Speaker
Description
Paramagnetic NMR measures the NMR chemical shift of paramagnetic complexes compared to their diamagnetic counterpart. This shift may be split into
dipolar terms, arising from the magnetic dipolar interaction between the metal center and the NMR active nucleus, and the contact term, due to the spin delocalization leaking from the paramagnetic center towards the ligand [1]. The separation of these two contributions is often a bottleneck [2]. In this presentation, we will analyze the magnetic properties of three tetravalent actinide hexanitrates [AnIV(NO3)6]2–complexes with An = U, Np, Pu, (see Figure below) based on paramagnetic NMR and ab initio calculations. With
pyritohedral symmetry (Th), the dipolar contribution is quenched by symmetry and consequently, the Actinide Induced Shifts (AIS) are from pure
contact origin. The AIS are calculated by quantum chemical tools, combining in good agreement with the experimental values. There is an alternation of sign on the ligands CAS based and DFT methods, due to spin polarization. The hyperfine coupling (HFC) constants are deduced from the AIS and calculated by unrestricted DFT methods. The high symmetry and the simple structure of the ligands allow a decomposition of the HFC constants in terms of molecular orbitals contributions. This reveals the differential role of the core and valence orbitals depending on the position of the probed nucleus on the ligand and on the actinide cation, leading to information about the actinide- metal bonding scheme.
