Browsing by Author "Prieto, Amy L. (Amy Lucia), committee member"
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Item Open Access Chloro-, aryl-, and perfluoroalkylfullerenes(Colorado State University. Libraries, 2009) Kuvychko, Igor V., author; Strauss, Steven H., advisor; Prieto, Amy L. (Amy Lucia), committee member; Hochheimer, Hans D., committee member; Rappé, Anthony K., 1952-, committee member; Elliott, Cecil Michael, committee memberThe preparation, properties, stability, and handling of three classes of fullerene derivatives, chloro-, aryl-, and perfluoroalkylfullerenes, were studied in detail, in some cases for the first time. The same general methodology was used throughout this work: i) analytical methodology was developed and optimized using internal standards of known composition and purity; ii) the preparation of new compounds was studied by mapping the space of different reaction conditions using the newly developed analytical methods; iii) the efficient synthetic methods for the preparation of individual pure compounds were developed based on the mapping; iv) the pure compounds were characterized and their stability, or lack thereof, was studied. The first detailed study of fullerene chlorination led to isolation and characterization of several new chlorofullerenes: o-C60Cl2; p-C60Cl2; C60Cl4; C60Cl10; C70Cl6; and C70Cl8. It was discovered that chlorofullerenes are generally photosensitive in solution, both in the presence and absence of air and moisture. Effective methods for handling chlorofullerenes were developed (including a specialized crystallization technique). The experimental findings and theoretical calculations revealed the fundamental patterns governing multiple additions to fullerene cages. The efficient preparation of aryl- and perfluoroalkylfullerenes from chlorofullerene synthons was developed, leading to high yields of aryl- and mixed perfluoroethyl/hydrofullerenes (i.e., C60(C2F5)5H and C60(C2F5)3H). The first example of an organometallic complex of a perfluoroalkylfullerene was prepared using C60(C2F5)5H as a synthon. The direct addition of thermally-generated CF3 radicals to fullerenes was also studied. A specialized reactor was designed and built in order to validate mechanistic hypotheses. Good agreement between the experimental observations and predictions based on the hypotheses was observed. It led to the development of a new approach to efficient synthesis of C60(CF3)n with low values of n, including an unprecedented selective synthesis of C60(CF3)2 from unpurified fullerene extract.Item Open Access Hyperpolarized and thermally polarized quadrupolar noble gas nuclei studied by nuclear magnetic resonance spectroscopy and magnetic resonance imaging(Colorado State University. Libraries, 2010) Stupic, Karl Francis, author; Meersmann, Thomas, advisor; Fisher, Ellen R., committee member; Szamel, Grzegorz, committee member; Prieto, Amy L. (Amy Lucia), committee member; Roberts, Jacob Lyman, committee memberThis dissertation consists of several studies of two quadrupolar nuclei, 83Kr and 131Xe, with nuclear spin states of I = 9/2 and I = 3/2, respectively. These nuclei possess a nuclear electric quadrupole moment that strongly interacts with the surrounding electric field gradient (EFG). The quadrupolar interactions in these noble gas atoms dominate the longitudinal (T1) spin relaxation. To fully study these nuclei, high non-equilibrium nuclear spin polarization, referred to as hyperpolarization (hp), is generated using spin exchange optical pumping (SEOP). By employing this technique, enhanced nuclear magnetic resonance (NMR) signals many orders of magnitude above that of a thermally polarized (Boltzmann distribution of spin states) sample are possible and allow for experiments where signal averaging over long periods of time is prohibited (i.e. in vivo). The gas phase 83Kr T1 is shown to be sensitive to the surface composition/chemistry and the surface-to-volume ratio in an ideal system of closest packed glass beads. Understanding the behavior of 83Kr in these conditions allows for its development as a surface sensitive probe that could provide information in opaque porous media environments. Similar relaxation behavior can be observed in 131Xe; however, the quadrupolar interactions experienced by 131Xe also induce an observable splitting in the NMR spectrum. This quadrupolar splitting is extremely sensitive to surfaces during periods of adsorption as well as to the magnetic field strength when a 131Xe atom is present in the bulk gas phase. As the influence on the quadrupolar splitting can be more readily observed than the relaxation of either 83Kr or 131Xe, the observed splitting in 131Xe NMR can provide helpful insights into quadrupolar behavior experienced by both nuclei. To develop a better understanding of the quadrupolar behavior, both 131Xe quadrupolar splitting and 83Kr relaxation are explored as functions of magnetic field strength, gas phase composition and co-adsorbing species. In closing, improvements in polarization of 83Kr from line-narrowed diode array lasers as well as new delivery techniques have provided improvements that allow for the implementation of variable flip angle FLASH imaging sequence in an excised, intact rat lung. Additionally, initial evidence suggests the T1 of 83Kr can differentiate between the regions of the lung (the trachea, the bronchi and bronchioles, and the alveoli), which has potential as a diagnostic tool for the biomedical community. Improvements in signal intensity are needed to achieve in vivo studies, additional enhancements are possible through improved SEOP and by using isotopically enriched gases.Item Open Access Rhodium-catalyzed cycloadditions between alkenyl isocyanates and alkynes: study of scope, mechanism and applications toward total synthesis(Colorado State University. Libraries, 2010) Friedman, Rebecca Ann Keller, author; Rovis, Tomislav, 1968-, advisor; Kennan, Alan J., committee member; Prieto, Amy L. (Amy Lucia), committee member; Shi, Yian, committee member; McNeil, Michael R., committee memberRhodium-catalyzed cycloadditions between alkenyl isocyanates and unsymmetrical, internal alkynes has been studied. A wide variety of alkynes have proven successful components in the [2+2+2] cycloaddition. Excellent yields and enantioselectivities have been achieved in the resulting indolizidinone products. Furthermore, a single regioisomer is obtained for the vast majority of alkynes subjected to reaction conditions. A logical explanation for the highly regioselective insertion for internal, unsymmetrical alkynes was provided. Small variations in the electronics and/or steric bulk of the alkyne substitution were sufficient to predictably control the insertion of the alkyne into the initial rhodacycle. Mechanistic insight into the rhodium-catalyzed [4+2+2] cycloaddition between dienyl isocyanates and alkynes has been achieved. A series of competition and slow addition experiments, alongside analysis of enantioselectivity and product formation, provided evidence for a proposed mechanism of the [4+2+2] cycloaddition. It was determined that the diene preferentially coordinates to the rhodium, in the presence of a terminal alkyne, to provide eight-membered bicyclic azocene products. Steps towards the total synthesis of natural product Secu'amamine A have been made. The bicyclic core of the molecule has been successfully synthesized utilizing rhodium-catalyzed [2+2+2] methodology developed within the Rovis group. Additionally, a successful, diastereoselective 1,4-reduction of the resulting vinylogous amide product and subsequent deprotection of an enyne side-chain provided an intermediate that is hypothetically two steps (an alpha-oxidation and 2+2+1 cycloaddition) away from Secu'amamine A.Item Open Access Sputter deposited hydroxyapatite thin films to enhance osseointegration(Colorado State University. Libraries, 2010) Riedel, Nicholas Alfred, author; Williams, John D., advisor; Popat, Ketul, advisor; Prieto, Amy L. (Amy Lucia), committee memberAs the demand for hip and knee replacements continues to grow, researchers look to increase the operational lifetimes of these implants. Many of these implants fail as a result of aseptic loosening caused from repeated loading of these joints. It is thought that implant life could be extended by improving the interface between the implant and natural tissue. To this effect, hydroxyapatite coatings have been demonstrated to improve implant to bone bonding and allow a more natural integration of the metallic substrates. This work explores the potential of using ion beam etching and sputter deposition to produce a hydroxyapatite thin film with a unique surface topography that would potentially enhance osseointegration. First, the effects of ion etching bare titanium were evaluated. Three ion energies (300 eV, 700 eV, and 1100 eV) were used to etch either as-received or polished substrates. Topographical changes were examined by scanning electron microscopy. Rat mesenchymal stem cells were differentiated to osteoblasts to test the biocompatibility of the surfaces with bone cells. It was found that ion etching the titanium increases cellular activity, and an ion energy of 700 eV appears to create the most beneficial topography. Hydroxyapatite thin films were then sputter deposited on titanium substrates etched at 700 eV. After the coatings were deposited, some of the hydroxyapatite films were re-etched in efforts to induce a unique topography. It was found that the hydroxyapatite coatings improved short term cell response but degraded over the course of the culture. Further investigation showed the as-sputtered coatings were amorphous. To prevent degradation of the coatings, annealed films were then prepared by heat treating at 600 °C for 2 hours. X-ray diffraction was used to confirm the presence of a crystalline hydroxyapatite phase. Films were immersed in culture media for four weeks, showing no signs of degradation. Ion etching performed on the substrates post annealing yielded a unique topography in the hydroxyapatite film. A final study was conducted evaluating the MSC response to the annealed and post-anneal etched films. It was found that the post-anneal etched hydroxyapatite coating had the highest cellular activity, indicating that this preparation may be an effective means to enhance osseointegration on medical implants.Item 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 memberThe 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.