The keto/enol tautomerism in acetoacetyl fluoride: properties, spectroscopy, and gas-phase and crystal structures of the enol form.

Tautomeric properties of acetoacetyl fluoride, CH(3)-C(O)-CH(2)-C(O)-F, were studied by IR (gas phase), Raman (liquid and solid), and NMR spectroscopy (neat liquid), gas electron diffraction (GED), X-ray crystallography, and quantum chemical calculations. The keto-enol tautomer possesses a much higher vapour pressure than the diketo form and therefore the keto-enol form strongly predominates in the gas phase. In the neat liquid state the thermally unstable compound tautomerizes at low temperatures slowly to yield the diketo form. Raman and NMR spectra show equilibrium with strong predominance of the diketo tautomer (>90%) in the liquid phase. Single crystals, grown at -90 °C from the gas phase, contain exclusively acetoacetyl fluoride in the keto-enol form and the molecules possess a planar skeleton with overall C(S) symmetry and with the O-H bond adjacent to the methyl group.