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.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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Subject: search.filters.subject.Photocatalysis

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    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.
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    TAILORING PROPERTIES AND FUNCTIONALITIES OF NANOSTRUCTURES THROUGH COMPOSITIONS, COMPONENTS AND MORPHOLOGIES
    (2013) WENG, LIN; Ouyang, Min; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The field of nanoscience and nanotechnology has made significant progresses over the last thirty years. Sophisticated nanostructures with tunable properties for novel physics and applications have been successfully fabricated, characterized and underwent practical test. In this thesis, I will focus on our recent efforts to develop new strategies to manipulate the properties of nanostructures. Particularly, three questions have been answered from our perspective, based on the nanomaterials synthesized: (1) How does the composition affect a novel nanostructure? We started from single-molecule precursors to reach nanostructures whose bulk counterparts only exist under extreme conditions. Fe3S and Fe3S2 are used as examples to demonstrate this synthetic strategy. Their potential magnetic properties have been measured, which may lead to interesting findings in astronomy and materials science. (2) How to achieve modularity control at nanoscale by a general bottom-up approach? Starting with reviewing the current status of this field, our recent experimental progresses towards delicate modularity control are presented by abundant novel heteronanostructures. An interesting catalytic mechanism of these nanostructures has also been verified, which involves the interaction between phonons, photons, plasmons, and excitons. (3) What can the morphology difference tell us about the inside of nanostructures? By comparing a series of data from three types of CdSe/CdS core-shell structures, a conclusion has been reached on the CdS growth mechanism on CdSe under different conditions, which also may lead to a solution to the asymmetry problem in the synthesis of CdSe/CdS nanorods. Finally this thesis is concluded by a summary and future outlook.
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    Design And Construction Of Low Power, Portable Photocatalytic Water Treatment Unit Using Light Emitting Diode
    (2006-05-03) chokshi, mihir k; Davis, Allen P; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Limited availability of mobile technology to disinfect drinking water at low cost led to the current research of using titanium dioxide (TiO2) photocatalysis for drinking water disinfection. New UV light emitting diodes (LEDs) have potential for application in this technology. The research was divided into three parts: immobilization of TiO2, optimization of coating and reactor using methyl orange and investigating disinfection efficiency for Escheriachia coli (ATCC 25922). Thin TiO2 films supplemented with Degussa P25, coated on glass beads and calcinated at 500 oC had 9.9 mm maximum and 2 mm average thickness, 0.28 m2/g BET surface area and was dominated by the anatase TiO2 phase. A reactor with LEDs degraded methyl orange with a first order rate constant of 0.39 hr-1 and 3 log10 E. coli removal was noted in 240 mins. With anticipated drops in LED cost, use of LEDs for TiO2 photocatalysis remains a promising disinfection technology.