Bond Dissociation Enthalpies of a Pinoresinol Lignin Model Compound

The pinoresinol unit is one of the principal interunit linkages in lignin. As such, its chemistry and properties are of major importance in understanding the behavior or the polymer. This work examines the homolytic cleavage of the pinoresinol system, representing the initial step in thermal degradation. The bond dissociation enthalpy of this reaction has been evaluated using M06-2X density functional calculations. Products that allow for extensive electron delocalization are energetically favored. Calculations on subsequent reactions reveal a preference of intermediates with two unpaired electrons over a proposed four unpaired electron structure. ■ INTRODUCTION Lignin accounts for approximately 25% by weight of terrestrial plants and results from the polymerization of radicals generated by the enzymatic dehydrogenation of the cinnamyl alcohols. Because of the number of resonance forms that the radicals can assume, a variety of interunit linkages occur (Figure 1), such that the polymer is amorphous, without a specific repeat unit. With renewed interest in alternative sources of energy and chemicals from sustainable sources has come a concomitant increase in work on the utilization of lignin in such applications. Among the processes that have been proposed for the conversion of lignin are the thermal methods, such as pyrolysis and oxidative thermostabilization. The chemical mechanisms associated with the thermal degradation of lignin have been the topic of both experimental and computational studies. The former includes work on phenethyl phenyl ether and derivatives thereof, representing the ubiquitous β−O−4 linkage. From this work, it was proposed that the initial steps in thermal degradation involved homolytic cleavage of carbon−carbon and ether bonds, with subsequent radicalmediated reactions. Methoxylated models have been extensively examined by Kawamoto and Saka and their co-workers, addressing the effect of structure and substitution on primary reaction products, secondary reactions, and more recently char formation. Computational work on the pyrolysis of lignin models has been dominated in the recent past by the work of Beste and coworkers, with contributions related to reaction selectivity and kinetics. Bond dissociation enthalpy calculations, as related to thermal degradation mechanisms, have been the subject of comprehensive reports and devoted to more specific models, including β−O−4 linkages, a phenylcoumaran structure, and dibenzodioxocin. It is in the spirit of the latter papers that the current work was undertaken. As indicated, there has been considerable computational work reported on reactions of the various open-chain structures of lignin. In contrast, there have been relatively few papers concerned with cyclic lignols. As such, the objective of this paper is to evaluate the bond dissociation enthalpies of homolytic ring-opening reactions of a pinoresinol model that may occur at elevated temperatures. Specific literature related to the behavior of this structure at elevated temperatures is limited, but a recent paper on the pyrolysis of kraft lignin proposed the formation of a quintet (four unpaired electrons) by homolysis of the two α−O linkages, followed by cleavage of the remaining β−β′ bond, resulting in two coniferyl alcohol radicals. The only computational work that has been reported for pinoresinol is concerned with its antioxidant activity and, therefore, the cleavage of the phenolic group rather than the interunit linkage. ■ EXPERIMENTAL SECTION The structures for the reactant and initial products are as shown in Figure 2, along with the labels for the rings in the systems and designations that are consistent with lignin chemistry nomenclature. In accordance with the previous experimental and computational literature, homolytic cleavage reactions resulting in neutral products are examined. A crystal structure has been reported for the pinoresinol reactant and was used as the starting point for the calculations, all of which were performed with Gaussian09, revision C01, executing on a SGI Ultraviolet 2000 or Dense Memory Cluster (DMC) administered by the Alabama Supercomputer Authority. The initial calculation was performed using the M06-2X density functional method with the 6-31+G(d) basis set and the ultrafine grid, consisting of 99 radial shells and 590 angular points per shell, with geometry optimization. On the basis of the geometry from this step, a second optimization with a frequency calculation was performed with the M06-2X method, the 6-311++G(d,p) basis set, and the ultrafine grid. 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 semi-empirical method, as implemented in Spartan. The lowest 10 conformers identified were optimized using M06-2X/6-31+G(d) with the ultrafine grid. The lowest energy Received: November 23, 2013 Revised: January 22, 2014 Published: January 23, 2014 Article