Structure and vibrational assignment of the enol form of α-bromo-acetylacetone

It is well known that the cis-enol form of α-diketones is characterized by a strong intramolecular hydrogen bond, IHB, [1,2]. The simplest members of this class of compounds are malonaldehyde (MA) and acetylacetone (AA), which have been the subject of many theoretical and experimental studies [3,4]. This hydrogen bond formation leads to an enhancement of the resonance conjugation of the p-electrons, which causes a marked tendency forequalization of the bond orders of the valence bonds in the resulting six-member chelating ring. Therefore, it seems that any parameter that affects the electron density of the chelating ring will change the hydrogen bond strength. The strength of such a bridge decreases when an electron withdrawing group replaces the hydrogen atom in position α or replaces the hydrogen atoms of the methyl groups in position β [5,6]. The electron-withdrawin groups tend to reduce the hydrogen bond strength by increasing the positive charges on the carbonyl oxygen atom [3-6]. Finally, electron withdrawing groups that are able to conjugate with the enol double bond increase the stability of the enol and strengthen its hydrogen bonding [3,7]. The complexity of these substituents effects is apparent when the sequence of enol contents on going from Cl to Br and to I is considered. A chloro substituent usually increases the enol content, but this becomes smaller for Br and I because of the predominant steric effects. According to the NMR proton chemical shifts, the trend in IHB strength is BrAA > AA > α-chloroacetylacetone,ClAA [8]. Therefore, studying the vibrational spectra and theoretical structure of BrAA and comparing the structural and vibrational properties with those of AA and ClAA may explain the discrepancies.