Browsing by Author "van Rijn, Raymond Michael"

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    Investigating mechanical responses to structural changes in crystalline materials
    (Stellenbosch : Stellenbosch University, 2023-11) van Rijn, Raymond Michael; Barbour, Leonard James; Loots, Leigh-Anne; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Previous work has established that the compound 2,7-dimethylocta-3,5-diyne-2,7-diol crystallizes to produce a range of inclusion compounds – the crystals of which exhibit elastic flexibility. In an attempt to tailor the Young’s modulus of the system, substitution of guest species, using both polar and apolar compounds, was carried out. Ultimately, little difference in elasticity was observed, regardless of the included guest species. Analysis of crystal structures revealed that little interaction takes place between guest molecules, or between host and guest. Thus, hydrogen bonding in the host framework is concluded to be the determining factor in crystal flexibility. Several other elastically flexible crystals were subsequently investigated to determine how their elastic moduli would change when temperature was varied. A relationship was identified between the change in elasticity and thermal expansion of the crystals. As the bending axis of a crystal expands in length, the Young’s modulus decreases. Thus, for crystals exhibiting positive thermal expansion, elasticity is reduced as temperature is decreased. Conversely, for a material displaying negative thermal expansion, decrease in temperature was found to produce an increase in elasticity. Greater intermolecular spacing likely allows for a greater degree of molecular reorientation that must occur to facilitate mechanical bending of crystals. Two of the compounds subjected to variable-temperature flexibility studies, Pd(acac)2 and Cu(acac)2, are isostructural and isomorphous, yet exhibit opposite thermal expansion characteristics along their crystallographic b axes. As such, they were identified as promising candidates to form solid solutions, whose thermal expansion behaviours could be tuned. A mixed crystal displaying near-zero thermal expansion was successfully produced, demonstrating the applicability of solid solutions in tailoring the thermal properties of molecular compounds. A reaction between 2,7-dimethylocta-3,5-diyne-2,7-diol and iodine was observed to produce a novel cumulene-type compound in high yield. It was established that light is required for the reaction to proceed, and can also be used to isomerize the cumulene in a cis to trans manner. Several other diyne compounds were found to react in analogous ways under the novel reaction conditions, provided they featured hydroxyl functional groups. Thus, it is proposed that formation of hydrogen-bonded adducts is responsible for halting the halogenation reaction upon formation of a cumulene.