On the Footprints of the Ring Normal Modes in Heterocycles: Pyridine and
Derivatives
Abstract
In the present study, the vibrational frequencies for pyridine,
quinoline, 1,8-naphthyridine and 1,10-phenathroline and their
corresponding fully fluorinated derivatives were investigated through
quantum chemical calculations. In parallel, the electronic structure and
the topology of these molecules were analysed in details by means of the
Natural Bond Orbital analysis (NBO) and the Bader Atom-In-Molecules
(AIM) theory. Among their vibrational normal modes, 7 ring related
normal modes (RNMs), defined for pyridine, were found traceable in the
heterocyclic pyridinic derivatives and their fluorinated compounds,
evidenced by the percentage contribution of the internal coordinates.
The shift of the vibrational frequencies from those of pyridine and the
disappearance of some RNMs in the derivatives were rationalised from the
perspectives of frontier Molecular Orbitals (MOs), maps of Molecular
Electrostatic Potential (MEP), NBO and Electron Localization Function
(ELF). These RNMs previously identified for pyridine were found out to
serve not only as distinctive labels in the characterization of the
vibrations of the heterocyclic derivatives of pyridine and corresponding
fluorine substitutes, but also useful references to study the potential
intermolecular interactions with these compounds involved.