Ab initio studies of structural features not easily amenable to experiment: Part V. Conformational analysis of keto- and enol-acetone

The structures of several conformations of keto- and enol-acetone were determined by unconstrained ab initio geometry refinements using the 4-21G basis set. The geometry of propene was also refined to compare it with enol-acetone. The structural consequences of hyperconjugation for the local geometries of the methyl groups were determined in all conformations. In the most stable form of keto-acetone, one hydrogen atom of each methyl group was found in an eclipsed arrangement with respect to the carbonyl group. The stability of this crowded structure has previously been rationalized in terms of aromatic π-electron delocalization. This result is in contrast to one of two previous gas electron diffraction studies. It is concluded that the electron diffraction data may not contain enough information to determine the exact conformational arrangement of the methyl groups in acetone. The calculated structures are found to be in excellent agreement with experiment. Uncertainties in calculated bond distances and bond angles are on the order of magnitude of 0.01–0.02 Å and 1–2°, respectively.