Experimental and computational evidence of U(VI)-OH-Si(OH)4 complexes under alkaline conditions: Implications for cement systems.

The complexation of uranyl hydroxides with orthosilicic acid was investigated by experimental and theoretical methods. Spectroluminescence titration was performed in a glovebox under argon atmosphere at pH 9.2, 10.5 and 11.5, with [U(VI)] = 10-6 and 5 × 10-6 mol kgw-1. The polymerization effects of silicic acid were minimized by ruling out samples with less than 90 % monomeric silicic acid present, identified via UV-Vis spectrometry using the molybdate blue method. Linear regression analysis based on time-resolved laser-induced fluorescence spectroscopy (TRLFS) results yielded the conditional stepwise formation constants of U(VI)-OH-Si(OH)4 complexes at 0.05 mol kgw-1 NaNO3. The main spectroscopic features - characteristic peak positions and decay-time - are reported for the first time for the UO2(OH)2SiO(OH)3- species observed at pH 9.2 and 10.5 and UO2(OH)2SiO2(OH)22- predominant at pH 11.5. Quantum chemical calculations successfully computed the theoretical luminescence spectrum of the complex UO2(OH)2SiO(OH)3- species, thus underpinning the proposed chemical model for weakly alkaline systems. The conditional stability constants were extrapolated to infinite dilution using the Davies equation, resulting in log10ß°(UO2(OH)2SiO(OH)3-) and log10ß°(UO2(OH)2SiO2(OH)22-). Implications for U(VI) speciation in the presence and absence of competing carbonate are discussed for silicate-rich environments expected in certain repository concepts for nuclear waste disposal.

Complexation of Np(V) with the Dicarboxylates, Malonate, and Succinate: Complex Stoichiometry, Thermodynamic Data, and Structural Information.

The complexation of Np(V) with malonate and succinate is studied by different spectroscopic techniques, namely, attenuated total reflection Fourier transform infrared (ATR FT-IR) and extended X-ray absorption fine-structure (EXAFS) spectroscopy, as well as by quantum chemistry to determine the speciation, thermodynamic data, and structural information of the formed complexes. For complex stoichiometries and the thermodynamic functions (log ßn°(Θ), ΔrHn°, ΔrSn°), near infrared absorption spectroscopy (vis/NIR) is applied. The complexation reactions are investigated as a function of the total concentration of malonate ([Mal2-]total) and succinate ([Succ2-]total), ionic strength [Im = 0.5-4.0 mol kg-1 Na+(Cl-/ClO4-)], and temperature (Θ = 20-85 °C). Besides the solvated NpO2+ ion, the formation of two Np(V) species with the stoichiometry NpO2(L)n1-2n (n = 1, 2, L = Mal2-, Succ2-) is observed. With increasing temperature, the molar fractions of both complex species increase and the temperature-dependent conditional stability constants log ßn'(Θ) at given ionic strengths are determined by the law of mass action. The log ßn'(Θ) are extrapolated to IUPAC reference-state conditions (Im = 0) according to the specific ion interaction theory (SIT), revealing thermodynamic log ßn°(Θ) values. For all formed complexes, [NpO2(Mal)-: log ß1°(25 °C) = 3.36 ± 0.11, NpO2(Mal)23-: log ß2°(25 °C) = 3.95 ± 0.19, NpO2(Succ)-: log ß1°(25 °C) = 2.05 ± 0.45, NpO2(Succ)23-: log ß2°(25 °C) = 0.75 ± 1.22], an increase of the stability constants with increasing temperature was observed. This confirmed an endothermic complexation reaction. The temperature dependence of the log ßn°(T) values is described by the integrated Van't Hoff equation, and the standard reaction enthalpies and entropies for the complexation reactions are determined. Furthermore, the sum of the specific binary ion-ion interaction coefficients Δεn°(Θ) for the complexation reactions are obtained as a function of the t from the respective SIT modeling as a function of the temperature. In addition to the thermodynamic data, the structures of the complexes and the coordination modes of malonate and succinate are investigated using EXAFS spectroscopy, ATR-FT-IR spectroscopy, and quantum chemical calculations. The results show that in the case of malonate, six-membered chelate complexes are formed, whereas for succinate, seven-membered rings form. The latter ones are energetically unfavorable due to the limited space in the equatorial plane of the Np(V) ion (as NpO2+ cation).

Spectroscopic characterisation and thermodynamics of the complexation of Np(V) with sulfate up to 200 °C.

In the present work the complexation of Np(V) with sulfate in aqueous solution is studied in a temperature range up to 200 °C by absorption spectroscopy. For this purpose, a new spectroscopic setup is implemented and tested for its suitability for Vis/NIR absorption spectroscopy at elevated temperatures. The complexation of Np(V) with sulfate is studied as a function of the total ligand concentration at various temperatures (T = 25-200 °C) and ionic strengths (Im(NaClO4) = 1.0-4.0 mol kg-1 NaClO4). The exclusive formation of NpO2(SO4)- up to 200 °C is confirmed by peak deconvolution and slope analyses. The thermodynamic stability constants log ß01(T) are obtained from linear regressions according to the specific ion interaction theory (SIT). A systematic increase of the log ß01(T) is observed with increasing temperature, resulting in a linear correlation of log ß01(T) with T-1. The magnitude of the increase is 1.9 logarithmic units at 200 °C in comparison to log ß01(25 °C) = 1.05 ± 0.16. Thus, the standard reaction enthalpy and entropy (ΔrH0m, ΔrS0m) are determined with the integrated Van't Hoff equation revealing ΔrH0m = 31.0 ± 1.0 kJ mol-1 and ΔrS0m = 123 ± 9 J mol-1 K-1. In addition, the stoichiometric sum of the specific binary ion-ion interaction coefficient (Δε01(T)) is determined up to 200 °C showing an insignificant temperature dependence. Thus, a temperature-independent ε(Na+, NpO2(SO4)-) = 0.07 ± 0.11 is calculated for the temperature range up to 200 °C. Comparison of the present results with literature data confirms the excellent applicability of the new high-temperature absorption spectroscopic setup for complexation studies up to 200 °C.

Thermodynamics and Structure of Neptunium(V) Complexes with Formate. Spectroscopic and Theoretical Study.

The temperature and ionic strength dependences of the complex formation of NpO2+ with formate in aqueous solution are studied by absorption spectroscopy (Im = 0.5-4.0 mol kg-1, T = 20-85 °C, [Form-]total = 0-0.65 mol kg-1), extended X-ray absorption fine structure spectroscopy (EXAFS) and quantum chemical methods. The complex stoichiometry and the thermodynamic functions of the complexation reactions are determined by peak deconvolution of the absorption spectra and slope analyses. Besides the solvated NpO2+ ion, two NpO2+ formate species (NpO2(Form)n1-n; n = 1, 2) are identified. Application of the law of mass action yields the temperature dependent conditional stability constants log ß'n(T) at a given ionic strength. These data are extrapolated to IUPAC reference state conditions (Im = 0) using the specific ion interaction theory (SIT). The results show, that log ß01(20 °C) = 0.67 ± 0.04 decreases by approximately 0.1 logarithmic units with increasing temperature, log ß02(20 °C) = 0.11 ± 0.11 increases by about 0.2 logarithmic units. The temperature dependence of the log ß0n(T) values is modeled with the integrated Van't Hoff equation yielding the standard reaction enthalpy ΔrH0 and entropy ΔrS0 of the complexation reactions. The results show that the formation of NpO2(Form) is exothermic (ΔrH01 = -2.8 ± 0.9 kJ mol-1) whereas the formation of NpO2(Form)2- is endothermic (ΔrH02 = 6.7 ± 4.1 kJ mol-1). Furthermore, the binary ion-ion interaction coefficients εT(i,k) of the formed complexes are determined in NaClO4 and NaCl media as a function of the temperature. The coordination mode of formate toward the metal ion is investigated by EXAFS spectroscopy and quantum chemical calculations. A coordination of the ligand via only one O atom of formate to the metal ion is identified.

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.

Coordination and Thermodynamics of Trivalent Curium with Malonate at Increased Temperatures: A Spectroscopic and Quantum Chemical Study.

The complexation of Cm(III) with malonate is studied by time-resolved laser fluorescence spectroscopy (TRLFS) in the temperature range from 25 to 90 °C. Three complexes ([Cm(Mal)n]3-2n, n = 1, 2, 3) are identified and their molar fractions are determined as a function of the ligand concentration, the ionic strength, and the temperature. A general shift of the chemical equilibrium toward higher complexes with increasing temperature is observed, with the [CmMal3]3- complex forming only at T > 40 °C. The conditional stability constants (log K'n(T)) are calculated and extrapolated to Im = 0 with the specific ion interaction theory (SIT). The log Kn0(T) values increase by 0.25 to 0.5 logarithmic unit in the studied temperature range. The temperature dependency of the log K°n(T) is fitted by the integrated Van't Hoff equation, yielding the thermodynamic functions ΔrH°m and ΔrS°m. The results show positive reaction enthalpies and entropies for each complexation step. While the ΔrH°n values are constant within their error range, the ΔrS°n values decrease successively with each ligand added. To explain this effect, quantum chemical calculations of binding energies and bond lengths of the different Cm(III) malonate species are performed. The results show that malonate is capable of stabilizing its end-on coordination mode to some extent by forming hydrogen bonds to first-shell water molecules. As a result, an equilibrium between side-on and end-on coordinated malonate ligands is present, with the latter becoming more pronounced for the higher complexes due to steric reasons.

4,4'-Di-tert-butyl-6-(1H-tetrazol-5-yl)-2,2'-bipyridine: modification of a highly selective N-donor ligand for the separation of trivalent actinides from lanthanides.

In the present work, the complexation and extraction behaviour of 4,4'di-tert-butyl-6-(1H-tetrazol-5-yl)-2,2'-bipyridine (HN4tbubipy) towards trivalent actinides (An(iii)) and lanthanides (Ln(iii)) is studied by spectroscopic methods, liquid-liquid extraction, and quantum chemical calculations. The ligand synthesis of HN4tbubipy as well as its application in coordination chemistry of the 4f elements is described. Reaction of HN4tbubipy with [Ln(NO3)3·6H2O] (Ln = Sm, Eu) results in [H2N4tbubipy]+[Ln(N4tbubipy)(NO3)3(H2O)]-. Both compounds have been characterized by single crystal X-ray diffraction. The solubility of the ligand in different organic solvents is determined, showing a high solubility in MeOH which decreases with the lipophilicity of the solvent. The pKa = 2.4 ± 0.2 of HN4tbubipy in EtOH (4.4 vol% H2O) is determined by absorption spectrophotometry. The complexation of Cm(iii) and Eu(iii) with HN4tbubipy is studied by time resolved laser fluorescence spectroscopy (TRLFS). For both metal ions the formation of the complexes [M(N4tbubipy)n]3-n with n = 2, 3 (M = Cm(iii), Eu(iii)) is observed. Slightly higher conditional stability constants for Eu(iii) (log ß'2(Eu(N4tbubipy)2+) = 8.9 ± 0.3, log ß'3(Eu(N4tbubipy)3) = 12.7 ± 0.5), compared to Cm(iii) (log ß'2(Cm(N4tbubipy)2+) = 8.5 ± 0.4 and log ß'3(Cm(N4tbubipy)3) = 12.4 ± 0.6) are determined. Thus, the ligand has no preference for the complexation of An(iii) over Ln(iii). Additionally, no significant extraction of Am(iii) and Eu(iii) is observed in liquid-liquid extraction experiments due to protonation of the ligand at the experimental conditions. The experimental studies are supported by quantum chemical calculations of the free ligand and the [M(N4tbubipy)3] complexes (M = Cm(iii), Gd(iii)). The results are in excellent agreement with the experimental data and provide a deeper understanding of the complexation properties of HN4tbubipy.

Combined EXAFS Spectroscopic and Quantum Chemical Study on the Complex Formation of Am(III) with Formate.

The complexation of Am(III) with formate in aqueous solution is studied as a function of the pH value using a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy, iterative transformation factor analysis (ITFA), and quantum chemical calculations. The Am LIII-edge EXAFS spectra are analyzed to determine the molecular structure (coordination numbers; Am-O and Am-C distances) of the formed Am(III)-formate species and to track the shift of the Am(III) speciation with increasing pH. The experimental data are compared to predictions from density functional calculations. The results indicate that formate binds to Am(III) in a monodentate fashion, in agreement with crystal structures of lanthanide formates. Furthermore, the investigations are complemented by thermodynamic speciation calculations to verify further the results obtained.

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.

An EXAFS spectroscopic study of Am(III) complexation with lactate.

The pH dependence (1-7) of Am(III) complexation with lactate in aqueous solution is studied using extended X-ray absorption fine-structure (EXAFS) spectroscopy. Structural data (coordination numbers, Am--O and Am--C distances) of the formed Am(III)-lactate species are determined from the raw k(3)-weighted Am LIII-edge EXAFS spectra. Between pH 1 and pH 6, Am(III) speciation shifts continuously towards complexed species with increasing pH. At higher pH, the amount of complexed species decreases due to formation of hydroxo species. The coordination numbers and distances (3.41-3.43 Å) of the coordinating carbon atoms clearly point out that lactate is bound `side-on' to Am(III) through both the carboxylic and the α-hydroxy function of lactate. The experimentally determined coordination numbers are compared with speciation calculations on the basis of tabulated thermodynamic stability constants. Both EXAFS data and thermodynamic modelling are in very good agreement. The EXAFS spectra are also analyzed by iterative transformation factor analysis to further verify the determined Am(III) speciation and the used structural model.

The complexation of Cm(III) with oxalate in aqueous solution at T = 20-90 °C: a combined TRLFS and quantum chemical study.

The complexation of Cm(III) with oxalate is studied in aqueous solution as a function of the ligand concentration, the ionic strength (NaCl), and the temperature (T = 20­90 °C) by time-resolved laser fluorescence spectroscopy (TRLFS) and quantum chemical calculations. Four complex species ([Cm(Ox)n](3­2n), n = 1, 2, 3, 4) are identified, and their molar fractions are determined by peak deconvolution of the emission spectra. The conditional log K'n(T) values of the first three complexes are calculated and extrapolated to zero ionic strength with the specific ion interaction theory approach. The [Cm(Ox)4](5­) complex forms only at high temperatures. Thus, the log K4(0)(T) value was determined at T > 60 °C. The log K1(0)(25 °C) = 6.86 ± 0.02 decreases by 0.1 logarithmic units in the studied temperature range. The log K2(0)(25 °C) = 4.68 ± 0.09 increases by 0.35, and log K3(0)(25 °C) = 2.11 ± 0.05 increases by 0.37 orders of magnitude. The log Kn(0)(T) (n = 1, 2, 3) values are linearly correlated with the reciprocal temperature. Thus, their temperature dependencies are fitted with the linear Van't Hoff equation yielding the standard reaction enthalpy (ΔrHm(0)) and standard reaction entropy (ΔrSm(0)) of the stepwise formation of the [Cm(Ox)n](3­2n) species (n = 1, 2, 3). Furthermore, the binary ion­ion interaction coefficients of the four Cm(III) oxalate species with Cl(­)/Na(+) are determined. The binding energies, bond lengths, and bond angles of the different Cm(III) oxalate complexes are calculated in the gas phase as well as in a box containing 1000 H2O molecules by ab inito calculations and molecular dynamics simulations, respectively.

The pH dependence of Am(III) complexation with acetate: an EXAFS study.

The complexation of acetate with Am(III) is studied as a function of the pH (1-6) by extended X-ray absorption fine-structure (EXAFS) spectroscopy. The molecular structure of the Am(III)-acetate complexes (coordination numbers, oxygen and carbon distances) is determined from the raw k(3)-weighted Am LIII-edge EXAFS spectra. The results show a continuous shift of Am(III) speciation with increasing pH value towards the complexed species. Furthermore, it is verified that acetate coordinates in a bidentate coordination mode to Am(III) (Am-C distance: 2.82 ± 0.03 Å). The EXAFS data are analyzed by iterative transformation factor analysis to further verify the chemical speciation, which is calculated on the basis of thermodynamic constants, and the used structural model. The experimental results are in very good agreement with the thermodynamic modelling.

Fulvic acid complexation of Eu(III) and Cm(III) at elevated temperatures studied by time-resolved laser fluorescence spectroscopy.

The interaction of Eu(III) and Cm(III) with three different aquatic fulvic acids (FA) was studied as a function of the temperature (T = 20-80 °C) in 0.1 M NaCl solution by time-resolved laser fluorescence spectroscopy. The speciation of both trivalent metal ions was determined by peak deconvolution of the recorded fluorescence spectra. For each studied metal ion-FA system only one complexed species is formed under the given experimental conditions. The stability constants at 20, 40, 60 and 80 °C (log ß'(T)) were determined according to the charge neutralization model. The log ß' (20 °C) for the different FAs show similar values (log ß(20 °C) = 5.60-6.29). The stability constants increase continuously with increasing temperature by approximately 0.3-1.0 orders of magnitude. The reaction enthalpies and entropies are derived from the integrated Van't Hoff equation. The results show that all investigated complexation reactions are endothermic and entropy-driven.

Combined time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure spectroscopy study on the complexation of trivalent actinides with chloride at T = 25-200 °C.

The complexation of trivalent actinides (An(III)) with chloride is studied in the temperature range from 25 to 200 °C by spectroscopic methods. Time-resolved laser fluorescence spectroscopy (TRLFS) is applied to determine the thermodynamic data of Cm(III)-Cl(-) complexes, while extended X-ray absorption fine structure spectroscopy (EXAFS) is used to determine the structural data of the respective Am(III) complexes. The experiments are performed in a custom-built high-temperature cell which is modified for the respective spectroscopic technique. The TRLFS results show that at 25 °C the speciation is dominated mainly by the Cm(3+) aquo ion. Only a minor fraction of the CmCl(2+) complex is present in solution. As the temperature increases, the fraction of this species decreases further. Simultaneously, the fraction of the CmCl2(+) complex increases strongly with the temperature. Also, the CmCl3 complex is formed to a minor extent at T > 160 °C. The conditional stability constant log ß'2 is determined as a function of the temperature and extrapolated to zero ionic strength with the specific ion interaction theory approach. The log ß°2(T) values increase by more than 3 orders of magnitude in the studied temperature range. The temperature dependency of log ß°2 is fitted by the extended van't Hoff equation to determine ΔrH°m, ΔrS°m, and ΔrC°p,m. The EXAFS results support these findings. The results confirm the absence of americium(III) chloride complexes at T = 25 and 90 °C ([Am(III)] = 10(-3) m, [Cl(-)] = 3.0 m), and the spectra are described by 9-10 oxygen atoms at a distance of 2.44-2.48 Å. At T = 200 °C two chloride ligands are present in the inner coordination sphere of Am(III) at a distance of 2.78 Å.

A spectroscopic study on the formation of Cm(III) acetate complexes at elevated temperatures.

The complexation of Cm(III) with acetate is studied by time resolved laser fluorescence spectroscopy (TRLFS) as a function of ionic strength, ligand concentration, temperature and background electrolyte (NaClO4, NaCl and CaCl2 solution). The speciation of Cm(III) is determined by peak deconvolution of the emission spectra. To obtain the thermodynamic stability constants (log K) for the formation of [Cm(Ac)n](3-n) (n = 1-3), the experimental data are extrapolated to zero ionic strength according to the specific ion interaction theory (SIT). The results show a continuous increase of the stability constants with increasing temperature (20-90 °C). The standard reaction enthalpies and entropies (ΔrH, ΔrS) of the respective reactions are derived from the integrated Van't Hoff equation. The results show that all complexation steps are endothermic and thus entropy driven (ΔrH and ΔrS > 0).