Single crystal X-ray structure determinations of six crystals 1-6 of b-diketones, the related DFT calculations as well as a systematic investigation, on the CSD (Cambridge Structural Database) files, of all acyclic b-diketones having at least one a-hydrogen, in both b-diketo and b-keto-enol tautomeric forms, are reported. In spite of the stabilization energy gained by the formation of strong intramolecular O-HÁ Á ÁO resonace assisted hydrogen bonds (RAHB) a certain number of non-enolized structures were retrieved. The structural data show that the steric and electronic properties of the substituents play a definite role in tuning the hydrogen bond strength and determining the enolic site but the driving force able to shift from the more common b-keto-enol tautomer to the b-diketo one can be only the steric hindrance of bulky groups. In this context the substituents in position 2 play a crucial role in establishing the tautomeric form. In fact, while the 2-unsubstituted b-diketones (or 2-substitued by a group linked by a sp 2 carbon) assume almost exclusively the b-keto-enol form with some exceptions for very bulky substituents, b-diketones carrying 2-alkyl substituents, in general, display the b-diketo tautomeric form. The only exceptions are the 2-alkyl curcumin derivatives where the planar b-keto-enol group is stabilized by extended p-conjugation within the whole molecule and by the absence of short contacts between the alkyl R 2 groups and R 1 or R 3 substituents. DFT calculations on the six compounds, 1-6, show that in the four more overcrowded structures, 3-6, the trans-b-diketo tautomer is more stable than the Z-b-keto-enol isomer unlike what happens for 1 and 2 where the Z-b-keto-enol isomer is the most stable by a few kcal mol À1 . Thereby, the occurrence of the trans-b-diketo tautomer for all compounds, in the crystal, can be interpreted in terms of the existence of a large activation energy in the mechanism to attain the Z-b-keto-enol isomer containing an intramolecular O-HÁ Á ÁO hydrogen bond.
