Low frequency vibrational spectra and internal rotation of 2‐chlorobuta‐1,3‐diene, propenoyl fluoride, and propenoyl chloride

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 chlori...