Dominique Guillaumont

Calculation of the absorption wavelength of dyes using time-dependent density-functional theory (TD-DFT)

The absorption wavelengths and oscillator strengths of a series of organic dyes important for the dye industry (indigo, azobenzene, phenylamine, hydrazone, anthraquinone, naphthoquinone and cationic dyes) were calculated using time-dependent density-functional theory. The results were compared with experimental data. TD-DFT correctly reproduced the visible absorption of the dyes.

Solving the hydration structure of the heaviest actinide aqua ion known: the californium(III) case.

The solution chemistry of actinide ions has been a fundamental question since the beginning of the nuclear technologies, given that the solvent stabilizes the high oxidation states of actinides. The development of procedures to avoid the migration of actinides from the already accumulated nuclear waste into natural water systems is a field of great activity. One of the primary properties of actinide ions in solution is their solvation, as it is intimately joined to complexation, precipitation, and resolution processes. The rareness and hazardousness of the heavier actinide elements, which steeply increase with the atomic number, has prevented a complete examination of the trends along the series, beyond the middle of the series. The curium cation Cm has often been considered as the heaviest actinide species characterized, and it has attracted much attention from both experimental and theoretical views in recent years. Systematic studies of the aqueous trivalent lanthanides have revealed a contraction of the metal–oxygen distance and a decrease of the total first coordination number along the series. Recent investigations using extended X-ray absorption fine structure (EXAFS) techniques have examined if this contraction takes place in a monotone or an irregular way along the series. The data available for the actinide series up to Cm indicates a similar contraction, 5, 7] although a conclusive answer cannot be given owing to the uncertainty of the structural data, particularly concerning the hydration number, and the scarce information on the second half of the series. Beyond the middle of the series, there is only one study reported for berkelium (Bk) and a preliminary EXAFS study for californium(III) carried out by one of us. Owing to the position of Cf in the actinide series, an accurate enough determination of the coordination number and Cf O distance could certainly shed light on the question of the actinide contraction. This objective gives the study a more fundamental than applied character, owing to the extreme rareness of this element. The most similar available crystallographic data of Cf with Cf O bonds is that of single crystals of Cf(IO3)3, which present a significantly distorted tricapped trigonal prism with a wide range of Cf O distances (2.353–2.921 ). This limited information does not meet the required level of accuracy for answering the question on the basis of a conventional EXAFS data analysis. Herein we present an alternative way to study this extreme case, by coupling new highly refined EXAFS data obtained in an actinide-dedicated beamline in the European Synchrotron Radiation Facility (ESRF, Grenoble), with the first Monte Carlo (MC) simulations of Cf in water. Specifically developed Cf OH2 intermolecular potentials based on ab initio quantum mechanical (QM) potential energy surfaces and the polarizable and flexible MCDHO water model have been used. Figure 1 shows the experimental and fitted k-weighted EXAFS spectra of a Cf aqueous solution using two model structures, the square antiprism configuration (SA; see Figure 2a), which represents an octacoordination of water

Theoretical study of an intermediate, a factor determining the quantum yield in photochromism of diarylethene derivatives

Experimental and theoretical joint study on the photochemical reaction mechanism of photochromic diarylethenes is presented. An intermediate, suggesting an important role for the determination of the quantum yield, is proposed by ab initio multiconfiguration self-consistent (MCSCF) calculation.

Complexation of lanthanides(III), americium(III), and uranium(VI) with bitopic N,O ligands: an experimental and theoretical study.

New functionalized terpyridine-diamide ligands were recently developed for the group actinide separation by solvent extraction. In order to acquire a better understanding of their coordination mode in solution, protonation and complexation of lanthanides(III), americium(III), and uranium(VI) with these bitopic N,O-bearing ligands were studied in homogeneous methanol/water conditions by experimental and theoretical approaches. UV-visible spectrophotometry was used to determine the protonation and stability constants of te-tpyda and dedp-tpyda. The conformations of free and protonated forms of te-tpyda were investigated using NMR and theoretical calculations. The introduction of amide functional groups on the terpyridine moiety improved the extracting properties of these new ligands by lowering their basicity and enhancing the stability of the corresponding 1:1 complexes with lanthanides(III). Coordination of these ligands was studied by density functional theory and molecular dynamics calculations, especially to evaluate potential participation of hard oxygen and soft nitrogen atoms in actinide coordination and to correlate with their affinity and selectivity. Two predominant inner-sphere coordination modes were found from the calculations: one mode where the cation is coordinated by the nitrogen atoms of the cavity and by the amide oxygen atoms and the other mode where the cation is only coordinated by the two amide oxygen atoms and by solvent molecules. Further simulations and analysis of UV-visible spectra using both coordination modes indicate that inner-sphere coordination with direct complexation of the three nitrogen and two oxygen atoms to the cation leads to the most likely species in a methanol/water solution.

Structure of early actinides(V) in acidic solutions

Abstract Protactinium occupies a key position in the actinide series between thorium and uranium. In aqueous acidic solution, it is stable at oxidation state (V), occurring either as an oxocation or as a naked ion, depending on the media. Very few structural information on the hydration sphere of Pa(V) in acidic medium is available, in particular in hydrofluoric acid. Combined EXAFS and theoretical calculations have been used in this work to characterize the protactinium coordination sphere at various HF concentrations. The correlation of the XAFS data with quantum chemical calculations provides complementary structural and electronic models from ab initio techniques. At HF concentrations from 0.5 to 0.05 M, both theoretical calculations and EXAFS data suggest that the protactinium coordination sphere is mainly composed of fluoride ions. At the lowest HF concentration, the occurrence of a monooxo bond is observed with EXAFS, in agreement with the literature. A comparison of these data with related neptunium(V) and plutonium(V) diooxocations in perchloric acid is also presented.

New insights into formation of trivalent actinides complexes with DTPA.

Complexation of trivalent actinides with DTPA (diethylenetriamine pentaacetic acid) was studied as a function of pcH and temperature in (Na,H)Cl medium of 0.1 M ionic strength. Formation constants of both complexes AnHDTPA(-) and AnDTPA(2-) (where An stands for Am, Cm, and Cf) were determined by TRLFS, CE-ICP-MS, spectrophotometry, and solvent extraction. The values of formation constants obtained from the different techniques are coherent and consistent with reinterpreted literature data, showing a higher stability of Cf complexes than Am and Cm complexes. The effect of temperature indicates that formation constants of protonated and nonprotonated complexes are exothermic with a high positive entropic contribution. DFT calculations were also performed on the An/DTPA system. Geometry optimizations were conducted on AnDTPA(2-) and AnHDTPA(-) considering all possible protonation sites. For both complexes, one and two water molecules in the first coordination sphere of curium were also considered. DFT calculations indicate that the lowest energy structures correspond to protonation on oxygen that is not involved in An-DTPA bonds and that the structures with two water molecules are not stable.

Theoretical chemical contribution to the simulation of the LIII X-ray absorption edges of uranyl, neptunyl and osmyl hydrates and hydroxides

XANES spectroscopy has long been used as a structural and electronic probe of a selected element. Phenomenological application of this technique to actinide cations has proved fruitful to characterize the actinide environment in both solid state and solution compounds. Although powerful XANES simulation codes have been developed, the use of such simulations in order to describe the valence orbitals of the actinide cation is still scarce. The very short life time of the core hole at the LIII edge as well as the low symmetry and large size of the coordination polyhedron are difficulties to be overcome in the analysis of the edge spectra. In this work, three simple molecules have been selected for their similar geometry that is typical of the trans dioxo actinyl compounds: [UO2(H2O)5]2+, [NpO2(H2O)5]2+, [NpO2(OH)4]2−. Additional comparison with a transition metal, the osmyl cation [OsO2(OH)4]2−, is also made. The cation LIII edges have been recorded and compared to edge calculations using FEFF8.2 code. This article is structured in two parts. In the first one, elaboration and optimization of a valid structural model cluster is carried out using molecular dynamics calculations. The influence of the water solvent molecules as well as the hydrogen atoms of the cations’ first coordination sphere are discussed. In the second part, Amsterdam quantum chemical calculations have been carried out on the four clusters and molecular energy levels are qualitatively compared to the data obtained from calculated XANES spectra.

Density Functional Theory Calculations of the Redox Potentials of Actinide(VI)/Actinide(V) Couple in Water

The measured redox potential of an actinide at an electrode surface involves the transfer of a single electron from the electrode surface on to the actinide center. Before electron transfer takes place, the complexing ligands and molecules of solvation need to become structurally arranged such that the electron transfer is at its most favorable. Following the electron transfer, there is further rearrangement to obtain the minimum energy structure for the reduced state. As such, there are three parts to the total energy cycle required to take the complex from its ground state oxidized form to its ground state reduced form. The first part of the energy comes from the structural rearrangement and solvation energies of the actinide species before the electron transfer or charge transfer process; the second part, the energy of the electron transfer; the third part, the energy required to reorganize the ligands and molecules of solvation around the reduced species. The time resolution of electrochemical techniques such as cyclic voltammetry is inadequate to determine to what extent bond and solvation rearrangement occurs before or after electron transfer; only for a couple to be classed as reversible is it fast in terms of the experimental time. Consequently, the partitioning of the energy theoretically is of importance to obtain good experimental agreement. Here we investigate the magnitude of the instantaneous charge transfer through calculating the fast one electron reduction energies of AnO2(H2O)n(2+), where An = U, Np, and Pu, for n = 4-6, in solution without inclusion of the structural optimization energy of the reduced form. These calculations have been performed using a number of DFT functionals, including the recently developed functionals of Zhao and Truhlar. The results obtained for calculated electron affinities in the aqueous phase for the AnO2(H2O)5(2+/+) couples are within 0.04 V of accepted experimental redox potentials, nearly an order of magnitude improvement on previous calculated standard potentials E(0) values, obtained using both DFT and high level multireference approaches.

Crystal and electronic structure of a mixed-valent Np(IV)-Np(V) compound: [BuMeIm]5[Np(NpO2)3(H2O)6Cl12].

A new mixed valent neptunium compound [BuMeIm](5)[Np(NpO(2))(3)(H(2)O)(6)Cl(12)] was synthesized by evaporation of an ethyl acetate-acetone mixture where the salt [BuMeIm](2)[NpCl(6)] was dissolved. The crystal structure was determined using single crystal X-ray diffraction; it consists of four crystallographically unique Np centers with two different coordination environments in two different oxidation states. In addition, five crystallographically independent dialkylimidazolium cations stabilize the crystal through C-H...Cl hydrogen bonds. [Np(NpO(2))(3)(H(2)O)(6)Cl(12)] unit presents a highly symmetric structure with the formation of three Np(IV)-Np(V)O(2)(+) bonds. According to the crystal structure and density functional theory (DFT) calculations, the stability of the three Np(4+)-NpO(2)(+) interactions is due to the presence of chloride ions around NpO(2)(+) which brings electronic charge into the system and also to the presence of water molecules which create hydrogen bond with chloride ions.

Thermodynamics and Structure of Actinide(IV) Complexes with Nitrilotriacetic Acid

Nitrilotriacetic acid, commonly known as NTA (N(CH(2)CO(2)H)(3)), can be considered a representative of the polyaminocarboxylic family. The results presented in this paper describe the thermodynamical complexation and structural investigation of An(IV) complexes with NTA in aqueous solution. In the first part, the stability constants of the An(IV) complexes (An = Pu, Np, U, and Th) have been determined by spectrophotometry. In the second part, the coordination spheres of the actinide cation in these complexes have been described using extended X-ray absorption fine structure spectroscopy and compared to the solid-state structure of (Hpy)(2)[U(NTA)(2)] x (H(2)O). These data are further compared to quantum chemical calculations, and their evolution across the actinide series is discussed. In particular, an interpretation of the role of the nitrogen atom in the coordination mode is proposed. These results are considered to be model behavior of polyaminocarboxylic ligands such as diethylenetriamine pentaacetic acid, which is nowadays the best candidate for a chelating agent in the framework of actinide decorporation for the human body.