Quantum-chemical calculations, tautomeric, thermodynamic, spectroscopic and X-ray studies of 3-(4-fluorophenylhydrazone)pentane-2,4-dione

The structural and tautomeric characteristics of 3-(4-fluorophenylhydrazone)pentane-2,4-dione were studied using IR, 1H and 13C NMR spectroscopy, X-ray diffraction analysis and quantum-chemical calculations. Crystallographic data shows that the compound has a hydrazone structure that contains intramolecular H-bonds assisted by resonance, with a N–O distance of 2.5872 Å. The effective atomic charges in the tautomeric forms of the colorant (enol-azo, keto-azo and hydrazone), was calculated using an MO LCAO quantum-chemical method employing the Hückel approximation; the compound was stabilized in the hydrazone form and displayed high reactivity. The thermodynamic functions of the proton dissociation constant of the colorant were determined by potentiometry; the dissociation process was unspontaneous, endothermic and entropically unfavorable. The findings reveal that the colorant has the same structure in both DMSO solution and the solid state.