SPIN-ORBIT EFFECT ON THE MAGNETIC SHIELDING CONSTANT - NIOBIUM HEXAHALIDES AND TITANIUM TETRAHALIDES

The Nb-93 and Ti-47 NMR chemical shifts of niobium hexahalides and tit anium tetrahalides are studied theoretically by the ab initio UHF/fini te 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 st udied previously, the SO effect is generally small for early transitio n metal halides. The origin of the chemical shifts lies in the d-orbit al contribution of the paramagnetic term and is due to the d-d excita tion mechanism. Soft d-orbitals adsorb the SO effect, leaving only a s mall net spin-orbit effect. The chemical shift shows a monotonic downf ield shift as the halogen ligand becomes heavier (inverse halogen depe ndence), 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, (C) 1997 Elsevier Science B.V.