Inductive effect of polar groups on CH and CF stretching vibrations of alkyl and perfluoroalkyl groups and its relation to adsorption

Abstract

The trifluoromethyl-stretching vibrations for a series of perfluorocarboxylic acids and dihydroperfluoro alcohols were determined using an infrared spectrophotometer, and the results were compared with published information on the methyl-stretching vibrations of the corresponding hydrocarbon compounds. The frequency shifts of the CF 3-stretching vibrations are opposite in direction to those of the CH 3-stretching vibrations. This opposite shift may be attributed to the inductive effect of highly electronegative fluorine atoms. As a result, the alkyl groups are expected to become somewhat electropositive whereas the perfluoroalkyl groups electronegative. These induced polarities of the nonpolar groups were confirmed by the surface potential measurements at an air-aqueous solution interface and by the electrocapillary and differential capacity measurements at a mercury-aqueous solution interface.