Actinoids and Lanthanoids Solvation – ACLASOLV

We have obtained a detalied understanding of lathanoids and actinoids solvation by coupling different theoretical methods with experiments. In particular, we have linked structural and electronic properties. The developping of polarizable force fields was couple with the determination of the nature of the interaction due to quantum chemistry calculations. This was the basis to understand at atomic level the analogies and the differences between the ions of the two series. We have developed new force fields to treat these systems composed by a lanthanoid or actinoid, solvent molecules (water but also organic solvents) and complexes (carbonates and silicates). Then we coupled our results with experiments. The same systems were studied at quantum chemistry level to obtain information on the nature of the interactions. A new point was the coupling between dynamics and electronic structure. Thanks to that, we have obtained quantitative information on solvation free energy and ion effect on acidity of surrounding molecules. Then, it will be possible to study systems containing actinoids for which experiments are very difficult (or impossible) to be performed. Actinoids are very difficult to handle and it is difficult to obtain some experimental data. Simulations show a very close analogy between the two series, in particular concerning solvation. We have now a quantitative map of this analogy that can be used to better understand the experiments done on lanthanoids (easier to do instead of actinoids).
Experimentally, time resolved laser fluorimetry, solubility measures, electrospray mass spectrometry and X-ray absorption spectroscopy were used.

We have characterized in details the hydration of actionoids(III) and Th(IV) developing new polarizable force fields. Due to a strong coupling with experiments (EXAFS in particular) we have understood different aspects and in particular the physical bases of analogies between actinoids and lanthanoids in liquid phase. A review article was published on Chemistry – A European Journal and it was the back-cover of that issue. This opened a new research line on the developing of more advanced theories on ion diffusion in solvents.
Then, studying complexes with carbonates and silicates we have shown that the interaction is mainly electrostatic also in this case and then it was possible to develop polarizable force fields from ionic radii of such ions obtained previously.
Finally, thanks to the ANR we renewed the collaboration with Paola D’Angelo (Università di Roma “La Sapienza”) that is an expert on EXAFS. We have also begun new collaboration with the groups of Laura Gagliardi and Christopher J. Cramer (University of Minnesota, USA) that are experts on multi-reference methods and on characterization of chemical bonds from quantum chemistry calculations. Also a new collaboration with J.-P.Simonin (UPMC) was done on the ion diffusion. Finally, a new collaboration with Enrico Bodo (Università di Roma “La Sapienza”), expert on non-aqueous solvents, was established.
We have obtained an international visibility on the subject, being invited to the show our results at the American Chemical Society conference in mars 2014.

Due to this project polarizable force fields are available to study systems containing actinoids and lanthanoids in liquid phase. They can be used to other systems to have other properties and/or for different thermodynamics conditions (often difficult to study experimentally). The study on non-aqueous solvents can open new perspectives: developing generalized theoretical methods for diffusion and application to liquid/liquid separation of such ions. These force fields and respective molecular dynamics simulations can be used to understand the capture of such ions by complexants in different solvents and then propose new separation procedures between lanthanoids and actinoids and between actinoids. Finally, the same approach can be extended to actinoids and lanthanoids in more structured systems, like ionic liquids and proteins.

A) Force field developing: 1) Polarizable Interaction Potential for Molecular Dynamics Simulations of Actinoids(III) in Liquid Water. J.Chem.Phys.135,044503(2011). 2) Unravelling the hydration structure of ThX4 (X=Br, Cl) water solutions by molecular dynamics simulations and X-ray absorption spectroscopy. J.Phys.Chem.B 116,6465(2012). 3) Hydration properties of Lanthanoid(III) carbonate complexes in liquid water determined by polarizable molecular dynamics simulations. Phys.Chem.Chem.Phys.16,3693(2014). 4) Developing polarizable potential for molecular dynamics of Cm(III)-carbonate complexes in liquid water. J.Mol.Model.20,2398(2014). 5) Uranyl-peroxide Nanocapsules in Aqueous Solution: Force Field Development and First Applications. J.Phys.Chem.C 118,24730(2014).
B) Lanthanoids/actinoids comparison: 1) Lanthanoids(III) and actinoids(III) in water : diffusion coefficients and hydration enthalpies from polarizable molecular dynamics simulations. Pure Appl.Chem.85,237(2013). 2) Hydration of Lanthanoids(III) and Actinoids(III): an Experimental/Theoretical Saga. Chem.Eur.J.18,11162(2012).
C) Theory/experiments coupling: 1) Varying the charge of small cations in liquid water: Structural, transport and thermodynamical properties. J.Chem.Phys.137,164501(2012). 2) K-edge XANES investigation of octakis(DMSO)lanthanoid(III) complexes in DMSO solution and solid iodides. Phys.Chem.Chem.Phys.15,8684(2013). 3) Hydration properties and ionic radii of actinide(III) ions in aqueous solution. Inorg.Chem.52,10318(2013).
D) Reactivity and electronic properties: 1) Electronic Structure and Bonding of Lanthanoid(III) Carbonates. Phys.Chem.Chem.Phys.14,14822(2012). 2) pKa of silicic acid in presence of La3+ using single sweep method coupled to DFT-based molecular dynamics. Mol.Phys.111,3478(2013). 3) Hydration properties of Cm(III) and Th(IV) combining coordination free energy profiles with electronic structure analysis. Phys.Chem.Chem.Phys.16,5824(2014).

The aim of this project is to describe the solvation of actinoids and lanthanoids at the microscopic level. To this end, novel molecular dynamics simulations will be coupled to DFT-based studies and experiments. Some of the experiments will be done in the context of this project in order to go beyond available experimental data from the literature. All simulations will be done within this project and they will be useful to better interpret these. Knowledge of the chemical properties of actinoids (and indirectly also of lanthanoids) has a strong societal impact for their use in nuclear energy processes.

While lanthanoids are present mainly at oxidation state III, actinoids have several oxidation states, especially the first elements in the series, often forming in water oxocations. In our project, we will focus on oxidation state III and some An(IV). Thus we can shed light on analogies and differences between the two series bare cations.

In particular, we will treat solvation in water, forming aqueous complexes with carbonates and silicates for An(III) and Ln(III), and in two different solvents, dimethylformamide (DMF) and dimethylsulfoxide (DMSO) for An(III) and (IV) and Ln(III). We will develop polarizable classical interaction potentials, using the same approach developed by us for Ln(III) hydration, that will be coupled with different experimental techniques: X-ray absorption spectroscopy (and EXAFS in particular), providing structural properties and time-resolved laser induced fluorescence spectroscopy (TRLFS) aimed to understand complexation with silicates. These last will be coupled to electrospray mass spectrometry (ESI-MS) to solve the question of nature of ligands, i.e. mono or poly silicates.

Our project is divided in three tasks where different ligands will be investigated and a fourth task where, making the use of DFT-based dynamics and using results of other tasks, we will be able to understand the analogies and differences between Ln(III) and An(III) in solution.
In particular in task 1 we will investigate An(III) hydration with classical molecular dynamics. In task 2 we will pass to aqueous complexes of An(III) and Ln(III) with carbonates and silicates by means of classical molecular dynamics and TRLFS experiments for silicates. In task 3 we will understand solvation behavior of An(III), An(IV) and Ln(III) in DMF and DMSO by coupling simulations with EXAFS experiments, providing also information on An(IV) properties in non-aqueous solvents. In parallel, starting after the achievement of task 1, we will develop task 4 that will provide us the microscopical basis of An(III)/Ln(III) analogies and differences. Further, by means of DFT-based simulations we will have an additional validation of classical force fields and we will give the basis for a possible employment of these methods to actinoids and lanthanoids reactivity.

Within this project, we will put together young researchers of both academical institutions and CEA on a subject that has fundamental key questions but also a connection with the field of nuclear waste management. The project coordinator, R.Spezia (CNRS, LAMBE) has experience on solvation by means of both classical and DFT-based dynamics. Within this project he will be able to build up a team on very heavy metal ions solvation that can be a reference in Europe, since it will put together experiments (that on actinoids are limited to few centers) with theoretical developments. R.Vuilleumier (CNRS, PASTEUR) will provide his competences in DFT-based dynamics. T.Vercouter (CEA, Saclay) works on complexation reactions for actinoids with environmentally-relevant inorganic ligands by spectroscopic and spectrometric techniques and he will carry out TRLFS experiments on Ln(III) and An(III) interacting with silicates in water. C.Fillaux and C.Den Auwer (CEA, Marcoule) will be the EXAFS experimentalists needed for the team, since they are experts on actinoids XAS.