Achieving the molecular structural identity and functional characterization of chromium(III) cyclohexaphosphate hydrate: Elucidation of quantum computational approach and supramolecular intermolecular interactions

A chromium(III) cyclohexaphosphate hydrate, [Cr(H 2 O) 6 ] 2 (P 6 O 18 )·9H 2 O, was synthesized, and its structural, spectroscopic, optical, and thermal properties were thoroughly investigated. Single-crystal X-ray diffraction revealed that the compound crystallizes in the cubic system with a P 4 ¯ 3n. space group. The structure is composed of linear chains of hexaaquachromium(III) cations, [Cr(H 2 O) 6 ] 3+ , and hexametaphosphate rings, [P 6 O 18 ] 6− , which are interconnected by a complex network of O-H···O hydrogen bonds. Vibrational spectroscopy, including IR and Raman, confirmed the presence of characteristic stretching and bending modes for both the [P 6 O 18 ] 6− anion and the coordinated and lattice water molecules. Optical analysis by UV-Vis spectroscopy identified a fundamental band gap of 3.8 eV, which is consistent with a possible contribution from ligand-to-metal charge transfer (LMCT) character and classifies the material as a wide-bandgap semiconductor. This property, combined with the observed d-d electronic transitions in the visible region, gives the compound potential for use as a photocatalyst and a photostable pigment. Thermal analysis (DTA/TGA) confirmed a multi-stage thermal decomposition process dominated by the stepwise loss of water molecules, with endothermic peaks at 412 K, 479 K, 525 K, and a final, most intense peak at 627 K, corresponding to the sequential removal of phosphate lattice and coordinated water. Density functional theory (DFT) calculations were performed using a finite molecular-cluster model at the B3LYP/LANL2DZ level to obtain molecular level descriptors of the electronic and nonlinear optical properties of chromium(III) cyclohexaphosphate hydrate. Frontier molecular orbital analysis of the finite molecular cluster model gives HOMO and LUMO energies of −3.540 and −3.069 eV, respectively, corresponding to a HOMO–LUMO energy difference (ΔE) of 0.470 eV. This molecular orbital energy separation is distinct from the experimentally determined optical bandgap of 3.8 eV for the crystalline material and is therefore interpreted only as a qualitative descriptor of the frontier electronic structure. The HOMO is predominantly localized over the phosphate oxygen atoms, whereas the LUMO has a greater contribution around the chromium coordination region, suggesting spatial donor–acceptor character within the molecular model. Natural bond orbital (NBO) analysis demonstrates strong second-order donor–acceptor interactions between oxygen lone pairs and antibonding orbitals, with significant stabilization energies, reflecting pronounced electron delocalization within the phosphate framework. Molecular electrostatic potential (MEP) mapping highlights highly negative regions localized on oxygen atoms, identifying them as key sites for intermolecular hydrogen bonding. Furthermore, the calculated nonlinear optical (NLO) parameters, including dipole moment and polarizability, indicate substantial electronic polarization and enhanced optical susceptibility. These computational results provide a qualitative molecular-level interpretation complementary to the experimental structural, spectroscopic, optical, and supramolecular findings.