Covalent Character of Cellobiose–Water Hydrogen Bonds Revealed by ELF and QTAIM for Enhanced Dewatering and Reactivity

Hydrogen bonding between cellulose and water plays a fundamental role in controlling cellulose reactivity, swelling, and accessibility in both natural and industrial processes. However, the precise nature of these interactions, whether purely electrostatic or partially covalent, remains a topic of debate. In this study, quantum chemical topology methods were applied to characterize hydrogen bonds formed between water molecules and a solvated cellobiose cluster. Geometry optimizations were carried out using density functional theory at the M06-2X/6-31+G(2d,2p) level, targeting four key hydration regions derived from an I β -cellulose surface model. Electron Localization Function (ELF) analysis revealed disynaptic valence basins (V(O,H)) bridging donor–acceptor pairs, with electron populations ranging from 1.68 to 1.79 e - and compact volumes (<6 Å 3 ) in the strongest bonds. These features indicate real-space electron sharing and partial covalent character in select O–H···O hydrogen bonds. Two-dimensional ELF and Laplacian maps of the electron density (∇ 2 ρ) revealed directionally localized regions of electron accumulation and depletion, supporting the presence of shared density channels along hydrogen-bond axes. Bond critical points, confirmed via Quantum Theory of Atoms in Molecules (QTAIM) analysis, were located in six of eight hydrogen bond pairs, with associated potential energy densities yielding estimated interaction energies of –23 to –40 kJ/mol. These findings provide quantum mechanical evidence that hydrogen bonding at the cellulose–water interface is not purely electrostatic but includes covalent-like contributions in geometrically favorable regions, with implications for fiber dewatering hydration and reactivity in cellulosic materials.