Solvent-and Concentration-Dependent Shifts in Proton Magnetic Resonance of Quaternary Ammonium Salts

Proton magnetic resonance spectra of some methylpiperidinium salts were taken in various solvents and at various concentrations. Chemical shifts of the N-methyl protons were strongly dependent on solvent and concentration. Those at the infinite dilution were dependent on solvent but not on the counter anion, indicating that the free cation exists as a solvated form. Those of the ion-paired forms were also dependent on solvent. The effects of the solvent and counter ion on the chemical shift are discussed.