Molecular Structure and Conformation of 1,1-Dichloro-2-propanone, CHCl2C(O)CH3, as Determined by Gas-Phase Electron Diffraction and ab Initio Molecular Orbital Calculations

The molecular structure and conformational composition of 1,1-dichloro-2-propanone (1,1-dichloroacetone) have been investigated by gas-phase electron diffraction at 60 °C. Results from ab initio molecular orbital calculations were used as constraints in the analysis. The experimental data are consistent with a model where the molecules exist predominantly with the hydrogen synperiplanar to the carbonyl group. Small amounts (up to 15%) of a second anticlinal form (C−Cl eclipsing C O) cannot be ruled out. Ab initio calculations showed that both synperiplanar and anticlinal forms are stable conformers with the former conformer lower in energy by 2.25 kcal mol -1 (MP2/6-311+G(d)). The geometrical parameters for the synperiplanar conformer ( r g and ∠ α ) obtained from least squares analyses of the electron diffraction data, using results from the theoretical calculations to provide some constraints in the model used, are r (C−H) av = 1.093(14) Å (average value), r (C O) = 1.196(6) Å, r (C 1 −C 2 ) = 1.524(4) Å, r (C 2 −C 3 ) = 1.496(4) Å, r (C 1 −Cl) av = 1.775(2) Å, ∠C 3 C 2 C 1 = 115.2(5)°, ∠C 1 C 2 O = 119.6(5)°, ∠C 3 C 2 O = 125.2(5), ∠C 2 C 1 Cl av = 110.6(5), φ(HC 1 C 2 O) = 2(8)°, and φ(HC 3 C 2 O) = 25.6° (ab initio value).