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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
51

Surface-enhanced Raman scattering and electron spectroscopic studies of copper and silver surfaces.

Ingram, Jani Cheri January 1990 (has links)
The overall goal of this research is to investigate the enhancement mechanisms associated with the surface enhanced Raman scattering (SERS) phenomenon. The approach taken in these investigations is to directly compare the SERS abilities of Cu and Ag surfaces using both SERS and electron spectroscopy. One set of experiments has been directed toward studying the contribution of the charge transfer (CT) mechanism to the SERS of pyridine adsorbed on Cu and Ag electrodes. These studies involve modifying the electrodes by depositing submonolayer amounts of Pb which serves to quench the SERS. By using a variety of excitation energies, the resulting quenching profiles track the CT process. Additionally, other electrochemical/SERS experiments have been pursued in order to probe the contribution of the CT mechanism. The second set of experiments involved measuring the optical properties of the Pb-modified Cu and Ag surfaces using electron energy loss spectroscopy in the reflection mode (REELS). Based on electromagnetic (EM) theory, the enhancement is, to a rough approximation, indirectly proportional to ε₂, the imaginary part of the dielectric constant. Thus, the goal of these studies is to determine the changes in ε₂ as a function of Pb coverage in order to determine the contribution of the EM mechanism to the SERS quenching profile. It was necessary to develop a method to determine optical constants from REELS data. A number of pure metals (Al, Cu, Ag, Au, Ti, V, Fe, Co, and Ni) were chosen to test the method. In all cases, our results compare well to the literature values with a relative standard deviation of 20% or less. Having established the method, the next step was to apply it to the Pb-modified Cu and Ag surfaces. Due to instrumental limitations, only semi-quantitative optical constants could be determined. From these values, it was found that the relative changes in the ε₂ values were larger for submonolayer coverages of Pb on Ag compared to Cu. The EM enhancements determined from these results did not predict the SERS-quenching behavior suggesting that other mechanisms must be considered.
52

High-resolution microwave spectroscopy of several weakly bound complexes.

Pauley, Deanne Josephine. January 1990 (has links)
High resolution rotational spectra were obtained for a several weakly-bound complexes (WBC) and one transition metal organometallic molecule, cyclopentadienylnickel nitrosyl (CpNiNO), using pulsed-beam, Fourier transform microwave spectroscopy. The weakly-bound complexes included the two structural isomers of N₂O-HF, the planar bent asymmetric N₂O-HCN complex, the planar bent asymmetric N₂O-HCl complex, and the "stacked" H₂S-SO₂, and H₂S-CO₂ complexes. In all of the cases with the exclusion of the CpNiNO molecule, additional isotopic measurements were obtained to aid in the spectral and structural analyses of the weakly-bound complexes. Analysis of rotational spectra was used to determine several spectroscopic constants. For each WBC and CpNiNO rotational constants and some quartic distortion parameters were determined. In the experimental studies performed on N₂O-HCN and N₂O-HCl additional quadrupole coupling components were determined from the data analysis. Structural analyses were performed on all of the WBC's. Isotopic Kraitchman analysis was used as a comparative guideline in helping to select the lowest energy vibrationally-averaged structure. High resolution (0.005 cm⁻¹) infrared spectra for CpNiNO were obtained with the Fourier transform spectrometer in the Kitt Peak McMath Solar Telescope. Absorption spectra were measured in the 1750-1880 cm⁻¹ and 2500-3700⁻¹ regions. A program written by Dr. Clive Jarman, a postdoc in Dr. Peter Bernath's laboratory, was used to perform Loomis-Wood analysis of 2 significant patterns in the 2500-3700 cm⁻¹ range. The series determined from the Loomis-Wood analysis are given in the dissertation.
53

The reactions of electron-rich monomers with electrophilic compounds: Methyl tricyanoethylenecarboxylate and trimethylsilyl esters.

Way, Tun-Fun. January 1990 (has links)
The experimental results of the current work has three parts. First, the cycloaddition and copolymerization of methyl tricyanoethylenecarboxylate 2 with electron-rich olefins, such as p-methoxystyene, trans-anethole, isobutyl vinyl ether and ethyl cis-propenyl ether are discussed. The nature of the cycloadduct is determined by the orientation of the electrophilic olefins. Copolymerization of 2 with p-methoxystyrene under free-radical initiation gave an alternating copolymer. Second, trimethylsilyl methanesulfonate and trimethylsilyl diphenylphosphate were used as initiators for cationic polymerization. In the presence or absence of hindered pyridine, trimethylsilyl diphenylphosphate and trimethylsilyl methanesulfonate did not initiate the polymerization of p-methoxystyrene, anethole, 4-isopropenylanisole, 1,3-dioxolane or trioxane. Only trimethylsilyl methanesulfonate was able to initiate the cationic polymerization of 1,3-dioxepane in the presence of a hindered base. A model study demonstrates fast desilylation of a carbocation β to a silicon by an oxygen-containing counterion. Finally, block copolydioxepane-polydimethylsiloxane has been synthesized by the "silyl sulfonate approach." In this approach, the nucleophilic macromer, lithium polydimethylsiloxanate, was reacted with chlorodimethylsilane or allylchlorodimethylsilane to produce the corresponding macromers with silylated end groups. They contained a labile substituent, an allyl or a proton, on silicon. These macromers were then converted to electrophilic macropolydimethylsiloxane arylsulfonate by reaction with an aryl or alkyl sulfonic acid. The sulfonate polydisiloxanes can initiate the cationic polymerization of 1,3-dioxepane to yield block polydimethylsiloxane-polydioxepane. This cationic polymerization did proceed in the presence of 2,6-di-t-butylpyridine, which would trap any acid impurities. / The experimental results of the current work has three parts. First, the cycloaddition and copolymerization of methyl tricyanoethylenecarboxylate 2 with electron-rich olefins, such as p-methoxystyene, trans-anethole, isobutyl vinyl ether and ethyl cis-propenyl ether are discussed. The nature of the cycloadduct is determined by the orientation of the electrophilic olefins. Copolymerization of 2 with p-methoxystyrene under free-radical initiation gave an alternating copolymer. Second, trimethylsilyl methanesulfonate and trimethylsilyl diphenylphosphate were used as initiators for cationic polymerization. In the presence or absence of hindered pyridine, trimethylsilyl diphenylphosphate and trimethylsilyl methanesulfonate did not initiate the polymerization of p-methoxystyrene, anethole, 4-isopropenylanisole, 1,3-dioxolane or trioxane. Only trimethylsilyl methanesulfonate was able to initiate the cationic polymerization of 1,3-dioxepane in the presence of a hindered base. A model study demonstrates fast desilylation of a carbocation β to a silicon by an oxygen-containing counterion. Finally, block copolydioxepane-polydimethylsiloxane has been synthesized by the "silyl sulfonate approach." In this approach, the nucleophilic macromer, lithium polydimethylsiloxanate, was reacted with chlorodimethylsilane or allylchlorodimethylsilane to produce the corresponding macromers with silylated end groups. They contained a labile substituent, an allyl or a proton, on silicon. These macromers were then converted to electrophilic macropolydimethylsiloxane arylsulfonate by reaction with an aryl or alkyl sulfonic acid. The sulfonate polydisiloxanes can initiate the cationic polymerization of 1,3-dioxepane to yield block polydimethylsiloxane-polydioxepane. This cationic polymerization did proceed in the presence of 2,6-di-t-butylpyridine, which would trap any acid impurities. / The experimental results of the current work has three parts. First, the cycloaddition and copolymerization of methyl tricyanoethylenecarboxylate 2 with electron-rich olefins, such as p-methoxystyene, trans-anethole, isobutyl vinyl ether and ethyl cis-propenyl ether are discussed. The nature of the cycloadduct is determined by the orientation of the electrophilic olefins. Copolymerization of 2 with p-methoxystyrene under free-radical initiation gave an alternating copolymer. Second, trimethylsilyl methanesulfonate and trimethylsilyl diphenylphosphate were used as initiators for cationic polymerization. In the presence or absence of hindered pyridine, trimethylsilyl diphenylphosphate and trimethylsilyl methanesulfonate did not initiate the polymerization of p-methoxystyrene, anethole, 4-isopropenylanisole, 1,3-dioxolane or trioxane. Only trimethylsilyl methanesulfonate was able to initiate the cationic polymerization of 1,3-dioxepane in the presence of a hindered base. A model study demonstrates fast desilylation of a carbocation β to a silicon by an oxygen-containing counterion. Finally, block copolydioxepane-polydimethylsiloxane has been synthesized by the "silyl sulfonate approach." In this approach, the nucleophilic macromer, lithium polydimethylsiloxanate, was reacted with chlorodimethylsilane or allylchlorodimethylsilane to produce the corresponding macromers with silylated end groups. They contained a labile substituent, an allyl or a proton, on silicon. These macromers were then converted to electrophilic macropolydimethylsiloxane arylsulfonate by reaction with an aryl or alkyl sulfonic acid. The sulfonate polydisiloxanes can initiate the cationic polymerization of 1,3-dioxepane to yield block polydimethylsiloxane-polydioxepane. This cationic polymerization did proceed in the presence of 2,6-di-t-butylpyridine, which would trap any acid impurities.
54

Low-energy collision phenomena in free jet expansions: Molecular relaxation theory and ion-molecule rate studies.

Randeniya, Lakshman Kumar. January 1990 (has links)
Theoretical and experimental development of a new kinetic method to measure the rate coefficients of ion-molecule reactions occurring in free jet expansions below 20K is presented. The method is successfully used to determine the temperature dependences of numerous bimolecular and termolecular ion-molecule reactions over the temperature range of 0.5-20K. A new theoretical method based on the generalized Boltzmann equation is developed to calculate macroscopic flow properties of pure molecular supersonic flows. The variation of the different temperature components, hydrodynamic speed and density of the free jet as a function of distance is presented assuming a Maxwellian anisotropic distribution function. This theory facilitates the kinetic analysis and the assignment of temperatures to the chemical reactions occurring in jets. Using the Boltzmann equation, the flow properties of a mixed atomic free jet expansion are also analyzed. The method is more general than previous treatments which assume a vanishingly small mole fraction for one component of the mixture. The presence of velocity slip arising from the difference in hydrodynamic speeds of the two components complicates this treatment. Expressions for the calculation of flow properties for an atomic mixture with an arbitrary composition are presented. Temperature dependences of the termolecular association rate coefficients for the reactions of, N₂⁺ + 2N₂, O₂⁺ + 2O₂ and NO⁺ + 2NO over the temperature range of 3-15K are presented. The results are discussed in the light of statistical phase space theory. For the reactions of N₂⁺ + 2N₂ and O₂⁺ + 2O₂ excellent agreement between theory and experiment is obtained. The kinetic analysis of NO⁺ + 2NO is complicated due to the competing charge transfer reaction. The observed temperature dependence for this reaction does not agree with the predictions of the statistical theory. The ternary association rate coefficients for the reaction, Ar⁺ + 2Ar, show a strong temperature dependence at very low temperatures (0.5-2.5K). Current statistical formulations cannot predict this temperature dependence and a comprehensive model for this reaction mechanism has yet to be developed. Three distinct temperature dependences are observed for the bimolecular reactions of N₂⁺ with CH₄, O₂ and n-H₂ at temperatures below 15K. Speculations are made regarding the interaction potential energy surfaces that may lead to the observed behaviors.
55

Functionalization of polyisobutylene, and, Synthesis and crystal structure of heterocyclic quinoimines.

Yahagi, Isao. January 1990 (has links)
Part I. Functionalization of polyisobutylene. Polyisobutylene(PIB), a large-scale commercial polymer, is of potential interest as the component of block copolymers with unique physical properties. Lochmann and Schlosser used super bases, a combination of n-BuLi and KOtBu, to abstract allylic hydrogens from simple olefins to give anions. We applied this super-base chemistry to PIB to introduce new functional groups, such as carboxylic acid, esters, and amino groups, into the polymer. The functionalization of PIB via PIB-Li, formed by reaction with the base, led to complications because the PIB-Li was too reactive and basic. PIB carboxylic acid was obtained by the reaction of PIB heterocuprate with carbon dioxide without any complications. Part II. Synthesis and crystal structure of heterocylic quinoimines. Various heterocyclic quinones which have oxazole rings as their component were synthesized. The transformation to the quinoimines was achieved by the reaction of quinones and triphenylarsinimines which were prepared by the reaction of triphenylarsine oxide and isocyanates. X-Ray analyses of quinoimines showed that the molecules were not planar. The arylimino groups were out of the plane of the molecule. Each quinoimine had an unique crystal structure.
56

Characterization of molecular semiconductor and multilayer molecular organic photoconductor interfaces by photoelectrochemistry and surface analytical techniques.

Danziger, James Lee. January 1990 (has links)
Organic semiconductor thin films are of interest to us for a variety of molecular electronic applications, including solar cells, chemical sensors, and nonlinear optical devices. We have been seeking unusual new ways of controlling the composition and long-range molecular structure of these materials through the use of vacuum deposition techniques which mimic, in some ways, those used for epitaxial layer growth in inorganic materials. Thin films of perylene tetracarboxylic dianhydride (PTCDA) have been examined as electrodes and photoelectrodes on both metal and metal oxide substrates. In contrast to most previous studies of phthalocyanine thin films, these materials behaved in such a way as to suggest n-type character, i.e. dark electron transfer reactions were facile in negative potential regions with solution redox couples, and little dark electrochemistry could be observed in regions of positive potentials. It is likely that junction formation occurs only as a result of illumination, with different rates of interfacial hole and electron injection and transport, at the PTCDA/electrolyte interface. Electron microscopy of the PTCDA films indicated that they were deposited as elongated crystallites, with relatively large spaces between individual crystallites, which strongly affected their dark and photoelectrochemical behavior, especially on Au substrates. Electrochemical polymerization of α-napthol was carried out to passivate sites that were electrochemically active in the dark, a treatment which greatly enhanced the overall electrochemical activity of these PTCDA thin films. A variety of p-n heterojunction-like structures, created from thin film molecular materials (vanadyl phthalocyanine (VOPc) and perylene tetra-carboxylic dianhydride (PTCDA)), have been nondestructively explored by photoelectrochemical techniques and UHV surface analytical techniques. Vacuum deposited bilayers and multilayers of these thin films behave like "p-n" diodes over a narrow potential window. The open circuit photopotential is determined by the junction potential formed at the Pc/PTCDA interface. It was found that the transient photocurrent (using a modulated light source) in multilayer VOPc/PTCDA assemblies was directly related to the number of interfaces present, consistent with the idea that exciton dissociation is localized primarily to such an interface, and is the photocurrent limiting process.
57

Design and synthesis of delta-opioid receptor selective enkephalin analogues.

Gehrig, Catherine Anne. January 1990 (has links)
A series of conformationally constrained analogues of (D-Pen², D-Pen⁵) enkephalin (DPDPE) was prepared by solid phase peptide synthesis and the opioid activity and selectivity of each analogue was assessed by guinea pig ileum and mouse vas deferens bioassays and rat brain radioreceptor binding assays. For example, the conformationally restricted, cyclic disulfide-containing enkephalin analogue (D-Pen², D-Pen⁵) enkephalin (DPDPE) was modified by addition of a methyl group at either the pro R or pro S position of the beta carbon of the phenylalanine-4 residue and by addition of a nitro group in the para position of the phenylalanine-4 position. Other modifications at the Phe⁴ position included methyl substitution on the aromatic ring and dehydration of the C(α)-Cᵦ bond. In addition the effect of beta, geminal dimethyl groups at position 2 was explored. These peptides demonstrate a wide range of selectivities, some of them being more selective than (DPDPE) for the delta opioid receptor. Two-dimensional NMR experiments have been performed on DPDPE and several of the more interesting analogues to determine their solution conformations. These included the absolute assignments of the Gly³ Cα protons, made possible by the preparation of (D-²H-Gly³) DPDPE. Possible φ backbone dihedral angles and χ₁ side chain rotamer populations were calculated in order to develop a model for the solution conformation in DMSO. These data combined with computer molecular modeling studies (energy minimization and quenched molecular dynamics) using the CHARMM computer programs have helped to elucidate the importance of side chain topography for selectivity at the delta opioid receptor and the conformational properties of the peptide backbone required for binding and transduction.
58

DNA binding studies of antitumor antibiotics and antitumor anthracene derivatives: Computer simulations and spectrophotometric titrations.

Hill, Gordon Craig. January 1990 (has links)
Quinocarcin binds to d(ATGCAT)₂ with a preferred direction of 3' and the R configuration at C4 of the drug. A mode of action involving ring opening of the oxazolidine ring to form an iminium ion which can then alkylate the N2 of guanine has been reinforced by the current computer modeling study. The absolute configuration for quinocarcin should be reversed based on the fact that the optical isomer of the structure arbitrarily assigned in the literature forms a much better binding complex to DNA. Anthramycin binds to the 2-amino group of guanine but its mechanism of action proceeds through a neutral imine. The 3' direction is again favored but for this molecule, the preferred configuration is S. This computer modeling study provided a basis for a 2D NMR study which confirmed that anthramycin forms a 3'S adduct when it binds to d(ATGCAT)₂. Bisantrene and R9 are synthetic anthracene derivatives with antitumor activity. Use of UV spectroscopy provided insight into the ability of these compounds to intercalate between the base pairs of double helical DNA. Standard Scatchard plot analysis proved useless in determining the binding parameters. A McGhee-von Hippel equation was able to describe a portion of the data but a smoothing spline function was able to describe the data completely. Naphthyridinomycin studies indicate that it too prefers a covalent adduct in which the direction is 3' and the configuration is R at C7. When the noncovalent drug binds to d(ATGCAT)₂ it may bind with either the C3a face or the C7 face closest to N2 of guanine. Iminium ion mechanisms have been proposed for the binding of naphthyridinomycin to N2 of guanine in the minor groove of DNA and the computer modeling presents evidence to support such mechanisms. Saframycin A binds much better to d(GATGCATC)₂ as a hydroquinone species but the quinone can still bind in the same site. The 3' direction is clearly preferred with the R configuration at C7. The hydrogen bonding network of the hydroquinone is conserved in the noncovalent, iminium ion, and covalent 3'R models after 32 ps of dynamics. Iminium ion mechanisms have been proposed for the binding of saframycin A to N2 of guanine in the minor groove of DNA and the computer modeling presents evidence to support such mechanisms.
59

A krypton VUV photoionization source for gas chromatography and mass spectrometry.

Homan, Mark Eugene. January 1990 (has links)
The use of a coaxial krypton discharge lamp to selectively photoionize samples in a complex matrix is described. The lamp produces a krypton line emission spectrum with available photon energies of 10.0 and 10.6 eV. When used as a photoionization mass spectrometer source, conditions can be created where molecular ions can be produced which do not undergo fragmentation. This effect can greatly simplify spectral interpretation in the case of either direct sample introduction or coelution from a chromatographic column. Mixtures containing 6 to 8 components can be analyzed by direct injection with detection limits for most of the components below the 100 x 10⁻⁹ g level. Data is also presented demonstrating the lamps ability to switch between photoionization conditions producing primarily molecular ions and conditions producing spectra similar to those produced by electron impact sources. This ability would allow two complimentary sets of mass spectrometric information to be gathered during a single run. Using the lamp as a photoionization GC detector, a LOD of 12 x 10⁻¹² g for benzene has also been determined.
60

Synthesis and reactivity of tantalum and tungsten alkyne complexes: Models for alkyne cyclization.

Wexler, Pamela Andrea. January 1990 (has links)
Intermediates in the cyclization reaction of alkynes have been isolated using the group 5 tantalum phenoxide reagents, Ta(DIPP)₂Cl₃(OEt₂) and Ta(DIPP)₃Cl₂(OEt₂) (DIPP = O-2,6-C₆H₃-i-Pr₂). The extent of cyclization has been effected by controlling the sterics at the metal center or the alkyne itself. Reducing the less congested bis phenoxide complex, Ta(DIPP)₂Cl₃(OEt₂), by two electrons in the presence of 2-butyne or 3-hexyne allowed the isolation of an arene complex, (C₆R₆)Ta(DIPP)₂Cl (R = Me, Et), which is formally classified as a 7-metallanorbornadiene. This complex can also be reduced by one more electron to produce a tanatalum (II) species that readily undergoes a one-electron addition reaction with halogenated reagents. This complex also underwent an intramolecular C-H activation of one of the alkyl groups on the arene ring. Attempts were made to try and generalize this cyclization and C-H activation chemistry to the group 6 metals. Tungsten phenoxide and mixed phenylimido-phenoxide reagents were synthesized for use in subsequent cyclization reactions. Reducing the bis phenoxide complex, W(DIPP)₂Cl₄, by two electrons in the presence of a variety of alkynes afforded the alkyne complexes W(DIPP)₂Cl₂(RC≡CR') (R = R' = Me, Et, Ph; R = CMe₃, R' = H). The mixed phenylimido-phenoxide complexes, W(NAr)(DMP)ₓCl₃₋ₓ (x = 1 or 2; NAr = N-2,6-C₆H₃-i-Pr₂; DMP = O-2,6-C₆H₃Me₂), were also reduced by two electrons in the presence of alkynes to afford adducts (i.e. W(NAr)(DMP)₂(EtC≡CEt)). These alkyne adduct complexes failed to undergo any cycloaddition reactions. Reduction of the tantalum tris phenoxide complex, Ta(DIPP)₃Cl₂(OEt₂), by two electrons in the presence of the bulky alkynes diphenylacetylene or trimethylsilyl-1-propyne afforded the isolation of the alkyne adducts (DIPP)₃Ta(PhC≡CPh) and (DIPP)₃Ta(Me₃SiC≡CMe) respectively. The alkyne adduct (DIPP)₃Ta(Me₃SiC≡CMe) undergoes regioselective cross-coupling reactions with smaller alkynes to afford metallacyclopentadienes. Metallacyclopentadienes can be formed directly from the reduction of the tris phenoxide complex in the presence of smaller alkynes (i.e. (DIPP)₃Ta(CEt=CEtCEt=CEt)). The alkyne adduct undergoes cyclization reactions with nitriles that contain α-hydrogens to yield metallacycloenamine complexes (DIPP)₃Ta(CSiMe₃=CMeC(=CHR)NH). The adduct also reacts with ketones to produce metallacyclic complexes with the formulation (DIPP)₃Ta(CSiMe₃=CMeC(RR')O).

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