Coordination and thermodynamic properties of aqueous protactinium(V) by first-principle calculations

Protactinium (Z = 91) is a very rare actinide with peculiar physico-chemical properties. Indeed, although one may naively think that it behaves similarly to either thorium or uranium by its position in the periodic table, it may in fact follow its own rules. Because of the quite small energy gap between its valence shells (in particular the 5f and 6d ones) and also the strong influence of relativistic effects on its properties, it is actually a challenging element for theoretical chemists. In this article, we combine experimental information, chemical arguments and standard first-principle calculations, complemented by implicit and explicit solvation, to revisit the stepwise complexation of aqueous protactinium(V) with sulfate and oxalate dianionic ligands (SO4^2- and C2O4^2-, respectively). From a methodological viewpoint, we notably conclude that it is necessary to at least saturate the coordination sphere of protactinium(V) to reach converged equilibrium constant values. Furthermore, in the case of single complexations (i.e. with one sulfate or oxalate ligand bound in the bidentate fashion), we show that it is necessary to maintain the coordination of one hydroxyl group, thought of in the [PaO(OH)]^3+ precursor, to obtain coherent complexation constants. Therefore, we predict that this hydroxyl group is maintained in the formation of 1:1 complexes while we confirm that it is withdrawn when coordinating three sulfate or oxalate ligands. Finally, we stress that this work is a first step toward the future use of theoretical predictions to elucidate the enigmatic chemistry of protactinium in solution

Approches combinées ab initio et par spectroscopie de luminescence résolue en temps pour l'étude des l'interactions uranium-ligand

Uranyl complexes have been the subject of many research works for fundamental chemistry of actinides, environmental issues, or nuclear fuel cycle processes. The formation of various uranium(VI) complexes, with ligands in solution must be characterized for a better understanding of U(VI) speciation. Uranyl-ligand interactions and symmetry of the complexes both affect the electronic structure of U(VI), and thus its luminescence properties. Time-resolved laser induced fluorescence spectroscopy (TRLFS) is one of the widely used techniques to get insights on the closest chemical environment of the uranyl ion in samples, owing to its high sensitivity and selectivity. However, the luminescence spectra fingerprints hold information within and beyond the first-coordination sphere of uranyl(VI), that needs to be more deeply investigated by supplementary techniques.A promising route for data interpretation consists in creating a synergy between TRLFS and ab initio-based interpretations. Luminescence spectra of uranyl complexes in solution typically show well-spaced vibronic progressions that overlap with the pure electronic transition from the excited state to the ground state. This has driven the theoretical methodology implementation. In the frame of this thesis, time-dependent density functional theory (TD-DFT) with hybrid and range-separated functionals is used to model the electronic structure of uranium(VI) complexes. This represents an effective theoretical approach with a reasonable computational cost and accuracy, compared with computationally expensive wave-function based methods, in a relativistic context. It enabled to characterize the main spectral parameters and the first low-lying excited state of uranyl compounds with different ligands and counterions after the photo-excitation, and to compute with a high accuracy the vibronic progression in order to guide the interpretation of experimental results.In particular we focused our efforts on characterizing the influence of the organic or inorganic closest chemical environment of the uranium(VI)-based complexes. We studied 1) the influence of the extracting agent such as Aliquate 336 and solvent effect on uranyl tetrahalides; 2) inorganic Ca2+ and Mg2+ counterions on uranyl triscarbonates; and 3) monoamide ligands (di-2-ethylhexyl-isobutyramide) on uranyl binitrate complexes. Their electronic structures and main spectroscopic properties have been estimated by both TRLFS and ab initio techniques. The theoretical approach enabled to calculate the main luminescence emissions of the complexes with the corresponding assignment of the electronic transitions and vibronic modes involved. For all the studied complexes, a good agreement between theory and experiment was found, allowing to build a full picture about the capabilities of the methods.Les complexes d'uranyle ont fait l'objet de nombreux travaux de recherche pour la chimie fondamentale des actinides, les enjeux environnementaux ou les procédés du cycle du combustible nucléaire. La formation de divers complexes d'uranium(VI), avec des ligands en solution doit être caractérisée pour une meilleure compréhension de la spéciation de U(VI). Les interactions uranyl-ligand et la symétrie des complexes modifient la structure électronique de U(VI) et donc ses propriétés de luminescence. La spectrofluorimétrie laser résolue en temps (SLRT) est l'une des techniques largement utilisées pour obtenir des informations sur l'environnement chimique proche de l'ion uranyle dans les échantillons, en raison de sa sensibilité et de sa sélectivité élevées. Cependant, les signatures des spectres de luminescence contiennent des informations liées à la première sphère de coordination de l'uranyle et au-delà, et méritent d'être étudiée de manière plus approfondie par des techniques adaptées.Une voie prometteuse pour l'interprétation des données consiste à créer une synergie entre SLRT et les interprétations ab initio. Les spectres de luminescence des complexes d'uranyle en solution montrent généralement des progressions vibroniques bien espacées qui se chevauchent avec la transition électronique pure provenant de l'état excité à l'état fondamental. Ceci a conduit la mise en œuvre de notre méthodologie théorique. Dans le cadre de cette thèse, la théorie fonctionnelle de la densité dépendante du temps (TD-DFT) avec des fonctionnelles hybrides et séparées par une plage est utilisée pour modéliser la structure électronique de complexes d'uranium(VI) dans un contexte relativiste. Cela a permis de caractériser les principaux paramètres spectraux et le premier état excité de plusieurs composés d'uranyle avec différents ligands et contre-ions, et de calculer avec une grande précision la progression vibronique afin de guider l'interprétation des résultats expérimentaux.En particulier, nous avons concentré nos efforts sur la caractérisation de l'influence de l'environnement chimique le plus proche des complexes à base d'uranium(VI). Nous avons étudié 1) l'influence d'un agent d'extraction tel que l'Aliquate 336 et l'effet du solvant sur les tétrahalogénures d'uranyle; 2) les contre-ions inorganiques Ca2+ et Mg2+ sur les triscarbonates d'uranyle ; et 3) les ligands monoamide (di-2-éthylhexyl-isobutyramide) sur les complexes de binitrate d'uranyle. Leurs structures électroniques et leurs principales propriétés spectroscopiques ont été estimées par les deux techniques SLRT et ab initio. L'approche théorique a permis de calculer les principales émissions de luminescence des complexes avec l'affectation correspondante des transitions électroniques et des modes vibroniques impliqués. Pour tous les complexes étudiés, un bon accord entre la théorie et l'expérience a été obtenu, permettant de construire une image plus complète des capacités des méthodes

Approches combinées ab initio et par spectroscopie de luminescence résolue en temps pour l'étude des l'interactions uranium-ligand

Uranyl complexes have been the subject of many research works for fundamental chemistry of actinides, environmental issues, or nuclear fuel cycle processes. The formation of various uranium(VI) complexes, with ligands in solution must be characterized for a better understanding of U(VI) speciation. Uranyl-ligand interactions and symmetry of the complexes both affect the electronic structure of U(VI), and thus its luminescence properties. Time-resolved laser induced fluorescence spectroscopy (TRLFS) is one of the widely used techniques to get insights on the closest chemical environment of the uranyl ion in samples, owing to its high sensitivity and selectivity. However, the luminescence spectra fingerprints hold information within and beyond the first-coordination sphere of uranyl(VI), that needs to be more deeply investigated by supplementary techniques.A promising route for data interpretation consists in creating a synergy between TRLFS and ab initio-based interpretations. Luminescence spectra of uranyl complexes in solution typically show well-spaced vibronic progressions that overlap with the pure electronic transition from the excited state to the ground state. This has driven the theoretical methodology implementation. In the frame of this thesis, time-dependent density functional theory (TD-DFT) with hybrid and range-separated functionals is used to model the electronic structure of uranium(VI) complexes. This represents an effective theoretical approach with a reasonable computational cost and accuracy, compared with computationally expensive wave-function based methods, in a relativistic context. It enabled to characterize the main spectral parameters and the first low-lying excited state of uranyl compounds with different ligands and counterions after the photo-excitation, and to compute with a high accuracy the vibronic progression in order to guide the interpretation of experimental results.In particular we focused our efforts on characterizing the influence of the organic or inorganic closest chemical environment of the uranium(VI)-based complexes. We studied 1) the influence of the extracting agent such as Aliquate 336 and solvent effect on uranyl tetrahalides; 2) inorganic Ca2+ and Mg2+ counterions on uranyl triscarbonates; and 3) monoamide ligands (di-2-ethylhexyl-isobutyramide) on uranyl binitrate complexes. Their electronic structures and main spectroscopic properties have been estimated by both TRLFS and ab initio techniques. The theoretical approach enabled to calculate the main luminescence emissions of the complexes with the corresponding assignment of the electronic transitions and vibronic modes involved. For all the studied complexes, a good agreement between theory and experiment was found, allowing to build a full picture about the capabilities of the methods.Les complexes d'uranyle ont fait l'objet de nombreux travaux de recherche pour la chimie fondamentale des actinides, les enjeux environnementaux ou les procédés du cycle du combustible nucléaire. La formation de divers complexes d'uranium(VI), avec des ligands en solution doit être caractérisée pour une meilleure compréhension de la spéciation de U(VI). Les interactions uranyl-ligand et la symétrie des complexes modifient la structure électronique de U(VI) et donc ses propriétés de luminescence. La spectrofluorimétrie laser résolue en temps (SLRT) est l'une des techniques largement utilisées pour obtenir des informations sur l'environnement chimique proche de l'ion uranyle dans les échantillons, en raison de sa sensibilité et de sa sélectivité élevées. Cependant, les signatures des spectres de luminescence contiennent des informations liées à la première sphère de coordination de l'uranyle et au-delà, et méritent d'être étudiée de manière plus approfondie par des techniques adaptées.Une voie prometteuse pour l'interprétation des données consiste à créer une synergie entre SLRT et les interprétations ab initio. Les spectres de luminescence des complexes d'uranyle en solution montrent généralement des progressions vibroniques bien espacées qui se chevauchent avec la transition électronique pure provenant de l'état excité à l'état fondamental. Ceci a conduit la mise en œuvre de notre méthodologie théorique. Dans le cadre de cette thèse, la théorie fonctionnelle de la densité dépendante du temps (TD-DFT) avec des fonctionnelles hybrides et séparées par une plage est utilisée pour modéliser la structure électronique de complexes d'uranium(VI) dans un contexte relativiste. Cela a permis de caractériser les principaux paramètres spectraux et le premier état excité de plusieurs composés d'uranyle avec différents ligands et contre-ions, et de calculer avec une grande précision la progression vibronique afin de guider l'interprétation des résultats expérimentaux.En particulier, nous avons concentré nos efforts sur la caractérisation de l'influence de l'environnement chimique le plus proche des complexes à base d'uranium(VI). Nous avons étudié 1) l'influence d'un agent d'extraction tel que l'Aliquate 336 et l'effet du solvant sur les tétrahalogénures d'uranyle; 2) les contre-ions inorganiques Ca2+ et Mg2+ sur les triscarbonates d'uranyle ; et 3) les ligands monoamide (di-2-éthylhexyl-isobutyramide) sur les complexes de binitrate d'uranyle. Leurs structures électroniques et leurs principales propriétés spectroscopiques ont été estimées par les deux techniques SLRT et ab initio. L'approche théorique a permis de calculer les principales émissions de luminescence des complexes avec l'affectation correspondante des transitions électroniques et des modes vibroniques impliqués. Pour tous les complexes étudiés, un bon accord entre la théorie et l'expérience a été obtenu, permettant de construire une image plus complète des capacités des méthodes

Chemistry and speciation of protactinium – a first principles study

National audienceIt is of fundamental interest to understand and predict the chemistry of rare radioelements. In this work, we focus on protactinium (Z = 91), an element that is sandwiched in between thorium and uranium in the periodic table. Protactinium may naturally occur in environment (protactini-um-231 results from the decay of naturally occurring uranium-235) and also appear in thorium-based nuclear fuel cycles. From a chemical point of view, protactinium is a crossing point in the actinide series [1] and its chemistry is hard to predict [2,3]. We hypothesize that relativistic quantum chemistry should allow us to understand the enigmatic chemistry of protactinium and even predict it. For our first study, we have chosen to focus on the coordination sphere of protactinium and on the computation of equilibrium constants for experimentally known systems [3–5]. The occur-rence of 1:1, 1:2 and 1:3 complexes of protactinium(V) with sulfate and oxalate ligands is thus studied by means of quantum mechanical calculations, in particular based on density functional theory. The solvent effects, inherent to solution chemistry, are introduced by means of a polariz-able continuum model [6] and the explicit treatment of water molecules (micro solvation).The coordination sphere of protactinium has been obtained by geometry optimizations per-formed both in the gas phase and in solution. It involves an oxygen atom from the Pa=O mono-oxo bond and also oxygen atoms from the bidentate ligands, and in some cases from additional water molecules. The computation of equilibrium constants and comparison with experimental data is more subtle. First, only apparent constants were experimentally determined. Since the oc-currence of a mono-oxo bond was confirmed by EXAFS [5] even in the case of the 1:3 complex with oxalate ligands (corresponding to the stronger complexation), we hypothesize that this bond is also present in all the studied complexes. Second, number of explicitly treated water molecules should not be randomly chosen, it should ideally (i) lead to saturation of the coordination sphere of protactinium and (ii) be sufficient to stabilize the anionic ligands. We find that adding CN+1 water molecules is enough to satisfy both conditions in all the six studied complexes. By doing so and computing ligand-exchange equilibrium constants, we reproduce well the experimental trends for the exchange of 2 and 3 ligands, while the exchange of only one ligand (1:1 complex-es) is still hard to reproduce from computations.We report recent progress concerning the basic chemistry of protactinium. We have shown that its coordination sphere may include up to 8 oxygen atoms (from the original mono-oxo bond and from ligand and solvent molecule complexation) and find an approximate way of determin-ing trends in equilibrium constants, opening the way for future predictions.[1]Wilson R. et al. (2018) Nat. Commun. 9, 622.[2]Wilson R. (2012) Nat. Chem. 4, 586.[3]Le Naour C. et al. (2019) Radiochim. Acta, 107, 979-991.[4]Le Naour C. et al. (2005) Inorg. Chem. 44, 9542.[5]Mendes M. et al. (2010) Inorg. Chem. 49, 9962-9971.[6]Barone et al. (1997) J. Chem. Phys. 107, 3210-3221.<br

Chemistry, spectroscopy and speciation of protactinium

International audienc

Chemistry, spectroscopy and speciation of protactinium

International audienc

A theoretical study of chemistry and speciation of protactinium

International audienc

Chemistry, spectroscopy and speciation of protactinium

International audienc

Chemistry and speciation of protactinium – a first principles study

National audienceIt is of fundamental interest to understand and predict the chemistry of rare radioelements. In this work, we focus on protactinium (Z = 91), an element that is sandwiched in between thorium and uranium in the periodic table. Protactinium may naturally occur in environment (protactini-um-231 results from the decay of naturally occurring uranium-235) and also appear in thorium-based nuclear fuel cycles. From a chemical point of view, protactinium is a crossing point in the actinide series [1] and its chemistry is hard to predict [2,3]. We hypothesize that relativistic quantum chemistry should allow us to understand the enigmatic chemistry of protactinium and even predict it. For our first study, we have chosen to focus on the coordination sphere of protactinium and on the computation of equilibrium constants for experimentally known systems [3–5]. The occur-rence of 1:1, 1:2 and 1:3 complexes of protactinium(V) with sulfate and oxalate ligands is thus studied by means of quantum mechanical calculations, in particular based on density functional theory. The solvent effects, inherent to solution chemistry, are introduced by means of a polariz-able continuum model [6] and the explicit treatment of water molecules (micro solvation).The coordination sphere of protactinium has been obtained by geometry optimizations per-formed both in the gas phase and in solution. It involves an oxygen atom from the Pa=O mono-oxo bond and also oxygen atoms from the bidentate ligands, and in some cases from additional water molecules. The computation of equilibrium constants and comparison with experimental data is more subtle. First, only apparent constants were experimentally determined. Since the oc-currence of a mono-oxo bond was confirmed by EXAFS [5] even in the case of the 1:3 complex with oxalate ligands (corresponding to the stronger complexation), we hypothesize that this bond is also present in all the studied complexes. Second, number of explicitly treated water molecules should not be randomly chosen, it should ideally (i) lead to saturation of the coordination sphere of protactinium and (ii) be sufficient to stabilize the anionic ligands. We find that adding CN+1 water molecules is enough to satisfy both conditions in all the six studied complexes. By doing so and computing ligand-exchange equilibrium constants, we reproduce well the experimental trends for the exchange of 2 and 3 ligands, while the exchange of only one ligand (1:1 complex-es) is still hard to reproduce from computations.We report recent progress concerning the basic chemistry of protactinium. We have shown that its coordination sphere may include up to 8 oxygen atoms (from the original mono-oxo bond and from ligand and solvent molecule complexation) and find an approximate way of determin-ing trends in equilibrium constants, opening the way for future predictions.[1]Wilson R. et al. (2018) Nat. Commun. 9, 622.[2]Wilson R. (2012) Nat. Chem. 4, 586.[3]Le Naour C. et al. (2019) Radiochim. Acta, 107, 979-991.[4]Le Naour C. et al. (2005) Inorg. Chem. 44, 9542.[5]Mendes M. et al. (2010) Inorg. Chem. 49, 9962-9971.[6]Barone et al. (1997) J. Chem. Phys. 107, 3210-3221.<br

Chemistry and speciation of protactinium – a first principles study

National audienceIt is of fundamental interest to understand and predict the chemistry of rare radioelements. In this work, we focus on protactinium (Z = 91), an element that is located in between thorium and uranium in the periodic table. It might be considered as an unpredictable actinide, because it be-haves differently than any other actinide and in particular its immediate neighbours. For instance, in solution, the +V oxidation state dominates, and related complexes display a mono-oxo bond (thorium would not while uranium would display trans-di-oxo bonds). For a first study, we start with two series of protactinium(V) complexes of similar nature, that have been already experimentally investigated, and that successively form in sulfuric acid and oxalic acidic media [1-3]. As experimental information, we may rely on the structure of one of the complexes, determined by EXAFS, and on apparent formation constants. Based on this in-formation, research hypotheses and a preliminary methodological study, we aim at revealing the structure and coordination of all the complexes and at defining a computational strategy to derive relative complexation constants (corresponding to ligand-exchange reactions). For instance, if the formed 1:1, 1:2 and 1:3 (Pa:L) complexes all mutually have similar natures, we may simply de-rive the relative complexation constants from the global formation constants of both the relevant complexes. Note that owing to the few available computational studies of protactinium complex-es and to the near degeneracy of the protactinium 5f and 6d shells, this study may prove to be quite challenging.From our computational models, geometry optimizations performed both in the gas phase and in solution lead to a first view on the actual coordination sphere of protactinium(V). It involves an oxygen atom from the Pa=O mono-oxo bond and also oxygen atoms from the bidentate sulfate and oxalate ligands, and in some cases from additional water molecules. The exchange formation constants are deduced and found to be in fair agreement with experiment for the reactions involving two or three bidentate ligands; i.e. when the ligands nearly fill by themselves the coordination sphere.[1]Le Naour C. et al. (2019) Radiochim. Acta, 107, 979-991.[2]Le Naour C. et al. (2005) Inorg. Chem. 44, 9542.[3]Mendes M. et al. (2010) Inorg. Chem. 49, 9962-9971.<br