X-ray Birefringence Imaging and other fundamental aspects of solid organic inclusion compounds

• Main Author: Edwards-Gau, Gregory R.
• Published: Cardiff University 2014
• Subjects: 547; QD Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.659273

This thesis presents new experimental techniques and utilizes these strategies in the analysis of solid organic inclusion compounds. This thesis also reports the production of a new series of co-crystals and examines their crystal structures. Chapter 1 acts as an introduction to the materials studied in this research. It explains the properties of inclusion compounds and lists the chemical materials used for these experiments. Chapter 2 explains the experimental techniques used in this research. Specifically it explains X-ray diffraction, X-ray birefringence and in-situ solid-state NMR. Chapter 3 presents a new technique for spatially resolved mapping of specific bond orientations in anisotropic solid materials using wide beam linearly polarized X-rays and an area detector. Earlier work with a focussed beam and a point detector showed the sensitivity of X-ray Birefringence to the orientation of specific energy-matching bonds inside a material, but these experiments only probed a small section of the crystal. Our wide beam imaging technique (X-ray Birefringence Imaging) shows similar sensitivity but allows us to investigate the full crystal simultaneously, which allows us to identify different domains within a single crystal. We apply this technique to a model material (1 bromocyclohexane/thiourea) which undergoes a low temperature phase transition and serves to demonstrate the usefulness of imaging techniques - in the high temperature phase the relevant C−Br bonds are isotropically disordered and no birefringence is observed, in the low temperature phase the relevant C−Br bonds are ordered but there are three possible orientations for the bromocyclohexane molecule so different regions of the crystal exhibit different birefringent signal. This behaviour is very clear on an imaging technique, but can appear highly confusing when using point-detector techniques. Chapter 4 utilizes X-ray Birefringence Imaging to investigate the dynamic rotational properties of guest molecules in a different set of solid organic inclusion compounds. By studying the known structures of 1,10-dibromodecane/urea and 1,8-dibromooctance/urea we have determined that XBI is a time-averaged and space-averaged technique. Additionally this chapter utilizes a Ge(555) analyzer instead of the Si(555) analyzer, which results in better spatial resolution and a different beam shape on the final images. Chapter 5 utilizes solid-state in-situ NMR to monitor crystallization processes as they occur and gain insight on competitive uptake of different guest molecules within the inclusion compound. These experiments use alkane and α,ω-dibromoalkane guest molecules inside urea inclusion compounds where the urea host structure (created in-situ) acts like a one-dimensional tunnel confining the guest. Every position within the urea tunnel is equivalent (a property of the incommensurate structure) which serves to simply the solid-state NMR spectra and means that for a given atom at the end of an alkane chain the only difference in NMR site comes from the neighbour molecule along the tunnel. This means in the solid phase we can observe peak splitting on certain atoms based on neighbour environment (e.g. the -CH3 in undecane will give a slightly different chemical shift if the neighbouring guest molecule is another undecane compared to if the neighbouring guest molecule is 1,8-dibromooctane) which in turn allows us to extract some information about the ordering within the inclusion compound. In these experiments we can also clearly distinguish between the same molecules in different phases, so as crystallization occurs we observe the loss of solution signal alongside the gain of solid signal. Additionally these experiments show no evidence of any intermediate structures or transition states. Chapter 6 describes a new set of organic co-crystals formed by reacting thiourea with α,ω-diiodoalkane chains and examines the crystal structures of these materials. Chapter 7 details further work and potential applications of this research. Digital data includes animated videos of the X-ray birefringence imaging data obtained in Chapter 3 and CIF files of the structures determined in Chapter 6.