A novel approach to refinement of proton positions in organic crystal structures

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Accurate determination of hydrogen positions within organic crystalline structures is essential for rational product design in many industrial sectors. While crystallography has been the gold standard for structure determination, many of the materials of interest cannot be produced in single crystal form and powder diffraction methods are limited in their ability to locate hydrogens. Neutron methods which require deuteration of the sample are also limited in application for this field. The project theme sits within the Manufacturing the Future and Healthcare themes of EPSRC.It has been shown how NEXAFS can be combined with Density Functional Theory (DFT) calculations to determine hydrogen bond lengths with accuracy comparable to high-resolution X-ray diffraction and neutron diffraction methods. Such NEXAFS refinement of proton and hydrogen positions is a radically novel way of determining the hydrogen positions in crystal structures formed by small organic molecules, and with the advent of high-throughput NEXAFS it will also be experimentally straightforward. It overcomes limitations of X-ray diffraction crystallography because it does not require crystals and those of neutron methods because it does not require deuteration.The proposed PhD project will use the High-Throughput Near-Ambient Pressure NEXAFS facility at Diamond Light Source for a programme of high-throughput C, N and O K-edge NEXAFS measurements on crystalline sample libraries focused on different donor/acceptor hydrogen bond pairs. After training at Leeds and Diamond in 2019/20 the PhD student will perform these studies, choosing libraries of representative crystal structures from the Cambridge Crystallography database. Where possible, the accuracy of the resulting crystal structure refinement will be validated through additional analysis with high-resolution X-ray and neutron diffraction.