Conformational analysis, barriers to internal rotation, vibrational assignment and ab initio calculations of chloroacetone

Abstract

The infrared (3500-20 cm −1) and Raman (3200-10 cm −1) spectra have been recorded for gaseous and solid chloroacetone (1-chloro-2-propanone), CH 2ClC(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 a gauche conformation having a “near cis” structure of C 1 symmetry (dih ClCCO=142°C) in the vapor but for the liquid a second conformer having a trans structure (chlorine atom oriented trans to the methyl group) with C s point group symmetry is present. From a study of the Raman spectrum of the liquid at variable temperatures, the trans conformation has been determined to be more stable than the gauche form by 1042±203 cm −1 (2.98±0.6 kcal mol −1 and is the only conformer present in the spectrum of the annealed solid. From ab initio calculations at the 3-21G* and 6-31G* basis set levels optimized geometries for both the gauche and trans conformers have been obtained and the potential surfaces governing internal rotation of the symmetric and asymmetric rotors have been obtained. The observed vibrational frequencies and assignments to the fundamental vibrations for both the gauche and trans conformers are compared to those calculated with the 3-21G* basis set. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.