INTERIOR HYDROXYLS OF THE SILICA-GEL SYSTEM AS STUDIED BY SI-29 CP-MAS NMR-SPECTROSCOPY

Silica gels were deuterium-exchanged with D2O at 25 and 100-degrees-C. Si-29 CP-MAS NMR results indicate that for a silica gel with lower surface area, 9.3% of the single silanols remains undeuterated at 25-degrees-C and 4.0% of the single silanols remains undeuterated at 100-degrees-C. For a higher surface-area silica gel, only 3.0% of the single silanols was not deuterated at 25-degrees-C. In all deuterium-exchanged samples of this study, there is little, if any, evidence of internal geminal silanols. On the basis of the measured T(Si-H) values and a qualitative study of the H-1-Si-29 dipolar interaction strength by an experiment in which the H-1-decoupling power was varied, it appears that the hydroxyl groups of the internal single silanols undergo rapid rotation about the Si-OH bond axis. H-1-H-1 and H-1-Si-29 dipolar dephasing (the former prior to H-1 --> Si-29 cross polarization) indicate that internal single silanols have a much smaller H-1-H-1 dipolar interaction than those experienced by single silanols on the surface and that at least some of the internal single silanols may be accompanied by trapped water.