Quantitative characterisation of the ring normal modes. Pyridine as a study case.

In the present work, the vibrational normal modes (NM) of pyridine were revisited. Quantum Chemical calculations were performed to help understand the true nature of some ring related vibrational normal modes (RNM) and how they may be correlated with the electronic structure on the ring. The 27 vibrational normal modes were decomposed into the molecular internal coordinates, and the interest was focused on 7 of them, involving the in-plane ring motion. The electronic structure was analysed through frontier Molecular Orbitals (MO), maps of Molecular Electrostatic Potential surfaces (MEPs) and Natural Bond Orbital (NBO) analysis in a dynamic manner, wherein, each vibration was scanned. The present investigation is aimed to provide the Reader with a quantitative characterisation of the RNMs of pyridine.