Dynamics and Molecular-Level Control of Solid State Transformations in Molecular Hydrates
Watts, Taylor Ann
Swift, Jennifer A
Organic solids are pervasive in the fine chemical industry. The diversity of solid state structures continues to expand with chemical and engineering efforts to design, synthesize, and use fit-for-purpose molecular materials. An important prerequisite in these efforts is a thorough understanding of solid state structure-property relationships wherein changes in structural features (e.g. chemical composition, molecular conformation, and intermolecular interactions) can have a dramatic impact on the bulk physiochemical properties of the material. Molecular hydrates are multi-component solids frequently encountered in the production of active pharmaceutical ingredients and agrochemicals. Hydrate formation is unsurprising given the ubiquity of water in the manufacturing, processing, and storage of these materials. However, for crystalline forms which contain labile solvent molecules as crystallographic components, solid state transformations due to solvent loss and/or sorption can occur under routine conditions. The prediction and control of molecular hydrate stability over a variety of processing conditions are crucial challenges in developing these materials for practical use. Meeting these challenges depends on a thorough understanding of the structures and dynamics associated with thermal- and moisture-induced solid form conversion in these materials.This dissertation focuses on the dynamic processes and form conversions associated with a prototypical channel hydrate, thymine hydrate (TH). In this work, the molecular motions associated with the dehydration of TH are characterized using techniques including time-resolved synchrotron powder x-ray diffraction and quasi-elastic neutron scattering. The design, preparation, and characterization of mixed crystal forms establish the conditions under which the TH lattice accepts compositional variation. In each case, a fraction of thymine molecules in the TH lattice were replaced with analogous, 5-X-uracil (X = H, CH2CH3, OH, NH2) substitutions. The compositions of the isomorphous lattices were found to be dependent on the chemical functionality and the ratio of the components in the growth solutions. Importantly, the lattice substitutions were found to dramatically alter the thermal stability and dehydration processes of the parent hydrate. The investigations presented herein highlight specific structural features that dictate water retention/sorption in TH while detailing a general design strategy for tuning hydrate thermal stability that may be transferable to other hydrate systems.
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