CALCULATION OF NMR SHIELDINGS AND OTHER PROPERTIES FOR 3 AND 5 COORDINATE SI, 3 COORDINATE-O AND SOME SILOXANE AND BOROXOL RING COMPOUNDS

Valuable information on the geometric and electronic structure of both predominant and defect species in the bulk and on the surface of amorphous SiO2 can be obtained from solid state 29Si and 17O NMR. Most previous NMR.studies have made assignments of structure types to NMR signals by relying on a comparison to crystalline reference materials. Using the quantum mechanical technique of Coupled Hartree-Fock Perturbation Theory (CHFPT) we can directly calculate NMR shielding tensors, σ, for Si and O and electric field gradients tensors, q for O in silicates. Previous CHFPT calculations on (SiH3)2O at 〈SiOSi = 180° to 140° semiquantitatively reproduced observed trends in σ for SiO2 polymorphs. These calculations have been improved and extended to 〈SiOSi = 120° in (SiH3)2O and to the small siloxane rings H4Si2O2, H6Si3O3, H8,Si4O4 which are models for the ring structures which may occur in the bulk and surface of amorphous SiO2. Thus, predictions can be made of the 17O and 29Si NMR characteristic of such species. We have also calculated NMR shieldings and other properties for three and five coordinate Si and for three coordinate O. The 29Si NMR shielding increases with coordination, as expected, and five coordinate Si shows an anisotropy which although large is still smaller than that inferred from spin-lattice relaxation times in silicate melts. O q values are also calculated for (BH2)2O and H3B3O6. The results are consistent with O EFGs and Raman frequencies observed in B2O3 glass, supporting a boroxol ring model. © 1990.