Atomic-scale structure of gel materials by solid-state NMR

Smith, M.E. and Holland, D. (2018) Atomic-scale structure of gel materials by solid-state NMR. In: Handbook of Sol-Gel Science and Technology. Springer International Publishing AG, Cham, pp. 1281-1322. ISBN 9783319320991

Abstract

The underlying principles of solid-state NMR spectroscopy are outlined with an emphasis on the physical origins of the interactions that affect NMR spectra so that an understanding of the structural information they convey is clearly understood. The fundamental components of the experimental approach are described. How the experimental data can be analyzed to provide structural characterization of sol-gel materials is illustrated through a series of examples from the literature. The short-range structural sensitivity of NMR means that it is an ideal probe of sol-gel materials since they are structurally disordered. Given the importance of silicates in sol-gel science, 29Si magic-angle spinning (MAS) NMR is a widely used nucleus in solid-state NMR studies of sol-gel materials. However, it is emphasized that to derive maximum benefit fromNMR characterization, a multinuclear approach is used, although each nucleus will have its own particular considerations which are presented. In this second edition, key advances in the experimental methodology (e.g., much higher applied magnetic fields, faster MAS rates, more sophisticated excitation approaches) since 2005 are outlined. The use of first-principles computational approaches to calculate NMR interaction parameters and hence better constrain structure provides an important additional dimension to the NMR approach. Materials where there has been a substantial expansion of sol-gel approaches since 2005 are included, with, for example, novel sol-gel schemes opening up preparation of phosphates where 31P MAS NMR is a sensitive structural probe. Another area where there has been substantial sol-gel activity since 2005 is in the preparation of bioactive calcium silicate-based materials, where multinuclear NMR is an ideal probe, including the use of 43Ca, a quadrupolar nucleus with a small magnetic moment, which has only really become readily accessible in recent years. © Springer International Publishing AG, part of Springer Nature 2018.