A DFT investigation on the chemisorption mechanism and coordinate bonding of chromium by polyphenol–silica composites: insights from NCI and QTAIM analyses

Chromium (Cr) pollution from industrial effluents requires the development of efficient, robust remediation technologies. This study investigated the molecular mechanism of Cr(VI) reduction and its subsequent immobilization on a god crown (Phaleria macrocarpa) (GC/Bt) composite, providing a comprehensive theoretical foundation for the experimentally observed high adsorption capacity. The elucidation of the three-stage mechanism reveals that the initial homogeneous reduction via proton-coupled electron transfer (PCET) and radical decarboxylation face substantial thermodynamic and kinetic hurdles. Furthermore, TD-DFT analysis confirmed the absence of electronic transitions in the infrared spectrum, directly rationalizing the slow experimental reduction rate when relying solely on IR irradiation. Consequently, the highly exergonic heterogeneous adsorption phase (ΔG_ads = −135.52 kcal/mol) on the composite active site serves as a crucial stabilizing step. This immobilization process occurs through the formation of stable bidentate chelates, controlled by the ligand-to-metal charge transfer (LMCT) mechanism. The irreversible nature of this coordination bond was robustly validated by the Mayer bond order (MBO), non-covalent interaction (NCI) visualization, and the quantum theory of atoms in molecules (QTAIM) parameters, which comprehensively confirmed the existence of a partial covalent character that permanently locked the heavy metal.

All quantum chemical calculations were performed using the density functional theory (DFT) at the B3LYP-D3BJ/def2-TZVPD level of theory, employing the conductor-like polarizable continuum model (CPCM) to simulate an implicit aqueous environment. Reactant, intermediate, and product structures were geometrically optimized and confirmed as true minima or transition states through harmonic frequency analysis, with reaction pathways validated via intrinsic reaction coordinate (IRC) calculations. Excited state analyses were conducted using time-dependent DFT (TD-DFT) to map the photochemical response. Topological and bonding analyses, including the Mayer bond order, reduced density gradient (RDG) for NCI mapping, and QTAIM parameters, were executed using Multiwfn. The ORCA software package was used for all computational modeling, and high-resolution 3D visualizations were rendered using visual molecular dynamics (VMD).