Heavy atom effect on 14 group nuclear shielding constant of SiX(4) and CH(4-n)X(n) (X=Cl, Br, I; n=1, 2, 3, 4)

A sum-over-states third-order perturbation theory using a semiempirical wavefunction parameteric method 3 was applied to Si-29 and C-13 nuclear magnetic shielding constants of SiX(4) and CH(4-n)X(n), (X=Cl, Pr, I; n=1, 2, 3, 4), respectively. The other new spin-orbit (LS) term, sigma(LS)(A), was proposed to come from the cross term involving the halogen spin-orbit coupling, Fermi I . S interaction and the external field-(carbon or silicon) orbit interaction. The values using the MO method predicted a substantial contribution (25%) of sigma(LS)(Si or C) to the sigma(LS)(X) as already given. The results agreed fairly well with the experimental values, indicating the importance of the spin-orbit effect on the chemical shifts of molecules containing halogen heavier atoms. The dominant contribution of electronic states to the LS shifts is due to the excitations from all occupied MOs to only all vacant sigma(Si(or C)-x)* antibonding MOs, since the Fermi interaction intervenes between the shift contributions.