Accurate First-Principle Prediction of (29)Si and (17)O NMR Parameters in SiO2 Polymorphs: The Cases of Zeolites Sigma-2 and Ferrierite.

The magnetic shielding tensors of silica polymorphs have been investigated by means of quantum chemical calculations. Several levels of theory, from Hartree-Fock to the last generation of Density Functional Theory based approaches, have been tested on predicting (29)Si and (17)O isotropic and principal components of the chemical shift tensors together with (17)O quadrupolar coupling constants. The NMR parameters have been computed on all known silica systems, namely, α-quartz, α-cristobalite, coesite, Sigma-2, and ferrierite zeolites. Besides, cluster based approaches have been compared to a hybrid Quantum-Mechanics/Molecular-Mechanics (QM/MM) method, within the ONIOM scheme. The convergence of computed (17)O NMR parameters with respect to cluster size is found to be system-dependent. Excellent agreement between computed and experimental data has been found for (29)Si NMR parameters of the different Si sites of silica polymorphs and of Sigma-2 and ferrierite zeolites.