Thermal kinetics and molecular modeling studies of ZnII-poly (vinyl alcohol-nicotinic acid) complexes

Abstract

Complexes of poly (vinyl alcohol-nicotinic acid) with ZnII (ZnII/PVA-NA) were synthesized by incipient wetness impregnation. The successful synthesis of PVA-NA and ZnII/PVA-NA was examined using FT-IR, TGA, XRD, HR-SEM, and 1H NMR spectroscopy. The 1H NMR resonance of HOCO-nicotinic moiety close to 11 ppm disappeared from the spectrum of PVA-NA, revealing conversion to ester is complete. The 1711 and 1326 cm-1 bands, assigned to νCO and νCN, respectively, shifted downward as the ring nitrogen atom of NA and carboxyl group of PVA chelate to the ZnII center in ZnII/PVA-NA. The changes related to activation enthalpy (∆H*), entropy (∆S*), and Gibbs free energy (∆G*) during the thermal degradation process of PVA-NA and ZnII/ PVA-NA copolymers were assessed. The ∆H* values given at the 4th step and a heating rate of 30 K min-1 for the ZnII/PVA-NA degradation were: Coats-Redfern: 3503.695, Horowitz-Metzger: 1545.92 and Kissinger: 6.455 kJ mol-1. TGA profile of ZnII/ PVA-NA run at 30 K min−1 indicated an increased activation energy (E*), as the degraded ligand species strongly bound to ZnII. The computational simulation study was carried out using the Builder module of the wave function, and the molecular structure for each compound was fully optimized. The HOMO and LUMO simulation, band-gap, and electrostatic potential (MEP) maps were also examined. Besides, the stability of docked ligand was assessed through a molecular dynamics simulation study, showing the high stability of the tested structures in the active sites. The antimicrobial activity for the tested ZnII/PVA-NA complexes against six pathogens indicated promising antibacterial and antifungal activities.