Nuclear magnetic resonance studies of alkali metal halide solutions

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Abstract
Chemical shifts of the nuclear resonances of 23Na, 39K, 87Rb and 133Cs in aqueous solutions of alkali halides and nitrates from concentrations of 0·2 molal up to saturation are recorded. The magnitudes of the shifts increase considerably with increasing atomic number of the cation, and shifts to both higher and lower fields relative to the cation at infinite dilution are observed. At concentrations below 4 molal the shift of the potassium, rubidium and caesium resonances varles linearly with the mean activity of the salt and the shifts have been expressed as activity shifts in p.p.m/activity. Anions show a definite series of shielding effects that is found for all cations, the order of increasing shielding is I−−−−2O3 −. Direct interaction between the ions is the predominant cause of the chemical shifts of the cation resonances in these solutions. Small shifts of the sodium resonances are consistent with the ‘positive’ hydration of this ion. Overlap repulsive forces between the closed shell ions enable a satisfactory explanation of the observed chemical shifts to be made.