Pyridine adsorption onto metal oxides: An ab initio study of model systems

In order to shed some light on the origin of the shifts of the vibrational frequencies of pyridine adsorbed on metal oxides, ab initio calculations have been performed. SCF methods may be used to assign and identify the pyridine vibrations of interest, although coupling of certain vibrational modes hamper the interpretation. The effect of the ligands on the vibrations of pyridine is rather complex. It appeared that the shifts are mainly due to the direct effect of the ligand on the force constants and to changes in the extent of coupling with the in-plane bending vibrations of the hydrogen atoms, and, in case of proton transfer, the N-H group. Protonation, however, severely influences the geometry of pyridine, which may also have a considerable effect on the vibrational frequencies.