Tautomerism in pyridinyl methyl β-diketones in the liquid and the solid state; a combined computational and experimental study

Three 1-(n-pyridinyl)butane-1,3-diones (nPM) (n = 2, 3, and 4) molecules were synthesized and their molecular structure, tautomerism, conformational stability, and intramolecular hydrogen bonding (IHB) of cis-enol forms, was investigated by a combination of computational and experimental methods. Density functional theory (DFT) calculations were used and the Atoms in Molecules (AIM) analysis indicated an intramolecular hydrogen bond strength as 17.9–20.3 kcal/mol using the B3LYP/6–311++G(d,p) level of theory. The calculated structures, NBO, and intramolecular hydrogen bond strengths for the stable cis-enol forms of nPM molecules were compared with each other and with those of acetylacetone (AA) and benzoylacetone (BA). The single crystal X-ray structure of 4PM confirmed that the tautomer with the OH group next to the pyridine ring best describes the structure in the solid-state. The vibrational bands of the title molecules were assigned to the corresponding normal modes, and the equilibrium between the two cis enol tautomers was estimated by means of IR, Raman and UV/Vis spectroscopy.