Solvation spectra. Part 27.—Medium effects on the proton magnetic resonance spectra of N-alkylpyridinium salts

Abstract

The effects of solvent, anion, concentration, added electrolytes and temperature on the n.m.r. spectra of N-alkylpyridinium (mainly 1,4-diethylpyridinium) cations are reported. The 2,6-ring proton resonance was particularly sensitive to the nature of the solvent and of the anion. In general, shifts correlate well with the solvent parameters Z and E_t and with the hydrogen-bonding ability of the anion. They are rationalized by assuming that the anion is held electrostatically above the positively charged nitrogen atom of the cation in an ion-pair structure, the tightness of which decreases with increasing solvent polarity and anion size, and that the anion interacts weakly with the neighbouring α-protons apparently by a mechanism similar to hydrogen-bonding. In aqueous solutions there is also evidence of cation-cation interactions.