Opportunities and challenges of applying advanced X-ray spectroscopy to actinide and lanthanide N-donor ligand systems.

N-donor ligands such as n-Pr-BTP [2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine] preferentially bind trivalent actinides (An3+) over trivalent lanthanides (Ln3+) in liquid-liquid separation. However, the chemical and physical processes responsible for this selectivity are not yet well understood. Here, an explorative comparative X-ray spectroscopy and computational (L3-edge) study for the An/Ln L3-edge and the N K-edge of [An/Ln(n-Pr-BTP)3](NO3)3, [Ln(n-Pr-BTP)3](CF3SO3)3 and [Ln(n-Pr-BTP)3](ClO4)3 complexes is presented. High-resolution X-ray absorption near-edge structure (HR-XANES) L3-edge data reveal additional features in the pre- and post-edge range of the spectra that are investigated using the quantum chemical codes FEFF and FDMNES. X-ray Raman spectroscopy studies demonstrate the applicability of this novel technique for investigations of liquid samples of partitioning systems at the N K-edge.

A Combined Study of Tc Redox Speciation in Complex Aqueous Systems: Wet-Chemistry, Tc K-/L3-Edge X-ray Absorption Fine Structure, and Ab Initio Calculations.

The combination of wet-chemistry experiments (measurements of pH, Eh, and [Tc]) and advanced spectroscopic techniques (K- and L3-edge X-ray absorption fine structure spectroscopy) confirms the formation of a very stable Tc(V)-gluconate complex under anoxic conditions. In the presence of gluconate and an excess of Sn(II) (at pe + pH ≈ 2), technetium forms a very stable Tc(IV)-gluconate complex significantly enhancing the solubility defined by TcO2(s) in hyperalkaline gluconate-free systems. A new setup for "tender" X-ray spectroscopy (spectral range, ∼2-5 keV) in transmission or total fluorescence yield detection mode based on a He flow cell has been developed at the INE Beamline for radionuclide science (KIT light source). This setup allows handling of radioactive specimens with total activities up to one million times the exemption limit. For the first time, Tc L3-edge measurements (∼2.677 keV) of Tc species in liquid (aqueous) media are reported, clearly outperforming conventional K-edge spectroscopy as a tool to differentiate Tc oxidation states and coordination environments. The coupling of L3-edge X-ray absorption near-edge spectroscopy measurements and relativistic multireference ab initio methods opens new perspectives in the definition of chemical and thermodynamic models for systems of relevance in the context of nuclear waste disposal, environmental, and pharmaceutical applications.

Trivalent Actinide Ions Showing Tenfold Coordination in Solution.

Trivalent actinides generally exhibit ninefold coordination in solution. 2,6-Bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP), a tridentate nitrogen donor ligand, is known to form ninefold coordinated 1:3 complexes, [An(nPr-BTP)3]3+ (An = U, Pu, Am, Cm) in solution. We report a Cm(III) complex with tenfold coordination in solution, [Cm(nPr-BTP)3(NO3)]2+. This species was identified using time-resolved laser fluorescence spectroscopy (TRLFS), vibronic side band spectroscopy (VSBS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Adding nitrate to a solution of the [Cm(nPr-BTP)3]3+ complex in 2-propanol shifts the Cm(III) emission band from 613.1 to 617.3 nm. This bathochromic shift is due to a higher coordination number of the Cm(III) ion in solution, in agreement with the formation of the [Cm(nPr-BTP)3(NO3)]2+ complex. The formation of this complex exhibits slow kinetics in the range of 5 to 12 days, depending on the water content of the solvent. Formation of a complex [Cm(nPr-BTP)3(X)]2+ was not observed for anions other than nitrate (X- = NO2-, CN-, or OTf-). The formation of the [Cm(nPr-BTP)3(NO3)]2+ complex was studied as a function of NO3- and nPr-BTP concentrations, and slope analyses confirmed the addition of one nitrate anion to the [Cm(nPr-BTP)3]3+ complex. Experiments with varied nPr-BTP concentration show that [Cm(nPr-BTP)3(NO3)]2+ only forms at nPr-BTP concentrations below 10-4 mol/L whereas for concentrations greater than 10-4 mol/L the formation of the tenfold species is suppressed and [Cm(nPr-BTP)3]3+ is the only species present. The presence of the tenfold coordinated complex is supported by VSBS, XPS, and DFT calculations. The vibronic side band of the [Cm(nPr-BTP)3(NO3)]2+ complex exhibits a nitrate stretching mode not observed in the [Cm(nPr-BTP)3]3+ complex. Moreover, XPS on [M(nPr-BTP)3(NO3)](NO3)2 (M = Eu, Am) yields signals from both non-coordinated and coordinated nitrate. Finally, DFT calculations reveal that the energetically most favored structure is obtained if the nitrate is positioned on the C2 axis of the D3 symmetrical [Cm(nPr-BTP)3]3+ complex with a bond distance of 413 pm. Combining results from TRLFS, VSBS, XPS, and DFT provides sound evidence for a unique tenfold coordinated Cm(III) complex in solution-a novelty in An(III) solution chemistry.

Tris-{hydridotris(1-pyrazolyl)borato}actinide Complexes: Synthesis, Spectroscopy, Crystal Structure, Bonding Properties and Magnetic Behaviour.

The isostructural compounds of the trivalent actinides uranium, neptunium, plutonium, americium, and curium with the hydridotris(1-pyrazolyl)borato (Tp) ligand An[η3 -HB(N2 C3 H3 )3 ]3 (AnTp3 ) have been obtained through several synthetic routes. Structural, spectroscopic (absorption, infrared, laser fluorescence) and magnetic characterisation of the compounds were performed in combination with crystal field, density functional theory (DFT) and relativistic multiconfigurational calculations. The covalent bonding interactions were analysed in terms of the natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) models.

Interdisciplinary Round-Robin Test on Molecular Spectroscopy of the U(VI) Acetate System.

A comprehensive molecular analysis of a simple aqueous complexing system-U(VI) acetate-selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry.

Exploring the electronic structure and speciation of aqueous and colloidal Pu with high energy resolution XANES and computations.

Pu L3 HR-XANES and FEFF9 computations provide evidence for band-like 6d states in colloidal Pu contrasting to narrow 6d states in molecular Pu(iv). Pu L3 HR-XANES is valuable for bond length estimation in plutonyl, whereas Pu M5 HR-XANES is an advanced tool for analysing Pu redox states and 5f unoccupied density of states.

Computing UV/vis spectra using a combined molecular dynamics and quantum chemistry approach: bis-triazin-pyridine (BTP) ligands studied in solution.

We report a combined computational and experimental study to investigate the UV/vis spectra of 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine (BTP) ligands in solution. In order to study molecules in solution using theoretical methods, force-field parameters for the ligand-water interaction are adjusted to ab initio quantum chemical calculations. Based on these parameters, molecular dynamics (MD) simulations are carried out from which snapshots are extracted as input to quantum chemical excitation-energy calculations to obtain UV/vis spectra of BTP ligands in solution using time-dependent density functional theory (TDDFT) employing the Tamm-Dancoff approximation (TDA). The range-separated CAM-B3LYP functional is used to avoid large errors for charge-transfer states occurring in the electronic spectra. In order to study environment effects with theoretical methods, the frozen-density embedding scheme is applied. This computational procedure allows to obtain electronic spectra calculated at the (range-separated) DFT level of theory in solution, revealing solvatochromic shifts upon solvation of up to about 0.6 eV. Comparison to experimental data shows a significantly improved agreement compared to vacuum calculations and enables the analysis of relevant excitations for the line shape in solution.

First structural characterization of Pa(iv) in aqueous solution and quantum chemical investigations of the tetravalent actinides up to Bk(IV): the evidence of a curium break.

More than a century after its discovery the structure of the Pa(4+) ion in acidic aqueous solution has been investigated for the first time experimentally and by quantum chemistry. The combined results of EXAFS data and quantum chemically optimized structures suggest that the Pa(4+) aqua ion has an average of nine water molecules in its first hydration sphere at a mean Pa-O distance of 2.43 Å. The data available for the early tetravalent actinide (An) elements from Th(4+) to Bk(4+) show that the An-O bonds have a pronounced electrostatic character, with bond distances following the same monotonic decreasing trend as the An(4+) ionic radii, with a decrease of the hydration number from nine to eight for the heaviest ions Cm(4+) and Bk(4+). Being the first open-shell tetravalent actinide, Pa(4+) features a coordination chemistry very similar to its successors. The electronic configuration of all open-shell systems corresponds to occupation of the valence 5f orbitals, without contribution from the 6d orbitals. Our results thus demonstrate that Pa(iv) resembles its early actinide neighbors.

Influence of hydrogen bonding on the structure of the (001) corundum-water interface. Density functional theory calculations and Monte Carlo simulations.

Density functional theory calculations and classical Monte Carlo simulations are applied to study the behavior of water in contact with a hydroxylated corundum (001) surface. Using DFT with periodic boundary conditions at T = 0 K, we systematically study the influence of the number of water molecules on the surface geometry and on the structure of the contact water layer. Only little effect of the thickness of the water layer on the geometry of the surface hydroxyl groups is observed. On the other hand, the molecules in the second layer have strong influence on the arrangement of water molecules in direct contact with the solid surface. In order to mimic macroscopic systems at room temperature, we perform inhomogeneous MC simulations of model corundum surface in contact with the water phase modeled by SPC/E model. The water molecules are classified according to their hydrogen-bonded partners into several groups. It is found that the preferential orientation of interfacial water molecules is primarily determined by the type of their hydrogen bonding. The hydroxyl groups at the corundum surface can serve as hydrogen bond donor or acceptor, depending on their orientation. No surface hydroxyls are found to coordinate two water molecules at the same time. On the other hand, water molecules coordinated by two different surface groups appear in MC simulations.

Further insights in the ability of classical nonadditive potentials to model actinide ion-water interactions.

Pursuing our efforts on the development of accurate classical models to simulate radionuclides in complex environments (Réal et al., J. Phys. Chem. A 2010, 114, 15913; Trumm et al. J. Chem. Phys. 2012, 136, 044509), this article places a large emphasis on the discussion of the influence of models/parameters uncertainties on the computed structural, dynamical, and temporal properties. Two actinide test cases, trivalent curium and tetravalent thorium, have been studied with three different potential energy functions, which allow us to account for the polarization and charge-transfer effects occurring in hydrated actinide ion systems. The first type of models considers only an additive energy term for modeling ion/water charge-transfer effects, whereas the other two treat cooperative charge-transfer interactions with two different analytical expressions. Model parameters are assigned to reproduce high-level ab initio data concerning only hydrated ion species in gas phase. For the two types of cooperative charge-transfer models, we define two sets of parameters allowing or not to cancel out possible errors inherent to the force field used to model water/water interactions at the ion vicinity. We define thus five different models to characterize the solvation of each ion. For both ions, our cooperative charge-transfer models lead to close results in terms of structure in solution: the coordination number is included within 8 and 9, and the mean ion/water oxygen distances are 2.45 and 2.49 Å, respectively, for Th(IV) and Cm(III).

Thorium nanochemistry: the solution structure of the Th(IV)-hydroxo pentamer.

Tetravalent thorium exhibits a strong tendency towards hydrolysis and subsequent polymerization. Polymeric species play a crucial role in understanding thorium solution chemistry, since their presence causes apparent solubility several orders of magnitude higher than predicted by thermodynamic data bases. Although electrospray mass spectrometry (ESI MS) identifies Th(IV) dimers and pentamers unequivocally as dominant species close to the solubility limit, the molecular structure of Th(5)(OH)(y) polymers was hitherto unknown. In the present study, X-ray absorption fine structure (XAFS) spectroscopy, high energy X-ray scattering (HEXS) measurements, and quantum chemical calculations are combined to solve the pentamer structure. The most favourable structure is represented by two Th(IV) dimers linked by a central Th(IV) cation through hydroxide bridges.

Modeling the hydration of mono-atomic anions from the gas phase to the bulk phase: the case of the halide ions F-, Cl-, and Br-.

In this work, we investigate the hydration of the halide ions fluoride, chloride, and bromide using classical molecular dynamics simulations at the 10 ns scale and based on a polarizable force-field approach, which treats explicitly the cooperative bond character of strong hydrogen bond networks. We have carried out a thorough analysis of the ab initio data at the MP2 or CCSD(T) level concerning anion/water clusters in gas phase to adjust the force-field parameters. In particular, we consider the anion static polarizabilities computed in gas phase using large atomic basis sets including additional diffuse functions. The information extracted from trajectories in solution shows well structured first hydration shells formed of 6.7, 7.0, and 7.6 water molecules at about 2.78 Å, 3.15 Å, and 3.36 Å for fluoride, chloride, and bromide, respectively. These results are in excellent agreement with the latest neutron- and x-ray diffraction studies. In addition, our model reproduces several other properties of halide ions in solution, such as diffusion coefficients, description of hydration processes, and exchange reactions. Moreover, it is also able to reproduce the electrostatic properties of the anions in solution (in terms of anion dipole moment) as reported by recent ab initio quantum simulations. All the results show the ability of the proposed model in predicting data, as well as the need of accounting explicitly for the cooperative character of strong hydrogen bonds to reproduce ab initio potential energy surfaces in a mean square sense and to build up a reliable force field.

Quantum chemical and molecular dynamics study of the coordination of Th(IV) in aqueous solvent.

In this work, we investigate the solvation of tetravalent thorium Th(IV) in aqueous solution using classical molecular dynamics simulations at the 10 ns scale and based on polarizable force-field approaches, which treat explicitly the covalent character of the metal-water interaction (and its inherent cooperative character). We have carried out a thorough analysis of the accuracy of the ab initio data that we used to adjust the force-field parameters. In particular, we show that large atomic basis sets combined with wave function-based methods (such as the MP2 level) have to be preferred to density functional theory when investigating Th(IV)/water aggregates in gas phase. The information extracted from trajectories in solution shows a well-structured Th(IV) first hydration shell formed of 8.25 ± 0.2 water molecules and located at about 2.45 ± 0.02 Å and a second shell of 17.5 ± 0.5 water molecules at about 4.75 Å. Concerning the first hydration sphere, our results correspond to the lower bounds of experimental estimates (which range from 8 to 12.7); however, they are in very good agreement with the average of existing experimental data, 2.45 ± 0.02 Å. All our results demonstrate the predictable character of the proposed approach, as well as the need of accounting explicitly for the cooperative character of charge-transfer phenomena affecting the Th(IV)/water interaction to build up reliable and accurate force-field approaches devoted to such studies.

Complexation of Cm(III) with fluoride in aqueous solution in the temperature range from 20 to 90 °C. A joint TRLFS and quantum chemical study.

The formation of hydrated CmF2+ and CmF2+ species in aqueous solutions are studied in the temperature range of 20−90 °C at different fluoride concentrations and at constant ionic strength as well as at constant fluoride concentration and different ionic strengths by means of time-resolved laser fluorescence spectroscopy (TRLFS). The molar fractions of the Cm3+ aqua ion, CmF2+, and CmF2+ species are determined by peak deconvolution of the emission spectra. An increase of the mono- and difluoro complexes is observed with increasing fluoride concentration and/or increasing temperature. Using the specific ion interaction theory (SIT), the thermodynamic stability constants log K10 (CmF2+) and log K20 (CmF2+) as well as the values of Δε1 and Δε2 are determined as a function of temperature. The log K10 values increase from 3.56 ± 0.07 to 3.98 ± 0.06 and the log K20 values increase from 2.20 ± 0.84 to 3.34 ± 0.21 with increasing temperature from 20 to 90 °C. The value of Δε1 determined at 25 °C is in good agreement with literature data and shows a negligible temperature dependency in the studied temperature range. The value of Δε2 also shows only a moderate variation in the studied temperature range. The thermodynamic standard state data (ΔrHm0, ΔrSm0, ΔrGm0) are determined from the temperature dependence of the equilibrium constants at Im = 0 using the integrated Van't Hoff equation. The fluorescence lifetime of the 6D'7/2(Cm3+) state is found to be constant at 63 ± 5 µs with increasing fluoride concentration. A model based on density functional theory (DFT) calculations is introduced to account for the additional quenching occurring through the near second sphere waters in the [Cm(H2O)8F]2+(H2O)18 complex.

Characterization of redox sensitive plutonium(III) complexed with alkylated 2,6-ditriazinylpyridine (BTP) in organic solution.

Spectroscopic, solvent extraction methods and computational chemistry are applied for the characterization of redox sensitive trivalent plutonium complexed with the N-donor extracting agent alkylated 2,6-ditriazinylpyridines (n-C3H7-BTP) in organic solution. The redox stabilization and speciation of Pu(III) is discussed. Extraction of Pu(III) with n-C3H7-BTP is compared to that for redox stable Am(III). The speciation of Pu(III) with n-C3H7-BTP in organic solution is studied by UV-Vis/NIR and XAFS spectroscopy. Only the 1:3 complex, [Pu(n-C3H7-BTP)3](3+) is observed to form, which is confirmed by EXAFS and solvent extraction. The [Pu(n-C3H7-BTP)3](3+) complex is compared to the isostructural U(III), Am(III), and Cm(III) complexes studied earlier.

Theoretical investigation of the water/corundum (0001) interface.

For the reliable long-term modeling of the actinide migration in geological formations, the adsorption/desorption properties and the reactivity of mineral surfaces must be understood at the molecular level. The adsorption of radioisotopes at mineral surfaces of the aquifer is an important process that leads to the retention of contaminants such as radionuclides. Their transport by the ground water is either retarded or even completely inhibited by the presence of such a surface. Accordingly, this subject is of main importance for the safety assessment of nuclear waste repositories. As part of a joint theoretical/experimental effort, the interaction of water with the corundum (0001) surface is studied using several theoretical methods (Moller-Plesset perturbation theory, coupled cluster singles doubles with triplet corrections, as well as density functional theory). We focus in this study on the determination of the bond lengths and tilt angles of the surface OH species and their respective vibrational frequencies. The theoretical results are confirmed by subsequent simulation of the interface selective nonlinear sum frequency spectra. The excellent agreement of the simulated with the experimental spectra allows an assignment of the observed peaks in the sum frequency spectra of the water/corundum (0001) interface on the basis of our theoretical data. In this theoretical study we are able to give a unique interpretation of the observed sum frequency spectra of the water/corundum (0001) interface.

Hydration of mineral surfaces probed at the molecular level.

By employing the nonlinear optical, interface selective experiment of sum frequency spectroscopy together with independent ab initio and density functional theory calculations, we determine the functional species of a corundum (001) surface: doubly coordinated OH groups which differ in their bond tilt angles. The interaction of the functional species with the adjacent water molecules is also observed. In a large pH range around the point of zero charge, the interaction is not controlled electrostatically but by hydrogen bonding. The functional species' tilt angles are crucial parameters, determining whether the species act as hydrogen bond donors or acceptors.

Characterization and comparison of Cm(III) and Eu(III) complexed with 2,6-di(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine using EXAFS, TRFLS, and quantum-chemical methods.

The complexation of Cm(III) and Eu(III) with 2,6-di(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (n-C3H7-BTP) in nonaqueous organic solution is studied with extended X-ray absorption spectroscopy. Bond lengths are the same in both complexes. Quantum-chemical calculations performed at different levels support this finding. On the other hand, the Cm.(n-C3H7-BTP)3 complex is formed at much lower ligand-to-metal concentration ratio than the Eu.(n-C3H7-BTP)3 complex, as shown by time-resolved laser-induced fluorescence spectroscopy. This is in good agreement with n-C3H7-BTP's high selectivity for trivalent actinides over lanthanides in liquid-liquid extraction.

Electron transfer in neptunyl(VI)-neptunyl(V) complexes in solution.

The rates and mechanisms of the electron self-exchange between Np(V) and Np(VI) in solution have been studied with quantum chemical methods and compared with previous results for the U(V)-U(VI) pair. Both outer-sphere and inner-sphere mechanisms have been investigated, the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. Solvent effects were calculated using the Marcus equation for the outer-sphere reactions and using a nonequilibrium PCM method for the inner-sphere reactions. The nonequilibrium PCM appeared to overestimate the solvent effect for the outer-sphere reactions. The calculated rate constant for the self-exchange reaction NpO2(+)(aq) + NpO2(2+)(aq) right harpoon over left harpoon NpO2(2+)(aq) + NpO2(+)(aq), at 25 degrees C is k = 67 M(-1) s(-1), in fair agreement with the observed rates 0.0063-15 M(-1) s(-1). The differences between the Np(V)-Np(VI) and the U(V)-U(VI) pairs are minor.