Control of Nuclear Spin Interactions in Solid-state NMR by MAS Sideband Manipulation

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful means of determining molecular structure, making significant contributions over the last 25 years to the study of biological molecules, such as proteins. More recently, NMR has also emerged as an important technique for studying complex molecular systems in the solid phase. Examples include membrane proteins, supramolecular assemblies, biological interfaces, polymers, molecular sieves, ionic conductors, nanocomposite materials and catalysts, systems which will underpin a host of scientific and technological developments in the future.Detailed information about molecular structure is obtained from orientation-dependent nuclear spin interactions, such as the chemical shift anisotropy (CSA). Measurements of these can be made from wideline NMR spectra of powdered solids which show singularities corresponding directly to the principal components of the tensor which describes the interaction. However, spectral overlap means that it is often necessary to resort instead to an analysis of the intensities of the rotational sidebands which appear in the magic angle spinning (MAS) spectrum, and this approach has become routine in the case of the CSA. Furthermore, it has been shown recently that analysis of a moderate number (approximately 8 / 10) of spinning sidebands usually gives more reliable results than fitting the wideline spectrum. A relatively low spinning rate is normally required to give this many sidebands, and so many two-dimensional MAS NMR experiments have been developed which separate the sideband manifolds from different sites and further improve resolution. In particular, we have shown how to record spectra in which the sideband intensities are identical to those expected for a sample spinning at some fraction of the actual MAS rate. This CSA amplification experiment represents a new approach to the measurement of spinning sideband intensities and is the starting point for the research programme proposed here.In the course of the research we will broaden the scope of CSA amplification to include other interactions, scaling them to make measurement of the corresponding tensors feasible in situations where conventional methods fail. We will make measurements of CSA parameters in systems which are challenging for conventional methods. We will compare the results with calculated tensors to extract structural information and to make assignments in poorly-resolved MAS spectra. In addition, we will investigate how experiments which use MAS sideband intensities to study molecular dynamics can benefit from the CSA amplification approach.