Browsing by Author "Levinger, Nancy E., committee member"

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Open Access
Analytical spectroscopy method development to study mechanisms of Alzheimer's and tuberculosis diseases
(Colorado State University. Libraries, 2020) Beuning, Cheryle Nicole, author; Crans, Debbie C., advisor; Levinger, Nancy E., committee member; Barisas, George, committee member; Fisher, Ellen R., committee member; Zabel, Mark, committee member
This dissertation covers the analytical method development created to study and enhance the knowledge of two specific disease mechanisms important to Alzheimer's disease and Mycobacterium tuberculosis. There are two parts in this dissertation where Part 1 is entitled Measurement of The Kinetic Rate Constants of Interpeptidic Divalent Transition Metal Ion Exchange in Neurodegenerative Disease. Part 2 is entitled The Electrochemistry of Truncated Menaquinone Electron Transporters with Saturated Isoprene Side Chains Important in Tuberculosis. These diseases appear on the World Health Organization's top 10 leading causes of death worldwide. The amyloid-beta (Aβ) peptides are associated with Alzheimer's disease, where neurodegeneration is caused by the aggregation of the peptide into senile plaques within neuronal synaptic cleft spaces. Alzheimer's disease currently has no cure due to its multi-causative pathology. One disease mechanism is the coordination of divalent metal ions to the peptide. Specifically, Aβ coordinates Cu(II) and Zn(II) ions that can enhance the aggregation of Aβ into plaques. These metal ions are highly regulated within the human body and are usually found bound to peptides and not as free ions. Therefore, the Aβ must sequester the metals from other proteins and peptides. The primary research in this dissertation advances fluorescence method development to measure interpeptidic Cu(II) exchange kinetics to be able to measure this type of disease mechanism. The small peptides GHK (Gly – His – Lys) and DAHK (Asp – Ala – His – Lys) both chelate Cu(II) with nM affinity, have biological relevance as they are motifs found in human blood like Aβ, and chelate Cu(II) with similar nitrogen-rich binding ligands as Aβ. By substituting non-coordinating lysine residues with fluorescent tryptophan, the interpeptidic exchange rates can be measured since tryptophan fluorescence is statically quenched when within 14 angstroms of a paramagnetic bound Cu(II). Thus Cu(II) transfer from Cu(H-1GHW) to either GHK or DAHK can be monitored by recovered GHW fluorescence as the Cu(II) is exchanged and second-order kinetic rate constants were determined. This methodology was then used to monitor the Cu(II) exchange from truncated Cu(Aβ1-16) and Cu(Aβ1-28) complexes to GHW and DAHW, where second-order reaction kinetic rate constants were determined. While in the exchanges between Cu(H-1GHW) with GHK/DAHK the second-order rate constants were on the magnitude of 102 or 101 M-1s-1, respectively, the exchanges from Cu(Aβ) complexes were 2-3 orders of magnitude larger, 104 M-1s-1 (to GHW and DAHW). These differences in rate constant magnitude arise from the fact that the affinity of GHW (KA = 1013 M-1) for Cu(II) is larger than Aβ (KA =1010 M-1). This method development is an important step to an accurate measurement of the interpeptidic exchange between Aβ peptides, including in their fibril and plaque formations. Since senile plaques are found in synaptic cleft spaces with nanometer distances between neurons, a model system was generated to study coordination reactions at the nanoscale. In order to do this, the metal ion would need to be released in a controlled manner. Studies of metal ion burst reactions through the use of photocages can simulate bursts of ions like those seen in the synaptic cleft. Zn(II) is often released in its ionic form within the synapse in its function as a neurotransmitter, so we simulated a burst of Zn(II) ions by using a photocage, [Zn(NTAdeCage)]- which releases Zn(II) when irradiated with light. The photocage was irradiated to release Zn(II) then we followed its reaction progress with an in situ chelator, Zincon, in reverse micelles and in bulk aqueous buffer. The coordination reaction was 1.4 times faster in an aqueous buffer than in reverse micelles, despite the Zn(II) and Zincon being closer in the nanoparticle. These observations suggested that there is an impact on coordination reactivity within a highly heterogeneous environment with a cell-like membrane, which is due to the partitioning of each ligand. We observe that the photocage stays in the water pool of the reverse micelle and the Zincon partitions into the membrane interface. Thus, the coordination reactivity is diminished, likely due to the need for Zn(II) to diffuse to the Zincon, crossing a highly organized Stern layer to encounter the Zincon. Whereas in aqueous buffer, these are free to encounter each other despite being hundreds of nanometers apart. These proof of concept studies are integral to studying initial binding dynamics of metal ions with peptides at the nanoscale present in cells and neuronal synapses. Tuberculosis is a pathogenic bacterium which despite having a curable medication, can be drug-resistant. Menaquinone (MK) analogs with regiospecific partial saturation in their isoprenyl side chain, such as MK-9(II-H2), are found in many types of bacteria, including pathogenic Mycobacterium tuberculosis and function as electron transport lipids cycling between quinone and quinol forms within the electron transport system. While the function of MK is well established, the role of regiospecific partial saturation in the isoprenyl side chain on MK remains unclear and may be related to the redox function. Recently, an enzyme in M. tuberculosis called MenJ was shown to selectively saturate the second isoprene unit of MK-9 to MK-9(II-H2). The knockout expression of this enzyme was shown to be essential to the survival of the bacterium. A series of synthesized truncated MK-n analogs were investigated using a systematic statistical approach to test the effects of regiospecific saturation on the redox potentials. Using principal component analysis on the experimental redox potentials, the effects of saturation of the isoprene tail on the redox potentials were identified. The partial saturation of the second isoprene unit resulted in more positive redox potentials, requiring less energy to reduce the quinone. While full saturation of the isoprene tail resulted in the most negative potentials measured, requiring more energy to reduce the quinone. These observations give insight into why these partially saturated menaquinones are conserved in nature.
Open Access
Contributions of gas-phase plasma chemistry to surface modifications and gas-surface interactions: investigations of fluorocarbon rf plasmas
(Colorado State University. Libraries, 2012) Cuddy, Michael F., author; Fisher, Ellen R., advisor; Levinger, Nancy E., committee member; Rickey, Dawn, committee member; Krummel, Amber, committee member; Yalin, Azer P., committee member
The fundamental aspects of inductively coupled fluorocarbon (FC) plasma chemistry were examined, with special emphasis on the contributions of gas-phase species to surface modifications. Characterization of the gas-phase constituents of single-source CF4-, C2F6-, C3F8-, and C3F6-based plasmas was performed using spectroscopic and mass spectrometric techniques. The effects of varying plasma parameters, including applied rf power (P) and system pressure (p) were examined. Optical emission spectroscopy (OES) and laser-induced fluorescence (LIF) spectroscopy were employed to monitor the behavior of excited and ground CFx (x = 1,2) radicals, respectively. Mass spectrometric techniques, including ion energy analyses, elucidated behaviors of nascent ions in the FC plasmas. These gas-phase data were correlated with the net effect of substrate processing for Si and ZrO2 surfaces. Surface-specific analyses were performed for post-processed substrates via x-ray photoelectron spectroscopy (XPS) and contact angle goniometry. Generally, precursors with lower F/C ratios tended to deposit robust FC films of high surface energy. Precursors of higher F/C ratio, such as CF4, were associated with etching or removal of material from surfaces. Nonetheless, a net balance between deposition of FC moieties and etching of material exists for each plasma system. The imaging of radicals interacting with surfaces (IRIS) technique provided insight into the phenomena occurring at the interface of the plasma gas-phase and substrate of interest. IRIS results demonstrate that CFx radicals scatter copiously, with surface scatter coefficients, S, generally greater than unity under most experimental conditions. Such considerable S values imply surface-mediated production of the CFx radicals at FC-passivated sites. It is inferred that the primary route to surface production of CFx arises from energetic ion bombardment and ablation of surface FC films. Other factors which may influence the observed CFx scatter coefficient include the surface with which the radical interacts, the vibrational temperature (ΘV) of the radical in its gas phase, and radical interactions in the gas phase. The analyses of ΘV in particular were extended to diatomic radicals from other plasma sources, including nitric oxide and fluorosilane systems, to gauge the contributions of vibrational energy to surface reactivity. In general, a monotonic increase in S is observed for CF, NO, and SiF radicals with increasing ΘV. Preliminary results for mixed plasma precursor systems (i.e. FC/H2, FC/O2) indicate that the choice of feed gas additives has a profound effect on surface modification. Hydrogen additions tend to promote FC film deposition through scavenging of fluorine atoms, whereas oxygen consumes polymerizing species, thus favoring etching regimes. Time-resolved optical emission spectroscopy (TR-OES) studies of gas-phase species elucidate the mechanisms by which these processes occur. Ultimately, the work presented herein expands the fundamental chemical and physical understanding of fluorocarbon plasma systems.
Open Access
Exploring gas-phase plasma chemistry and plasma-surface interactions: progress in plasma-assisted catalysis
(Colorado State University. Libraries, 2020) Hanna, Angela R., author; Fisher, Ellen R., advisor; Levinger, Nancy E., committee member; Rappe, Anthony, committee member; Bradley, R. Mark, committee member
To view the abstract, please see the full text of the document.
Open Access
Impact of iron and redox chemistry on the environmental fate and transport of metalloids and radionuclides
(Colorado State University. Libraries, 2014) Troyer, Lyndsay D., author; Borch, Thomas, advisor; Ladanyi, Branka M., committee member; Levinger, Nancy E., committee member; Henry, Charles S., committee member; Kelly, Eugene F., committee member
Millions of cubic meters of uranium (U) mine tailings worldwide and millions of gallons of contaminated groundwater are the result of U mining and milling activity. Arsenic can occur at up to 10 weight percent in U ore, so both U and As can be released during U mining. Although these elements commonly occur together, little research into their redox behavior when present in the same environmental system has been performed. The goal of this research is to gain an improved understanding of how redox chemistry affects U and As speciation and complexation when the two elements are present together as co-contaminants. The North Cave Hills in Harding County, South Dakota is an abandoned U mine where overburden has been left open to weathering and transport since mining began in 1955. The exposed overburden has resulted in above-background level concentrations of U and As in sediments and groundwater in the surrounding wetlands. We conducted a field-scale study to investigate U and As redox chemistry at the North Cave Hills by taking sediment samples from the tailings pile and the down gradient watershed in order to assess U and As fate and transport. As sediments pass through anoxic zones at the field site, U is immobilized as reduction takes place but As can simultaneously be released into surface waters as reductive dissolution of Fe minerals also occurs. A laboratory-based study was conducted in order to examine the redox chemistry of U and As in North Cave Hills sediments under controlled conditions. Upon microbial reduction of sulfate and formation of mackinawite in batch systems, U(VI) and As(V) were reduced to nano- UO2 and a reduced As-sulfide mineral phase respectively during biostimulation by three different electron donors. When these systems were exposed to air for 24 hours, mackinawite protected U and As from oxidation and little change in their solid-phase speciation was observed. While mackinawite was shown to play a role in reduction, we could not determine if direct microbial reduction of U and As was also taking place in the systems. In order to further explore the reduction of U(VI) and As(V) by mackinawite, an experiment was set up to determine if As(V) prevented U(VI) reduction, especially following the formation of uranyl arsenate precipitates. As(V) only had an impact on the extent of U reduction at concentrations higher than would occur in most environmental systems. When As(V) concentrations were high, U(VI) was shown to be resistant to reduction because of the precipitation of a uranyl arsenate mineral phase. The findings in this dissertation contribute important information that will improve our current understanding of U and As redox behavior that will lead to improved remediation strategies to effectively prevent the mobilization of both elements in environmental systems.
Open Access
Interactions of biologically active molecules, cofactors, and drugs with model membranes
(Colorado State University. Libraries, 2021) Van Cleave, Cameron, author; Crans, Debbie C., advisor; Van Orden, Alan K., committee member; Levinger, Nancy E., committee member; Roess, Deborah A., committee member
The cell membrane is important for the structure, function, and overall homeostasis of the cell. It consists mainly of phospholipids which have different physicochemical and material properties. As such, molecular interactions between the membrane's components and its environment are of importance. This manuscript explores the interactions of different classes of molecules with model membrane systems to gain a fundamental understanding. Chapter 1 provides background on the cell membrane and current models as well as an introduction to lipoquinones and small molecule drugs. Chapter 2 discusses the interactions of menadione and menadiol with Langmuir monolayers and reverse micelles. Menadiol and menadione are representative of the headgroup of menaquinones, a class of electron transporter, hence they are redox active. We hypothesized that the respective locations of menadione and menadiol within the membrane would vary due to their different physicochemical properties. We used Langmuir monolayers and NMR of reverse micelles to explore the location and association of menadione and menadiol with model membrane interfaces. Chapter 3 investigates the location, association, and conformation of truncated menaquinones with Langmuir monolayers and simulated bilayers. Previous work found that truncated menaquinones fold at the interface of a reverse micelle, so we hypothesized that subtle differences in folding would cause variations in location and association with phospholipids. We used a combination of Langmuir monolayers and molecular dynamics simulations to probe location, association, and conformation of truncated menaquinone homologues, MK-1 through MK-4, in a phospholipid membrane. Chapter 4 explores the pH-dependent effects of two anti-tubercule molecules at the membrane interface. Recent studies have suggested that pyrazinoic acid behaves as a protonophore and we further explored this suggestion while simultaneously exploring physicochemical properties of pyrazinoic acid and pyrazinamide. This chapter utilized a combination of Langmuir monolayers, NMR, and fluorescence leakage studies to characterize the molecular interactions of pyrazinoic acid with model membranes so that POA could be compared to a previous study with benzoic acid, a known protonophore.
Open Access
Investigations of nitrogen oxide plasmas: fundamental chemistry and surface reactivity and Monitoring student perceptions in a general chemistry recitation
(Colorado State University. Libraries, 2016) Blechle, Joshua M., author; Fisher, Ellen R., advisor; Levinger, Nancy E., committee member; Henry, Charles S., committee member; Weinberger, Christopher R., committee member
Part I of this dissertation focuses on investigations of nitrogen oxide plasma systems. With increasing concerns over the environmental presence of NxOy species, there is growing interest in utilizing plasma-assisted conversion techniques. Advances, however, have been limited because of the lack of knowledge regarding the fundamental chemistry of these plasma systems. Understanding the kinetics and thermodynamics of processes in these systems is vital to realizing their potential in a range of applications. Unraveling the complex chemical nature of these systems, however, presents numerous challenges. As such, this work serves as a foundational step in the diagnostics and assessment of these NxOy plasmas. The partitioning of energy within the plasma system is essential to unraveling these complications as it provides insight into both gas and surface reactivity. To obtain this information, techniques such as optical emission spectroscopy (OES), broadband absorption spectroscopy (BAS), and laser induced fluorescence (LIF) were utilized to determine species energetics (vibrational, rotational, translational temperatures). These temperature data provide mechanistic insight and establish the relationships between system parameters and energetic outcomes. Additionally, these data are also correlated to surface reactivity data collected with the Imaging of Radicals Interacting with Surfaces (IRIS) technique. IRIS data demonstrate the relationship between internal temperatures of radicals and their observed surface scatter coefficients (S), the latter of which is directly related to surface reactivity (R) [R = 1-S]. Furthermore, time-resolved (TR) spectroscopic techniques, specifically TR-OES, revealed kinetic trends in NO and N2 formation from a range of precursors (NO, N2O, N2/O2). By examining the rate constants associated with the generation and destruction of various plasma species we can investigate possible mechanistic implications. All told, such data provides unparalleled insight into the chemistry of these plasma systems. Part II of this work is focused on understanding the efficacy of a general chemistry recitation program. Such programs can be an valuable tool for improving students’ problem solving skills and understanding using methods that are difficult to implement in large lecture settings. Here, general chemistry students at Colorado State University participated in a variety of recitation activities throughout the first semester of a 2-semester general chemistry sequence, including peer-led exercises, games, and scaffolded worksheets. Through weekly surveys, students were asked to evaluate and assess recitation activities for both interest and effectiveness as part of their course homework. Also included in these survey assignments were content questions relevant to the weekly themes, providing a measure of student learning of recitation topics. Student opinions were correlated with content retention, and these data were compared against student responses to a pre-survey administered before the first recitation session. This analysis allows for monitoring students’ expectations of recitation courses and how well those expectations are met through the various types of activities employed. Ultimately, this work has found that students have positive feeling with respect to individual assignments, but that perspectives on chemistry and the course in general decrease dramatically from the beginning to the end of the semester. Thus, this work can serve as a significant starting points for future efforts to monitor and record student perceptions in the general chemistry recitation classroom, leading to further investigation into the source of changing attitudes and the role that week-to-week activities have on global course attitudes.
Open Access
Part 1, executed electronic state decomposition of energetic molecules. Part 2, conformation specific reactivity of radical cation intermediates of bioactive molecules
(Colorado State University. Libraries, 2010) Bhattacharya, Atanu, author; Bernstein, Elliot R., advisor; Levinger, Nancy E., committee member; Van Orden, Alan K., committee member; Szamel, Grzegorz, committee member; Bartels, Randy A., committee member
Energetic materials have a wide variety of industrial, civil, and military applications. They include a number of organic compounds such as RDX (1,3,5- trinitroheahydro-s-triazine), HMX (octahydro-1,3,5,7-tetranitro-l ,3,5,7-tetrazocine), DAAF (3,3'-diamino-4,4'-azoxyfurazan), DAATO35 (3,3'-azobis(6-amino-l,2,4,5- tetrazine)-mixed N-oxides), etc. These materials release huge chemical energy during their decomposition. The decomposition of energetic materials is initiated with a shock or compression wave or a spark. Such events in solids generate molecules in the excited electronic states. Hence, in order to maximize release of the stored chemical energy in the most efficient and useful manner and to design new energetic materials, the unimolecular decomposition mechanisms and dynamics from excited electronic states should be understood for these systems. This thesis describes understanding about unimolecular decomposition of energetic materials from their excited electronic states. A few fundamental questions at molecular level dealing with electronic excitation of energetic materials are addressed here: (a) what happens immediately after electronic excitation of energetic molecules?; (b) how is excess energy partitioned among product molecules following electronic excitation?; (b) what are the mechanism and dynamics of molecular decomposition?; (d) does nonadiabatic chemistry (a process that span multiple electronic potential energy surfaces) through conical intersection (crossing of different potential energy surfaces) dominate system behavior? Both energy and time resolved spectroscopic techniques are used in this effort. The product internal state (rotational and vibrational) distributions are probed using mass and energy resolved spectroscopic techniques using time-of-flight mass spectrometry (TOFMS) and laser induced fluorescence (LIF) spectroscopy. Analyzing the product internal state distributions, the mechanisms of unimolecular decomposition of energetic molecules from excited electronic states are determined. The femtosecond pump-probe spectroscopic technique is utilized to determine ultrafast decomposition dynamics of these molecules. From a theoretical point of view, multiconfigurational methodologies such as, CASSCF and CASMP2 are used to model the processes involving excited electronic states of energetic molecules. Influence of nonadiabatic chemistry in the overall decomposition of energetic molecules is also theoretically judged. The primary energetic systems whose nonadiabatic chemistry discussed here are the nitramine (e.g., RDX, HMX), furazan (e.g., DAAF), and tetrazine-N-oxide (e.g., DAATO3.5) based energetic species. A number of model systems, which are simple analogue molecules of the large and more complex energetic materials, are studied in detail to understand nonadiabatic energetic behavior of a single energetic moiety of particular class. Subsequently, the decomposition mechanism for more complex energetic systems are studied and compared with that of their model systems. Nitramine energetic materials and model systems undergo nitro-nitrite isomerization followed by IV NO elimination. Nitramine energetic materials dissociates in the ground state generating rotationally cold (20 K) distribution of the NO product. Nitramine model systems dissociates in the excited state surface producing rotationally hot (-120 K) distribution of the NO product. The nitro-nitrite isomerization happens through conical intersection. Furazan based model molecules (e.g., furazan) possess two different pathways of decomposition: ring contraction and ring opening. These two pathways are electronically nonadiabatic. The ring contraction mechanism generates rotationally cold (20 K) product NO and the ring opening mechanism generates rotationally hot (100 K) product NO. Furazan based energetic material (DAAF), however, dissociates only through a ring contraction mechanism. Thus nonadiabatic pathways control the decomposition of furazan based molecules. Decomposition of tetrazine-2,4-dioxide based molecules involves a ring contraction mechanism through (Si/So)ci, producing only rotationally cold (20 K) but vibrationally hot (1200 K) distributions of the NO product. Tetrazine-l,4-dioxde undergoes similar decomposition pathway through (Si/So)ci; however, it produces rotationally hotter (50 K) but vibrationally colder distribution of the NO product. Thus the relative position of the oxygen atoms attached to the tetrazine ring is important parameter along with their nonadiabatic chemistry controlling their final energetic reactivity. Decomposition dynamics of all energetic materials is faster than 180 fs. Considering the influence of conical intersections in the excited electronic state decomposition of energetic materials, rotationally cold N2 product is predicted to be the major decomposition product of high nitrogen content energetic species. The present work infers that generation of internally cold product is an important characteristics of a true energetic molecule. Presence of low lying chemically relevant conical intersections provides a direct pathway of ultrafast decomposition chemistry of energetic molecules. The energy barrier to the low lying chemically relevant conical intersection, in principle, would be a point of interest to make a system more or less energetic.
Open Access
Rare sugars in soils: insights on their presence, persistence, and potential for carbon sequestration
(Colorado State University. Libraries, 2024) Lemas, Stephanie, author; Boot, Claudia M., advisor; Conant, Richard, committee member; von Fischer, Joe, committee member; Levinger, Nancy E., committee member
Carbon (C) is a fundamental element in the biosphere, cycling through all its natural pools. However, due to human activity, the flux of C into the atmosphere has accelerated, impacting the climate in significant and consequential ways. Awareness of this has prompted world-wide research into different mitigation strategies, including both reducing the flux of C into the atmosphere and active carbon dioxide removal (CDR) from the atmosphere. Soil represents a substantial C reservoir with the capacity to store large amounts of C. Our research focuses on the role of chiral molecules, specifically rare sugars, to enhance the storage of C in soil. To assess the feasibility of this idea, we designed an experiment to test whether soil microorganisms were able to consume and respire 14 rare sugars. We found that some rare sugars showed very little or repressed respiration, but that most showed moderate or high respiration rates. This finding prompted the hypothesis that soil microorganisms have evolved the capacity to grow on rare sugars because those rare sugars are present in the soil. To test this, we designed another experiment to check for the presence of rare sugars, specifically rare hexoses, in soils. Hexose sugars are among the most important small molecules in nature, in part because they are essential sources of energy for most cells. D-glucose, the most abundant hexose, is well-known due to its roles in both cellular respiration and photosynthesis; however, D-glucose is far from the only hexose in nature. Plants and microorganisms produce not only D-glucose, but also D-fructose, D-galactose, and D-mannose, and they contribute these hexoses to soils in different proportions. These four hexoses are considered common in soil and have been incredibly well-studied, but they represent only a small fraction of the hexoses possible in soil systems. In this study, we used gas chromatography-mass spectrometry (GC-MS) to measure hexoses in diverse soils crowdsourced from across the contiguous United States. In addition to the four common hexoses, we identified a fifth hexose: a rare ketohexose corresponding to the overlapping retention times of psicose and tagatose ("PsiTag"). This rare ketohexose, or possible mixture of these two rare ketohexoses, was present in every soil we sampled. To the best of our knowledge, this is the first time a rare ketohexose has been identified in a soil extract. The ubiquitous presence of a rare hexose in soils shifts the paradigm and challenges the narrative about which hexoses are truly common. These two experiments explore the potential of rare sugars for soil C sequestration via bio-transformative CDR (BtCDR) by examining their presence and the persistence in soils. As a result of our investigation, we determined that each rare sugar falls into one of two categories: having high turnover in soil, and therefore low C sequestration potential, or having high recalcitrance in soil and high C sequestration potential. Experimental data suggest that PsiTag and L-fructose have high recalcitrance in soil, but that L-glucose has high turnover in soil. Further research is needed to verify these findings and explore additional rare sugars. Although preliminary data indicate that rare sugar monosaccharides would not serve as effective long-term C sinks in soil, we believe that rare sugars may have a yet unknown role to play in soil C dynamics. This thesis sheds new light into the previously uninvestigated presence of rare sugars in soil, their implications for sustainable C storage, and their potential contributions to a holistic approach to climate change mitigation.
Open Access
Steady-state and time-resolved spectroscopy to probe the effects of confinement on Cy3 and the dynamics of AOT/iso-octane reverse micelles
(Colorado State University. Libraries, 2010) McPhee, Jeffrey, author; Van Orden, Alan K., advisor; Levinger, Nancy E., committee member; Barisas, B. George, committee member; Prieto, Amy L., committee member; Luger, Karolin, committee member
This dissertation describes the use of steady-state and time-resolved spectroscopy to probe the effects of localized confinement on the water soluble dye Cyanine-3 (Cy3) and the dynamics of intermicellar interactions using fluorescence correlation spectroscopy (FCS). The first set of experiments presents a wide range of steady-state and time-resolved spectroscopy data which indicate that the Cy3 molecules form H-aggregates at concentrations so dilute (nM) that this behavior is not observed in bulk aqueous solution. This unique behavior allowed for a series of FCS and dynamic light scattering measurements to be performed on the same system. These results indicate the formation of a transient reverse micelle dimer, whose lifetime has been identified to be on the order of 15 μs. Furthermore, preliminary experiments are presented on the same reverse micelle system containing the Rhodamine 6G and the results are consistent with those obtained for Cy3 in the reverse micelles. Lastly, fluorescence resonance energy transfer within the reverse micelles was investigated using Cy3 and Cy5. The preliminary results suggest that FRET may be occurring within this extremely confined environment. The work as a whole provides insight into the nature of confinement as well as the dynamics occurring within the world of reverse micelles.
Open Access
Synthesis and characterization of iridium model, and cobalt and nickel, industrial Ziegler-type hydrogenation catalysts and their precursors
(Colorado State University. Libraries, 2011) Alley, William Morgan, author; Finke, Richard G., advisor; Chen, Eugene Y.-X., committee member; Elliott, C. Michael, committee member; Levinger, Nancy E., committee member; Kipper, Matthew J., committee member
Following a comprehensive critical review of the pertinent literature, the research presented herein is focused on the synthesis of an Ir precursor used to model industrial Ziegler-type hydrogenation catalysts, and on catalyst characterization using both the Ir model, and genuine Co and Ni, industrial catalyst materials. The studies include: (i) the synthesis, characterization, and initial catalytic investigation of Ir (and Rh) compounds for use as models for the industrial Co and Ni Ziegler-type hydrogenation catalysts; (ii) characterization of the Ziegler-type hydrogenation catalyst made from the Ir precursor; and (iii) characterization of the authentic industrial Co and Ni Ziegler-type hydrogenation catalysts. The synthesis and definitive characterization of Ir (and Rh) precatalysts designed to facilitate investigation into the homogeneous versus heterogeneous nature of Ziegler-type hydrogenation catalysts is described herein. Additionally, the ability of these Ir (and Rh) precatalysts to form active Ziegler-type hydrogenation catalysts upon combination with AlEt3 is demonstrated. The homogeneous versus heterogeneous nature of the Ir Ziegler-type hydrogenation catalyst is investigated using several complementary analytical methods plus kinetic studies. Initial active catalyst solutions contain a variety of Ir species ranging from mono-Ir compounds to nanometer-scale Irn clusters, but on average are subnanometer, Ir~4-15 species. However, crystalline Ir(0)~40-150 nanoclusters are rapidly formed when the solutions are put under pressurized H2 gas, and these larger, "Ziegler nanoclusters" are shown to be the most active catalysts, an important result in comparison to all the prior, extensive literature of these important industrial catalysts. The homogeneous versus heterogeneous nature of the authentic industrial Co- and Ni-based Ziegler-type hydrogenation catalysts are investigated using an approach parallel to that used for the Ir system, and are compared to the results from the Ir model system. The metal cluster species are essentially the same pre- and posthydrogenation; they comprise a broad distribution of Mn cluster sizes from subnanometer to nanometer in scale, with average diameters of about one nanometer, and with some amount of unreduced mono-metallic complexes also present dependent on the Al/M ratio. These findings support the primary working hypothesis present in the most recent literature, namely that Ziegler-type hydrogenation catalysis is enacted by "Ziegler nanoclusters" (as defined herein), nanoclusters of size M4 in the case of the industrial Co and Ni system.
Open Access
Synthesis and characterization of sterically and electronically tuned ligands toward magnetic control of iron and cobalt complexes
(Colorado State University. Libraries, 2015) Klug, Christina M., author; Shores, Matthew P., advisor; Rappé, Anthony K., committee member; Ackerson, Christopher J., committee member; Levinger, Nancy E., committee member; Wu, Mingzhong, committee member
Presented within this dissertation are the syntheses and characterizations of iron and cobalt complexes featuring ligands designed to tune the magnetic properties. Two key magnetic phenomena are of interest: spin crossover and single-molecule magnetism. Both of these topics are known to be significantly influenced by subtle changes in coordination and inter- and intramolecular interactions. The overarching goal is to understand how the magnetic properties of the metal center can be controlled via electronic and steric modifications. In Chapter 1, I offer a brief introduction into the background and motivation of the works presented in this dissertation in the realm of spin crossover and single-molecule magnetism. The first section of this chapter is focused on spin crossover and how host:guest interactions can be exploited to alter the magnetic behavior of first-row transition metals. Examples of Fe(II) complexes that display anion-dependent spin state behaviors in both the solid-state and in solution are discussed. Functionalized tripodal Schiff-base ligands are placed into context as an extension of previous research into tripodal ligands for use as metal-based anion-receptors and tripodal spin crossover complexes. The second section of Chapter 1 gives a brief introduction into single-molecule magnetism. An examination of mononuclear Co(II) complexes displaying slow magnetic relaxation and application of acetylide-bridged metal centers to enhance magnetic communication are also given. In Chapter 2, I discuss the preparation and characterizations of a Fe(II) complex coordinated by the alcohol functionalized hexadentate tripodal iminopyridine L6-OH with varying anions. Solid-state magnetic susceptibility measurements of [FeL6-OH]X2 (X = OTf-, Br-, I-, or BPh4-) reveal an anion-dependence on the magnetic behavior. Magnetostructural correlations indicate that stronger hydrogen-bonding interactions are achieved with larger anions, which are better able to undergo bifurcated interactions with the hydroxyl groups from two of the arms. Removal of the tether between the ligand arms leads to the formation of [Fe(L2)2](OTf)2, a bis(tridentate) complex that remains high spin at all temperatures. Variable temperature magnetic measurements in d3-methanol reveal that the high spin state of [FeL6-OH]2+ persists regardless of the anion down to 183 K. In Chapter 3, attempts towards synthesizing the heteroarmed tris(imine) [FeL556]2+ and analogous bis(imine)-mono(amine) [FeL556-NH]2+ complexes are discussed. Several routes are attempted to synthesize the tris-iminopyridine species including selective deprotonation of tris(2-aminoethyl)amine*3 HCl, in situ complex formation via metal-templated self-assembly, and use of presynthesized ligands. Analyses of the reaction mixtures by mass spectrometry suggest that mixtures of products are formed regardless of the method. An anion and solvent dependence leads to preferential formation of the low-spin species [FeL5-ONHtBu]2+, while using solvents such as acetonitrile and ethanol lead to increased production of the desired [FeL556]2+. To test if anion-dependent magnetic behavior can be observed with this ligand type, the comparable complex [FeL556-NH]2+ was synthesized and characterized. Variable temperature solution measurements in d3-acetonitirile suggest that host:guest interactions in solution induce a stabilization of the low-spin state for [FeL556-NH]2+ as indicated by a decrease in susceptibility at lower temperatures for the Cl- salt. In Chapter 4, the preparation, structural, and magnetic characterizations for a family of Fe(II) complexes of tripodal ligands based on L5-ONHtBu are presented. The series of ligands aim to tune the ligand field by selectively reducing imines to amines, producing the ligands L5-(NH)x (x = 1 - 3, number of amines). In the solid state, the three Fe(II) complexes formed are high spin, but significant differences in the structural distortion of both the coordination environment of the Fe(II) center as well as the anion-binding pocket of the amides are noted. In solution, the complexes [FeL5-(NH)3]2+ and [FeL5-NH]2+ are high spin between 183 and 308 K in d6-acetone but interestingly, [FeL5-(NH)2]2+ undergoes a spin-state change with decreasing temperature. Variable temperature studies in d6-acetone and anion titrations in d3-acetonitrile at room temperature monitored by Evans' method of [FeL5-(NH)2]2+ show host:guest interactions stabilize the high spin state. These studies suggest a viable method of ligand tuning for spin-state control by host:guest interactions. In Chapter 5, I discuss the structural and magnetic properties of [Co5-ONHtBu]X2 (X = Cl-, Br-, I-, and ClO4-). These hexadentate Co(II) complexes vary only in the charge-balancing anion, but marked differences in their magnetic properties are observed. Investigation of the magnetic anisotropy of the various salts reveal that the chloride salt possesses the most axial anisotropy, which manifests as an exhibition of slow magnetic relaxation under application of an external field. To my knowledge this is the first example of anion-binding influencing the magnetic anisotropy and 'turning on' single-molecule magnet-like behavior. Lastly, Chapter 6 describes the syntheses and magnetic properties of a series of mono-and dinuclear Fe(III) complexes bridged by ethynylmesitylene ligands. Inclusion of steric bulk onto the bridging-aryl ligand is predicted to increase orbital overlap between the singly-occupied molecular orbital of the metal center and the π-system of the aryl linker. The addition of methyl groups to the aryl ring cements the desired equatorial ligand orientation with respect to the π-system. This leads to an increase in ferromagnetic coupling between the metal centers.
Open Access
Synthesis and characterization of uranium(IV) compounds: from mononuclear complexes to multinuclear assemblies
(Colorado State University. Libraries, 2011) Newell, Brian S., author; Shores, Matthew P., advisor; Anderson, Oren P., advisor; Chen, Eugene Y., committee member; Levinger, Nancy E., committee member; Wu, Mingzhong, committee member
This dissertation describes the synthesis of multinuclear compounds that possess magnetically-coupled actinide, namely uranium-238, clusters. These assemblies are supported by both acetylide-type ligands as well as triamidoamine or softer phosphine ligands. Synthetic inorganic chemists have been able to synthesize molecules and clusters with increased spin, S, or axial anisotropy, D, in an effort to augment the spin-reversal barriers and create better single-molecule magnets (SMMs). However, efforts to simultaneously increase these parameters are complicated. One potential route utilizes heavy atoms as a result of their larger single-ion anisotropy and believed ability to modulate the magnetism of other systems. My research is placed in this context in Chapter 1, where recent efforts to incorporate heavy atoms into expanded clusters are discussed. In Chapter 2, the preparation and magnetic property investigations of a structurally related family of mono-, di- and trinuclear U(IV) aryl acetylide complexes are presented. The reaction between [(NN′3)UCl] and lithiated aryl acetylides leads to the formation of hexacoordinate compounds. In contrast, combining the uranacycle [(bit-NN′3)U] (bit-NN′3 = [N(CH2CH2NSitBuMe2)2(CH2CH2SitBuMeCH2]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords pentacoordinate arylacetylide complexes, where NN′3 = [N(CH2CH2NSitBuMe2)3]. The measured magnetic susceptibilities for these compounds trend toward non-magnetic ground states at low temperatures. Nevertheless, the di- and trinuclear pentacoordinate compounds appear to display weak magnetic communication between the uranium centers. This communication is modeled by fitting of the DC magnetic susceptibility data, using the spin Hamiltonian. Geometry-optimized Stuttgart/6-31g* B3LYP hybrid DFT calculations were carried out (spin-orbit coupling omitted) on model complexes and the electrochemistry of the monomeric phenylacetylide complex exhibits a reversible redox couple at -1.02 V versus [Cp2Fe]+/0, assignable to an oxidation of U(IV) to U(V). Efforts to study the magnetic correlations as a result of cubic ligands fields are presented in Chapter 3, whereby a neutral bidentate phosphine ligand was utilized. In the course of structurally characterizing previously reported complexes based on the 1,2-bis(dimethylphosphino)ethane)) (dmpe) ligand ([(dmpe)2UCl4] (3.1) and [(dmpe)2UMe4] (3.2)), we found that adjusting the U:dmpe ratio leads to an unprecedented species. Whereas the use of two or three equivalents of dmpe relative to UCl4 produces 3.1 as a blue-green solid, use of a 1:1 dmpe:UCl4 stoichiometry yields [(dmpe)4U4Cl16]•2CH2Cl2•(3.3•2CH2Cl2) as a green solid. In turn, 3.3 is used to prepare a mixed-chelating ligand complex featuring the bidentate ligand 4,4′-dimethyl-2,2′-bipyridine (dmbpy), [(dmpe)(dmbpy)UCl4] (3.4). The measured magnetic susceptibilities for 3.1-3.4 trend toward non-magnetic ground states at low temperatures. In Chapter 4, we hypothesized that preparing complexes that contain U(IV) in a cubic ligand field environment, using acetylide ligands, might allow for the isolation of compounds exhibiting enhanced magnetic coupling. In that vein, we report the synthesis and characterization of [(dmpe)2U(CCPh)4] (4.1) (CCPh = phenylacetylide) and [(dmpe)2U(CCPh)5(Li∙Et2O)] (4.2). No reproducible magnetic susceptibility data were obtained and a discussion about these difficulties is presented. In the course of studying the crystal structure of the mixed-chelating ligand complex [(dmpe)(dmbpy)UCl4] (3.4) an interesting effect on the U-Cl⋯H was observed. Several computation methods were utilized to determine that the M-Cl⋯HC distance based on approach angles is suggestive that Cl is acting more like chlorine and less like chloride. This provides a route to study U-L bonding and is presented in Chapter 5. Finally, in Chapter 6, efforts to synthesize a mixed-metal complex are discussed and preliminary characterization of a dinuclear ethynylbenzene 5f-3d complex (6.3) is presented. While an unambiguously paramagnetic metal-complex was not isolated, initial electrochemical studies indicate a redox process takes place. A short discussion about the temperature dependence of the magnetic susceptibility is given.
Open Access
The role of plasma-surface interactions in process chemistry: mechanistic studies of a-CNx deposition and SF6/O2 etching of silicon
(Colorado State University. Libraries, 2010) Stillahn, Joshua Michael, author; Fisher, Ellen R., advisor; Bernstein, E. R. (Elliot R.), committee member; Dandy, David S., committee member; Levinger, Nancy E., committee member; Prieto, Amy L. (Amy Lucia), committee member
The molecular level chemistry of a-CNx deposition in plasma discharges was studied with emphasis on the use of CH3CN and BrCN as single source precursors for these films. Characterization of the global deposition behavior in these systems indicates that the resulting films are relatively smooth and contain significant levels of N-content, with N/C > 0.3. Notably, films obtained from BrCN plasmas are observed to delaminate upon their exposure to atmosphere, and preliminary investigation of this behavior is presented. Detailed chemical investigation of the deposition process focuses primarily on the contributions of CN radicals, which were characterized from their origin in the gas phase to their reaction at the a-CNx film surface. Laser-induced fluorescence studies suggest that CN is formed through electron impact dissociation of the precursor species and that this breakdown process produces CN with high internal energies, having rotational and vibrational temperatures on the order of 1000 K and 5000 K, respectively. Measurement of CN surface reactivity coefficients in CH3CN plasmas show that CN reacts with a probability of ~94%, irrespective of the deposition conditions; this information, combined with gas phase and film characterization data, leads to the conclusion that CN internal energies exert a strong influence on their surface reactivity and that these surface reactions favor their incorporation into the a-CNx film. Moreover, this correlation is shown to hold for several other plasma radicals studied in our lab, suggesting the potential for developing a general model for predicting surface interactions of activated gas phase species. This dissertation also presents results from studies of SF6/O2 etching of Si. Addition of O2 to the feed gas leads to the generation of SO2, among other species, and gas phase characterization data suggest that SO2 may act as a sink for atomic S, preventing the reformation of SOxFy (y > 0) and thus promoting generation of atomic F. The surface scatter coefficient of SO2 was also measured in an effort to understand its role in the formation of gas phase species. These measurements suggest that SO2 does not undergo surface reaction during etching and therefore does not contribute to the generation of gaseous SOxFy species.
Open Access
Visualizing dynamics using 100 kHz 2D IR spectroscopy and microscopy
(Colorado State University. Libraries, 2018) Tracy, Kathryn Marie, author; Krummel, Amber T., advisor; Levinger, Nancy E., committee member; Szamel, Grzegorz, committee member; Krueger, David A., committee member; Fisk, John D., committee member
2D IR spectroscopy is a nonlinear optical method with the ability to characterize condensed phase chemical systems. It offers information regarding structure and dynamics of chemical systems. Recent efforts have been made to resolve spatially the molecular structure and dynamics of heterogeneous samples, which shows the feasibility of ultrafast 2D IR microscopy. To image more efficiently, we have moved away from the Ti:sapphire based laser systems and OPA systems that operate at one to several kHz typically used in 2D IR spectroscopy. Instead, for the first time we have demonstrated higher repetition rate, 2D IR spectroscopy at 100 kHz. Achieving this higher repetition rate was accomplished by utilizing advances in diode pumped ytterbium oscillators and amplifiers, and is based on an OPCPA utilizing Mg:PPLN followed by DFG in ZGP. Using this system, we have for the first time, demonstrated the interfacing of IR compatible microfluidics with 2D IR spectroscopy to examine the solvatochromic pseudo-halide anion, cyanate in cosolvent environments. This high repetition rate source also provided a path to 2D IR microscopy experiments that explore the dynamics of complex, heterogeneous, chemical systems. We have shown the chemical dynamics in a room temperature ionic liquid microdroplet. Spatially resolved time-dependent 2D IR signals reveal three regions with different chemical dynamics—the bulk, the interface, and a region between the bulk and interface. This demonstration provides proof-of-concept to use 2D IR microscopy on a wide array of additional chemical systems.