The stability, kinetics and inter-fragment electron communication of the tautomers of twelve selected β-diketone...

Keto-enol equilibrium is known to depend on the difference in the free energy between the keto and enol tautomers and is greatly influenced by the nature of the substituents, temperature, and the polarity of the solvents. New insight was gained into the series of twelve differently substituted β-diketones (A-L), showing that the keto form of each β-diketone has a lesser tendency towards resonance formation, compared to their enol forms. For molecules G and H (which contain an electron-withdrawing CF₃ substituent), the experimentally reported high tendency towards the enol tautomer, was computationally traced to a high level of the alternative enolic resonance weight also in their keto structure, as well as to the highly favourable enolic inter-fragment stability of the energy of interaction. Computational results further showed that the polar solvent dioxane enhances the enol form of these twelve molecules more effectively than water and chloroform media. The abundance of either the enol or keto tautomer, was also found to be dependent on the competitive ratio of both the forward and reverse reaction rate constants (namely the computed values for K_forward/K_reverse). High similarity was observed between the experimental and computed UV spectra of the selected molecules in their enol forms, which provides further evidence supporting predictions for the most favourable position of the enolization (for unsymmetrical molecules E and G-L with two possible enol positions), as well as confirming the previously reported trend of their experimental equilibrium K_e/k values.