Knowledge Bank

The complete infrared and Raman spectra of benzoyl chloride have been recorded. In addition, the spectra of benzoyl bromide and benzoyl chloride-$d5$ have been observed to aid in the assignment. Benzene ring modes have been assigned by the use of other mono-substituted benzene ring assignments and isotopic shift data. Vibrations of the -COCl group were assigned by comparison with the spectra of the corresponding -COBr compound. of particular interest in the benzoyl chloride assignment is the observed doublet in the carbonyl stretching region of the spectrum. Two explanations have been previously proposed. One proposed Fermi Resonance between the carbonyl stretching fundamental and the overtone of a fundamental. The second argues that the doublet is a manifestation of rotational isomers, differing in that the -COCl groups are coplanar and non-coplanar with the benzene ring. Our results indicate that neither of the previously proposed explanations for the doublet in the carbonyl region are satisfactory. Rather, the explanation appears to involve Fermi Resonance of the carbonyl stretching mode with a combination band, one of whose components is a hydrogen mode.