Ion-pair formation by tetra-alkylammonium ions in methanol. A nuclear magnetic resonance study

Abstract

The proton resonance multiplet for N-methylene protons of tetrabutylammonium ions in methanol shifts to low fields as the concentration of salt is increased. These shifts have been analysed in terms of contact ion-pair formation involving anion interaction with the inner methylene groups. The magnitude of the shift is a function of the size of the anion and the strength of its interaction with the solvent. The results suggest that ca. 25 % of the primary solvation of the anion is lost on ion-pair formation, which for the halide ions corresponds to a reduction in the primary solvation number from 4 to 3. These results correlate well with the concentration dependence of the hydroxyl proton resonance of the solvent, which indicates a similar reduction of anion solvation on ion-pair formation. The shifts have been used to estimate values for the degree of dissociation for various ion-pairs over a range of concentrations.