Browsing by Author "Neilson, James R., committee member"

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Open Access
Air quality implications from oxidation of anthropogenic and biogenic precursors in the troposphere
(Colorado State University. Libraries, 2019) Link, Michael F., author; Farmer, Delphine, advisor; Fisher, Ellen R., committee member; Neilson, James R., committee member; Jathar, Shantanu H., committee member; Ravishankara, Akkihebbal R., committee member
Oxidation chemistry in the troposphere drives the formation of air pollutants, harmful to human health and the natural world. Emissions from both anthropogenic and biogenic sources control the ways in which air pollution is formed and thus understanding the chemistry of the oxidation of these emissions enhances our ability to predict how air quality evolves in the future. Experiments simulating tropospheric oxidation chemistry on anthropogenic point sources show that identifying unique chemical processes resulting in air pollution allow for a greater specificity in how to pursue strategies for pollution mitigation policy with regional and hemispheric implications. This thesis focuses on the implementation of advancements in instrumentation and experimental techniques to understand how tropospheric oxidation of anthropogenic and biogenic precursors can produce air pollution. First, we subject vehicle exhaust to simulated tropospheric oxidation and quantify the formation of particulate matter and a toxic gas, isocyanic acid. We estimate how important oxidation of vehicle emissions are for these atmospheric pollutants for the South Coast Air Basin of California and the Seoul Metropolitan Region. Second, we investigate the propensity for isoprene to produce formic and acetic acid in laboratory oxidation experiments. We find that isoprene is likely a major source of formic acid in biogenically-influenced environments, however the exact mechanisms for formation remain unclear. Lastly, we use chemical ionization mass spectrometer measurements to quantify the fraction of oxidized carbon allocated to gas-phase organic acids from isoprene oxidation in laboratory experiments. Through comparison with field measurements from a forest in Alabama to a forest in Colorado we determine high levels of isoprene in Alabama are responsible for high levels of organic acids compared to Colorado. We also observe that influences of anthropogenic NOₓ suppress the formation of gas-phase organic acids suggesting as NOₓ levels decrease throughout the US in the future organic acids produced from oxidation from isoprene are likely to increase.
Open Access
Brillouin light scattering study of linear and nonlinear spin waves in continuous and patterned magnetic thin films
(Colorado State University. Libraries, 2014) Liu, Hau-Jian Jason, author; Buchanan, Kristen S., advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Neilson, James R., committee member
This thesis focuses on the use of the Brillouin light scattering (BLS) technique to measure spin waves or magnons in thin films. BLS is an experimental technique that measures the inelastically scattered light from photon-magnon interactions. Broadly, three different experiments are presented in this thesis: the measurements of spin wave properties in iron cobalt (FeCo), yttrium iron garnet (YIG), and microstructures involving Permalloy (Ni80Fe20) and cobalt nickel (CoNi). First, conventional backward scattering BLS was used to measure the spin waves in a set of Fe65Co35 films that were provided by Seagate Technologies. By fitting the spin wave frequencies that were measured as a function of the external magnetic field and film thickness, the quantum mechanical parameter responsible for short range order, known as the exchange parameter, was determined. Second, nonlinear spin waves were measured in YIG using conventional forward scattering BLS with time resolution. Two nonlinear three wave processes were observed, namely, the three magnon splitting and confluence. The nonlinear power threshold, the saturation magnetization, and the film thickness were determined independently using network analyzer measurements. The spin wave group velocities were determined from the space- and time-resolved BLS data and compared to calculations from the dispersion relations. Back calculations showed the location where the three magnon splitting process took place. Lastly, spin waves in Permalloy and CoNi microstrips were measured using a recently developed micro-BLS. The micro-BLS, with a spatial resolution of 250 nm, allows for measuring the effects on the lateral confinement of spin waves in microstrips. The confinement of spin waves led to modifications to the dispersion relations, which were compared against the spin wave frequencies obtained from the micro-BLS. The Permalloy experiments shows non-reciprocity in surface spin wave modes with opposite wavevectors and provides a quantitative measure of the difference in excitation efficiency between the surface spin wave and the backward volume spin wave modes. Measurements were also conducted in the Permalloy microstrips at zero external magnetic field, showing evidence that propagating spin waves can be observed by exploiting the effects of shape anisotropy. Finally, preliminary measurements were done on CoNi microstrips with perpendicular anisotropy. A magnetic signal was detected, however further investigation will be needed to determine the exact origin of the observed signal and to definitively answer the question as to whether or not BLS can be used to measure spin waves in perpendicularly magnetized films. Overall, the experiments and results presented in this thesis show that BLS is a useful tool for measuring spin wave properties in magnetic thin films.
Open Access
Developing paper-based devices for mapping agricultural pesticides and environmental contaminants
(Colorado State University. Libraries, 2021) Menger, Ruth F., author; Henry, Charles S., advisor; Borch, Thomas, advisor; Ravishankara, A. R., committee member; Neilson, James R., committee member; Trivedi, Pankaj, committee member
The detection of environmental contaminants is important to ensure the health of both humans and the environment. Currently, detection is done by instrumentation like liquid or gas chromatography coupled with mass spectrometry. While sensitive and selective for multiple analytes, these instruments suffer from disadvantages like large size, high sample cost, and the need for a trained analyst to run the samples. As an alternative, microfluidic paper-based analytical devices (µPADs) are becoming more common as inexpensive, fast, easy to use devices to detect and quantify a variety of analytes. My research has been focused on developing µPADs for three different analytes: pesticides, PFAS, and heavy metals. In order to ensure proper crop protection and pest management, it is important to manage and optimize pesticide application. Currently, this is done by water-sensitive papers, which often inaccurately portray the presence of pesticide due to humidity and extraneous water droplets that are not pesticide. In Chapter 2, I have developed a method that uses filter paper to capture a fluorescent tracer dye that has been mixed with the pesticide and then sprayed over the crop. The filter papers are imaged with a lightbox and Raspberry Pi camera system and then analyzed to determine percent coverage. After optimization and validation of the method to WSP, the filter paper method was used to evaluate pesticide distribution in a citrus grove in Florida (Chapter 3). The data from these field studies was used to make recommendations for which application method is best for the different types of pesticides. Paper-based devices are inherently limited by the inability to control fluid properties like mixing. In order to incorporate mixing but also retain a small device that does not require external power to initial flow, a microfluidic device was fabricated out of two glass slides. A staggered herringbone pattern is laser ablated into the slides, and a channel is formed by double-sided adhesive (Chapter 4). Mixing was quantified using blue and yellow dyes. A reaction between horseradish peroxidase and hydrogen peroxide was used as a representative enzymatic reaction and also to determine enzyme kinetics. Since the microfluidic device is made of glass, it is also compatible with non-aqueous solvents. Paper-based devices do not work well with organic solvents because the hydrophobic wax on the paper is dissolved by the solvent. In Chapter 5, the dissertation returns to traditional µPADs for environmental contaminants. Per- and polyfluoroalkyl substances (PFAS) are class of compounds that are highly persistent, toxic, bioaccumulative, and ubiquitous. While multiple instrument-based methods exist for sensitive and selective detection in a variety of matrices, there is a huge need for a fast, inexpensive, and easy-to-use sensor for PFAS detection. This would enable widespread testing of drinking water supplies, ensuring human health. A µPAD was developed for the detection of perfluorooctane sulfonate (PFOS) where the ion-pairing of PFOS and methylene green forms a purple circle. The diameter of the purple circle can be measured by the naked eye with a ruler or with the help of a smartphone to correlate the diameter back to PFOS concentration. At a cost of cents per sample, this µPAD enables fast and inexpensive detection of PFOS to ensure safe drinking water. A common issue with environmental µPADs is the relatively high limits of detection compared to what is needed for regulatory purposes. It can be challenging to lower the limits of detection without incorporating an external pretreatment and/or preconcentration step. As µPADs are small and handle only a small volume of sample (<120 µL), there is the possibility of increasing the sample capacity of the device but without significantly increasing the device size or analysis time. By adding multiple layers of absorbent filter paper underneath radial device for heavy metal detection, the sample volume increased to 1 mL, decreasing the limit of detection for a radial copper detection card from 100 ppb to 5 ppb (Chapter 6). The research presented here achieves the goal of developing µPADs for environmental contaminants. They can be used in different ways to visualize the presence of the contaminant for monitoring and management purposes, ultimately ensuring human and environmental health.
Open Access
Effects of the addition of boron – nitride nanoplatelets to hydroxyapatite: processing, testing, and characterization
(Colorado State University. Libraries, 2017) Aguirre, Trevor G., author; Holland, Troy B., advisor; Radford, Donald W., committee member; Neilson, James R., committee member
Bioceramics range in biocompatibility from inert oxides that do not react with the body to the other extreme of materials that completely absorbed by the human body, but are prone to failure by fracture. Limited fracture toughness (KIC) and flexural strength (σFS) are major factors limiting wider scale application as structural implant materials. KIC and σFS of ceramics can be improved through grain size refinement and through the addition of various reinforcement materials. The bioceramic hydroxyapatite (HA), the primary inorganic component of bone, has excellent osteoconductivity which offers a suitable surface for new bone growth and integration but suffers from low KIC. To improve the KIC of HA we used boron nitride nanoplatelets (BNNPs), a strong and biocompatible material, making them excellent candidate for use in the human body. However, these materials have been shown to cause embrittlement of the material they are incorporated in; thus, it becomes important to understand the effect of BNNPs through analysis of the failure statistics of tested samples. Using spark plasma sintering to create these materials HA – BNNP composites with 0.5, 1.0 and 2.0 wt% BNNPs were fabricated. Sample grain sizes were measured to evaluate the effect the BNNPs had on the microstructure and the flexural strength, fracture toughness, and hardness were tested to observe the effect BNNP had on the mechanical properties of HA and as well as the failure statistics. To analyze the failure statistics of the HA BNNP composites the Weibull Distribution was used because studies have shown that the Normal Distribution does not accurately report the failure statistics of brittle materials. This work summarizes the effect of the addition of BNNPs to spark plasma sintered HA. The results of this study show that BNNPs are capable of increasing flexural strength and fracture toughness through grain size refinement but BNNPs lead to a measurable decrease in the reliability of the material, which is indicative of the predictability of measured material property value and yields information about the flaw distributions in these materials.
Open Access
Fundamental research into gold nanocluster properties
(Colorado State University. Libraries, 2021) Window, Phillip S., author; Ackerson, Christopher J., advisor; Neilson, James R., committee member; Kennan, Alan J., committee member; Peersen, Olve, committee member
Gold materials are popular for research into many applications with their interesting properties, such as magnetism, bio-inactivity, and other size-dependent properties. As the size of the gold material decreases from a bulk material to the nanoscale, new properties are introduced moving through different size regimes. As the particle size reaches the 2-3 nm range and move into the quantum-confined particle range, the most interesting particle changes occur and gold nanomaterials have extremely interesting research potential. These materials exist between the bulk and molecular systems and have similar properties to both; however, they are different enough from both of these to have their own unique application possibilities. Some properties of gold nanoclusters can be attributed more to the core or more to the ligand layer of the nanocluster. Certain properties, like electronics and magnetism, are due to the superatomic electron count and electronic structure from the core and depend on the number of gold atoms in the nanocluster. Extensive research has been done on investigating and altering these properties in small nanoclusters, however, larger nanoclusters have hardly been studied as they can be more difficult to work with. Within this work is investigated the magnetism and thus electronic structure of Au102(SPh)44 and Au133(tBBT)52 in different oxidation states. Paramagnetism up to two unpaired electrons is observed with both these nanoclusters through solution phase magnetic studies. Through this, electronic structure information has been obtained to elucidate the behavior of unique superatomic 1G and 1H orbitals. Looking at the outside of a nanocluster structure, interactions of nanoclusters with other nanoclusters, molecules, surfaces, and solvents are all due to the ligand layer of the nanocluster. Investigations of the ligand layer have been performed extensively through many techniques. However, further studies are always helpful since controlling the ligand layer is essential for functionalization for potential applications. Within this work is investigated the interactions of Au25(SR)18 with other Au25 nanoclusters in both solution and solid phase, as well as ligand exchange reactions of Au133(tBBT)52. Studies on Au25(SR)18 within solution include investigations of a supramolecular assembly, or supercluster, formed solely of the nanocluster itself with control over its growth and size. Studies on Au25(SR)18 within the solid-phase include controlled crystallization techniques that result in different solid-phase structures with previously unseen properties. Ligand exchange studies have also been expanded from small nanocluster materials only in previously published studies to the large nanocluster, Au133(tBBT)52. Within this dissertation, some of the first empirical studies into the oxidation state- dependent properties of large gold nanoclusters, Au102(SPh)44 and Au133(tBBT)52, were performed. This betters the field's understanding of how many unpaired electron spins these large gold nanocluster can sustain at room temperature and further elucidates the behavior of superatomic electronic structure and behavior based on electron count. Furthermore, this dissertation presents the first investigations into the formation of supramolecular assemblies of gold nanocluster as recyclable materials, and more interactions of gold nanoclusters based on ligand layer interactions through polymorphism studies and ligand exchange studies. These investigations all help understand how to control the ligand layer for future applications of gold nanoclusters and nanoparticles, from molecular to bulk materials.
Open Access
Insight into alternative battery technologies using 3D configurations, protective coatings, and characterization of resistive properties
(Colorado State University. Libraries, 2025) Windsor, Daniel S., author; Prieto, Amy L., advisor; Neilson, James R., committee member; Shores, Matthew P., committee member; Bandhauer, Todd M., committee member
The omni presence of lithium-ion batteries (LIBs) have revolutionized the modern world due to this technology's implementation as an energy storage device in smart phones, wearable electronics, and electric vehicles. Lithium-ion batteries are well suited for these applications owing to the light weight of these systems and their ability to store a large amount of charge. For these reasons, LIBs are classified as energy dense systems, which describes the amount of energy a technology can store per unit mass. A battery metric where LIBs struggle in terms of performance is power density, or the amount of power a technology can produce per unit mass. These systems, also, require expensive feedstock materials that are geographically isolated which has profound impacts on economics and supply chain considerations for LIBs. Thus, if rechargeable batteries are to continue to advance, alternative battery configurations and chemistries must be studied. Chapter 1 describes the field of LIBs, in terms of the advantages and disadvantages of this technology. This discussion is followed by brief mentions of some of the champion materials found in the anodes, cathodes, and electrolytes currently implemented in LIBs. The discussion on the champion materials for LIBs also covers the drawbacks of each material, and ways in which future investigations can improve their performance. This is then followed by a section which highlights how alternative battery configurations and chemistries can address some of the inherent disadvantages of the LIBs system. This chapter concludes with a discussion on some important soft skills the author learned during the completion of this degree. Chapter 2 covers the development and advances made in the field of 3D batteries. This chapter begins with an introduction of the 3D battery field and includes a section which discusses the current advances made in the literature. This is then followed by a discussion on the computational advances made in the field of 3D batteries, where there is a critical need to develop digital twins of 3D batteries to better understand the chemo-mechanical dynamics of these complex systems. The following portion of this chapter covers the development of 3D batteries through the lens of critical performance metrics, being power density, energy density, and cyclability and scalability. For 3D batteries, this chapter identified that improvements in energy density is the area where further advances are most needed. Finally, this chapter discuss efforts being made in industry toward the commercialization of these 3D battery systems. Chapter 3 covers an investigation into the fundamental effect of a polymer protective coating, cyclized-polyacrylonitrile (cPAN), on the Na-ion (de)insertion chemistry of antimony-based anodes in sodium-ion batteries (NIBs). This investigation was able to determine that the cPAN coating had the most pronounced effect on the early cycle (cycles 1-10) Na-ion (de)insertion chemistry of the antimony-based anodes. The interfacial resistance was, also, diminished by the presence of the cPAN protective layer which implies that the cPAN helps to facilitate Na-ion transport at the electrode-electrolyte interface. Chapter 4 discusses a practical and beginners' approach to the learning electrochemical impedance spectroscopy (EIS) for rechargeable batteries. This chapter begins with a simple deconvolution of the EIS acronym, such that the reader has a deeper understanding of how each component of the acronym combines to create this technique. The chapter continues by discussing how to preform both qualitative and quantitative EIS analyses on rechargeable batteries, and finishes with a discussion on the EIS specifics of rechargeable battery systems. Chapter 5 covers the future areas in which the work presented in Chapter 3 can be extended. In particular this chapter discusses the critical need to quantify the SEI products of a cPAN coated antimony electrode, as early cycle numbers, and ways in which cPAN can be applied to high surface area substrates to ideally formulate a 3D sodium-ion battery.
Open Access
Investigations into photocatalysis and electronic structure for transition metal and actinide complexes
(Colorado State University. Libraries, 2018) Higgins, Robert F., author; Shores, Matthew P., advisor; Rappé, Anthony K., committee member; Neilson, James R., committee member; McNally, Andrew, committee member; Wu, Mingzhong, committee member
Presented herein are investigations into the electronic structure of various metal complexes and how they effect reactivity. The first chapters are centered on how [Cr(Ph2phen)3]3+ reacts as a photooxidant. The latter part of this work concerns magnetic properties of various first row transition metal and actinide complexes. In Chapter 1, I provide a background on how understanding electronic structure of transition metal complexes has motivated later work in reactivity. This Chapter also includes a detailed background in photoredox catalysis and different electronic structures of Ru-, Ir- and Cr-containing photosensitizers. It ends with a lead-in to our initial hypotheses and motivations for using Cr as a paramagnetic, Earth-abundant congener to Ru photosensitizers in photoredox manifolds. Chapters 2-4 illustrate our mechanistic studies into transformations using Cr as a photooxidant to perform [4+2] cycloaddition reactions between (trans and cis)-anethole and dienes. Chapter 2 focuses on the interactions of oxygen (O2) in the reaction of trans-anethole and isoprene mediated by [Cr(Ph2phen)3]3+. We determined three separate, yet invaluable roles that oxygen performs in this reaction, which include: (1) protection of the catalyst through excited-state energy-transfer giving 1O2, (2) 1O2 oxidation of the reduced form of the catalyst, regenerating the ground state species and giving 2O2•- as well as (3) 2O2•- reduction of the radical cation of the [4+2] product, completing the catalytic cycle. In Chapter 3, I discuss the association that trans-anethole and similar dienophiles show with [Cr(Ph2phen)3]3+ and how this affects the overall reactivity. Interestingly, diamagnetic analogues do not show the same association. Finally, in Chapter 4, trans-anethole is replaced with cis-anethole to determine how the overall reactivity changes. These data are supported by reactivity, kinetic and quenching studies to probe the reactivity. Chapters 5-7 concern similar mechanistic details involving [Cr(Ph2phen)3]3+ in photocatlytic cycloaddition reactions, except that trans-anethole, which is electron-rich, is replaced by 4-methoxychlacone, which is electron-poor. Chapter 5 discusses the synthetic utility of this reaction manifold and initial mechanistic details of the transformation, which reveal an orthogonal mechanism which proceeds through energy transfer when compared to the reactivity of trans-anethole with [Cr(Ph2phen)3]3+. In Chapter 6, the observation of enhanced regioselectivity that is observed when [Cr(Ph2phen)3]3+ is used is investigated, specifically in comparison to all other Cr- and Ru-photooxidants attempted. This regioselectivity is manifested in the stabilization of a one-bond intermediate, as well as an association between 4-methoxychalcone and [Cr(Ph2phen)3]3+. To conclude this section, Chapter 7 focuses on the interesting solution-phase equilibria of 4-methoxychalcone and how the association of 4-methoxychlacone with itself and [Cr(Ph2phen)3]3+ impacts the overall reaction mechanism. Chapter 8 provides an interesting method of using ferrocenium as an inexpensive and abundant electron-transfer reagent in reactions similar to common photoredox reactions. This uncommon reaction pathway provides an interesting reactivity compared to traditional pericyclic reactions. The remaining Chapters (9-13) explore the magnetic properties and electronic structures of a variety of first-row and actinide complexes and clusters. Chapter 9 focuses on spin-state switching through oxidation chemistry of both iron and nitrogen atoms in organometallic complexes. The ground states of these complexes can be controllably tuned through sequential oxidation reactions. In Chapter 10, I present the synthesis and magnetic properties of mono- and bis-terpyridine Co(II) complexes. These Co complexes display a variety of coordination geometries which affect their dynamic magnetic properties. Chapter 11 focuses on the reactivity and magnetic properties of a family of U-acetylide species, where interesting redox chemistry is noted upon addition of redox-inactive crown ether molecules. In Chapter 12, I discuss the magnetic properties of 3 different families of uranium complexes measured in collaboration with Prof. Suzanne Bart's group at Purdue University. Finally, in Chapter 13, I give some broad conclusions about what was learned in the mechanistic studies of Cr-photocatalysis and possible interesting avenues for future work.
Open Access
Part 1: Formation and nucleophilic interception of α,β-unsaturated platinum carbenes. Part 2: Efforts toward controlling magnetic properties of cobalt and iron coordination complexes
(Colorado State University. Libraries, 2017) Ozumerzifon, Tarik J., author; Shores, Matthew P., advisor; Kennan, Alan J., committee member; Neilson, James R., committee member; Achter, Jeffrey D., committee member
Presented in this dissertation are a series of studies describing the use of transition metals in several different applications. Part 1 concerns the development of novel platinum(II)-catalyzed reaction manifolds toward C-C bond formation, as well as the formal synthesis of a natural product. Meanwhile, Part 2 describes three separate efforts toward modulation of either single-molecule magnet properties in cobalt(II) or spin state control of iron(II) coordination complexes. The first chapter is a general introduction to single-molecule magnetism (SMM) and spin crossover, as these topics specifically relate to Co(II) and Fe(II) complexes, respectively. The physical origins of both phenomena are discussed, as well as some general terminology that are used throughout Chapters 3-5. Chapter 2 describes the use of Pt(II) salts in alkyne activation reactivity. The vinylogous addition of carbon nucleophiles into α,β-unsaturated platinum carbenes is discussed, and the optimization and scope of enol incorporation is provided. This is followed by a description of how Pt(II) catalysis enables the rapid formal synthesis of frondosin B, a sesquiterpene natural product. In Chapter 3, the synthesis and characterization of several salts of a trigonal prismatic cobalt(II) complex are detailed. The capping ligand used in these podands is cis-,cis-,1,3,5-triaminocyclohexane (tach), a rigid backbone which dictates coordination geometry and the iminopyridine contains pendant tert-butylamide moieties which are meant to enable guest association. The single-molecule magnet behavior (measured via slow magnetic relaxation) of these compounds is also explored, where the cation-binding tetraphenylborate salt shows slow magnetic relaxation at both zero and applied dc fields. A brief discussion of theoretical considerations of the effect of trigonal distortion on axial anisotropy is also presented, which suggests systems in an intermediate twisting geometry may give rise to guest-dependent magnetic properties in SMMs. Chapter 4 presents initial efforts toward the development of an Fe(II) system which can undergo a spin-state switch upon addition of a reagent. The chemoselective process is intended to be the result of an irreversible ligand modification. The first target toward this goal is manifested in desilylation of a 5-siloxy substituted podand. Spectroscopic and spectrometric as well as electrochemical and magnetic data indicate qualitatively that ligand desilylation is occurring as a function of fluoride addition, affecting a decrease in high-spin:low-spin ratio. Last, Chapter 5 details the systematic study of electronic character of 5-pyridyl substitution in the Fe(II) tren iminopyridine tripodal system. The Fe(II) species magnetic susceptibility and Ni(II) analog d-d transition energy data are compared to the Hammett parameter of the respective substituent, which define the complexes' electron-donating or -withdrawing properties. Overall, electron withdrawing substituents at this position lead to stabilization of the HS state. A comparison of these iminopyridine complexes to Fe(II) podands which undergo spin crossover is provided in an effort to explain the observed low-spin behavior of these complexes.
Open Access
Reaction development and mechanistic investigation of rhodium-catalyzed pyridine synthesis via C-H activation
(Colorado State University. Libraries, 2014) Neely, Jamie M., author; Rovis, Tomislav, advisor; McNally, Andrew, committee member; Fisk, John D., committee member; Neilson, James R., committee member; Inamine, Julia M., committee member
Described herein are two complementary rhodium-catalyzed methods for the synthesis of substituted pyridines from unsaturated oxime derivatives and alkenes. In the first, formal [4+2] cycloaddition of O-pivaloyl α, β-unsaturated oxime esters and activated terminal alkenes was discovered to proceed in high yields and with excellent selectivity for 6-substituted pyridine products. Mechanistic experiments were found to be consistent with a reversible C-H activation step and a C-N bond forming, N-O bond cleaving process en route to pyridine formation. Rhodium-catalyzed coupling using unactivated alkene substrates was shown to present important information regarding the influence of the alkene component on product distribution. In a second method, access to 5-substituted pyridine derivatives was achieved by decarboxylative annulation of α, β-unsaturated oxime esters and β-substituted acrylic acid derivatives. In this case, carboxylic acids were found to serve as traceless activating groups for selective alkene incorporation. A wealth of mechanistic insight was gained by identification of and decomposition studies regarding catalytically relevant rhodium complexes.
Open Access
Rhodium-catalyzed cycloadditions to construct nitrogen heterocycles and progress towards the synthesis of ionomycin
(Colorado State University. Libraries, 2014) Oberg, Kevin Martin, author; Rovis, Tomislav, advisor; Kennan, Alan J., committee member; Ferreira, Eric M., committee member; Neilson, James R., committee member; Kanatous, Shane B., committee member
The ability to construct molecules in a rapid, atom-economical fashion is a major goal of organic chemistry. This work describes four topics; pyridone synthesis, mechanistic understanding in [2+2+2] cycloadditions, pyrimidinone synthesis, and progress towards ionomycin. The first chapter describes the synthesis of 4,6-substituted 2-pyridones and 3,5-substituted 4-pyridones from the rhodium-catalyzed [2+2+2] cycloaddition of two alkynes and an isocyanate. Our group demonstrated that an enantioselective rhodium-catalyzed [2+2+2] cycloaddition of alkenyl isocyanates and alkynes generates indolizidinone and quinolizidinone products. Although trends for product and regioselectivity were established, the underlying mechanism was unclear. The second chapter describes X-ray analysis of rhodium·phosphoramidite complexes in conjunction with other mechanistic work to elucidate a theory that explains product and regioselectivity in this reaction. This system is amazing in that it illuminates the factors contributing to oxidative cycloadditions in a spectacular fashion by delivering two different products. The third chapter describes the enantioselective synthesis of pyrimidinones from a rhodium-catalyzed [4+2] cycloaddition of α, β-unsaturated imines and isocyanates. The final chapter describes our group's progress toward the synthesis of ionomycin using rhodium-catalyzed desymmetrization of anhydrides with zinc nucleophiles.
Open Access
The development and applications of light-gated cobalt catalysis
(Colorado State University. Libraries, 2017) Ruhl, Kyle E., author; Rovis, Tomislav, advisor; McNally, Andrew, committee member; Neilson, James R., committee member; Kipper, Matthew J., committee member
Transition metals are an important natural resource and an essential component of many industrial processes and applications. Examples of these include air-quality control, electronics manufacture, agriculture, pharmaceuticals, and petro-chemistry. Within the field of synthetic chemistry, transition metal catalysts minimize waste, decrease expense, and enable rapid construction of small molecules. Over the past decade, transition-metal-based polypyridyl complexes have been the cornerstone of photo-redox catalysis which facilitate electron transfer and allow synthetic chemists to functionalize inert functionalities using visible-light energy. The first chapter of this work introduces rhodium- and cobalt-catalyzed [2+2+2] cycloadditions as well as photo-redox catalysis. The following chapter covers our group's progress toward the merger of photo-redox and cobalt catalysis as well as the multi-disciplinary approach we have used to understand mechanism. The third chapter explores light-gated catalysis and its importance for spatially and temporally resolved methods. Finally, the fourth chapter focuses on the applications of light-gated cobalt catalysis. We have found a light-gated cobalt catalyst to temporally control the [2+2+2] cycloaddition, and when combined with photolithography, enable a spatially resolved method for arene formation.
Open Access
The effects of point defects and microstructure on the pseudo-elasticity of ThCr2Si2-type crystals
(Colorado State University. Libraries, 2018) Bakst, Ian Nathaniel, author; Weinberger, Christopher R., advisor; Ma, Kaka, committee member; Neilson, James R., committee member; Radford, Donald W., committee member
Ternary intermetallic compounds with the ThCr2Si2-type structure, which are known for their high-temperature superconductivity, have recently garnered interest due to the discovery of a pseudo-elastic mechanical response to compression along the c-axis. However, the effects of point defects and doping on this response remain unknown. In this work, these effects are investigated with density functional theory (DFT) in conjunction with continuum-scale models. DFT simulations of hydrostatic and uniaxial compression of pure ThCr2Si2-type crystals were conducted. The magnetic phase transition of CaFe2As2 was reproduced, while LaRu2P2 exhibited a continuous transition into its collapsed tetragonal phase. The two-phase DFT data was used to build a continuum-scale, thermodynamically-driven composite model which predicts the pseudo-elastic response of a large sample under displacement control and load control scenarios. Strain along the c-axis was shown to be the critical parameter in predicting crystal collapse. Then, DFT simulations of defected or doped unit cells were conducted to investigate their energetics and mechanical responses to compression. In some cases, the addition of vacancies effectively suppressed the pseudo-elastic response of the crystals. Simulations of crystals doped with varying concentrations revealed alterations of the mechanical properties as well. Tunable variability of the phase change with respect to dopant concentration was predicted in disordered doped structures, while multiple phase changes were predicted in ordered doped structures. Composite models were then built with the DFT data to predict the response of a sample comprised of multiple microstructures. The models predict a wide range of variability in the mechanical behavior and provide insight into how impurities and defects can be used to tune the response of these materials.
Open Access
Thermoplastic electrodes for detection of biomarkers
(Colorado State University. Libraries, 2021) McCord, Cynthia P., author; Henry, Charles S., advisor; Reynolds, Melissa M., committee member; Neilson, James R., committee member; Bark, David L., Jr., committee member
To view the abstract, please see the full text of the document.
Open Access
Vortex rectification and phase slips in superconducting granular aluminum
(Colorado State University. Libraries, 2020) Maughan, Weston F., II, author; Field, Stuart B., advisor; Gelfand, Martin, committee member; Buchanan, Kristen, committee member; Neilson, James R., committee member
Superconductivity is a unique and interesting phenomenon that manifests as a new phase of matter in a wide variety of materials. The most well-known property of superconductors is that they exhibit perfect conductivity when cooled below a critical temperature Tc. In addition to their perfect conductivity, superconductors exhibit the equally fundamental Meissner effect that expels magnetic fields from the interior of the material. While applications of a material that exhibits perfect conductivity, such as generating large magnetic fields via electromagnets or transmitting a large current with zero dissipation, are highly desired, the subtle details of flux penetration into mesoscopic samples may also be exploited to realize useful devices, or as a testbed to understand one-dimensional superconductivity. In this work, the nature of superconductivity in granular aluminum was explored in two mesoscopic sample classes: first, by studying Abrikosov vortices in films, and then by studying dissipation from phase slips in one-dimensional nanowires. The penetration of an applied field is possible in film sample geometries, even though the Meissner effect generally expels flux. This penetration occurs in type-II superconductors via quantized flux bundles through normal regions or domains of the superconductor called vortices. The behavior and dynamics of these vortices are of significant interest as they can be exploited to realize fluxonic devices that perform circuit operations analogous to the operations performed with electrons in electronics. One method to influence the motion of vortices within a superconductor in order to realize a fluxonic device is to introduce a periodic potential landscape that causes an easy and a hard direction for vortex motion. In other words, the vortex motion is rectified. By realizing a so-called vortex ratchet with such a potential landscape, vortices may easily be introduced or removed from the superconductor by driving them in the easy or hard directions respectively. We begin by studying the rectification properties of both symmetrically and asymmetrically thickness-modulated superconducting films. These thickness modulations were fabricated with an elegant method of angle-sputter deposition of granular aluminum onto a glass substrate that has a sinusoidal modulation in its thickness. We then explored the rectification of these symmetric and asymmetric films by studying the motion of vortices using cryogenic transport measurements. In these measurements, vortices are driven in both directions across a modulated sample and the resulting voltages are measured. Differences in the voltages corresponding to motion in opposite directions imply that the vortices move more readily in one direction, that is, that there is an overall rectification in their motion. While these measurements performed with the symmetric washboard film seemed to exhibit reversibility in the transport properties, the asymmetric washboard exhibited a mild asymmetry that was much smaller than expected. This result indicates that the potential landscape is influenced by another source in addition to the asymmetric thickness modulation. To better understand these effects, we tested the influence of the sample edges on the nucleation of vortices with two multi-segment films. These multi-segment films were fabricated in either an 8- or 14-probe geometry where each segment shares a vertical reference edge, while the opposing edges between pairs of voltage leads contain tapers of varying lengths which were fabricated lithographically. Clear rectification effects are observed with cryogenic transport measurements of these samples, with enhanced rectification for longer taper lengths showing the importance of the sample edge geometry on vortex motion. Following these studies in superconducting films, we explored the nature of dissipation in one-dimensional superconducting nanowires. Recent advancements in laboratory fabrication techniques have reduced the accessible size scale of superconducting samples into the nanometer regime. As a result, superconductors can be fabricated that exhibit one-dimensional superconductivity, in which the complex superconducting order parameter ψ is restricted to fluctuations along the length of the nanowire because its cross-sectional dimensions are smaller than ξ. Experiments performed with these nanowires exhibit a non-zero resistance even when the samples were cooled below Tc. This dissipation is understood as due to thermal fluctuations that cause |ψ| to vanish in a small segment of the wire of length ∼ξ, allowing the superconducting phase to "slip" by ±2π, resulting in a voltage pulse. However, several experimental studies have observed excess nanowire resistance at low temperatures that cannot be described with this thermal fluctuation model alone. Some researchers have proposed that macroscopic quantum tunneling events lead to the excess resistance, while other studies claim that nanowire inhomogeneities influence the thermally activated phase slip rate. In order to provide insight into the origin of the excess nanowire resistance, we performed cryogenic scanning experiments to map the local phase-slip rate along a superconducting nanowire. This was achieved by scanning either a dielectric or a magnetic tip with a home- built cryogenic atomic force microscope (cryo-AFM) to locally perturb superconductivity along a granular aluminum nanowire, while simultaneously measuring the nanowire resistance. This required the construction and characterization of the cryo-AFM along with a method of locating nanowire samples at cryogenic temperatures. We then fabricated one-dimensional granular aluminum nanowires with electron beam lithographic (EBL) techniques. We scanned these nanowires with the cryo-AFM and found that a dielectric tip does not locally perturb superconductivity enough to cause a measurable change in the wire resistance. However, repeating this experiment with either a magnetic tip or another material may plausibly elucidate the origins of the low-temperature nanowire dissipation.
Open Access
"You are young and can afford to do something stupid": fostering an understanding of electronic spin in chemistry
(Colorado State University. Libraries, 2021) Joyce, Justin P., author; Shores, Matthew P., advisor; Rappé, Anthony K., advisor; Neilson, James R., committee member; Reynolds, Melissa M., committee member; Ross, Kathryn A., committee member
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