A Coupled EXAFS-Molecular Dynamics Study on PuO2+ and NpO2+ Hydration: The Importance of Electron Correlation in Force-Field Building

The physicochemical properties of the monovalent actinyl cations, PuO₂⁺ and NpO₂⁺, in water have been studied by means of classical molecular dynamic simulations. A specific set of cation-water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The TIP4P water model was adopted. Given the paramagnetic character of these actinyls, the cation-water interaction energies were computed from highly correlated wave functions using the NEVPT2 method. It is shown that the multideterminantal character of the wave function has a relevant effect on the main distances of the hydrated molecular cations. Several structural, dynamical, and energetic properties of the aqueous solutions have been obtained and analyzed. Structural RDF analysis gives An-O_yl distances of 1.82 and 1.84 Å and An-O(water) distances of 2.51 and 2.53 Å for PuO₂⁺ and NpO₂⁺ in water, respectively. Experimental EXAFS spectra from dilute aqueous solutions of PuO₂⁺ and NpO₂⁺ are revisited and analyzed, assuming tetra- and pentahydration of the actinyl cations. Simulated EXAFS spectra have been computed from the snapshots of the MD simulations. Good agreement with the experimental information available is found. The global analysis leads us to conclude that both PuO₂⁺ and NpO₂⁺ cations in water are stable pentahydrated aqua ions.

Force Field Parameterization of Actinyl Molecular Cations Using the 12-6-4 Model

In this work, a set of 12-6-4 force fields (FFs) parameters were developed for the actinyl molecular cations, AnO2n+ (n = 1, 2), from uranium to plutonium for classical molecular dynamics (MD) for four water models: TIP3P, SPC/E, OPC3, and TIP4Pew. Such a non-bonded potential model taking into account the induced dipole between the metallic center and the surrounding molecules has shown better performances for various cations than the classic 12-6 non-bonded potentials. The parametrization method proposed elsewhere for metallic cations has been extended to these molecular cations. In contrast to the actinyl 12-6 FFs from the literature, the new models reproduce correctly both solvation and thermodynamic properties, thanks to the inclusion of the induced dipole term (C4). The transferability of such force fields was assessed by performing MD simulations of carbonato actinyl species, which are highly implicated in actinide migration or actinide extraction from seawater. A highly satisfying agreement was found when comparing the EXAFS signals computed from our MD simulation to the experimental ones. The set of FFs developed here opens new possibilities for the study of actinide chemistry.

Hydrophobic core formation and secondary structure elements in uranyl(VI)-binding peptides

Cyclic peptides as well as a modified EF-hand motif of calmodulin have been newly designed to achieve high affinity towards uranyl(VI). Cyclic peptides may be engineered to bind uranyl(VI) to its backbone under acidic conditions, which may enhance its selectivity. For the modified EF-hand motif of calmodulin, strong electrostatic interactions between uranyl(VI) and negatively charged side chains play an important role in achieving high affinity; however, it is also essential to have a secondary structure element and formation of hydrophobic cores in the metal-bound state of the peptide.

Combining EXAFS and Computer Simulations to Refine the Structural Description of Actinyls in Water

EXAFS spectroscopy is one of the most used techniques to solve the structure of actinoid solutions. In this work a systematic analysis of the EXAFS spectra of four actinyl cations, [UO2]2+, [NpO2]2+, [NpO2]+ and [PuO2]2+ has been carried out by comparing experimental results with theoretical spectra. These were obtained by averaging individual contributions from snapshots taken from classical Molecular Dynamics simulations which employed a recently developed [AnO2]2+/+ -H2O force field based on the hydrated ion model using a quantum-mechanical (B3LYP) potential energy surface. Analysis of the complex EXAFS signal shows that both An-Oyl and An-OW single scattering paths as well as multiple scattering ones involving [AnO2]+/2+ molecular cation and first-shell water molecules are mixed up all together to produce a very complex signal. Simulated EXAFS from the B3LYP force field are in reasonable agreement for some of the cases studied, although the k= 6-8 Å-1 region is hard to be reproduced theoretically. Except uranyl, all studied actinyls are open-shell electron configurations, therefore it has been investigated how simulated EXAFS spectra are affected by minute changes of An-O bond distances produced by the inclusion of static and dynamic electron correlation in the quantum mechanical calculations. A [NpO2]+-H2O force field based on a NEVPT2 potential energy surface has been developed. The small structural changes incorporated by the electron correlation on the actinyl aqua ion geometry, typically smaller than 0.07 Å, leads to improve the simulated spectrum with respect to that obtained from the B3LYP force field. For the other open-shell actinyls, [NpO2]2+ and [PuO2]2+, a simplified strategy has been adopted to improve the simulated EXAFS spectrum. It is computed taking as reference structure the NEVPT2 optimized geometry and including the DW factors of their corresponding MD simulations employing the B3LYP force field. A better agreement between the experimental and the simulated EXAFS spectra is found, confirming the a priori guess that the inclusion of dynamic and static correlation refine the structural description of the open-shell actinyl aqua ions.

Rational Construction of Porous Metal-Organic Frameworks for Uranium(VI) Extraction: The Strong Periodic Tendency with a Metal Node

Although metal-organic frameworks (MOFs) have been reported as important porous materials for the potential utility in metal ion separation, coordinating the functionality, structure, and component of MOFs remains a great challenge. Herein, a series of anionic rare earth MOFs (RE-MOFs) were synthesized via a solvothermal template reaction and for the first time explored for uranium(VI) capture from an acidic medium. The unusually high extraction capacity of UO₂²⁺ (e.g., 538 mg U per g of Y-MOF) was achieved through ion-exchange with the concomitant release of Me₂NH₂⁺, during which the uranium(VI) extraction in the series of isostructural RE-MOFs was found to be highly sensitive to the ionic radii of the metal nodes. That is, the uranium(VI) adsorption capacities continuously increased as the ionic radii decreased. In-depth mechanism insight was obtained from molecular dynamics simulations, suggesting that both the accessible pore volume of the MOFs and hydrogen-bonding interactions contribute to the strong periodic tendency of uranium(VI) extraction.

The role of cations in uranyl nanocluster association: a molecular dynamics study

Actinyl ions can self-assemble in aqueous solution to form closed cage clusters ranging from 1.5 to 4.0 nm in diameter. The self-assembly, stability, and behavior of the nanoclusters depend on the nature of the aqueous environment, such as the pH and cations present. In this work, a classical force field for [(UO2)20(O2)30]20- (U20) peroxide nanoclusters in aqueous solution was developed from quantum-mechanical calculations. Using molecular dynamics simulations, the preferred binding sites of six cations (Li+, Na+, K+, Rb+, Cs+, and Ca2+) to the nanocluster were determined. Replica exchange molecular dynamics was used to equilibrate the structure and determine the equilibrium distribution of cations and water with respect to the nanocluster cage. In addition, the free energy barriers associated with cations entering the cluster were computed. Finally, the association of two cages was investigated by computing the free energy as a function of intercage distance. The free energy profiles reveal that the nanoclusters prefer to be associated when neutralized with divalent cations, but do not associate when neutralized with monovalent cations. This could explain the formation of tertiary structures observed experimentally.

Molecular Dynamics Simulation Studies on Structure, Dynamics, and Thermodynamics of Uranyl Nitrate Solution at Various Acid Concentrations

The structural and dynamical characteristics of uranyl ions in an aqueous acidic environment are of immense importance in the field of nuclear fuel reprocessing. In view of that, the structural and dynamical behavior of the uranyl ion in water has been investigated by performing molecular dynamics (MD) simulations using different force fields. All the force fields have depicted similar structural and dynamical properties except the free energy of hydration where the Guilbaud-Wipff (GW) model performs well over the others. The calculated density using MD simulations is found to be in excellent agreement with the measured experimental density, which ensures the accuracy of the adopted GW force field. The calculated surface tension and shear viscosity are seen to be increased with uranyl nitrate concentrations. At a higher concentration of about 4.0 mol/L, the supersaturation effect has been captured by an inflection in the plot of surface tension and shear viscosity against concentration because of the solution heterogeneity, which was correlated by an inflection in the scattering intensity observed by performing the dynamic light scattering experiment. The binding mode of nitrate ions with the uranyl ion is found to be concentration-dependent, and at higher concentration, it is predominantly monodentate.

The influence of humic substances on uranium biomineralization induced by Bacillus sp. dwc-2

In this paper, the influence of humic acid (HA) and fulvic acid (FA) on biomineralization behaviour was evaluated. The results showed HA and FA did not obviously inhabit or promote the precipitation of U-phosphate minerals. The data from molecular dynamic simulation indicated that the free energy for the dissociation of uranyl the PO₄^3- -uranyl was 202.49 kJ/mol, which was much larger than that form HA-uranyl (88.3 kJ/mol). These simulated results revealed the less competitiveness of HA and FA with PO₄^3- for uranyl and explained why HA and FA had less impacted on the formation of U-phosphate minerals. However, the influence of HA/FA on the morphology was obvious, the microstructure of the bio-minerals changed from small particles to lamellar stacking structure with the addition of HA or FA. The findings of this study are helpful for us to gain a better understanding natural U-phosphate biomineralization behaviour.

Ion association with tetra-n-alkylammonium cations stabilizes higher-oxidation-state neptunium dioxocations

Extended-coordination sphere interactions between dissolved metals and other ions, including electrolyte cations, are not known to perturb the electrochemical behavior of metal cations in water. Herein, we report the stabilization of higher-oxidation-state Np dioxocations in aqueous chloride solutions by hydrophobic tetra-n-alkylammonium (TAA⁺) cations—an effect not exerted by fully hydrated Li⁺ cations under similar conditions. Experimental and molecular dynamics simulation results indicate that TAA⁺ cations not only drive enhanced coordination of anionic Cl^– ligands to Np^V/VI but also associate with the resulting Np complexes via non-covalent interactions, which together decrease the electrode potential of the Np^VI/Np^V couple by up to 220 mV (ΔΔG = −22.2 kJ mol⁻¹). Understanding the solvation-dependent interplay between electrolyte cations and metal–oxo species opens an avenue for controlling the formation and redox properties of metal complexes in solution. It also provides valuable mechanistic insights into actinide separation processes that widely use quaternary ammonium cations as extractants or in room temperature ionic liquids.

The electrochemical behaviour of redox-active metal cations foremost depends on the metal centre’s inner-sphere coordination environment. Here the authors show that electrolyte cations unexpectedly stabilize higher-oxidation-state neptunium dioxocations in water through extended-coordination sphere interactions.

UO22+ structure in solvent extraction phases resolved at molecular and supramolecular scales: a combined molecular dynamics, EXAFS and SWAXS approach

We developed a polarizable force field for unraveling the UO₂²⁺ structure in both aqueous and solvent extraction phases.

Bonding trends across the series of tricarbonato-actinyl anions [(AnO2)(CO3)3]4- (An = U-Cm): the plutonium turn

Actinyl-tricarbonato anions [(AnO₂)(CO₃)₃]^4- (An = U-Cm) in various environments were investigated using theoretical approaches of quantum-mechanics, molecular-mechanics and cluster-models. Cations and solvent molecules in the 2^nd coordination sphere affect the equatorial An←O_eq bonds more than the axial An[triple bond, length as m-dash]O_ax bonds. Common actinide contraction is found for calculated and experimental axial bond lengths of ₉₂U to ₉₄Pu, though no longer for ₉₄Pu to ₉₆Cm. The tendency of U to Pu forming actinyl(vi) species dwindles away toward Cm, which already features the preferred An^III/Ln^III oxidation state of the later actinides and all lanthanides. The well known change from d-type to typical U-Pu-Cm type and then to f-type behavior is labeled as the plutonium turn, a phenomenon that is caused by f-orbital energy-decrease and f-orbital localization with increase of both nuclear charge and oxidation state, and a non-linear variation of effective f-electron population across the actinide series. Both orbital and configuration mixing and occupation of antibonding 5f type orbitals increase, weakening the AnO_ax bonds and reducing the highest possible oxidation states of the later actinides.

A molecular dynamics investigation of actinyl–ligand speciation in aqueous solution

Actinyl ions (AnO₂ⁿ⁺), the form in which actinides are commonly found in aqueous solution, are important species in the nuclear fuel cycle.

Crown ether complexes of actinyls: a computational assessment of AnO2(15-crown-5)2+ (An = U, Np, Pu, Am, Cm)

Computational characterization of AnO₂²⁺–(15-crown-5) complexes (An = U, Np, Pu, Am, and Cm) reveals actinyl insertion coordination to crown ether.

A Combined Experimental and Theoretical Study on the Extraction of Uranium by Amino-Derived Metal-Organic Frameworks through Post-Synthetic Strategy

A novel carboxyl-functionalized metal-organic framework for highly efficient uranium sorption was prepared through a generic postsynthetic strategy, and this MOF's saturation sorption capacity is found to be as high as 314 mg·g^-1. The preliminary application illustrated that the grafted free-standing carboxyl groups have notably enhanced the sorption of uranyl ions on MIL-101. In addition, we have performed molecular dynamics simulation combined with density functional theory calculations to investigate the molecular insights of uranyl ions binding on MOFs. The high selectivity and easy separation of the as-prepared material have shown tremendous potential for practical applications in the nuclear industry or radioactive water treatment, and the functionalized MOF can be extended readily upon the versatility of click chemistry. This work provides a facile and purposeful approach for developing MOFs toward a highly efficient and selective extraction of uranium(VI) in aqueous solution, and it further facilitates the structure-based design of nanomaterials for radionuclide-containing-medium pretreatment.

Dynamics of Humic Acid and Its Interaction with Uranyl in the Presence of Hydrophobic Surface Implicated by Molecular Dynamics Simulations

This work targeted a molecular level of understanding on the dynamics of humic acid (HA) and its interaction with uranyl in the presence of hydrophobic surface mimicked by a carbon nanotube (CNT), which also represents a potential intruder in the environment accompanying with the development of nanotechnology. In aqueous phase, uranyl and HA were observed to build close contact spontaneously, driven by electrostatic interaction, leading to a more compact conformation of HA. The presence of CNT unfolds HA via π-π interactions with the aromatic rings of HA without significant perturbation on the interaction strength between HA and uranyl. These results show that the hydrophilic uranyl and the hydrophobic CNT influence the folding behavior of HA in distinct manners, which represents two fundamental mechanisms that the folding behavior of HA may be modulated in the environment, that is, uranyl enhances the folding of HA via electrostatic interactions, whereas CNT impedes its spontaneous folding via van der Waals (vdW) interactions. The work also provides molecular level of evidence on the transformation of a hydrophobic surface into a hydrophilic one via noncovalent functionalization by HA, which in turn affects the migration of HA and the cations it binds to.

Molecular dynamics simulation study of distribution and dynamics of aqueous solutions of uranyl ions: the effect of varying temperature and concentration

Investigating the characteristics of actinyl ions has been of great interest due to their direct relevance in the nuclear fuel cycle. All-atom molecular dynamics simulations have been employed to study the orientational structure and dynamics of aqueous solutions of uranyl ions of various concentrations. The orientational structure of water around a uranyl ion has been thoroughly investigated by calculating different orientational probability distributions corresponding to different molecular axes of water. The orientational distribution of water molecules in the first coordination shell of a uranyl ion is found to be markedly different from that in bulk water. Analysis of counterion distribution around the uranyl ion reveals the presence of nitrate ions along with water molecules in the first solvation shell. From the comparison of the number of coordinated water and nitrate ions at various uranyl nitrate concentrations, it is evident that these two species compete for occupying the first solvation shell of the uranyl ion. Orientational dynamics of water molecules about different molecular axes of water in the vicinity of uranyl ions have also been investigated and decreasing orientational mobility of water with increasing uranyl concentration has been found. However, it is observed that the orientational dynamics remains more or less the same whether we consider all the water molecules in the aqueous solution or only the solvation shell water molecules. The effect of temperature on the translational and orientational characteristics of the aqueous uranyl solutions has also been studied in detail.

Uranyl Solvation by a Three-Dimensional Reference Interaction Site Model

We report an implementation of the three-dimensional reference interaction site model (3D RISM) that in particular addresses the treatment of the long-range Coulomb field of charged species, represented by point charges and/or a distributed charge density. A comparison of 1D and 3D results for atomic ions demonstrates a reasonable accuracy, even for a moderate size of the unit cell and a moderate grid resolution. In an application to uranyl complexes with 4-6 explicit aqua ligands and an implicit bulk solvent modeled by RISM, we show that the 3D technique is not susceptible to the deficiencies of the 1D technique exposed in our previous work [Li, Matveev, Krüger, Rösch, Comp. Theor. Chem. 2015, 1051, 151]. The 3D method eliminates the artificial superposition of explicit aqua ligands and the RISM medium and predicts essentially the same values for uranyl and uranyl-water bond lengths as a state-of-the-art polarizable continuum model. With the first solvation shell treated explicitly, the observables are nearly independent of the order of the closure relationship used when solving the set of integral equations for the various distribution functions. Furthermore, we calculated the activation barrier of water exchange with a hybrid approach that combines the 3D RISM model for the bulk aqueous solvent and a quantum mechanical description (at the level of electronic density functional theory) of uranyl interacting with explicitly represented water molecules. The calculated result agrees very well with experiment and the best theoretical estimates.

Spectroscopic markers for uranium(vi) phosphates: a vibronic study

Uranyl phosphate minerals are studied by optical and vibrational spectroscopy which provide robust markers to discriminate among environmentally hazardous uranium phases.

Dynamics of actinyl ions in water: a molecular dynamics simulation study

The dynamics of actinyl ions (AnO₂ⁿ⁺) in aqueous solutions is important not only for the design of advanced separation processes but also for understanding the fate of actinides in the environment.