Isomerism and Hydrogen Bonding in the Cis-enol Forms of 1-(n-pyridyl)butane-1,3-diones: A Theoretical Study

Molecular structure, isomerism, conformational stability and intramolecular hydrogen bonding (IHB) of cis-enol forms of 1-(n-pyridyl)butane-1,3-diones (nPBD) (n = 2, 3, or 4) have been investigated by means of density functional theory (DFT) calculations. Energy differences for all possible nPBD cis-enol forms of isomers with respect to the most stable form of the corresponding isomer have been estimated in the gas phase and solution. AIM results (performed at the B3LYP/6-311++G** level) suggest 75.19-84.77 kJ mol-1 for the strength of intramolecular hydrogen bond in these systems, as a medium hydrogen bond strength. Theoretical structure, NBO and intramolecular hydrogen bond strength for the stable cis-enol forms of nPBD have been compared with each other and also with those of acetylacetone (AA), benzoylacetone (BA), and triflouroacetylacetone (TFAA) molecules. The hydrogen bond strength and molecular stability are investigated by applying the NBO, topological analysis, geometry calculations, and spectroscopic results. The correlation between IHB and some parameters related to hydrogen bonding have been also investigated.