Spectroscopic study of hydrogen bonding in the enol form of β-diketones—I. Vibrational assignment and strength of the bond

Abstract

The i.r. and Raman spectra of the trans-enol form of some β-diketones and their deuterated analogues in α position are examined in the 4000-50cm −1 range and an approximate assignment is given for the fundamental vibrations. The ν as (OH) and ν(CO) values suggest that the intramolecular H-bond strength increases in the order: hexafluoroacetylacetone < trifluoroacetylacetone < acetylacetone < α - Cl - acetylacetone < benzoylacetone < dibenzoylmethane < tetraacetylethane. The observed relative Raman intensities of the ν(CC) and ν(CC) bands also support this feature. The spectroscopic results show a great change in the internal modes of the β-diketones upon deuteration; this behaviour indicates a greater localization of the π-electron system of the chelate ring and a weakening of the H-bond on deuteration. The ν s (OH) vibration seems to be involved in one of the in-plane ring deformational modes.