On the ``sagging pattern'' of 29Si NMR shieldings in the series SiH_a F_b, a+b=4.

Ab initio coupled Hartree-Fock perturbation theory employing large Gaussian orbital basis sets has been used to calculate the 29Si NMR shielding constants σ in the series SiHaFb, a+b=4. Calculated isotropic shieldings σ reproduce the observed “sagging pattern” (minimum shielding for SiH3F) of chemical shifts δ. For the gauge origin at Si, there is a smooth increase in the diamagnetic shielding σd from SiH4 to SiF4 but an abrupt increase in the paramagnetic shielding σp in going from SiH4 to SiH3F with little change from SiH3F to SiF4.