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

Experimental data on the thermodynamics and reaction mechanism of the inner-sphere. uoride exchange reaction (UO22+)-O-17 + UO2F+ reversible arrow (UO2F+)-O-17 + 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)[UAO2F(OH2)(4)(+)] center dot center dot center dot [UBO2(OH2)(5)(2+)] and [UAO2F(OH2)(4)(+)] center dot center dot center dot [UBO2(OH2)(5)(2+)]; 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. uoride exchange takes place via the formation and breaking of a U - F - U bridge that is the rate determining step. The calculated activation enthalpy Delta H-not equal = 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. uoride bridge is formed as an insertion of UAO2F(OH2)(4)(+) into UBO2(OH2)(5)(2+) followed by a subsequent transfer of water from the first to the second coordination sphere of U-B.