Abstract--The i.r. and Raman spectra of the trans-enol form of some /3-diketones and their deuterated analogues in a position are examined in the 4000-50cm -~ range and an approximate assignment is given for the fundamental vibrations. The v=,(OH) and v(C~)) values suggest that the intramolecular H-bond strength increases in the order: hexafluoroacetylacetone<trifluorc~ acetylacetone < acetylacetone < a - CI - acetylacetone < benzoylacetone < dibenzoylmethane < tetraacetylethane. The observed relative Raman intensities of the v(C=C) and v(C--C) bands also support this feature. The spectroscopic results show a great change in the internal modes of the ~-diketones upon deuteration; this behaviour indicates a greater localization of the ~r-electron system of the chelate ring and a weakening of the H-bond on deuteration. The v,(OH) vibration seems to be involved in one of the in-plane ring deformational modes.

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