LONG-RANGE NONBONDED FORCES AND ROTATIONAL ISOMERISM IN ACYL HALIDES.

Author Institution: Monsanto Research CorporationThe infrared spectra of a series of aliphatic acyl chlorides and several acyl bromides have been studied in the $2000-250 cm^{-1}$ region. The Raman spectra of a number of these compounds have also been recorded. Several strong bands which may be assigned to carbon--halogen stretching frequencies are noted in the $750-500 cm^{-1}$ range. The number and position of these bands may be related to the structures of the various compounds. Specifically, they can be related to the number and form of rotational isomers about both the alpha and beta carbon--carbon bonds. The fact that the conformation of substituents on the beta carbon atom affects the frequency of the carbon--halogen stretching vibration provides experimental evidence of nonbonded intramolecular forces operative over relatively large distances. An assignment of the various carbon--halogen stretching vibrations to specific molecular conformation is proposed. These assignments are based on comparison of the spectra of the liquid and crystalline materials, known relative conformer stabilities and possible rotational isomers of various structures. Work supported by the U.S. Air Force under Contract AF 33(615)-1565