Ab initio calculations of 29Si NMR chemical shifts for some gas phase and solid state silicon fluorides and oxides

Ab initio coupled Hartree–Fock perturbation theory (CHFPT) employing near Hartree–Fock basis sets has been used to calculate the 29Si NMR chemical shifts of SiF4(Td), SiF^-₅ (D3h), SiF^{- 2}₆ (Oh), and SiO^{- 4}₄ (in both Td and C3v geometries). The increase in the shielding constant, sigma, from SiF4 to SiF^{- 2}₆ is calculated to be 114 ppm, compared to an experimental value of about 75 ppm, while the decrease in sigma between SiF4 to SiO^{- 4}₄ is calculated to be 43 ppm, compared to an experimental value of about 40 ppm. These results indicate that CHFPT applied to properly chosen molecular cluster models for solids can accurately reproduce experimental trends in solid state NMR chemical shifts. Calculations on SiF4 and SiO^{- 4}₄ for different internuclear distances also establish that the chemical shift is very weakly dependent on distance, suggesting that observed correlations between chemical shift and average Si–O distance in silicates arise indirectly, through the relationship between degree of polymerization and the average Si–O bond distance. By contrast, a C3v distorted SiO^{- 4}₄ tetrahedron (one bond 1.584 Å long and three bonds 1.651 Å long) gives an anisotropy in the chemical shift tensor of 54 ppm, with the larger value of sigma occurring along the direction of the short Si–O bond, in reasonable agreement with available experiment. The calculations also support the use of the simple superimposed free atom approximation for the diamagnetic contribution to sigma. For the Td species the paramagnetic contribution is shown to be dominated by excitations from the t2 sigma orbitals to the sigma* orbitals, as suggested previously on qualitative grounds. It may therefore be possible to interpret trends in sigma by focusing on the energies and compositions of the t2 sigma orbitals.