Rates and mechanisms of water exchange of UO22+(aq) and UO2(oxalate)F(H2O)2-: A variable-temperature 17O and 19F NMR study

Ildikó Farkas, I. Bányai, Zoltán Szabó, Ulf Wahlgren, Ingmar Grenthe

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Abstract

This study consists of two parts: The first part comprised an experimental determination of the kinetic parameters for the exchange of water between UO2(H2O)52+ and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO2(H2O)52+, UO2(H2O)42+, and UO2(H2O)62+ 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 this might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using 17O NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 °C: k(ex) = (1.30 ± 0.05) x 106 s-1; ΔH(+) = 26.1 ± 1.4 kJ/mol; ΔS(+) = -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 UO22+ 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 UO2(H2O)42+ and UO2(H2O)52+ are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in ΔU for the reactions UO2(H2O)52+ → UO2(H2O)42+ + H2O and UO2(H2O)52+ + H2O → UO2(H2O)62+ 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, UO2(H2O)52+ → UO2(H2O)4(H2O)2+ and UO2(H2O)5(H2O)2+ → UO2(H2O)62+, 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, we suggest that the water exchange in UO2(H2O)52+ follows a dissociative interchange mechanism. The rates of exchange of water in UO2(oxalate)F(H2O)2- (and UO2(oxalate)F2(H2O)2- studied previously) are much slower than in the aqua ion, k(ex) = 1.6 x 104 s-1, an effect which we assign to hydrogen bonding involving coordinated water and fluoride. The kinetic parameters for the exchange of water in UO2(H2O)52+ and quenching of photo excited (*)UO2(H2O)52+ are very near the same, indicating similar mechanisms.

Original languageEnglish
Pages (from-to)799-805
Number of pages7
JournalInorganic Chemistry
Volume39
Issue number4
DOIs
Publication statusPublished - Feb 21 2000

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Oxalates
oxalates
Nuclear magnetic resonance
nuclear magnetic resonance
Water
water
Temperature
temperature
Kinetic parameters
Thermodynamics
thermodynamics
Hydrogen bonds
kinetics
Ligands
ligands
Geometry
Solvation
Interchanges
Chemical shift
geometry

ASJC Scopus subject areas

  • Inorganic Chemistry

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Rates and mechanisms of water exchange of UO22+(aq) and UO2(oxalate)F(H2O)2- : A variable-temperature 17O and 19F NMR study. / Farkas, Ildikó; Bányai, I.; Szabó, Zoltán; Wahlgren, Ulf; Grenthe, Ingmar.

In: Inorganic Chemistry, Vol. 39, No. 4, 21.02.2000, p. 799-805.

Research output: Contribution to journalArticle

Farkas, Ildikó ; Bányai, I. ; Szabó, Zoltán ; Wahlgren, Ulf ; Grenthe, Ingmar. / Rates and mechanisms of water exchange of UO22+(aq) and UO2(oxalate)F(H2O)2- : A variable-temperature 17O and 19F NMR study. In: Inorganic Chemistry. 2000 ; Vol. 39, No. 4. pp. 799-805.
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T1 - Rates and mechanisms of water exchange of UO22+(aq) and UO2(oxalate)F(H2O)2-

T2 - A variable-temperature 17O and 19F NMR study

AU - Farkas, Ildikó

AU - Bányai, I.

AU - Szabó, Zoltán

AU - Wahlgren, Ulf

AU - Grenthe, Ingmar

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N2 - This study consists of two parts: The first part comprised an experimental determination of the kinetic parameters for the exchange of water between UO2(H2O)52+ and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO2(H2O)52+, UO2(H2O)42+, and UO2(H2O)62+ 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 this might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using 17O NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 °C: k(ex) = (1.30 ± 0.05) x 106 s-1; ΔH(+) = 26.1 ± 1.4 kJ/mol; ΔS(+) = -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 UO22+ 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 UO2(H2O)42+ and UO2(H2O)52+ are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in ΔU for the reactions UO2(H2O)5/·2+ → UO2(H2O)42+ + H2O and UO2(H2O)52+ + H2O → UO2(H2O)62+ 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, UO2(H2O)52+ → UO2(H2O)4(H2O)2+ and UO2(H2O)5(H2O)2+ → UO2(H2O)62+, 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, we suggest that the water exchange in UO2(H2O)52+ follows a dissociative interchange mechanism. The rates of exchange of water in UO2(oxalate)F(H2O)2- (and UO2(oxalate)F2(H2O)2- studied previously) are much slower than in the aqua ion, k(ex) = 1.6 x 104 s-1, an effect which we assign to hydrogen bonding involving coordinated water and fluoride. The kinetic parameters for the exchange of water in UO2(H2O)52+ and quenching of photo excited (*)UO2(H2O)52+ are very near the same, indicating similar mechanisms.

AB - This study consists of two parts: The first part comprised an experimental determination of the kinetic parameters for the exchange of water between UO2(H2O)52+ and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO2(H2O)52+, UO2(H2O)42+, and UO2(H2O)62+ 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 this might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using 17O NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 °C: k(ex) = (1.30 ± 0.05) x 106 s-1; ΔH(+) = 26.1 ± 1.4 kJ/mol; ΔS(+) = -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 UO22+ 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 UO2(H2O)42+ and UO2(H2O)52+ are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in ΔU for the reactions UO2(H2O)5/·2+ → UO2(H2O)42+ + H2O and UO2(H2O)52+ + H2O → UO2(H2O)62+ 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, UO2(H2O)52+ → UO2(H2O)4(H2O)2+ and UO2(H2O)5(H2O)2+ → UO2(H2O)62+, 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, we suggest that the water exchange in UO2(H2O)52+ follows a dissociative interchange mechanism. The rates of exchange of water in UO2(oxalate)F(H2O)2- (and UO2(oxalate)F2(H2O)2- studied previously) are much slower than in the aqua ion, k(ex) = 1.6 x 104 s-1, an effect which we assign to hydrogen bonding involving coordinated water and fluoride. The kinetic parameters for the exchange of water in UO2(H2O)52+ and quenching of photo excited (*)UO2(H2O)52+ are very near the same, indicating similar mechanisms.

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