Two-dimensional silicon-29 nuclear magnetic resonance spectroscopic study of chemical exchange pathways in potassium silicate solutions

The results of two-dimensional silicon-29 nuclear magnetic resonance exchange spectroscopy (EXSY) experiments on ²⁹Si isotopically enriched potassium silicate solutions are reported The EXSY spectra reveal individual silicate anion exchange pathways and provide evidence for the occurrence of at least six single-step exchange processes in moderately concentrated potassium silicate solutions. Four are intermolecular, involving the monomeric anion, Q⁰, the formation and dissociation of the dimer, Q₂¹; the formation and dissociation of the linear trimer, Q₂¹Q²; the formation and dissociation of the linear tetramer, Q₂¹Q²²; and the formation and dissociation of the substituted cyclic trimer, Q²Q³Q₂². The remaining two are intermolecular and consist of ring opening/ring closing reactions of the cyclic trimer, Q₃², and an internal rearrangement of the bicyclic pentamer, Q²Q₃²Q₂². There is also evidence for the occurrence of additional exchange processes, although the congested nature of the ²⁹Si NMR spectrum introduces some uncertainties. Contrary to the generally accepted picture derived from linewidth considerations, the cyclic trimer appears to be rather labile, undergoing rapid ring opening and closing reactions, whereas the cyclic tetramer is considerably less reactive.