H atom gem to the carboxylic group with a bond dissociation enthalpy of 75.0 kcal/mol. Carboxylation and dehydration channels are found to incur enthalpies of activations at 71.0 kcal/mol and 72.8 kcal/mol; respectively. Calculated pressure-dependent reaction rate constants, i.e.; k (P, T) values, indicates that water elimination and H elimination from the carbon bearing the carboxylic group dominates the unimolecular decomposition of proline at all temperatures and pressures. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As combustion of biomass is associated with a CO2-emission neutrality, it has attracted a mounted interest during the last few years. However, thermal treatment of biomass results in the emission of a wide spectra of trace pollutants. In particular, emission of nitrogen-containing species pose serious environmental and health problems. It is believed that these compounds are released during the course of devolatilisation and char burning [1]. In addition to inorganic nitrates and ammonium ions, heterocyclic nitrogenated compounds represents a significant fraction of the total N-content in biomass [2]. These compounds include purines, pyrimidines and pyrroles. Pyrolysis of N-cyclic compounds emits NH3, HCN and HCNO and NOx [3]. Chemistry of nitrogen conversion during combustion of biomass has been investigated by utilising simplified models of surrogate compounds; including pyrrole, pyridine and nitromethane [4]. As demonstrated by Lucassen et al. [5] an accurate description of nitrogen transformation and fate requires considering a nitrogen–oxygen cyclic compound such as morpholine. Our recent theoretical analysis supports experimental findings with regard to the dominance of unimolecular-decomposition pathways even under pure oxidative conditions [6]. While morpholine serves as a good representative model for cyclic oxygen–nitrogen containing, proline in particular accounts for the presence of a carboxylic group as in amino acids. Carbocyclic ⇑ Corresponding author at: Process Safety and Environmental Protection Research Group, School of Engineering, The University of Newcastle, NSW 2308, Australia. E-mail address: [email protected] (M. Altarawneh). 2210-271X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2013.05.034
groups often result from the rupture of ether linkages in cellulose and lignin; common structural entities in biomass. Proline is a major amino acids in various types of biomass such as tobacco [7]. Accordingly, pyrolysis of proline has been extensively studied [8]. Degradation of proline was postulated to follow general mechanisms encountered in self-decomposition of amino acids; namely, decarboxylation, dehydration and homolysis of the aliphatic side [9]. Pyrolysis of proline was found to produce numerous poly aromatic hydrocarbons (PAHs) and nitrogenated-PAHs such as 2,5-diketopiperazine, benzonitrile and pyrole [8]. Thermal behaviour of the decomposition of proline was investigated with the aid of a thermogravity [10]. An overall rate of unimolecular decomposition was determined to be 1.54 1014T1.1 exp (20200/T) s1. A robust kinetic model for the pyrolysis of proline, as a model representative of cyclic a-amino acids in biomass, necessitates detailed mechanistic pathways that govern its initial decomposition. To this end, we report in this study a theoretical investigation into primary exit channels involved in the pyrolysis of proline. It is hoped that this study will provide useful information in the pursuit of better understanding of nitrogen behaviour upon thermal utilisation of biomass.
2. Computational details Gaussian03 suite of programs [11] was used to perform all structural and energy calculations were carried out at the composite chemistry model of CBS-QB3 [12] as implemented in the Gaussian03 suite of programmes. The CBS-QB3 performs initial optimisation and frequency estimations at the B3LYP/6-311G(2d,p) level of theory. This is followed by successive single point energy calculations at very accurate theoretical levels. Thermochemical
46
S. Rawadieh et al. / Computational and Theoretical Chemistry 1018 (2013) 45–49
Fig. 1. Bond dissociation enthalpies for direct C–H, O–H and C–OH bond fission (Dr Ho298:15 values) in the proline molecule. All values are in kcal/mol at 298.15 K.
and kinetics parameters were derived with the aid of the Chemrate code [13]. Master equation analysis from RRMK theory [14] was utilised to compute pressure-dependent reaction rate constants for key reactions. In order to simulate a moderate collision environment, DEdown was set to 500 cm1 in the energy transfer model. Deployed Lennard–Jones parameters for proline were adopted from corresponding values of pyrazine at r = 5.350 Å and k/eb = 307.0 K [15]. These parameters serve to describe physical interaction between pairs of proline molecules. Detailed descriptions of pressure-dependent kinetics in general and physical significance of parameters deployed in the Chemrate code are given by Carstensen and Dean [16]. 3. Results and discussions 3.1. Stationary point determinations Fig. 1 depicts plausible channels for direct fissions of C–H, N–H, O–H and C–H bonds. As shown in Fig. 1, the proline molecule
posses four distinct C–H bonds. An H atom loss from the C atom gem to the C(O)OH group represents the lowest enthalpy demanding pathway. Formation of M7 radical via this route is associated with an endothermicity of 75.0 kcal/mol (298.15 K). Reaction enthalpies for fission of the other three C–H bonds are significantly larger and in the range of 91.6–98.1 kcal/mol. Calculated Dr Ho298:15 for these C–H bonds are very similar to corresponding values of the morpholine molecule. Unimolecular loss of an H atom from the NH group requires a sizable enthalpy reaction at 93.2 kcal/ mol. Fission of the O–H hydroxyl bond is found to be endothermic by 83.0 kcal/mol and produces the M1 radical. Concluded from Fig. 1 is that reaction of the proline molecule with the O/H radical pool will most likely result in the formation of the M7 as a major intermediate. Initial oxidation of the proline molecule will further proceed via addition of oxygen molecule to the M7 and subsequent transformation of the resulted M7–OO moiety. Fig. 2 shows closed-shell elimination pathways encountered in the initial decomposition of proline. These pathways features
Fig. 2. Stationary point determinations in the unimolecular decomposition of proline. All values are in kcal/mol at 298.15 K.
S. Rawadieh et al. / Computational and Theoretical Chemistry 1018 (2013) 45–49
47
Fig. 3. Optimized geometries for intermediates and products involved in the unimolecular decomposition of the proline molecule. Interatomic distances are in Å.
self-ejection of H2, CO2 and H2O molecules. Elimination of H2 molecule from the cyclic ring is associated with activation enthalpies of 96.2, 96.3 and 81.9 kcal/mol; via the transition structures TS2, TS3 and TS4; respectively. A lower activation enthalpy for TS4 in reference to TS2 and TS3 is in accord with reported Dr Ho298:15 values in Fig. 1 with regard to the preferential loss of H atom from C gem to the carboxylic group. Formation of the stable molecules M9, M10 and M12 amounts to endothermecity of 17.2–23.8 kcal/mol. Elimination of H2O and CO2 molecules through the four-centre transition structures of TS5 and TS6 marks the two comply accepted dominant channels in the pyrolysis of alkylated carboxylic acids; namely dehydration and decarboxylation; respectively. These two reactions are associated with enthalpic activations of 71.0 and 72.8 kcal/mol; respectively. Formation of the M5 moiety through decarboxylation is coupled with a trivial exothermicity of 5.4 kcal/mol whereas the product of the dehydration channel (M6) resides 38.9 kcal/mol; above the entrance channel. No transition structures could be found for the elimination HC(O)OH group and for the formation of the M14 structure. Optimised geometries for all structures and transition states are given in Figs. 3 and 4; respectively.
3.2. Kinetic consideration Modified Arrhenius parameters at the high-pressure limit and at 1 atm are given in Table 1 for the most important initial pathways. Reaction rate constants H fission leading to the formation of M7 was calculated by setting the value of activation enthalpy to the value of the reaction enthalpy, i.e., 75.0 kcal/mol. Arrhenius parameter for this reaction in particular was obtained from a minimum energy point that resembles the longest C H elon at which the structure still holds a single imaginary frequency along the reaction coordinate. Reaction rate constants for the other six channels were calculated within the formalism of the conventional transition state theory (TST) with a treatment for tunnelling effects based on a one-dimensional Eckart functional. Arrhenius plots at the high-pressure limit and 1 atm are given in Fig. 5a and b; respectively. As shown in Fig. 5a, two channels mainly dominate the decomposition fate of the proline molecule. The dehydration pathway predominates throughout intermediate and low temperatures (T > 850 K). After this temperature, fission of C–H accompanied with the formation of the M7 becomes the most important channel. Decarboxylation channel leading to the formation of the
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S. Rawadieh et al. / Computational and Theoretical Chemistry 1018 (2013) 45–49
Fig. 4. Optimized geometries for transition states involved in the unimolecular decomposition of the proline molecule. Interatomic distances are in Å.
Table 1 Modified Arrhenius parameters for initial exit channels in the unimolecular decomposition of proline. Product
1 atm A (s
M8 M9 M10 M12 M13 M15 H + M7
1
a
High P-limit ) 41
1.70 10 4.11 1023 4.32 1032 7.19 1031 2.11 1021 3.64 1022 3.97 1020
Ea
n
A (s1)
Ea
n
120.969 87.1091 103.903 103.921 75.0363 76.9578 77.9108
8.70 3.47 5.91 5.64 2.47 2.77 2.14
7.63 108 1.08 108 5.80 109 3.37 108 2.91 1010 1.19 1011 9.27 1010
110.18 81.7582 96.2048 96.0249 71.2447 73.0108 75.0548
1.43 1.40 1.20 1.63 0.92 0.79 1.41
M5 molecule contribute slightly by 4–6%. As shown in Fig. 5b, the onset temperature for the dominance of the dehydration channel is shifted to a lower temperature, i.e.; 500 K. k (P, T) values for the three most important reactions are plotted in Fig. 6. As shown in Fig. 6, all channels exhibit a very weak pressure-dependent behaviour. In fact, k (P, T) values reach that of k (P1, T) values at a pressure as low as 0.001 atm for most reaction channels. Our calculated reaction rate constants are significantly lower than the experimentally determined overall rate for the decay of proline. The experimental rate constant could be influenced by a profound catalytic reactor-wall effects and/or the presence of unexpected parallel bimolecular reactions. For instance, formation of 2,5-diketopiperazine as a major intermediate in the pyrolysis of proline [10] supports the hypothesis of the possible involvement of parallel reactions. Along the same line of enquiry, we have shown recently that bimolecular self-reactions were necessary to explain the experimentally determined reaction rate constant for the decomposition of acetamide [17]. 3.3. Thermochemical parameters Thermochemical parameters for all products and transition structures are given in Table 2. These data are useful for kinetic modelling. Barrier for internal rotation of the O–H group in the proline molecule was found to amount to 7.2 kcal/mol. Subsequently, it was treated as a harmonic oscillator rather than a
b
Fig. 5. Branching ratios for the three most important channels at the high P-limit (a) and 1 atm (b).
hindered rotation. Enthalpies of formation for all species were calculated based on the experimental Df Ho298:15 for proline (87.5 kcal/mol), H (52.1 kcal/mol), OH (9.3 kcal/mol), and H2O (57.8 kcal/mol) [18] along with calculated Dr Ho298:15 values in Figs. 1 and 2. However, it is worthwhile mentioning that more accurate values for calculated Df Ho298:15 could be achieved by applying isodesmic work reaction schemes. To the best of our
S. Rawadieh et al. / Computational and Theoretical Chemistry 1018 (2013) 45–49
49
Acknowledgement
4.E-07 M13
k (s -1)
3.E-07
This study was supported by a Grant of computing time from the National Computational Infrastructure (NCI), Australia (Project Id = De3).
M15 M7
2.E-07
References 1.E-07
0.E+00 1E-09
0.0000001
0.00001
0.001
0.1
10
1000
P (atm) Fig. 6. k (P, T) values for the three most important initial channels.
Table 2 Thermochemical parameters for products, and intermediates. Compound So298:15 (cal/ mol)
Df Ho298:15 C oP (T) (cal/mol) (kcal/ 300 500 800 mol)
Proline M7 M8 M9 M10 M12 M13 M15
87.52 64.65 59.46 63.76 70.33 66.18 48.64 92.88
90.75 88.99 89.39 88.54 88.78 87.02 83.49 74.37
31.10 31.60 29.83 29.78 29.10 29.83 26.18 20.78
49.09 47.86 45.57 45.64 44.95 45.45 40.52 35.64
65.86 62.99 59.72 59.71 59.42 59.57 54.11 50.50
1000 1200 1500 2000 72.88 69.32 65.51 65.46 65.37 65.38 59.86 56.93
77.98 73.92 69.68 69.62 69.64 69.57 64.03 61.65
83.30 78.71 74.07 73.95 74.04 73.91 68.34 66.58
88.51 83.40 78.24 78.195 78.30 78.19 72.59 71.39
knowledge, experimental Dr Ho298:15 values for potential reference species in plausible formulated isodesmic reactions are not available in the literature. We recommend using calculated Df Ho298:15 values in Table 2 until more accurate theoretical or experimental values become available. 4. Conclusions Structural and thermochemical properties are obtained for all species involved in the unimolecular decomposition of the proline molecule. Dr Ho298:15 values are calculated for all distinguished bonds in the proline molecule. C–H fission from carbon atom gem to the carboxylic group, decarboxylation and dehydration are found to be associated very similar energetic requirements in the range of 71.0–75.0 kcal/mol. Calculated k (P, T) values show that the region of dominance of the decarboxylation channel is shifted a higher temperature as pressure increases.
[1] A. Williams, J.M. Jones, L. Ma, M. Pourkashanian, Pollutants from the combustion of solid biomass fuels, Prog. Energy Combust. Sci. 38 (2) (2012) 113–137. [2] A. Frassoldati, T. Faravelli, E. Ranzi, A wide range modeling study of formation and nitrogen chemistry in hydrogen combustion, Int. J. Hydrogen Energy 31 (15) (2006) 2310–2328. [3] J.A. Miller, C.T. Bowman, Mechanism and modeling of nitrogen chemistry in combustion, Prog. Energy Combust. Sci. 15 (4) (1989) 287–338. [4] G. Stubenberger, R. Scharler, S. Zahirovic´, I. Obernberger, Experimental investigation of nitrogen species release from different solid biomass fuels as a basis for release models, Fuel 87 (6) (2008) 793–806. [5] A. Lucassen, N. Labbe, P.R. Westmoreland, K. Kohse-Höinghaus, Combustion chemistry and fuel-nitrogen conversion in a laminar premixed flame of morpholine as a model biofuel, Combust. Flame 158 (9) (2011) 1647–1666. [6] M. Altarawneh, B.Z. Dlugogorski, A mechanistic and kinetic study on the decomposition of morpholine, J. Phys. Chem. A 116 (29) (2012) 7703–7711. [7] E.B. Higman, I. Schmeltz, W.S. Schlotzhauer, Products from the thermal degradation of some naturally occurring materials, J. Agric. Food Chem. 18 (4) (1970) 636–639. [8] R.K. Sharma, W.G. Chan, J.I. Seeman, M.R. Hajaligol, Pyrolysis of a-amino acids. Fuel. Chem. Divi. Rep, 47 (2002) 398–399. [9] M. Altarawneh, A.a.H. Al-Muhtaseb, M.H. Almatarneh, R.A. Poirier, N.W. Assaf, K.K. Altarawneh, Theoretical investigation into competing unimolecular reactions encountered in the pyrolysis of acetamide, J. Phys. Chem. A 115 (48) (2011) 14092–14099. [10] T. Wanjun, W. Cunxin, C. Donghua, An investigation of the pyrolysis kinetics of some aliphatic amino acids, J. Anal. Appl. Pyrol. 75 (1) (2006) 49–53. [11] M.J.T. Frisch, G.W. Schlegel, H.B. Scuseria, G.E. Robb, M.A. Cheeseman, J.R. Montgomery, J.A. Vreven Jr., T. Kudin, K.N. Burant, J. C.; et al., Gaussian 03, revision C.02, Gaussian, Inc, Wallingford, CT, 2004. [12] L.A. Curtiss, P.C. Redfern, K. Raghavachari, V. Rassolov, J.A. Pople, Gaussian-3 theory using reduced M[o-slash]ller-Plesset order, J. Chem. Phys. 110 (10) (1999) 4703–4709. [13] V. Mokrushin, V. Bedanov, W. Tsang, M. Zacariah, V. Knyazev, ChemRate; version 1.19, NIST, Gaithersburg, MD, 2002. [14] R.G. Gilbert, S.C. Smith, Theory of Unimolecular and Recombination Reactions, Oxford, Blackwell Scientific, 1990. [15] T.J. Bevilacqua, R.B. Weisman, Collisional vibrational relaxation of a triplet state: energy-dependent energy loss from T pyrazine, J. Chem. Phys. 98 (8) (1993) 6316–6326. [16] H.-H. Carstensen, A.M. Dean, The Kinetics of Pressure-Dependent Reactions, in: Comprehensive Chemical Kinetics, W.C. Robert (Ed.), Elsevier, 2007, pp. 101– 184 (Chapter 4). [17] M. Altarawneh, K. Altarawneh, A theoretical study on the bimolecular reactions encountered in the pyrolysis of acetamide, J. Phys. Org. Chem. 25 (5) (2012) 431–436. [18] H.Y. Afeefy, J.F. Liebman, S.E. Stein, P.J. Linstrom, W.G. Mallard, NIST Chemistry WebBook, NIST Standard Reference, Database Number 692005.
Contents lists available at SciVerse ScienceDirect
Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Theoretical study on the unimolecular decomposition of proline Saleh Rawadieh a, Ibrahem Altarawneh a, Heba B. Alateyat b, Mohammednoor Altarawneh a,c,⇑ a
Chemical Engineering Department, Al-Hussein Bin Talal University, Ma’an, Jordan Princess Alia University College, Al-Balqa’ Applied University, Jordan c Process Safety and Environmental Protection Research Group, School of Engineering, The University of Newcastle, NSW 2308, Australia b
a r t i c l e
i n f o
Article history: Received 22 April 2013 Received in revised form 26 May 2013 Accepted 27 May 2013 Available online 20 June 2013 Keywords: Proline Amino acids CBS-QB3 RRKM theory
a b s t r a c t As a representative model compound for cyclic amino acids in biomass, initial reactions in the decomposition of proline are thoroughly investigated herein. The weakest bond in the proline molecule is found to be the H atom gem to the carboxylic group with a bond dissociation enthalpy of 75.0 kcal/mol. Carboxylation and dehydration channels are found to incur enthalpies of activations at 71.0 kcal/mol and 72.8 kcal/mol; respectively. Calculated pressure-dependent reaction rate constants, i.e.; k (P, T) values, indicates that water elimination and H elimination from the carbon bearing the carboxylic group dominates the unimolecular decomposition of proline at all temperatures and pressures. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As combustion of biomass is associated with a CO2-emission neutrality, it has attracted a mounted interest during the last few years. However, thermal treatment of biomass results in the emission of a wide spectra of trace pollutants. In particular, emission of nitrogen-containing species pose serious environmental and health problems. It is believed that these compounds are released during the course of devolatilisation and char burning [1]. In addition to inorganic nitrates and ammonium ions, heterocyclic nitrogenated compounds represents a significant fraction of the total N-content in biomass [2]. These compounds include purines, pyrimidines and pyrroles. Pyrolysis of N-cyclic compounds emits NH3, HCN and HCNO and NOx [3]. Chemistry of nitrogen conversion during combustion of biomass has been investigated by utilising simplified models of surrogate compounds; including pyrrole, pyridine and nitromethane [4]. As demonstrated by Lucassen et al. [5] an accurate description of nitrogen transformation and fate requires considering a nitrogen–oxygen cyclic compound such as morpholine. Our recent theoretical analysis supports experimental findings with regard to the dominance of unimolecular-decomposition pathways even under pure oxidative conditions [6]. While morpholine serves as a good representative model for cyclic oxygen–nitrogen containing, proline in particular accounts for the presence of a carboxylic group as in amino acids. Carbocyclic ⇑ Corresponding author at: Process Safety and Environmental Protection Research Group, School of Engineering, The University of Newcastle, NSW 2308, Australia. E-mail address: [email protected] (M. Altarawneh). 2210-271X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2013.05.034
groups often result from the rupture of ether linkages in cellulose and lignin; common structural entities in biomass. Proline is a major amino acids in various types of biomass such as tobacco [7]. Accordingly, pyrolysis of proline has been extensively studied [8]. Degradation of proline was postulated to follow general mechanisms encountered in self-decomposition of amino acids; namely, decarboxylation, dehydration and homolysis of the aliphatic side [9]. Pyrolysis of proline was found to produce numerous poly aromatic hydrocarbons (PAHs) and nitrogenated-PAHs such as 2,5-diketopiperazine, benzonitrile and pyrole [8]. Thermal behaviour of the decomposition of proline was investigated with the aid of a thermogravity [10]. An overall rate of unimolecular decomposition was determined to be 1.54 1014T1.1 exp (20200/T) s1. A robust kinetic model for the pyrolysis of proline, as a model representative of cyclic a-amino acids in biomass, necessitates detailed mechanistic pathways that govern its initial decomposition. To this end, we report in this study a theoretical investigation into primary exit channels involved in the pyrolysis of proline. It is hoped that this study will provide useful information in the pursuit of better understanding of nitrogen behaviour upon thermal utilisation of biomass.
2. Computational details Gaussian03 suite of programs [11] was used to perform all structural and energy calculations were carried out at the composite chemistry model of CBS-QB3 [12] as implemented in the Gaussian03 suite of programmes. The CBS-QB3 performs initial optimisation and frequency estimations at the B3LYP/6-311G(2d,p) level of theory. This is followed by successive single point energy calculations at very accurate theoretical levels. Thermochemical
46
S. Rawadieh et al. / Computational and Theoretical Chemistry 1018 (2013) 45–49
Fig. 1. Bond dissociation enthalpies for direct C–H, O–H and C–OH bond fission (Dr Ho298:15 values) in the proline molecule. All values are in kcal/mol at 298.15 K.
and kinetics parameters were derived with the aid of the Chemrate code [13]. Master equation analysis from RRMK theory [14] was utilised to compute pressure-dependent reaction rate constants for key reactions. In order to simulate a moderate collision environment, DEdown was set to 500 cm1 in the energy transfer model. Deployed Lennard–Jones parameters for proline were adopted from corresponding values of pyrazine at r = 5.350 Å and k/eb = 307.0 K [15]. These parameters serve to describe physical interaction between pairs of proline molecules. Detailed descriptions of pressure-dependent kinetics in general and physical significance of parameters deployed in the Chemrate code are given by Carstensen and Dean [16]. 3. Results and discussions 3.1. Stationary point determinations Fig. 1 depicts plausible channels for direct fissions of C–H, N–H, O–H and C–H bonds. As shown in Fig. 1, the proline molecule
posses four distinct C–H bonds. An H atom loss from the C atom gem to the C(O)OH group represents the lowest enthalpy demanding pathway. Formation of M7 radical via this route is associated with an endothermicity of 75.0 kcal/mol (298.15 K). Reaction enthalpies for fission of the other three C–H bonds are significantly larger and in the range of 91.6–98.1 kcal/mol. Calculated Dr Ho298:15 for these C–H bonds are very similar to corresponding values of the morpholine molecule. Unimolecular loss of an H atom from the NH group requires a sizable enthalpy reaction at 93.2 kcal/ mol. Fission of the O–H hydroxyl bond is found to be endothermic by 83.0 kcal/mol and produces the M1 radical. Concluded from Fig. 1 is that reaction of the proline molecule with the O/H radical pool will most likely result in the formation of the M7 as a major intermediate. Initial oxidation of the proline molecule will further proceed via addition of oxygen molecule to the M7 and subsequent transformation of the resulted M7–OO moiety. Fig. 2 shows closed-shell elimination pathways encountered in the initial decomposition of proline. These pathways features
Fig. 2. Stationary point determinations in the unimolecular decomposition of proline. All values are in kcal/mol at 298.15 K.
S. Rawadieh et al. / Computational and Theoretical Chemistry 1018 (2013) 45–49
47
Fig. 3. Optimized geometries for intermediates and products involved in the unimolecular decomposition of the proline molecule. Interatomic distances are in Å.
self-ejection of H2, CO2 and H2O molecules. Elimination of H2 molecule from the cyclic ring is associated with activation enthalpies of 96.2, 96.3 and 81.9 kcal/mol; via the transition structures TS2, TS3 and TS4; respectively. A lower activation enthalpy for TS4 in reference to TS2 and TS3 is in accord with reported Dr Ho298:15 values in Fig. 1 with regard to the preferential loss of H atom from C gem to the carboxylic group. Formation of the stable molecules M9, M10 and M12 amounts to endothermecity of 17.2–23.8 kcal/mol. Elimination of H2O and CO2 molecules through the four-centre transition structures of TS5 and TS6 marks the two comply accepted dominant channels in the pyrolysis of alkylated carboxylic acids; namely dehydration and decarboxylation; respectively. These two reactions are associated with enthalpic activations of 71.0 and 72.8 kcal/mol; respectively. Formation of the M5 moiety through decarboxylation is coupled with a trivial exothermicity of 5.4 kcal/mol whereas the product of the dehydration channel (M6) resides 38.9 kcal/mol; above the entrance channel. No transition structures could be found for the elimination HC(O)OH group and for the formation of the M14 structure. Optimised geometries for all structures and transition states are given in Figs. 3 and 4; respectively.
3.2. Kinetic consideration Modified Arrhenius parameters at the high-pressure limit and at 1 atm are given in Table 1 for the most important initial pathways. Reaction rate constants H fission leading to the formation of M7 was calculated by setting the value of activation enthalpy to the value of the reaction enthalpy, i.e., 75.0 kcal/mol. Arrhenius parameter for this reaction in particular was obtained from a minimum energy point that resembles the longest C H elon at which the structure still holds a single imaginary frequency along the reaction coordinate. Reaction rate constants for the other six channels were calculated within the formalism of the conventional transition state theory (TST) with a treatment for tunnelling effects based on a one-dimensional Eckart functional. Arrhenius plots at the high-pressure limit and 1 atm are given in Fig. 5a and b; respectively. As shown in Fig. 5a, two channels mainly dominate the decomposition fate of the proline molecule. The dehydration pathway predominates throughout intermediate and low temperatures (T > 850 K). After this temperature, fission of C–H accompanied with the formation of the M7 becomes the most important channel. Decarboxylation channel leading to the formation of the
48
S. Rawadieh et al. / Computational and Theoretical Chemistry 1018 (2013) 45–49
Fig. 4. Optimized geometries for transition states involved in the unimolecular decomposition of the proline molecule. Interatomic distances are in Å.
Table 1 Modified Arrhenius parameters for initial exit channels in the unimolecular decomposition of proline. Product
1 atm A (s
M8 M9 M10 M12 M13 M15 H + M7
1
a
High P-limit ) 41
1.70 10 4.11 1023 4.32 1032 7.19 1031 2.11 1021 3.64 1022 3.97 1020
Ea
n
A (s1)
Ea
n
120.969 87.1091 103.903 103.921 75.0363 76.9578 77.9108
8.70 3.47 5.91 5.64 2.47 2.77 2.14
7.63 108 1.08 108 5.80 109 3.37 108 2.91 1010 1.19 1011 9.27 1010
110.18 81.7582 96.2048 96.0249 71.2447 73.0108 75.0548
1.43 1.40 1.20 1.63 0.92 0.79 1.41
M5 molecule contribute slightly by 4–6%. As shown in Fig. 5b, the onset temperature for the dominance of the dehydration channel is shifted to a lower temperature, i.e.; 500 K. k (P, T) values for the three most important reactions are plotted in Fig. 6. As shown in Fig. 6, all channels exhibit a very weak pressure-dependent behaviour. In fact, k (P, T) values reach that of k (P1, T) values at a pressure as low as 0.001 atm for most reaction channels. Our calculated reaction rate constants are significantly lower than the experimentally determined overall rate for the decay of proline. The experimental rate constant could be influenced by a profound catalytic reactor-wall effects and/or the presence of unexpected parallel bimolecular reactions. For instance, formation of 2,5-diketopiperazine as a major intermediate in the pyrolysis of proline [10] supports the hypothesis of the possible involvement of parallel reactions. Along the same line of enquiry, we have shown recently that bimolecular self-reactions were necessary to explain the experimentally determined reaction rate constant for the decomposition of acetamide [17]. 3.3. Thermochemical parameters Thermochemical parameters for all products and transition structures are given in Table 2. These data are useful for kinetic modelling. Barrier for internal rotation of the O–H group in the proline molecule was found to amount to 7.2 kcal/mol. Subsequently, it was treated as a harmonic oscillator rather than a
b
Fig. 5. Branching ratios for the three most important channels at the high P-limit (a) and 1 atm (b).
hindered rotation. Enthalpies of formation for all species were calculated based on the experimental Df Ho298:15 for proline (87.5 kcal/mol), H (52.1 kcal/mol), OH (9.3 kcal/mol), and H2O (57.8 kcal/mol) [18] along with calculated Dr Ho298:15 values in Figs. 1 and 2. However, it is worthwhile mentioning that more accurate values for calculated Df Ho298:15 could be achieved by applying isodesmic work reaction schemes. To the best of our
S. Rawadieh et al. / Computational and Theoretical Chemistry 1018 (2013) 45–49
49
Acknowledgement
4.E-07 M13
k (s -1)
3.E-07
This study was supported by a Grant of computing time from the National Computational Infrastructure (NCI), Australia (Project Id = De3).
M15 M7
2.E-07
References 1.E-07
0.E+00 1E-09
0.0000001
0.00001
0.001
0.1
10
1000
P (atm) Fig. 6. k (P, T) values for the three most important initial channels.
Table 2 Thermochemical parameters for products, and intermediates. Compound So298:15 (cal/ mol)
Df Ho298:15 C oP (T) (cal/mol) (kcal/ 300 500 800 mol)
Proline M7 M8 M9 M10 M12 M13 M15
87.52 64.65 59.46 63.76 70.33 66.18 48.64 92.88
90.75 88.99 89.39 88.54 88.78 87.02 83.49 74.37
31.10 31.60 29.83 29.78 29.10 29.83 26.18 20.78
49.09 47.86 45.57 45.64 44.95 45.45 40.52 35.64
65.86 62.99 59.72 59.71 59.42 59.57 54.11 50.50
1000 1200 1500 2000 72.88 69.32 65.51 65.46 65.37 65.38 59.86 56.93
77.98 73.92 69.68 69.62 69.64 69.57 64.03 61.65
83.30 78.71 74.07 73.95 74.04 73.91 68.34 66.58
88.51 83.40 78.24 78.195 78.30 78.19 72.59 71.39
knowledge, experimental Dr Ho298:15 values for potential reference species in plausible formulated isodesmic reactions are not available in the literature. We recommend using calculated Df Ho298:15 values in Table 2 until more accurate theoretical or experimental values become available. 4. Conclusions Structural and thermochemical properties are obtained for all species involved in the unimolecular decomposition of the proline molecule. Dr Ho298:15 values are calculated for all distinguished bonds in the proline molecule. C–H fission from carbon atom gem to the carboxylic group, decarboxylation and dehydration are found to be associated very similar energetic requirements in the range of 71.0–75.0 kcal/mol. Calculated k (P, T) values show that the region of dominance of the decarboxylation channel is shifted a higher temperature as pressure increases.
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