J. R. Durig, J. S. Church

The i.r. spectra of gaseous and solid 1,1,1-trifluoro-2-propanone were recorded between 4000 and 15 cm⁻¹. The Raman spectra of the gas, liquid and solid were recorded between 300 and 10 cm⁻¹. A vibrational assignment is presented based on C_s symmetry and the results of a preliminary normal coordinate analysis are discussed. The methyl torsion was assigned to a broad weak band in the i.r. spectrum and the frequency of 120 cm⁻¹ yields a barrier to internal rotation of 392 ± 14 cm⁻¹ (1.12 kcal mol⁻¹). The perfluoromethyl torsion was observed as a strong broad infrared band centered at about 40 cm⁻¹ from which a barrier of ∼320 cm⁻¹ (920 cal mol⁻¹) was calculated. These results are compared to the similar quantities in related molecules.

J. R. Durig, J. S. Church, D. A. C. Compton

The infrared and Raman spectra of gaseous 2‐chlorobuta‐1,3‐diene, propenoyl fluoride, and propenoyl chloride have been recorded below 300 cm⁻¹. It was observed that the low frequency spectra of these three molecules were quite similar. The infrared spectrum of 2‐chlorobuta‐1,3‐diene exhibited a series of sharp torsional transitions whereas in the Raman spectrum a B‐type band was observed. These torsional frequencies have been assigned to the s‐trans conformer and no evidence for a higher energy conformer was observed. In the infrared and Raman spectra of the propenoyl halides, bands assigned to the torsional series of both conformers have been observed. The analyses of these spectra were simplified by comparison with that of the 2‐chlorobuta‐1,3‐diene. Asymmetric potential functions and values for ΔH for propenoyl fluoride and propenoyl chloride were calculated. The enthalpy differences of 36 and 215 cm⁻¹ between the lowest energy levels of the s‐trans and s‐cis wells for the propenoyl fluoride and chloride, respectively, were obtained.