Ab initio calculations of oxygen nuclear quadrupole coupling constants and oxygen and silicon NMR shielding constants in molecules containing Si-O bonds

Ab initio coupled Hartree—Fock perturbation theory (CHFPT) calculations employing large gaussian basis sets have been used to evaluate the electric field gradient at O, qo, and the NMR shieldings at O and Si, σo and σSi, in the molecules SiO, SiO2, Si2O2, H2SiO, SiO−44, Si(OH)4 and (H3Si)2O. Species containing Si bonded to three or fewer atoms have small NMR shieldings at both O and Si while those with four-coordinate Si have systematically larger O and Si shieldings. A significant positive correlation is observed between calculated O and Si NMR shieldings. Reduction of the Si---O distance in SiO4−4 and H3SiOSiH3 gives a significant reduction in the magnitude of qo and a small increase in σ. Anisotropies in σSi are large for two- and three-coordinate Si (200–900 ppm) but for (H3Si)2O the σSi anisotropy is only ≈60 ppm. Anisotropies in σo are generally larger than those in σSi, with values larger than 200 ppm for both SiO−44 and (H3Si)2O. Values of qo for SiO4−4 and (H3Si)2O are in qualitative agreement with experimentally determined values for nesosilicates and SiO2 polymorphs, respectively, but all the qo values appear to be exaggerated at the Hartree—Fock level. Also, qo values are not greatly different for the exotic species SiO, Si2O2, etc. compared to the typical silicate models SiO4−4 and (H3Si)2O. Calculated isotropic chemical shifts yield good values for the Si chemical shift differences of SiF4, SiO4−4 and (H3Si)2O and for the O chemical shift difference of SiO4−4 and H2O. For SiO4−4 the paramagnetic contribution to the Si shift, σpSi is dominated by contributions from the t2 symmetry Si---O bonding orbital and σpO is dominated by contributions from the t2 symmetry O 2p non-bonding orbital.