On the development of polarizable and Lennard-Jones force fields to study hydration structure and dynamics of actinide(III) ions based on effective ionic radii

Going beyond Radial Hydration Models: The Hidden Structures of Chloride and Iodide Aqua Ions Revealed by the Use of Lone Pairs

A novel model of hydration for the chloride and iodide ions in water is proposed, which overcomes the limitations of conventional radial models. A new approach, based on a representation of the halide lone pairs, highlighted a subset of first shell water molecules featuring preferential strong interactions with the ion lone pairs, giving rise to tetrahedral hydration structures in both Cl- and I- aqueous solutions. By adopting a novel descriptor correlated to the halide-water interaction energy, we were able to split the conventional first solvation shell into a tight first hydration shell, composed of water molecules strongly interacting with the ions via hydrogen bonds, and a loose first shell containing molecules that are only slightly perturbed by the halide electrostatic charge. The picture emerging from our findings indicates that lone pairs play an important role in the description of systems where hydrogen bonds are the main interactions taking place in the solvation process.

Architector for high-throughput cross-periodic table 3D complex building

Rare-earth and actinide complexes are critical for a wealth of clean-energy applications. Three-dimensional (3D) structural generation and prediction for these organometallic systems remains a challenge, limiting opportunities for computational chemical discovery. Here, we introduce Architector, a high-throughput in-silico synthesis code for s-, p-, d-, and f-block mononuclear organometallic complexes capable of capturing nearly the full diversity of the known experimental chemical space. Beyond known chemical space, Architector performs in-silico design of new complexes including any chemically accessible metal-ligand combinations. Architector leverages metal-center symmetry, interatomic force fields, and tight binding methods to build many possible 3D conformers from minimal 2D inputs including metal oxidation and spin state. Over a set of more than 6,000 x-ray diffraction (XRD)-determined complexes spanning the periodic table, we demonstrate quantitative agreement between Architector-predicted and experimentally observed structures. Further, we demonstrate out-of-the box conformer generation and energetic rankings of non-minimum energy conformers produced from Architector, which are critical for exploring potential energy surfaces and training force fields. Overall, Architector represents a transformative step towards cross-periodic table computational design of metal complex chemistry.

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.

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.

Molecular Dynamics Simulations of Complexation of Am(III) with a Preorganized Dicationic Ligand in an Ionic Liquid

Preorganized ligands with imidazolium arms have been found to be highly selective in extracting Am(III) into ionic liquids (ILs), but the detailed structure and mechanism of the complexation process in the ionic solvation environment are unclear. Here, we carry out molecular dynamics simulation of the complexation of Am(III) with a preorganized 1,10-phenanthroline-2,9-dicarboxamide complexant (L) functionalized with alkyl chains and imidazolium cations in the butylmethylimidazolium bistriflimide ([BMIM][NTf₂]) IL. Both Am:L (1:1) and Am:L₂ (1:2) complexes are examined. In the absence of the ligand, Am(III) is found to be coordinated by six NTf₂ anions via nine O donors in the first solvation shell. In the Am:L complex, Am(III) is coordinated to the ligand via two O donors and four NTf₂ anions via seven O donors in the first coordination shell. In the Am:L₂ complex, Am(III) is coordinated to the two ligands via four O donors and four NTf₂ anions via five O donors. The imidazolium arms of the ligands play an important role in the secondary solvation environment by attracting NTf₂ anions closer to the metal center. As a result, we find that the binding free energy for the second L²⁺ ligand is twice that for the first L²⁺ ligand, making the Am:L₂ complex significantly more stable than the Am:L complex. This work highlights the multiple factors and tunability in using preorganized ligands with charged functional groups in an ionic solvation environment, which could hold the key to achieving desired selectivity in ion extraction efficiency.

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.

Determination of thermodynamic state variables of liquids from their microscopic structures using an artificial neural network

In this work we implement a machine learning method to predict the thermodynamic state of a liquid using only its microscopic structure provided by the radial distribution function (RDF). The main goal is to determine the equation of state of the system. The goal is achieved by predicting the density, temperature or both at the same time using only the RDF. We implement and train a machine learning feed forward artificial neural network (ANN) to address the different cases of interest where single or simultaneous predictions are done. Due to its versatility, in this study the Lennard-Jones (LJ) fluid is used as the reference system. The ANN is trained in a wide range of densities and temperatures, covering the liquid-vapour coexistence, liquid phase and supercritical states. We show that the overall percentage relative error of most of the predictions in different cases of study is around 3%. As a practical case of study we use the ANN predictions to determine the pressure equation of state for different isotherms and we found a very good agreement with respect to the exact results. Our ANN implementation is a versatile and useful tool to predict thermodynamic state variables when some information is unknown and, consequently, to enhance the thermodynamic description of liquids.

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

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

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.

Density functional theory (DFT) calculations of VI/V reduction potentials of uranyl coordination complexes in non-aqueous solutions

Of particular interest within the +6 uranium complexes is the linear uranyl(vi) cation and it forms numerous coordination complexes in solution and exhibits incongruent redox behavior depending on coordinating ligands. In this study, to determine the reduction potentials of uranyl complexes in non-aqueous solutions, a hybrid density functional theory (DFT) approach was used in which two different DFT functionals, B3LYP and M06, were applied. Bulk solvent effects were invoked through the conductor-like polarizable continuum model. The solute cavities were described with the united-atom Kohn-Sham (UAKS) cavity definition. Inside the cavity the dielectric constant matches the value of a vacuum and outside the cavity the dielectric constant value is the same as that of the solvent of interest, for example, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dichloromethane (DCM), acetonitrile and pyridine. With the help of the Nernst equation the calculated reduction potentials with respect to the ferrocene (Fc) reference electrode are converted into reduction free energies (RFEs). Uranyl complexes of organic ligands which range from mono- to hexa-dentate coordination modes were investigated in non-aqueous solutions of DMSO, DMF, DCM, acetonitrile and pyridine solutions. The effect of the spin-orbit correction and the reference electrode correction on the RFEs and various methods such as the direct method and the isodesmic reaction model were explored. Overall, our computational determination of RFEs of uranyl complexes in various non-aqueous solutions demonstrates that the RFEs can be obtained within ∼0.2 eV of experimental values.

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

Preface: Special Topic: From Quantum Mechanics to Force Fields

This Special Topic issue entitled "From Quantum Mechanics to Force Fields" is dedicated to the ongoing efforts of the theoretical chemistry community to develop a new generation of accurate force fields based on data from high-level electronic structure calculations and to develop faster electronic structure methods for testing and designing force fields as well as for carrying out simulations. This issue includes a collection of 35 original research articles that illustrate recent theoretical advances in the field. It provides a timely snapshot of recent developments in the generation of approaches to enable more accurate molecular simulations of processes important in chemistry, physics, biophysics, and materials science.

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

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