Computational study on the prototropic tautomerism between simple oxo-, thio-, carbon-, aza-hydrazones, and their respective azines

Most hydrazone compounds prefer the azine tautomeric states. However, oxygen-/sulfur-substituted compounds prefer hydrazone tautomers. In this study, density functional theory at M062X level with the basis set of 6–311 + + g(2d, 2p), with MP2/cc-pVTZ for reference, was used to investigate the different tautomeric mechanisms between hydrazone and azine forms with oxygen, sulfur, carbon, and nitrogen as negative centers. The energetic stabilities are in the order as oxygen-