Proton chemical shifts of water in cationic hydration complexes and their contribution to water shifts in electrolyte solutions

Abstract

The proton chemical shift of water in hydration complexes of the cations Sn⁴⁺, Al³⁺, Ga³⁺, In³⁺, Mg²⁺, and Be²⁺ has been measured at low temperatures. The shifts are strongly downfield and correlate with shift increments calculated to arise from the electric field of the ion. The correlation indicates that the total cationic water shift contains two contributions; a downfield one from the electric field and a smaller upfield one from a structural effect. The correlation is used to estimate the proton shift of hydration water of the larger Group II and Group I cations where proton exchange is always too fast to allow a separate resonance for the co-ordinated water to be observed. The contribution of cationic hydration to the molal shifts of salt solutions is thus obtained and comparison with measured molal shifts allows estimates to be made of the anionic contribution, which is upfield and arises from solvent structure-breaking. A three-site model is developed to explain the solution shifts and previous measurements are shown to be consistent with a hydration number of 6 for all the ions Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, of 4 for the alkali-metal ions, and of zero for Me₄N⁺. The sulphate anion is also probably tetrahydrated.