DENSITY FUNCTIONAL THEORY INVESTIGATION AND MOLECULAR DOCKING ANALYSIS OF HYDROGEN-BONDED COMPLEXES OF CYANURIC ACID WITH ORTHOPHOSPHORIC ACID: STRUCTURAL, ELECTRONIC, AND SPECTROSCOPIC CHARACTERIZATION

Abstract

Two hydrogen-bonded molecular complexes of cyanuric acid (C₃H₃N₃O₃, CYA) with orthophosphoric acid (H₃PO₄) in 1 : 1 and 1 : 3 molar ratios were investigated by a combination of density functional theory (DFT) calculations, molecular docking (ArgusLab), and Fourier-transform infrared (FTIR) spectroscopy. Full geometry optimizations and harmonic frequency calculations were performed at the B3LYP/3-21G level using Gaussian 16. Both complexes were confirmed as true energy minima. The 1:1 complex features two intermolecular hydrogen bonds — N3–H···O(P) (H···O = 1.564 Å; N···O = 2.628 Å) and O(C=O)···H–O(P) (H···O = 1.528 Å) — while the 1:3 complex displays a symmetric, C₃-like arrangement in which each of the three carbonyl/N–H pairs of the triazine ring engages one H₃PO₄ molecule through equivalent dual hydrogen bonds. The HOMO–LUMO energy gaps of 5.672 eV and 5.955 eV for the 1:1 and 1 : 3 complexes, respectively, indicate moderate reactivity and enhanced kinetic stability upon progressive phosphorylation. Global reactivity descriptors — ionization potential, electron affinity, chemical hardness, chemical potential, and electrophilicity index — reveal quantitative differences between the two assemblies. Mulliken population analysis shows significant polarization at N, C=O, and P=O sites. Molecular docking calculations in ArgusLab yielded binding energies of −11.3 kcal/mol (1 : 1) and −27.6 kcal/mol (1 : 3), consistent with the formation of stable supramolecular adducts. Calculated vibrational frequencies reproduce the experimentally observed red-shifts of ν(N–H), ν(C=O), and ν(P=O) bands with good fidelity, supporting the proposed interaction mode. These results advance quantitative understanding of triazine–phosphate supramolecular chemistry relevant to flame-retardant design, crystal engineering, and biomedical materials.