π-ELECTRON DENSITIES IN THE PYRIDINIUM CATION THE EFFECT OF THE COUNTERION ON THE PROTON MAGNETIC RESONANCE SPECTRUM

Proton shifts relative to internal benzene of pyridine hydrochloride, hydrobromide, and hydriodide in methylene chloride, acetonitrile, nitromethane, and formic acid are reported. A model involving a hydrogen-bonded ion pair between the pyridinium cation and the counterion is discussed. It is assumed that the counterion reduces considerably the π-electron polarization in the pyridinium ion calculated for the isolated ion by the V.E.S.C.F. method. If the electric fields arising from the π-electrons, the counterion, and the positive charge in the N—H+ sigma bond are taken into consideration, the observed order of the C—H proton shifts is predicted. The N—H+ proton shifts are sensitive to the counterion, the solvent, and traces of water. In formic acid the N—H+ proton shifts of the hydrochloride and hydrobromide are characteristic of the solvent only. In these two cases the N—H+ proton does not exchange rapidly and an 14N—H+ coupling constant of 67.6 ± 0.5 c/s is found.