Exploiting ¹H→²⁹Si Cross-Polarization Features for Structural Characterization of Inorganic Materials

High-resolution solid state ²⁹Si NMR spectroscopy was frequently used during the last 25 years in structural studies of inorganic materials including zeolites [1–3], minerals [1–3], glasses [3], and cement-based systems [4]. Its important place in structure determination of these materials relies on the fact that ²⁹Si NMR spectra permit a precise determination of the ²⁹Si isotropic chemical shift in different silicon environments of powdered samples. Indeed, for materials with silicon in tetrahedral coordination, the isotropic position of resonance signal provides immediately the degree of condensation of SiO₄ terahedra (Q ⁽ⁿ⁾, with n = 0, 1, 2, 3, 4) and informs about Si-O-Si bond angles and Si-O bond lengths [1–3]. Although the easiest way to record the quantitative ²⁹Si NMR spectrum is a direct excitation by a single pulse, this cannot be reasonably applied in most cases due to extremely long ²⁹Si longitudinal relaxation times. To avoid this inconvenience, one can take advantage of magnetization transfer from protons to ²⁹Si spins.