Determining the geometry of strongly hydrogen-bonded silanols in a layered hydrous silicate by solid-state nuclear magnetic resonance

High-resolution solid-state NMR spectroscopy is exploited to obtain structural constraints involving strongly hydrogen-bonded silanols in octosilicate, a prominent member of the layered hydrous sodium silicates. Proton-silicon cross-polarization dynamics reveals that octosilicate contains two types of Q3 silicons present in hydrogen-bonded –Si–O–H⋯O–Si– and –Si–O−–type sites which can only be distinguished by their different abilities to cross polarize and the different mobilities of neighboring hydrous species. The theoretical analysis of the oscillating components of the polarization transfer buildup curves suggests that the model of heteronuclear pairs is an adequate description of the quantum spin system within hydrogen-bonded –Si–O–H⋯O–Si– fragments. We also show that dipolar modulated, slow speed magic-angle Si29 NMR spectrum provides unique geometric information on strongly hydrogen-bonded silanols. The dipolar modulated spinning sidebands contain all the information necessary to determine the internuclear Si⋯H distances as well as the magnitude and orientation of the principal elements of the Si29 chemical shielding tensor in the molecular frame. The data provide definite proof of the intralayer character of strongly hydrogen-bonded silanol groups in a bridging, albeit not symmetric, position between neighboring tetrahedra. The approach developed in this work may be useful to obtain structural information on related layered alkali metal silicates, silica gels as well as on other classes of microporous materials.