Revisiting the identification of structural units in aqueous silicate solutions by two-dimensional silicon-29 INADEQUATE

Si-29-Si-29 INADEQUATE experiments have been successfully used on sodium aqueous silicate partially enriched (19% of Si-29 isotope) solutions in order to gain connectivity information and to help in spectral assignments. The structures of almost all known species from the literature were confirmed by this technique. Moreover, accurate scalar J couplings were extracted. The two-dimensional (2D) spectra demonstrated additional cross correlations for some Si-Si bonds never reported before, representing additional oligomeric species. They reveal five silicate cages with the following connectivity sets: Q(2)-Q(2)-Q(3) or Q(3 Delta)-Q(3 Delta)-Q(3) for one species, Q(2)-Q(2) or Q(3 Delta)-Q(3 Delta) for three species (Q(3 Delta) indicates a three-connected silicon in a three-membered cycle), and Q(3)-Q(3) for one species. All possible molecular graphs for anions containing silicon up to 11 atoms, matching these sequential connnectivities, were enumerated. Additionally, a direct comparison between resonances observed in the 2D experiments with that obtained in the ID spectrum allows us to assign single-sited silicate anions. Only a few species reported before were not detected, most probably due to their too low concentration in our solutions.