Proton having a small mass and radius shows various characteristic phenomena such as hydrogen bonding. Recently, many researches are devoted to very strong hydrogen bonds, because of their important role in biochemical reactions, and enzyme catalysis as transition state [1]. It is well known that the cis-enol form of β-diketones is characterized by a strong intramolecular hydrogen bond [2]. The NMR and vibrational spectroscopy techniques have been intensively used to study the hydrogen bond strength and also keto– enol equilibrium in these compounds [3]. In the present work, we measure the theoretical and experimental (in several solvents at room temperature) 1H NMR chemical shift for the symmetrical β-diketones, such as 2,4-pentanedione (AA) (X= CH3, Y= CH3), 1,3-diphenyl-1,3-propanedione (DBM) (X= Ph, Y= Ph), hexafluoro-acetylacetone (HFAA), (X= CF3, Y= CF3), and assymetrical compound like 1-Phenyl-1, 3-butanedione(BA) (X= Ph, Y= CH3), 4,4-Dimethyl-1-phenylpentane-1,3-dione, (DMPD) (X= Ph, Y= t-butyl), and 1,1,1-trifluoro-2,4-pentanedione (TFAA) (X= CF3, Y= CH3), to investigate hydrogen bonding strength. Some chelated cis-enol form could be considered for the mentioned compounds. The theoretical results have been done by means of DFT calculations at B3LYP level with different basis sets and regression coefficient applied to find the best basis set(s) which is in more agreement to experimental data.

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