Pyridine's ring normal modes as footprints for its derivatives: The impact of perfluorination

The effects of full fluorination of pyridine, quinoline, 1,8-naphthyridine and 1,10-phenathroline were investigated through quantum mechanical calculations using pyridine as a reference. Among their vibrational normal modes, 6 out of 7 ring related normal modes (RNMs) classified and well characterized for pyridine in our recent work, were recognized in not only the heterocyclic pyridine derivatives, but also their fully fluorinated compounds, based on the coefficient contribution of internal coordinates. Upon perfluorination, the wavenumbers of RNM1 and 2 indicate a red shift mainly driven by the enhanced reduced mass, whereas the wavenumbers of RNM6 and 7 experience a blue shift primarily due to the fundamental change in the electronic structure reflected in the frontier Molecular Orbitals (MOs), maps of Molecular Electrostatic Potential (MEP), NBO and Electron Localization Function (ELF). Topological analysis indicated a non-trivial bond critical and ring critical point involving two non-covalently bonded fluorine atoms in perfluorinated quinoline, 1,8-naphthyridine and 110-phenathroline.