Renewal of "Advancing Quantum Crystallography: Visualisation and Characterisation of Chemical Reactions via Diffraction Experiments": From Model Systems to Real Systems – from Molecules to Materials

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Quantum Crystallography is an integrative combination of diffraction experiments and wavefunctions into single research tools. It has developed into an independent new research field in the last few years. Our contribution to the field is the method X-ray Wavefunction Refinement (XWR). In the original Emmy Noether project, we have shown how XWR can solve long-standing problems of X-ray structure determination and experimental electron-density research, especially concerning the correct and accurate description of hydrogen atoms and polar bonds. We have now successfully finalised method and software development in XWR for molecular chemistry.In the first part of the renewal project, we thus focus on applications of XWR to chemical problems related to hydrogen atoms that cannot be solved with any other approach. We will investigate prototropic tautomerism in quinolone N-oxides, which is decisive for the mode of action of their antibiotic activity. We will also study C-H bond activation through agostic interactions in titanium amides. In both classes of compounds, the accurate localisation of hydrogen atoms and subsequent energetic and bonding analyses have not been possible so far, but XWR offers ideal capabilities to do so and hence to answer the open questions.In the second part, XWR will be extended to materials science. Many materials of technical interest are extended solids, i.e. periodic compounds for which molecular wavefunctions are not suitable. Moreover, they do not form single crystals large enough for routine X-ray structure determination, i.e. only powder diffraction is viable. Therefore, we will develop periodic XWR by utilising the techniques of extremely localised molecular orbitals and Wannier functions leading to extremely localised Wannier functions, ELWFs. In addition, we will develop an iterative XWR-Rietveld refinement which can perform an improved structure determination from powder samples. By the combination of both newly developed methods, application of XWR to powders of extended solids will be possible. This will for the first time allow the study of material properties from powders experimentally on the subatomic level focused on bonding aspects. We will begin with an investigation of thermoelectric properties of a skutterudite-type and thermochromic properties of a mullite-type material.