Spin-orbit effect on the magnetic shielding constant: niobium hexahalides and titanium tetrahalides

The 93Nb and 47Ti NMR chemical shifts of niobium hexahalides and titanium tetrahalides are studied theoretically by the ab initio UHF/finite perturbation method including the spin-orbit (SO) interaction. The calculated chemical shifts agree well with experiment for both the Nb and Ti compounds. In contrast to the halides of main-group elements studied previously, the SO effect is generally small for early transition metal halides. The origin of the chemical shifts lies in the d-orbital contribution of the paramagnetic term and is due to the d-d∗ excitation mechanism. Soft d-orbitals adsorb the SO effect, leaving only a small net spin-orbit effect. The chemical shift shows a monotonic downfield shift as the halogen ligand becomes heavier (inverse halogen dependence), in contrast to the normal halogen dependence observed for the halides of main-group elements. It is expected that such a result is common to other transition metal halides in which the transition metal atom has an open d subshell.