Hydrogen bonding effects in perfluorinated polyamides: An investigation based on infrared spectroscopy and density functional theory calculations

The IR spectra of solutions of a perfluorinated polyamide in different solvents are analyzed in the spectral range of “free NH” stretching modes, in order to study the effects of hydrogen bonding in determining the unusual spectral behaviour observed for these systems. The presence of different bands in the spectral range associated to NH bonds with the H atom not involved in hydrogen bonding and their evolution with the polymer concentration has shown that free NH bonds have a different nature depending on the occurrence of hydrogen bonding on neighbour atoms and on the configuration of the molecular complexes in which they are involved. These differences have been interpreted on the basis of DFT (B3LYP/6-311++G**) computations on suitable molecular models that allowed a qualitative and quantitative rationalization of the chemical-physical effects involved. The computational analysis points out the subtle intramolecular and intermolecular vibrational effects occurring in these systems and brings to an interpretation and assignment of the IR spectra which goes over the interpretations usually adopted when dealing with the vibrational spectroscopy of hydrogen bonded polymers and molecules.