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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Item Exploring Mechanisms and Predicting Reactivity of Transition Metal-Catalyzed and Photocatalyzed Radical and Polar Organic Transformations(2023) Martin, Robert Thompson; Davis, Jeffery; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The creation of protocols to form novel C-C and C-heteroatom bonds has been the primary goal of organic synthesis since its inception. Chemists have long harnessed both radical and polar reactivities, often as complementary paths to construct these bonds to yield more complex molecular architectures. However, compared to the development of synthetic protocols, development of mechanistic models and enriching of mechanistic understanding of many organic reactions has been limited. Computational studies into the mechanisms of organic transformations provide an avenue by which mechanisms of reactions can be better understood and new patterns of reactivity can be predicted. Herein, quantum-mechanical computational methods e.g., density functional theory (DFT) have been employed in the pursuit of understanding the mechanisms of a series of radical and polar reaction schemes. Specifically, DFT calculations were employed to understand the mechanism and origins of selectivity of two nickel(I)-catalyzed olefin functionalizations. These studies demonstrate a catalyst-control scheme by which selectivity can be induced by the steric properties of the catalyst (Chapter 1). Following this work, two photocatalytic transformations which yield difluorinated products were studied thoroughly with computations. First, a synthesis of difluorinated lactone derivatives revealed a long-lived radical intermediate and motivated mechanistic experiments to isolate this radical. Next, a synthesis of difluorinated oxindole derivatives demonstrated the ability of arenethiols to act as photocatalysts (Chapter 2). Then, computations were used to rigorously explore a copper-catalyzed reductive cross-coupling of imine and allenamides. Specifically, computations were employed to explore the mechanism of the transformation and the origins of stereoselectivity and the divergent formation of urea and diamine products (Chapter 3). Finally, two computational investigations into the mechanisms of transformations catalyzed by first-row transition metals are detailed. In particular, a nickel-catalyzed hydroarylation of gem-difluoroalkenes is explored computationally to determine the order of steps in the reaction. In addition, the mechanism of a cobalt(I)-catalyzed allylic substitution is considered to ascertain the nature of the transformation as either radical or polar (Chapter 4). Given the complexity of the mechanisms of these transformations, computational studies provide an alternative route to acquire useful mechanistic understanding that can support or explain observed experiments and suggest further mechanistic experiments that could provide stronger evidence for a given mechanistic proposal.Item AB INITIO MODELING OF THE SELECTIVITY AND REACTIVITY OF BOTH THERMAL AND LIGHT MEDIATED ORGANIC AND ORGANOMETALLIC TRANSFORMATIONS(2022) Dykstraa, Ryan Henry; Gutierrez, Osvaldo; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The mechanism of a reaction is the collection of events that take place that lead to the products of a chemical transformation. Though there are some events in a chemical reaction that can be observed by experiment, such long-lived intermediates, many of the events are too short lived to be measured. Due to these restrictions and the advancements in the development of moderately scaling computational tools, it is becoming commonplace to use quantum mechanical software packages to model the mechanism of a reaction. Here, I used quantum mechanical calculations alongside experimental evidence provided by multiple collaborators to understand the reactivity of both heat- and light-mediated organic transformations. In chapter 2, I investigated the role of electron donor-acceptor complexes in the generation of alkyl and acyl radicals in the presence of visible light. In addition, the pathways to the experimentally observed products, alkyl and acyl thioethers, were modeled. The lowest energy pathway to product, post-radical generation, was radical addition to the radical electron donor-acceptor complex. For a photoredox-catalyzed method to cyclopropanes from a novel halomethyl radical precursor (Chapter 3), computations strongly supported a redox-neutral reductive radical/polar crossover mechanism over radical pathways, consistent with experimental trends. Investigation of the isomerization of cinnamyl chloride to cyclopropane via a commonly used photoredox catalyst (Chapter 4) revealed that the reaction was mediated via dexter energy transfer between photocatalyst and substrate over the more commonly proposed electron transfer, affording diastereoselective product formation. A dual nickel/photoredox-catalyzed coupling of sulfinate salts and aryl halides gave a mixture of aryl sulfide and aryl sulfone products (Chapter 5), suggesting that disproportionation of sulfone radical was leading to the formation of thiyl radical. Modeling the product determining steps indicated that the product distribution was controlled by radical addition of the thiyl radical to the nickel(II) species versus reductive elimination of the sulfone bound to the nickel(III) catalyst. A bicyclo[1.1.1]pentane diborylated with pinacolboryl groups, one at the arm and head position, was found to have reactivity only at the bridgehead position (Chapter 6). Calculations of a hydrozone coupling reaction performed by the Qin group found that the reactivity was due to the unique hybridization of the bridgehead position as well as increased steric interactions at the arm position. Finally, a sulfoxide synthesized from a sulfinate salt could be activated with Grignard reagent, affording coupling of the substituents originally bound to the sulfoxide. DFT calculations validated the role of the sulfurane intermediate acting as a mediator to the coupled product.Item First Principle Computational Study of Fast Ionic Conductors(2018) He, Xingfeng; Mo, Yifei; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fast ionic conductors have great potential to enable novel technologies in energy storage and conversion. However, it is not yet understood why only a few materials can deliver exceptionally higher ionic conductivity than typical solids or how on can design fast ion conductors following simple principles. In this dissertation, I applied first principles computational method to understanding the fast ionic diffusion within fast ionic conductors and I demonstrated a conceptually simple framework for guiding the design of super-ionic conductor materials. I studied Na0.5Bi0.5TiO3 (NBT) as the model material for oxygen ionic conductor. The structure-property relationship of the NBT materials is established. Based on the newly gained materials understanding, our first principles computation predicted that Na and K were promising dopants to increase oxygen ionic conductivity. The newly designed NBT materials with A-site Na and K substituted A sites exhibited a many-fold increase in the ionic conductivity at 900K comparing to that in the experimental compound. We demonstrated that the concerted migration mechanism with low energy barrier is the universal mechanism of fast ionic diffusion in a broad range of ionic conducting materials. Our theory provides a conceptually simple framework for guiding the design of super-ionic conductor materials, that is, inserting mobile ions into high-energy sites to activate concerted ion conduction with lower migration barriers. We demonstrated this strategy by designing a number of novel fast Li-ion conducting materials to activate concerted migration with reduced diffusion barrier. We identified the common features of crystal structural framework for lithium SICs. Based on the determined attributes, we performed a high-throughput screening of all lithium-containing oxide and sulfide compounds. The screening revealed several crystal structures that are potential to be fast ion conductors. Through aliovalent doping, we modified the Li content of these structures which resulting in different Li sublattice within the structure and we found a number of lithium super- ionic conductors that are predicted to have Li+ conductivities greater than 0.1 mS/cm at 300K.Item INTEGRATED MODELING OF RELIABILITY AND PERFORMANCE OF 4H-SILICON CARBIDE POWER MOSFETS USING ATOMISTIC AND DEVICE SIMULATIONS(2015) Perinthatta Ettisserry, Devanarayanan; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)4H-Silicon Carbide (4H-SiC) power MOSFET is a promising technology for future high-temperature and high-power electronics. However, poor device reliability and performance, that stem from the inferior quality of 4H-SiC/SiO2 interface, have hindered its development. This dissertation investigates the role of interfacial and near-interfacial atomic defects as the root cause of these key concerns. Additionally, it explores device processing strategies for mitigating reliability-limiting defects. In order to understand the atomic nature of material defects, and their manifestations in electrical measurements, this work employs an integrated modeling approach together with experiments. Here, the electronic and structural properties of defects are analyzed using first-principles hybrid Density Functional Theory (DFT). The insights from first-principles calculations are integrated with conventional physics-based modeling techniques like Drift-Diffusion and Rate equation simulations to model various device characteristics. Subsequently, the atomic-level models are validated by comparison with experiments. From device reliability perspective, this dissertation models the time-dependent worsening of threshold voltage (Vth) instability in 4H-SiC MOSFETs operated under High-Temperature and Gate-Bias (HTGB) conditions. It proposes a DFT-based oxygen-vacancy hole trap activation model, where certain originally ‘electrically inactive’ oxygen vacancies are structurally transformed under HTGB stress to form electrically ‘active’ switching oxide hole traps. The transients of this atomistic process were simulated with inputs from DFT. The calculated time-evolution of the buildup of positively charged vacancies correlated well with the experimentally measured time-dependence of HTGB-induced Vth instability. Moreover, this work designates near-interfacial single carbon interstitial defect in SiO2 as an additional switching oxide hole trap that could cause room-temperature Vth instability. This work employs DFT-based molecular dynamics to develop device processing strategies that could mitigate reliability-limiting defects in 4H-SiC MOSFETs. It identifies Fluorine treatment to be effective in neutralizing oxygen vacancy and carbon-related hole traps, unlike molecular hydrogen. Similarly, Nitric Oxide passivation is found to eliminate carbon-related defects. From device performance perspective, this dissertation proposes a methodology to identify and quantify channel mobility-limiting interfacial defects by integrating Drift-Diffusion simulations of 4H-SiC power MOSFET with DFT. It identifies the density of interface trap spectrum to be composed of three atomically distinct defects, one of which is potentially carbon di-interstitial defect.