On the Mechanism of Oxygen Exchange Between Uranyl(VI) Oxygen and Water in Strongly Alkaline Solution as Studied by 17O NMR Magnetization Transfer

Abstract

The mechanism, rate constant, and activation parameters for the exchange between uranyl(VI) oxygen and water oxygen in tetramethyl ammonium hydroxide solution, TMA-OH, have been determined using (17)O NMR magnetization transfer technique. In the concentration range investigated, the predominant complex is UO(2)(OH)(4)(2-). The experimental rate equation, rate = k(ex)[TMA-OH](free)[U(VI)](2)(total) indicates that the exchange takes place via a binuclear complex or transition state with the stoichiometry [(UO(2)(OH)(4)(2-))(UO(2)(OH)(5)(3-)]. The rate-determining step most likely takes place between the axial "yl" oxygens and the equatorial hydroxides. The experimental Gibbs energy of activation, DeltaG(++) = 60.8 +/- 2.4 kJ/mol is in good agreement with the value, DeltaA(++) approximately DeltaG(++) = 52.3 +/- 5.4 kJ/mol, found by Buhl and Schreckenbach in a recent Car-Parrinello molecular dynamics study, indicating that their proposed "shuttle" mechanism may be applicable also on the proposed binuclear transition state.