An ab initio calculation of 17O and 29Si NMR parameters for SiO2 polymorphs

Ab initio molecular orbital calculations (Hartree-Fock, KF and density functional theories, DFTs) have been carried out for SiO2 polymorphs coesite, low cristobalite, and alpha-quartz, in order to investigate the reliability of this method for predicting Si-29 and O-17 nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the Si-29 and O-17 chemical shifts (delta(i)(Si) and delta(i)(O)), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the delta(i)(Si) and delta(i)(O) have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated delta(i)(Si) values agree well (within +/- 1 ppm) with experimental data. The calculated O-17 electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in delta(i)(O) for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting Si-29 and O-17 NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method. (C) 2000 Elsevier Science B.V. All rights reserved.