Electronic Properties, Conformational Flexibility, and Reactivity Descriptors of 1-(2'-Hydroxybenzylidene)-5-(1'-Pyridylethylidene)carbonohydrazone: A Combined DFT and Experimental Assessment

Abstract

In this study, the structural and electronic properties, as well as the chemical reactivity, of the Schiff base hydrazone 1-(2'-hydroxybenzylidene)-5-(1'-pyridylethylidene)carbonohydrazone were comprehensively investigated using density functional theory (DFT) at the B3LYP/6-311G(d,p) level. Geometric optimisation revealed excellent agreement with experimental X-ray diffraction (XRD) data, confirming the quasi-coplanarity of the π-conjugated carbonohydrazone backbone and the presence of a stabilising intramolecular hydrogen bond (O18–H28⋯N3). A conformational discrepancy observed in the D(8,12,16,25) torsion angle (-24.60° calculated vs. 168.70° experimental) was explicitly attributed to crystal-packing forces and intermolecular interactions in the solid state. A frequency scaling factor of 0.967 was applied to the calculated harmonic vibrational frequencies, providing improved correlation with experimental FTIR spectra, particularly for the characteristic ν(C=O) (1675 cm-1 calc. vs. 1694 cm-1 exp.) and azomethine ν(C=N) (1615 cm-1 calc. vs. 1619 cm-1 exp.) bands. Mulliken atomic charge analysis, complemented by electrostatic potential considerations, highlighted potential nucleophilic sites on the electronegative heteroatoms (O18, N4, N1, O5) and electrophilic regions on the azomethine and carbonyl carbon centres. The investigation of frontier molecular orbitals (HOMO and LUMO) indicated efficient intramolecular charge transfer from the salicylaldehyde moiety towards the pyridine ring and the hydrazone backbone, with an energy gap (ΔE = 4.22 eV) reflecting high kinetic stability and moderate chemical polarisability. Global reactivity descriptors (I, A, η, S, χ, µ, ω), calculated within Koopmans' approximation, confirmed the overall electronic stability of the molecule.