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Structural properties, X-ray birefringence and crystal growth of solid organic inclusion compounds

Palmer, Benjamin Alexander 2012. Structural properties, X-ray birefringence and crystal growth of solid organic inclusion compounds. PhD Thesis, Cardiff University.
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This thesis presents several new insights into the behaviour and properties of solid organic inclusion compounds and explores some of the potential applications of these properties. The thesis considers the structural properties, X-ray birefringence and crystal growth of urea and thiourea inclusion compounds. Chapter 1 provides an introduction to organic solid inclusion compounds, surveying the physico-chemical properties and applications of urea and thiourea inclusion compounds. In Chapter 2, the basic theory of the experimental techniques utilized in this thesis is introduced. These techniques include single-crystal and powder X-ray diffraction, Raman microspectrometry and differential scanning calorimetry. Chapter 3 presents a novel strategy for retrospectively mapping the growth history of a crystal. The new experimental strategy allows insights to be gained on the evolution of crystal growth processes by analysis of crystals recovered at the end of crystallization. The feasibility of the strategy is demonstrated by considering the crystal growth of a urea inclusion compound containing a binary mixture of guest molecules. After the crystal has finished growing, the composition of the crystal is determined using confocal Raman microspectrometry, and is interpreted to yield insights into the growth history of the crystal. In Chapter 4, new insights into the phase transition behaviour of thiourea inclusion compounds are established. The structural properties of the bromocyclohexane/thiourea inclusion compound are determined using both single-crystal and powder X-ray diffraction over a range of temperatures above and below a first-order phase transition. The results demonstrate marked contrasts to the phase transition behaviour in the prototypical cyclohexane/thiourea inclusion compound, demonstrating that relatively small changes in molecular geometry (in this case bromine substitution) can have a profound influence on the structural properties of the low-temperature phase in such materials. This observation reflects the fine energetic balances that pertain in such materials and the role of small and subtle changes in intermolecular interactions involving the host and guest components. Chapter 5 reports the first definitive demonstration of the phenomenon of X-ray birefringence, reporting a material that exhibits essentially ideal birefringence behaviour at X-ray energies near the Br K-edge. The designed material, the 1-bromoadamantane/thiourea inclusion compound gives experimental behaviour in excellent agreement with theoretical predictions for the dependence of transmitted X-ray intensity on both X-ray energy and crystal orientation. The results vindicate the potential to exploit this phenomenon to establish a detailed understanding of molecular polarization, particularly as an experimental strategy to determine the orientational distributions of specific bonds in solids, for example, in the case of partially ordered materials or materials that undergo order-disorder phase transitions. Building upon the fundamentally important observations of Chapter 5, Chapter 6 demonstrates that measurements of X-ray birefringence can be used to characterise changes in molecular polarization and bond orientation in an anisotropic material. For the bromocyclohexane/thiourea inclusion compound, measurements of X-ray birefringence are used to determine the changes in the orientational distribution of the C–Br bonds of the guest molecules, associated with an order-disorder phase transition in this material. Best-fits to simulated data based on a structural model were performed, allowing quantitative structural information on the guest molecules to be established. The structural properties determined from X-ray birefringence correlate exquisitely with those obtained independently from diffraction data, demonstrating the validity of the structural model and the reliability of this novel experimental technique. These observations represent the basis of a new technique for determining information on the structural properties of materials, where diffraction methods may be unsuitable. The technique has huge potential to be utilised in the exploration and discovery of new materials and in principle could be applied to any anisotropic system. In Chapter 7, the first example of an incommensurate thiourea inclusion compound is reported. X-ray diffraction studies reveal that the tunnel inclusion compound formed between 1-tert-butyl-4-iodobenzene and thiourea has an incommensurate relationship between the periodicities of the host and guest substructures along the tunnel axis, representing the first reported case of an incommensurate thiourea inclusion compound. Finally in Chapter 8, some general conclusions and outlooks for the field are stated. This chapter conveys some of the overarching concepts and questions which motivated the diverse studies presented in this thesis and also expresses some thoughts on the future outlook for the field.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:04

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