AB-INITIO CALCULATIONS OF THE GEOMETRIES AND THE NUCLEAR-MAGNETIC-RESONANCE SHIELDING CONSTANTS OF THE SILANE DERIVATIVES (CH3)(4-N)SICLN AND (SIH3)(4-N)SIHN (N=0-4)

Optimized geometries of the molecules SiX(4) (X=H, CH3, SiH3, Cl) are presented using Hartree-Fock SCF calculations with several basis sets and MP2/6-31G* calculations (except for X=Cl). The Si-H bond length converges and gives the experimental value already when using SCF/DZ calculations. The use of polarization functions is required for the calculations of Si-C and Si-Cl bond lengths. Even though the inclusion of correlation effects leads to a negligibly small increase of the Si-H and Si-C bond lengths, it also leads to a remarkable shortening of the Si-Si bond length to a value of 234 pm, comparable with the experimental value for the Si-Si bond in disilane. SCF/DZ2P geometries of the compounds (CH3)(4-n)SiCln and (SiH3)(4-n)SiHn (n = 0-4) have been used as input data for the calculations of NMR shielding constants. Si-29 NMR shielding constants are presented for these compounds as calculated by the IGLO method. For the silyl silane series, a very good agreement is observed between experimental and calculated Si-29 chemical shifts. The calculated silicon chemical shifts in the methylchlorosilane series show an increasing deviation from experimental values with increasing number of chlorine atoms. The silicon chemical shifts are due mainly to the changes of the shielding contributions of the bonds from the silicon atom of interest to its nearest neighbours. The contributions of the inner shells and of the chlorine lone pairs to the substituent dependence of the Si-29 chemical shifts are small. The dependence of the Si-29, C-13, and H-1 shielding constants on the geometry has been studied for tetramethylsilane (TMS), which is the preferred reference compound for the respective NMR chemical shifts.