Rates and mechanisms of water exchange of UO{sub 2}{sup 2+}(aq) and UO{sub 2}(oxalate)F(H{sub 2}O){sub 2}{sup {minus}}: A variable-temperature {sup 17}O and {sup 19}F NMR study
This study consists of two parts: The first part comprised an experimental determination of the kinetic parameters for the exchange of water between UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO{sub 2}(H{sub 2}O){sub 5}{sup 2+}, UO{sub 2}(H{sub 2}O){sub 4}{sup 2+}, and UO{sub 2}(H{sub 2}O){sub 6}{sup 2+} and the thermodynamics of their reactions with water. In the second part the authors made an experimental study of the rate of water exchange in uranyl complexes and investigated how this might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using {sup 17}O NMR, with Tb{sup 3+} as a chemical shift reagent, gave the following kinetic parameters at 25 C: k{sub ex} = (1.30 {+-} 0.05) x 10{sup 6} s{sup {minus}1}; {Delta}H{sup {double{underscore}dagger}} = 26{sub {sm{underscore}bullet}4} kJ/mol; {Delta}S{sup {double{underscore}dagger}} = {minus}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{sub 2}{sup 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{sub 2}(H{sub 2}O){sub 4}{sup 2+} and UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} are 29.8 and {minus}2.4 kcal/mol, respectively. A comparison of the change in {Delta}U for the reactions UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} {r{underscore}arrow} UO{sub 2}(H{sub 2}O){sub 4}{sup 2+} + H{sub 2}O and UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} + H{sub 2}O and UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} + H{sub 2}O and UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} + H{sub 2}O {r{underscore}arrow} UO{sub 2}(H{sub 2}O){sub 6}{sup 2+} indicates that the thermodynamics favors the second (associative) reaction in gas phase at 0 K, while the thermodynamics of water transfer between the first and second coordination spheres, UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} {r{underscore}arrow} UO{sub 2}(H{sub 2}O){sub 4}(H{sub 2}O){sup 2+} and UO{sub 2}(H{sub 2}O){sub 5}(H{sub 2}O){sup 2+} {r{underscore}arrow} UO{sub 2}(H{sub 2}O){sub 6}{sup 2+}, favors 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 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, it appears that the water exchange in UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} follows a dissociative interchange mechanism. The rates of exchange of water in UO{sub 2}(oxalate)F(H{sub 2}O){sub 2}{sup {minus}} (and UO{sub 2}(oxalate)F{sub 2}(H{sub 2}O){sup 2{minus}} studied previously) are much slower than in the aqua ion, k{sub ex} = 1.6 x 10{sup 4} s{sup {minus}1}, an effect which is assigned to hydrogen bonding involving coordinated water and fluoride. The kinetic parameters for the exchange of water in UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} and quenching of photo excited *UO{sub 2}(H{sub 2}O){sub 5}{sup 2+} are very near the same, indicating similar mechanisms.
- Research Organization:
- Royal Inst. of Tech., Stockholm (SE)
- OSTI ID:
- 20017572
- Journal Information:
- Inorganic Chemistry, Vol. 39, Issue 4; Other Information: PBD: 21 Feb 2000; ISSN 0020-1669
- Country of Publication:
- United States
- Language:
- English
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