Characterizations of F-superconductors and selected F-compounds, amorphous carbon and (VO)2P2O7 by 19F, 13C, 1H, 31P NMR and a new probe for multiple pulse, MAS and DAS NMR

[superscript]19F nuclear magnetic resonance (NMR) studies were performed on the polycrystalline inorganic conductor Ag[subscript]2F and the polycrystalline insulators AgF, YOF, EuOF, YF[subscript]3, CuF[subscript]2, BaF[subscript]2 and KF. The Ag[subscript]2F spectrum showed a [superscript]19F pea clearly shifted downfield with respect to AgF. The two types of nonequivalent F\u27s in YF[subscript]3 have the same isotropic chemical shift. Comparing the signals from the insulators and Ag[subscript]2F with the signals found in so-called 1-2-3 type superconductors of the formula RBa[subscript]2Cu[subscript]3O[subscript] 7-xF[subscript] x (R = Y and Eu) type, no detectable evidence for a Knight shifted built-in fluorine signal was found in the 1-2-3 type superconductors. The only signal found was from the BaF[subscript]2 which is an impurity in the superconductors;Amorphous carbon has been studied by [superscript]13C and [superscript]1H NMR with techniques of static single excitation, magic angle spinning and cross-polarization/magic angle spinning (CP/MAS). A spin-lattice relaxation time 0.7s was obtained by the progressive saturation method. Two different components are clearly seen in the spectrum of the static sample. The downfield component corresponds to sp[superscript]2-like carbons which comprise 93.6% of the total signal, and has an axially symmetric shift tensor with [sigma][subscript] ǁ = -28 ppm, [sigma][subscript]⊥ = 209 ppm and [macron][sigma] = 130 ppm. The upfield component corresponds to sp[superscript]3-like carbons which comprise 6.4% of the total signal and has a symmetric shift tensor with [macron][sigma] = 62 ppm. Two thirds of carbons are not seen in the static measurement because of high concentration of unpaired electrons leading to severe inhomogeneous line broadening. Those carbons are detected by magic angle spinning method in which sidebands spread over a range of 2000 ppm with the first moment located at 130 (±5) ppm. The upper limit of the fraction of hydrogenated carbons is 1.5%;The temperature dependence of the chemical shift of [superscript]31P in (VO)[subscript]2P[subscript]2O[subscript]7 has been studied. A Curie-Law-type temperature dependence of the isotropic chemical shift was observed. This makes (VO)[subscript]2P[subscript]2O[subscript]7 a viable internal temperature standard in high temperature NMR;A stable rotor-stator system is designed that is suitable for NMR experiments of homonuclear decoupling, magic angle spinning, and dynamic angle spinning experiments. A maximum speed of 16 KHz is achieved. The angle of rotation can be changed by 45° in 9 ms with no loss in rotation stability when spinning at 10 KHz

Report / Institute für Physik

In this report the Institutes of Physics of the Universität Leipzig present their scientific activities and major achievements in the year 2003

Aerospace medicine and biology: A continuing bibliography with indexes, supplement 128, May 1974

This special bibliography lists 282 reports, articles, and other documents introduced into the NASA scientific and technical information system in April 1974

Multi-Acquisition and Multi-Dimensional Earth's Field Nuclear Magnetic Resonance Spectroscopy

In this thesis we investigate the ways in which the sensitivity, resolution and overall performance of an Earth's field NMR system can be improved without significantly compromising its simplicity, portability or affordability. We investigate the limits of the information obtainable using this device and present a range of methods for calculating and analyzing NMR spectroscopy experiments detected in the Earth's magnetic field. We demonstrate significant improvements in the performance of a commercial Earth's field NMR device, the Terranova-MRI, through several apparatus developments. First-order shimming is added to the system in order to counter any local inhomogeneity of the Earth's field. The spectral resolution of the instrument is further improved through the introduction of a field locking system to counter the natural temporal drift in the magnitude of the Earth's magnetic field. External noise interference is reduced through the use of Faraday screening, effectively increasing the signal-to-noise ratio (SNR) performance of the device. We explore three signal enhancement methodologies for optimizing the SNR performance of the system. Prepolarization, with an electromagnet as well as a permanent magnet array, is considered and compared to dynamic nuclear polarization (DNP) and hyperpolarization via optical pumping. We present a detailed theoretical discussion of DNP in low-fields and demonstrate the application of this technique for signal enhancement in EFNMR. An apparatus for performing DNP in the Earth's field is presented and optimized. A density matrix approach to simulating one- and two-dimensional Earth's field NMR experiments is presented. These numerical simulations, along with a perturbation theory approach to calculating one-dimensional EFNMR spectra of tightly coupled heteronuclear systems, are explored and compared to experimental spectra of the tetrahydroborate and ammonium ions. These systems are of particular interest for NMR detected in the Earth's field because they contain strongly coupled nuclei of differing spin, a situation previously unexplored in the literature. Multi-dimensional Earth's field NMR spectroscopy methods, in particular the correlation spectroscopy (COSY) experiment, are implemented and optimized through the use of shimming, field stabilization and noise screening. The 2D COSY spectrum of monofluorobenzene is analyzed and compared to calculated spectra in order to determine the indirect spin-spin coupling constants of this molecule in the Earth's magnetic field. A 2D COSY spectrum of 1,4-difluorobenzene is also presented and compared to simulation. The SNR performance of COSY in the Earth's field is greatly improved through the use of DNP for signal enhancement. A high-quality, 2D COSY EFNMR spectrum with DNP acquired from 2,2,2- trifluoroethanol is presented and compared to simulation. The particular features of this spectrum, which result from the use of DNP for signal enhancement, are discussed with reference to a density matrix simulation and to a one-dimensional spectrum calculated using perturbation theory. The strong indirect spin-spin coupling regime in fields weaker than the Earth's magnetic field is explored through exact calculations and density matrix simulations of a 13C-enriched methyl group. A novel multi-dimensional EFNMR method for observing such spectra is discussed. This experiment allows for the resolution of strongly coupled NMR spectra both in the Earth's magnetic field, in the directly detected domain, and in weaker fields, in the indirectly detected domain. In the final section of this thesis, residual dipolar coupling is observed by EFNMR for the first time in a system of poly-[gamma]-benzyl-L-glutamate (PBLG) in dichloromethane. The form of the EFNMR spectrum of this liquid crystalline system is discussed and compared to equivalent high-field (9.4T) spectra

Effect of minor impurities on the electrical properties of Se and Se containing glasses

A number of selenium glasses doped with halogens (Cl, Br, I), in the range of few p.p.m. up to U at %, were prepared in an oxygen-free atmosphere by melt quenching. D.C. conductivity as a function of the temperature was measured and for the higher impurity concentration samples the measurement of the thermoelectric power, D.C. photoconductivity, electron spin resonance and optical absorption coefficient was also undertaken. Other supporting experiments included infrared spectrometry, differential scanning calorimetry, energy dispersive analysis of X-Rays and field effect. X-Ray diffraction was used to check the lack of long-range order in each sample. A general review of amorphous semiconductors is included with particular reference to their electronic and optical properties and the importance of defect states. Results demonstrate the large effect of halogens on conductivity and that the conduction remains p-type with no observable E.S.R. signal. Data obtained are compared with those available in the literature and discussed within the framework of the CFQ-Mott theory. Tentative interpretation of the results is made via a "dangling bonds model" assuming a single particle acceptor level. Additional complementary studies of a number of chalcogenide glasses doped with Br are also included. The effect of several other additives (Ag, Cu, Tl, Na, Sb, Al, Bi) on the D.C. conductivity and other properties of vitreous selenium is briefly investigated

Development and application of techniques in solid-state NMR

The purpose of this thesis is to describe research into the extension and development of methods in solid state NMR, and their application to chemical systems where, for various reasons, analysis was difficult, incomplete, or impossible by other spectroscopic techniques. In particular silicon-29, sodium-23, fluorine-19 and proton nuclei have been used as molecular probes in inorganic silicates, macrocyclic polyether crown complexes and fluoropolymer chemical systems. The sodium-23 chemical shift and lineshape have been shown to be sensitive to coordination and local symmetry. The cross-polarization experiment has also been applied to the sodium-23 nucleus, and shown to exhibit the same steric selectivity which has been observed in carbon-13 andsilicon-29 experiments. A probe has been configured at 4.7 Tesla to perform proton and fluorlne-19 CRAMPS experiments. Multiple pulse sequences and CRAMPS experiments have been used to investigate the molecular motions and chemical environments of the systems investigated. Conclusions are drawn upon the results which support the utility of these methods in application to difficult solid state systems

Doctor of Philosophy

dissertationThis work presents the results of various investigations using various techniques of hyperpolarizing the nuclei of atoms. Hyperpolarization implies magnetic order in excess of the thermal order obtained naturally as described by Curie's law. The main portion of this work presents the results of a detailed experimental exploration of predictions arising from a new model of transverse nuclear spin relaxation in quantum systems, based on possible manifestations of microscopic chaos in quantum systems. Experiments have been carried out on a number of hyperpolarized xenon samples, each di ering in its relative percentage of xenon isotopes in order to vary the homonuclear and heteronuclear dipole couplings in the spin system. The experiments were performed under a variety of conditions in an attempt to observe the behaviors predicted by the model. Additionally, much more extensive measurements were made on a number of samples of solid CaF2 in both single crystal and powder forms. These samples, although thermally polarized, were observed with superior signal to noise ratios than even the hyperpolarized xenon solids, allowing for more precise measurements for comparison to the theory. This work thus contains the rst experimental evidence for the majority of the model's predictions. Additionally, this work contains the rst precise measurements of the frequency-shift enhancement parameters for 129Xe and krypton in the presence of spin-polarized Rb. The determination of these important numbers will be useful to many groups who utilize spinexchange optical pumping in their labs. This work built on the prior knowledge of a precise number for the frequency-shift enhancement parameter of 3He in Rb vapor. Finally, I detail work using NMR to detect nuclear-spin polarization enhancement in silicon phosphorus by a novel, photo-induced hyperpolarization technique developed by the Boehme research group at the University of Utah. Signiif cant nuclear polarization enhancements were observed by the Boehme group due to electron-photon interactions in semiconductor soilds; these enhancements were observed by their e ffects on the ambient electrons and measured with electron spin resonance techniques. The work described here details experiments to observe the enhanced nuclear polarization by directly measuring the intensity increase in an NMR measurement

Carbon and deuterium nuclear magnetic resonance in solids

In Chapter I we present the results on a study of cross polarization dynamics, between protons and carbon-13 in adamantane, by the direct observation of the dilute, carbon-13, spins. These dynamics are an important consideration in the efficiency of proton enhancement double-resonance techniques and they also provide good experimental models for statistical theories of cross relaxation. In order to test these theories we present a comparison of the experimental and theoretical proton dipolar fluctuation correlation time {tau}{sub c}, which is experimentally 110 {+-} 15 {micro}sec and theoretically 122 {micro}sec for adamantane. These double resonance considerations provide the background for extensions to deuterium and double quantum effects discussed in Chapter II. In Chapter II an approach to high resolution nmr of deuterium in solids is described. The m = 1 {yields} -1 transition is excited by a double quantum process and the decay of coherence Q({tau}) is monitored. Fourier transformation yields a deuterium spectrum devoid of quadrupole splittings and broadening. If the deuterium nuclei are dilute and the protons are spin decoupled, the double-quantum spectrum is a high resolution one and yields information on the deuterium chemical shifts {Delta}{omega}. The relationship Q({tau}) {approx} cos 2{Delta}{omega}{tau} is checked and the technique is applied to a single crystal of oxalic acid dihydrate enriched to {approx} 10% in deuterium. The carboxyl and the water deuterium shifts are indeed resolved and the anisotropy of the carboxyl shielding tensor is estimated to be {Delta}{sigma} = 32 {+-} 3 ppm. A complete theoretical analysis is presented. The extension of cross relaxation techniques, both direct and indirect, to proton-deuterium double resonance is also described. The m = 1 {yields} -1 double quantum transition and the m = {+-} 1 {yields} 0 single quantum transitions may all be polarized and we present the derivation of the Hartmann-Hahn cross polarization conditions for each case. In addition the dynamics of the double quantum process, for monodeutero benzene are discussed, giving proton dipolar fluctuation correlation times, and spin heat capacities for the double quantum transition

Solid-State Proton Nuclear Magnetic Resonance Studies of Hydrogen Site Occupancies, Electronic Structure Properties, and Diffusion Behavior in Transition Metal Hydrides

Solid-state NMR techniques have been used to measure the proton lineshapes, Knight shifts, and relaxation times in several transition metal hydrides. The objective of these studies is to obtain a better understanding of the roles of host metal structure and substitutional alloying on the hydrogen site occupancy, the electronic structure properties, and diffusion mechanisms. An improved method for observing the rigid-lattice proton lineshapes and extracting the second moments has been developed. Comparisons of the experimental second moments for polycrystalline samples with the values calculated from Van Vleck formulas for nuclear dipolar interactions have indicated that only tetrahedral interstitial sites are occupied by the protons in TiHx, ZrHx, crystalline TiCuH0.94, Ti2CuH1.9, and Zr2PdHx (with x &lt; 2) while both tetrahedral and octahedral sites can be occupied in amorphous a-TiCuH1.4, Ti2CuH2.6, and Zr2PdHx (x &gt; 2). The proton Knight shifts and low-temperature spin-lattice relaxation times have been related to the local densities of electron states at the Fermi levels N(EF) in Ti1-yVyHx, TiCr1.8Hx, TiCr1.9Hx, TiCuHx, Ti2CuHx, ZrHx, and Zr2PdHx. The dominant conduction electron hyperfine interaction for protons is a transferred "core-polarization" of the paired hydrogen 1s electrons through spin exchange with the unpaired metal d electrons. The proton NMR parameters have confirmed that decreases in N(EF) through a Jahn-Teller type mechanism are associated with the temperature and composition dependent tetragonal distortions in Ti1-yVyH1.95 and ZrHx (where 1.75 ≤ x ≤ 2.0). The proton NMR results are consistent with recent band theory calculations and photoemission spectra. Unusual N(EF) increases with hydrogen content, which are supported by independent magnetic susceptibility data, have been observed in TiCr1.8Hx, TiCr1.9Hx, and Ti2CuHx. The proton parameters suggest that significant differences in N(EF) for the crystalline and amorphous phases of TiCuHx and Zr2PdHx may reflect a smearing of energy levels in the disordered phases. The proton rotating-frame relaxation times for Ti1-yCuyHx indicate both crystal structure and hydrogen site occupancies greatly influence diffusion behavior. A significant enhancement in hydrogen mobility for amorphous a-TiCuH1.4 has been confirmed; but, short range order is probably retained in the structure of a-TiCuH1.4. Reductions in activation energies are observed when octahedral sites exist in the diffusion paths between tetrahedral sites.</p