Structure and dynamics of the keto and enol forms of acetylacetone in the solid state

The tunneling and librational dynamics of the methyl groups of acetyl-acetone were investigated by inelastic and quasielastic neutron scattering at ambient and high pressure (4 kbar) for a variety of isotopic compounds. Samples, prepared by quenching the liquid, are shown to consist of a mixture of keto and enol forms of the molecule. This fact explains difficulties in the data analysis of previous studies. In the present work the contributions of the two forms could be separated, by preparing pure enol samples as well as keto-enriched samples. Two inequivalent methyl groups are identified for the enol form with barrier heights of the hindering potential in the range of 220–800 K. These potential barriers are fairly sensitive to deuteration of the nonmethyl protons and to disorder in the crystal. In contrast, for the keto form the potential is insensitive to these factors. These differences reflect the influence of the hydrogen-bonded proton on the methyl group dynamics in the enol molecule.