Significance of the changes in the temperature dependence of the proton chemical shift of water brought about by dissolution of electrolytes

Abstract

A model is proposed for the hydration of anions and cations based upon the influence the ions may have upon solvent polarization, and thus upon the solvent proton chemical shift parameters. Spherical cations usually form a centro-symmetric solvation shell of water molecules whose electric moments cannot interact with bulk solvent and whose proton chemical shift is, therefore, independent of temperature and is determined principally by the cation. If the cation is not spherical, and has a multipolar charge distribution (e.g., Me₄N⁺), then the solvation shell is not centro-symmetric and the water electrical interactions are not reduced. The solvation shell chemical shift will then show normal temperature dependence, and is determined both by interaction with surrounding water and with the cation. Such a model also seems to describe the behaviour of the second hydration shell of highly charged aquo cations such as Al(H₂O)³⁺₆. In contrast, anions do not form a centro-symmetric structure, possibly because the wide separation of the hydrogen atoms in water prevents there being a unique water orientation relative to an anion, but influence water motion in the solvation shell in such a way that all electrical interactions are reduced. This, for instance, produces a small reduction in the temperature dependence of chemical shift of the water.

The relevance of this model to results obtained for the proton chemical shifts over a range of temperatures of a group of perchlorate salt solutions is discussed. The salts of the cations Ba²⁺, Sr²⁺, Pb²⁺, Mg²⁺, Cd²⁺, Zn²⁺, Al³⁺, Ga³⁺ and In³⁺ all reduce the temperature dependence of shift to an extent consistent with cationic hexahydration. The changes in chemical shift observed are, in most cases, close to those predicted using simple electric field calculations, though solutions of In³⁺ and Cd²⁺ have larger effects than can be accounted for on this basis alone. The behaviour of a zinc chloride solution was also investigated and was found to have a small influence upon the temperature dependence of chemical shift as might be expected for a solution containing a large proportion of non spherical, complex cations.