Quantum Chemical and Molecular Dynamics Study of the Coordination of Th(IV) in Aqueous Solvent

Stability of the protactinium(V) mono-oxo cation probed by first-principle calculations

This study explores the distinctive behavior of protactinium (Z=91) within the actinide series. In contrast to neighboring elements like uranium or plutonium, protactinium in the pentavalent state diverges by not forming the typical dioxo protactinyl moiety PaO2 + in aqueous phase. Instead, it manifests as a monooxo PaO3+ cation or a Pa5+ . Employing first-principle calculations with implicit and explicit solvation, we investigate two stoichiometrically equivalent neutral complexes: PaO(OH)2 (X)(H2 O) and Pa(OH)4 (X), where X represents various monodentate and bidentate ligands. Calculating the Gibbs free energy for the reaction PaO(OH)2 (X)(H2 O)→Pa(OH)4 (X), we find that the PaO(OH)2 (X)(H2 O) complex is stabilized with Cl- , Br- , I- , NCS- , NO3 - , and SO4 2- ligands, while it is not favored with OH- , F- , and C2 O4 2- ligands. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) methods reveal the Pa mono-oxo bond as a triple bond, with significant contributions from the 5f and 6d shells. Covalency of the Pa mono-oxo bond increases with certain ligands, such as Cl- , Br- , I- , NCS- , and NO3 - . These findings elucidate protactinium's unique chemical attributes and provide insights into the conditions supporting the stability of relevant complexes.

Geometries, interaction energies and bonding in [Po(H2O)n]4+ and [PoCln]4-n complexes

Polonium (Z = 84) is one of the rarest elements on Earth. More than a century after its discovery, its chemistry remains poorly known and even basic questions have not yet been satisfactorily addressed. In this work, we perform a systematic study of the geometries, interactions energies and bonding in basic polonium(IV) species, namely the hydrated [Po(H₂O)_n]⁴⁺ and chlorinated [PoCl_n]^4-n complexes by means of gas-phase electronic structure calculations. We show that while up to nine water molecules can fit in the first coordination sphere of the polonium(IV) ion, its coordination sphere can already be filled with eight chloride ligands. Capitalising on previous theoretical studies, a focused methodological study based on interaction energies and bond distances allows us to validate the MP2/def2-TZVP level of theory for future ground-state studies. After discussing the similarities and differences between complexes with the same number of ligands, we perform topological analyses of the MP2 electron densities in the quantum theory of atoms in molecules (QTAIM) fashion. While the water complexes display typical signatures of closed-shell interactions, we reveal large Po-Cl delocalisation indices, especially in the hypothetical [PoCl]³⁺ complex. This "enhanced" covalency opens the way for a significant spin-orbit coupling (SOC) effect on the corresponding bond distance, which has been studied using two independent approaches (i.e. one a priori and one a posteriori). We finally conclude by stressing that while the SOC may not affect much the geometries of high-coordinated polonium(IV) complexes, it should definitely not be neglected in the case of low-coordinated ones.

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.

Hierarchical phenomena in multicomponent liquids: simulation methods, analysis, chemistry

Complex, multicomponent, solutions have often been studied solely through the lens of specific applications of interest. Yet advances to both simulation methodologies (enhanced sampling, etc.) and analysis techniques (network analysis algorithms and others), are creating a trove of data that reveal transcending characteristics across vast compositional phase space. This perspective discusses technical considerations of the reliable and accurate simulations of complex solutions, followed by the advances to analysis algorithms that elucidate coupling of different length and timescale behavior (hierarchical phenomena). The different manifestations of hierarchical phenomena are presented across an array of solution environments, emphasizing fundamental and ongoing science questions. With a more advanced molecular understanding in hand, a quintessential application (solvent extraction) is discussed, where significant opportunities exist to re-imagine the technical scope of an established technology.

Properties of the tetravalent actinide series in aqueous phase from a microscopic simulation self-consistent engine

In the context of nuclear fuel recycling and environmental issues, the understanding of the properties of radio-elements with various approaches remains a challenge regarding their dangerousness. Moreover, experimentally, some issues are also of importance; first, it is imperative to work at sufficiently high concentrations to reach the sensitivities of the analytical tools, however this condition often leads to precipitation for some of them; second, stabilizing specific oxidation states of some actinides remains a challenge, thus making it difficult to extract general trends across the actinide series. Complementary to experiments, modeling can be used to unbiasedly probe the actinide's properties in an aquatic environment and offers a predictive tool. We report the first molecular dynamics simulations based on homogeneously built force fields for the whole series of the tetravalent actinides in aqueous phase from ThIV to BkIV and including PuIV. The force fields used to model the interactions among the constituents include polarization and charge donation microscopic effects. They are built from a self-consistent iterative ab initio based engine that can be included in future developments as an element of a potential machine learning procedure devoted to generating accurate force fields. The comparison of our simulated hydrated actinide properties to available experimental data shows the model robustness and the relevance of our parameter assignment engine. Moreover, our simulated structural, dynamical and evolution of the hydration free energy data show that, apart from AmIV and CmIV, the actinide properties change progressively along the series.

Density Functional Theory Investigations on the Mechanism of Formation of Pa(V) Ion in Hydrous Solutions

Due to the enormous threat of protactinium to the environment and human health, its disposal and chemistry have long been important topics in nuclear science. [PaO(H₂O)₆]3+ is proposed as the predominant species in hydrous and acidic solutions, but little is known about its formation mechanism. In this study, density functional theory (DFT) calculations demonstrate a water coordination-proton transfer-water dissociation mechanism for the formation of PaO3+ in hydrous solutions. First, Pa(V) ion preferentially forms hydrated complexes with a coordination number of 10. Through hydrogen bonding, water molecules in the second coordination sphere easily capture two protons on the same coordinated H₂O ligand to form [PaO(H₂O)₉]3+. Water dissociation then occurs to generate the final [PaO(H₂O)₆]3+, which is the thermodynamic product of Pa(V) in hydrous solutions.

Linking Solution Structures and Energetics: Thorium Nitrate Complexes

Seeking predictive insights into how metal-ion speciation impacts solution chemistry as well as the composition and structure of solid-precipitates, thorium correlations, with both solvent and other solute ions, were quantitatively probed in a series of acidic, nitrate/perchlorate solutions held at constant ionic strength. Difference pair-distribution functions (dPDF), obtained from high-energy X-ray scattering (HEXS) data, provide unprecedented structural information on the number of Th ligating ions in solution and how they change with increasing nitrate concentration. A fit of the end member solution, Th (4 m perchloric acid and no nitrate), reveals a homoleptic Th aqua ion with 10 waters in its first coordination shell. Analyses of the acidic solutions containing nitrate reveal exclusively bidentate NO₃^- complexation with Th, consistent with published solid-state M^IV nitrate structures, where M^IV = Ce, Th, U, Np, Pu. Metrical fits of Th coordination as a function of nitrate concentration are used to calculate Th-NO₃ stability constants, information important to a molecular-scale description of reaction energetics. The coordination environments of Th in solution were compared with single-crystal structures obtained from their precipitates, Th(NO₃)₄(H₂O)₄ and Th(NO₃)₄(H₂O)₃·(H₂O)₂. Relative stabilities of the solid-state compounds, assessed based on the results of molecular quantum chemical calculations, reveal the importance of including an accurate description of complexed waters when predicting relative energetics of dissolved ions in aqueous solution.

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.

Computational study of Th(4+) and Np(4+) hydration and hydrolysis of Th(4+) from first principles

The aqueous solvation of Th and Np in the IV oxidation state was examined using cluster models generated by Monte Carlo simulations and density functional theory embedded within the COSMO continuum model to approximate the effect of bulk water. Our results suggest that the coordination number (CN) for both Th(IV) and NP(IV) should be 9, in accordance to some experimental and theoretical results from the literature. The structural values for average oxygen-metal distances are within 0.01 Å compared to experimental data, and also within the experimental error. The calculated ΔG _Sol⁰ are in very good agreement with experimental reported values, with deviations at CN = 9 lower than 1% for both Th(IV) and Np(IV). The hydrolysis constants are also in very good agreement with experimental values. Finally, this [corrected] methodology has the advantage of using a GGA functional (BP86) that not only makes the calculations more affordable computationally than hybrid functional or ab initio molecular dynamics simulations (Car-Parrinello) calculations, but also opens the perspective to use resolution of identity (RI) calculations for more extended systems.

Structural, dynamical, and transport properties of the hydrated halides: How do At(-) bulk properties compare with those of the other halides, from F(-) to I(-)?

The properties of halides from the lightest, fluoride (F(-)), to the heaviest, astatide (At(-)), have been studied in water using a polarizable force-field approach based on molecular dynamics (MD) simulations at the 10 ns scale. The selected force-field explicitly treats the cooperativity within the halide-water hydrogen bond networks. The force-field parameters have been adjusted to ab initio data on anion/water clusters computed at the relativistic Möller-Plesset second-order perturbation theory level of theory. The anion static polarizabilities of the two heaviest halides, I(-) and At(-), were computed in the gas phase using large and diffuse atomic basis sets, and taking into account both electron correlation and spin-orbit coupling within a four-component framework. Our MD simulation results show the solvation properties of I(-) and At(-) in aqueous phase to be very close. For instance, their first hydration shells are structured and encompass 9.2 and 9.1 water molecules at about 3.70 ± 0.05 Å, respectively. These values have to be compared to the F(-), Cl(-), and Br(-) ones, i.e., 6.3, 8.4, and 9.0 water molecules at 2.74, 3.38, and 3.55 Å, respectively. Moreover our computations predict the solvation free energy of At(-) in liquid water at ambient conditions to be 68 kcal mol(-1), a value also close the I(-) one, about 70 kcal mol(-1). In all, our simulation results for I(-) are in excellent agreement with the latest neutron- and X-ray diffraction studies. Those for the At(-) ion are predictive, as no theoretical or experimental data are available to date.

Structure, Stability, and Electronic Properties of Dimethyl Sulfoxide and Dimethyl Formammide Clusters Containing Th(4.)

By using accurate density functional theory calculations, we have studied the complexes of Th(4+) with dimethyl-sulfoxide (DMSO) and dimethyl-formammide (DMF) molecules. These solvents are prototypes for oxygen-donor organic environments in which the oxygen atom is connected to S and C atom, respectively. Extended structural, energetic, and electronic structure analysis has been performed to provide a complete picture of the physical properties at the basis of the interaction of Th(4+) with the two solvents. By using a cluster grow approach, we have found that, very likely, the first solvation shell contains nine molecules in the case of DMSO, while it contains eight molecules for DMF. The theoretical results shown here are in agreement with experimental data taken from the literature.

First structural characterization of Pa(iv) in aqueous solution and quantum chemical investigations of the tetravalent actinides up to Bk(IV): the evidence of a curium break

More than a century after its discovery the structure of the Pa(4+) ion in acidic aqueous solution has been investigated for the first time experimentally and by quantum chemistry. The combined results of EXAFS data and quantum chemically optimized structures suggest that the Pa(4+) aqua ion has an average of nine water molecules in its first hydration sphere at a mean Pa-O distance of 2.43 Å. The data available for the early tetravalent actinide (An) elements from Th(4+) to Bk(4+) show that the An-O bonds have a pronounced electrostatic character, with bond distances following the same monotonic decreasing trend as the An(4+) ionic radii, with a decrease of the hydration number from nine to eight for the heaviest ions Cm(4+) and Bk(4+). Being the first open-shell tetravalent actinide, Pa(4+) features a coordination chemistry very similar to its successors. The electronic configuration of all open-shell systems corresponds to occupation of the valence 5f orbitals, without contribution from the 6d orbitals. Our results thus demonstrate that Pa(iv) resembles its early actinide neighbors.

Understanding the structure and electronic properties of Th4+-water complexes

We present a systematic quantum chemistry study of the [Th(H2O)n]4+ (n = 1 to 10) complexes to gain insight into their electronic structure and properties: the effect of the ligand distribution on the valence shells of the thorium(IV) ion is studied by means of the electron localization function (ELF) topological analysis. Particular care is given to the study of the mono-aqua complex both at its equilibrium geometry, using various tools such as energy decomposition analyses (EDA), and along its dissociation pathway. Indeed, as several electronic states cross the Th4 +-H2O0 ground state along the minimum energy path, we demonstrate that the diabatic representation implemented in MOLPRO is able to generate reference potential energy surfaces that will lead to the evaluation of diabatic dissociation curves. The calculated diabatic interaction energy curve will allow for a consistent parameterization of new generation force fields dedicated to heavy metals based on quantum chemistry.

Force field development for actinyl ions via quantum mechanical calculations: an approach to account for many body solvation effects

Advances in computational algorithms and methodologies make it possible to use highly accurate quantum mechanical calculations to develop force fields (pair-wise additive intermolecular potentials) for condensed phase simulations. Despite these advances, this approach faces numerous hurdles for the case of actinyl ions, AcO2(n+) (high-oxidation-state actinide dioxo cations), mainly due to the complex electronic structure resulting from an interplay of s, p, d, and f valence orbitals. Traditional methods use a pair of molecules (“dimer”) to generate a potential energy surface (PES) for force field parametrization based on the assumption that many body polarization effects are negligible. We show that this is a poor approximation for aqueous phase uranyl ions and present an alternative approach for the development of actinyl ion force fields that includes important many body solvation effects. Force fields are developed for the UO2(2+) ion with the SPC/Fw, TIP3P, TIP4P, and TIP5P water models and are validated by carrying out detailed molecular simulations on the uranyl aqua ion, one of the most characterized actinide systems. It is shown that the force fields faithfully reproduce available experimental structural data and hydration free energies. Failure to account for solvation effects when generating PES leads to overbinding between UO2(2+) and water, resulting in incorrect hydration free energies and coordination numbers. A detailed analysis of arrangement of water molecules in the first and second solvation shell of UO2(2+) is presented. The use of a simple functional form involving the sum of Lennard-Jones + Coulomb potentials makes the new force field compatible with a large number of available molecular simulation engines and common force fields.

A QMCF-MD investigation of the structure and dynamics of Ce4+ in aqueous solution

A quantum-mechanical charge-field molecular dynamics simulation has been performed for a tetravalent Ce ion in aqueous solution. In this framework, the complete first and second hydration spheres are treated by ab initio quantum mechanics supplemented by an electrostatic embedding technique, making the construction of non-Coulombic solute-solvent potentials unnecessary. During the 10 ps of simulation time, the structural aspects of the solution were analyzed by various methods. Experimental results such as the mean Ce-O bond distance and the predicted first-shell coordination number were compared to the results obtained from the simulation resolving some ambiguities in the literature. The dynamics of the system were characterized by mean ligand residence times and frequency/force constant calculations. Furthermore, Ce-O and Ce-H angular radial distribution plots were employed, yielding deeper insight into the structural and dynamical aspects of the system.

Thermal and solvent effects on NMR indirect spin-spin coupling constants of a prototypical Chagas disease drug

NMR J-couplings across hydrogen bonds reflect the static and dynamic character of hydrogen bonding. They are affected by thermal and solvent effects and can therefore be used to probe such effects. We have applied density functional theory (DFT) to compute the NMR (n)J(N,H) scalar couplings of a prototypical Chagas disease drug (metronidazole). The calculations were done for the molecule in vacuo, in microsolvated cluster models with one or few water molecules, in snapshots obtained from molecular dynamics simulations with explicit water solvent, and in a polarizable dielectric continuum. Hyperconjugative and electrostatic effects on spin-spin coupling constants were assessed through DFT calculations using natural bond orbital (NBO) analysis and atoms in molecules (AIM) theory. In the calculations with explicit solvent molecules, special attention was given to the nature of the hydrogen bonds formed with the solvent molecules. The results highlight the importance of properly incorporating thermal and solvent effects into NMR calculations in the condensed phase.