Dipolar and Contact Paramagnetic NMR Chemical Shifts in An IV Complexes with Dipicolinic Acid Derivatives.

Actinide +IV complexes (An = Th, U, Np, and Pu) with two dipicolinic acid derivatives (DPA and Et-DPA) have been studied by H and C NMR spectroscopies and first-principles calculations. The Fermi contact and dipolar contributions to the actinide-induced shifts (AIS) are evaluated from a temperature dependence analysis, combined with results. It allows an experimental estimation of the axial anisotropy of the magnetic susceptibility Δχ and of the hyperfine coupling constants of the NMR-active nuclei. Due to the compactness of the coordination sphere, the magnetic anisotropy of the paramagnetic center is small, and this makes the contact contribution to be the dominant one, even on the remote atoms. The sign of the hyperfine coupling constants and related spin densities is alternating on the nuclei of the ligand cycle, denoting a preponderant spin polarization mechanism. This is well reproduced by unrestricted density functional theory (DFT) calculations. Those values are furthermore slightly decreasing in the actinide series, which indicates a small decrease of the covalency from U to Pu.