Ordering of Silicate Layers by Hydrogen-Bonding Networks:  Solid State NMR Investigation of the Perfect Three-Dimensional Registration in the Layer Silicate RUB-18

Solid state 1H and 29Si NMR spectroscopies were applied to the layer silicate RUB-18 in order to investigate the hydrogen bonding network providing the three-dimensional ordering of the silicate layers. Two 1H NMR signals corresponding to water molecules and strongly hydrogen-bonded silanol groups were resolved, the relative intensities of which are in good agreement with the recently proposed structure. By means of two-dimensional heteronuclear correlation NMR, the protons in hydroxyl groups are clearly identified as the source of polarization transfer to the 29Si nuclei of the silicate layer. The dynamics of the exchange process between the protons in hydroxyl groups and those in the water molecules were investigated by two-dimensional 1H exchange spectroscopy, giving insight into the nature of the exchange processchemical exchange or spin diffusion. In addition, the ratio of exchanging protons of the two species has been found to differ from the stoichiometric ratio. 29Si cross-polarization/magic angle spinning (CP MAS) NMR spectra taken at temperatures down to 140 K lead to enhanced resolution in the spectral region of Q4 silicons, whereas the signal from Q3 silicons remains unaffected. The potential applicability of solid-state NMR methods for measuring internuclear distances in this material is discussed.