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

Abstract 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.