Theoretical Study on the Thermodynamic Properties and Self

J. Phys. Chem. A 2010, 114, 11751–11760

11751

Theoretical Study on the Thermodynamic Properties and Self-Decomposition of Methylbenzenediol Isomers Mohammednoor Altarawneh,*,† Ala’a H. Al-Muhtaseb,† Bogdan Z. Dlugogorski,‡ Eric M. Kennedy,‡ and John C. Mackie‡,§ Department of Chemical Engineering, Faculty of Engineering, Al-Hussein Bin Talal UniVersity, Ma’an-Jordan, Process Safety and EnVironment Protection Research Group, School of Engineering, The UniVersity of Newcastle, Callaghan, NSW 2308, Australia, School of Chemistry, The UniVersity of Sydney, Australia ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: September 4, 2010

Alkylated hydroxylated aromatics are major constituents of various types of fuels, including biomass and low-rank coal. In this study, thermochemical parameters are obtained for the various isomeric forms of methylbenzenediol isomers in terms of their enthalpies of formation, entropies, and heat capacities. Isodesmic work reactions are used in quantum chemical computations of the reaction enthalpies for O-H and H2C-H bond fissions and the formation of phenoxy- and benzyl-type radicals. A reaction potential energy on the singlet-state surface surface is mapped out for the unimolecular decomposition of the 3-methylbenzene-1,2diol isomer. According to the calculated high pressure-limit reaction rate constants, concerted hydrogen molecule elimination from the methyl group and the hydroxyl group, in addition to intermolecular H migration from the hydroxyl group, dominates the unimolecular decomposition at low to intermediate temperatures (T e 1200 K). At higher temperatures, O-H bond fission and concerted water elimination are expected to become the sole decomposition pathways. 1. Introduction Alkylated aromatics are major and growing constituents of liquid fuels such as jet fuels and gasoline. Toluene is regarded as the most significant alkylated aromatic liquid fuel component. Under oxidative conditions, where hydroxyl radicals are abundant across wide ranges of temperatures, hydroxylated substituted isomers of these alkylated aromatics can be formed via the reaction of the parent aromatic compound with hydroxyl radicals. Hydroxylated aromatics can also originate from oxidation reactions of intermediates that involve the oxygen molecule. For example, phenol is reported to be the main aromatic compound from the oxidation of benzene,1 and ortho quinone methide is also found to be an important intermediate in the oxidation of toluene.2 Furthermore, dihydroxylated benzenes, namely hydroquinone compounds, are major products from the pyrolysis and the oxidation of various types of biomass, including tobacco.3,4 It is believed that these hydroquinone compounds are sourced from breakdown of the ether linkages of lignin; a major constituent in biomass.5,6 The formation of o- and p-quinone methides from the pyrolysis of wood and low-rank coal indicates the presence of hydroxyl-substituted alkyl aromatics in these types of fuels.7-9 Recently, a great deal of research has been carried out to understand many key reaction processes in the oxidation and pyrolysis of benzene-substituted aromatics, such as benzene and dihydroxylated benzene, as model compounds for major constituents in biomass. For instance, the decomposition of toluene commences with the formation of the benzyl radical, yet, the presence of appreciable concentrations of H/O radicals could initiate the decomposition process via the formation of a * To whom correspondence should be addressed. E-mail: mn.Alt@ ahu.edu.jo. Phone: +962 3 2179000. † Al-Hussein Bin Talal University. ‡ The University of Newcastle. § The University of Sydney.

methylphenyl radical.10 Also, it has been found that the presence of two adjacent hydroxyl groups in some of the hydroxylated benzene isomers affords additional decomposition pathways that do not exist in the case of phenol. In our recent study on the self-decomposition of 1,2-dihydroxybenzene, direct expulsion of two hydroxyl H atoms forming a water molecule was found to be the dominant channel.11 It will be insightful to see the effect of introducing a methyl group on the decomposition behavior of dihydroxylated benzene isomers, a likely condition in many oxidation and combustion systems, especially in biomass. The aim of the present theoretical work is twofold. First, we intend to obtain thermo chemical parameters for all possible methylbenzenediol isomers and their derived radicals, in terms of standard heat of formation, entropy and heat capacity. Second, we wish to examine the self-decomposition behavior of a model compound for these isomers. We expect that this theoretical investigation will assist in the construction of reaction models for the decomposition of methylbenzenediol isomers and their derived radicals. 2. Computational Details The Gaussian 03 suite of programs12 was used to carry out all structural optimizations at the B3LYP13/GTLarge level of theory. The GTLarge14 basis set is a modified version of the commonly used basis set of 6-311+G(3df,2p), where more polarization functions are included in the second (3d,2f) and the first (2df) row atoms, in addition to some core polarizations. Standard enthalpies of formation were calculated using isodesmic work reactions. Values obtained for the standard enthalpies of formation at the B3LYP/GTLarge level for dihydroxylated benzene isomers were found to be very close to the values obtained by using the composite theoretical method G3B3// B3LYP/6-31G(d).15 Despite the development of various DFT functional that are capable of characterizing transition states

10.1021/jp1054765  2010 American Chemical Society Published on Web 10/08/2010

11752

J. Phys. Chem. A, Vol. 114, No. 43, 2010

Altarawneh et al.

Figure 1. Optimized structures for Methylbenzenediol isomers at the B3LYP/GTLarge level. Distances are in Å.

geometries and barrier heights, the common B3LYP functional retains its success in accurately predicting bond energies, especially in aromatic compounds. Accordingly, enthalpies of bond dissociation in this study are calculated at the B3LYP/ GTLarge level of theory.

The nature of located stationary points was distinguished either as minima or transition states (TS) through the analysis of the vibrational frequencies where a transition structure contains only one imaginary frequency along the specified reaction coordinate. Intrinsic reaction coordinate calculations

Figure 2. Optimized structures for radicals A at the B3LYP/GTLarge level. Distances are in Å.

Self-Decomposition of Methylbenzenediol Isomers

J. Phys. Chem. A, Vol. 114, No. 43, 2010 11753

Figure 3. Optimized structures for radicals B at the B3LYP/GTLarge level. Distances are in Å.

(IRC) were performed, where necessary, to connect reactants and products with their corresponding transition structures. B3LYP/6-311+G(d,p) energies and optimized geometries were used to map out decomposition pathways for one selected isomer. Rate constants, k(T), were derived based on the conventional transition state theory (TST)16 according to:

k(T) ) k(T) × kTST(T) where kTST(T) is the rate constant derived by TST, and κ(T) is the transmission coefficient that compensates for the quantum tunneling corrections accounted for with the one-dimensional Eckart functional.17 The main advantage for the use of the simple Eckart functional is that it has the potential to account for analytical transmission probabilities.17 Rate constant calculations were carried out using The Rate code18 installed at the CSEO resource (http://www.cseo.net). Internal rotations in methylbenzenediol isomers are treated as hindered rotors in the calculations of entropies and heat capacities. This treatment requires that vibrational frequencies associated with the assigned internal rotors are replaced with the barriers of rotation, moments of inertia of the rotors, and the rotational symmetry number. The STATHERM code19 is deployed to carry out the calculations of entropies and heat capacities. 3. Results and Discussions 3.1. Optimized Structures. The optimized structures for the six possible methylbenzenediol isomers are depicted in Figure

1, whereas Figures 2 and 3 show structures of the corresponding radicals, all optimized at the B3LYP/GTLarge level of theory. The most important molecular parameters are shown in Figures 1-3. Full structures are available in the Supporting Information. Overall, introducing two hydroxyl groups and a methyl group is found to induce only minor changes in the geometries with respect to the parent toluene or dihydroxybenzene, respectively. For example, the C-CH3 bond in the methylbenzenediol isomers ranges from 1.503 to 1.507 Å; while the corresponding distance in toluene is 1.507 Å. In an analogy with the dihydroxylated benzenes, the methylbenzenediol isomers can exhibit syn-anti and anti-anti conformers with respect to the orientation of the two hydroxyl hydrogen atoms. The difference in the thermodynamic stability between these two possible conformers is only apparent in the case of the presence of the two hydroxyl groups in an ortho position. In the case of dihydroxylated benzenes, the strong hydrogen bonding renders the syn-anti form of 1,2-dihydroxybenzene more stable than the anti-anti counterpart by about 4.0 kcal/mol. Accordingly, only the syn-anti conformers of the 3-methyl and 4-methyl 1,2-diol were optimized. The hydrogen bonding in these two isomers elongates the hydroxyl H group pointing to the adjacent hydroxyl group by 0.005 Å in reference to the other O-H bonds where the two hydroxyl groups are not in an ortho position. In spite of this small difference, the six methylbenzenediol isomers are found to display, to a large extent, very similar geometrical features. Two categories of resonance stabilized radicals can arise from the methylbenzenediol isomers. The first group resembles

11754


Altarawneh et al.

TABLE 1: ∆fH°298 (kcal/mol) for Methylbenzenediol Isomers and Their Derived Radicals 2-methylbenzene-1,3-diol 5-methylbenzene-1,3-diol 4-methylbenzene-1,3-diol 2-methylbenzene-1,4-diol 3-methylbenzene-1,2-diol 4-methylbenzene-1,2-diol R1a R1b R2a R2b R3a R3b R4a R4b R5a R5b R6a R6b a

Rxn-1a

Rxn-1b

exp

-73.13 -74.02 -72.90 -71.53 -74.04 -73.39 -35.04 -37.90 -36.11 -37.95 -35.83 -39.27 -34.88 -42.29 -36.36 -46.11 -36.31 -47.43

-75.69 -76.58 -75.46 -73.94 -74.03 -73.37 -37.60 -40.56 -38.67 -40.65 -38.40 -42.00 -37.1 -45.12 -36.33 -44.85 -36.29 -45.67

-71.31 ( 0.62a -71.32 ( 0.79a -70.29 ( 0.88a -69.69 ( 1.00a -71.48 ( 0.38b -71.26 ( 0.38b

Reference 21. b Reference 22.

dihydroxylated benzyl radicals and is sourced from the fission of one of the three H2C-H bonds. The second group represents methyl monohydroxylated phenoxy radicals and is produced via the breakage of one of the two hydroxyl O-H bonds. For clarity, the first- and second-group radicals are given symbols from R(1-6)a and R(1-6)b, respectively. Optimized structures of R(1-6)a and R(1-6)b, respectively, are displayed in Figures 2 and 3, respectively. The C-CH2 distances in the R(1-6)a radicals are very close to the corresponding distance in the benzyl radical (1.400 Å). Generally, all geometries are essentially very close to the corresponding distances and angles of their parent molecules as given in Figure 1. From the given structures in Figure 2, R1a and R2a possess C2V symmetry, while the remaining radicals belong to the symmetry point group of Cs. The similar O-H bond elongation (0.005 Å) in R5a and its parent molecule (3-methylbenzene-1,2-diol) indicates that the hydrogen bonding is not affected by the presence of the partial spin density at the neighboring CH2 group. The phenolic C-O distances in the R(1-6)b radicals are also very close to the equilibrium phenolic bond in the phenoxy radical (1.250 Å). All radicals in this group have a Cs symmetry point group. Despite the expected existence of small steric effects due to the flanking hydroxyl/methyl groups, all the considered structures in Figures 1-3 have a planar structure. 3.2. Thermochemistry. 3.2.1. Enthalpies of Formation. Enthalpies of formation for the methylbenzenediol isomers, the radicals R(1-6)a and the radicals R(1-6)b have been evaluated from isodesmic work reactions using the B3LYP/GTLarge method. Reactions Rxn-1a and Rxn-1b are used to calculate the standard enthalpies of formation of the methylbenzenediol isomers. Reaction Rxn-1a utilizes literature experimental enthalpy values for benzene (19.8 ( 0.2 kcal/mol),20 toluene (12.00 ( 0.26 kcal/mol),20 and phenol (-23.03 ( 0.14 kcal/mol).21 3-methylbenzene-1,2-diol is used as an example of the target species:

Table 1 gives the calculated enthalpies of formation (∆fH°298). The experimental standard enthalpies of formation for methylbenzenediol isomers listed in this table range from -69.69 ( 1.0 to -71.32 ( 0.79 kcal/mol.22,23 The values calculated from Rxn-1a differ from their corresponding experimental ones by 1.82-2.60 kcal/mol. Taking into account the marginal error in the theoretical method, along with the uncertainty in the enthalpy of formation of the reference species, the calculated and experimental enthalpies based on Rxn-1a can be considered to be in satisfactory agreement. The most stable isomers are 5-methylbenzene-1,3-diol and 3-methylbenzene-1,2-diol (-74.00 kcal/mol). The six isomers have a very comparable thermal stability within a range of 2.5 kcal/mol. In reaction Rxn-1b, the literature experimental enthalpy values for the three isomeric forms of dihydroxybenzene (C6H4(OH)2) are used (-65.9 ( 0.30 kcal/mol for o-C6H4(OH)2, -68.0 ( 0.30 kcal/mol for m- C6H4(OH)2 and -66.20 ( 0.30 kcal/mol for p- C6H4(OH)2).24a,b In the isodesmic reaction scheme of Rxn1b, the ortho isomer is used as an illustrative example. Generally, the enthalpies values obtained from Rxn-1b are not as close to the experimental values as those values calculated by (using Rxn-1a). We have used enthalpy values from Sabbah and Buluhu24a for the reference species, that is, dihydroxybenzenes, in reaction Rxn-1b. However, examination of literature values25 reveals sets of values, respectively higher by 1.8, 2.2, and 2.3 kcal/mol for ortho-, meta- and para-dihydroxybenzene. Similarly, reactions Rxn-2a and Rxn-2b, respectively, serve to obtain enthalpies of formation for the radicals a and b. In reaction Rxn-2a, the experimental enthalpy values for benzyl radical (49.5 kcal/mol), phenol, and benzene are used, whereas in reaction Rxn-2b phenol is replaced with one of the three isomeric forms of dihydroxybenzene (o-C6H4(OH2)2 is given as an illustrative example):

On the basis of the values obtained from reaction Rxn-2a, all radicals have very similar enthalpies of formation (-34.88 to -36.36 kcal/mol). As discussed above for the parent species, the corresponding enthalpies computed from reaction Rxn-2b are lower by about 2.50-3.00 kcal/mol. It is clear that hydrogen bonding is responsible for the additional stability of R5a and R6a over the other four radicals in this category. To calculate the enthalpies of formation for the phenoxytype radicals in group b, two isodesmic reactions are used, Rxn3a and Rxn-3b:


As shown in the reaction schemes, the phenoxy radical (13.9 ( 1.0 kcal/mol)26 is used in Rxn-3a, whereas in Rxn-3b the three isomeric semiquinone radicals are used (-36.4 ( 2.0 kcal/ mol for o-C6H4(OH)O, -31.1 ( 2.0 kcal/mol for m-C6H4(OH)O · and -36.0 ( 2.5 kcal/mol for p- C6H4(OH)O · ).24b As the fission of the hydroxyl H requires less energy, radicals in group (b) have more negative enthalpies of formation. For instance, based on the values obtained from Rxn-3a, the most stable radical (R6b) is associated with an enthalpy of formation of -47.43 kcal/mol, whereas the enthalpy of formation of the least stable isomer (R1b) amounts to -37.9 kcal/mol. To the best of our knowledge, these values are the first estimates for the enthalpies of formation of these radicals, and there are no other values in the literature with which to compare. Because of possible uncertainties in the reference enthalpies of the dihydroxybenzenes, enthalpies of formation computed using the isodesmic reactions Rxn-1a, Rxn-2a, and Rxn-3a are regarded as our best possible estimates for methylbenzenediol isomers, radicals a and radicals b respectively. 3.2.2. Entropies and Heat Capacities. Three vibrational frequencies in each methylbenzenediol isomer are found to correspond to internal rotations of the two OH groups about the C-OH axis and to the rotation of the methyl group. As an example of these, Table 3 lists the vibrational frequencies for the 3-methylbenzene-1,2-diol isomer wherein the three vibrational frequencies that correspond to the three internal rotations are marked in bold. These modes are treated as hindered rotors in the calculation of entropy and heat capacity. Potential energy profiles for these internal rotations in the six methylbenzenediol isomers are calculated at the B3LYP/6-31G(d,p) level. Figure 4 displays the torsional angles for three considered rotors in the 3-methylbenzene-1,2-diol isomer, in addition to the energy profiles for the OH and CH3 rotations in phenol and toluene, respectively. The calculated standard entropies and heat capacities for the six methylbenzenediol isomers are given in Table 2. Treating these modes as free rotors rather than hindered rotors would result in an increase in the calculated S°298 by about 4.0-6.0 cal/(mol K). 3.2.3. Bond Dissociation Enthalpies of the O-H and H-CH2 Bonds in Methylbenzenediol Isomers. Reaction enthalpies for the formation of radicals (a) and radicals (b) through C-CH2 and O-H bond fission, respectively, are considered here. A method to provide an accurate estimate for the O-H fission in phenol was proposed by da Silva et al.26 This method is based on using isodesmic reactions that deploy reference species with well-known enthalpies of formation (errors not greater than (0.50 kcal/mol) while maintaining the same number of radical species on both sides of the reaction. Implementing this methodology for the two considered reactions in the case of methylbenzenediol (C6H3(OH)2CH3) yields:


Figure 4. Internal rotors’ potentials for the two hydroxyl Hs in 3-methylbenzene-1,2-diol (a and b with the same magnitude), the rotation of the methyl group in 3-methylbenzene-1,2-diol (c), the hydroxyl H rotation in phenol (d), and the methyl group rotation in toluene (e) at the B3LYP/6-31G(d,p) level.

C6H3(OH)2CH3 + X• f R1a + XH

(Rxn-4a)

C6H3(OH)2CH3 + X• f R1a + XH

(Rxn-4b)

Then, the bond dissociation enthalpy (BDH) is calculated as:

BDH(H2C-H) ) ∆rxnH°298(Rxn-4a) + ∆fH°298(H) + ∆fH°298(X) - ∆fH°298(XH) BDH(O-H) ) ∆rxnH°298(Rxn-4b) + ∆fH°298(H) + ∆fH°298(X) - ∆fH°298(XH) ° (Rxn-4a) and∆rxnH298 ° (Rxn-4b) are the B3LYP/ where ∆rxnH298 GTlarge calculated enthalpies of the reactions Rxn-4a and Rxn4b, respectively. In order to increase the accuracy of the calculated bond enthalpy, similar type of radicals is used on each side of the isodesmic reactions. In reaction Rxn-4a, X is considered to be one of the carbon-centered radicals of C2H5, C3H5 or C6H5CH2 radicals; and in Rxn-4b, X is considered to be one of the oxygen-centered radicals of HO2, CH3O, or C2H3O radicals. Tables 4 and 5 list the calculated ∆rxnH°298(Rxn-4a) and ∆rxnH°298(Rxn-4b) for the three chosen radicals, respectively. The reaction enthalpies for the H-CH2 bond fission in methylbenzenediol isomers are given in Table 4 as the average values obtained from the corresponding three (Rxn-5a) reactions. On the basis of the results in Table 6, the fission of the H-CH2 bond in the six isomers requires 89.0-91.0 kcal/mol at 298 K. Table 7 lists the enthalpies of dissociation of the O-H bond in the six methylbenzenediol isomers. Bond enthalpies for the fission of one O-H bond in the six methylbenzenediol isomers range from 88.97 kcal/mol for fission in 2-methylbenzene-1,3diol and formation of R1b to 77.0 for the O-H bond fission in 4-methylbenzene-1,2-diol and formation of R6b. 3.3. Decomposition of 3-Methylbenzene-1,2-diol. 3.3.1. Potential Energy Surface. Unimolecular decomposition of methylbenzenediol isomers are expected to have comparable features with unimolecular decomposition pathways of the analogous compounds of catechol, phenol, and toluene. However, the presence of adjacent methyl and hydroxyl groups warrant

11756


Altarawneh et al.

TABLE 2: S°298 in cal/(mol K) and C°(T) in cal/(mol K) for Methylbenzenediol Isomers p Cp°(T) 2-methylbenzene-1,3-diol 5-methylbenzene-1,3-diol 4-methylbenzene-1,3-diol 2-methylbenzene-1,4-diol 3-methylbenzene-1,2-diol 4-methylbenzene-1,2-diol

S°298K 86.04 86.21 86.56 88.43 85.93 84.83

300 K 30.34 36.0 36.2 35.8 35.9 32.90

TABLE 3: Vibrational Frequencies (cm-1) of 3-Methybenzene-1,2-diol (B3LYP/GTlarge)a 123.5251 243.5392 318.2635 507.4891 573.3050 785.0884 952.8411 1064.6789 1185.7474 1309.1121 1423.6437 1517.9279 1662.0376 3111.8116 3191.4870 a

148.1855 285.0379 446.3438 536.9473 691.9889 843.2594 962.9636 1103.7860 1244.6820 1334.2604 1483.8986 1539.6138 3033.2268 3155.7143 3796.8525

212.4346 302.5209 496.1096 573.2852 743.4215 877.9470 1036.4424 1164.7647 1262.3619 1385.0086 1484.4821 1633.9878 3077.4040 3168.9785 3851.1732

Values listed in bold correspond to internal rotations.

investigating the presence of potential additional channels. In this section, the unimolecular decomposition of 3-methylbenzene-1,2-diol as a model for the methylbenzenediol isomers that contain ortho-positioned methyl and hydroxyl group. This isomer, in particular, has been chosen because it contains the catechol moiety (1,2-dihydroxybenzene) that is used widely as a model compound for structural entities present in lignin.27,28 Self-decomposition of 3-methylbenzene-1,2-diol is expected to involve similar initial channels to those involved in the decomposition of toluene, phenol, and catechol. The most important initial channels in these compounds are: fission of the benzyl-H bond in toluene,10 formation of cyc-C6H6O in phenol,29 and direct elimination of a water molecule in catechol.11 In view of the models presented for the self-decomposition of these three analogous compounds, Figure 5 depicts plausible initial decomposition channels in the decomposition of 3-methylbenzene-1,2-diol calculated at the B3LYP/6311+G(d,p) level of theory. Channels A and B represent direct fission of the O-H bond and the H2C-H bond through endoergic reactions of 72.9 and 85.5 kcal/mol, respectively. Both bond fissions occur without passing an intrinsic barrier. The products from channels A and B are R5b and R5a, respectively. Channels C and D involve H migration from the methyl group to the neighboring C atoms bearing a hydroxyl group and an H atom, respectively. The calculated reaction barriers for channels C and D amount to 91.3 and 92.9 kcal/mol, respectively, characterized by the transition structures TS1 and TS2; respectively. Geometries for all transition structures are given in Figure 6. Channel F represents a concerted elimination of a hydrogen molecule from the two hydroxyl groups. Another direct elimination of a hydrogen molecule can also take place in channel I through TS7. As shown in the structure of TS7 (Figure 6), a methyl H departs the molecule in a concerted movement with a hydroxyl H. Hydrogen molecule elimination in channel I is associated with a reaction barrier that is 13.0 kcal/mol lower than the barrier encountered in the hydrogen elimination in channel F.

500 K 47.20 47.55 47.25 48.00 48.49 50.01

800 K 63.00 63.42 63.02 62.98 63.53 65.45

1000 K 69.61 69.32 69.63 69.20 69.80 71.84

1400 K 78.00 77.60 78.02 77.10 77.77 79.96

TABLE 4: Reaction Enthalpies (kcal/mol) Used in the Estimation of the H2C-H Bond in Methylbenzenediol Isomers

∆rxnH°298(Rxn-4a) 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

2-methylbenzene-1,3-diol + C2H5 f R1a + C2H6 2-methylbenzene-1,3-diol + C3H5 f R1a + C3H6 2-methylbenzene-1,3-diol + C6H5CH2 f R1a + C6H6CH3 5-methylbenzene-1,3-diol + C2H5 f R2a + C2H6 5-methylbenzene-1,3-diol + C3H5 f R2a + C3H6 5-methylbenzene-1,3-diol + C6H5CH2 f R2a + C6H6CH3 4-methylbenzene-1,3-diol + C2H5 f R3a + C2H6 4-methylbenzene-1,3-diol + CH3O f R3a + C3H6 4-methylbenzene-1,3-diol + CH2 ) CHO f R3a + C6H5CH3 2-methylbenzene-1,4-diol + C2H5 f R4a + C2H6 2-methylbenzene-1,4-diol + CH3O f R4a + C3H6 2-methylbenzene-1,4-diol + CH2dCHO f R4a + C6H5CH3 3-methylbenzene-1,2-diol + C2H5 f R5a + C2H6 3-methylbenzene-1,2-diol + CH3O f R5a + CH3OH 3-methylbenzene-1,2-diol + CH2dCHO f R5a + C6H5CH3 4-methylbenzene-1,2-diol + C2H5 f R6a + H2O2 4-methylbenzene-1,2-diol + CH3O f R6a + CH3OH 4-methylbenzene-1,2-diol + CH2dCHO f R6a + C6H5CH3

-11.37 4.50 0.58 -11.54 4.33 0.41 -12.39 3.47 -0.44 -12.80 3.07 -0.84 -11.76 4.11 0.19 -12.62 3.25 -0.66

The expulsion of the hydrogen molecule via TS5 is associated with an activation barrier of 75.5 kcal/mol. Departure of a water molecule in channel G produces a methyl substituted fivemembered ring as shown in Figure 5 passing through the transition structure of TS6 located 76.8 kcal/mol above the entrance channel. Hydroxyl H migration to the neighboring C atom bearing a hydrogen atom takes place through the transition structure TS4, which resides 66.2 kcal/mol above the entrance channel. Finally, the C-CH3 bond fission is considered in channel J. C-CH3 bond fission is reported to demands 97.2



TABLE 5: Reaction Enthalpies (kcal/mol) Used in the Estimation of the O-H Bond in Methylbenzenediol Isomers

∆rxnH°298(Rxn-4b) 1 2 3

1 2 3

1 2 3

1 2 3

1 2 3

1 2 3

2-methylbenzene-1,3-diol + HOO f R1b + H2O2 2-methylbenzene-1,3-diol + CH3O f R1b + CH3OH 2-methylbenzene-1,3-diol + CH2dCHO f R1b + CH2dCHOH 5-methylbenzene-1,3-diol + HOO f R2b + H2O2 5-methylbenzene-1,3-diol + CH3O f R2b + CH3OH 5-methylbenzene-1,3-diol + CH2dCHO f R2b + CH2dCHOH 4-methylbenzene-1,3-diol + HOO f R3b + H2O2 4-methylbenzene-1,3-diol + CH3O f R3b + CH3OH 4-methylbenzene-1,3-diol + CH2dCHO f R3b + CH2dCHOH 2-methylbenzene-1,4-diol + HOO f R4b + H2O2 2-methylbenzene-1,4-diol + CH3O f R4b + CH3OH 2-methylbenzene-1,4-diol + CH2dCHO f R4b + CH2dCHOH 3-methylbenzene-1,2-diol + HOO f R5b + H2O2 3-methylbenzene-1,2-diol + CH3O f R5b + CH3OH 3-methylbenzene-1,2-diol + CH2dCHO f R5b + CH2dCHOH 4-methylbenzene-1,2-diol + HOO f R6b + H2O2 4-methylbenzene-1,2-diol + CH3O f R6b + CH3OH 4-methylbenzene-1,2-diol + CH2dCHO f R6b + CH2dCHOH

-0.63 -17.49 1.46

0.22 -16.64 2.31

-2.22 -19.08 -0.13

TABLE 6: Bond Dissociation Energies (BDH) (kcal/mol) for the H2C-H Bond in Methylbenzenediol Isomers reaction No. (from Table 5)

radical produced

BDH(H2C-H)

1 2 3 average 1 2 3 average 1 2 3 average 1 2 3 average 1 2 3 average 1 2 3 average

R1a

89.13 92.92 90.19 90.75 88.96 92.45 90.02 90.47 88.11 92.59 89.16 89.95 87.71 91.19 88.75 89.22 88.74 92.23 89.79 90.25 88.89 92.37 88.94 90.00

R2a

R3a

R4a

R5a

R6a

TABLE 7: Bond Dissociation Energies (BDH) (kcal/mol) for the O-H Bond in Methylbenzenediol Isomers -6.62 -23.47 -4.52

-8.41 -25.27 -6.33

-9.90 -26.75 -7.81

kcal/mol to take place, that is, 11.7 and 24.3 kcal/mol, higher than the energy needed in the O-H and H2C-H bond fissions, respectively. 3.3.2. High-Pressure Limit Reaction Rate Constants. Thermal rate constants at the high-pressure limit were calculated using the located transition structures. Modified Arrhenius parameters for the eight initial channels are given in Table 8. For the H2C-H fission reaction, which does not have a discrete TS, the experimental A factor obtained for the reaction (toluene f benzyl + H) was applied.30 Similarly, the A factor calculated by Zhu and Bozzelli31 for the O-H dissociation in phenol was used for the O-H bond fission in 3-methylbenzene-1,2-diol. The calculated enthalpies of reaction at 298.15 K in this study

reaction No. (from Table 5)

radical produced

BDH(O-H)

1 2 3 average 1 2 3 average 1 2 3 average 1 2 3 average 1 2 3 average 1 2 3 average

R1b

87.20 87.09 86.62 86.97 88.05 87.94 87.47 87.82 85.61 85.49 85.03 85.38 81.22 81.10 80.64 80.99 79.42 79.30 78.84 79.18 77.94 77.82 77.36 77.70

R2b

R3b

R4b

R5b

R6b

were deployed as the activation energy (72.7 kcal/mol for the O-H bond fission and 85.5 kcal/mol for the H2C-H fission reaction). To assess the contribution of each channel to the breakdown of 3-methylbenzene-1,2-diol, branching ratios based on the calculated thermal rate constants are calculated and plotted in Figure 7. As illustrated in Figure 7, at low temperature, decomposition of the parent molecule occurs solely through direct hydrogen molecule expulsion via channel I and hydroxyl H migration to the neighboring C atom through channel E. The relative importance of channels I and E decreases as temperature increases. In an analogy with phenol and catechol, the product of channel E is expected to give cyc-C5H4CH3(OH) subsequent to CO elimination.

11758


Altarawneh et al.

Figure 5. Reaction pathway map for the unimolecular decomposition of 3-methylbenzene-1,2-diol on the singlet-state surface. Values in italic are the activation energy, and values in bold are the reaction energies, both at the B3LYP/6-311+G(d,p).

Figure 6. Optimized structures for the transition structures at the B3LYP/6-311+G(d,p) level. Distances are in Å.



TABLE 8: Modified Arrhenius Parameters for Key Reactions, Forward (f) (A in s-1 and Ea(f)/R in K) and Reverse (r) (A in cm3/(molecule · s) and Ea (r)/R in K), in the Unimolecular Decomposition of 3-Methylbenzene-1,2-diol reaction

3-methylbenzene-1,2-diol 3-methylbenzene-1,2-diol 3-methylbenzene-1,2-diol 3-methylbenzene-1,2-diol 3-methylbenzene-1,2-diol 3-methylbenzene-1,2-diol 3-methylbenzene-1,2-diol 3-methylbenzene-1,2-diol a

A(f)

f f f f f f f f

A+H B+H C D E F + H2 G + H2O I + H2 a

7.63 × 10

14

n(f)

Ea(f)/R

A(r)

n(r)

Ea(r)/R

0.0

36 600

3.15 × 1015 5.46 × 108 7.70 × 1011 3.86 × 1011 2.00 × 1012

0.0 1.36 0.0 0.0 0.0

43 000 45 500 44 700 29 300 36 200

1.86 × 109 8.68 × 1011 5.54 × 1011 4.30 × 10-14

1.24 0.0 0.0 0.0

26 500 27 500 23 500 16 500

7.58 × 1012

0.85

39 400

1.31 × 10-22

3.11

23 800

31 000

-22

2.58

11 300

8.34 × 10

11

0.0

3.77 × 10

The A factor is taken from ref 31.

Acknowledgment. This study has been funded by the Australian Research Council and supported by a grant of computing time from the Australian Centre of Advanced Computing and Communications (AC3). Supporting Information Available: Calculated total energies, zero-point energies, Cartesian coordinates, moments of inertia, and vibrational frequencies of all structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 7. Estimated branching ratio for product yields from the unimolecular decomposition of 3-methylbenzene-1,2-diol based on the high-pressure limit rate constants in Table 7.

Throughout the intermediate temperature range (800-1200 K), channel A, O-H bond fission, dominates the unimolecular decomposition. In view of the well-established mechanism for the phenoxy radical decomposition, the exit pathway for the product of channel A (R6b) must involve ring contraction/CO elimination. As the temperature increases, water elimination via the dehydration channel G becomes competitive with O-H bond fission. Due to the substantial energy barrier required for hydrogen molecule elimination from the two hydroxyl H through channel F, the formation of methyl benzoquinone is unimportant at all temperatures. Contribution of channels C and D is also found to be negligible at all temperatures. Conclusions Optimized geometries, vibrational frequencies, internal rotor potentials, and thermochemical properties are calculated for the various isomeric forms of methylbenzenediol isomers and their phenoxy and benzyl type radicals. Bond dissociation enthalpies are obtained using isodesmic work reactions. The calculated enthalpies of formation for methylbenzenediol isomers are found to be relatively close to the available literature data. Unimolecular decomposition of the 3-methylbenzene-1,2-diol is investigated in view of the unimolecular decomposition pathways of the analogous compounds of catechol, phenol and toluene. The presence of an ortho methyl group is found to facilitate concerted elimination of a hydrogen molecule; a channel that is important over a wide range of temperatures.

(1) Alzueta, M. U.; Oliva, M.; Glarborg, P. Int. J. Chem. Kinet. 2000, 32, 498. (2) da Silva, G.; Chen, C. C.; Bozzelli, J. W. J. Phys. Chem. A 2007, 111, 8663. (3) Truong, H.; Lomnicki, S.; Dellinger, B. Chemosphere 2008, 71, 107. (4) Lomnicki, S.; Truong, H.; Dellinger, B. Chemosphere 2008, 73, 629. (5) Maskos, Z.; Dellinger, B. Energy Fuels 2008, 22, 1675. (6) Maskos, Z.; Khachatryan, L.; Dellinger, B. Energy Fuels 2005, 19, 2466. (7) Dorrestijn, E.; Epema, O. J.; van Scheppinger, W. B.; Mulder, P. J. Chem. Soc., Perkin Trans. 2 1998, 1173. (8) Dorrestijn, E.; Laarhoven, L.; Arends, I.; Mulder, P. J. Anal. Appl. Pyrolys. 2000, 54, 153. (9) Paul, G. C.; Gajewski, J. J. J. Org. Chem. 1993, 58, 5060. (10) da Silva, G.; Moore, E. E.; Bozzelli, J. W. J. Phys. Chem. A 2009, 113, 10264. (11) Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. J. Phys. Chem. A 2009. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Gaussian, Inc.: Wallingford, CT, 2001. (13) Becke, A. D. J. Chem. Phys. 1996, 104, 1040. (14) Montgomery, J. A.; Jr. n.; Ochterski, J. W.; Petersson, G. A. J. Chem. Phys. 2000, 112, 6532. (15) Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. J.Phys. Chem. A 2009, 114, 1098. (16) Eyring, H. J. Chem. Phys. 1935, 3, 107. (17) Eckart, C. Phys. ReV. 1930, 35, 1303. (18) Duncan, W. T.; Bell, R. L.; Truong, T. N. J. Comput. Chem. 1998, 19, 1309. (19) Mackie, J. C. STATHERM Code, 2000. (20) Roux, M. V.; Temprado, M.; Chickos, J. S.; Nagano, Y. J. Phys. Chem. Ref. Data 2008, 37, 1855. (21) Cox, J. D. Pure Appl. Chem. 1961, 2, 125.

11760


(22) Manuel, A. V.; da Silva, R.; Ana, I. M. C.; Ferreira, L. J. Chem. Thermodyn 2009, 41, 1096. (23) Riberio Da Silva, M. D. M. C.; Riberio Da Silva, M. A. V.; Pilich, G. J. Chem. Thermodyn 1984, 15, 1194. (24) (a) Sabbah, R.; Buluku, E. Can. J. Chem. 1991, 69, 481. (b) Fattahi, A.; Kass, S. R.; Liebman, J. F.; Matos, M. A. R.; Miranda, M. S.; Morais, V. M. F. J. Am. Chem. Soc. 2005, 127, 6116. (25) Afeefy, H. Y. Liebman, J. F. and Stein, S. E. Neutral Thermochemical Data in NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G. , Eds.; National Institute of Standards and Technology: Gaithersburg MD, 20899; http:// webbook.nist.gov (retrieved April 30, 2010).

Altarawneh et al. (26) da Silva, G.; Chen, C. C.; Bozzelli, J. W. Chem. Phys. Lett. 2006, 424, 42. (27) Wornat, M. J.; Ledesma, E. B.; Marsh, N. D. Fuel 2001, 80, 1711. (28) Ledesma, E. B.; Marsh, N. D.; Sandrowitz, A. K.; Wornat, M. J. Proc. Combust. Inst. 2002, 29, 2299. (29) Xu, Z. F.; Lin, M. C. J. Phys. Chem. A 2006, 110, 1672. (30) Braun-Unkhoff, M.; Frank, P.; Just, T. Proc. Combust. Inst. 1989, 22, 1053. (31) Zhu, L.; Bozzelli, J. W. J. Phys. Chem. A 2003, 107, 3696.

JP1054765

Theoretical Study on the Thermodynamic Properties and Self

Recommend Documents