Proton Magnetic Resonance Shifts in Aqueous Solutions of Paramagnetic Metal Ions

Abstract

The nuclear magnetic resonance of the water proton in aqueous solutions of paramagnetic metal ions is shifted by the isotropic hyperfine interaction AI·S with the electron spin of the metal. These shifts have been measured for solutions of Cr³⁺, Fe³⁺, Mn²⁺, Co²⁺, Ni²⁺, and Cu²⁺ between 0° and 100°C. Complete averaging of the shift due to rapid exchange of H₂O in and out of the solvation shell occurred for Co²⁺, Cu²⁺, and Mn²⁺. However, the averaging of the shift was not complete for Cr³⁺, Fe³⁺ (and possibly Ni²⁺) and the kinetic parameters for the exchange process had to be considered in order to evaluate A. From the temperature dependence of the proton shifts in Cr³⁺ and Fe³⁺ solutions, we determined the pseudo first‐order rate constants k and the activation energies ΔE for the proton exchange between solvation shell and bulk. These values are: 8.7×10⁴ sec⁻¹ and 7.5 kcal mole⁻¹ for Cr³⁺, 2.8×10⁶ sec⁻¹ and 12.1 kcal mole⁻¹ for Fe³⁺. The proton exchange reaction proceeds via the acid dissociation of the hydrated ions. From the measured values of A which are all positive, it is shown that the interaction arises from d electrons in d_xy, d_xz, and d_yz, which can only form π bonds with the oxygen of the water. The π bonding is able to proceed via two orbitals on the water, one in the H₂O plane and one perpendicular to it. These two orbitals transfer spin to the hydrogen 1s orbitals with similar efficiency but opposite sign, the dominant path being through the orbital in the plane. Following McDonald and Phillips, it is shown how the water proton shifts can be used to measure the number of sites in the first hydration shell of Co²⁺ which are occupied by H₂O. For Co²⁺ bound to inorganic phosphates or to RNA, this number is five, indicating that Co²⁺ in these complexes makes only one bond at a time to RNA or to inorganic phosphates.