Raman spectroscopy of uranyl rare earth carbonate kamotoite-(Y)

Raman spectroscopy at 298 and 77K has been used to study the mineral kamotoite-(Y), a uranyl rare earth carbonate mineral of formula Y(2)(UO(2))(4)(CO(3))(3)(OH)(8).10-11H(2)O. The mineral is characterised by two Raman bands at 1130.9 and 1124.6 cm(-1) assigned to the nu(1) symmetric stretching mode of the (CO(3))(2-) units, while those at 1170.4 and 862.3 cm(-1) (77K) to the deltaU-OH bending vibrations. The assignment of the two bands at 814.7 and 809.6 cm(-1) is difficult because of the potential overlap between the symmetric stretching modes of the (UO(2))(2+) units and the nu(2) bending modes of the (CO(3))(2-) units. Only a single band is observed in the 77K spectrum at 811.6 cm(-1). One possible assignment is that the band at 814.7 cm(-1) is attributable to the nu(1) symmetric stretching mode of the (UO(2))(2+) units and the second band at 809.6 cm(-1) is due to the nu(2) bending modes of the (CO(3))(2-) units. Bands observed at 584 and 547.3 cm(-1) are attributed to water librational modes. An intense band at 417.7 cm(-1) resolved into two components at 422.0 and 416.6 cm(-1) in the 77K spectrum is assigned to an Y(2)O(2) stretching vibration. Bands at 336.3, 286.4 and 231.6 cm(-1) are assigned to the nu(2) (UO(2))(2+) bending modes. U-O bond lengths in uranyl are calculated from the wavenumbers of the uranyl symmetric stretching vibrations. The presence of symmetrically distinct uranyl and carbonate units in the crystal structure of kamotoite-(Y) is assumed. Hydrogen-bonding network related to the presence of water molecules and hydroxyls is shortly discussed.     

Chemical equilibria in the binary and ternary uranyl(VI)-hydroxide-peroxide systems

The composition and equilibrium constants of the complexes formed in the binary U(VI)-hydroxide and the ternary U(VI)-hydroxide-peroxide systems have been studied using potentiometric and spectrophotometric data at 25 °C in a 0.100 M tetramethylammonium nitrate medium. The data for the binary U(VI) hydroxide complexes were in good agreement with previous studies. In the ternary system two complexes were identified, [UO(2)(OH)(O(2))](-) and [(UO(2))(2)(OH)(O(2))(2)](-). Under our experimental conditions the former is predominant over a broad p[H(+)] region from 9.5 to 11.5, while the second is found in significant amounts at p[H(+)] < 10.5. The formation of the ternary peroxide complexes results in a strong increase in the molar absorptivity of the test solutions. The absorption spectrum for [(UO(2))(2)(OH)(O(2))(2)](-) was resolved into two components with peaks at 353 and 308 nm with molar absorptivity of 16200 and 20300 M(-1) cm(-1), respectively, suggesting that the electronic transitions are dipole allowed. The molar absorptivity of [(UO(2))(OH)(O(2))](-) at the same wave lengths are significantly lower, but still about one to two orders of magnitude larger than the values for UO(2)(2+)(aq) and the binary uranyl(VI) hydroxide complexes. It is of interest to note that [(UO(2))(OH)(O(2))](-) might be the building block in cluster compounds such as [UO(2)(OH)(O(2))](60)(60-) studied by Burns et al. (P. C. Burns, K. A. Kubatko, G. Sigmon, B. J. Fryer, J. E. Gagnon, M. R. Antonio and L. Soderholm, Angew. Chem. 2005, 117, 2173-2177). Speciation calculations using the known equilibrium constants for the U(vi) hydroxide and peroxide complexes show that the latter are important in alkaline solutions even at very low total concentrations of peroxide, suggesting that they may be involved when the uranium minerals Studtite and meta-Studtite are formed by α-radiolysis of water. Radiolysis will be much larger in repositories for spent nuclear fuel where hydrogen peroxide might contribute both to the corrosion of the fuel and to transport of uranium in a ground water system.     

Chemical equilibria in the uranyl(vi)-peroxide-carbonate system; identification of precursors for the formation of poly-peroxometallates

The focus of this study is on the identification of precursors in solution that might act as building blocks when solid uranyl(vi) poly-peroxometallate clusters containing peroxide and hydroxide bridges are formed. The precursors could be identified by using carbonate as an auxiliary ligand that prevented the formation of large clusters, such as the ones found in solids of fullerene type. Using data from potentiometric and NMR ((17)O and (13)C) experiments we identified the following complexes and determined their equilibrium constants: (UO(2))(2)(O(2))(CO(3))(4)(6-), UO(2)(O(2))CO(3)(2-), UO(2)(O(2))(CO(3))(2)(4-), (UO(2))(2)(O(2))(CO(3))(2)(2-), (UO(2))(2)(O(2))(2)(CO(3))(2-) and [UO(2)(O(2))(CO(3))](5)(10-). The NMR spectra of the pentamer show that all uranyl and carbonate sites are equivalent, which is only consistent with a ring structure built from uranyl units linked by peroxide bridges with the carbonate coordinated "outside" the ring; this proposed structure is very similar to [UO(2)(O(2))(oxalate)](5)(10-) identified by Burns et al. (J. Am. Chem. Soc., 2009, 131, 16648; Inorg. Chem., 2012, 51, 2403) in K(10)[UO(2)(O(2))(oxalate)](5)·(H(2)O)(13); similar ring structures where oxalate or carbonate has been replaced by hydroxide are important structure elements in solid poly-peroxometallate complexes. The equivalent uranyl sites in (UO(2))(2)(O(2))(2)(CO(3))(2-) suggest that the uranyl-units are linked by the carbonate ion and not by peroxide.     

Redox energetics and kinetics of uranyl coordination complexes in aqueous solution

Detailed voltammetric results for five uranyl coordination complexes are presented and analyzed using digital simulations of the voltammetric data to extract thermodynamic (E(1/2)) and heterogeneous electron-transfer kinetic (k(0) and alpha) parameters for the one-electron reduction of UO(2)(2+) to UO(2)(+). The complexes and their corresponding electrochemical parameters are the following: [UO(2)(OH(2))(5)](2+) (E(1/2) = -0.169 V vs Ag/AgCl, k(0) = 9.0 x 10(-3) cm/s, and alpha = 0.50); [UO(2)(OH)(5)](3-) (-0.927 V, 2.8 x 10(-3) cm/s, 0.46); [UO(2)(C(2)H(3)O(2))(3)](-) (-0.396 V, approximately 0.1 cm/s, approximately 0.5); [UO(2)(CO(3))(3)](4-) (-0.820 V, 2.6 x 10(-5) cm/s, 0.41); [UO(2)Cl(4)](2-) (-0.065 V, 9.2 x 10(-3) cm/s, 0.30). Differences in the E(1/2) values are attributable principally to differences in the basicity of the equatorial ligands. Differences in rate constants are considered within the context of Marcus theory of electron transfer, but no specific structural change(s) in the complexes between the two oxidation states can be uniquely identified with the underlying variability in the heterogeneous rate constants and electron-transfer coefficients.     

A Raman spectroscopic study of the uranyl tellurite mineral schmitterite

Raman spectra of schmitterite measured at 298 and 77K are presented and discussed in detail and in part in comparison with published IR spectrum of synthetic schmitterite. U-O bond lengths in uranyls, calculated with the empirical relations R(U-O)=f[nu(1)(UO(2))(2+)]A and R(U-O)=f[nu(3)(UO(2))(2+)] A, are close to those inferred from the X-ray single crystal structure of synthetic schmitterite and agree also with the data for other natural and synthetic uranyl tellurites.     

Evidence for the formation of UO2(NO3)4(2-) in an ionic liquid by EXAFS

The complexation between uranium(vi) and nitrate ions in a hydrophobic ionic liquid (IL), namely [BMI][NO(3)] (BMI = 1-butyl-3-methylimidazolium(+)), is investigated by EXAFS spectroscopy. It was performed by dissolution of uranyl nitrate UO(2)(NO(3))(2)·6H(2)O or UO(2)(Tf(2)N)(2) (Tf(2)N = bis(trifluoromethylsulfonyl)imide (CF(3)SO(2))(2)N(-)). The formation of the complex UO(2)(NO(3))(4)(2-) is evidenced.     

A Raman spectroscopic study of the uranyl sulphate mineral johannite

Raman spectroscopy at 298 and 77K has been used to study the secondary uranyl mineral johannite of formula (Cu(UO2)2(SO4)2(OH)2 x 8H2O). Four Raman bands are observed at 3593, 3523, 3387 and 3234cm(-1) and four infrared bands at 3589, 3518, 3389 and 3205cm(-1). The first two bands are assigned to OH- units (hydroxyls) and the second two bands to water units. Estimations of the hydrogen bond distances for these four bands are 3.35, 2.92, 2.79 and 2.70 A. A sharp intense band at 1042 cm(-1) is attributed to the (SO4)2- symmetric stretching vibration and the three Raman bands at 1147, 1100 and 1090cm(-1) to the (SO4)2- anti-symmetric stretching vibrations. The nu2 bending modes were at 469, 425 and 388 cm(-1) at 77K confirming the reduction in symmetry of the (SO4)2- units. At 77K two bands at 811 and 786 cm(-1) are attributed to the nu1 symmetric stretching modes of the (UO2)2+ units suggesting the non-equivalence of the UO bonds in the (UO2)2+ units. The band at 786cm(-1), however, may be related to water molecules libration modes. In the 77K Raman spectrum, bands are observed at 306, 282, 231 and 210cm(-1) with other low intensity bands found at 191, 170 and 149cm(-1). The two bands at 282 and 210 cm(-1) are attributed to the doubly degenerate nu2 bending vibration of the (UO2)2+ units. Raman spectroscopy can contribute significant knowledge in the study of uranyl minerals because of better band separation with significantly narrower bands, avoiding the complex spectral profiles as observed with infrared spectroscopy.     

Sequestering ability of polycarboxylic ligands towards dioxouranium(VI)

In this paper we report a comparison on the sequestering ability of some polycarboxylic ligands towards dioxouranium(VI) (UO(2)(2+), uranyl). Ligands taken into account are mono- (acetate), di- (oxalate, malonate, succinate and azelate), tri- (1,2,3-propanetricarboxylate) and hexa-carboxylate (1,2,3,4,5,6-benzenehexacarboxylate). The sequestering ability of polycarboxylic ligands towards UO(2)(2+) was quantified by a new approach expressed by means of a sigmoid Boltzman type equation and of a empirical parameters (pL(50)) which defines the amount of ligand necessary to sequester 50% of the total UO(2)(2+) concentration. A fairly linear correlation was obtained between pL(50) or log K(110) (log K(110) refers to the equilibrium: UO(2)(2+)+L(z-)=UO(2)L((2-z)); L=generic ligand) and the polyanion charges. In order to complete the picture, a tetra-carboxylate ligand (1,2,3,4-butanetetracarboxylate) was studied in NaCl aqueous solutions at 0相似文献   

Raman observation of the interactions between NH4+, SO4(2-), and H2O in supersaturated (NH4)2SO4 droplets

High signal-to-noise ratio (S/N) Raman spectra of (NH(4))(2)SO(4) droplets deposited on a quartz substrate were obtained from dilute to supersaturated states upon decreasing the relative humidity (RH). When the molar water-to-solute ratio (WSR) decreases from 16.8 to 3.2, the v(1)-SO(4)(2-) band changes very little, that is, showing a red-shift of only about 1 cm(-1) (from 979.9 to 978.8 cm(-1)) and an increase of its full width at half-maximum (fwhm) from 8.3 to 9.8 cm(-1). Other vibration modes such as v(2)- and v(4)-SO(4)(2-) bands appear almost constantly at 452 and 615 cm(-1). Such kind of a spectroscopic characteristic is different from previous observation on other cations, indicating that the interactions between SO(4)(2-) and NH(4)+ in supersaturated states are similar to those between SO(4)(2-) and H(2)O in dilute states. After fitting the Raman spectra with Gaussian functions in the spectral range of 2400-4000 cm(-1), we successfully extracted six components at positions of 2878.7, 3032.1, 3115.0, 3248.9, 3468.4, and 3628.8 cm(-1), respectively. The first three components are assigned to the second overtone of NH(4)+ umbrella bending, the combination band of NH(4)+ umbrella bending and rocking vibrations, and the NH(4)+ symmetric stretching vibration, while the latter three components are from the strongly, weakly, and slightly hydrogen-bonded components of water molecules, respectively. With a decrease of the RH, the proportion of the strongly hydrogen-bonded components increases, while that of the weakly hydrogen-bonded components decreases in the droplets. The coexistence of strongly, weakly, and slightly hydrogen-bonded water molecules must hint at a similar hydrogen-bonding network of NH(4)+, SO(4)(2-), and H(2)O to that of pure liquid water in supersaturated (NH(4))(2)SO(4) droplets.     

Role of the uranyl oxo group as a hydrogen bond acceptor

Density functional theory calculations have been used to evaluate the geometries and energetics of interactions between a number of uranyl complexes and hydrogen bond donor groups. The results reveal that although traditional hydrogen bond donors are repelled by the oxo group in the [UO(2)(OH(2))(5)](2+) species, they are attracted to the oxo groups in [UO(2)(OH(2))(2)(NO(3))(2)](0), [UO(2)(NO(3))(3)](-), and [UO(2)Cl(4)](2-) species. Hydrogen bond strength depends on the equatorial ligation and can exceed 15 kcal mol(-1). The results also reveal the existence of directionality at the uranyl oxo acceptor, with a weak preference for linear U═O---H angles.     

Raman spectroscopic study of the molecular structure of the uranyl mineral zippeite from Jáchymov (Joachimsthal), Czech Republic

Raman spectra at 298 and 77K and infrared spectra of the uranyl sulfate mineral zippeite from Jáchymov (Joachimsthal), Czech Republic, K(0.6)(H(3)O)0.4[(UO(2))6(SO(4))3(OH)7].8H2O, were studied. Observed bands were tentatively attributed to the (UO(2))2+ and (SO(4))2- stretching and bending vibrations, the OH stretching vibrations of water molecules, hydroxyls and oxonium ions, and H(2)O, oxonium, and delta U-OH bending vibrations. Empirical relations were used for the calculation of U-O bond lengths in uranyl R (A)=f(nu(3) or nu(1)(UO(2))2+). Calculated U-O bond lengths are in agreement with U-O bond lengths from the single crystal structure analysis and those inferred for uranyl anion sheet topology of uranyl pentagonal dipyramidal coordination polyhedra. The number of observed bands supports the conclusion from single crystal structure analysis that at least two symmetrically distinct U6+ (in uranyls) and S6+ (in sulfates), water molecules and hydroxyls may be present in the crystal structure of the zippeite studied. Strong to very weak hydrogen bonds present in the crystal structure of zippeite studied were inferred from the IR spectra.     

Uranyl and strontium salt solvation in room-temperature ionic liquids. A molecular dynamics investigation

Using molecular dynamics simulations, we compare the solvation of uranyl and strontium nitrates and uranyl chlorides in two room-temperature ionic liquids (ILs): [BMI][PF(6)] based on 1-butyl-3-methylimidazolium(+),PF(6)(-) and [EMI][TCA] based on 1-ethyl-3-methylimidazolium(+),AlCl(4)(-). Both dissociated M(2+),2NO(3)(-) and associated M(NO(3))(2) states of the salts are considered for the two cations, as well as the UO(2)Cl(2) and UO(2)Cl(4)(2)(-) uranyl complexes. In a [BMI][PF(6)] solution, the "naked" UO(2)(2+) and Sr(2+) ions are surrounded by 5.8 and 10.1 F atoms, respectively. The first-shell PF(6)(-) anions rotate markedly during the dynamics and are coordinated, on the average, monodentate to UO(2)(2+) and bidentate to Sr(2+). In an [EMI][TCA] solution, UO(2)(2+) and Sr(2+) coordinate 5.0 and 7.4 Cl atoms of AlCl(4)(-), respectively, which display more restricted motions. Four Cl atoms sit on a least motion pathway of transfer to uranyl, to form the UO(2)Cl(4)(2)(-) complex. The free NO(3)(-) anions and the UO(2)Cl(4)(2)(-) complex are surrounded by imidazolium(+) cations ( approximately 4 and 6-9, respectively). The first shell of the M(NO(3))(2) and UO(2)Cl(2) neutral complexes is mostly completed by the anionic components of the IL, with different contributions depending on the solvent, the M(2+) cation, and its counterions. Insights into energy components of solvation are given for the different systems.     

Surface complexation modeling of uranium(VI) sorbed onto lanthanum monophosphate

Sorption/desorption are basic processes in the field of contaminant transport. In order to develop mechanistically accurate thermodynamic sorption models, the simulation of retention data has to take into account molecular scale informations provided by structural investigations. In this way, the uranyl sorption constants onto lanthanum monophosphate (LaPO(4)) were determined on the basis of a previously published structural investigation. The surface complexation modeling of U(VI) retention onto LaPO(4) has been performed using the constant capacitance model included in the FITEQLv3.2 program. The electrical behavior of the solid surface was investigated using electrophoretic measurements and potentiometric titration experiments. The point of zero charge was found to be 3.5 and surface complexation modeling has made it possible to calculate the surface acidity constants. The fitting procedure was done with respect to the spectroscopic results, which have shown that LaPO(4) presents two kinds of reactive surface sites (lanthanum atoms and phosphate groups). The uranyl sorption edges were determined for two surface coverages: 40 and 20% of the surface sites that are occupied, assuming complete sorption. The modeling of these experimental data was realized by considering two uranyl species ("free" uranyl and uranyl nitrate complex) sorbed only onto phosphate surface groups according to the previously published structural investigation. The obtained sorption constants present similar values for both surface complexes and make it possible to fit both sorption edges: logK(U)=9.4 for z.tbnd;P(OH)(2)+UO(2)(2+)<-->z.tbnd;P(OH)(2)UO(2)(2+) and logK(UN)=9.7 for z.tbnd;P(OH)(2)+UO(2)NO(3)(+)<-->z.tbnd;P(OH)(2)UO(2)NO(3)(+).     

Mono- and dinuclear complexes of sulfones with the tetrachlorides of group 4

The reactions of dialkyl sulfones [R(2)SO(2): R = Me, Et, Ph, R(2)=-(CH(2))(4)-] with the metal tetrachlorides of Group 4 [MCl(4): M = Ti, Zr, Hf] give different products mainly depending on the sulfone/M molar ratio. Compounds of formula [M(2)Cl(8)(R(2)SO(2))(2)][M = Ti, R(2)=-(CH(2))(4)-; M = Zr, R = Et, R = Ph] and [MCl(4)(R(2)SO(2))(2)](sulfone/M = 2)[M = Ti, R = Me; M = Zr, R = Me, R = Ph, R(2)=-(CH(2))(4)-; M = Hf, R = Me, R(2)=-(CH(2))(4)-] have been obtained. By X-ray diffraction methods the dinuclear titanium and zirconium adducts, [Ti(2)Cl(8)(mu-sulfolane-O,O')(2)] and [Zr(2)Cl(8)(mu-Ph(2)SO(2)-O,O')(2)] have been established to contain bridging sulfone and hexacoordinated metal centres, while the mononuclear zirconium complex [ZrCl(4)(Me(2)SO(2))(2)] has cis-monodentate sulfones in a slightly distorted octahedral geometry. The reaction between TiCl(4) and sulfolane (tetrahydrothiophene 1,1-dioxide) in SOCl(2) affords the 1:1 adduct independent of the sulfone/Ti molar ratio. Ligand-exchange and inter-conversion between mononuclear and dinuclear species have been observed by NMR, while the spectral features of the SO(2) moiety have been assigned by IR- and Raman spectroscopies.     

Effect of nitrate, perchlorate, and water on uranyl(VI) speciation in a room-temperature ionic liquid: a spectroscopic investigation

Room-temperature ionic liquids form potentially important solvents in novel nuclear waste reprocessing methods, and the solvation, speciation, and complexation behaviors of lanthanides and actinides in these solvents are of great current interest. In the study reported here, the coordination environment of uranyl(VI) in solutions of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf(2)N]) containing perchlorate, tetrabutylammonium nitrate, and water was investigated using Raman, ATR-FTIR, and NMR spectroscopies in order to better understand the role played in uranyl(VI) solution chemistry in room-temperature ionic liquids by water and other small, weakly complexing ligands. The (2)H NMR chemical shift for water in a solution of uranyl perchlorate hexahydrate in [EMIM][Tf(2)N] appears at 6.52 ppm, indicating that water is coordinated to uranyl(VI). A broad ν(OH) stretching mode at 3370 cm(-1) in the ATR-FTIR spectrum shows that this coordinated water is engaged in hydrogen bonding with water molecules in a second coordination sphere. A significant upfield shift in the (2)H NMR signal for water and the appearance of distinct ν(as)(HOH) (at 3630 cm(-1)) and ν(s)(HOH) (at 3560 cm(-1)) vibrational bands in the ATR-FTIR spectra show that coordinated water is displaced by nitrate upon formation of the UO(2)(NO(3))(2) and UO(2)(NO(3))(3)(-) complexes. The Raman spectra indicate that perchlorate complexed to uranyl(VI) is also displaced by nitrate. Our results indicate that perchlorate and water, though weakly complexing ligands, do have a role in uranyl(VI) speciation in room-temperature ionic liquids and that Raman, infrared, and NMR spectroscopies are valuable additions to the suite of tools currently used to study the chemical behavior of uranyl(VI)-ligand complexes in these solvents.     

Crystallization pathways of sulfate-nitrate-ammonium aerosol particles

Crystallization experiments are conducted for aerosol particles composed of aqueous mixtures of (NH(4))(2)SO(4)(aq) and NH(4)NO(3)(aq), (NH(4))(2)SO(4)(aq) and NH(4)HSO(4)(aq), and NH(4)NO(3)(aq) and NH(4)HSO(4)(aq). Depending on the aqueous composition, crystals of (NH(4))(2)SO(4)(s), (NH(4))(3)H(SO(4))(2)(s), NH(4)HSO(4)(s), NH(4)NO(3)(s), 2NH(4)NO(3) x (NH(4))(2)SO(4)(s), and 3NH(4)NO(3) x (NH(4))(2)SO(4)(s) are formed. Although particles of NH(4)NO(3)(aq) and NH(4)HSO(4)(aq) do not crystallize even at 1% relative humidity, additions of 0.05 mol fraction SO(4)(2-)(aq) or NO(3)(-)(aq) ions promote crystallization, respectively. 2NH(4)NO(3) x (NH(4))(2)SO(4)(s) and (NH(4))(3)H(SO(4))(2)(s) appear to serve as good heterogeneous nuclei for NH(4)NO(3)(s) and NH(4)HSO(4)(s), respectively. 2NH(4)NO(3) x (NH(4))(2)SO(4)(s) crystallizes over a greater range of aqueous compositions than 3NH(4)NO(3) x (NH(4))(2)SO(4)(s). An infrared aerosol spectrum is provided for each solid based upon a linear decomposition analysis of the recorded spectra. Small nonzero residuals occur in the analysis because aerosol spectra depend on particle morphology, which changes slightly across the range of compositions studied. In addition, several of the mixed compositions crystallize with residual aqueous water of up to 5% particle mass. We attribute this water content to enclosed water pockets. The results provide further insights into the nonlinear crystallization pathways of sulfate-nitrate-ammonium aerosol particles.     

The resonance Raman effect of uranyl chloride in dimethyl sulfoxide

A study was carried out of the resonance Raman scattering spectra of uranyl chloride (UO2Cl2) in dimethyl sulfoxide ((CH3)2SO) (DMSO) under laser excitation of the UO2(2+) ion in resonance with the 1sigma(g)+ --> 1phi(g) Laport-forbidden f-f electronic transitions span from 530 to 450 nm by using ten output lines of the argon-ion laser at room temperature. The resonance Raman excitation profile of the totally symmetric stretching vibrational mode of uranyl observed at 832 cm(-1) is presented and analyzed in terms of transform theory within the non-Condon model to give relatively good agreement with experimental results. The disagreement between the experimental data and the calculated resonance Raman excitation profile, at the long-wave part of the the 1sigma(g)+ --> 1phi(g) electronic transitions, may be referred to interference between the weak scattering from the neighboring forbidden electronic states (1delta(g)) and strong preresonance scattering from allowed electronic states at higher levels. An amount of change in the experimental resonance Raman excitation profile of the uranyl-DMSO system depends considerably upon the ligands (L) bound to the uranyl group. Elongation of the U-O equilibrium bond length resulting from the 1sigma(g)+ --> 1phi(g) electronic transitions is related to the magnitude of the change in the excitation profile of UO2L2 (L = NO3, CH3COO, Cl) type uranyl compounds in (DMSO).     

U(VI) uranyl cation-cation interactions in framework germanates

The isomorphous compounds NH(4)[(UO(6))(2)(UO(2))(9)(GeO(4))(GeO(3)(OH))] (1), K[(UO(6))(2)(UO(2))(9)(GeO(4))(GeO(3)(OH))] (2), Li(3)O[(UO(6))(2)(UO(2))(9)(GeO(4))(GeO(3)(OH))] (3), and Ba[(UO(6))(2)(UO(2))(9)(GeO(4))(2)] (4) were synthesized by hydrothermal reaction at 220 °C. The structures were determined using single crystal X-ray diffraction and refined to R(1) = 0.0349 (1), 0.0232 (2), 0.0236 (3), 0.0267 (4). Each are trigonal, P(3)1c. 1: a = 10.2525(5), c = 17.3972(13), V = 1583.69(16) ?(3), Z = 2; 2: a = 10.226(4), c = 17.150(9), V = 1553.1(12) ?(3), Z = 2; 3: a = 10.2668(5), c = 17.0558(11), V = 1556.94(15) ?(3), Z = 2; 4: a = 10.2012(5), c = 17.1570(12), V = 1546.23(15) ?(3), Z = 2. There are three symmetrically independent U sites in each structure, two of which correspond to typical (UO(2))(2+) uranyl ions and the other of which is octahedrally coordinated by six O atoms. One of the uranyl ions donates a cation-cation interaction, and accepts a different cation-cation interaction. The linkages between the U-centered polyhedra result in a relatively dense three-dimensional framework. Ge and low-valence sites are located within cavities in the framework of U-polyhedra. Chemical, thermal, and spectroscopic characterizations are provided.     

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.     

Speciation and coordination chemistry of uranyl(VI)-citrate complexes in aqueous solution

The pH dependence of uranyl(VI) complexation by citric acid was investigated using Raman and attenuated total reflection FTIR spectroscopies and electrospray ionization mass spectrometry. pH-dependent changes in the nu(s)(UO(2)) envelope indicate that three major UO(2)(2+)-citrate complexes with progressively increasing U=O bond lengths are present over a range of pH from 2.0 to 9.5. The first species, which is the predominant form of uranyl(VI) from pH 3.0 to 5.0, contains two UO(2)(2+) groups in spectroscopically equivalent coordination environments and corresponds to the [(UO(2))(2)Cit(2)](2)(-) complex known to exist in this pH range. At pH values >6.5, [(UO(2))(2)Cit(2)](2)(-) undergoes an interconversion to form [(UO(2))(3)Cit(3)](3)(-) and (UO(2))(3)Cit(2). ESI-MS studies on solutions of varying uranyl(VI)/citrate ratios, pH, and solution counteranion were successfully used to confirm complex stoichiometries. Uranyl and citrate concentrations investigated ranged from 0.50 to 50 mM.