The C(2v) structure of enolic acetylacetone.

It has often been postulated that the lowest energy enolic form of Acetylacetone (AcAc) assumes C(s) symmetry, i.e., has a double-minimum potential possibly exhibiting a low barrier to internal proton transfer and not a single minimum, C(2v). Recent theoretical calculations and experimental work support the C(s) hypothesis but the literature on this fascinating molecule is divided. Toward this objective, the high-resolution rotational spectra of enolic acetylacetone and 3 isotopologues have been obtained, revealing C(2v) symmetry. The two methyl groups exhibit a very low barrier to internal rotation, thus making AcAc internally highly dynamic.