Article pubs.acs.org/EF
Bond Dissociation Enthalpies of a Dibenzodioxocin Lignin Model Compound Thomas Elder* USDA-Forest Service, Southern Research Station, Pineville, Louisiana, United States S Supporting Information *
ABSTRACT: The initial steps in the thermal degradation of dibenzodioxocin, a relatively recently discovered seven-memberedring linkage in lignin, have been evaluated using density functional calculations. The bond dissociation enthalpy of the various ring-opening reactions is closely related to the delocalization of the unpaired electrons in the products. It also appears as if these are barrierless reactions.
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INTRODUCTION Lignin is the most globally abundant, naturally occurring aromatic polymer, accounting for about 25% by weight of woody plants. The polymer is derived from the enzymatic dehydrogenation of the cinnamyl alcohols, the products of which may be described by several resonance forms. As a consequence, the polymer is amorphous and irregular, with several different interunit linkages, consisting of carbon−carbon and ether bonds. The dibenzodioxocin linkage (Figure 1) in lignin was first identified in 1995,1 and subsequently a crystal structure was reported.2 In the intervening years, there have been numerous reports with respect to its chemistry.3−6 A recent contribution,7 in which experimental and computational methods are applied to diobenzodioxocin, is particularly germane to the current paper. Products of thermal degradation and energy barriers associated with bond cleavage are reported in the referenced work. The current interest in alternative and renewable sources of energy and chemicals has resulted in a renewed interest in lignin, particularly as part of the biorefinery concept. In this context, the thermal degradation of lignin has been examined using both experiment and calculation to elucidate mechanistic details of these reactions. While the experimental literature on lignin pyrolysis is voluminous, some notable examples of proposed mechanisms based on model compound studies come from Saka and co-workers,8−11 Britt and co-workers,12,13 and, recently, Holmelid et al.14 In general, the experimental evidence indicates that the initial steps are homolytic bond cleavage reactions. In addition to the experimental work, computational methods have been applied to the proposed reactions through examinations of bond dissociation enthalpies, 15−21 kinetics,22−24 and reaction selectivity.25,26 While bond dissociation enthalpies for a wide range of dilignols have been reported,15,17−19 there has been relatively little work done on the fused ring structures, such as pinoresinol, phenylcoumaran, or dibenzodioxocins. The exceptions to this generalization are the recent papers by Younker et al.21 and Gardrat et al.7 The objective of the current work is to apply contemporary computational methods to evaluate the energetics, electronics, and geometry associated with the ring-opening © 2013 American Chemical Society
Figure 1. Reactant and products under study in the current work. Received: May 31, 2013 Revised: July 7, 2013 Published: July 8, 2013 4785
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Table 1. Bond Dissociation Enthalpy and Interatomic Distances for Each of the Ring-Opened Products optimized 5−5′ α−O α−β β−O
conformational search
enthalpy (kcal mol−1)
distance (Å)
enthalpy (kcal mol−1)
distance (Å)
114.27 45.79 71.98 57.13
3.40 3.54 3.22 3.37
113.62 42.22 67.81 57.28
5.15 4.49 4.78 6.10
reactions, such as may occur in the initial stages of lignin pyrolysis, of a dibenzodioxocin model.
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METHODS
The reactants and products (the designations of which are based on lignin nomenclature) under consideration in the current work are as shown in Figure 1. All calculations were done using Gaussian 09, Revision C.01,27 as implemented on SGI-Altix and SGI-UV supercomputers administered by the Alabama Supercomputer Authority. A crystal structure has been reported for dibenzodioxocin, upon which the reactant in this paper is based.2 The geometry of the crystal structure was optimized as a singlet at the M06-2X level of theory, using the 6-31+G(d) basis set and the ultrafine grid consisting of 99 radial shells and 590 angular points per shell. Furthermore, to verify the quality of the crystal structure geometry, it was used as the starting point for a 500-step Monte Carlo search, with optimization using the PM3 semiempirical method as implemented in Spartan.28 The lowest 10 conformers identified were optimized using M06-2X/6-31+G(d) with the ultrafine grid. The lowest-energy conformation was used as the input for an optimization and frequency calculation using M062X/6-311++G(d,p) with the ultrafine grid. The products of the reaction were evaluated using the triplet electronic configuration. Although calculations on such open-ring products could be done as either triplets or singlet diradicals, the former is justified based on previous results21 for phenylcoumaran lignin models in which small singlet−triplet gaps and similar bond dissociation enthalpies were found for both configurations. Two sets of calculations were performed on the ring-opened products. In the first, beginning with an interatomic distance of 2.5 Å, a complete geometry optimization was performed at the M06-2X/631+G(d) level with an ultrafine grid, followed by an optimization and frequency calculation using M06-2X/6-311++G(d,p) also with the ultrafine grid. The second set of calculations involved a conformational search, as previously described for the dibenzodioxocin parent compound, with corresponding density functional theory optimizations and frequency calculations. The latter were performed under the assumption that, upon ring-opening, and at elevated temperatures, the structure may undergo considerable conformational changes as evaluated by the Monte Carlo search and subsequent minimizations. Parenthetically, it was found that, if bonds were simply deleted, the optimization would revert to the original structure, accounting for the initiation of the open-ring structure calculations at a distance of 2.5 Å.
Figure 2. Geometries of reactant and products.
Table 2. Ring Plane Angles, in deg, and Inter-Ring Distances in Å (in Parentheses) for the Reactant and Each of the Ring-Opened Products conformer reactant 5−5′ α−O α−β β−O
optimized conformational optimized conformational optimized conformational optimized conformational
search search search search
A−B 52.66 73.90 76.46 69.06 44.85 65.71 65.81 68.99 47.80
(4.291) (4.555) (6.022) (4.303) (4.315) (4.295) (4.290) (4.303) (4.306)
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A−C 42.16 55.70 79.32 1.90 9.90 63.09 81.08 40.81 42.75
(4.301) (4.191) (5.049) (3.395) (3.824) (4.750) (6.058) (4.091) (4.504)
B−C 75.77 32.81 26.03 70.94 39.80 87.06 20.16 58.23 10.00
(6.580) (5.622) (6.631) (6.042) (4.940) (6.733) (3.678) (6.219) (3.686)
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Bond dissociation enthalpies were calculated as the difference in enthalpy of the ring-opened products and the dibenzodioxocin reactant.
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RESULTS AND DISCUSSION After conformational searching and geometry optimization of the dibenzodioxocin reactant, the lowest-energy conformer identified was that based on optimization of the original crystal structure. The enthalpies at 298 K for each bond cleavage, and final interatomic distances for the open positions, are as shown in Table 1. All of the reactions are endothermic, the highest of which, perhaps not surprisingly, is associated with the 5−5′ bond, between two aromatic carbons. Albeit, markedly lower, the other carbon−carbon bond, between the α- and β-carbons is the next highest. The lower enthalpies of dissociation occur with cleavage of the ether bonds. Within the latter, the α−O bond exhibits a lower dissociation enthalpy than the β−O, which is consistent with open-chain models,18 and the α−O values are also comparable to that reported for phenylcoumaran models.21 The β−O bond in the current work is found to be more labile than that for dilignols in the literature, whereas the BDE for the 5−5′ linkage is comparable to that previously reported.18−20 In general, conformational searching identified conformers with lower enthalpies of reaction. The exception to this generalization is the product of β−O cleavage for which the search found a structure that is very slightly less stable then the optimized conformer. Not surprisingly, the interatomic distances are greatest for the conformational search results, being as high as 6.10 Å, whereas the structures that were allowed to optimize after ring-opening ranged from 3.22 to 3.54 Å. The optimized structures for the dibenzodioxocin reactant and each of the products are as shown in Figure 2 (Cartesian coordinates for each structure are provided in the Supporting Information). Among the more striking observations is the variability in conformation of the aromatic rings for the openring structures, due to bond rotations resulting in differences in dihedral angles. Table 2 shows the plane angles formed between each ring and the distance between the center of each ring. The angles formed between rings A and B vary from ∼45° to 76°, the highest of which occurring with cleavage of the 5−5′ bond. Similarly, the A−B inter-ring distances exhibit only small variation, except for the 5−5′ product, which is not unsurprisingly larger. The most striking feature of the A−C plane angle results occurs with α−O bond cleavage. As can be seen in Figure 2 and quantitatively in Table 2, these rings are approaching parallel, and in close proximity for both α−O products. It is also interesting to note that the distance between the rings (∼3.4−3.8 Å) is in close agreement with the value previously reported for a parallel benzene dimer.29 The largest A−C separation occurs with conformational searching of the α−β product. The optimized α−β and α−O ring-opened products exhibit B−C plane angles of 87.06° and 70.94°, respectively, approximating the T-conformation of benzene dimers, but with larger separations (∼6 Å) than those reported for the latter.29 Because of the latter observation, the preferred conformation notwithstanding, little stabilization would be expected to occur. The lowest-energy structure identified by conformational searching of the product of β−O bond cleavage was found to have a B−C plane angle of 10.00° and an inter-ring distance of 3.686 Å, again approaching a parallel conformation.
Figure 3. Dihedral angles of reactant and products.
Plots of the dihedral angles formed by the atoms associated with the seven-membered ring (see Figure 1 for the atom 4787
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Figure 4. Spin density and orbital plots for optimized structures.
somewhat similar trends, except for the 21−26−2−27 and 20−15−1−14 dihedrals, on either side of the α- and β-carbons. Figure 4 shows the unpaired spin density and plots for orbitals α-112 and α-113, the highest-energy α-orbitals for the triplet systems of the optimized open-ring products, whereas Figure 5 shows these data for the low-energy conformers identified by conformational searching. The spin density plots are similar for both sets of products, and the orbital plots when taken together generally mirror the unpaired spin densities. The exception to this generalization is the 5−5′ open-ring product with, as would be expected, high spin densities at the 5-carbon positions, but the plots for orbitals 112 and 113 are typical of aromatic systems. While this might suggest that these are not singly occupied, a further examination of lower-energy orbitals did not reveal plots that were spatially similar to the unpaired spin density results. On the basis of the spin densities, the α−O cleavage products exhibit the highest degree of delocalization, whereas the open 5−5′ structures, with the spin density largely confined to the open aromatic carbons, exhibit the least. These observations are also consistent with the bond dissociation enthalpies, which show that the former is the
numbering) are as shown in Figure 3. It can be seen that, while the absolute values for the optimized products differ from those for the original dibenzodioxocin, the trends are very similar, with correlation coefficients of 0.91−0.97. These results would indicate that relative changes in the torsional angles are small upon initial ring-opening. In contrast, the geometries of the product conformers identified by conformational searching are somewhat different. The exceptions to this generalization are the α−O and β−O products. The former exhibits very close correspondence between the optimized ring-opened product and the low-energy conformers. The latter tracks closely, with the apparent exception of the 27−14−1−15 dihedral angle. This, however, is an artifact of how the dihedral angle is determined. The raw value, −175°, results from clockwise rotation above 180°, such that the dihedral angle could also be recorded as +185°, making the two conformers much more consistent internally and with the dibenzodioxocin reactant. The products of 5−5′ bond cleavage are quite different from each other, except notably the 15−20−21−26 dihedral angle corresponding to the broken 5−5′ bond, perhaps reflecting the similarity in the A−B plane angles. The α−β products exhibit 4788
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Figure 5. Spin density and orbital plots for optimized structures of low-energy conformers.
determined using semiempirical and density functional calculations. At the highest level (M06-2X/6-31G(d,p), values of 35.8 and 49.0 kcal mol−1 were found for α−O and β−O, respectively. In the current work, the bond dissociation enthalpies associated with these reactions (at the M06-2X/6-311++G(d,p) level) were calculated to be 45.79 and 57.13 kcal mol−1, with optimization and 42.22 and 57.28 kcal mol−1 with conformational searching. In other related work,30 homolytic cleavage reactions of 2-phenethyl phenyl ether at the CBS-QB3 level were found to not exhibit significant potential energy surface barriers, such that the activation energies are approximated by the bond dissociation enthalpies. Similarly, calculations on phenethyl phenyl ether using M06-2X/6-311++G(d,p) found that the dissociation reaction did not exhibit a transition state,20 reaching the same conclusion regarding the correlation between activation energy and bond dissociation enthalpies. These results were also invoked in the interpretation of phenylcoumaran ring-opening reactions,21 and would account for the correspondence between the current results and those of Gardrat et al.7 Furthermore, per the selectivity arguments of
lowest and the latter is the highest. Furthermore, the parallel conformation of the rings and orbital plots for the α−O structure are indicative of π-stacking, which may result in additional energetic stabilization. Not unsurprisingly, the products of β−O and α−β cleavage exhibit intermediate degrees of electron delocalization and dissociation enthalpies. It is also of interest that, with conformational searching, the α−β product identified has a decreased bond dissociation enthalpy, a more parallel conformation of the B−C rings (20.16 vs 87.06°), and smaller inter-ring separation (3.678 vs 6.773 Å). With respect to the β−O products, conformational searching also reveals a structure with a parallel (10.00°) B−C plane angle, a close proximity of the rings (3.686 Å), and orbital plots indicative of π-stacking. Energetically, however, this conformation actually has a slightly higher bond dissociation enthalpy than the corresponding optimized structure. It might be concluded, therefore, that delocalization of the unpaired electrons is a more important predictor of stability than geometry or orbital interactions. In the recent literature,7 transition-state barriers of α−O and β−O bond cleavage for a dibenzodioxocin model were 4789
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Younker et al.,21 and given the lability of the α−O bond cleavage relative to the other ring-opening reactions, this would be virtually the exclusive product of these reactions. Experimental work7 reports that, at elevated temperatures, dibenzodioxocin initially degrades to 4-vinyl-guaiacol and a 5−5′ linked diphenolic compound. On the basis of the current results, it could be proposed that the first step in such a process would be the cleavage of the α−O bond, followed by the β−O bond of the ring-opened product, leading to 4-vinyl guaiacol. Secondary reactions of the 5−5′ diphenolic product are reported to result in a number of guaiacol derivatives.
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CONCLUSIONS These data show that the cleavage of the α−O bond in dibenzodioxocin is energetically favored in the initial stages of thermal degradation and that the other products of ringopening reactions would be present in vanishingly small amounts. Furthermore, the stability of this product can be traced to increased delocalization of the unpaired electrons, and a geometry that is amenable to π-stacking, which is also supported by plots of the singly occupied molecular orbitals. Additionally, based on the current calculations and previous work,7,21,30 the ring-opening reaction appears to be barrierless. These results may be of utility in evaluating subsequent steps in the decomposition of dibenzodioxocin in particular and lignin in general.
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ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates for each structure. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The author is indebted to Dr. Ariana Beste, for her comments and suggestions, and Dr. David C. Young, for his support in the performance of this work. The author declares no competing financial interest.
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REFERENCES
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