Contribution of Molecular Dynamics in pNMR for the Structural Determination of An^V and An^VI Complexes in Solution

Abstract

In this study, we propose to use classical molecular dynamics (MD) coupled with 1 H NMR spectroscopy to study the conformations of different actinyl An VI (An = U, Np, 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 1 H 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.

Are Actinyl Cations Good Probes for Structure Determination in Solution by NMR?

“…In the case of the 1:2 complex, the MD study revealed all of the possible conformations that can be found in solution through two different modes of coordination in the equatorial plane of the cation. 25 MD simulations were in agreement with EXAFS results. The different structures differ by the number of oxygen atoms in the first coordination sphere of the actinide cation, five or six.…”

“…With regard to the 1:1 complexes from U VI to Pu VI , MD simulations show that two water molecules are in the first coordination sphere with the ether and two amide oxygens of the TEDGA coordinated (conformation denoted 2W) . In addition to this 5c structure, another complex with a third water molecule but only two amide oxygens in the first coordination sphere was observed by MD simulations (conformation denoted 3W).…”

“…Small standard deviations are, however, obtained for the 1:1 complex. As shown by MD simulation, 1:2 complexes exist in a large number of conformations with two different coordination modes, and one can expect a mixture of them in solution while the 1:1 stoichiometry presents a reduced number of conformers . The selected conformations [6c­(TTT), 6c­(CCC), and 5c­(TTT)] are consequently not representative enough of the mixture in solution, and this can explain the quite large standard deviations.…”

“…Previous molecular dynamics (MD) simulations showed that in the 1:2 complexes in acetonitrile, the oxygen atom (O2) appears mainly above or below the coordination plane and not coordinated to the metallic center on the nanosecond scale. 25 Consequently, the 1:2 complexes may exist in solution in a dissymmetric structure with a coordination number of five instead of six as shown by SC-XRD. Nevertheless, different representative conformations of the 1:2 complex differing in their ethyl chain orientations and coordination numbers were taken into account.…”

“…With regard to the 1:1 complexes, MD simulations also show that the first coordination sphere of the actinyl is mainly completed by two water molecules rather than by three with a tridendate TEDGA ligand. 25 The collected 1 H NMR shifts of the 1:1 and 1:2 complexes are analyzed within the dipolar approximation like in the previous study using MD simulations. On the basis of the conformational information drawn from MD, we focus here on the magnetic susceptibility tensors from an ab initio point of view to obtain explanations at the electronic level.…”