Tautomerism, molecular structure, intramolecular hydrogen bond of α-methyl and ethyl substituted Acetylacetone; A theoretical study.

A β-dicarbonyl compound, with at least one alpha proton, predominantly exists as conjugated cis-enol form, stabilized by an intramolecular hydrogen bond. We considered all enol and keto forms for 3-methylacetylacetone, 3-Meacac, and 3-ethylacetylacetone, 3-Etacac, by considering the conformations of methyl groups around C-C bonds, and C-O bonds, with respect to the plane of the molecule. Upon the substitution of methyl and ethyl group in the α position and because of the larger steric effect of this group with the hydrogen atoms of methyl groups in β positions, which pushes the methyl groups toward oxygen atoms, this effect, causes to reduce the O…O and O…H distances decrease, while the OHO bond angle and O-H bond length increase [2] (see Table. 1). Therefore, the IHB strength of 3-Meacac and 3-Etacac increases compared to that of Acac. AIM results confirm that the IHB in 3-Meacac, 3-Etacac are stronger than Acac. This result is in agreement with the EHB values, which are obtained by the AIM method (23.29, 23.89 and 18.07 kcal/mol, in 3-Meacac, 3-Etacac and Acac, respectively).