Calculating NMR chemical shifts for paramagnetic metal complexes from first-principles

Paramagnetic effects on NMR shifts (pNMR) for paramagnetic metal complexes are calculated from first-principles, without recourse to spin Hamiltonian parameters. A newly developed code based on complete active space (CAS) and restricted active space (RAS) techniques in conjunction with treating spin-orbit (SO) coupling via state interaction is applied to ¹³C NMR shifts of actinyl tris-carbonate complexes, specifically [UO₂(CO₃)₃]^5- and [NpO₂(CO₃)₃]^4-. The experimental pNMR shifts as well as the sizable difference of the ¹³C NMR shift for these iso-electronic species are well reproduced by the calculations. Approximations to the pNMR shift equations using spin Hamiltonian parameters or the magnetic susceptibility are calculated for the same systems at the same level of theory, and it is shown how the approximations relate to the ab initio data.