Dissertation Defense: Taylor Watts
Candidate Name: Taylor Watts
Major: Chemistry
Advisor: Jennifer Swift, Ph.D.
Title: Dynamics and Molecular-Level Control of Solid State Transformations in Molecular Hydrates
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 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, the introduction of a labile solvent molecule as a crystallographic component often leads to solid state transformations as a function of water loss and/or sorption. As such, the prediction and control over the stability of molecular hydrates under a variety of processing conditions are crucial challenges in developing these materials for practical use. These efforts depend on a thorough understanding the structural features and dynamics involved in the thermal- and moisture-induced solid form conversions in
these materials.
This dissertation focuses on the dynamic processes associated with a prototypical channel hydrate, thymine hydrate (TH), and its form conversions. In this work, the molecular motions involved in the dehydration of TH are characterized using techniques including time-resolved synchrotron powder x-ray diffraction and quasi-elastic neutron scattering. Further, we establish the conditions under which TH accepts compositional variation and discuss the design, preparation, and characterization of resulting multi-component solids. In each case, a fraction of thymine molecules in the TH lattice are replaced using one of several 5-substituted uracil molecules. 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 affect the thermal stability of the parent hydrate. The investigations presented herein highlight specific structural features that dictate water retention/sorption in a channel hydrate while detailing a design strategy for tuning the thermal stability of other analogous systems.