UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

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Subject: search.filters.subject.Crystallography

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    Tuning Crystallographic and Magnetic Symmetry in Lithium Transition Metal Phosphates and Thiophosphates
    (2022) Diethrich, Timothy; Rodriguez, Efrain E.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Ferroic ordering needs no introduction; ferromagnetic, ferroelectric, and ferroelastic materials have had a significant impact on the materials science community for many years. While these three main types of ferroic ordering are well known, there is a fourth and final, lesser known ferroic ordering known as ferrotoroidicity. A ferrotoroidic material undergoes a spontaneous, physical alignment of toroidal moments under a critical temperature. This study is focused on broadening our current understanding of ferrotoroidics by studying two families of materials: LiMPO4, and Li2MP2S6, where M = Fe, Mn, and Co. While these two materials initially appear to be similar in some regards, many differences can be observed as a deeper dive is taken into their crystallography and magnetic structures. For a toroidal moment to exist, a specific orientation of magnetic moments is required, because of this, only certain magnetic point groups are allowed. For example, LiFePO4 has an “allowed” magnetic point group of m’mm, while it’s delithiated counterpart FePO4 has a “forbidden” magnetic point group of 222. This work has found that by using a new selective oxidation technique, lithium concentration can be controlled in the Li1−xFexMn1−xPO4 solid solution series. Neutron powder diffraction and representational analysis were used to find the magnetic point groups of each member of this series. In the end, each structure was solved and the largest transition temperature to date was reported for a potential ferrotoroidic material. The magnetic exchange interactions can be used to describe the magnetic phase changes that occuracross the Li1−xFexMn1−xPO4 series. The second group of materials in this study is the lithium transition metal thiophosphates of the formula Li2MP2S6, where M = Fe, Co. The structure of Li2FeP2S6 has been previously studied but no magnetic properties of this material have been reported. In addition, neither the structural nor magnetic properties have been reported for the cobalt analog. Single crystalXRD was used to confirm the previously reported crystal structure of Li2FeP2S6 and to find the novel crystal structure of Li2CoP2S6; both crystallize in a trigonal P31m space group. While isostructural in some regard, there are some crucial differences between these materials. The site occupancies are different, resulting in non-trivial charge balances and a unique thiophosphate distortion. Originally, these materials were chosen because their nuclear structure was predicted to host long-range antiferromagnetic order and potentially ferrotoroidic order. Contrary to expectations, magnetic susceptibility and field dependent measurements demonstrated paramagnetic behavior for both the iron and the cobalt sample down to 2 K. This result was further confirmed by a lack of magnetic reflections in the time-of-flight neutron powder diffraction data. While the phosphates and the thiophosphates demonstrated very different structural andmagnetic results, they both remain relevant materials for not only ferrotoroidics, but also magnetoelectrics, spintronics, quantum materials, and much more.
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    Building Block Synthesis and Recognition Properties of Cucurbit[n]uril (n = 7, 8) Derivatives
    (2015) Vinciguerra, Brittany Marie; Isaacs, Lyle; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Molecular containers have been a topic of interest for chemists since the discovery of crown ethers and their molecular recognition properties in the late 1960’s. Since then, the field of molecular containers has expanded rapidly to include many high affinity and highly selective host molecules. Chapter 1 introduces common molecular containers and goes on to discuss the CB[n] family of molecular containers. The CB[n] family are an exemplary group of hosts because they exhibit extremely high affinities (Ka values up to 1017 M-1) and high selectivity towards their guests which make them excellent candidates for many supramolecular applications. In order to maximize the use of CB[n], it became important to access specialized and functionalized derivatives to cater to various applications and chemistry. Early functionalization routes were limited by a lack of mechanistic understanding, but the mechanistic work of the Isaacs, Kim, and Day groups led to more successful syntheses. Chapter 2 discusses a building block synthesis towards water-soluble CB[7] derivatives Me2CB[7] and CyCB[7]. The recognition properties of Me2CB[7] are investigated as well as its use in drug solubilization. It is found that Me2CB[7], though 10 times more water soluble than CB[7], is able to solubilize drugs only as well as CB[7]. Additionally, a route towards a monofunctionalized CB[7] derivative, Cl-CB[7], bearing a primary chloride which is able to undergo further functionalization to a clickable azide by SN2 chemistry is presented. A click reaction with a small alkyne is performed resulting in a self-associating host whose self-assembly process is further investigated. Chapter 3 discusses a building block synthesis towards the first water-soluble CB[8] hosts Me4CB[8] and Cy2CB[8]. Mechanistic details of the CB[8] formation are elucidated from contrasting experiments and the recognition properties of the CB[8] derivatives are investigated by 1H NMR spectroscopy and X-ray crystallography. The CB[8] derivatives are investigated as potential drug solubilizing agents and it is found that they are able to solubilize several larger pharmaceutical molecules whereas CB[8] is water insoluble.
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    Kinetic and Structual Characterization of Glutamine-Dependent NAD Synthetases
    (2010) Resto, Melissa; Gerratana, Barbara; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Multifunctional enzymes catalyzing successive reactions have evolved several mechanisms for the transport of intermediates between active sites. One mechanism, substrate channeling, allows the transport of the intermediate without releasing it into the solvent. Members of the glutamine amidotransferase (GAT) family often utilize substrate channeling for the transport of intermediates. GAT enzymes hydrolyze glutamine to ammonia, which is transported to an acceptor domain preventing wasteful hydrolysis of glutamine and increasing the efficiency of the reaction. Many GAT enzymes utilize molecular tunnels to shuttle ammonia between active sites. Often GAT enzymes synchronize the active site through conformational changes that occur during catalysis. Glutamine-dependent NAD synthetases are GAT enzymes and catalyze the last step in the biosynthesis of NAD, utilizing nicotinic acid adenine dinucleotide (NaAD), ATP and glutamine. Steady-state kinetic characterizations and stoichiometric analysis of NAD synthetase from Mycobacterium tuberculosis (NAD synthetaseTB) revealed a substrate channeling mechanism for ammonia transport and tight coordination of the active sites resulting in an enzyme that is highly efficient in the use of glutamine. The crystal structure of NAD synthetaseTB has revealed a 40 Å tunnel that connects the active sites and is postulated to play a role in the synchronized activities. Several regions of the enzyme were identified that may be important for regulation, such as the YRE loop which contacts the glutamine active site and key regions of the tunnel. Mutations of tunnel residues, such as D656A, show that interruption of important interactions can result in compromise in transfer of ammonia or active site communication. Phylogenetic analysis revealed that glutamine-dependent NAD synthetases have different levels of regulation. Three groups of enzymes were identified represented by NAD synthetase from M. tuberculosis, S. cerevisiae (NAD synthetaseYeast) and Thermotoga maritima (NAD synthetaseTM). Steady-state kinetic characterizations and stoichiometric analysis of NAD synthetaseTM has revealed a compromised coordination of the active sites compared to the highly synchronized NAD synthetaseTB and the moderate synchronization of NAD synthetaseYeast. Sequence alignment of these groups has allowed identification of residues that line the tunnel that may be responsible for the differences observed in active site coordination and are, therefore, important for active site communication.