Structural, electronic, spectroscopic, and thermodynamic properties of gentisic acid: solvent and temperature effects using DFT and TD-DFT approaches

Gentisic acid (GA) is a naturally occurring phenolic compound known for its anti-inflammatory, antioxidant, antimicrobial, and anticancer properties. Despite some reports, a systematic assessment of solvent- and temperature-dependent electronic, vibrational, and thermodynamic properties has not yet been investigated. In this study, we employed semi-empirical PM6, Hartree-Fock, and DFT (B3LYP/6-311++G(d,p)) with IEF−PCM Continuum solvation to examine the effect of solvent polarity. The results indicate that increasing solvent polarity modifies the electronic structure of GA and narrows the HOMO−LUMO energy gap. The natural bond orbital (NBO) analysis reveals that oxygen lone-pair donation and $$\pi$$-electron delocalization are important contributors to the molecular stabilization. Charge distribution analyses displayed non-uniform redistribution of electron density in the gas phase and aqueous environment. The TD-DFT calculations show slight solvent-dependent shifts, with the UV−Vis data showing consistency with the previous experimental data. The FTIR vibrational frequencies were also evaluated and show good agreement with previous experimental measurements. The calculated thermodynamic properties over the temperature range of 100−1000 K show systematic increase in enthalpy, heat capacity, and entropy in both the gas and aqueous phases. The gas phase thermodynamic properties are slightly greater than those in water, due to enhanced translational and rotational degrees of freedom in the gas phase. In general, the results of this work provide a systematic computational description of solvent- and temperature-dependent behavior of GA, offering valuable insights into its potential relevance to drug development and molecular spectroscopy.