Long-Range Nonbonded Interactions and Rotational Isomerism in Acyl Halides

The infrared spectra of the liquid acyl halides possess a number of carbon—halogen stretching vibrations that are due to the existence of discrete rotational isomers in the liquid state. The only satisfactory explanation for the number of these vibrations is that rotational isomers formed by internal rotation about both the alpha and beta carbon—carbon bonds possess different carbon-halogen stretching frequencies. This implies that alkyl substituents on the beta carbon interact with the halogen atom and provides the first spectroscopic evidence of nonbonded forces operating over so great a chain distance. An assignment of carbon—halogen stretching frequencies to the possible rotational isomers is proposed and the implications of the results concerning the nature of nonbonded forces in this class of compounds is discussed.