H, Si and Sn double and triple resonance NMR spectroscopy of the small-pore framework sodium stannosilicate NaSnSiO·2HO.

The small-pore framework sodium stannosilicate AV-10, chemical composition NaSnSiO·2HO and known crystallographic structure, was synthesized by hydrothermal crystallization. This stannosilicate is built up of a three-dimensional network of corner-shared SiO tetrahedra and SnO octahedra. The SnO sites are linked to six SiO tetrahedra (Sn(6Si)) while each of the two crystallographically different SiO units are connected to two SnO and SiO units (Si(2Si,2Sn)). This material was used as model compound for developing a solid-state MAS NMR strategy aimed on the challenges and possibilities for structural studies, particularly considering the short and medium range order to verify the connectivity of SiO and SnO of such compounds despite the low natural abundances of 4.68% for Si and 8.59% for Sn nuclei as a real challenge. Si{Sn} and Sn{Si} REDOR (Rotational-Echo Double-Resonance) NMR measurements after H cross-polarization (CP) were carried out. The REDOR curves show a significant change after the "normal" quadratic short time evolution from which both (i) the shortest internuclear Si - Sn distances (and vice versa) and (ii) the number of corner-sharing SiO tetrahedra around the SnO octahedra (and vice versa) can be obtained. Based on these data, optimized Si{Sn} and Sn{Si} REPT-HMQC (Recoupled Polarization Transfer-Heteronuclear Multiple-Quantum Correlation, again after H CP) experiments were implemented, which directly show those heterogroup connectivity as correlation peaks in a 2D spectrum. This information was also obtained using 2DSi{Sn}-J-Coupling NMR experiments. Furthermore, 2DSi INADEQUATE NMR experiments are also feasible, showing the connectivity of SiO tetrahedra. The combination of REDOR, REPT-HMQC, J-Coupling and INADEQUATE experiments yielded a complete analysis of the short and medium range structure of this microporous stannosilicate, in agreement with the previously published structure obtained Ab Initio from powder X-Ray diffraction data (XRD).