College of Computer, Mathematical & Natural Sciences

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Effective October 4, 2010, the University established the College of Computer, Mathematical, and Natural Sciences (CMNS) by integrating the former colleges of Chemical and Life Sciences (CLFS) and Computer Mathematical and Physical Sciences (CMPS).

The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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1/f noise and Luttinger liquid phenomena in carbon nanotubes
(2007-08-03) Tobias, David; Lobb, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Carbon nanotubes (CNTs) provide an ideal medium for testing the behavior of one-dimensional electron systems and are promising candidates for electronic applications such as sensors or field-effect transistors. This thesis describes the use of low frequency resistance fluctuations to measure both the properties of the one-dimensional electron system in CNTs, and the sensitivity of CNT devices to their environment. Low frequency noise was measured in CNTs in field effect transistor (FET) geometry. CNTs have a large amount of surface area relative to their volume and are expected to be strongly affected by their environment, leading to speculation that CNTs should have large amounts of 1/f noise. My measurements indicate that the noise level is in the same range as that of traditional FETs, an encouraging result for possible electronic applications. The temperature dependence of 1/f noise from 1.2 K to 300 K can be used to extract the characteristic energies of the fluctuators responsible for the noise. The characteristic energies allows for the elimination of structural and electronic transitions within the CNT itself as possible sources of 1/f noise in CNTs, leaving the motion of defects in the gate dielectric, or possibly strongly physisorbed species, as the likely culprits. Another form of low frequency noise found in CNTs is random telegraph signal (RTS), which manifests as the alternation between two current states at a stable voltage bias. In CNTs, this phenomenon occurs due to the tunneling of electrons into and out of the CNT from a nearby defect, and thus provides a way to probe the tunneling density of states of the CNT itself. The tunneling density of states in turn provides information on the strength of the electron-electron interaction in CNTs. Due to the one-dimensional structure of CNTs their electronic state is expected to be a Luttinger liquid, which should manifest as a power-law suppression of the tunneling density of states at the Fermi energy. The power law exponent is measured in both the temperature dependence and energy dependence of the tunneling rates. In agreement with theory, the power-law exponent is significantly larger in semiconducting CNTs than found in previous experiments on metallic CNTs. The RTS can also be used as a "defect thermometer" to probe the electron temperature of the CNT. The effect of the bias voltage on the electron temperature provides a means to determine the energy relaxation length for the electrons in the CNT.
1620 GEOGRAPHOS AND 433 EROS: SHAPED BY PLANETARY TIDES?
(University of Chicago Press, 1999) BOTTKE, W. F. JR.; RICHARDSON, D. C.; MICHEL, P.; LOVE, S. G.
Until recently, most asteroids were thought to be solid bodies whose shapes were determined largely by collisions with other asteroids. Recent work by Burns and others has shown that many asteroids may be little more than rubble piles, held together by self-gravity ; this means that their shapes may be strongly distorted by tides during close encounters with planets. Here we report on numerical simulations of encounters between an ellipsoid-shaped rubble-pile asteroid and Earth. After an encounter, many of the simulated asteroids develop the same rotation rate and distinctive shape as 1620 Geographos (i.e., highly elongated with a single convex side, tapered ends, and small protuberances swept back against the rotation direction). Since our numerical studies show that these events occur with some frequency, we suggest that Geographos may be a tidally distorted object. In addition, our work shows that 433 Eros, which will be visited by the NEAR spacecraft in 1999, is much like Geographos, suggesting that it too may have been molded by tides in the past.
182W and HSE constraints from 2.7 Ga komatiites on the heterogeneous nature of the Archean mantle
(Elsevier - Geochimica et Cosmochimica Acta, 2018) Puchtel, Igor S; Blichert-Toft, Janne; Touboul, Mathieu; Walker, Richard J.
While the isotopically heterogeneous nature of the terrestrial mantle has long been established, the origin, scale, and longevity of the heterogeneities for different elements and isotopic systems are still debated. Here, we report Nd, Hf, W, and Os isotopic and highly siderophile element (HSE) abundance data for the Boston Creek komatiitic basalt lava flow (BCF) in the 2.7 Ga Abitibi greenstone belt, Canada. This lava flow is characterized by strong depletions in Al and heavy rare earth elements (REE), enrichments in light REE, and initial epsilon143Nd = +2.5 ± 0.2 and initial epsilon176Hf = +4.2 ± 0.9 indicative of derivation from a deep mantle source with time-integrated suprachondritic Sm/Nd and Lu/Hf ratios. The data plot on the terrestrial Nd-Hf array suggesting minimal involvement of early magma ocean processes in the fractionation of lithophile trace elements in the mantle source. This conclusion is supported by a mean mu142Nd = -3.8 ± 2.8 that is unresolvable from terrestrial standards. By contrast, the BCF exhibits a positive 182W anomaly (mu182W = +11.7 ± 4.5), yet is characterized by chondritic initial gamma187Os = +0.1 ± 0.3 and low inferred source HSE abundances (35 ± 5% of those estimated for the present-day Bulk Silicate Earth, BSE). Collectively, these characteristics are unique among Archean komatiite systems studied so far. The deficit in the HSE, coupled with the chondritic Os isotopic composition, but a positive 182W anomaly, are best explained by derivation of the parental BCF magma from a mantle domain characterized by a predominance of HSE-deficient, differentiated late accreted material. According to the model presented here, the mantle domain that gave rise to the BCF received only ~35% of the present-day HSE complement in the BSE before becoming isolated from the rest of the convecting mantle until the time of komatiite emplacement at 2.72 Ga. These new data provide strong evidence for a highly heterogeneous Archean mantle in terms of absolute HSE abundances and W isotopic composition, and also indicate slow mixing, on a timescale of at least 1.8 billion years. Additionally, the data are consistent with a stagnant-lid plate tectonic regime in the Hadean and Archean, prior to the onset of modern-style plate tectonics.
1991 Human-Computer Interaction Laboratory Video Reports
(2000-06-23) Plaisant, Catherine (Editor)
Introduction - Ben Shneiderman, Scheduling home control devices - Catherine Plaisant, Ben Shneiderman, Touchscreen toggles - Catherine Plaisant , A home automation system - Reuel Launey (Custom Command Systems), PlayPen II (now known as PenPlay II) : A novel fingerpainting program - Andrew Sears, Ben Shneiderman, Touchscreen keyboards - Andrew Sears, Ben Shneiderman, Pie menus - Don Hopkins, Three interfaces for browsing tables of contents - Rick Chimera (Also cross-referenced as CAR-TR-791)
1992 Human-Computer Interaction Laboratory Video Reports
(1998-10-15) Plaisant, Catherine (Editor)
Introduction - Ben Shneiderman, [3:00], Dynamic Queries: database searching by direct manipulation - Ben Shneiderman, Chris Williamson, Christopher Ahlberg, [10:55], Treemaps for visualizing hierarchical information - Ben Shneiderman, Brian Johnson, Dave Turo, [11:25], Three strategies for directory browsing - Rick Chimera, [10:30], Filter-Flow metaphor for boolean queries - Degi Young, Ben Shneiderman, [6:35], The AT&T Teaching Theater: active learning through computer supported collaborative courseware - Kent Norman, [8:25], ACCESS: an online public access catalog at the Library of Congress - Gary Marchionini, [8:15] Remote Direct Manipulation: a telepathology workstation - Catherine Plaisant, Dave Carr, [7:30], Guiding automation with pixels: a technique for programming in the user interface - Richard Potter, [11:50] (Also cross-referenced as CAR-TR-792)
1993 Human-Computer Interaction Laboratory Video Reports
(1998-10-15) Plaisant, Catherine (Editor)
Introduction and table of contents - Ben Shneiderman, [4:00] Dynamaps: dynamic queries on a health statistics atlas - Catherine Plaisant and Vinit Jain, [6:34], Hierarchical visualization with Treemaps: making sense of pro basketball data - Dave Turo, [10:47], TreeViz: file directory browsing - Brian Johnson, [10:04], HyperCourseware: computer integrated tools in the AT&T Teaching Theater - Kent Norman, [7:08], Improving access to medical abstracts: Grateful Med Interface prototype - Gary Marchionini, [6:08], Layout appropriateness: guiding interface desi gn with simple task descriptions - Andrew Sears, [4:00] (Also cross-referenced as CAR-TR-793)
1994 Human-Computer Interaction Laboratory Video Reports
(1998-10-15) Plaisant, Catherine (Edited by); Reesch, John (Video by)
80 minute video demonstrations of the past year's research Topics are: Introduction and table of contents - Ben Shneiderman, [3:18] Visual information seeking using the FilmFinder - Christopher Ahlberg, Ben Shneiderman, [6:12] Organization overviews and role management-Inspiration for future desktop environments - Catherine Plaisant, Ben Shneiderman, [9:39] Visual decision-making: using treemaps for the analytic hierarchy process - Toshiyuki Asahi, Ben Shneiderman, David Turo, [8:34] Visual information management for satellite network configuration-Catherine Plaisant, Harsha Kumar, Marko Teittinen, Ben Shneiderman, [8:49] Graphical macros: a technique for customizing any application using pixel-pattern matching-Richard Potter, [9:49] Education by engagement and construction: can distance learning be better than face to face?- Ben Shneiderman, [15:00] Dynamic queries demos: revised HomeFinder and text version plus health statistics atlas-Ben Shneiderman, [9:40] Dynamic Queries are user controlled displays of visual or textual information. Ben Shneiderman presents the HomeFinder (developed by Chris Williamson), followed by the text version (Vinit Jain) and the Health Statistics Atlas (Catherine Plaisant and Vinit Jain). CHI '94 slide and video show- [9:12]Open House '94 Video (Also cross-referenced as CAR-TR-794)
1995 Human-Computer Interaction Laboratory Video Reports
(2000-07-21) Plaisant, Catherine (Edited by)
49 minute video of the labs work over the past year. Topics are: Introduction and table of contents - Ben Shneiderman Using Dynamic Queries for Youth Services Information - Anne Rose, Ajit Vanniamparampil Life-Lines: Visualizing Personal Histories - Brett Milash, Catherine Plaisant, Anne Rose Dynamic Queries and Pruning for Large Tree Structures - Harsha Kumar Browsing Anatomical Image Databases : the Visible Human - Flip Korn, Chris North Spinning Your Web: WWW Interface Design Issues - Vince Boisselle BizView : Managing Business and Network Alarms - Catherine Plaisant, Wei Zhao and Rina Levy Animated Specifications Using Interaction Object Graphs - David Carr WinSurfer: Treemaps for Replacing the Windows File Manager - Marko Teittinen (Also cross-referenced as CAR-TR-795)
1996 HCIL Video Reports
(1998-12-05) Plaisant, Catherine
Elastic Windows for Rapid Multiple Window Management Life-Lines: Visualizing Personal Histories Designing Interfaces for Youth Services Information Management Query Previews in Networked Information Systems the Case of EOSDIS Baltimore Learning Communities Table of Contents of the 1995 HCIL Video Reports Table of Contents of the 1994 HCIL Video Reports Visual Information Seeking using the FilmFinder (Extract from the HCIL1994 Video Report
A 20-YEAR CLIMATOLOGY OF GLOBAL ATMOSPHERIC METHANE FROM HYPERSPECTRAL THERMAL INFRARED SOUNDERS WITH SOME APPLICATIONS
(2022) Zhou, Lihang; Warner, Juying; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Atmospheric Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2), and accounts for approximately 20% of the global warming produced by all well-mixed greenhouse gases. Thus, its spatiotemporal distributions and relevant long-term trends are critical to understanding the sources, sinks, and global budget of atmospheric composition, as well as the associated climate impacts. The current suite of hyperspectral thermal infrared sounders has provided continuous global methane data records since 2002, starting with the Atmospheric Infrared Sounder (AIRS) onboard the NASA EOS/Aqua satellite launched on 2 May 2002. The Cross-track Infrared Sounder (CrIS) was launched onboard the Suomi National Polar Orbiting Partnership (SNPP) on 28 October 2011 and then on NOAA-20 on 18 November 2017. The Infrared Atmospheric Sounding Interferometer (IASI) was launched onboard the EUMETSAT MetOp-A on 19 October 2006, followed by MetOp-B on 17 September 2012, then Metop-C on 7 November 2018. In this study, nearly two decades of global CH4 concentrations retrieved from the AIRS and CrIS sensors were analyzed. Results indicate that the global mid-upper tropospheric CH4 concentrations (centered around 400 hPa) increased significantly from 2003 to 2020, i.e., with an annual average of ~1754 ppbv in 2003 and ~1839 ppbv in 2020. The total increase is approximately 85 ppbv representing a +4.8% change in 18 years. More importantly, the rate of increase was derived using satellite measurements and shown to be consistent with the rate of increase previously reported only from in-situ observational measurements. It further confirmed that there was a steady increase starting in 2007 that became stronger since 2014, as also reported from the in-situ observations. In addition, comparisons of the methane retrieved from the AIRS and CrIS against in situ measurements from NOAA Global Monitoring Laboratory (GML) were conducted. One of the key findings of this comparative study is that there are phase shifts in the seasonal cycles between satellite thermal infrared measurements and ground measurements, especially in the middle to high latitudes in the northern hemisphere. Through this, an issue common in the hyperspectral thermal sensor retrievals were discovered that was unknown previously and offered potential solutions. We also conducted research on some applications of the retrieval products in monitoring the changes of CH4 over the selected regions (the Arctic and South America). Detailed analyses based on local geographic changes related to CH4 concentration increases were discussed. The results of this study concluded that while the atmospheric CH4 concentration over the Arctic region has been increasing since the early 2000s, there were no catastrophic sudden jumps during the period of 2008-2012, as indicated by the earlier studies using pre-validated retrieval products. From our study of CH4 climatology using hyperspectral infrared sounders, it has been proved that the CH4 from hyperspectral sounders provide valuable information on CH4 for the mid-upper troposphere and lower stratosphere. Future approaches are suggested that include: 1) Utilizing extended data records for CH4 monitoring using AIRS, CrIS, and other potential new generation hyperspectral infrared sensors; 2). Improving the algorithms for trace gas retrievals; and 3). Enhancing the capacity to detect CH4 changes and anomalies with radiance signals from hyperspectral infrared sounders.
THE 3' UTR OF TURNIP CRINKLE VIRUS INTERACTS LOCALLY AND DISTALLY TO REGULATE TRANSCRIPTION AND TRANSLATION OF THE VIRUS
(2012) Young, Megan Yoke Len; Simon, Anne E; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Turnip crinkle virus (TCV) is a 4054 b positive-strand RNA virus of the genus Carmovirus in the Family Tombusviridae. Upon entry into cells, TCV is translated using host translational machinery to produce its RNA-dependent RNA polymerase (RdRp). The RNA is proposed to undergo a conformational rearrangement, mediated by recruitment of the RdRp to the 3' ends of the viral RNA, which represses translation and promotes negative-strand synthesis. A second RNA switch is proposed to occur that inhibits minus-strand synthesis and promotes recruitment of the RdRp to the 3' ends of negative-strands for the asymmetrical production of positive-strands. Within the 3' UTR of TCV is a tRNA-shaped structure (TSS) that is capable of binding ribosomes and overlaps with structures necessary for translational enhancement. The RdRp has been shown to bind within this region and result in a widespread conformational shift. The binding of RdRp to the 3' end of the virus is very sensitive to perturbations of sequence or structure, with many mutations resulting in non-specific binding of the RdRp. The elements within the 3' UTR have been shown to be very interactive with alterations affecting the structure of regions hundreds of bases away. A second-site mutation study indicated that regions upstream of the 3' UTR may also be interacting with the 3' UTR. Some second-site mutations located in this upstream region were found to increase accumulation in protoplasts and additional studies are under way to explain this phenomenon. The 3' viral contribution in a luciferase reporter construct was increased to incorporate the second-site mutations. While the second-site mutations had little effect on translation, it was surprising to find that extension of the viral 3' sequence enhanced translation. Translational enhancement was mapped to just an additional twenty bases and further study revealed that a hairpin (H3) is important for viral translation and accumulation and may also be interacting with the 3' UTR.
3D Ionospheric Effects on HF Propagation and Heating
(2015) Zawdie, Katherine A.; Papadopoulos, Konstantinos; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The thesis uses a three-dimensional, first-principles model of the ionosphere in combination with High Frequency (HF) raytracing model to address key topics related to the physics of HF propagation and artificial ionospheric heating. In particular: 1. Explores the effect of the ubiquitous electron density gradients caused by Medium Scale Traveling Ionospheric Disturbances (MSTIDs) on high-angle of incidence HF radio wave propagation. Previous studies neglected the all-important presence of horizontal gradients in both the cross- and down-range directions, which refract the HF waves, significantly changing their path through the ionosphere. The physics-based ionosphere model SAMI3/ESF is used to generate a self-consistently evolving MSTID that allows for the examination of the spatio-temporal progression of the HF radio waves in the ionosphere. 2. Tests the potential and determines engineering requirements for ground- based high power HF heaters to trigger and control the evolution of Equatorial Spread F (ESF). Interference from ESF on radio wave propagation through the ionosphere remains a critical issue on HF systems reliability. Artificial HF heating has been shown to create plasma density cavities in the ionosphere similar to those that may trigger ESF bubbles. The work explores whether HF heating may trigger or control ESF bubbles. 3. Uses the combined ionosphere and HF raytracing models to create the first self-consistent HF Heating model. This model is utilized to simulate results from an Arecibo experiment and to provide understanding of the physical mechanism behind observed phenomena. The insights gained provide engineering guidance for new artificial heaters that are being built for use in low to middle latitude regions. In accomplishing the above topics: (i) I generated a model MSTID using the SAMI3/ESF code, and used a raytrace model to examine the effects of the MSTID gradients on radio wave propagation observables; (ii) I implemented a three- dimensional HF heating model in SAMI3/ESF and used the model to determine whether HF heating could artificially generate an ESF bubble; (iii) I created the first self-consistent model for artificial HF heating using the SAMI3/ESF ionosphere model and the MoJo raytrace model and ran a series of simulations that successfully modeled the results of early artificial heating experiments at Arecibo.
3D Magnetic Imaging using SQUIDs and Spin-valve Sensors
(2016) Jeffers, Alex; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
We have used 2 µm by 4 µm thin-film Cu-Mn-Ir spin-valve sensors and high Tc YBa2Cu3O7-x dc SQUIDs to take magnetic images of test samples with current paths that meander between 1 and 5 metallization layers separated by 1 µm to 10 µm vertically. I describe the development and performance of a 3D magnetic inverse for reconstructing current paths from a magnetic image. I present results from this inverse technique that demonstrate the reconstruction of the 3D current paths from magnetic images of samples. This technique not only maps active current paths in the sample but also extracts key parameters such as the layer-to-layer separations. When imaging with 2 µm by 4 µm spin-valve sensors I typically applied currents of 1 mA at 95 kHz and achieved system noise of about 200 nT for a 3 ms averaging time per pixel. This enabled a vertical resolution of 1 µm and a lateral resolution of 1 µm in the top layers and 3 µm in the bottom layer. For our roughly 30 µm square SQUID sensors, I typically applied currents of 1 mA at 5.3 kHz, and achieved system noise of about 200 pT for a 3 ms averaging time per pixel. The higher sensitivity compared to the spin-valve sensor allowed me to resolve more deeply buried current paths.
5-MODIFIED LIPOPHILIC G-QUADRUPLEXES: STRUCTURAL STUDIES AND APPLICATIONS AS SCAFFOLDS FOR [2+2] PHOTOCYCLOADDITIONS
(2019) Sutyak, Keith Brandon; Davis, Jeffery T; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Guanosine self-assembly is a powerful tool for constructing nanostructures and materials with intriguing properties. Guanosine’s nucleobase has a hydrogen bond donor and acceptor face that in the presence of an appropriate templating cation can self-assemble to form larger G-quadruplex structures. Through the addition of a nonpolar solubilizing group on the 2, 3, or 5 position of guanosine’s ribose sugar, the G-quadruplex may be formed in organic solvents. Not surprisingly, any modification made to the guanine nucleobase can have a substantial effect on the self-assembly of the G-quadruplex core. These alterations can significantly alter or destroy the ability of the guanine base to form larger H-bond structures. For this reason, a major target for the synthetic modification of the G-quadruplex is the ribose sugar. Modifications to the sugar can be used to add new functionality or groups capable of modulating the G-quadruplexes stability and structure. The 5-position is easily esterified and is an ideal target for incorporating new functionality to the G-quadruplex, not not much was known about how esterification of the 5-position of guanosine impacted self-assembly. In this thesis, a series of 5-modified aryl esters of guanosine were synthesized G 21-G 25, comprising either activating or de-activating substituents on the aromatic rings, to systematically investigate how the addition of an ester to the 5- position of guanosine affects the self-assembly G-quadruplex. The identity of the 5-aryl ester was found to have a direct impact on the molecularity (how many Gs are in each assembly) and stability of the G-quadruplexes. The information gained from these experiments was applied to rationally design a new G-quadruplex capable of templating a [2+2] photocycloaddition reaction. With an increased knowledge of how modifications to the 5-position affect the structural integrity of the G-quadruplex, we applied this information to rationally design a new G-quadruplex system capable of templating a [2+2] photocycloaddition reaction. To achieve this goal a series of 5-cinnamate esters G 26-G 29 were synthesized and studied. Each derivative G 26- G 29 when self-assembled into a G-quadruplex an photoirradiated underwent a photocycloadditon reaction ins high yields with good stereoselectivity. These same compounds when disassembled only underwent trans-cis photoisomerization. Highlighting the need for the G-quadruplex self-assembly.
Ab initio Lattice Dynamics and Infrared Dielectric Response
(2004-11-24) lawler, hadley Mark; Shirley, Eric L.; Drew, H. Dennis; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Methods for theoretically evaluating lattice dynamics, anharmonic effects and related optical properties from first principles are designed and implemented. Applications of density-functional theory and the pseudopotential approximation are adapted, via the Born-Oppenheimer approximation, the Hellmann-Feynman force theorem, and wave-commensurate supercells, to a direct calculation of the Born-von Karman force constants. With a symmetry analysis and interpolation of Born-von Karman force constants, the complete phonon spectra are obtained for the cubic systems Ar, Si, Ge, and diamond, and for the stacked hexagonal system, graphite. The phonon spectra for the polar materials GaAs and GaP, in which the degeneracy between longitudinal and transverse optical modes is lifted, are also calculated. The splitting is a consequence of the macroscopic field associated with long-range Coulombic interactions and longitudinal displacements. Diagramatically-derived expressions for the finite lifetime of the Raman mode arising from phonon-phonon interactions are calculated for Si, Ge, and diamond from first principles, and agree with experiment to within uncertainty. The infrared absorption spectra of GaAs and GaP are calculated from first principles through the phonon anharmonic self-energy (phonon-phonon interaction) and the Born effective charges (photon-phonon interaction). Several aspects of the spectra are in detailed agreement with the experimental spectra, including the strong temperature dependence of the far-infrared absorption due to the onset of difference processes; the linewidth and asymmetric lineshape of the reststrahlen; the spectral structure of the absorption by two-phonon modes, and overall oscillator strengths. The theory allows for the identification of narrow spectral transmission bands with an ionic mass mismatch in the case of GaP. Analytic and complete calculations are performed for the ion-ion displacement correlation function in solid Ar, and agree well. The correlations are evaluated for arbitrary lattice vector and Cartesian displacement directions, and their pressure dependence leads to the conjecture that anharmonic effects are less prominent at higher pressures.
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.
ABOVE THE CLOUDS: 1-D MODELING OF OBSERVATIONS OF TIDALLY LOCKED EXTRASOLAR WORLDS
(2019) Afrin Badhan, Mahmuda; Deming, L. Drake; Domagal-Goldman, Shawn D.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Unique and exotic planetary environments give us an opportunity to understand how planetary systems form and evolve over their lifetime, by placing our planetary system in the context of vastly different extrasolar systems. With orbital separations a fraction of the Mercury-Sun distance, these close-in planets provide us with valuable insights regarding interactions between the stellar and planetary atmospheres. Further, observational biases actually allow such planets to be the first to be observed via transit spectroscopy. Observed spectrophotometric signatures from transit measurements can reveal spectrally active species in a planet’s atmosphere. Present observational technologies can also shed light on the atmosphere’s structure and dynamics. Future missions will allow us to constrain these properties with unprecedented accuracy, and are also being designed to observe increasingly smaller, cooler and less extreme planets. The eventual goal, after all, is to identify a world like our own. To interpret the observations with any certainty, however, we must build robust atmospheric models that sufficiently factor both physical and chemical processes expected in those atmospheres. 3-D climate modeling has shown that tidally-locked Earth-like planets, at the inner edge of M dwarf habitable zones, may retain water-vapor-rich stratospheres. However, flaring M dwarfs have strong UV activity, which may photodisassociate H2O. Using synthetical stellar UV within a 1-D photochemical model, I assess whether water vapor loss driven by high stellar UV would affect its detectability in JWST/MIRI transmission spectroscopy. I pseudo-couple a 3-D climate model to our 1-D model to achieve this. In a follow-up study, I also compute 125 additional atmospheric states by varying the Earth-like planet’s orbital distance (thus moistness) and methane production rates. I check for and quantify the simultaneous presence of detectable ozone and methane in an otherwise abiotic anoxic atmosphere. I have also implemented techniques to robustly quantify atmospheric properties of hot Jupiters from data-driven retrievals and built a versatile template for hot Jupiter atmospheres within our 1-D photochemical modeling tool, which was previously only valid for cool rocky worlds. I sketch out a plan for using this work towards mapping non-equilibrated (non-LTE) emissions from methane in the upper atmospheres of observable giants.
Absolutely Continuous Spectrum for Parabolic Flows/Maps
(2016) Simonelli, Lucia Dora; Forni, Giovanni; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This work is devoted to creating an abstract framework for the study of certain spectral properties of parabolic systems. Specifically, we determine under which general conditions to expect the presence of absolutely continuous spectral measures. We use these general conditions to derive results for spectral properties of time-changes of unipotent flows on homogeneous spaces of semisimple groups regarding absolutely continuous spectrum as well as maximal spectral type; the time-changes of the horocycle flow are special cases of this general category of flows. In addition we use the general conditions to derive spectral results for twisted horocycle flows and to rederive spectral results for skew products over translations and Furstenberg transformations.
Abstract Elementary Classes with Lowenheim-Skolem Number Cofinal with Omega
(2008-08-03) Johnson, Gregory Mitchell; Kueker, David W; Mathematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
An abstract elementary class is a class $\aec$ of structures for some vocabulary $L$ together with a ``strong substructure'' relation $\prec_{\aec}$ on $\aec$ satisfying certain axioms. Abstract elementary classes include elementary classes with elementary substructure and classes axiomatizable in $L_{\infty,\omega}$ with elementary substructure relative to some fragment of $L_{\infty,\omega}$. For every abstract elementary class there is some number $\kappa$, called the L\"owenheim-Skolem number, so that every structure in the class has a strong substructure of cardinality $\leq \kappa$. We study abstract elementary classes with L\"owenheim-Skolem number $\kappa$, where $\kappa$ is cofinal with $\omega$, which have finite character. We generalize results obtained by Kueker for $\kappa=\omega$. In particular we show that $\aec$ is closed under $L_{\infty,\kappa}$-elementary equivalence and obtain sufficient conditions for $\aec$ to be $L_{\infty,\kappa}$-axiomatizable. The results depend on developing an appropriate concept of $\kappa$-a.e.
The Abundance and Distribution of Transparent Exopolymer Particles in the Turbidity Maximum Region of Chesapeake Bay
(2010) Malpezzi, Michael A.; Crump, Byron C; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Transparent exopolymer particle (TEP) concentrations were measured in the turbidity maximum (ETM) region of Chesapeake Bay during eight research cruises over a two-year period. TEP concentrations ranged from <100 to >2500 ug XG eq l^-1 and accounted for an estimated average of 31% ± 14 of POC. Spatially averaged TEP and chl a concentrations were positively correlated over the two year period, although these parameters were rarely correlated within cruises. Peak TEP concentrations were often separated from chl a maxima, suggesting that formation and concentration processes are more responsible for TEP concentrations than the proximity to precursor source material. Significant correlations between TEP and phaeophytin, POC, DOC, TSS and level of stratification were observed during some sampling periods. Settling tube experiments revealed a positive correlation between TEP concentration and the fraction of settling particulate matter. A hypothetical model for TEP formation and concentration in estuaries is proposed.

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