EFFECTS OF H-1-H-1-SPIN EXCHANGE IN THE SI-29 CP-MAS NMR-SPECTRA OF THE SILICA SURFACE

Four Si-29 CP-MAS NMR pulse sequences have been used to study the effects of H-1 spin exchange among various H-1 spin reservoirs on a silica gel surface. Both H-1-Si-29 dipolar-dephasing experiments and proton-coupled Si-29 CP-MAS results indicate that various hydroxyl groups of silanols in silica gel undergo rapid H-1 spin exchange and that the most strongly coupled protons provide the dominant source of cross polarization to geminal-silanol silicons. H-1-H-1 dipolar dephasing prior to H-1 --> Si-29 cross polarization shows a rapid H-1 spin exchange between H-1 reservoirs of single-silanol groups and of geminal-silanol groups; however, the H-1 spin exchange rate is too fast to be measured by this strategy. A slower H-1-H-1 dipolar-dephasing decay due to H-1 spin exchanges is found for the H-1 spin reservoir of single silanol groups. A pulse sequence that utilizes a H-1-H-1 dipolar-dephasing period followed by an evolution period for z-axis spin exchange prior to H-1 --> Si-29 cross polarization provides a measure of a proton (chemical) exchange rate. The results are discussed in terms of beta-cristobalite models of the silica surface.