Study of Perfluoroalkyl Chain-Specific Band Shift in Infrared Spectra on the Chain Length

The CF2 symmetric stretching vibration (νs(CF2)) band of a perfluoroalkyl (Rf) group in an infrared (IR) spectrum exhibits a unique character, i.e., an apparent high wavenumber shift with increasing the chain length, which is an opposite character to that of the CH stretching vibration band of a normal alkyl chain. To reveal the mechanism of the unusual IR band shift, two vibrational characters of an Rf chain are focused: (1) a helical conformation of an Rf chain, (2) the carbon (C) atoms having a smaller mass than the fluorine (F) atom dominantly vibrate as a coupled oscillator leaving F atoms stay relatively unmoved. These indicate that a ‘coupled oscillation of the skeletal C atoms’ of an Rf chain should be investigated considering the helical structure. In the present study, therefore, the coupled oscillation of the Rf chain dependent on the chain length is investigated by Raman spectroscopy, which is suitable for investigating a skeletal vibration. The Raman active νs(CF2) band is found to be split into two bands, and the splitting is readily explained by considering the helical structure and length with respect to group theory, and the unusual IR peak shift is concluded to be explained by the helical length.