Accurate First-Principle Prediction of 29Si and 17O NMR Parameters in SiO2 Polymorphs: The Cases of Zeolites Sigma-2 and Ferrierite
ABSTRACT 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 29Si and 17O isotropic and principal components of the chemical shift tensors together with 17O quadrupolar coupling constants. The NMR parameters have been computed on all known silica systems, namely, R-quartz, R-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 17O NMR parameters with respect to cluster size is found to be system-dependent. Excellent agreement between computed and experimental data has been found for 29Si NMR parameters of the different Si sites of silica polymorphs and of Sigma-2 and ferrierite zeolites.