Theoretical Insights into Hydrogen Bonding, IR Spectra, and Reactivity of Pimelic Acid in Mixed Solvents using DFT Calculations

This study explores the structural, vibrational, and electronic behavior of pimelic acid (PA) in different solvent environments—pure, binary (PA–water and PA–ethanol), and ternary (PA–water–ethanol)—using density functional theory (DFT) at the B3LYP/6-311+G(d,p) level. Geometry optimization, vibrational frequency analysis, and molecular orbital calculations were performed to examine hydrogen bonding interactions, IR spectral shifts, and global reactivity descriptors. Results show that the –OH bond distance and stretching frequencies are significantly affected by solvation. Stronger hydrogen bonding was observed in the PA–ethanol complex than in PA–water, with the ternary PA–water–ethanol system (PAWE) exhibiting cooperative effects from both solvents. Intermolecular H-bond distances and frequency shifts confirmed the presence and strength of solute–solvent interactions. Frontier molecular orbital (FMO) analysis revealed solvent-induced variations in the HOMO–LUMO gap, indicating changes in reactivity and stability. Global descriptors—electronegativity, chemical hardness, softness, and electrophilicity—further support the influence of solvent polarity on the reactivity profile of PA. These findings provide theoretical insight into solvation effects on PA, contributing to a better understanding of their chemical behavior in mixed solvent systems.