Spin—orbit effect on the magnetic shielding constant using the ab initio UHF method: silicon tetrahalides

The ²⁹Si NMR chemical shifts of silane, SiH ₄, and silicon tetrahalides, SiX ₄ (X = F, Cl, Br and I) and SiXI ₃ (X = Cl and Br), are calculated by the ab initio unrestricted Hartree-Fock/finite perturbation method including the spin-orbit (SO) interaction proposed previously. The SO effect is included through the effective core potentials for the halogens. The chemical shifts calculated with the SO effects show good agreement with experiment for all the compounds studied. The SO effects of the halogen ligands, especially of bromine and iodine, are large and move the chemical shift to higher magnetic field. The inverse halogen dependence on the substitution of F by Cl is derived from the paramagnetic term, but the normal halogen dependence on the substitution from Cl to I is caused mainly by the SO effect.