DFT, Atoms in molecule and Non-Covalent Interactions Study of the Intramolecular Hydrogen Bonding in Dienaminones and Dienaminothiones

Abstract

This study presents a detailed computational investigation of the tautomerism and intramolecular hydrogen bonding in selected dienaminones and dienaminothiones derived from β-diketones. Density Functional Theory (DFT) calculations employing the long-range corrected hybrid functional wB97XD with the cc-pVTZ basis set reveal that the most stable forms of these compounds are six-membered chelated tautomers stabilized by resonance-assisted intramolecular hydrogen bonding (RAHB) involving N–H···O or N–H···S interactions. Alternative tautomers, such as those with O–H or S–H groups, were found to be significantly less stable.Atoms in Molecules (AIM) topological analysis confirms that the energetically preferred tautomers adopt a cis-configuration around the ethylene (–CH₂–CH₂–) bridge. Non-Covalent Interaction (NCI) analysis, supported by Reduced Density Gradient (RDG) isosurfaces, provides further evidence for the presence and strength of intramolecular hydrogen bonds in the N–H containing tautomers, while indicating their absence or weakness in the corresponding O–H and S–H analogues.