Abstract

The lecture will highlight three experimental strategies that we have developed for exploring crystallization pathways and structural properties of solids: (i) in-situ solid-state NMR techniques for understanding the time-evolution of crystallization processes, (ii) the study of X-ray birefringence for spatially resolved mapping of the distribution of molecular orientations in materials, and (iii) structure determination of organic materials when single crystals cannot be prepared.

Our in-situ solid-state NMR technique [1-3] for studying crystallization pathways exploits the ability of NMR to selectively detect the solid phase in heterogeneous solid/liquid systems of the type that exist during crystallization from solution. We have shown that this technique can establish the sequence of solid phases formed during crystallization processes [1] and can be exploited in the discovery of new polymorphs [2]. Our most recent development is an in-situ NMR technique [3] that yields simultaneous information on the time-evolution of both the solid phase and the liquid phase during crystallization. This new strategy (called “CLASSIC NMR ” [3]) extends significantly the scope and capability of in-situ NMR for gaining fundamental insights on the evolution of crystallization processes.

Following our earlier studies of the phenomenon of X-ray birefringence [4,5], we recently reported [6] a new experimental set-up that allows spatially revolved measurements of X-ray birefringence to be carried out in “imaging mode”. In many respects, this technique (called X-ray Birefringence Imaging) represents the X-ray analogue of the polarizing optical microscope. The lecture will describe the first results obtained using this technique, demonstrating the utility and potential of X-ray Birefringence Imaging as a sensitive technique for imaging the local orientational properties of anisotropic materials [6]. Inter alia, the technique can be applied to characterize changes in molecular orientational ordering associated with solid-state phase transitions and to determine the size, spatial distribution and temperature dependence of domain structures in materials.

Finally, although single-crystal X-ray diffraction (XRD) is a very powerful technique for determining crystal structures, the requirement for a single crystal is a limitation on the scope of this technique. For materials that cannot be grown as suitable single crystals, structure determination must be tackled instead from powder XRD data. However, structure determination from powder XRD data is much more challenging than from single-crystal XRD data, particularly in the case of organic materials. Indeed, as recently as the early 1990s, no organic crystal structure had ever been determined directly from powder XRD data. Since that time, developments in methodology (particularly the direct-space strategy for structure solution [7]) are such that crystal structures of organic materials of moderate complexity can now be determined relatively routinely from powder XRD data [8-10]. The lecture will give an overview of the current opportunities for carrying out structure determination of organic materials directly from powder XRD data, with examples from chemical, materials, pharmaceutical and biological sciences.

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