paramagnetic NMR shifts in actinide complexes : beyond the lanthanide model – ACTIpNMR

The presence of covalency in complexes of the 4f and 5f elements has been a source of intense research and controversy. This covalency have been probed by many spectroscopies, but there has been little use of NMR. The NMR shifts of paramagnetic substances have two contributions; the dipolar term arises from the dipolar magnetic interaction between the electronic magnetic moment of the paramagnetic center and the nuclear magnetic moment of the nucleus of interest, while the contact term is caused by a non zero spin density at the nucleus and is a signature of a spin density delocalization between the metal center and the ligand ; it probes the degree of covalency between the paramagnetic center and the ligand. For lanthanide complexes, these two contributions are unraveled thanks to a model proposed by Bleaney in the 70's, based on temperature and position dependence of the shifts. But this model is not suitable for actinides due to the larger interaction of the actinide orbitals with the ligands. The aim of the actipNMR project is to derive a model valid for actinides.

To achieve that, the consortium gathers a team of experimentalists expert of pNMR in actinide complexes, a team of developers of relativistic quantum chemistry tools and a team of quantum chemists specialist in molecular magnetism. pNMR and X-ray data will be collected in a complete series of actinide complexes with the DOTAM ligand, new codes devoted to the calculation of pNMR shifts in actinide chemistry will be developed, the pNMR shifts will be computed in the whole series and the temperature dependence analyzed. This temperature dependence will be modeled either as a refinement of Bleaney's model or using a new paradigm, describing the properties of the complex thanks to statistical descriptors. This model should permit the discrimination between contact and dipolar contributions from experimental data, and consequently, to probe the degree of covalency in the late 5f actinides with N-donors ligand. This should impact the choice of the ligand for extraction processes.