Rates and mechanisms of water exchange of UO2(2+)(aq) and UO2(oxalate)F(H2O)2-: a variable-temperature 17O and 19F 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 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 17O NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 degrees C: kex = (1.30 +/- 0.05) x 10(6) s(-1); deltaH(not equal to) = 26.1 +/- 1.4 kJ/mol; deltaS(not equal to) = -40 +/- 5J 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 UO2(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 UO2(H2O)4(2+) and UO2(H2O)5(2+) are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in deltaU 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)F2(H2O)2- studied previously) are much slower than in the aqua ion, kex = 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)52+ and quenching of photo excited *UO2(H2O)5(2+) are very near the same, indicating similar mechanisms.     

Reactivity of the "yl"-bond in uranyl(VI) complexes. 1. Rates and mechanisms for the exchange between the trans-dioxo oxygen atoms in (UO2)2(OH)2 2+ and mononuclear UO2(OH)n 2-n complexes with solvent water

The stoichiometric mechanism, rate constant, and activation parameters for the exchange of the "yl"-oxygen atoms in the dioxo uranium(VI) ion with solvent water have been studied using 17O NMR spectroscopy. The experimental rate equation, (-->)v= k(2obs)[UO2(2+)]tot2/[H+]2, is consistent with a mechanism where the first step is a rapid equilibrium 2U(17)O2(2+) + 2H2O<==>(U(17)O2)2(OH)2(2+) + 2H+, followed by the rate-determining step (U(17)O2)2(OH)2(2+) + H2O<==>(UO2)2*(OH)2(2+) + H2(17)O, where the back reaction can be neglected because the (17)O enrichment in the water is much lower than in the uranyl ion. This mechanism results in the following rate equation (-->)v= d[(UO2)2(OH)2(2+)]/dt = k(2,2)[(UO2)2(OH)2(2+)] = k(2,2*)beta(2,2)[UO2(2+)]2/[H + ]2; with k(2,2) = (1.88 +/- 0.22) x 10(4) h(-1), corresponding to a half-life of 0.13 s, and the activation parameters DeltaH++ = 119 +/- 13 kJ mol-1 and DeltaS++ = 81 +/- 44 J mol(-1) K(-1). *Beta(2,)2 is the equilibrium constant for the reaction 2UO2(2+) + 2H2O<==>(UO2)2(OH)2(2+) + 2H+. The experimental data show that there is no measurable exchange of the "yl"-oxygen in UO2(2+), UO2(OH)+, and UO2(OH)4(2-)/ UO2(OH)5(3-), indicating that "yl"-exchange only takes place in polynuclear hydroxide complexes. There is no "yl"-exchange in the ternary complex (UO2)2(mu-OH)2(F)2(oxalate)2(4-), indicating that it is also necessary to have coordinated water in the first coordination sphere of the binuclear complex, for exchange to take place. The very large increase in lability of the "yl"-bonds in (UO2)2(OH)2(2+) as compared to those of the other species is presumably a result of proton transfer from coordinated water to the "yl"-oxygen, followed by a rapid exchange of the resulting OH group with the water solvent. "Yl"-exchange through photochemical mediation is well-known for the uranyl(VI) aquo ion. We noted that there was no photochemical exchange in UO2(CO3)3(4-), whereas there was a slow exchange or photo reduction in the UO2(OH)4(2-) / UO2(OH)5(3-) system that eventually led to the appearance of a black precipitate, presumably UO2.     

Rates and mechanism of fluoride and water exchange in UO(2)F(5)(3-) and [UO(2)F(4)(H(2)O)](2-) studied by NMR spectroscopy and wave function based methods

The reaction mechanism for the exchange of fluoride in UO(2)F(5)(3-) and UO(2)F(4)(H(2)O)(2-) has been investigated experimentally using (19)F NMR spectroscopy at -5 degrees C, by studying the line broadening of the free fluoride, UO(2)F(4)(2-)(aq) and UO(2)F(5)(3-), 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 UO(2)F(5)(3-). The theoretical results indicate that the fluoride exchange pathway of lowest activation energy, 71 kJ/mol, in UO(2)F(5)(3-) 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 UO(2)F(6)(4-) 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 UO(2)F(4)(H(2)O)(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.     

The mechanism for water exchange in [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-), as studied by quantum chemical methods.

The mechanisms for the exchange of water between [UO(2)(H(2)O)(5)](2+), [UO(2)(oxalate)(2)(H(2)O)](2)(-)(,) and water solvent along dissociative (D), associative (A) and interchange (I) 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, Delta U(++), for the water exchange in [UO(2)(H(2)O)(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 Delta H(++) = 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 approximately 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 [UO(2)(oxalate)(2)(H(2)O)](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, Delta U(++), 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 [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-) indicates that the entering/leaving water molecules are located outside the plane formed by the spectator ligands.     

"yl"-Oxygen exchange in uranyl(VI) ion: a mechanism involving (UO2)2(μ-OH)2(2+) via U-O(yl)-U bridge formation

Szabó and Grenthe (Inorg. Chem. 2007, 46, 9372-9378) suggested from NMR spectroscopy that the "yl"-oxygen exchange in dioxo uranium(VI) ion in acidic solution occurs via an OH-bridged binuclear complex (UO(2))(2)(μ-OH)(2)(2+). Here, an "yl"-oxygen exchange pathway involving the (UO(2))(2)(μ-OH)(2)(2+) is studied by B3LYP density functional theory calculations. The oxygen exchange takes place via an intramolecular proton shuttle between the oxygen atoms in (UO(2))(2)(μ-OH)(2)(H(2)O)(6)(2+). The direct proton transfer from the hydroxo bridge or from the coordinating water to the "yl"-oxygen in (UO(2))(2)(μ-OH)(2)(H(2)O)(6)(2+) appears to be negligible because of an exceedingly high activation barrier (～170 kJ mol(-1)). The exchange mechanism in (UO(2))(2)(μ-OH)(2)(H(2)O)(6)(2+) can be described by a multistep pathway that leads to the formation of an oxo bridge between two uranyl(VI) centers (U-O(yl)-U bridge). The activation enthalpy Δ(?)H of the reaction obtained at the B3LYP level is 94.7 kJ mol(-1) and is somewhat larger than the experimental value of 80 ± 14 kJ mol(-1). However, the discrepancy between theory and experiment is at the acceptable level. The formation of an oxo bridge between the two uranyl(VI) centers was found to be the key step in proton shuttling, indicating that uranyl(VI) complexes with a stable oxo bridge (such as trinuclear (UO(2))(3)(μ(3)-O)(OH)(3)(+)) may have even faster "yl"-oxygen exchange rates than (UO(2))(2)(μ-OH)(2)(2+).     

Potentiometric and multinuclear NMR study of the binary and ternary uranium(VI)-L-fluoride systems, where L is alpha-hydroxycarboxylate or glycine

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 1H, 17O, and 19F 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 UO2LF3(2-) 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 UO2(2+)(aq). The ligand-exchange dynamics of UO2(O-CH2-COO)2F3- and the activation parameters for the fluoride exchange in D2O (k(obs) = 12 s(-1), deltaH(double dagger) = 45.8 +/- 2.2 kJ mo(-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, k1 < 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 19F NMR spectroscopy, indicate the formation of one major ternary complex, UO2LF3(2-), and one binary complex, UO2L2 (L = H2N-CH2COO-), with chelate-bonded glycine; log beta(9) = 13.80 +/- 0.05 for the equilibrium UO2(2+) + H2N-CH2COO- + 3F- = UO2(H2N-CH2COO)F3(2-) and log beta(11) = 13.0 +/- 0.05 for the reaction UO2(2+) + 2H2N-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.     

Predicting the energy of the water exchange reaction and free energy of solvation for the uranyl ion in aqueous solution

The structures and vibrational frequencies of UO2(H2O)4(2+) and UO2(H2O)5(2+) have been calculated using density functional theory and are in reasonable agreement with experiment. The energies of various reactions were calculated at the density functional theory (DFT) and MP2 levels; the latter provides the best results. Self-consistent reaction field calculations in the PCM and SCIPCM approximations predicted the free energy of the water exchange reaction, UO2(H2O)4(2+) + H2O <--> UO2(H2O)5(2+). The calculated free energies of reaction are very sensitive to the choice of radii (O and H) and isodensity values in the PCM and SCIPCM models, respectively. Results consistent with the experimental HEXS value of -1.19 +/- 0.42 kcal/mol (within 1-3 kcal/mol) are obtained with small cavities. The structures and vibrational frequencies of the clusters with second solvation shell waters: UO2(H2O)4(H2O)8(2+), UO2(H2O)4(H2O)10(2+), UO2(H2O)4(H2O)11(2+), UO2(H2O)5(H2O)7(2+), and UO2(H2O)5(H2O)10(2+), were calculated and are in better agreement with experiment as compared to reactions involving only UO2(H2O)4(2+) and UO2(H2O)5(2+). The MP2 reaction energies for water exchange gave gas-phase results that agreed with experiment in the range -5.5 to +3.3 kcal/mol. The results were improved by inclusion of a standard PCM model with differences of -1.2 to +2.7 kcal/mol. Rearrangement reactions based on an intramolecular isomerization leading to a redistribution of water in the two shells provide good values in comparison to experiment with values of Delta G(exchange) from -2.2 to -0.5 kcal/mol so the inclusion of a second hydration sphere accounts for most solvation effects. Calculation of the free energy of solvation of the uranyl cation yielded an upper bound to the solvation energy of -410 +/- 5 kcal/mol, consistent with the best experimental value of -421 +/- 15 kcal/mol.     

Determination of the equilibrium formation constants of two U(VI)-peroxide complexes at alkaline pH

The formation of uranyl-peroxide complexes was studied at alkaline media by using UV-Visible spectrophotometry and the STAR code. Two different complexes were found at a H(2)O(2)/U(VI) ratio lower than 2. A graphical method was used in order to obtain the formation constants of such complexes and the STAR program was used to refine the formation constants values because of its capacity to treat multiwavelength absorbance data and refining equilibrium constants. The values obtained for the two complexes identified were: UO(2)(2+) + H(2)O(2) + 4OH(-) UO(2)(O(2))(OH)(2)(2-) + 2H(2)O: log β°(1,1,4) = 28.1 ± 0.1 (1). UO(2)(2+) + 2H(2)O(2) + 6OH(-) UO(2)(O(2))(2)(OH)(2)(4-) + 4H(2)O: log β°(1,2,6) = 36.8 ± 0.2 (2). At hydrogen peroxide concentrations higher than 10(-5) mol dm(-3), and in the absence of carbonate, the UO(2)(O(2))(2)(OH)(2)(4-) complex is predominant in solution, indicating the significant peroxide affinity of peroxide ions for uranium and the strong complexes of uranium(VI) with peroxide.     

Experimental and theoretical investigations of IR spectra and electronic structures of the U(OH)2, UO2(OH), and UO2(OH)2 molecules

Reactions of laser-ablated U atoms and H2O2 molecules produce UO2, H2UO2, and UO2(OH)2 as major products and U(OH)2 and HU(O)OH as minor products. Complementary information is obtained from similar reactions of U atoms with D2O2, with H2 + O2 mixtures, and with H2O in excess Ar. Through extensive relativistic density functional theory calculations, we have determined the geometry structures and ground states of these U species with a variety of oxidation states U(II), U(IV), U(V), and U(VI). The calculated vibrational frequencies, IR intensities, and isotopic frequency ratios are in good agreement with the experimental values, thus supporting assignments of the observed matrix IR spectra. We propose that the reactions proceed by forming an energized [U(OH)4] intermediate from reactions of the excited U atom with two H2O2 molecules. Because of the special stability of the U(VI) oxidation state, this intermediate decomposes to the UO2(OH)2 molecule, which reveals a distinctive difference between the chemistries of U and Th, where the major product in analogous Th reactions is the tetrahedral Th(OH)4 molecule owing to the stable Th(IV) oxidation state.     

Electron transfer in uranyl(VI)-uranyl(V) complexes in solution

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 UO(2)(+)(aq) + UO(2)(2+)(aq) <=>UO(2)(2+)(aq) + UO(2)(+)(aq), at 25 degrees C, 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 M(3+) and M(4+) actinide species that have eight or nine water ligands in the first coordination sphere.     

Cation-cation interactions in Sr5(UO2)20(UO6)2O16(OH)6(H2O)6 and Cs(UO2)9U3O16(OH)5

Two novel U6+ compounds, Sr5(UO2)20(UO6)2O16(OH)6(H2O)6 (SrFm) and Cs(UO2)9U3O16(OH)5 (CsFm), have been synthesized by mild hydrothermal reactions. The structures of SrFm (orthorhombic, C2221, a = 11.668(1), b = 21.065 (3), c = 13.273 A, V = 3532.5(1) A3, Z = 2) and CsFm (trigonal, R3c, a = 11.395(2), c = 43.722(7) A, V = 4916.7(1) A3, Z = 6) are rare examples of uranyl compounds that contain cation-cation interactions where an O atom of one uranyl ion is directly linked to another uranyl ion. Both structures are complex frameworks. SrFm contains sheets of polyhedra that are linked through cation-cation interactions with uranyl ions located between the sheets. CsFm possesses an unusually complex framework of vertex- and edge-sharing U6+ polyhedra that incorporates cation-cation interactions.     

(CH3)2NH(CH2)2NH(CH3)2][(UO2)2F2(HPO4)2]: a new organically templated layered uranium phosphate fluoride--synthesis, structure, characterization, and ion-exchange reactions

A new organically templated layered uranium phosphate fluoride, [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)][(UO(2))(2)F(2)(HPO(4))(2)] has been synthesized by hydrothermal reaction of UO(3), H(3)PO(4), HF, and (CH(3))(2)NCH(2)CH(2)N(CH(3))(2) at 140 degrees C. [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)][(UO(2))(2)F(2)(HPO(4))(2)] has a layered crystal structure consisting of seven-coordinated UO(5)F(2) pentagonal bipyramids and four-coordinated HPO(4) tetrahedra. Each anionic layer containing three-, four-, and six-membered rings is separated by [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)](2+) cations. The [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)](2+) cations may be readily exchanged with the M(2+) ions (M = Ba, Sr and Ca) in water to give high crystalline AE(UO(2))(2)(PO(4))(2).6H(2)O (AE = Ca, Sr, Ba).     

Theoretical study of the oxygen exchange in uranyl hydroxide. An old riddle solved?

A multistep mechanism for the experimentally observed oxygen exchange [Inorg. Chem. 1999, 38, 1456] of UO2(2+) cations in highly alkaline solutions is suggested and probed computationally. It involves an equilibrium between [UO2(OH)4](2-) and [UO2(OH)5](3-), followed by formation of the stable [UO3(OH)3 x H2O](3-) intermediate that forms from [UO2(OH)5](3-) through intramolecular water elimination. The [UO3(OH)3 x H2O](3-) intermediate facilitates oxygen exchange through proton shuttling, retaining trans-uranyl structures throughout, without formation of the cis-uranyl intermediates proposed earliar. Alternative cis-uranyl pathways have been explored but were found to have activation energies that are too high. Relativistic density functional theory (DFT) has been applied to obtain geometries and vibrational frequencies of the different species (reactants, intermediates, transition states, products) and to calculate reaction paths. Two different relativistic methods were used: a scalar four-component all-electron relativistic method and the zeroeth-order regular approximation. Calculations were conducted for both gas phase and condensed phase, the latter treated using the COSMO continuum model. An activation energy of 12.5 kcal/mol is found in solution for the rate-determining step, the reaction of changing the four-coordinated uranyl hydroxide to the five-coordinated one. This compares favorably to the experimental value of 9.8 +/- 0.7 kcal/mol. Activation energies of 7.8 and 5.1 kcal/mol are found for the hydrogen transfer between equatorial and axial oxygens through a water molecule in [UO3(OH)3 x H2O](3-) in the gas phase and condensed phase, respectively. Contrary to previously proposed mechanisms that resulted in high activation barriers, we find energies that are low enough to facilitate the reaction at room temperature. For the activation energies, two approximate DFT methods, B3LYP and PBE, are compared. The differences in activation energies are only about 1-2 kcal/mol for these methods.     

Structure of [UO2Cl4]2- in acetonitrile

The complex formation of uranyl UO(2)(2+) with chloride ions in acetonitrile was studied by UV-vis and U L(III) EXAFS spectroscopy. The investigations unambiguously point to the existence of a [UO(2)Cl(4)](2-) species in solution with D(4)(h)() symmetry. The distances in the U(VI) coordination sphere are U-O(ax) = 1.77 +/- 0.01 Angstroms and U-Cl = 2.68 +/- 0.01 Angstroms.     

Structure, equilibrium and ligand exchange dynamics in the binary and ternary dioxouranium(VI)-ethylenediamine-N,N'-diacetic acid-fluoride system: A potentiometric, NMR and X-ray crystallographic study

The structure, thermodynamics and kinetics of the binary and ternary uranium(VI)-ethylenediamine-N,N'-diacetate (in the following denoted EDDA) fluoride systems have been studied using potentiometry, 1H, 19F NMR spectroscopy and X-ray diffraction. The UO2(2+)-EDDA system could be studied up to -log[H3O+] = 3.4 where the formation of two binary complexes UO2(EDDA)(aq) and UO2(H3EDDA)3+ were identified, with equilibrium constants logbeta(UO2EDDA) = 11.63 +/- 0.02 and logbeta(UO2H3EDDA3+) = 1.77 +/- 0.04, respectively. In the ternary system the complexes UO2(EDDA)F-, UO2(EDDA)(OH)- and (UO2)2(mu-OH)2(HEDDA)2F2(aq) were identified; the latter through 19F NMR. 1H NMR spectra indicate that the EDDA ligand is chelate bonded in UO2(EDDA)(aq), UO2(EDDA)F- and UO2(EDDA)(OH)- while only one carboxylate group is coordinated in UO2(H3EDDA)3+. The rate and mechanism of the fluoride exchange between UO2(EDDA)F- and free fluoride was studied by 19F NMR spectroscopy. Three reactions contribute to the exchange; (i) site exchange between UO2(EDDA)F- and free fluoride without any net chemical exchange, (ii) replacement of the coordinated fluoride with OH- and (iii) the self dissociation of the coordinated fluoride forming UO2(EDDA)(aq); these reactions seem to follow associative mechanisms. (1)H NMR spectra show that the exchange between the free and chelate bonded EDDA is slow and consists of several steps, protonation/deprotonation and chelate ring opening/ring closure, the mechanism cannot be elucidated from the available data. The structure (UO2)2(EDDA)2(mu-H2EDDA) was determined by single crystal X-ray diffraction and contains two UO2(EDDA) units with tetracoordinated EDDA linked by H2EDDA in the "zwitterion" form, coordinated through a single carboxylate oxygen from each end to the two uranium atoms. The geometry of the complexes indicates that there is no geometric constraint for an associative ligand substitution mechanism.     

Co(III)离子在二氧化铅电极上的阳极形成

文献上曾报导过Co(Ⅱ)离子具有加速PbO_2电极上氧阳极析出过程的作用,并表明当Co(Ⅱ)离子存在时,氧的析出过程有可能通过表面吸附的高价钴氧化物氧化水分子而形成.本文通过浓硫酸溶液中Co(Ⅱ)阳极氧化为Co(Ⅲ)以及O_2阳极析出动力学的研究,表明了O_2的析出和Co(Ⅲ)的形成是通过吸附在电极表面的高价钴(Ⅳ)的OH 基配合物分别氧化水分子和Co(Ⅱ)离子形成的,它与Cr(Ⅲ)、Mn(Ⅱ)离子的阳极氧化过程相类似,而不是通过Co(Ⅱ)离子直接放电形成的.本文利用文献所述的研究方法.在固定硫酸浓度(3.4mol·kg~(-1))下,研究CoSO_4浓度(0.05—0.35mol·drn~(-3))对于O_2和Co(Ⅲ)阳极形成过程分别的影响.在固定CoSO_4(0.2mol·     

Dioxouranium(VI)-carboxylate complexes. Speciation of UO2(2+)-1,2,3-propanetricarboxylate system in NaCl(aq) at different ionic strengths and at t=25 degrees C

The formation constants of dioxouranium(VI)-1,2,3-propanetricarboxylate [tricarballylate (3-), TCA] complexes were determined in NaCl aqueous solutions at 0 < or = I/mol L(-1) < or = 1.0 and t=25 degrees C, by potentiometry, ISE-[H+] glass electrode. The speciation model obtained at each ionic strength includes the following species: ML-, MLH0, ML2(4-) and ML2H3- (M = UO2(2+) and L = TCA). The dependence on ionic strength of protonation constants of 1,2,3-propanetricarboxylate and of the metal-ligand complexes was modeled by the SIT (Specific ion Interaction Theory) approach and by the Pitzer equations. The formation constants at infinite dilution are [for the generic equilibrium p UO22+ + q (L3-) + r H+ = (UO2(2+))p(L)qHr(2p-3q+r); betapqr]: log beta110 = 6.222 +/- 0.030, log beta111 = 11.251 +/- 0.009, log beta121 = 7.75 +/- 0.02, log beta121 = 14.33 +/- 0.06. The sequestering ability of 1,2,3-propanetricarboxylate towards UO2(2+) was quantified by using a sigmoid Boltzman type equation.     

Crystallographic controls on uranyl binding at the quartz/water interface

Molecular dynamics methods were used to simulate UO(2)(OH)(2)(0) binding to pairs of oxo sites (O(S)) on three low-index planes of α-SiO(2) in contact with water. Differences in binding site distributions on the (001), (010) and (101) planes produced distinct sets of stable U inner-sphere species. Steric constraints prevented bidentate coordination to the (001) surface, resulting in a mononuclear monodentate complex, [UO(2)(OH)(2)(H(2)O)(n)O(S)] (90% for n=1 and 10% for n=2 over 5 ns production runs). Binuclear bidentate coordination, [UO(2)(OH)(2)(H(2)O)(n)(O(S))(2)], was however favored on the (010) (99% for n=0 and 1% for n=1) and the (101) (72% for n=0 and 28% for n=1) planes. These results underscore a predominant four-coordinated equatorial shell for U when complexed to the quartz/water interface. Potential of mean force calculations uncovered a diversity of metastable outer- and inner-sphere complexes at local energy minima up to ～0.4 nm from the surface. These calculations point to important differences in both energetic requirements and mechanisms for the approach of UO(2)(OH)(2)(0) to different quartz surfaces. Binding strengths are affected by binding site distribution, steric freedom, U hydration and OH orientation, and increase in the order (001) (3.7 kJ mol(-1)) < (101) (5.6 kJ mol(-1)) < (010) (6.5 kJ mol(-1)). A general binding mechanism involves (1) formation of monodentate outer-sphere complexes, (2) removal of oxo-bound waters, (3) formation of one (monodentate), then two (bidentate) direct U-O(S) bonds (inner-sphere), and (4) expulsion of excessive waters from the equatorial shell of U.     

Calculating geochemical reaction pathways--exploration of the inner-sphere water exchange mechanism in Al(H2O)6(3+)(aq) + nH2O with ab Initio calculations and molecular dynamics

We have simulated exchange of inner-sphere and bulk water molecules for different sizes of Al3+(aq) clusters, Al(H2O)63+ + nH2O for n = 0, 1, 6, or 12, with ab initio and molecular dynamics simulations, in order to understand how robust the ab initio method is for identifying hydrolytic reaction pathways of particular importance to geochemistry. In contrast to many interfacial reactions, this particular elementary reaction is particularly simple and well-constrained by experiment. Nevertheless, we find that a rich array of parallel reaction pathways depend sensitively on the details of the solvation sphere and structure and that larger clusters are not necessarily better. Inner-sphere water exchange in Al3+(aq) may occur through two Langford-Gray dissociative pathways, one in which the incoming and outgoing waters are cis, the other in which they are trans to one another. A large majority of exchanges in the molecular dynamics simulations occurred via the trans mechanism, in contrast to the predictions of the ab initio method. In Al(H2O)63+ + H2O, the cis mechanism has a transition state of 84.3 kJ/mol, which is in good agreement with previous experimental and ab initio results, while the trans mechanism has only a saddle point with two negative frequencies, not a transition state, at 89.7 kJ/mol. In addition to the exchange mechanisms, dissociation pathways could be identified that were considerably lower in energy than experiment and varied considerably between 60 and 100 kJ/mol, depending on the particular geometry and cluster size, with no clear relation between the two. Ab initio calculations using large clusters with full second coordination spheres (n = 12) were unable to find dissociation or exchange transition states because the network of hydrogen bonds in the second coordination sphere was too rigid to accommodate the outgoing inner-sphere water. Our results indicate that caution should surround ab initio simulation of complicated dynamic processes such as hydrolysis, ion exchange, and interfacial reactions that involve several steps. Dynamic methods of simulation need to accompany static methods such as ab initio calculation, and it is best to consider simulated pathways as hypotheses to be tested experimentally rather than definitive properties of the reaction.     

Role of solvation in the reduction of the uranyl dication by water: a density functional study

We have studied the solvation of uranyl, UO(2)(2+), and the reduced species UO(OH)(2+) and U(OH)(2)(2+) systematically using three levels of approximation: direct application of a continuum model (M1); explicit quantum-chemical treatment of the first hydration sphere (M2); a combined quantum-chemical/continuum model approach (M3). We have optimized complexes with varying numbers of aquo ligands (n = 4-6) and compared their free energies of solvation. Models M1 and M2 have been found to recover the solvation energy only partially, underestimating it by approximately 100 kcal/mol or more. With our best model M3, the calculated hydration free energy Delta(h)G degrees of UO(2)(2+) is about -420 kcal/mol, which shifts to about -370 kcal/mol when corrected for the expected error of the model. This value agrees well with the experimentally determined interval, -437 kcal/mol < Delta(h)G degrees < -318 kcal/mol. Complexes with 5 and 6 aquo ligands have been found to be about equally favored with models M2 and M3. The same solvation models have been applied to a two-step reduction of UO(2)(2+) by water, previously theoretically studied in the gas phase. Our results show that the solvation contribution to the reaction free energy, about 60 kcal/mol, dominates the endoergicity of the reduction.