Binding modes of oxalate in UO2(oxalate) in aqueous solution studied with first-principles molecular dynamics simulations. Implications for the chelate effect

Car-Parrinello molecular dynamics simulations are reported for aqueous UO(2)(H(2)O)(n)(C(2)O(4)) (n = 3, 4), calling special attention to the binding modes of oxalate and the thermodynamics of the so-called chelate effect. Based on free energies from thermodynamic integration (BLYP functional), the κ(1),κ(1')-binding mode of the oxalate (with one O atom from each carboxylate coordinating) is more stable than κ(2) (2 O atoms from the same carboxylate) and κ(1) forms by 23 and 39 kJ mol(-1), respectively. The free energy of binding a fourth water ligand to UO(2)(H(2)O)(3)(κ(1)-C(2)O(4)) is computed to be low, 12 kJ mol(-1). Changes of the hydration shell about oxalate during chelate opening are discussed. Composite enthalpies and free energies, obtained from both experiment and quantum-chemical modeling, are proposed for the formation of monodentate UO(2)(H(2)O)(4)(κ(1)-C(2)O(4)). These data suggest that the largest entropy change in the overall complex formation occurs at this stage, and that the subsequent chelate closure under water release is essentially enthalpy-driven.     

A theoretical study of the fluoride exchange between UO2F+(aq) and UO2 2+(aq)

Experimental data on the thermodynamics and reaction mechanism of the inner-sphere fluoride exchange reaction U17O2(2+) + UO2F+ <==> U17O2F+ + UO2(2+) have been compared with different intimate reaction mechanisms using quantum chemical methods. Two models have been tested that start from the outer sphere complexes, (H2O)[U(A)O2F(OH2)4+]...[U(B)O2(OH2)5(2+)] and [U(A)O2F(OH2)4+]...[U(B)O2(OH2)5(2+)]; the geometry and energies of the intermediates and transition states along possible reaction pathways have been calculated using different ab initio methods, SCF, B3LYP and MP2. Both the experimental data and the theoretical results suggest that the fluoride exchange takes place via the formation and breaking of a U-F-U bridge that is the rate determining step. The calculated activation enthalpy DeltaH( not equal) = 30.9 kJ mol(-1) is virtually identical to the experimental value 31 kJ mol(-1); however this agreement may be a coincidence as we do not expect a larger accuracy than 10 kJ mol(-1) with the methods used. The calculations show that the fluoride bridge is formed as an insertion of U(A)O2)F(OH2)4+ into U(B)O2(OH2)5(2+) followed by a subsequent transfer of water from the first to the second coordination sphere of U(B).     

Gas-phase uranyl, neptunyl, and plutonyl: hydration and oxidation studied by experiment and theory

The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An(VI)O(2)(ClO(4))(2) (An = U, Np, Pu): U(V)O(2)(+), Np(V)O(2)(+), Pu(V)O(2)(+), U(VI)O(2)(OH)(+), and Pu(VI)O(2)(OH)(+); abundances of the actinyl ions reflect the relative stabilities of the An(VI) and An(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO(2)(+)·(H(2)O)(4) (An = U, Np, Pu) and AnO(2)(OH)(+)·(H(2)O)(3) (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO(2)(OH)(+) were substantially faster than for the AnO(2)(+), reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization of hot adducts. Dioxygen addition resulted in UO(2)(+)(O(2))(H(2)O)(n) (n = 2, 3), whereas O(2) addition was not observed for NpO(2)(+) or PuO(2)(+) hydrates. DFT suggests that two-electron three-centered bonds form between UO(2)(+) and O(2), but not between NpO(2)(+) and O(2). As formation of the UO(2)(+)-O(2) bonds formally corresponds to the oxidation of U(V) to U(VI), the absence of this bonding with NpO(2)(+) can be considered a manifestation of the lower relative stability of Np(VI).     

EXAFS investigation of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution

The local structure of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution was investigated by U-L(3) and Th-L(3) EXAFS spectroscopy for total sulfate concentrations 0.05 < or = [SO(4)(2-)] < or = 3 M and 1.0 < or = pH < or = 2.6. The sulfate coordination was derived from U-S and Th-S distances and coordination numbers. The spectroscopic results were combined with thermodynamic speciation and density functional theory (DFT) calculations. In equimolar [SO(4)(2-)]/[UO(2)(2+)] solution, a U-S distance of 3.57 +/- 0.02 Angstrom suggests monodentate coordination, in line with UO(2)SO(4)(aq) as the dominant species. With increasing [SO(4)(2-)]/[UO(2)(2+)] ratio, an additional U-S distance of 3.11 +/- 0.02 Angstrom appears, suggesting bidentate coordination in line with the predominance of the UO(2)(SO(4))(2)(2-) species. The sulfate coordination of Th(IV) and U(IV) was investigated at [SO(4)(2-)]/[M(IV)] ratios > or = 8. The Th(IV) sulfato complex comprises both, monodentate and bidentate coordination, with Th-S distances of 3.81 +/- 0.02 and 3.14 +/- 0.02 Angstrom, respectively. A similar coordination is obtained for U(IV) sulfato complexes at pH 1 with monodentate and bidentate U-S distances of 3.67 +/- 0.02 and 3.08 +/- 0.02 Angstrom, respectively. By increasing the pH value to 2, a U(IV) sulfate precipitates. This precipitate shows only a U-S distance of 3.67 +/- 0.02 Angstrom in line with a monodentate linkage between U(IV) and sulfate. Previous controversially discussed observations of either monodentate or bidentate sulfate coordination in aqueous solutions can now be explained by differences of the [SO(4)(2-)]/[M] ratio. At low [SO(4)(2-)]/[M] ratios, the monodentate coordination prevails, and bidentate coordination becomes important only at higher ratios.     

How amidoximate binds the uranyl cation

This study identifies how the amidoximate anion, AO, interacts with the uranyl cation, UO(2)(2+). Density functional theory calculations have been used to evaluate possible binding motifs in a series of [UO(2)(AO)(x)(OH(2))(y)](2-x) (x = 1-3) complexes. These motifs include monodentate binding to either the oxygen or the nitrogen atom of the oxime group, bidentate chelation involving the oxime oxygen atom and the amide nitrogen atom, and η(2) binding with the N-O bond. The theoretical results establish the η(2) motif to be the most stable form. This prediction is confirmed by single-crystal X-ray diffraction of UO(2)(2+) complexes with acetamidoxime and benzamidoxime anions.     

Uranyl complexes with diamide ligands: a quantum mechanics study of chelating properties in the gas phase

We report a quantum mechanical study on the complexes of UO(2)(2+) with diamide ligands L of malonamide and succinamide type, respectively, forming 6- and 7-chelate rings in their bidentate coordination to uranium. The main aims are to (i) assess how strong the chelate effect is (i.e., the preference for bi- versus monodentate binding modes of L), (ii) compare these ligands as a function of the chelate ring size, and (iii) assess the role of neutralizing counterions. For this purpose, we consider UO(2)L(2+), UO(2)L(2)(2+), UO(2)L(3)(2+), and UO(2)X(2)L type complexes with X(-) = Cl(-) versus NO(3)(-). Hartree-Fock and DFT calculations lead to similar trends and reveal the importance of saturation and steric repulsions ("strain") in the first coordination sphere. In the unsaturated UO(2)L(2+), UO(2)L(2)(2+), and UO(2)Cl(2)L complexes, the 7-ring chelate is preferred over the 6-ring chelate, and bidentate coordination is preferred over the monodentate one. However, in the saturated UO(2)(NO(3))(2)L complexes, the 6- and 7-chelating ligands have similar binding energies, and for a given ligand, the mono- and bidentate binding modes are quasi-isoenergetic. These conclusions are confirmed by the calculations of free energies of complexation in the gas phase. In condensed phases, the monodentate form of UO(2)X(2)L complexes should be further stabilized by coordination of additional ligands, as well as by interactions (e.g., hydrogen bonding) of the "free" carbonyl oxygen, leading to an enthalpic preference for this form, compared to the bidentate one. We also considered an isodesmic reaction exchanging one bidentate ligand L with two monoamide analogues, which reveals that the latter are clearly preferred (by 23-14 kcal/mol at the HF level and 24-12 kcal/mol at the DFT level). Thus, in the gas phase, the studied bidentate ligands are enthalpically disfavored, compared to bis-monodentate analogues. The contrast with trends observed in solution hints at the importance of "long range" forces (e.g., second shell interactions) and entropy effects on the chelate effect in condensed phases.     

Gaseous rust: thermochemistry of neutral and ionic iron oxides and hydroxides in the gas phase

The experimental and theoretical thermochemistry of the gaseous neutral and ionic iron oxides and hydroxides FeO, FeOH, FeO(2), OFeOH, and Fe(OH)(2) and of the related cationic water complexes Fe(H(2)O)(+), (H(2)O)FeOH(+), and Fe(H(2)O)(2)(+) is analyzed comprehensively. A combination of data for the neutral species with those of the gaseous ions in conjunction with some additional measurements provides a refined and internally consistent compilation of thermochemical data for the neutral and ionic species. In terms of heats of formation at 0 K, the best estimates for the gaseous, mononuclear FeO(m)H(n)(-/0/+/2+) species with m = 1, 2 and n = 0-4 are Delta(f)H(FeO(-)) = (108 +/- 6) kJ/mol, Delta(f)H(FeO) = (252 +/- 6) kJ/mol, Delta(f)H(FeO(+)) = (1088 +/- 6) kJ/mol, Delta(f)H(FeOH) = (129 +/- 15) kJ/mol, Delta(f)H(FeOH(+)) = (870 +/- 15) kJ/mol, Delta(f)H(FeO(2)(-)) = (-161 +/- 13) kJ/mol, Delta(f)H(FeO(2)) = (67 +/- 12) kJ/mol, Delta(f)H(FeO(2)(+)) = (1062 +/- 25) kJ/mol, Delta(f)H(OFeOH) = (-84 +/- 17) kJ/mol, Delta(f)H(OFeOH(+)) = (852 +/- 23) kJ/mol, Delta(f)H(Fe(OH)(2)(-)) = -431 kJ/mol, Delta(f)H(Fe(OH)(2)) = (-322 +/- 2) kJ/mol, and Delta(f)H(Fe(OH)(2)(+)) = (561 +/- 10) kJ/mol for the iron oxides and hydroxides as well as Delta(f)H(Fe(H(2)O)(+)) = (809 +/- 5) kJ/mol, Delta(f)H((H(2)O)FeOH(+)) = 405 kJ/mol, and Delta(f)H(Fe(H(2)O)(2)(+)) = (406 +/- 6) kJ/mol for the cationic water complexes. In addition, charge-stripping data for several of several-iron-containing cations are re-evaluated due to changes in the calibration scheme which lead to Delta(f)H(FeO(2+)) = (2795 +/- 28) kJ/mol, Delta(f)H(FeOH(2+)) = (2447 +/- 30) kJ/mol, Delta(f)H(Fe(H(2)O)(2+)) = (2129 +/- 29) kJ/mol, Delta(f)H((H(2)O)FeOH(2+)) = 1864 kJ/mol, and Delta(f)H(Fe(H(2)O)(2)(2+)) = (1570 +/- 29) kJ/mol, respectively. The present compilation thus provides an almost complete picture of the redox chemistry of mononuclear iron oxides and hydroxides in the gas phase, which serves as a foundation for further experimental studies and may be used as a benchmark database for theoretical studies.     

Solvation of uranyl-CMPO complexes in dry vs. humid forms of the [BMI][PF6] ionic liquid. A molecular dynamics study

The solvation of the [UO(2)(NO(3))(CMPO)](+) and [UO(2)(NO(3))(2)(CMPO)(2)] complexes (CMPO = octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide) is investigated by molecular dynamics in the "dry" and "humid" forms of a room temperature ionic liquid (IL) based on the 1-butyl-3-methylimidazolium (BMI(+)) cation and the hexafluorophosphate (PF(6)(-)) anion. The simulations reveal the importance of the solvent anions in "dry" conditions and of water molecules in the "humid" solvent. For the [UO(2)(NO(3))(CMPO)](+) complex, the monodentate vs. bidentate coordination modes of CMPO are compared, and the first solvation shell of uranyl is completed by 1-3 PF(6)(-) anions in the dry IL and by 2-3 water molecules in the humid IL, leading to a total coordination number close to 5. The energy analysis shows that interactions with the IL stabilize the [UO(2)(NO(3))(bi)(CMPO)(mono)](+) form (with bidentate nitrate and monodentate CMPO) in the dry IL and the [UO(2)(NO(3))(mono)(CMPO)(mono)](+) form (with monodentate nitrate and CMPO) in the humid IL. The extracted compound characterized by EXAFS is thus proposed to be the [UO(2)(NO(3))(mono)(CMPO)(mono)(H(2)O)(3)](+) species. Furthermore we compare the [UO(2)(NO(3))(2)(CMPO)(2)] complex in its associated and dissociated forms ([UO(2)(NO(3))(mono)(CMPO)(mono)](+) + CMPO + NO(3)(-)) and discuss the results in the context of uranyl extraction by CMPO to ionic liquids.     

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.     

Structural studies of uranium and thorium complexes with 4,5-dihydroxy-3,5-benzenesdisulfonate (Tiron) at low and neutral pH by X-ray absorption spectroscopy

We have determined the structure of uranyl, UO(2)(2+), and Th(4+) complexes formed in aqueous solution with 4,5-dihydroxy-3,5-benzenedisulfonate (Tiron) as function of pH and concentration. At equimolar concentrations of 0.05 M UO(2)(2+) and Tiron, the predominant species was found to be aqueous uranyl at pH = 2.0. At pH = 6.0, the formation of a 3:3 UO(2)(2+):Tiron trimer (proposed in earlier studies) was observed. In this structure, bidentate catecholate complexation to Tiron as well as oxygen bridging between uranyl units is detected. Th(4+) structural changes were observed both as a function of pH and Th:L (L = Tiron) ratio. At Th:L = 1:1 and pH = 1.4, a monomeric complex is observed with each Th center complexing monodentate to approximately 2 sulfonate functional groups. At pH 4.0 similar sulfonate ligation is observed along with oligomer formation. At pH 6.0 thorium hydrolysis products are detected, with little evidence for inner-sphere Tiron coordination. When the Th:L is changed to 1:2 at pH = 6.0, a stable oligomeric complex is formed that dominates the speciation for Th:L ratios up to 1:5. This complex is characterized by bidentate catechol and monodentate sulfonate ligation to Tiron along with oxygen bridging between Th(4+) atoms and is consistent with the formation of the 2:3 Th:L polymeric species proposed from earlier work. At a Th:L ratio of 1:10, Th(4+) complexation is dominated by bidentate catechol ligation and the formation of a monomeric Th(Tiron)(x) species, where x > or = 2.     

Ions generated from uranyl nitrate solutions by electrospray ionization (ESI) and detected with fourier transform ion-cyclotron resonance (FT-ICR) mass spectrometry

Electrospray ionization (ESI) of uranyl nitrate solutions generates a wide variety of positively and negatively charged ions, including complex adducts of uranyl ions with methoxy, hydroxy, and nitrate ligands. In the positive ion mode, ions detected by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry are sensitive to instrumental tuning parameters such as quadrupole operating frequency and trapping time. Positive ions correspond to oligomeric uranyl nitrate species that can be characterized as having a general formula of [(UO(2))(n)(A)(m)(CH(3)OH)(s)](+) or [(UO(2))(n)(O)(A)(m)(CH(3)OH)(s)](+) with n = 1-4, m = 1-7, s = 0 or 1, and A = OH, NO(3), CH(3)O or a combination of these, although the formation of NO(3)-containing species is preferred. In the negative ion mode, complexes of the form [(UO(2))(NO(3))(m)](-) (m = 1-3) are detected, although the formation of the oxo-containing ions [(UO(2))(O)(n)(NO(3))(m)](-) (n = 1-2, m = 1-2) and the hydroxy-containing ions [(UO(2))(OH)(n)(NO(3))(m)](-) (n = 1-2, m = 0-1) are also observed. The extent of coordinative unsaturation of both positive and negative ions can be determined by ligand association/exchange and H/D exchange experiments using D(2)O and CD(3)OD as neutral reaction partners in the gas-phase. Positive ions are of varying stability and reactivity and may fragment extensively upon collision with D(2)O, CD(3)OD and N(2) in sustained off-resonance irradiation/collision-induced dissociation (SORI-CID) experiments. Electron-transfer reactions, presumably occurring during electrospray ionization but also in SORI-CID, can result in reduction of U(VI) to U(V) and perhaps even U(IV).     

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).     

Uranyl binary and ternary chelates of tenoxicam Synthesis, spectroscopic and thermal characterization of ternary chelates of tenoxicam and alanine with transition metals

Ternary Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and UO(2)(II) chelates with tenoxicam (Ten) drug (H(2)L(1)) and dl-alanine (Ala) (HL(2)) and also the binary UO(2)(II) chelate with Ten were studied. The structures of the chelates were elucidated using elemental, molar conductance, magnetic moment, IR, diffused reflectance and thermal analyses. UO(2)(II) binary chelate was isolated in 1:2 ratio with the formula [UO(2)(H(2)L)(2)](NO(3))(2). The ternary chelates were isolated in 1:1:1 (M:H(2)L(1):L(2)) ratios and have the general formulae [M(H(2)L(1))(L(2))(Cl)(n)(H(2)O)(m)].yH(2)O (M=Fe(III) (n=2, m=0, y=2), Co(II) (n=1, m=1, y=2) and Ni(II) (n=1, m=1, y=3)); [M(H(2)L(1))(L(2))](X)(z).yH(2)O (M=Cu(II) (X=AcO, z=1, y=0), Zn(II) (X=AcO, z=1, y=3) and UO(2)(II) (X=NO(3), z=1, y=2)). IR spectra reveal that Ten behaves as a neutral bidentate ligand coordinated to the metal ions via the pyridine-N and carbonyl-O groups, while Ala behaves as a uninegatively bidentate ligand coordinated to the metal ions via the deprotonated carboxylate-O and amino-N. The magnetic and reflectance spectral data confirm that all the chelates have octahedral geometry except Cu(II) and Zn(II) chelates have tetrahedral structures. Thermal decomposition of the chelates was discussed in relation to structure and different thermodynamic parameters of the decomposition stages were evaluated.     

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.     

Aqueous coordination chemistry and photochemistry of uranyl(VI) oxalate revisited: a density functional theory study

Using density functional theory (DFT) calculations, we revisited a classical problem of uranyl(VI) oxalate photochemical decomposition. Photoreactivities of uranyl(VI) oxalate complexes are found to correlate largely with ligand-structural arrangements. Importantly, the intramolecular photochemical reaction is inhibited when oxalate is bound to uranium exclusively in chelate binding mode. Previously proposed mechanisms involving a UO(2)(C(2)O(4))(2)(2-) (1:2) complex as the main photoreactive species are thus unlikely to apply, because the two oxalic acids are bound to uranium in a chelating binding mode. Our DFT results suggest that the relevant photoreactive species are UO(2)(C(2)O(4))(3)(4-) (1:3) and (UO(2))(2)(C(2)O(4))(5)(6-) (2:5) complexes binding uranium in an unidentate fashion. These species go through decarboxylation upon excitation to the triplet state, which ensues the release of CO(2) and reduction of U(vi) to U(v). The calculations also suggest an alternative intermolecular pathway at low pH via an electron transfer between the excited state *UO(2)(2+) and hydrogen oxalate (HC(2)O(4)(-)) which eventually leads to the production of CO and OH(-) with no net reduction of U(VI). The calculated results are consistent with previous experimental findings that CO is only detected at low pH while U(IV) is detected only at high pH.     

The complexation of uranium(VI) and atmospherically derived CO2 at the ferrihydrite-water interface probed by time-resolved vibrational spectroscopy

The sorption reactions of uranium(VI) at the ferrihydrite(Fh)-water interface were investigated in the absence and presence of atmospherically derived CO(2) by time-resolved in situ vibrational spectroscopy. The spectra clearly show that a single uranyl surface species, most probably a mononuclear bidentate surface complex, is formed irrespective of the presence of atmospherically derived CO(2). The character of the carbonate surface species correlates with the presence of the actinyl ions and changes from a monodentate to a bidentate binding upon sorption of U(VI). From the in situ sorption experiments under mildly acid conditions, the formation of a ternary surface complex is derived where the carbonate ligands coordinate bidentately to the uranyl moiety (≡UO(2)(O(2)CO)(x)). Furthermore, the release reaction of the carbonate ligands from the ternary surface complex is found to be considerably retarded compared to those from the pristine surface suggesting a tighter bonding of the carbonate ions in the ternary complex. Simultaneous sorption of U(VI) and atmospherically derived carbonate onto pristine Fh shows formation of binary monodentate carbonate surface complexes prior to the formation of the ternary complexes.     

Uranyl and/or rare-earth mellitates in extended organic-inorganic networks: a unique case of heterometallic cation-cation interaction with U(VI)═O-Ln(III) bonding (Ln = Ce, Nd)

A series of uranyl and lanthanide (trivalent Ce, Nd) mellitates (mel) has been hydrothermally synthesized in aqueous solvent. Mixtures of these 4f and 5f elements also revealed the formation of a rare case of lanthanide-uranyl coordination polymers. Their structures, determined by XRD single-crystal analysis, exhibit three distinct architectures. The pure lanthanide mellitate Ln(2)(H(2)O)(6)(mel) possesses a 3D framework built up from the connection of isolated LnO(6)(H(2)O)(3) polyhedra (tricapped trigonal prism) through the mellitate ligand. The structure of the uranyl mellitate (UO(2))(3)(H(2)O)(6)(mel)·11.5H(2)O is lamellar and consists of 8-fold coordinated uranium atoms linked to each other through the organic ligand giving rise to the formation of a 2D 3(6) net. The third structural type, (UO(2))(2)Ln(OH)(H(2)O)(3)(mel)·2.5H(2)O, involves direct oxygen bondings between the lanthanide and uranyl centers, with the isolation of a heterometallic dinuclear motif. The 9-fold coordinated Ln cation, LnO(5)(OH)(H(2)O)(3), is linked to the 7-fold coordinated uranyl (UO(2))O(4)(OH) (pentagonal bipyramid) via one μ(2)-hydroxo group and one μ(2)-oxo group. The latter is shared between the uranyl bonding (U═O = 1.777(4)/1.779(6) ?) and a long Ln-O bonding (Ce-O = 2.822(4) ?; Nd-O = 2.792(6) ?). This unusual linkage is a unique illustration of the so-called cation-cation interaction associating 4f and 5f metals. The dinuclear motif is then further connected through the mellitate ligand, and this generates organic-inorganic layers that are linked to each other via discrete uranyl (UO(2))O(4) units (square bipyramid), which ensure the three-dimensional cohesion of the structure. The mixed U-Ln carboxylate is thermally decomposed from 260 to 280 °C and then transformed into the basic uranium oxide (U(3)O(8)) together with U-Ln oxide with the fluorite structural type ("(Ln,U)O(2)"). At 1400 °C, only fluorite type "(Ln,U)O(2)" is formed with the measured stoichiometry of U(0.63)Ce(0.37)O(2) and U(0.60)Nd(0.40)O(2-δ).     

"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+).     

Adsorption and Precipitation of Aqueous Zn(II) on Alumina Powders

The products of aqueous Zn(II) sorption on high-surface-area alumina powders (Linde-A) have been studied using XAFS spectroscopy as a function of Zn(II) sorption density (Gamma=0.2 to 3.3 μmol/m(2)) at pH values of 7.0 to 8.2. Over equilibration times of 15-111 h, we find that at low sorption densities (Gamma=0.2-1.1 μmol/m(2)) Zn(II) forms predominantly inner-sphere bidentate surface complexes with AlO(6) polyhedra, whereas at higher sorption densities (Gamma=1.5 to 3.5 μmol/m(2)), we find evidence for the formation of a mixed-metal Zn(II)-Al(III) hydroxide coprecipitate with a hydrotalcite-type local structure. These conclusions are based on an analysis of first- and second-neighbor interatomic distances derived from EXAFS spectra collected under ambient conditions on wet samples. At low sorption densities the sorption mechanism involves a transformation from six-coordinated Zn-hexaaquo solution complexes (with an average Zn-O distance of 2.07 ?) to four-coordinated surface complexes (with an average Zn-O distance of 1.97 ?) as described by the reaction identical withAl(OH(a))(OH(b))+Zn (H(2)O)(6)(2+)--> identical withAl(OH(a)') (OH(b)')Zn(OH(c)')(OH(d)'+4H(2)O+zH(+), where identical withAl(OH(a))(OH(b)) represents edge-sharing sites of Al(O,OH,OH(2))(6) octahedra to which Zn(O,OH,OH(2))(4) bonds in a bidentate fashion. The proton release consistent with this reaction (z=a-a'+b-b'+4-c'-d'), and with bond valence analysis falls in the range of 0 to 2 H(+)/Zn(II) when hydrolysis of the adsorbed Zn(II) complex is neglected. This interpretation suggests that proton release is likely a strong function of the coordination chemistry of the surface hydroxyl groups. At higher sorption densities (1.5 to 3.5 μmol/m(2)), a high-amplitude, second-shell feature in the Fourier transform of the EXAFS spectra indicates the formation of a three-dimensional mixed-metal coprecipitate, with a hydrotalcite-like local structure. Nitrate anions presumably satisfy the positive layer charge of the Al(III)-Zn(II) hydroxide layers in which the Zn/Al ratio falls in the range of 1 : 1 to 2 : 1. Our results for the higher Gamma-value sorption samples suggest that Zn-hydrotalcite-like phases may be a significant sink for Zn(II) in natural or catalytic systems containing soluble alumina compounds. Copyright 2000 Academic Press.     

The OH* + CH3SH reaction: support for an addition-elimination mechanism from ab initio calculations

Several intermediates for the CH(3)SH + OH(*) --> CH(3)S(*) + H(2)O reaction were identified using MP2(full) 6-311+g(2df,p) ab initio calculations. An adduct, CH(3)S(H)OH(*), I, with electronic energy 13.63 kJ mol(-1) lower than the reactants, and a transition state, II(double dagger), located 5.14 kJ mol(-1) above I, are identified as the entrance channel for an addition-elimination reaction mechanism. After adding zero-point and thermal energies, DeltaH(r,298) ( degrees )(reactants --> I) = -4.85 kJ mol(-1) and DeltaH(298) (double dagger)(I --> II(double dagger)) = +0.10 kJ mol(-1), which indicates that the potential energy surface is broad and flat near the transition state. The calculated imaginary vibrational frequency of the transition state, 62i cm(-1), is also consistent with an addition-elimination mechanism. These calculations are consistent with experimental observations of the OH(*) + CH(3)SH reaction that favored an addition-elimination mechanism rather than direct hydrogen atom abstraction. An alternative reaction, CH(3)SH + OH(*) --> CH(3)SOH + H(*), with DeltaH(r,298) ( degrees ) = +56.94 kJ mol(-1) was also studied, leading to a determination of DeltaH(f,298) ( degrees )(CH(3)SOH) = -149.8 kJ mol(-1).