Far infrared spectra and barriers to internal rotation of benzaldehyde, benzoyl fluoride, benzoyl chloride and acetophenone

The far infrared (250-40 cm−1) spectra of gaseous benzaldehyde, benzoyl fluoride, benzoyl chloride and acetophenone have been recorded. The fundametnal CHO torsion for benzaldehyde has been observed at 110.85 cm−1 with three excited states at 109.51, 106.52 and 104.17 cm−1 along with several “hot bands” arising from the low frequency bending modes. The corresponding fundamental for benzoyl fluoride has been observed at 63.36 cm−1 with one well defined excited state at 61.91 cm−1. Similarly, bands observed at 44.6 and 49.5 cm−1 in the spectra of benzoyl chloride and acetophenone, respectively, have been assigned to the fundamental CXO torsions of these molecules. These data have allowed for the determination of the twofold barrier which governs the internal rotation of the CXO mojety and have been found to be 1611 cm−1 (4.61 kcal mol−1), 1739 cm−1 (4.97 kcal mol−1), 1162 cm−1 (3.32 kcal mol−1) and 1103 cm−1 (3.15 kcal mol−1) for the aldehyde, fluoride, chloride and ketone, respectively. These results are compared to previously obtained values for two of the molecules and to some corresponding barriers for several related molecules.