Molecular dynamics to rationalize EXAFS experiments: A dynamical model explaining hydration behaviour across the lanthanoid(III) series

Electronic Structure of Ytterbium(III) Solvates-a Combined Spectroscopic and Theoretical Study

The wide range of optical and magnetic properties of lanthanide(III) ions is associated with their intricate electronic structures which, in contrast to lighter elements, is characterized by strong relativistic effects and spin-orbit coupling. Nevertheless, computational methods are now capable of describing the ladder of electronic energy levels of the simpler trivalent lanthanide ions, as well as the lowest energy term of most of the series. The electronic energy levels result from electron configurations that are first split by spin-orbit coupling into groups of energy levels denoted by the corresponding Russell-Saunders terms. Each of these groups are then split by the ligand field into the actual electronic energy levels known as microstates or sometimes m_J levels. The ligand-field splitting directly informs on the coordination geometry and is a valuable tool for determining the structure and thus correlating the structure and properties of metal complexes in solution. The issue with lanthanide complexes is that the determination of complex structures from ligand-field splitting remains a very challenging task. In this paper, the optical spectra-absorption, luminescence excitation, and luminescence emission-of ytterbium(III) solvates were recorded in water, methanol, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF). The electronic energy levels, that is, the microstates, were resolved experimentally. Subsequently, density functional theory calculations were used to model the structures of the solvates, and ab initio relativistic complete active space self-consistent field calculations (CASSCF) were employed to obtain the microstates of the possible structures of each solvate. By comparing the experimental and theoretical data, it was possible to determine both the coordination number and solution structure of each solvate. In water, methanol, and N,N-dimethylformamide, the solvates were found to be eight-coordinated and have a square antiprismatic coordination geometry. In DMSO, the speciation was found to be more complicated. The robust methodology developed for comparing experimental spectra and computational results allows the solution structures of homoleptic lanthanide complexes to be determined.

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.

Metal Ion Modeling Using Classical Mechanics

Metal ions play significant roles in numerous fields including chemistry, geochemistry, biochemistry, and materials science. With computational tools increasingly becoming important in chemical research, methods have emerged to effectively face the challenge of modeling metal ions in the gas, aqueous, and solid phases. Herein, we review both quantum and classical modeling strategies for metal ion-containing systems that have been developed over the past few decades. This Review focuses on classical metal ion modeling based on unpolarized models (including the nonbonded, bonded, cationic dummy atom, and combined models), polarizable models (e.g., the fluctuating charge, Drude oscillator, and the induced dipole models), the angular overlap model, and valence bond-based models. Quantum mechanical studies of metal ion-containing systems at the semiempirical, ab initio, and density functional levels of theory are reviewed as well with a particular focus on how these methods inform classical modeling efforts. Finally, conclusions and future prospects and directions are offered that will further enhance the classical modeling of metal ion-containing systems.