AN NMR INVESTIGATION OF THE ACTINIDE IONS AND THEIR COMPLEXES

Abstract

We currently use several advanced NMR techniques in order to fully characterize actinide ions and their complexes in water or in organic solvents. The dispersion of the longitudinal relaxation time T1 of solvent nuclei with the magnetic field (NMRD) yields information on the magnetic properties and on the dynamic behavior of paramagnetic species. 17O NMR allows the measurement of the water exchange times and 1H and 13C spectra yield information on the solution structures of the complexes and on the covalency of their coordination bonds. The application of NMR in actinide science will be illustrated with studies on the U, Np, Pu and Cm ions in different oxidation states and on their complexes. For instance, Cm3+ ion is the actinide analogue of Gd3+ but is not in a pure 8S state as indicated by much lower relaxation rates and much shortened electronic relaxation rates. In keeping with EPR studies1, Cm3+ does not have a perfectly spherical distribution of its unpaired electronic spins because of a much stronger spin-orbit coupling. Moreover, the Cm3+ relaxivity originates from three different processes: a dipolar coupling between the nuclear and electronic spins, a delocalization of unpaired electronic spins into the solvent orbitals (contact interaction) and a Curie contribution. Each process gives rise to an inflection point in the NMRD curves and the contact interaction reflects the partial covalency of the coordination bonds formed by Cm3+. A contact contribution is also observed in the NMR spectra of Cm3+ complexes. The sensitivity of NMR to the exact nature of the ground state of actinide ions is also illustrated by detailed studies on the U, Np and Pu ions in different oxidation states. For instance, a comparison of the NMRD curves of the 5f2 ions U4+, NpO2+ and PuO22+ indicates that the two dioxo cations have abnormally long electronic relaxation times. However, well-resolved 1H NMR spectra of their complexes can be obtained provided the solution species are sufficiently rigid. It will be shown that NpO2+ and PuO22+ induce dipolar paramagnetic shifts from which the solution structure can be deduced