| 1. |
- Farkas, I., et al.
(author)
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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
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In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 39:4, s. 799-805
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Journal article (peer-reviewed)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.
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| 2. |
- Toraishi, T., et al.
(author)
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Complexation of Th(IV) and various lanthanides(III) by glycolic acid; potentiometric, C-13-NMR and EXAFS studies
- 2002
-
In: Journal of the Chemical Society. Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1472-7773 .- 1364-5447. ; :20, s. 3805-3812
-
Journal article (peer-reviewed)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.
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| 3. |
- Macak, P., et al.
(author)
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Electron transfer in neptunyl(VI)-neptunyl(V) complexes in solution
- 2005
-
In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 109:22, s. 4950-4956
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Journal article (peer-reviewed)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.
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| 4. |
- Moll, H., et al.
(author)
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Solution coordination chemistry of uranium in the binary UO22+-SO42- and the ternary UO22+-SO42--OH- system
- 2006
-
In: Radiochimica Acta. - : Walter de Gruyter GmbH. - 0033-8230 .- 2193-3405. ; 88:11-sep, s. 559-566
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Journal article (peer-reviewed)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.
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| 5. |
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| 6. |
- Privalov, Timofei, et al.
(author)
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Electron transfer in uranyl(VI)-uranyl(V) complexes in solution
- 2004
-
In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 126:31, s. 9801-9808
-
Journal article (peer-reviewed)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.
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| 7. |
- Privalov, Timofei, et al.
(author)
-
Reduction of uranyl(VI) by iron(II) in solutions : An ab initio study
- 2003
-
In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:4, s. 587-592
-
Journal article (peer-reviewed)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.
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| 8. |
- Privalov, Timofei, et al.
(author)
-
Structure and thermodynamics of uranium(VI) complexes in the gas phase : A comparison of experimental and ab initio data
- 2002
-
In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 106:46, s. 11277-11282
-
Journal article (peer-reviewed)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.
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| 9. |
- Real, F., et al.
(author)
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Spectroscopy and photochemistry of the uranyl(VI)
- 2006
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In: RECENT PROGRESS IN COMPUTATIONAL SCIENCES AND ENGINEERING, VOLS 7A AND 7B. - : VSP BV-C/O BRILL ACAD PUBL. - 9789004155428 ; , s. 979-
-
Conference paper (peer-reviewed)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.
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| 10. |
- Schimmelpfennig, B., et al.
(author)
-
Ab initio studies of Np and Pu complexes and reactions in the gas phase : Structures and thermodynamics
- 2003
-
In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:45, s. 9705-9711
-
Journal article (peer-reviewed)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.
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| 11. |
- Szabo, Zoltan, et al.
(author)
-
Potentiometric and multinuclear NMR study of the binary and ternary uranium(VI)-L-fluoride systems, where L is alpha-hydroxycarboxylate or glycine
- 2000
-
In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 39:22, s. 5036-5043
-
Journal article (peer-reviewed)abstract
- Equilibria, structures, and ligand-exchange dynamics in binary and ternary U(VI)-L-F- systems, where L is glycolate, alpha -hydroxyisobutyrate, or glycine, have been investigated in 1.0 M NaClO4 by potentiometry and H-1, O-17, and F-19 NMR spectroscopy. L may be bonded in two ways: either through the carboxylate end or by the formation of a chelate. In the glycolate system, the chelate is formed by proton dissociation from the -alpha hydroxy group at around pH 3, indicating a dramatic increase, a factor of at least 10(13), of its dissociation constant on coordination to uranium(VI). The L exchange in carboxylate-coordinated UO2LF32- follows an Eigen-Wilkins mechanism, as previously found for acetate. The water exchange rate, k(aq) = 4.2 x 10(5) s(-1), is in excellent agreement with the value determined earlier for UO22+(aq). The ligand-exchange dynamics of UO2(O-CH2-COO)(2)F-3 and the activation parameters for the fluoride exchange in D2O (k(obs) = 12 s(-1), DeltaH(double dagger) = 45.8 +/- 2.2 kJ mol(-1), and DeltaS(double dagger) = -55.8 +/- 3.6 J K-1 mol(-1)) are very similar to those in the corresponding oxalate complex, with two parallel pathways, one for fluoride and one for the alpha -oxocarboxylate. The same is true for the L exchange in UO2(O-CH2-COO)(2)(2-) and UO2(oxalate)(2)(2-), The exchange of alpha -oxocarboxylate takes place by a proton-assisted chelate ring opening followed by dissociation. Because we cannot decide if there is also a parallel H+-independent pathway, only an upper limit for the rate constant, k(1) < 1,2 s(-1), can be given. This value is smaller than those in previously studied ternary systems. Equilibria and dynamics in the ternary uranium(VI)-glycine-fluoride system, investigated by F-19 NMR spectroscopy, indicate the formation of one major ternary complex, UO2LF32- and one binary complex, UO2L2 (L = H2N-CH2COO-), with chelate-bonded glycine; log beta>(*) over bar * (9) = 13.80 +/- 0.05 for the equilibrium UO22+ + H2N-CH2COO- + 3F(-) = UO2(H2N-CH2COO)F-3(2-) and log beta>(*) over bar * (11) = 13.0 +/- 0.05 for the reaction UO22+ + 2H(2)N-CH2COO- = UO2(H2N-CH2COO)(2). The glycinate exchange consists of a ring opening followed by proton-assisted steps. The rate of ring opening, 139 +/- 9 s(-1), is independent of both the concentration of H+ and the solvent, H2O or D2O.
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| 12. |
- Szabo, Zoltan, et al.
(author)
-
Solution coordination chemistry of actinides : Thermodynamics, structure and reaction mechanisms
- 2006
-
In: Coordination chemistry reviews. - : Elsevier BV. - 0010-8545 .- 1873-3840. ; 250:08-jul, s. 784-815
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Research review (peer-reviewed)abstract
- The emphasis of this review is on the combination of experimental and theoretical methods to obtain microscopic information on the chemistry of actinides in aqueous solution. A brief discussion is given of some important experimental methods that provide information on the equilibrium constants and constitution of actinide complexes in solution, their structure and the rate and mechanism of ligand substitution reactions. The microscopic perspective is provided by a comparison of experimental data with those obtained using quantum chemical methods; the emphasis is here on structure and reaction mechanisms. Most of the experimental data refer to the chemistry of uranium, thorium and curium, but this information can be generalized to other actinides as their chemistry is often very similar in a given oxidation state. The first step in the analysis of complex formation in solution is based on equilibrium analytical methods; the discussion is here focused on those requiring macro amounts of actinides, as these are necessary in the methods used to obtain structure (large angle X-ray scattering, extended X-ray absorption spectroscopy and NMR) and dynamic (NMR, relaxation and stopped-flow methods) information. Finally, some comments are made on how the molecular understanding of complex formation between UO2 (2+) and small ligands may be of importance in naturally occurring ligands like humic and fulvic acids and biomolecules, such as amino acids, proteins and nucleotides.
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| 13. |
- Szabo, Zoltan, et al.
(author)
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Structure and dynamics in the complex ion (UO2)(2)(CO3)(OH)(3)(-)
- 2000
-
In: Journal of the Chemical Society. Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1472-7773 .- 1364-5447 .- 1470-479X. ; :18, s. 3158-3161
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Journal article (peer-reviewed)abstract
- The structure and ligand exchange dynamics of the ternary complex (UO2)(2)(CO3)(OH)(3)(-) have been investigated by EXAFS and NMR spectroscopy. Very broad signals can be observed in both the C-13 and the O-17 NMR spectra. The EXAFS data show the presence of 1.3 +/- 0.3 short uranium-oxygen distances at 2.26 Angstrom, consistent with single bonded hydroxide and 3.9 +/- 0.6 distances at 2.47 Angstrom for the other ligands in the first co-ordination shell. There is also evidence for a U ... U interaction at 3.90 Angstrom. Based on the EXAFS and NMR data we suggest the presence of three isomers with different bridge arrangements, the dominant one, C, contains 80% of the uranium and the minor ones A and B, 5 and 15%, respectively. The ligand exchange reactions between these isomers are slow. The NMR data indicate that the main reactions involve intramolecular exchanges between isomers with different positions of the non-bridging ligands in A, B and C. We suggest that these take place through water exchange as discussed earlier for other ternary uranium(VI) complexes.
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| 14. |
- Toraishi, T., et al.
(author)
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Mechanisms of ligand exchange reactions, A quantum chemical study of the reaction UO22+(Aq)+HF(Aq) -> UO2F+(Aq)+H+(Aq)
- 2003
-
In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:44, s. 9456-9462
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Journal article (peer-reviewed)abstract
- The thermodynamics and the reaction mechanism for the reaction UO22+(aq) + HF(aq) --> UO2F+(aq) + H+(aq) in water solution has been studied using quantum chemical methods. The solvent was modeled using the polarized medium method (CPCM) with additional water molecules in the second coordination sphere of the complexes studied. The overall reaction was divided into three steps that were analyzed separately. The quantum chemical study was made on the reaction step [UO2(H2O)(5)(2+)],HF(H2O)(n) --> [UO2F (H2O)(4)(+)],H3O+ (H2O)(n), with n = 1 and 2, where the species in the second coordination sphere are located outside the square brackets. The formation of the precursor complex and dissociation of the successor complex were described by the Fuoss equation. The geometry of the different precursor and successor complexes was in good agreement with known bond distances, and strong F---H---O, and/or O---H---O hydrogen bonds are an important structure element in all of them. The Gibbs energy, enthalpy, and entropy of reaction was calculated using the electronic energy at the MP2 level in the solvent, with thermal functions calculated at the SCF/B3LYP levels using the gas-phase geometry. The calculated Gibbs energy of reaction for n = 2 at 298.15 K was -35 kJ/moI at the HF and -25 kJ/moI at the B3LYP level after correction for a known systematic error in the HF bond energy; this compares favorably with the experimental value, -11 kJ/mol. The ligand exchange mechanism was explored by identification of a transition state where HF from the second sphere enters the first coordination sphere in an associative reaction. It was not possible to identify the same transition state from the successor side, indicating that the reaction mechanism consists of at least two steps. We suggest that the rate determining step is the entry of HF from the second to the first coordination sphere, with practically no bond-breaking as indicated by the small change in the H-F distance between precursor and transition state. This suggestion is supported by the experimentally observed reverse H/D isotope effect. The quantum chemical activation energy DeltaU(not equal) was 34 kJ/mol, close to the experimental activation enthalpy DeltaH(not equal) = 38 kJ/mol.
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| 15. |
- Vallet, V., et al.
(author)
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Rates and mechanism of fluoride and water exchange in UO2F53- and UO2F4(H2O) (2-) studied by NMR spectroscopy and wave function based methods
- 2002
-
In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 41:21, s. 5626-5633
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Journal article (peer-reviewed)abstract
- The reaction mechanism for the exchange of fluoride in UO2F53- and UO2F4(H2O)(2-) has been investigated experimentally using F-19 NMR spectroscopy at -5 degreesC, by studying the line broadening of the free fluoride, UO2F42-(aq) UO2F53-, and theoretically using quantum chemical methods to calculate the activation energy for different pathways. The new experimental data allowed us to make a more detailed study of chemical equilibria and exchange mechanisms than in previous studies. From the integrals of the different individual peaks in the new NMR spectra, we obtained the stepwise stability constant K-5 = 0.60 +/- 0.05 M-1 for UO2F53-. The theoretical results indicate that the fluoride exchange pathway of lowest activation energy, 71 kJ/mol, in UO2F53- is water assisted. The pure dissociative pathway has an activation energy of 75 kJ/mol, while the associative mechanism can be excluded as there is no stable UO2F64- intermediate. The quantum chemical calculations have been made at the SCF/MP2 levels, using a conductor-like polarizable continuum model (CPCM) to describe the solvent. The effects of different model assumptions on the activation energy have been studied. The activation energy is not strongly dependent on the cavity size or on interactions between the complex and Na+ counterions. However, the solvation of the complex and the leaving fluoride results in substantial changes in the activation energy. The mechanism for water exchange in UO2F4(H2O)(2-) has also been studied. We could eliminate the associative mechanism, the dissociative mechanism had the lowest activation energy, 39 kJ/mol, while the interchange mechanism has an activation energy that is approximately 50 kJ/mol higher.
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| 16. |
- Vallet, V., et al.
(author)
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Solvent effects on uranium(VI) fluoride and hydroxide complexes studied by EXAFS and quantum chemistry
- 2001
-
In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 40:14, s. 3516-3525
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Journal article (peer-reviewed)abstract
- The structures of the complexes UO2Fn(H2O)(5-n)(2-n), n = 3-5, have been studied by EXAFS. All have pentagonal bipyramid geometry with U-F of and U-H2O distances equal to 2.26 and 2.48 Angstrom, respectively. On the other hand the complex UO2(OH)(4)(2-) has a square bipyramid geometry both in the solid state and in solution. The structures of hydroxide and fluoride complexes have also been investigated with wave function based and DFT methods in order to explore the possible reasons for the observed structural differences. These studies include models that describe the solvent by using a discrete second coordination sphere, a model with a spherical, or shape-adapted cavity in a conductor-like polarizable continuum medium (CPCM), or a combination of the two. Solvent effects were shown to give the main contribution to the observed structure variations between the uranium(VI) tetrahydroxide and the tetrafluoride complexes. Without a solvent model both UO2(OH)(4)(H2O)(2-) and UO2F4(H2O)(2-) have the same square bipyramid geometry, with the water molecule located at a distance of more than 4 Angstrom from uranium and with a charge distribution that is very near identical in the two complexes. Of the models tested, only the CPCM ones are able to describe the experimentally observed square and pentagonal bipyramid geometry in the tetrahydroxide and tetrafluoride complexes. The geometry and the relative energy of different isomers of UO2F3(H2O)(2)(-) are very similar, indicating that they are present in comparable amounts in solution. All calculated bond distances are in good agreement with the experimental observations, provided that a proper model of the solvent is used.
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| 17. |
- Vallet, V., et al.
(author)
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Structure and bonding in solution of dioxouranium(VI) oxalate complexes : Isomers and intramolecular ligand exchange
- 2003
-
In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 42:6, s. 1982-1993
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Journal article (peer-reviewed)abstract
- Structural isomers of [UO2(oxalate)(3)](4-), [UO2(oxalate)F-3](3-), [UO2(oxalate)(2)F](3-), and [UO2(oxalate)(2)(H2O)](2-) have been studied by using EXAFS and quantum chemical ab initio methods. Theoretical structures and their relative energies were determined in the gas phase and in water using the CPCM model. The most stable isomers according to the quantum chemical calculations have geometries consistent with the EXAFS data, and the difference between measured and calculated bond distances is generally less than 0.05 Angstrom. The complex [UO2(oxalate)(3)](4-) contains two oxalate ligands forming five-membered chelate rings, while the third is bonded end-on to a single carboxylate oxygen. The most stable isomer of the other two complexes also contains the same type of chelate-bonded oxalate ligands. The activation energy for ring opening in [UO2(oxalate)F-3](3-), DeltaU(double dagger) = 63 kJ/mol, is in fair agreement with the experimental activation enthalpy, DeltaH(double dagger) = 45 +/- 5 kJ/mol, for different [UO2(PiCOlinate)F-3](2-) complexes, indicating similar ring-opening mechanisms. No direct experimental information is available on intramolecular exchange in [UO3(oxalate)(3)](4-). The theoretical results indicate that it takes place via the tris-chelated intermediate with an activation energy of AV = 38 kJ/mol; the other pathways involve multiple steps and have much higher activation energies. The geometries and energies of dioxouranium(VI) complexes in the gas phase and solvent models differ slightly, with differences in bond distance and energy of typically less than 0.06 Angstrom and 10 kJ/mol, respectively. However, there might be a significant difference in the distance between uranium and the leaving/entering group in the transition state, resulting in a systematic error when the gas-phase geometry is used to estimate the activation energy in solution. This systematic error is about 10 kJ/mol and tends to cancel when comparing different pathways.
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| 18. |
- Vallet, V., et al.
(author)
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The mechanism for water exchange in UO2(H2O)(5) (2+) and UO2(oxalate)(2)(H2O) (2-), as studied by quantum chemical methods
- 2001
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 123:48, s. 11999-12008
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Journal article (peer-reviewed)abstract
- The mechanisms for the exchange of water between [UO2(H2O)(5)](2+), [UO2(oxalate)(2)(H2O)](2-), and water solvent along dissociative (D), associative (A) and interchange (1) pathways have been investigated with quantum chemical methods. The choice of exchange mechanism is based on the computed activation energy and the geometry of the identified transition states and intermediates. These quantities were calculated both in the gas phase and with a polarizable continuum model for the solvent. There is a significant and predictable difference between the activation energy of the gas phase and solvent models: the energy barrier for the D-mechanism increases in the solvent as compared to the gas phase, while it decreases for the A- and I-mechanisms. The calculated activation energy, AW, for the water exchange in [UO2(H2O)(5)](2+) is 74, 19, and 21 kJ/mol, respectively, for the D-, A-, and I-mechanisms in the solvent, as compared to the experimental value DeltaH(double 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 the transition states is small, indicating a lifetime for the intermediate approximate to 10(-10) s, making it very difficult to distinguish between the A- and I-mechanisms experimentally. There is no direct experimental information on the rate and mechanism of water exchange in [UO2(oxalate)(2)(H2O)](2-)containing two bidentate oxalate ions. The activation energy and the geometry of transition states and intermediates along the D-, A-, and I-pathways were calculated both in the gas phase and in a water solvent model, using a single-point MP2 calculation with the gas phase geometry. The activation 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 formed by the spectator ligands.
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