| 1. |
- Farkas, I., et al.
(författare)
-
Rates and mechanisms of water exchange of UO22+(aq) and UO2(oxalate)F(H2O)(2)(-) : A variable-temperature O-17 and F-19 NMR study
- 2000
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 39:4, s. 799-805
-
Tidskriftsartikel (refereegranskat)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)(5)(2+) and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO2(H2O)(5)(2+), UO2(H2O)(4)(2+), and UO2(H2O)(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 this might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using O-17 NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 degrees C: k(ex) = (1.30 +/- 0.05) x 10(6) s(-1); Delta H double dagger = 26.(1) +/- 1.(4) kJ/mol; Delta S double dagger = -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)(4)(2+) and UO2(H2O)(5)(2+) are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in Delta U for the reactions UO2(H2O)(5)(2+) --> UO2(H2O)(4)(2+) + H2O and UO2(H2O)(5)(2+) + H2O --> UO2(H2O)(6)(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, UO2(H2O)(5)(2+) --> UO2(H2O)(4)(H2O)(2+) and UO2(H2O)(5)(H2O)(2+) --> UO2(H2O)(6)(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, we suggest that the water exchange in UO2(H2O)(5)(2+) follows a dissociative interchange mechanism. The rates of exchange of water in UO2(oxalate)F(H2O)(2-) (and UO2(oxalate)F-2(H2O)(2-) studied previously) are much slower than in the aquation, k(ex) = 1.6 x 10(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 UO2(H2O)(5)(2+) and quenching of photo excited *UO2(H2O)(5)(2+) are very near the same, indicating similar mechanisms.
|
|
| 2. |
- Toraishi, T., et al.
(författare)
-
Complexation of Th(IV) and various lanthanides(III) by glycolic acid; potentiometric, C-13-NMR and EXAFS studies
- 2002
-
Ingår i: Journal of the Chemical Society. Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1472-7773 .- 1364-5447. ; :20, s. 3805-3812
-
Tidskriftsartikel (refereegranskat)abstract
- The complex formation of tetravalent thorium and various trivalent lanthanides by glycolate HOCH2CO2- = A(-), has been investigated by potentiometry, C-13-NMR spectroscopy and EXAFS. The potentiometric data were used to deduce the stoichiometry and equilibrium constants for the reactions pM(n+) (aq) + rA(-) reversible arrow M(p)H(-q)A(r)(np - q - r) + qH(+) at 25 degreesC, in an ionic medium with a constant concentration of Na+ equal to 3.00 M. Mononuclear complexes Th(HOCH2CO2-)(n); n = 1-4, were identified in the -log[H+] range 2.5-4.5. The equilibrium constants of these complexes obtained using a least-squares analysis of the experimental data agree well with previously published information; these test solutions also contain dinuclear ternary complexes Th(2)H(-2)A(r), r = 2, 4 and 6. The complex formation in the pH range 5-10 was studied at high and constant concentrations of glycolate, 0.50, 0.75 and 1.0 M, respectively. Under these conditions, in addition to the dinuclear species, also tetranuclear complexes M(4)H(-q)A(8) are formed, where q varies from 6 to 13 and 6 to 8 for the Th(IV) and Ln(III) systems, respectively. C-13 NMR spectra show that coordinated and free glycolate are in fast exchange at pH 4.5, while at higher pH there are two separate narrow peaks both in the CH2 and CO2- regions for the coordinated ligand, indicating slow exchange between two equally populated sites. The peak integrals correspond to two bonded ligands per metal for both Th(IV) and Ln(III). EXAFS data were used to deduce bond distances within the tetranuclear Th complexes. These data together with the NMR-data indicate that the tetranuclear complexes have a cubane-like core M-4 (OCH2CO2)(4) to which additional glycolate, oxyacetate and hydroxide ligands are coordinated. The identification of new structure and bonding characteristics of alpha-hydroxycarboxylates, in particular at higher pH, may be used to explore new separation schemes between actinides in different oxidation states, but also for group separations between lanthanide(III) and actinide(III) ions.
|
|
| 3. |
- Macak, P., et al.
(författare)
-
Electron transfer in neptunyl(VI)-neptunyl(V) complexes in solution
- 2005
-
Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 109:22, s. 4950-4956
-
Tidskriftsartikel (refereegranskat)abstract
- The rates and mechanisms of the electron self-exchange between Np(V) and Np(VI) in solution have been studied with quantum chemical methods and compared with previous results for the U(V)-U(VI) pair. Both outer-sphere and inner-sphere mechanisms have been investigated, the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. Solvent effects were calculated using the Marcus equation for the outer-sphere reactions and using a nonequilibrium PCM method for the inner-sphere reactions. The nonequilibrium PCM appeared to overestimate the solvent effect for the outer-sphere reactions. The calculated rate constant for the self-exchange reaction NpO2+(aq)+NpO22+(aq) &REVARR; NPO22+(aq)+NPO2+(aq), at 25° C is k = 67 M-1 s(-1), in fair agreement with the observed rates 0.0063-15 M-1 s-1. The differences between the Np(V)-Np(VI) and the U(V)-U(VI) pairs are minor.
|
|
| 4. |
- Moll, H., et al.
(författare)
-
Solution coordination chemistry of uranium in the binary UO22+-SO42- and the ternary UO22+-SO42--OH- system
- 2006
-
Ingår i: Radiochimica Acta. - : Walter de Gruyter GmbH. - 0033-8230 .- 2193-3405. ; 88:11-sep, s. 559-566
-
Tidskriftsartikel (refereegranskat)abstract
- The structure and reaction dynamics in the systems UO22+-SO42- and UO22+-SO42--OH- were investigated using EXAFS and O-17-NMR spectroscopy. Uranium Lm edge EXAFS indicated a bidentate coordination mode of sulfate to uranyl. In solution, this is characterized by an U-S distance of 3.11 Angstrom. Approximately 5 oxygen atoms were observed in the equatorial plane at 2.39-2.43 Angstrom. The kinetics in the binary uranyl sulfate system can be described by four dominant exchange reactions: (1) UO22++SO(4)(2-)reversible arrow UO2SO4(k(1)), (2) U*O-2(2+)+UO(2)SO(4)reversible arrowU*O2SO4+UO22+(k(2)), (3) UO22++UO2(SO4)(2)(2-)reversible arrow 2UO(2)SO(4)(k(3)), and (4) UO2SO4+SO42-reversible arrowUO2(SO4)(2)(2-)(k(4)). These reactions have rate constants indicating that the exchange is not of the simple Eigen-Wilkins type. Ternary uranyl sulfate hydroxide species were characterized by their O-17 chemical shift and by potentiometry. There are no separate signals for the possible isomers of the ternary species indicating that they are in fast exchange with each other.
|
|
| 5. |
|
|
| 6. |
- Privalov, Timofei, et al.
(författare)
-
Electron transfer in uranyl(VI)-uranyl(V) complexes in solution
- 2004
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 126:31, s. 9801-9808
-
Tidskriftsartikel (refereegranskat)abstract
- The rates and mechanisms of the electron self-exchange between U(V) and U(VI) in solution have been studied with quantum chemical methods. Both outer-sphere and inner-sphere mechanisms have been investigated; the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. The calculated rate constant for the self-exchange reaction UO2+(aq) + UO22+(aq)UO22+(aq) + UO2+(aq), at 25 degreesC, is k = 26 M-1 s(-1). The lower limit of the rate of electron transfer in the inner-sphere complexes is estimated to be in the range 2 x 10(4) to 4 x 10(6) M-1 s(-1), indicating that the rate for the overall exchange reaction may be determined by the rate of formation and dissociation of the binuclear complex. The activation energy for the outer-sphere model calculated from the Marcus model is nearly the same as that obtained by a direct calculation of the precursor- and transition-state energy. A simple model with one water ligand is shown to recover 60% of the reorganization energy. This finding is important because it indicates the possibility to carry out theoretical studies of electron-transfer reactions involving M3+ and M4+ actinide species that have eight or nine water ligands in the first coordination sphere.
|
|
| 7. |
- Privalov, Timofei, et al.
(författare)
-
Reduction of uranyl(VI) by iron(II) in solutions : An ab initio study
- 2003
-
Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:4, s. 587-592
-
Tidskriftsartikel (refereegranskat)abstract
- The reduction of uranyl U(VI) by Fe(II) in solution has been studied by quantum chemical methods, where the pH dependence of the reaction was simulated by using different numbers of coordinated hydroxide ions. The geometries for the binuclear U(VI)-Fe(II) precursor and the U(V)-Fe(III) successor complexes were optimized at the SCF level, and the reaction energies were calculated at the correlated level using the MP2 method. Effective core potentials were used throughout. Solvent effects were obtained by the polarizable continuum model. The accuracy of the solvent model was investigated for the binuclear complexes with two hydroxide bridges, and the accuracy of the MP2 method was assessed by comparing with CASPT2 and CCSD(T) calculations on the smallest complexes. The general trends in geometry and reaction energy are consistent with experiment.
|
|
| 8. |
- Privalov, Timofei, et al.
(författare)
-
Structure and thermodynamics of uranium(VI) complexes in the gas phase : A comparison of experimental and ab initio data
- 2002
-
Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 106:46, s. 11277-11282
-
Tidskriftsartikel (refereegranskat)abstract
- Ab initio methods were applied for the calculation of the total energy and the molar entropy and heat capacity of the compounds UO2F2, UO2(OH)(2), UF6, and UO3 in the gas phase with the purpose to obtain thermodynamic data for reactions that can be compared with experimental values. The total energy, geometry, and vibration frequencies were calculated at different levels of accuracy: second-order perturbation theory (MP2), coupled cluster theory (CCSD(T)), and density functional theory (B3LYP). Our results agree well with experimental values and previous theoretical results. Additionally, the transition state Of UO2F2(g) was studied and the value of the barrier for the inversion of the fluoride atoms was calculated.
|
|
| 9. |
- Real, F., et al.
(författare)
-
Spectroscopy and photochemistry of the uranyl(VI)
- 2006
-
Ingår i: RECENT PROGRESS IN COMPUTATIONAL SCIENCES AND ENGINEERING, VOLS 7A AND 7B. - : VSP BV-C/O BRILL ACAD PUBL. - 9789004155428 ; , s. 979-
-
Konferensbidrag (refereegranskat)abstract
- The U - O bond in the uranyl(VI) ion is in general considered to be kinetically inert in thermal reactions, such as isotope exchange reactions. However, a fast isotope exchange can take place in photochemical reactions under UV irradiation as exemplified by the reaction UO22++H-2*O -> U*O-2(2+) + H2O (1). *O denotes oxygen enriched in the isotope 170 or O-18. Under daylight, no exchange takes place, or the rate of exchange is very slow. This suggests that the reaction involves the excited-states of the uranyl(VI). In order to explore the mechanism of exchange it is necessary to have information on the chemistry of the excited states and their electronic structure. it seems reasonable to assume that "yl"-isotope exchange is related to the U - O-yl, stretching modes and following the work of Pierloot and van Besien(1) we have therefore studied the a(g)- and a(u)-modes of the U-O-yl bond using different methods such as CASPT(2,3), standard MRCI calculations, TD-DFT 133LYP(4,5) and DFT-MRCI6. These benchmark calculations are used to provide a computational cost effective model for the study of the photochemistry of actinide compounds. We suggest that the first step in the "yl" exchange involves transfer of a proton from a coordinated water molecule to the "yl" oxygen ions. We have investigated this reaction for a model system involving the uranyl ion and a single water-molecule. We have followed the reaction profile in the ground state, in the luminescent state (3)Delta(g) (sigma u (1)f(delta)(1)) and in a higher lying excited state (3)Gamma(g), which corresponds to the excitation from the highest occupied pi(u), orbitals to the f(delta) orbital. This results in a distorted uranyl structure and the 3 F. state seems to be a good candidate for the photochemically "active" state where the increase of the U-O-yl, bond-length modifies the electronic density in the uranyl ion, so that the distant "yl" oxygen become more negatively charged and a stronger proton acceptor.
|
|
| 10. |
- Schimmelpfennig, B., et al.
(författare)
-
Ab initio studies of Np and Pu complexes and reactions in the gas phase : Structures and thermodynamics
- 2003
-
Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:45, s. 9705-9711
-
Tidskriftsartikel (refereegranskat)abstract
- Reaction enthalpies for the reactions 2MO(3)(g) + MF6(g) --> 3MO(2)F(2)(g), MO2F2(9) + 2H(2)O(g) --> MO2(OH)(2)(g) + 2HF(g), MF6(g) + 2H(2)O(g) --> MO2F2(9) + 4HF(g), MO3(g) + H2O(g) --> MO2(OH)(2)(g), and MF6(g) + 3H(2)O (9) --> MO3(g) + 6HF(g) have been calculated at the CCSD(T) level for M = U and Np and at the MP2 level for M = U, Np, and Pu. The results are compared with previous calculated reaction enthalpies for M = U. The errors in the calculated reaction enthalpies are estimated to be below 20 kJ/mol for Np and about 50 kJ/mol for Pu.
|
|
