The mechanism for water exchange in [UO2(H2O)(5)](2+) and [UO2(oxalate)(2)(H2O)](2-), as studied by quantum chemical methods

The mechanisms for the exchange of water between [UO2(H2O)(5)](2+), [UO2(ox alate)(2)(H2O)](2-), and water solvent along dissociative (D), associative (A) and interchange (1) pathways have been investigated with quantum chemic al methods. The choice of exchange mechanism is based on the computed activ ation energy and the geometry of the identified transition states and inter mediates. These quantities were calculated both in the gas phase and with a polarizable continuum model for the solvent. There is a significant and pr edictable difference between the activation energy of the gas phase and sol vent models: the energy barrier for the D-mechanism increases in the solven t as compared to the gas phase, while it decreases for the A- and I-mechani sms. The calculated activation energy, AW, for the water exchange in [UO2(H 2O)(5)](2+) is 74, 19, and 21 kJ/mol, respectively, for the D-, A-, and I-m echanisms in the solvent, as compared to the experimental value DeltaH(doub le dagger) = 26 +/- 1 kJ/mol. This indicates that the D-mechanism for this system can be ruled out. The energy barrier between the intermediates and t he transition states is small, indicating a lifetime for the intermediate a pproximate to 10(-10) s, making it very difficult to distinguish between th e A- and I-mechanisms experimentally. There is no direct experimental infor mation on the rate and mechanism of water exchange in [UO2(oxalate)(2)(H2O) ](2-)containing two bidentate oxalate ions. The activation energy and the g eometry of transition states and intermediates along the D-, A-, and I-path ways were calculated both in the gas phase and in a water solvent model, us ing a single-point MP2 calculation with the gas phase geometry. The activat ion energy, AW, in the solvent for the D-, A-, and I-mechanisms is 56, 12, and 53 kJ/mol, respectively. This indicates that the water exchange follows an associative reaction mechanism. The geometry of the A- and I-transition states for both [UO2(H2O)(5)](2+) and [UO2(oxalate)(2)(H2O)](2-) indicates that the entering/leaving water molecules are located outside the plane fo rmed by the spectator ligands.