Computational Study of Bond Dissociation Enthalpies for Substituted β-O-4 Lignin Model Compounds

The biopolymer lignin is a potential source of valuable chemicals. Phenethyl phenyl ether (PPE) is representative of the dominant β-O-4 ether linkage. DFT is used to calculate the Boltzmann-weighted carbon–oxygen and carbon–carbon bond dissociation enthalpies (BDEs) of substituted PPE. These values are important for understanding lignin decomposition. Exclusion of all conformers that have distributions of less than 5 % at 298 K impacts the BDE by less than 1 kcal mol−1. We find that aliphatic hydroxyl/methylhydroxyl substituents introduce only small changes to the BDEs (0–3 kcal mol−1). Substitution on the phenyl ring at the ortho position substantially lowers the CO BDE, except in combination with the hydroxyl/methylhydroxyl substituents, for which the effect of methoxy substitution is reduced by hydrogen bonding. Hydrogen bonding between the aliphatic substituents and the ether oxygen in the PPE derivatives has a significant influence on the BDE. CCSD(T)-calculated BDEs and hydrogen-bond strengths of ortho-substituted anisoles, when compared with M06-2X values, confirm that the latter method is sufficient to describe the molecules studied and provide an important benchmark for lignin model compounds.