ABSTRACT
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425
Underivatized Silica . . . . . . . . . . . . . . . . . . . . . . . . . 428 29Si NMR Patterns . . . . . . . . . . . . . . . . . . . . . . . . 428
Linewidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
Dehydration of Silica Gel . . . . . . . . . . . . . . . . 429
Fumed Silica . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 1H NMR Studies . . . . . . . . . . . . . . . . . . . . . . . . . 431 17O NMR Experiments . . . . . . . . . . . . . . . . . . . . 435
The Interplay between 1H and 29Si Spins in
Silica Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 436
Structural Models . . . . . . . . . . . . . . . . . . . . . . . . . 439
Derivatized Silicas . . . . . . . . . . . . . . . . . . . . . . . . 440
Silylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440
Alkylsilane-Modified Silica . . . . . . . . . . . . . . . . . 442
Other Surface Modifications . . . . . . . . . . . . . . . . 443
Substitutionally-Modified Silicas . . . . . . . . . . . . 450
Summary and Conclusions . . . . . . . . . . . . . . . . . . . 453
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . 453
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453
The solid-state NMR revolution of the past quarter century has had an enormous impact of the state of
knowledge of silica and modified silicas. 1H ! 29Si cross polarization (CP) with magic-angle spinning (MAS) is an invaluable approach for observing local silicon environments on the silica surface. 1H NMR
approaches distinguish clustered (hydrogen-bonded) and isolated surface silanols. Correlations between 29Si and 1H NMR behaviors in silicas have led to detailed structural models of the silica surface based
on intersections of OH-terminated 100 and 111 faces of b-cristobalite. Other nuclides (e.g., 1H, 2H, 13C, and 15N), along with 29Si, provide valuable approaches for the characterization of local structure and
motion in derivatized (e.g., silylated) silicas. 17O NMR shows great promise for structure elucidation in
modified silicas in which some of the Si sites of silica are replaced by other metal centers.
