Ab initio Coupled Hartree-Fock calculation of the 29Si NMR shielding constants in SiH_4, Si_2H_6, Si_2H_4 and H_2SiO

Ab initio coupled Hartree-Fock perturbation theory (CHFPT) using large, flexible bases of gaugeless Gaussian orbitals is used to calculate the 29Si chemical shielding tensors, σαβ, of SiH4, Si2H6, Si2H4 and H2SiO. Si2H4 is deshielded with respect to SiH4 by an average of 134 ppm and the range of principal axis values of the shielding tensor is 291 ppm. In contrast, SiH6 is shielded with respect to SiH4, by 21 ppm on the average and its range of principal axis shielding values is only 23 ppm. The large deshielding and anisotropy of Si2H4 arise primarily from excitations of electrons from the Si-Si sigma bonding (σb) MO into the Si-Si π* orbital induced by the magnetic field component lying in the molecular plane and perpendicular to the Si-Si axis.