Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculation of bromoacetone

Raman (3200—10 cm−1) and infrared (3500—20 cm−1) spectra were recorded for gaseous and solid bromoacetone (1‐bromopropan‐2‐one), CH2BrC(O)CH3. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. These data were interpreted on the basis that the molecule exists predominantly in a gauche conformation of C1 symmetry (dih ∢ BrCCO = 111.8°) in the vapor. In the liquid a second conformer having a trans structure (bromine atom oriented trans to the methyl group) with Cs point group symmetry is also present, and the trans form is the only conformer present in the annealed solid. From a study of the Raman spectrum of the liquid with variable temperatures the trans conformer was determined to be more stable than the gauche form by 385 ± 61 cm−1 (1.09 ± 0.17 kcal mol−1). A complete vibrational assignment is proposed for both conformers. From ab initio Hartree‐Fock gradient calculations with the 4—31G*/MIDI‐4* basis set, optimized geometries for both the gauche and trans conformers were obtained. Similar calculations were also carried out for fluoroacetone and chloroacetone utilizing the 6—31G* basis set with electron correlation with secondorder perturbation. The conformational stabilities, barriers to internal rotation and observed fundamental vibrational frequencies for both the gauche and trans conformers of bromoacetone are compared with those calculated with the STO‐3G* basis set. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.