Keto-enol tautomerism of β-diketo molecules in the presence of graphitic materials through π-π stacking.

The pseudo aromatic structures of the enol forms of β-diketo molecules are stabilized on the surface of graphitic materials through π-π interaction. This phenomenon has been studied through a relative binding energy calculation using density functional theory. The intermolecular interaction as well as the relative stability of the keto or enol tautomer is also influenced by the functional groups attached to the graphitic materials. The theoretical results are supported by spectroscopic evidence. Our study with three different graphitic materials, with a comparable extent of π-electrons and acid functionalities, reveals that π-π interaction is the main governing factor for the stability of the enol forms. Then comes the role of intermolecular H-bonding between the adsorbate and adsorbent. This can stabilize both the keto and enol tautomers, according to the arrangements of the functional groups and the geometry of the β-diketo molecules. Acid groups on the adsorbent can enhance enolization through H-bonding, but an excess of functional groups may decrease the possibility of π-π interaction by disrupting the π-clouds of the graphitic surface and pushing the adsorbate and adsorbent away from each other beyond a π-π stacking distance. In that situation, H-bonding becomes crucial for determining the relative stability. Our results indicate that graphitic materials with acid functionalities across their edges, and ample π-cloud, are the most suitable catalysts for enolization.