Theoretical Study of the Structural Properties of Plutonium(IV) and (VI) Complexes

Prediction of Redox Potentials for U, Np, Pu, and Am in Aqueous Solution

The redox properties of the actinides in aqueous solution are important for fuel production/reprocessing and understanding the environmental impact of nuclear waste. The redox potentials for U, Np, Pu, and Am in oxidation states from 0 up to VII (as appropriate) in aqueous solutions have been predicted at the density functional theory level with the B3LYP functional, Stuttgart small core pseudopotential basis sets for the actinides, and explicit (30H2O molecules)/implicit treatment of the aqueous solvent using the self-consistent reaction field COSMO and SMD approaches for the implicit solvation. The predictions of the structural parameters of clusters incorporating first and second solvation shells are consistent with the available experimental data. Our results are typically within 0.2 V of the available experimental data using two explicit solvation shells with an implicit solvent model. The use of the PW91 functional substantially improved the prediction of the Pu(VI/V) redox couple. The redox couples for An(VI/IV) and An(V/IV) which involve the addition of protons and removal of the actinyl oxygens led to slightly larger differences from an experiment. The An(IV/0) and An(III/0) couples were reliably predicted with our approach. Predictions of the unknown An(II/I) redox potentials were negative, consistent with expectations, and predictions for unknown An(VII/VI), An(III/II), and An(II/0) redox couples improve prior estimates.

Unveiling the Reduction Mechanism of Pu(IV) by Acetaldoxime

The separation of plutonium (Pu) from spent nuclear fuel was achieved by effectively adjusting the oxidation state of Pu from +IV to +III in the plutonium uranium reduction extraction (PUREX) process. Acetaldoxime (CH3CHNOH) as a free salt reductant can rapidly reduce Pu(IV), but the reduction mechanism remains indistinct. Herein, we explore the reduction mechanism of two Pu(IV) ions by one CH3CHNOH molecule, where the second Pu(IV) reduction is the rate-determining step with the energy barrier of 19.24 kcal mol-1, which is in line with the experimental activation energy (20.95 ± 2.34 kcal mol-1). Additionally, the results of structure and spin density analyses demonstrate that the first and second Pu(IV) reduction is attributed to hydrogen atom transfer and hydroxyl ligand transfer, respectively. Analysis of localized molecular orbitals unveils that the reduction process is accompanied by the breaking of the Pu-OOH bond and the formation of the OOH-H and C-OOH bonds. The reaction energies confirm that the reduction of Pu(IV) by acetaldoxime is both thermodynamically and kinetically accessible. In this work, we elucidate the reduction mechanism of Pu(IV) with CH3CHNOH, which provides a theoretical understanding of the rapid reduction of Pu(IV).

Plutonium complexes in water: new approach to ab initio modeling

Geometry optimization and the electronic structure calculations of Pu Z+ complexes (Z = 3–6) in water solution have been performed, within the framework of the DMol3 and Relativistic Discrete-Variational (RDV) methods. For the simulation of Pu Z+ molecular environment in aqueous solution we used 22 and 32 water molecules randomly distributed around cation. To model the effect of bulk solvent environment we used COSMO (Conductor-like Screening Model) potential for water (ε = 78.54). The obtained results showed that this approach allows the modeling of water dissociation and the formation of hydrolysis products. Our previously suggested scheme for the calculation of interaction energies between selected fragments of multi-molecular systems provides the quantitative estimation of the interaction strengths between plutonium in various oxidation states and each ligand in the first and second coordination shells in water solution.

Insights from quantum chemical calculations into inner and outer-sphere complexation of plutonium(IV) by monoamide and carbamide extractants

The strong influence of the structure of amide derivatives on their extraction properties has been demonstrated in several studies in the literature. To investigate and rationalize the influence of the nature and length of the monoamide alkyl chains on Pu(iv) extraction/complexation, a theoretical study was performed using the Density Functional Theory (DFT) method in the scalar relativistic framework. For that, the geometries for the inner/outer-sphere complexes and interaction energies of [Pu(NO3)4] and [Pu(NO3)6]2- with different ligands have been calculated. For both inner and outer-sphere complexes, it is found that the introduction of a bulky alkyl group on the carbonyl side strongly diminishes the complexation energy. This is fully consistent with monamide extraction properties. The influence of the bulkiness of the alkyl group is as or even more important for outer than for inner-sphere interactions. This result was unexpected when considering that there are less flexibility and stronger steric constraints in the inner sphere compared to the outer one. However, this can be attributed to specific electrostatic interactions between the two outer-sphere amide ligands and two nitrate ions of [Pu(NO3)6]2-. By increasing the polarity of the solution, such interactions diminish and the outer-sphere ligands move away from [Pu(NO3)6]2-. Consequently, the solvent effects were found to be very significant for outer-sphere complexation while rather small for inner-sphere complexation. This gives the key possibility to tune the substituent effect by changing the polarity of the solution. As for carbamide ligands, it was found that the weak interactions (dispersion) have remarkable effects on both inner and outer-sphere complexations.

Activating Water and Hydrogen by Ligand-Modified Uranium and Neptunium Complexes: A Density Functional Theory Study

Organometallic uranium complexes that can activate small molecules are well-known. In contrast, there are no known organometallic trans-uranium species capable of small-molecule transformations. Using density functional theory, we previously showed that changing actinide-ligand bonds from U-O groups to Np-N- (amide/imido) bonds makes redox small-molecule activation more energetically favorable for Np species. Here, we determine how general this ligand-modulation strategy is for affecting small-molecule activation in Np species. We focus on two reactions, one involving redox transformation of the actinide(s) and the other involving no change in the oxidation state of the actinide(s). Specifically, we considered the hydrogen evolution reaction (HER) from H₂O by actinide tris-aryloxide species. We also considered H₂ capture and hydride transfer by actinide siloxide and silylamide complexes. For the HER, the barriers for Np(III) systems are much higher than those of U(III). The overall reaction energies are also much worse. An-O → An-N substitutions marginally improve the barriers by 1-4 kcal/mol and more substantially improve the reaction energies by 9-15 kcal/mol. For H₂ capture and hydride transfer, the reaction energies for the U and Np species are similar. For both actinides, like-for-like An-O → An-N substitutions lead to improved reaction energies. Interestingly, in a recent report, it seemingly appears that U-O (siloxide) → U-N (silylamide) leads to complete shutdown of reactivity for H₂ capture and hydride transfer. This observation is reproduced and explained with calculations. The ligand environments of the siloxide and silylamide that were compared are vastly different. The steric environment of the siloxide is conducive for reactivity while the particular silylamide is not. We conclude that small-molecule activation with organometallic neptunium species is achievable with a guided choice of ligands. Additional emphasis should be placed on ligands that can allow for improved transition state barriers.

Resolving the kinetics of individual aqueous reaction steps of actinyl (AnO2 + and AnO2 2+; An=U, Np, and Pu) tricarbonate complexes with ferrous iron and hydrogen sulfide from first principles

The solubility and mobility of actinides (An), like uranium, neptunium, and plutonium, in the environment largely depends on their oxidation states. Actinyls (AnV,VIO2 +/2+ (aq)) form strong complexes with available ligands, like carbonate (CO3 2−), which may inhibit reduction to relatively insoluble AnIVO2(s). Here we use quantum-mechanical calculations to explore the kinetics of aqueous homogeneous reaction paths of actinyl tricarbonate complexes ([AnO2(CO3)3]5−/4−) with two different reductants, [Fe(OH)2(H2O)4]0 and [H2S(H2O)6]0. Energetically-favorable outer-sphere complexes (OSC) are found to form rapidly, on the order of milliseconds to seconds over a wide actinyl concentration range (pM to mM). The systems then encounter energy barriers (E a), some of which are prohibitively high (>100 kJ/mol for some neptunyl and plutonyl reactions with Fe2+ and H2S), that define the transition from outer- to inner-sphere complex (ISC; for example, calculated E a of ISC formation between UO2 + and UO2 2+ with Fe2+ are 35 and 74 kJ/mol, respectively). In some reactions, multiple OSCs are observed that represent different hydrogen bonding networks between solvent molecules and carbonate. Even when forming ISCs, electron transfer to reduce An6+ and An5+ is not observed (no change in atomic spin values or lengthening of An–Oax bond distances). Proton transfer from bicarbonate and water to actinyl O was tested as a mechanism for electron transfer from Fe2+ to U6+ and Pu6+. Not all proton transfer reactions yielded reduction of An6+ to An5+ and only a few pathways were energetically-favorable (e. g. H+ transfer from H2O to drive Pu6+ reduction to Pu5+ with ΔE = −5 kJ/mol). The results suggest that the tricarbonate complex serves as an effective shield against actinide reduction in the tested reactions and will maintain actinyl solubility at elevated pH conditions. The results highlight reaction steps, such as inner-sphere complex formation and electron transfer, which may be rate-limiting. Thus, this study may serve as the basis for future research on how they can be catalyzed by a mineral surface in a heterogeneous process.

Inner to outer-sphere coordination of plutonium(iv) with N,N-dialkyl amide: influence of nitric acid

N,N-Dialkylamides are extensively studied as alternative organic ligands to achieve the extraction and separation of uranium(vi) and plutonium(iv). We report here the coordination structures of the plutonium(iv) ion with N,N-di(2-ethylhexyl)-n-butanamide as a function of nitric acid concentration in the aqueous phase. The coordination structure of Pu(iv) evolves gradually with increasing nitric acid concentration from an inner-sphere with two coordinated amide ligands toward an outer-sphere hexanitrate complex with only nitrate ions in the first coordination sphere and protonated amide ligands in the outer shell.

On the multi-reference nature of plutonium oxides: PuO22+, PuO2, PuO3 and PuO2(OH)2

Actinide-containing complexes present formidable challenges for electronic structure methods due to the large number of degenerate or quasi-degenerate electronic states arising from partially occupied 5f and 6d shells. Conventional multi-reference methods can treat active spaces that are often at the upper limit of what is required for a proper treatment of species with complex electronic structures, leaving no room for verifying their suitability. In this work we address the issue of properly defining the active spaces in such calculations, and introduce a protocol to determine optimal active spaces based on the use of the Density Matrix Renormalization Group algorithm and concepts of quantum information theory. We apply the protocol to elucidate the electronic structure and bonding mechanism of volatile plutonium oxides (PuO₃ and PuO₂(OH)₂), species associated with nuclear safety issues for which little is known about the electronic structure and energetics. We show how, within a scalar relativistic framework, orbital-pair correlations can be used to guide the definition of optimal active spaces which provide an accurate description of static/non-dynamic electron correlation, as well as to analyse the chemical bonding beyond a simple orbital model. From this bonding analysis we are able to show that the addition of oxo- or hydroxo-groups to the plutonium dioxide species considerably changes the π-bonding mechanism with respect to the bare triatomics, resulting in bent structures with a considerable multi-reference character.

Complexation of An(vi) with succinic acid in aqueous acid solutions: uranyl vs. plutonyl

Due to stronger electrostatic interaction in a uranyl–succinate system, complex species of U(vi) with succinate are more stable than the ones of Pu(vi).

Ligand size dependence of U–N and U–O bond character in a series of uranyl hexaphyrin complexes: quantum chemical simulation and density based analysis

A quantum chemical and density based analysis of bonding in uranyl hexaphyrin complexes, looking for trends in stability and covalency.

Exploring the Interaction Natures in Plutonyl (VI) Complexes with Topological Analyses of Electron Density

The interaction natures between Pu and different ligands in several plutonyl (VI) complexes are investigated by performing topological analyses of electron density. The geometrical structures in both gaseous and aqueous phases are obtained with B3LYP functional, and are generally in agreement with available theoretical and experimental results when combined with all-electron segmented all-electron relativistic contracted (SARC) basis set. The Pu- O y l bond orders show significant linear dependence on bond length and the charge of oxygen atoms in plutonyl moiety. The closed-shell interactions were identified for Pu-Ligand bonds in most complexes with quantum theory of atoms in molecules (QTAIM) analyses. Meanwhile, we found that some Pu-Ligand bonds, like Pu-OH(-), show weak covalent. The interactive nature of Pu-ligand bonds were revealed based on the interaction quantum atom (IQA) energy decomposition approach, and our results indicate that all Pu-Ligand interactions is dominated by the electrostatic attraction interaction as expected. Meanwhile it is also important to note that the quantum mechanical exchange-correlation contributions can not be ignored. By means of the non-covalent interaction (NCI) approach it has been found that some weak and repulsion interactions existed in plutonyl(VI) complexes, which can not be distinguished by QTAIM, can be successfully identified.

Understanding the interactions of neptunium and plutonium ions with graphene oxide: scalar-relativistic DFT investigations

Due to the vast application potential of graphene oxide (GO)-based materials in nuclear waste processing, it is of pivotal importance to investigate the interaction mechanisms between actinide cations such as Np(V) and Pu(IV, VI) ions and GO. In this work, we have considered four types of GOs modified by hydroxyl, carboxyl, and carbonyl groups at the edge and epoxy group on the surface, respectively. The structures, bonding nature, and binding energies of Np(V) and Pu(IV, VI) complexes with GOs have been investigated systematically using scalar-relativistic density functional theory (DFT). Geometries and harmonic frequencies suggest that Pu(IV) ions coordinate more easily with GOs compared to Np(V) and Pu(VI) ions. NBO and electron density analyses reveal that the coordination bond between Pu(IV) ions and GO possesses more covalency, whereas for Np(V) and Pu(VI) ions electrostatic interaction dominates the An-OG bond. The binding energies in aqueous solution reveal that the adsorption abilities of all GOs for actinide ions follow the order of Pu(IV) > Pu(VI) > Np(V), which is in excellent agreement with experimental observations. It is expected that this study can provide useful information for developing more efficient GO-based materials for radioactive wastewater treatment.

Theoretical study of plutonium(IV) complexes formed within the PUREX process: a proposal of a plutonium surrogate in fire conditions

We present a relativistic quantum chemical study to determine the best surrogate for plutonium(IV) to be used in experimental investigations of the behavior of plutonium-nitrate-TBP in fire conditions that might occur in the nuclear fuel refining process known as PUREX. In this study geometries and stabilities of Pu(NO3)6(2-) and Pu(NO3)4(TBP)2 complexes were compared to that of equivalent complexes of selected elements from the lanthanide and actinide series (Ce, Th, U) chosen on the basis of similar ionic radii and stability as tetravalent species. PBE and PBE0 DFT functionals have proven to be sufficient and affordable for qualitative studies, performing as good as the wave function based correlated method MP2. On the basis of our results, cerium(IV) appears to be a good surrogate for plutonium(IV).

Investigations on preferential Pu(IV) extraction over U(VI) by N,N-dihexyloctanamide versus tri-n-butyl phosphate: evidence through small angle neutron scattering and DFT studies

Straight chain amide N,N-dihexyloctanamide (DHOA) has been found to be a promising alternative extractant to tri-n-butyl phosphate (TBP) for the reprocessing of irradiated uranium- and thorium-based fuels. Unlike TBP, DHOA displays preferential extraction of Pu(IV) over U(VI) at higher acidities (≥3 M HNO3) and poor extraction at lower acidities. Density functional theory (DFT) based calculations have been carried out on the structures and relative binding energies of U(VI) and Pu(IV) with the extractant molecules. These calculations suggest that the differential hardness of the two extractants is responsible for the preferential binding/complexation of TBP to uranyl, whereas the softer DHOA and the bulky nature of the extractant lead to stronger binding/complexation of DHOA to Pu(IV). In conjunction with quantum chemical calculations, small angle neutron scattering (SANS) measurements have also been performed for understanding the stoichiometry of the complex formed that leads to relatively lower extraction of Th(IV) (a model for Pu(IV)) as compared to U(VI) using DHOA and TBP as the extractants. The combined experimental and theoretical studies helped us to understand the superior complexation/extraction behavior of Pu(IV) over U(VI) with DHOA.

Coordination and hydrolysis of plutonium ions in aqueous solution using Car-Parrinello molecular dynamics free energy simulations

Car-Parrinello molecular dynamics (CPMD) simulations have been used to examine the hydration structures, coordination energetics, and the first hydrolysis constants of Pu(3+), Pu(4+), PuO2(+), and PuO2(2+) ions in aqueous solution at 300 K. The coordination numbers and structural properties of the first shell of these ions are in good agreement with available experimental estimates. The hexavalent PuO2(2+) species is coordinated to five aquo ligands while the pentavalent PuO2(+) complex is coordinated to four aquo ligands. The Pu(3+) and Pu(4+) ions are both coordinated to eight water molecules. The first hydrolysis constants obtained for Pu(3+) and PuO2(2+) are 6.65 and 5.70, respectively, all within 0.3 pH unit of the experimental values (6.90 and 5.50, respectively). The hydrolysis constant of Pu(4+), 0.17, disagrees with the value of -0.60 in the most recent update of the Nuclear Energy Agency Thermochemical Database (NEA-TDB) but supports recent experimental findings. The hydrolysis constant of PuO2(+), 9.51, supports the experimental results of Bennett et al. [Radiochim. Acta 1992, 56, 15]. A correlation between the pKa of the first hydrolysis reaction and the effective charge of the plutonium center was found.