Conformational analysis, barriers to internal rotation, ab initio calculations and vibrational assignment for 1,1-dichloroacetone

Abstract

The IR (3500-40 cm −1) and Raman (3100-20 cm −1) spectra have been recorded for gaseous and solid 1,1-dichloroacetone (1,1-dichloro-2-propanone), CHCl 2C(O)CH 3. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the molecule exists predominantly in the trans form (hydrogen atom oriented trans to the methyl group) in the vaporphase. A second conformer having one chlorine atom lying in the CC(O)C plane eclipsing the CO bond (hydrogen atom gauche to the methyl group) is also found in very low abundance in the vapor. This conformer becomes more abundant in the liquid, and is the only form present in the annealed solid. From ab initio calculations at both the 3–21G* and 6–31G* basis set levels optimized geometries for both the trans and gauche conformers have been obtained. A potential surface governing internal rotation of the asymmetric top has been calculated at the 3–21G* level. The barrier to interconversion of the trans form to the gauche conformer at this level is calculated to be 1224 cm −1 (3.50 kcal mol −1), with the trans form being more stable by 305 cm −1 (0.87 kcal mol −1) and the barrier between the two equivalent gauche forms is 546 cm −1 (1.56 kcal mol −1). An assignment of the fundametal vibrations is proposed which is supported by frenquecy calculations at the 3–21G* level. From the torsional frequency of 125 cm −1, the barrier to internal rotation of the methyl group for the trans conformer is calculated to be 396 cm −1 (1.13 kcal mol −1). The results are compared to some similar quantities for some related molecules.