Rates and Mechanisms of Water Exchange of UO_2~(2+)(aq) and UO_2(oxalate)F(H_2O)_2~-: A Variable-Temperature ~(17)O and ~(19)F NMR Study

摘要

This study consists of two parts: The first part comprised an experimental determination of the kinetic parameters for the exchange of water between UO_2(H_2O)_5~(2+) and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO_2(H_2O)_5~(2+), UO_2(H_2O)_6~(2+) and the thermodynamics of their reactions with water. In the second part we made an experimental study of the rate of water exchange in uranyl complexes and investigated how tis might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using ~(17)O NMR, with Tb~(3+) as a chemical shift reagent, gave the following kinetic parameters at 25degC: K_(ex)=(1.30+-0.05)X10~6 s~(-1); #DELTA#H=26_.1+-1_.4 kJ/mol; #DELTA#S=-40+-5 J/(K mol). Additional mechanistic indicators were obtained from the known coordination geometry of U(VI) complexes with unidentate ligands and from the theoretical calculations. A survey of the literature shows that there are no known isolated complexes of UO_2~(2+) with unidentate ligands which have a coordination number larger than 5. This was corroborated by quantum chemical calculations which showed that the energy gains by binding an additional water to UO_2(H_2O)_4~(2+) and UO_2(H_2O)_5~(2+) are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in #DELTA#U for the reactions UO_2(H_2O)_5~(2+)->UO_2(H_2O)_4~(2+)+H_2O and UO_2O(H_2O)_5~(2+)+H_2O->UO_2(H_2O)_6~(2+) indicates that the thermodynamics favors the second (associative) reaction in gas phase at 0 K, while the thermodynamis of water transfer between the first and second coordination spheres, UO_2(H_2O)_5~(2+)->UO_2(H_2O)_4(H_2O)~(2+) and UO_2(H_2O)_5(H_2O_~(2+)->UO_2(H_2O)_6~(2+), favior the first (dissociative) reaction. The energy difference between the associative and dissociative reactions is small, and solvation has to be included in ab initio models in order to allow quantitative comparisons comparisons between experimental data and theory. Theoretical calculations of the activation energy were not possible because of the excessive computing time required. On the basis of theoretical and experimental studies, we suggest that the water exchange in UO_2(H_2O)_5~(2+) follows a dissociative interchange mechanism. The rates of exchange of water in UO_2(oxalate)F(H_2O)_2~- (and UO_2(oxalate)F_2(H_2O)~(2-) studied previously) are much slower than in the aqua ion, K_(ex)=1.6X10~4 s~(-1), an effect which we assign to hydrogen bonding involving coordinated water and fluoride. The kinetic parameters for the exchange of water in UO_2(H_2O)_5~(2+) and quenching of photo excited UO_2(H_2O)_5~(2+) are very near the same, indicating similar mechanisms.