Zinc Sensing with a Pyridine-Based Lanthanide Contrast Agent: Structural Analysis in Aqueous Solution

Zinc is an important physiological cation, and its misregulation is implicated in various diseases. It is therefore important to be able to image zinc by non-invasive methods such as Magnetic Resonance Imaging (MRI). In this work, we have successfully synthesized a novel Gd3+-based complex specifically for Zn2+ sensing by MRI. Using a combination of NMR, luminescence, potentiometric, and relaxivity experiments, completed with DFT calculations, we demonstrate that incorporating a short linker between the Zn2+ sensing unit and the Gd3+ complex leads to unique behavior of the system in the absence of Zn2+. A significant increase in efficacy of the system is observed upon Zn2+ binding, and importantly, the complex is highly selective for Zn2+ relative to other physiological cations. A comprehensive structural study reliably determines the microscopic parameters at the origin of the Zn2+ response, primarily an increase in the number of water molecules directly coordinated to Gd3+ upon Zn2+ binding. Crucially, the system maintains a strong response to Zn2+ binding in the presence of Human Serum Albumin, highlighting its potential for biological applications.

Neodymium(III) Aqua Ion as a Model System in Ab Initio Crystal Field Analysis Beyond Point Charges and Crystal Field Theory

Correlating the molecular structure with the electronic structure of lanthanide(III) solvates is a challenging task. Neodymium(III) in aqueous solution serves as an appealing and straightforward model to address this issue. Herein, the experimentally determined electronic structure of neodymium(III) in water is compared with ab initio calculated electronic structures based on various models of its molecular structure. This comparison enables the determination of the most reliable molecular structure. The findings reveal that the molecular structure of the neodymium(III) aqua ion that best aligns with its electronic structure corresponds to a nine coordinated neodymium(III) complex, surrounded by 17 water molecules in the second coordination sphere. The role of second-sphere water molecules was investigated by calculating the crystal field splitting of the five Kramers doublets within the 4I9/2 low-energy multiplet for several calculated molecular structures with coordination numbers of eight, nine, and ten. The results demonstrated that the shape of the donor molecular orbitals plays a critical role in the crystal field splitting of the neodymium(III) ion. Furthermore, the findings confirmed that the orientation of the donating orbitals, specifically the orientation of the O-H bonds in water, is essential for accurately describing the electronic structure. Finally, manual alteration of the Nd-O bond lengths revealed that CAS(3,7)CF calculations tend to underestimate the crystal field strength.

Computational Explorations of Th4+ First Hydrolysis Reaction Constants: Insights from Ab Initio Molecular Dynamics and Density Functional Theory Calculations

The fundamental hydrolysis behavior of tetravalent actinide cations (An4+) with a high charge is crucial for understanding their solution chemistry, particularly in nuclear fuel reprocessing and environmental behavior. Using Th4+ as a reference of the An4+ series, this work employed both the periodic model and the cluster model to calculate the first hydrolysis reaction constant (pKa1) of the Th4+ aqua ion and conducted a detailed evaluation of these approaches. In the periodic model, ab initio molecular dynamics (AIMD) simulations of Th4+ in the explicit solvation environment are conducted, using metadynamics and constrained molecular dynamics to calculate pKa1 values. Metadynamics simulations with sufficient sampling yielded a value of 5.02, aligning with the experimental values (4.12-4.97). Moreover, AIMD results reveal further Grotthuss-type proton transfers and changes in the solvent structures, which are important for accurately modeling the hydrolysis process. In the cluster model, density functional theory calculations are performed on isolated hydrate clusters to obtain pKa1 values, describing solvation effects through the cluster-continuum model. Based on insights from the periodic models, particularly regarding further proton transfer, the cluster model was modified and tested using different functionals and similar cations (La3+and Ac3+). The pKa1 values obtained in the cluster model also show good agreement with the experimental values. The current computational approaches provide a comprehensive understanding of Th4+ hydrolysis and a reference framework for studying the hydrolysis of other lanthanide and actinide ions.

DFT-Based Polarizable Ion Models for Molten Rare-Earth Chlorides: From Lanthanum to Europium

We developed a systematic polarizable force field for molten trivalent rare-earth chlorides, from lanthanum to europium, based on first-principle calculations. The proposed model was employed to investigate the local structure and physicochemical properties of pure molten salts and their mixtures with sodium chloride. We computed densities, heat capacities, surface tensions, viscosities, and diffusion coefficients and disclosed their evolution along the lanthanide series, filling the gaps for poorly studied elements, such as promethium and europium. The analysis of the local arrangement of chloride anions around lanthanide cations revealed broad coordination number distributions with a typical [from 6 to 9]-fold environment, the maximum of which shifts toward lower values with the increase of atomic number as well as upon dilution of the salt in sodium chloride. The neighboring lanthanide chloride complexes were found to be connected by sharing a corner or an edge of the corresponding polyhedra.

Remarkable Effect of Stereoisomerism on the Am(III)/Ln(III) Solvent Extraction. New Ligands for Highly Efficient Separation of Americium

Two novel 1,10-phenanthroline-2,9-diamide ligands were constructed on the basis of 2-phenylpyrrolidine and obtained as pure diastereomers. These ligands demonstrated advanced properties in liquid-liquid extraction tests. They revealed high efficiency of americium(III) extraction alongside with the record values of selectivity in the separation of americium from light lanthanides from strongly acidic media. An abrupt increase of extraction efficiency when moving along the lanthanide series from lanthanum to lutetium was observed. The examination of the extraction behavior of pure diastereomeric forms revealed noticeable differences in their selectivity while maintaining the overall extraction trend. The explanation of the discovered patterns was elucidated by a comprehensive study of the ability of the ligands to bind lanthanide nitrates in solutions. All the data collected (UV-vis and NMR titration, X-ray analysis of resulting complexes, solvation numbers estimation) were supported by quantum chemical calculation. These data clearly indicated that in the case of light lanthanides the formation of 1 : 1 complexes is most preferable. At the same time, complexes with heavy lanthanides, such as ytterbium and lutetium, exist as ionic pairs which may consist of [L2M]z+ cations counterbalanced by metallate anions, which may result in the formation of the species of unusual composition like L2M2 or even L4M5 clusters.

Exploring the Dynamic Coordination Sphere of Lanthanide Aqua Ions: Insights from r2SCAN-3c Composite-DFT Born-Oppenheimer Molecular Dynamics Studies

Born-Oppenheimer molecular dynamics (BOMD) simulations were performed to investigate the structure and dynamics of the first hydration shells of five trivalent lanthanide ions (Ln3+) at room temperature. These ions are relevant in various environments, including the bulk aqueous solution. Despite numerous studies, accurately classifying the molecular geometry of the first hydration sphere remains a challenge. To addres this, a cluster microsolvation approach was employed to study the interaction of Ln3+ ions (La, Nd, Gd, Er, and Lu) with up to 27 explicit water molecules. Electronic structure calculations were performed with the composite r2SCAN-3c method. The results demonstrate that this method offers an optimal balance between precision and computational efficiency. Specifically, it accurately predicts average Ln-O distances (MAE = 0.02 Å) of the first hydration sphere and preferred coordination numbers (CN) for the different lanthanide cations as compared to reported data in bulk. Highly dynamic first hydration shells for the examined Ln3+ ions were found, with noticeable and rapid rearrangements in their coordination geometries, some of which can be recognized as the tricapped trigonal prism (TTP) and the capped square antiprism (CSAP) for CN = 9, and as the square antiprism (SAP), the bicapped trigonal prism (BTP), and the trigonal dodecahedron (DDH) for CN = 8. However, ca. 70% of the nonacoordinated configurations did not meet the criteria of TTP or CSAP structures. For CN = 8, the percentage of configurations that could not be assigned to SAP, BTP, or DDH was lower, around 30%. The theoretical EXAFS spectra obtained from the BOMD simulations are in good agreement with the experimental data and confirm that model microsolvated environments accurately represent the near-solvation structure of these trivalent rare-earth ions. Moreover, this demonstrates that the faster dynamics of the first hydration shell can be studied separately from the dynamics of water exchange in the bulk aqueous solution.

Development of Multiscale Force Field for Actinide (An3+) Solutions

A multiscale force field (FF) is developed for an aqueous solution of trivalent actinide cations An3+ (An = U, Np, Pu, Am, Cm, Bk, and Cf) by using a 12-6-4 Lennard-Jones type potential considering ion-induced dipole interaction. Potential parameters are rigorously and automatically optimized by the meta-multilinear interpolation parametrization (meta-MIP) algorithm via matching the experimental properties, including ion-oxygen distance (IOD) and coordination number (CN) in the first solvation shell and hydration free energy (HFE). The water solvent models incorporate an especially developed polar coarse-grained (CG) water scheme named PW32 and three widely used all-atom (AA) level SPC/E, TIP3P, and TIP4P water schemes. Each PW32 is modeled as two bonded beads to represent three neighboring water molecules, the simulation efficiency of which is 1 to 2 orders of magnitude higher than that of AA waters. The newly developed FF shows high accuracy and transferability in reproducing the IOD, CN, and HFE of An3+. The molecular structure and water exchange dynamics of the first An3+ hydration shell and the ionic (van der Waals) radii are reinvestigated in this work. Moreover, the new FF can readily be transferred to other popular FFs, as it has practicably predicted the permeability of An3+ in a graphene oxide filter within the framework of optimized potentials for liquid simulations (OPLS)-AA FF. It holds promise for applications in exploring actinide aqueous solutions with multiscale computational overhead.

Local Coordination Environment of Lanthanides Adsorbed onto Cr- and Zr-based Metal-Organic Frameworks

Separating individual lanthanide (Ln) elements in aqueous mixtures is challenging. Ion-selective capture by porous materials, such as metal-organic frameworks (MOFs), is a promising approach. To design ion-selective MOFs, molecular details of the Ln adsorption complexes within the MOFs must be understood. We determine the local coordination environment of lanthanides Nd(III), Gd(III), and Lu(III) adsorbed onto Cr(III)-based terephthalate MOF (Cr-MIL-101) and Zr(IV)-based Universitet in Oslo MOFs (UiO-66 and UiO-68) and their derivatives. In the Cr(III)- and Zr(IV)-based MOFs, Ln adsorb as inner-sphere complexes at the metal oxo clusters, regardless of whether the organic linkers are decorated with amino groups. Missing linkers result in favorable Ln binding sites at oxo clusters; however, Ln can coordinate to metal sites even with linkers in place. MOF functionalization with phosphonate groups led to Ln chemisorption onto these groups, which out-compete metal cluster sites. Ln form monodentate and bidentate and mononuclear and binuclear surface complexes. We conclude that MOFs for ion-selective Ln capture can be designed by a combination of (1) maximizing metal-lanthanide interactions via shared O atoms at the metal oxo cluster sites, where mixed oxo clusters can lead to ion-selective Ln adsorption, and (2) functionalizing MOFs with Ln-selective groups capable of out-completing the metal oxo cluster sites.

Modeling separation of lanthanides via heterogeneous ligand binding

Individual lanthanide elements have physical/electronic/magnetic properties that make each useful for specific applications. Several of the lanthanides cations (Ln3+) naturally occur together in the same ores. They are notoriously difficult to separate from each other due to their chemical similarity. Predicting the Ln3+ differential binding energies (ΔΔE) or free energies (ΔΔG) at different binding sites, which are key figures of merit for separation applications, will help design of materials with lanthanide selectivity. We apply ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) to calculate ΔΔG for Ln3+ coordinated to ligands in water and embedded in metal-organic frameworks (MOFs), and ΔΔE for Ln3+ bonded to functionalized silica surfaces, thus circumventing the need for the computational costly absolute binding (free) energies ΔG and ΔE. Perturbative AIMD simulations of water-inundated simulation cells are applied to examine the selectivity of ligands towards adjacent Ln3+ in the periodic table. Static DFT calculations with a full Ln3+ first coordination shell, while less rigorous, show that all ligands examined with net negative charges are more selective towards the heavier lanthanides than a charge-neutral coordination shell made up of water molecules. Amine groups are predicted to be poor ligands for lanthanide-binding. We also address cooperative ion binding, i.e., using different ligands in concert to enhance lanthanide selectivity.

Hydration Structure of 102No2+: A Density Functional Theory-Molecular Dynamics Study

The hydration structure of No2+, the divalent cation of nobelium in water, was investigated by ab initio molecular dynamics (MD) simulations. First, a series of benchmark calculations were performed to validate the density functional theory (DFT) calculation methods for a molecule containing a No atom. The DFT-MD simulation of the hydration structure of No2+ was conducted after the MD method was validated by simulating the hydration structures of Ca2+ and Sr2+, whose behavior was previously reported to be similar to that of No2+. The model cluster containing M2+ (M = Ca, Sr, or No) and 32 water molecules was used for DFT-MD simulation. The results showed that the hydration distance of No2+ was intermediate between those of Ca2+ and Sr2+. This trend in the hydration distance is in good agreement with the elution position trend obtained in a previous radiochemical experiment. The calculated No-O bond lengths in the optimized structure of [No(H2O)8]2+ was 2.59 Å, while the average No-O bond length of [No(H2O)8]2+ in water by DFT-MD was 2.55 Å. This difference implies the importance of dynamic solvent effects, considering the second (and further) coordination sphere in the theoretical calculation of solution chemistry for superheavy elements.

Lanthanide transport in angstrom-scale MoS2-based two-dimensional channels

Rare earth elements (REEs), critical to modern industry, are difficult to separate and purify, given their similar physicochemical properties originating from the lanthanide contraction. Here, we systematically study the transport of lanthanide ions (Ln3+) in artificially confined angstrom-scale two-dimensional channels using MoS2-based building blocks in an aqueous environment. The results show that the uptake and permeability of Ln3+ assume a well-defined volcano shape peaked at Sm3+. This transport behavior is rooted from the tradeoff between the barrier for dehydration and the strength of interactions of lanthanide ions in the confinement channels, reminiscent of the Sabatier principle. Molecular dynamics simulations reveal that Sm3+, with moderate hydration free energy and intermediate affinity for channel interaction, exhibit the smallest dehydration degree, consequently resulting in the highest permeability. Our work not only highlights the distinct mass transport properties under extreme confinement but also demonstrates the potential of dialing confinement dimension and chemistry for greener REEs separation.

Understanding the Complexation Behavior of Carbamoylphosphine Oxide Ligands with Representative f-Block Elements

The complexation behavior of carbamoylmethylphosphine oxide ligands (CMPO), a bifunctional phosphine oxide, and their substituted derivatives with Ce(III), Eu(III), Th(IV), U(VI), and Am(III) was probed at the density functional theory (DFT) level. The enhanced extraction of trivalent rare earth elements by the 2-diphenylphosphinylethyl derivative over the conventional CMPO ligand is identified due to the availability of an additional P═O donor group in the former. In addition, the orbital and dispersive interactions play a vital role in the preference of Th(IV) over U(VI) during extraction using CMPO ligands. The better complexing ability of ligands having long alkyl chain substituents at the P atom is justified due to the observed enhanced dispersion interactions in these systems.

Ion solvation as a predictor of lanthanide adsorption structures and energetics in alumina nanopores

Adsorption reactions at solid-water interfaces define elemental fate and transport and enable contaminant clean-up, water purification, and chemical separations. For nanoparticles and nanopores, nanoconfinement may lead to unexpected and hard-to-predict products and energetics of adsorption, compared to analogous unconfined surfaces. Here we use X-ray absorption fine structure spectroscopy and operando flow microcalorimetry to determine nanoconfinement effects on the energetics and local coordination environment of trivalent lanthanides adsorbed on Al2O3 surfaces. We show that the nanoconfinement effects on adsorption become more pronounced as the hydration free energy, ΔGhydr, of a lanthanide decreases. Neodymium (Nd3+) has the least exothermic ΔGhydr (-3336 kJ·mol-1) and forms mostly outer-sphere complexes on unconfined Al2O3 surfaces but shifts to inner-sphere complexes within the 4 nm Al2O3 pores. Lutetium (Lu3+) has the most exothermic ΔGhydr (-3589 kJ·mol-1) and forms inner-sphere adsorption complexes regardless of whether Al2O3 surfaces are nanoconfined. Importantly, the energetics of adsorption is exothermic in nanopores only, and becomes endothermic with increasing surface coverage. Changes to the energetics and products of adsorption in nanopores are ion-specific, even within chemically similar trivalent lanthanide series, and can be predicted by considering the hydration energies of adsorbing ions.

Theoretical Insights on the Complexation of Americium(III) and Europium(III) with Diglycolamide- and Dimethylacetamide-Functionalized Calix[4]arenes

Separation of minor actinides from lanthanides is one of the biggest challenges in spent fuel reprocessing due to the similar physicochemical properties of trivalent lanthanides (Ln(III)) and actinides (An(III)). Therefore, developing ligands with excellent extraction and separation performance is essential at present. As an excellent pre-organization platform, calixarene has received more attention on Ln(III)/An(III) separation. In this work, we systematically explored the complexation behaviors of the diglycolamide (DGA)/dimethylacetamide (DMA)-functionalized calix[4]arene extractants for Eu(III) and Am(III) using relativistic density functional theory (DFT). These calix[4]arene-derived ligands were obtained by functionalization with two or four binding units at the narrow edge of the calix[4]arene platform. All bonding nature analyses suggested that the Eu-L complexes possess stronger interaction compared to Am-L analogues, resulting in the higher extraction capacity of the these calix[4]arene ligands toward Eu(III). Thermodynamic analysis demonstrates that these pre-organized ligands on the calix[4]arene platform with four binding units yield better extraction abilities than the single ligands. Although DMA-functionalized ligands show stronger complexation stability for metal ions, in acidic solutions, the calix[4]arene ligands with DGA binding units have better extraction performance for Eu(III) and Am(III) due to the basicity of the DMA ligand. This work enabled us to gain a deeper understanding of the bonding properties between supramolecular ligands and lanthanides/actinides and afford useful insights into designing efficient supramolecular ligands for separating Ln(III)/An(III).

Complexation of a Nitrilotriacetate-Derived Triamide Ligand with Trivalent Lanthanides: A Thermodynamic and Crystallographic Study

Non-heterocyclic N-donor nitrilotriacetate-derived triamide ligands are one of the most promising extractants for the selective extraction separation of trivalent actinides over lanthanides, but the thermodynamics and mechanism of the complexation of this kind of ligand with actinides and lanthanides are still not clear. In this work, the complexation behaviors of N,N,N',N',N″,N″-hexaethylnitrilotriacetamide (NTAamide(Et)) with four representative trivalent lanthanides (La³⁺, Nd³⁺, Eu³⁺, and Lu³⁺) were systematically investigated by using ¹H nuclear magnetic resonance (¹H NMR), ultraviolet-visible (UV-vis) and fluorescence spectrophotometry, microcalorimetry, and single-crystal X-ray diffractometry. ¹H NMR spectroscopic titration of La³⁺ and Lu³⁺ indicates that two species of 1:2 and 1:1 metal-ligand complexes were formed in NO₃^- and ClO₄^- media. The stability constants of NTAamide(Et) with Nd³⁺ and Eu³⁺ obtained by UV-vis and fluorescence titration show that the complexing strength of NTAamide(Et) with Nd³⁺ is lower than that with Eu³⁺ in the same anionic medium, while that of the same lanthanide complex is higher in ClO₄^- medium than in NO₃^- medium. Meanwhile, the formation reactions for all metal-ligand complexes are driven by both enthalpy and entropy. The structures of lanthanide complexes in the single ClO₄^- and NO₃^- medium and the mixed one were determined to be [LnL₂(MeOH)](ClO₄)₃ (Ln = La, Nd, Eu, and Lu), [LaL₂(EtOH)₂][La(NO₃)₆], and [LaL₂(NO₃)](ClO₄)₂, separately. The average bond lengths of lanthanide complexes decrease gradually with the decrease in ionic radii of Ln³⁺, indicating that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, which coincides with the trend of the complexing strength obtained by spectroscopic titration. This work not only reveals the thermodynamics and mechanism of the complexation between NTAamide ligands and lanthanides but also obtains the periodic tendency of complexation between them, which may facilitate the separation of trivalent lanthanides from actinides.

Direct Observation of Contact Ion-Pair Formation in La3+ Methanol Solution

An approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to carry out a comparative study about the solvation properties of dilute La(NO₃)₃ solutions in water and methanol, with the aim of elucidating the still elusive coordination of the La³⁺ ion in the latter medium. The comparison between these two systems enlightened a different behavior of the nitrate counterions in the two environments: while in water the La(NO₃)₃ salt is fully dissociated and the La³⁺ ion is coordinated by water molecules only, the nitrate anions are able to enter the metal first solvation shell to form inner-sphere complexes in methanol solution. The speciation of the formed complexes showed that the 10-fold coordination is preferential in methanol solution, where the nitrate anions coordinate the La³⁺ cations in a monodentate fashion and the methanol molecules complete the solvation shell to form an overall bicapped square antiprism geometry. This is at variance with the aqueous solution where a more balanced situation is observed between the 9- and 10-fold coordination. An experimental confirmation of the MD results was obtained by La K-edge XAS measurements carried out on 0.1 M La(NO₃)₃ solutions in the two solvents, showing the distinct presence of the nitrate counterions in the La³⁺ ion first solvation sphere of the methanol solution. The analysis of the extended X-ray absorption fine structure (EXAFS) part of the absorption spectrum collected on the methanol solution was carried out starting from the MD results and confirmed the structural arrangement observed by the simulations.

The formation of contact ion pairs between the La³⁺ ions and the nitrate anions has been demonstrated in diluted methanol solution using a combined approach using Molecular Dynamics simulations and X-ray absorption spectroscpy.

Insights into the solution structure of the hydrated uranyl ion from neutron scattering and EXAFS experiments

The solution structure of 1.0 M Uranyl Chloride has been determined by the EPSR modelling of a combination of neutron scattering and EXAFS data. The experimental data show an equilibrium in solution between [UO₂(H₂O)₅]²⁺ and [UO₂Cl(H₂O)₄]⁺ with a stability constant of 0.23 ± 0.03 mol^-1 dm^-3. A much smaller fraction of the neutral [UO₂Cl₂(H₂O)₃] ion is also observed. The data also show, for the first time in solution, that the uranyl ion is a very poor hydrogen bond acceptor, but the coordinated waters show enhanced hydrogen bond ability compared to the bulk water.

Interplay of physically different properties leading to challenges in separating lanthanide cations - an ab initio molecular dynamics and experimental study

Lanthanide elements have well-documented similarities in their chemical behavior, which make the valuable trivalent lanthanide cations (Ln3+) particularly difficult to separate from each other in water. In this work, we apply ab initio molecular dynamics simulations to compare the free energies (ΔGads) associated with the adsorption of lanthanide cations to silica surfaces at a pH condition where SiO- groups are present. The predicted ΔGads for lutetium (Lu3+) and europium (Eu3+) are similar within statistical uncertainties; this is in qualitative agreement with our batch adsorption measurements on silica. This finding is remarkable because the two cations exhibit hydration free energies (ΔGhyd) that differ by >2 eV, different hydration numbers, and different hydrolysis behavior far from silica surfaces. We observe that the similarity in Lu3+ and Eu3+ ΔGads is the result of a delicate cancellation between the difference in Eu3+ and Lu3+ hydration (ΔGhyd), and their difference in binding energies to silica. We propose that disrupting this cancellation at the two end points, either for adsorbed or completely desorbed lanthanides (e.g., via nanoconfinment or mixed solvents), will lead to effective Ln3+ separation.

Stabilization of hydrated AcIII cation: the role of superatom states in actinium-water bonding

225Ac-based radiopharmaceuticals have the potential to become invaluable in designated cancer therapy. However, the limited understanding of the solution chemistry and bonding properties of actinium has hindered the development of existing and emerging targeted radiotherapeutics, which also poses a significant challenge in the discovery of new agents. Herein, we report the geometric and electronic structural properties of hydrated AcIII cations in the [AcIII(H2O) n ]3+ (n = 4-11) complexes in aqueous solution and gas-phase using density functional theory. We found that nine water molecules coordinated to the AcIII cation is the most stable complex due to an enhanced hydration Gibbs free energy. This complex adopts a closed-shell 18-electron configuration (1S 21P 61D 10) of a superatom state, which indicates a non-negligible covalent character and involves H2O → AcIII σ donation interaction between s-/p-/d-type atomic orbitals of the Ac atom and 2p atomic orbitals of the O atoms. Furthermore, potentially existing 10-coordinated complexes need to overcome an energy barrier (>0.10 eV) caused by hydrogen bonding to convert to 9-coordination. These results imply the importance of superatom states in actinide chemistry generally, and specifically in AcIII solution chemistry, and highlight the conversion mechanism between different coordination numbers.

Structures and Free Energies of Cerium Ions in Acidic Electrolytes

The Ce³⁺/Ce⁴⁺ redox potential changes with the electrolyte, which could be due to unequal anion complexation free energies between Ce³⁺ and Ce⁴⁺ or a change in the solvent electrostatic screening. Ce complexation with anions and solvent screening also affect the solubility of Ce and charge transfer kinetics for electrochemical reactions involving waste remediation and energy storage. We report the structures and free energies of cerium complexes in seven acidic electrolytes based on Extended X-ray Absorption Fine Structure, UV-vis, and Density Functional Theory calculations. Ce³⁺ coordinates with nine water molecules as [Ce(H₂O)₉]³⁺ in all studied electrolytes. However, Ce⁴⁺ complexes with anions in all electrolytes except HClO₄. Thus, our results suggest that Ce⁴⁺-anion complexation leads to the large shifts in standard redox potential. Long range screening effects are smaller than the anion complexation energies but could be responsible for changes in the Ce solubility with acid.

On the Hydration of the Rare Earth Ions in Aqueous Solution

The totally symmetric stretching mode $$\nu_{1}$$ν1 Ln–(OH2) of the first hydration shells of all the rare earth (RE) ions across the series from lanthanum to lutetium has been measured on dilute aqueous perchlorate solutions at room temperature. An S-shaped relationship has been found between the $$\nu_{1}$$ν1 Ln–(OH2) peak positions and the Ln–(OH2) bond distances of the lanthanide(III) aqua ions. While the light rare earth ions form nona-hydrates, the heavy ones form octa-hydrates and the rare earth ions in the middle of the series show non integer hydration numbers between 9 and 8. A relationship between wavenumber positions $$\nu_{1}$$ν1 Ln–(OH2) and the Ln–(OH2) bond distances of the RE hydrates has been given. Recent quantum mechanical calculations support the given interpretation.

Ion Association in Lanthanide Chloride Solutions

A better understanding of the solution chemistry of the lanthanide (Ln) salts in water would have wide ranging implications in materials processing, waste management, element tracing, medicine and many more fields. This is particularly true for minerals processing, given governmental concerns about lanthanide security of supply and the drive to identify environmentally sustainable processing routes. Despite much effort, even in simple systems, the mechanisms and thermodynamics of LnIII association with small anions remain unclear. In the present study, molecular dynamics (MD), using a newly developed force field, provide new insights into LnCl3 (aq) solutions. The force field accurately reproduces the structure and dynamics of Nd3+ , Gd3+ and Er3+ in water when compared to calculations using density functional theory (DFT). Adaptive-bias MD simulations show that the mechanisms for ion pairing change from dissociative to associative exchange depending upon cation size. Thermodynamics of association reveal that whereas ion pairing is favourable, the equilibrium distribution of species at low concentration is dominated by weakly bound solvent-shared and solvent-separated ion pairs, rather than contact ion pairs, reconciling a number of contrasting observations of LnIII -Cl association in the literature. In addition, we show that the thermodynamic stabilities of a range of inner sphere and outer sphere LnCl x ( 3 - x ) + coordination complexes are comparable and that the kinetics of anion binding to cations may control solution speciation distributions beyond ion pairs. The techniques adopted in this work provide a framework with which to investigate more complex solution chemistries of cations in water.

On the Hydration of Heavy Rare Earth Ions: Ho3+, Er3+, Tm3+, Yb3+ and Lu3+-A Raman Study

Raman spectra of aqueous Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺-perchlorate solutions were measured over a large wavenumber range from 50-4180 cm^-1. In the low wavenumber range (terahertz region), strongly polarized Raman bands were detected at 387 cm^-1, 389 cm^-1, 391 cm^-1, 394 cm^-1, and 396 cm^-1, respectively, which are fairly broad (full widths at half height at ~52 cm^-1). These isotropic Raman bands were assigned to the breathing modes, ν₁ Ln-O of the heavy rare earth (HRE) octaaqua ions, [Ln(H₂O)₈]³⁺. The strong polarization of these bands (depolarization degree ~0) reveals their totally symmetric character. The vibrational isotope effect was measured in Yb(ClO₄)₃ solutions in H₂O and D₂O and the shift of the ν₁ mode in changing from H₂O to D₂O further supports the character of the band. The Ln-O bond distances of these HRE ions (Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺) follow the order of Ho-O > Er-O > Tm-O > Yb-O > Lu-O which correlates inversely with the band positions of the breathing modes of their corresponding octaaqua ions [Ln(OH₂)₈]³⁺. Furthermore, the force constants, k_Ln-O, were calculated for these symmetric stretching modes. Ytterbium perchlorate solutions were measured over a broad concentration range, from 0.240 mol·L^-1 to 2.423 mol·L^-1, and it was shown that with increasing solute concentration outer-sphere ion pairs and contact ion pairs were formed. At the dilute solution state (~0.3 mol·L^-1), the fully hydrated ions [Yb(H₂O)₈]³⁺ exist, while at higher concentrations (C_T > 2 mol·L^-1), ion pairs are formed. The concentration behavior of Yb(ClO₄)₃ (aq) shows similar behavior to the one observed for La(ClO₄)₃(aq), Ce(ClO₄)₃(aq) and Lu(ClO₄)₃(aq) solutions. In ytterbium chloride solutions in water and heavy water, representative for the behavior of the other HRE ions, 1:1 chloro-complex formation was detected over the concentration range from 0.422-3.224 mol·L^-1. The 1:1 chloro-complex in YbCl₃(aq) is very weak, diminishing rapidly with dilution and vanishing at a concentration < 0.4 mol·L^-1.

Local Structure Study of Lanthanide Elements by X-Ray Absorption Near Edge Structure Spectroscopy

This paper describes a systematic study of the spectra and local structures of lanthanide (Ln) L-edge XANES. We found that Ln L₁ and L₃ -edge XANES spectra exhibit characteristic features correlated to their local symmetry through experimental and theoretical simulations. We also propose a simple local structure index criterion for a combination of XANES study and theoretical simulation. Possible solutions of intrinsic problems such as low resolution of characteristic features in the Ln L-edge XANES and site distributions are also discussed.

Solvation structure of lanthanide(iii) bistriflimide salts in acetonitrile solution: a molecular dynamics simulation and EXAFS investigation

Lanthanide³⁺ ions in acetonitrile solutions of bistriflimide salts form 10-fold coordination complexes composed of both solvent molecules and counterions

Structural and thermodynamic aspects of hydration of Gd(iii) systems

A first systematic experimental study on the thermodynamic description of the hydration equilibrium of Gd(iii) compounds is presented.

Perspectives of multiscale rare earth crystal materials

Both multisize and multiweight effects are proposed to characterize multiscale rare earth crystal materials.

Hydration and Ion Pair Formation in Aqueous Lu3+- Solution

Aqueous solutions of Lu³⁺- perchlorate, triflate and chloride were measured by Raman spectroscopy. A weak, isotropic mode at 396 cm⁻¹ (full width at half height (fwhh) at 50 cm⁻¹) was observed in perchlorate and triflate solutions. This mode was assigned to the totally symmetric stretching mode of [Lu(OH₂)₈]³⁺, ν₁LuO₈. In Lu(ClO₄)₃ solutions in heavy water, the ν₁LuO₈ symmetric stretch of [Lu(OD₂)₈]³⁺ appears at 376.5 cm⁻¹. The shift confirms the theoretical isotopic effect of this mode. In the anisotropic scattering of aqueous Lu(ClO₄)₃, five bands of very low intensity were observed at 113 cm⁻¹, 161.6 cm⁻¹, 231 cm⁻¹, 261.3 cm⁻¹ and 344 cm⁻¹. In LuCl₃ (aq) solutions measured over a concentration range from 0.105–3.199 mol·L⁻¹ a 1:1 chloro-complex was detected. Its equilibrium concentration, however, disappeared rapidly with dilution and vanished at a concentration < 0.5 mol·L⁻¹. Quantitative Raman spectroscopy allowed the detection of the fractions of [Lu(OH₂)₈]³⁺, the fully hydrated species and the mono-chloro complex, [Lu(OH₂)₇Cl]²⁺. In a ternary LuCl₃/HCl solution, a mixtrure of chloro-complex species of the type [Lu(OH₂)_8−nCl_n]⁺³⁻ⁿ (n = 1 and 2) were detected. DFT geometry optimization and frequency calculations are reported for Lu³⁺- water cluster in vacuo and with a polarizable dielectric continuum (PC) model including the bulk solvent implicitly. The bond distance and angle for [Lu(OH₂)₈]³⁺ within the PC are in good agreement with data from structural experiments. The DFT frequencies for the Lu-O modes of [Lu(OH₂)₈]³⁺ and its deuterated analog [Lu(OD₂)₈]³⁺ in a PC are in fair agreement with the experimental ones. The calculated hydration enthalpy of Lu³⁺ (aq) is slightly lower than the experimental value.

Anion dependent ion pairing in concentrated ytterbium halide solutions

We have studied ion pairing of ytterbium halide solutions. THz spectra (30-400 cm^-1) of aqueous YbCl₃ and YbBr₃ solutions reveal fundamental differences in the hydration structures of YbCl₃ and YbBr₃ at high salt concentrations: While for YbBr₃ no indications for a changing local hydration environment of the ions were experimentally observed within the measured concentration range, the spectra of YbCl₃ pointed towards formation of weak contact ion pairs. The proposed anion specificity for ion pairing was confirmed by supplementary Raman measurements.

Quantum Chemistry Study on the Extraction of Trivalent Lanthanide Series by Cyanex301: Insights from Formation of Inner- and Outer-Sphere Complexes

The extraction of lanthanide series by Cyanex301, i.e., bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HC301), has been modeled by density functional theory calculation, taking into account the formation of both inner- and outer-sphere complexes. The inner-sphere complex Ln(C301)₃ and the outer-sphere complex Ln(H₂O)₉(C301)₃ are optimized, followed by the analysis of interaction energy, bond length, Laplacian bond orders, and Mulliken populations. The covalency degree increases in Ln-S and Ln-O bonds in the inner- and outer-sphere complexes, respectively, as the lanthanide series is traversed. Mulliken population analysis indicates the important role of the 5d-orbital participation in bonding in the formation of inner- and outer-sphere complexes. Two thermodynamic cycles regarding the formation of inner- and outer-sphere complexes are established to calculate the extraction Gibbs free energies (ΔG _extr), and relaxed potential energy surface scan is utilized to model the kinetic complexation of C301 anion with hydrated metal ions. Light lanthanides can form both inner- and outer-sphere complexes, whereas heavy lanthanides only form outer-sphere complexes in biphasic extraction. After adopting the data of forming inner-sphere complex for light Ln(III) and that of forming outer-sphere complexes for heavy Ln(III), the trend of the calculated -ΔG _extr agrees very well with that of the experimental distribution ratios on crossing the Ln(III) series. Results from this work help to theoretically understand the extraction behavior of Cyanex301 with respect to different Ln(III).

Raman spectroscopic characterization of light rare earth ions: La3+, Ce3+, Pr3+, Nd3+ and Sm3+ - hydration and ion pair formation

Raman spectra of aqueous La³⁺, Ce³⁺, Pr³⁺, Nd³⁺ and Sm³⁺ - perchlorate solutions were measured and weak strongly polarized Raman bands were detected at 343 cm^-1, 344 cm^-1, 347 cm^-1, 352 cm^-1 and 363 cm^-1, respectively. The full width at half height for these bands is quite broad (∼50 cm^-1) in the isotropic spectrum and the band width increases with increasing solute concentration. The polarized Raman bands were assigned to the breathing modes of the nona-aqua ions of the mentioned rare earth ions. Published structural results confirmed that these ions exist as nona-hydrates in aqueous solutions [Ln(H₂O)₉]³⁺. The Ln-O bond distances of these rare earth ions correlate well with the band positions of the nona-aqua ions [Ln(OH₂)₉]⁺³ (Ln = La³⁺, Ce³⁺, Pr³⁺, Nd³⁺ and Sm³⁺) and the force constants were calculated for these breathing modes. The strength of the force constants increase with decreasing the Ln-O bond distances (La-O > Ce-O > Pr-O > Nd-O > Sm-O). While the fully hydrated ions are stable in dilute perchlorate solutions (∼0.2 mol L^-1), in concentrated perchlorate solutions outer-sphere ion pairs and contact ion pairs are formed (C > 1.5 mol L^-1). In a hydrate melt at 161 °C of Ce(ClO₄)₃ plus 6H₂O, the contact ion pairs are the dominate species. The Raman bands of the ligated perchlorate and the Ce-O breathing mode of the partially hydrated ion pair at 326 cm^-1 were measured and characterized. In cerium chloride solutions chloro-complex formation was detected over the measured concentration range from 0.270-2.167 mol L^-1. The chloro-complexes in CeCl₃(aq) are weak and diminish rapidly with dilution and disappear at a concentration <0.1 mol L^-1. In a CeCl₃ solution, with additional HCl, a series of chloro-complex species of the type [Ce(OH₂)_9-nCl_n]^+3-n (n = 1, 2) were detected.