Structure and Dynamics of Uranyl(VI) and Plutonyl(VI) Cations in Ionic Liquid/Water Mixtures via Molecular Dynamics Simulations

Contribution of Molecular Dynamics in pNMR for the Structural Determination of AnV and AnVI Complexes in Solution

In this study, we propose to use classical molecular dynamics (MD) coupled with ¹H NMR spectroscopy to study the conformations of different actinyl An^VI (An = U, Np, and Pu) and An^V (An = Np) complexes with tetra-ethyl dyglicolamide (TEDGA) ligands in order to have a better representation of such complexes in solution. Molecular dynamics simulations showed its effectiveness in interpreting the experiments by the calculation of geometric factors needed for the determination of magnetic properties of these complexes. We demonstrated that different conformations of the An^V and An^VI complexes with TEDGA exist in solution with different coordination modes, which is experimentally confirmed by ¹H NMR and EXAFS spectroscopies. Furthermore, MD simulations provide additional insights into the structures of complexes in solution since conformations with fast exchanges, which are not accessible from NMR experiments, have been observed by MD simulations.

Advancing the Am Extractant Design through the Interplay among Planarity, Preorganization, and Substitution Effects

Advancing the field of chemical separations is important for nearly every area of science and technology. Some of the most challenging separations are associated with the americium ion Am(III) for its extraction in the nuclear fuel cycle, ²⁴¹Am production for industrial usage, and environmental cleanup efforts. Herein, we study a series of extractants, using first-principle calculations, to identify the electronic properties that preferentially influence Am(III) binding in separations. As the most used extractant family and because it affords a high degree of functionalization, the polypyridyl family of extractants is chosen to study the effects of the planarity of the structure, preorganization of coordinating atoms, and substitution of various functional groups. The actinyl ions are used as a structurally simplified surrogate model to quickly screen the most promising candidates that can separate these metal ions. The down-selected extractants are then tested for the Am(III)/Eu(III) system. Our results show that π interactions, especially those between the central terpyridine ring and Am(III), play a crucial role in separation. Adding an electron-donating group onto the terpyridine backbone increases the binding energies to Am(III) and stabilizes Am-terpyridine coordination. Increasing the planarity of the extractant increases the binding strength as well, although this effect is found to be rather weak. Preorganizing the coordinating atoms of an extractant to their binding configuration as in the bound metal complex speeds up the binding process and significantly improves the kinetics of the separation process. This conclusion is validated by the synthesized 1,2-dihydrodipyrido[4,3-b;5,6-b]acridine (13) extractant, a preorganized derivative of the terpyridine extractant, which we experimentally showed was four times more effective than terpyridine at separating Am³⁺ from Eu³⁺ (SF_Am/Eu ∼ 23 ± 1).

On the Coordination Chemistry of the lanthanum(III) Nitrate Salt in EAN/MeOH Mixtures

A thorough structural characterization of the La(NO₃)₃ salt dissolved into several mixtures of ethyl ammonium nitrate (EAN) and methanol (MeOH) with EAN molar fraction χ_EAN ranging from 0 to 1 has been carried out by combining molecular dynamics (MD) and X-ray absorption spectroscopy (XAS). The XAS and MD results show that changes take place in the La³⁺ first solvation shell when moving from pure MeOH to pure EAN. With increasing the ionic liquid content of the mixture, the La³⁺ first-shell complex progressively loses MeOH molecules to accommodate more and more nitrate anions. Except in pure EAN, the La³⁺ ion is always able to coordinate both MeOH and nitrate anions, with a ratio between the two ligands that changes continuously in the entire concentration range. When moving from pure MeOH to pure EAN, the La³⁺ first solvation shell passes from a 10-fold bicapped square antiprism geometry where all the nitrate anions act only as monodentate ligands to a 12-coordinated icosahedral structure in pure EAN where the nitrate anions bind the La³⁺ cation both in mono- and bidentate modes. The La³⁺ solvation structure formed in the MeOH/EAN mixtures shows a great adaptability to changes in the composition, allowing the system to reach the ideal compromise among all of the different interactions that take place into it.

The structural properties of the La(NO₃)₃ salt dissolved into EAN/methanol mixtures were characterized by molecular dynamics and X-ray absorption spectroscopy. The La³⁺ solvation shell undergoes significant changes with increasing the ionic liquid content of the mixture, progressively losing methanol molecules to accommodate more and more nitrate anions. The La³⁺ solvation structure shows great adaptability to composition changes, passing from a 10-fold bicapped square antiprism geometry in pure methanol to a 12-coordinated icosahedral complex in EAN.

Predicting lanthanide coordination structures in solution with molecular simulation

The chemical and physical properties of lanthanide coordination complexes can significantly change with small variations in their molecular structure. Further, in solution, coordination structures (e.g., lanthanide-ligand complexes) are dynamic. Resolving solution structures, computationally or experimentally, is challenging because structures in solution have limited spatial restrictions and are responsive to chemical or physical changes in their surroundings. To determine structures of lanthanide-ligand complexes in solution, a molecular simulation approach is presented in this chapter, which concurrently considers chemical reactions and molecular dynamics. Lanthanide ion, ligand, solvent, and anion molecules are explicitly included to identify, in atomic resolution, lanthanide coordination structures in solution. The computational protocol described is applicable to determining the molecular structure of lanthanide-ligand complexes, particularly with ligands known to bind lanthanides but whose structures have not been resolved, as well as with ligands not previously known to bind lanthanide ions. The approach in this chapter is also relevant to elucidating lanthanide coordination in more intricate structures, such as in the active site of enzymes.

Synthesis of Anhydrous Acetates for the Components of Nuclear Fuel Recycling in Dialkylimidazolium Acetate Ionic Liquids

A series of anhydrous acetate salts with uranium {[C₂C₁im][UO₂(OAc)₃] (1), [C₂C₂im][UO₂(OAc)₃] (2), and [C₄C₁im][UO₂(OAc)₃] (3)}, lanthanides {[C₂C₂im]₂[La(OAc)₅] (4) and [C₂C₁im]₂[Nd(OAc)₅] (5)}, and strontium {[C₂C₁im]_n[Sr(OAc)₃]_n (6)} (where C₂C₁im = 1-ethyl-3-methylimidazolium, C₂C₂im = 1,3-diethylimidazolium, C₄C₁im = 1-butyl-3-methylimidazolium, and OAc = acetate) have been prepared and structurally characterized. Both lanthanides and strontium are common components of the nuclear fuel waste, and their separation from uranium is an important but still challenging task. A new synthetic approach with dialkylimidazolium acetate ionic liquids (ILs) as the solvent has been developed for the direct synthesis of homoleptic acetates from the corresponding hydrates and, unexpectedly, hardly soluble f-element oxides. Although the group of characterized compounds shows perfect structural variability, all actinide and lanthanide metal ions form monomeric complex anions where the metal cation coordinates to five ligands including two oxygen atoms in the case of uranium, as is commonly observed for uranyl compounds. Crystallographic analyses revealed that the complex [UO₂(OAc)₃]^- anions possess rather standard D_3h symmetry featuring a hexagonal-bipyramidal coordination environment, while the lanthanide anions [Ln(OAc)₅]^2- are fully asymmetric and the Ln³⁺ cations are 10-coordinated in the form of a distorted bicapped tetragonal antiprism. This is the first report of lanthanide ions coordinated in this fashion. For Sr²⁺, 9-fold coordination through oxygen atoms in the form of a strongly distorted tricapped trigonal prism is observed. The crystallization of anhydrous, homoleptic, anionic acetate complexes from such a large variety of different metal salts appears to be due to the properties of dialkylimidazolium acetate ILs themselves, including enhanced basicity from the high concentration of free anions and their greater affinity for hydrogen-bonding solutes relative to metal cations.

Unraveling the solvation geometries of the lanthanum(iii) bistriflimide salt in ionic liquid/acetonitrile mixtures

La(Tf₂N)₃ in C₈(mim)₂(Tf₂N)₂/acetonitrile mixtures forms 10-fold coordination complexes composed of both acetonitrile molecules and Tf₂N⁻ anions.

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.

Solvation of Zn2+ ion in 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids: a molecular dynamics and X-ray absorption study

Molecular dynamics simulations and X-ray absorption spectroscopy were employed to study Zn²⁺ ions in [C_nmim][Tf₂N] (n = 2, 4).

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.

Impacts of Oxo Interactions on Np(V) Crown Ether Complexes

Intermolecular interactions between the oxo group of an actinyl cation and other metal cations (i.e., cation-cation interactions) are dependent on the strength of the actinyl bond. These cation-cation interactions are prominently observed for the neptunyl cation [Np(V)O₂]⁺ and are sufficiently stable enough to explore using a variety of chemical techniques. Herein, we investigate these intermolecular interactions in the neptunyl 18-crown-6 system, because this macrocyclic ligand provides both stable coordination and the proper sterics to engage the oxo group in bonding with both low-valent metal cations and neighboring neptunyl units. We report the structural and spectroscopic characterization of five neptunyl, [Np(V,VI)O₂]^+,2+, compounds: Np1a ([NpO₂(18-crown-6)]ClO₄), Np1b ([NpO₂(18-crown-6)]AuCl₄), Na-Np ([Np(V)O₂(18-crown-6)(Na(H₂O)(18-crown-6)][Np(VI)O₂Cl₄], Np-Np ([NpO₂(18-crown-6)](NpO₂Cl₂NO₃)], and Np-Cl (NpO₂Cl(H₂O)_1.75). Each of these compounds were prepared from the ambient reactions of Np(V) in HX (where X = Cl, NO₃) with the 18-crown-6 ether molecule. Structural information obtained from single-crystal X-ray diffraction data was paired with solid-state and solution Raman spectroscopy to provide information on the interaction of the neptunyl oxo atom with neighboring cations. Neptunyl (Np═O) bond lengths are not perturbed upon interaction with the Na⁺ cation (Na-Np), but elongation is observed upon formation of a neptunyl-neptunyl interaction (Np-Np). This is also the first structurally characterized isolated, molecular complex that contains a simple T-shaped neptunyl-neptunyl interaction. Raman spectroscopy indicates little perturbation to the neptunyl bond until the formation of the neptunyl-neptunyl motif, which also results in activation of the ν₃ asymmetric stretch. Additional spectroscopic studies indicated that the neptunyl 18-crown-6 inclusion complexes form in solution and persist in the presence of other low-valence cations.

Uranyl Carbonate Complexes in Aqueous Solution and Their Ligand NMR Chemical Shifts and 17O Quadrupolar Relaxation Studied by ab Initio Molecular Dynamics

Dynamic structural effects, NMR ligand chemical shifts, and ¹⁷O NMR quadrupolar relaxation rates are investigated in the series of complexes UO₂²⁺, UO₂(CO₃)₃^4-, and (UO₂)₃(CO₃)₆^6-. Car-Parrinello molecular dynamics (CPMD) is used to simulate the dynamics of the complexes in water. NMR properties are computed on clusters extracted from the CPMD trajectories. In the UO₂²⁺ complex, coordination at the uranium center by water molecules causes a decrease of around 300 ppm for the uranyl ¹⁷O chemical shift. The final value of this chemical shift is within 40 ppm of the experimental range. The UO₂(CO₃)₃^4- and (UO₂)₃(CO₃)₆^6- complexes show a solvent dependence of the terminal carbonate ¹⁷O and ¹³C chemical shifts that is less pronounced than that for the uranyl oxygen atom. Corrections to the chemical shift from hybrid functionals and spin-orbit coupling improve the accuracy of chemical shifts if the sensitivity of the uranyl chemical shift to the uranyl bond length (estimated at 140 ppm per 0.1 Å from trajectory data) is taken into consideration. The experimentally reported trend in the two unique ¹³C chemical shifts is correctly reproduced for (UO₂)₃(CO₃)₆^6-. NMR relaxation rate data support large ¹⁷O peak widths, but remain below those noted in the experimental literature. Comparison of relaxation data for solvent-including versus solvent-free models suggest that carbonate ligand motion overshadows explicit solvent effects.

Systematic analysis of structural and topological properties: new insights into PuO2(H2O)n2+ (n = 1–6) complexes in the gas phase

In this study, the equilibrium, electronic structures, bonding and topological properties of PuO₂(H₂O)_n²⁺ (n = 1–6) complexes were systematically investigated.

The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

Atomistic molecular dynamics simulations of aqueous ionic liquid mixtures were performed in order to compare the resulting Kirkwood–Buff integrals with experimental data and the corresponding integrals derived by an inverse Kirkwood–Buff approach.

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.

Isolation of Uranyl Dicyanamide Complexes from N-Donor Ionic Liquids

An ionic liquid (IL) approach for soft-donor f-element chemistry has been demonstrated by the isolation of several new uranyl dicyanamide complexes through reactions of UO2(NO3)2·6H2O with dicyanamide ([N(CN)2](-))-containing ILs. The [N(CN)2](-) ions are able to rapidly substitute uranium's O-donor ligands, as evidenced by single-crystal X-ray diffraction studies on two anhydrous adducts of UO2(NO3)2 with [N(CN)2](-) ILs as well as by IR and NMR spectroscopic studies on solutions of UO2(NO3)2 in these ILs. By contrast, the slow reaction of UO2(OAc)2·2H2O with a nitrile-functionalized imidazolium dicyanamide IL in solvent and the reaction of UO2(NO3)2·6H2O with NaN(CN)2 at elevated temperature resulted in irreversible hydrolysis. The reaction of UO2SO4 with [N(CN)2](-) ions in an acidified aqueous solution resulted in the crystallization of a [UO2](2+) complex with biuret, a N(CN)2](-) hydrolysis product. [N(CN)2](-) ions in the form of ILs react rapidly with [UO2](2+) at room temperature, allowing ligand substitution with [N(CN)2](-) to out-compete the slower hydrolysis reaction, enabling the isolation of uranyl dicyanamide complexes and challenging assumptions regarding the affinity of uranium for O-donors.

Solvent effects of 1-ethyl-3-methylimidazolium acetate: solvation and dynamic behavior of polar and apolar solutes

We study the solvation properties of the ionic liquid 1-ethyl-3-methylimidazolium acetate ([eMIM]⁺[ACE]⁻) and the resulting dynamic behavior for differently charged model solutes at room temperature via atomistic molecular dynamics (MD) simulations of 500 ns length.

Water in ionic liquids at electrified interfaces: the anatomy of electrosorption

Complete removal of water from room-temperature ionic liquids is nearly impossible. For the electrochemical applications of ionic liquids, how water is distributed in the electrical double layers when the bulk liquids are not perfectly dry can potentially determine whether key advantages of ionic liquids, such as a wide electrochemical window, can be harnessed in practical systems. In this paper, we study the adsorption of water on electrode surfaces in contact with humid, imidazolium-based ionic liquids using molecular dynamics simulations. The results revealed that water molecules tend to accumulate within sub-nanometer distance from charged electrodes. At low amount of water in the bulk, the distributions of ions and of electrostatic potential in the double layer are affected weakly by the presence of water, but the spatial distribution of water molecules is strongly dependent on both. The preferential positions of water molecules in double layers are determined by the balance of several factors: the tendency to follow the positions of the maximal absolute value of the electrical field, the association with their ionic surroundings, and the propensity to settle at positions where more free space is available. The balance between these factors changes with charging the electrode, but the adsorption of water generally increases with voltage. The ion specificity of water electrosorption is manifested in the stronger presence of water near positive electrodes (where anions are the counterions) than near negative electrodes (where cations are counterions). These predictions await experimental verification.

Amphiphilic interactions of ionic liquids with lipid biomembranes: a molecular simulation study

Current bottlenecks in the large-scale commercial use of many ionic liquids (ILs) include their high costs, low biodegradability, and often unknown toxicities. As a proactive effort to better understand the molecular mechanisms of ionic liquid toxicities, the work herein presents a comprehensive molecular simulation study on the interactions of 1-n-alkyl-3-methylimidazolium-based ILs with a phosphatidylcholine (PC) lipid bilayer. We explore the effects of increasing alkyl chain length (n = 4, 8, and 12) in the cation and anion hydrophobicity on the interactions with the lipid bilayer. Bulk atomistic molecular dynamics (MD) simulations performed at millimolar (mM) IL concentrations show spontaneous insertion of cations into the lipid bilayer regardless of the alkyl chain length and a favorable orientational preference once a cation is inserted. Cations also exhibit the ability to "flip" inside the lipid bilayer (as is common for amphiphiles) if partially inserted with an unfavorable orientation. Moreover, structural analysis of the lipid bilayer show that cationic insertion induces roughening of the bilayer surface, which may be a precursor to bilayer disruption. To overcome the limitation in the timescale of our simulations, free energies for a single IL cation and anion insertion have been determined based on potential of mean force calculations. These results show a decrease in free energy in response to both short and long alkyl chain IL cation insertion, and likewise for a single hydrophobic anion insertion, but an increase in free energy for the insertion of a hydrophilic chloride anion. Both bulk MD simulations and free energy calculations suggest that toxicity mechanisms toward biological systems are likely caused by ILs behaving as ionic surfactants. [Yoo et al., Soft Matter, 2014].

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.