NMR-Relaxation Mechanisms of O17 in Aqueous Solutions of Paramagnetic Cations and the Lifetime of Water Molecules in the First Coordination Sphere

TLDR

The study extends previous laboratory investigations of Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺ in aqueous solutions. The authors examined the temperature and frequency dependence of O¹⁷ T₂ relaxation in these solutions and compared the resulting water exchange rates with other physical measurements to assess bonding characteristics. Experimentally, the temperature dependence of O¹⁷ T₂ matched predictions of the modified Bloch equations, and the study quantified water exchange rates, activation energies, electronic T₁s, and coupling constants for all ions, revealing a tetrahedral Co²⁺(H₂O)₄ species near 100 °C and a distorted octahedral Cu²⁺(H₂O)₆ coordination with an axial/equatorial exchange ratio of ~105.

Abstract

An investigation was made of the temperature and frequency dependence of T2 for O17 in aqueous solutions containing Mn2+, Fe2+, Co2+, Ni2+, and Cu2+. This represented an extension of the studies previously performed in this laboratory on these ions. Virtually all of the temperature effects predicted by the modified Bloch equations for a two-species system were verified experimentally. Rates of exchange of water molecules between the bulk of the solution and the first coordination sphere of the paramagnetic cations were determined for all the ions studied. Activation energies for exchange were measured and electronic T1's and coupling constants were determined in some cases. Evidence was found for a tetrahedral Co2+(H2O)4 species in aqueous solutions near 100°C. The data for cupric ion were interpreted in terms of six coordinated water molecules in a distorted octahedron, with a ratio of ∼105 existing for the axial-water-exchange rate over that of the equatorial waters. The rates of exchange were compared with other physical measurements and the nature of the bonding was considered.