In the cis-enol forms, because of the spatial shape of the molecule, it is possible for hydrogen atom of the hydroxyl groups to lie at a suitable distance and proper orientation for encountering an intramolecular hydrogen bond (IHB) and this stabilizes the cis-enol form of β-dicarbonyl. Involved hydroxyl, carbonyl, and alkene functional groups of both enol forms contain six atoms that can contribute in a π-electronic resonance conjugation to induce a ring like structure referred to as a chelated ring. Any factor, which affects on the electronic properties of the chelated ring, can change the IHB strength and the tautomerization equilibrium. This fact has been established by theoretical and experimental studies such as quantum-mechanical calculations, IR, Raman, microwave, and NMR spectroscopies, X-ray, electron and neutron diffraction measurements on AA and a few of its derivatives.

Keywords