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 H-1 and Si-29 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 H-1 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 Si-29 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 H-1 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. Si-29 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 Q(4) silicons, whereas the signal from Q(3) silicons remains unaffected. The potential applicability of solid-state NMR methods for measuring internuclear distances in this material is discussed.