Behavior of the βO4 bond in chlorinated lignin models: a computational theoretical approach

This work uses a computational approach to evaluate the effect of replacing hydroxyl groups with chlorine at the α and γ positions on the βO4 bond in lignin models of the HH, GG, and SS types (H – hydroxyphenyl; G – guaiacyl; S – syringyl). The lowest energy conformations were obtained using the implicit aqueous GFN2-xTB level theory, followed by geometric optimizations using density functional theory (DFT) and thermochemical data. Electronic density analyses were performed using QTAIM and ELF. The results indicate that chlorination favors cleavage of the βO4 bond, reducing stability compared to unsubstituted models, as evidenced by ΔG and ΔH values, with the effect most pronounced in doubly substituted structures and the SS model. Topological analyses revealed local changes in electron density near the βO4 bond; however, ρ and ELF did not always show the same correlation trend between electron density and the thermochemical values assessed via multivariate methods, highlighting the need to use both tools to better understand the data generated by the substitution effect. Taken together, the data suggest that chlorination is a promising strategy for weakening the βO4 bond.