The connectivity of silicon sites in silicate glasses, as determined by two-dimensional ²⁹Si nuclear magnetic resonance spectroscopy

The application of two-dimensional silicon-29 homonuclear shift correlated magic angle spinning nuclear magnetic resonance (COSY MAS NMR) spectroscopy to silicate glasses prepared using silica isotopically enriched in silicon-29 is reported. Two sodium silicate glass compositions and a sodium phospho-silicate glass were examined. The results suggest that a substantial proportion of the four coordinate silicate sites in sodium silicate glasses Q²/Q³ and Q³/Q⁴ are interconnected on a nearest neighbor level. Experiments with the sodium silicate glasses prepared with varying levels of isotopic enrichment reveal a correlation between the intensity of the cross peak and the percentage of silicon-29 in the sample, confirming this observation. No cross peaks were observed for the sodium phospho-silicate glass, implying that the Q⁴ and six coordinate silicate sites present are not linked together at the nearest neighbor level.