Browsing by Author "Engel, Emile R."

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    Guest effects on the solid-state dynamics of selected inclusion compounds
    (Stellenbosh : Stellenbosch University, 2016-12) Engel, Emile R.; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Guest inclusion has always been a central feature of supramolecular chemistry. The present work describes guest effects on particular properties of selected inclusion compounds. Guest replacement is demonstrated as a means of modifying thermal expansion behaviour and porosity. The body of this thesis consists of three manuscripts (two published and one under review). The first manuscript describes a nitromethane solvate of 18-crown-6 that was investigated by means of temperature-resolved single-crystal X-ray diffraction. The compound exhibits exceptionally large positive thermal expansion in two axial directions and exceptionally large negative thermal expansion along the third. The underlying mechanism relies exclusively on weak electrostatic interactions to yield a linear thermal expansion coefficient of −129×10−6 𝐾−1, which is, to the best of our knowledge, the largest negative value yet recorded for an organic inclusion compound. Our discovery of a reported analogue with acetonitrile led us to prepare an isoskeletal series of compounds for further inquiry. The second manuscript demonstrates that guest replacement in a series of isoskeletal organic inclusion compounds can produce remarkable changes in thermal expansion behaviour. The three inclusion compounds have 18-crown-6 as host molecule and nitromethane, acetonitrile or iodomethane as guests. Along principal axis X1 the linear component of thermal expansion is negative for the nitromethane and acetonitrile solvates but zero for the iodomethane solvate. The compounds show varying degrees of large volumetric thermal expansion, with coefficients of 378(22), 226(3) and 256(8)×10−6 𝐾−1 for the nitromethane, acetonitrile and iodomethane solvates, respectively. Crystal structure analysis and computational methods were used to elucidate general features of the underlying mechanism of thermal expansion for the series. Interestingly, the thermosalient effect was observed for the acetonitrile version. To our knowledge this is the first example of thermosalience reported for an inclusion compound. The success with molecular organic crystals prompted similar experimentation with a different class of materials. As inorganic inclusion compounds, MOFs are an obvious choice for experiments involving guest replacement because of their proven capability for guest exchange, and the great interest in MOFs as potential porous sorbents for molecular storage and separation in industry. The final manuscript describes a non-interpenetrated MOF with a paddle-wheel SBU that has been activated by direct thermal evacuation, guest exchange with a volatile solvent, and supercritical CO2 drying. Conventional thermal activation results in a mixture of crystalline phases and some amorphous content. Exchange with a volatile solvent and subsequent vacuum activation produces a pure breathing phase with high sorption capacity, selectivity for CO2 over N2 and CH4, and substantial hysteresis. Supercritical drying can be used to access a guest-free open phase. Pressure-resolved differential scanning calorimetry was used to investigate the systematic loss of sorption capacity by the breathing phase as a function of successive cycles of sorption and desorption.