Exploring the effects of temperature and solvent polarity on the molecular structure, thermodynamic features, and photophysical properties of pyrazole: A DFT and TD-DFT approach

This study explored how temperature and solvent polarity influence the structural, electronic, thermodynamic, and spectroscopic properties of pyrazole, a five-membered aromatic compound known for its biological activities. Semiempirical (PM3), Hartree–Fock (HF/aug-cc-pVDZ), and density functional theory (B3LYP with basis sets 3TO-3G*, 6-31G+ (d, p), 6-311+G(d,p), and aug-cc-pVDZ) methods were employed to examine pyrazole in gas and solvent environments. The findings showed that solvent interactions notably affected the molecular geometry, FT-IR spectra, solvation energy, dipole moments, HOMO–LUMO energy gaps, and reactivity. The HOMO–LUMO gap decreased from 7.0557 eV in the gas phase to 7.000 eV in water. The dipole moment increased with increasing solvent polarity from 2.27 D in the gas phase to 4.51 D in water, while the solvation energy also varied accordingly. As the temperature increased from 100 to 1000 K, the molecular vibrations intensified, altering the thermodynamic properties and reducing the molecular stability. UV–vis spectra revealed redshifts in more polar solvents, indicating changes in π–π* transitions. The charge distribution and reactive sites were analyzed via density of states, electrostatic potential maps, reactivity indices, and Mulliken charges. Overall, this study quantifies how environmental conditions alter pyrazole properties and their suitability for various applications.