Vibration spectra of hydrocarbon molecules - II. Skeletal frequencies in certain branched paraffins

Abstract

Saturated paraffins containing the terminal grouping (CH 3 ) 3 C... show strong absorption bands in the infra-red at 1250 and 1200 cm. -1 , while those containing the terminal grouping (CH 3 ) 2 CH... show strong bands at 1170 and 1145 cm. -1 . It has been shown that these and other characteristic frequencies should be assigned as vibrations of the carbon skeleton of these structural units. Further, by using a simple valency force field on the model molecules X 3 YZ and X 2 WZ (where X is a CH 3 group, Y a carbon atom, W a CH group, and Z a large mass representing the remainder of the molecule), satisfactory agreement between observed and calculated frequencies is obtained. The implications of these results in the more general problem of the interpretation of the vibration spectra of hydrocarbons is considered. In particular, it is emphasized that such skeletal frequencies are at least partly responsible for the intense and complex absorption spectra of branched paraffins, in the interpretation of which stress has hitherto been placed mainly on the role of vibrations of the hydrogen atoms.