Anion Shifts in the Hydroxyl Proton Resonance of Methanol

Abstract

NUCLEAR magnetic resonance studies of interactions between ions and solvent in electrolyte solutions have been limited by the need to use an arbitrary standard when dividing salt shifts into separate ionic contributions, because absolute individual ion shifts could not be measured^1,2. Here we report further evidence supporting our suggestion³ that the residual shift of the methanol hydroxyl proton resonance for solutions of electrolytes showing a resolved signal for solvent molecules directly bonded to cations can be taken as a measure of the anion shift. A range of salt shifts in the hydroxyl proton resonance of methanol was measured at −69° C arid from these the individual ion shifts were obtained using the previously measured perchlorate anion shift³. This procedure yielded a value of −13.5 Hz for the molal shift of the acetate anion. The acetate ion shift was also obtained directly by measuring the residual shift of magnesium acetate–methanol solutions at −69° C, when a separate signal from methanol molecules in the magnesium solvation shell was observed. The residual shift was in the correct direction (downfield shifts are regarded as negative and upfield shifts as positive) and its molal value per acetate ion was −11.5 Hz, in fair agreement with that value.