Conformational stability and vibrational analyses of 1,1,3-trichloroacetone CHCl₂-CO-CH₂Cl

The conformational stability of 1,1,3-trichloroacetone was investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. The calculated potential energy curves of the molecule at DFT-B3LYP level were consistent with two distinct minima that correspond to cis-cis and gauche-gauche (G-gl) conformers in the order of decreasing relative stability. The cis-cis conformation with C_s symmetry was calculated to have an imaginary frequency and a near cis-cis with C_l symmetry form was predicted to be the true lower energy minimum for 1,1,3- trichloroacetone. A decrease of about 0.3 Debye in the total dipole moment was calculated as going from the low energy Ncc conformer to the high energy Ggl form. This small change in the dipole moment that supported by noticeable change in several of the skeletal bond and dihedral angles with changing conformation indicated that steric forces play more important role than dipolar interactions in controlling conformational stability of the molecule. The vibrational frequencies of 1,1,3-trichloroacetone in its stable forms were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental infrared and Raman spectra of the molecule.