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Describe the proposed research in about 200 words.Solid-state NMR (nuclear magnetic resonance) is capable of addressing molecular structural problems which are difficult to access any other way. For example, it is possible to use solid-state NMR measurements to estimate the dipole-dipole couplings, and hence distances, between neighbouring magnetic nuclei. This allows solid-state NMR to obtain high-resolution local molecular structural information on non-crystalline molecular systems such as ceramics, glasses, fibres, and membrane proteins, often with an accuracy of a few picometres. Unfortunately, many such solid-state NMR methods are not sufficiently robust, for example when the nuclei are relatively far from each other, or experience large chemical shift interactions or radio-frequency field imperfections, or when the nuclei involved have electric quadrupole interactions. The aim of this proposal is to apply a symmetry theory invented in our group to develop solid-state recoupling methods which are more robust, precise, and reliable, particularly at medium to long-range distances. One application of quadrupolar spin recoupling is to study hydrogen bonding between 17 O and 1H nuclei. The broad aim is to provide the community with new tools for the investigation of molecular structure on those scientifically and technologically important materials which do not yield high-quality diffraction data.