Effect of ions on the 17O and 1H NMR chemical shifts of water

Abstract

The ¹⁷O and ¹H NMR chemical shifts of water, correspondingly the δ(¹⁷OH₂) and δ(¹H₂O) values, have been measured in aqueous solutions of various salts. The effect of these electrolytes on the δ(¹⁷OH₂) and δ(¹H₂O) values is discussed. The individual ionic ¹⁷O and ¹H molal chemical shifts, δ(¹⁷OH₂)_m and δ(¹H₂O)_m values, respectively, are evaluated and correlated with the different properties of ions. In general, within a series of ions of one type, an increase in ionic radius causes an increase in δ(¹⁷OH₂) (deshielding) and a decrease in δ(¹H₂O) (increased shielding). This was observed (with exceptions) for the cations as well for the anions. There is no general correlation between the δ(¹⁷OH₂)_m and δ(¹H₂O)_m for all ions considered. The contribution of specific ion–solvent interactions on the δ(¹⁷OH₂) and δ(¹H₂O) values is important for the complex anions (SO₄²⁻, NO₃⁻, ClO₄⁻, SCN⁻) and protonated or deprotonated water clusters. Compared with the Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, and Na⁺, K⁺, Rb⁺, Cs⁺ ions, the relatively small Be²⁺ and Li⁺ ions behave differently. The δ(¹⁷OH₂) and δ(¹H₂O) values are largely specific quantities and the division of electrolytes by their effect on the water δ(¹⁷OH₂) and δ(¹H₂O) into water ‘structure breakers’ or ‘structure makers’ is confusing and not well founded.