A theoretical study of the fluoride exchange between UO2F+(aq) and UO22+(aq)

Abstract

Experimental data on the thermodynamics and reaction mechanism of the inner-sphere fluoride exchange reaction U¹⁷O₂²⁺ + UO₂F⁺ ⇌ U¹⁷O₂F⁺ + UO₂²⁺ have been compared with different intimate reaction mechanisms using quantum chemical methods. Two models have been tested that start from the outer sphere complexes, (H₂O)[U_AO₂F(OH₂)₄⁺]⋯[U_BO₂(OH₂)₅²⁺] and [U_AO₂F(OH₂)₄⁺]⋯[U_BO₂(OH₂)₅²⁺]; the geometry and energies of the intermediates and transition states along possible reaction pathways have been calculated using different ab initio methods, SCF, B3LYP and MP2. Both the experimental data and the theoretical results suggest that the fluoride exchange takes place via the formation and breaking of a U–F–U bridge that is the rate determining step. The calculated activation enthalpy ΔH^≠ = 30.9 kJ mol⁻¹ is virtually identical to the experimental value 31 kJ mol⁻¹; however this agreement may be a coincidence as we do not expect a larger accuracy than 10 kJ mol⁻¹ with the methods used. The calculations show that the fluoride bridge is formed as an insertion of U_AO₂F(OH₂)₄⁺ into U_BO₂(OH₂)₅²⁺ followed by a subsequent transfer of water from the first to the second coordination sphere of U_B.