Proton chemical shift and hydration of the hydronium ion in aqueous acids

Abstract

A new four-site model is developed to describe the chemical shifts of aqueous acid solutions which takes into account the structure-breaking effect of ions. The model predicts the differing values obtained from different acids for the chemical shift of H₃O⁺ and also predicts closely the apparent variations with temperature of the measured shift. The electric field of the ion H₃O⁺ produces only small chemical shifts in any hydrogen-bonded water and n.m.r. chemical-shift studies are probably insensitive to the presence of such species as H₅O₂⁺ or H₉O₄⁺. Measurements at different temperatures are consistent only with the formulation H₃O⁺.