Ligand NMR Chemical Shift Calculations for Paramagnetic Metal Complexes: 5f1 vs 5f2 Actinides.

Ligand paramagnetic NMR (pNMR) chemical shifts of the 5f1 complexes UO2(CO3)35- and NpO2(CO3)34-, and of the 5f2 complexes PuO2(CO3)34- and (C5H5)3UCH3 are investigated by wave function theory calculations, using a recently developed sum-over-states approach within complete active space and restricted active space paradigm including spin-orbit (SO) coupling [J. Phys. Chem. Lett. 2015, 20, 2183-2188]. The experimental 13C pNMR shifts of the actinyl tris-carbonate complexes are well reproduced by the calculations. The results are rationalized by visualizing natural spin orbitals (NSOs) and spin-magnetizations generated from the SO wave functions, in comparison with scalar relativistic spin densities. The analysis reveals a complex balance between spin-polarization, spin and orbital magnetization delocalization, and spin-compensation effects due to SO coupling. This balance creates the magnetization due to the electron paramagnetism around the nucleus of interest, and therefore the pNMR effects. The calculated proton pNMR shifts of the (C5H5)3UCH3 complex are also in good agreement with experimental data. Because of the nonmagnetic ground state of (C5H5)3UCH3, the 1H pNMR shifts arise mainly from the magnetic coupling contributions between the ground state and low-energy excited states belonging to the 5f manifold, along with the thermal population of degenerate excited states at ambient temperatures.