Elucidation of reactive sites of wood modified with acetic anhydride: Insights from density functional theory calculations

Density functional theory (DFT) was employed to investigate the interaction of cellulose and lignin with acetic anhydride for explaining the wood modification process. Atoms in molecules (AIM) and reduced density gradient (RDG) along with non-covalent interaction (NCI) plots were used to analyse the intermolecular bonding characteristics. Cellulose was modelled with a cellobiose unit (dimer of glucose) and dibenzodioxocin was used to represent lignin model. This typical lignin model has three predominant linkages such as β-O-4, α-O-4 and 5-5’. All DFT calculations were performed at dispersion-corrected wB97X-D/6-311g(d,p) level of theory. The obtained results revealed that interaction energy of cellobiose-acetic anhydride was higher (about 20 kJ mol-1) than lignin-acetic anhydride. Structural analysis demonstrated that acetic anhydride undergoes a conformational change in lignin-acetic anhydride conformer to avoid steric repulsion from lignin aromatic moieties. Hydrogen bonds are studied to provide detailed information about strength of the interaction between cellobiose/lignin and acetic anhydride using AIM, RDG and NCI. The results showed that H-bond between cellobiose-acetic anhydride is stronger than lignin-acetic anhydride, and those H-bonds have a non-covalent character. It is observed from AIM analysis that electron density and its Laplacian for cellobiose-acetic anhydride is two-fold higher than lignin-acetic anhydride. A similar result was found in RDG analysis, and the calculated eigen value from electron density is more negative for cellulose-acetic anhydride case. This work suggested that acetic anhydride strongly bound to cellobiose during acetylation of wood rather than lignin model, and the detailed investigated data provides the interaction mechanism of acetic anhydride treatment of wood to some extent.