17O nuclear magnetic resonance in aqueous solutions of Cu2+ : The combined effect of Jahn–Teller inversion and solvent exchange on relaxation rates

The dynamics of water molecules in Cu2+ aqueous solutions were investigated by 17O NMR at several temperatures and magnetic fields. A theory is developed for scalar relaxation in the presence of Jahn–Teller inversion of the aquaion and water exchange between the aquaion and the bulk, for a model where the inversion is much faster than the exchange and the scalar coupling is negligible for axially bound water molecules. It is shown that this model is consistent with the experimental results, while the standard expressions, ignoring the influence of the Jahn–Teller inversion, are clearly inconsistent with the observed longitudinal relaxation rates. The inversion process is characterized by τ 298i=(5.1±0.6)×10−12 s and Ei=3.5±1.5 kJ mol−1. The water exchange process is characterized by k298ex=(4.4±0.1)×109 s−1, ΔH‡=11.5±0.3 kJ mol−1 and ΔS‡=−21.8±0.9 J K−1 mol−1.