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|Title:||S-band electron spin echo and multifrequency EPR investigations of disordered solids|
|Author(s):||Brown, David R.|
|Doctoral Committee Chair(s):||Belford, R. Linn|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Electron spin echo (ESE) methods are particularly useful for measuring weak electron-nuclear couplings in paramagnetic solids. Frequencies that correspond to nuclear transitions are manifested in the electron spin echo envelope modulation (ESEEM) patterns recorded from a sample. Information regarding the distance between the unpaired electron spin and neighboring magnetic nuclei and about the number of nuclei which interact with the unpaired electron spin can be obtained from the analysis of ESEEM data.
In the limit of weak nuclear hyperfine coupling, i.e., the nuclear Zeeman interaction is much greater than the hyperfine interaction, the ESEEM depth can be enhanced by performing the experiment at a lower EPR excitation frequency. This modulation depth enhancement is demonstrated in chapter 1 of this thesis, by making a comparison of an ESEEM pattern recorded from a sample of vitrinite (a coal maceral) at a more commonly employed X-band EPR excitation frequency ($\sim$9 GHz) with an ESEEM pattern acquired at an S-band EPR excitation frequency ($-$3 GHz). The achievement of enhanced modulation depth (relative to X-band) was the primary impetus behind the construction of the S-band (2-4 GHz) pulsed EPR spectrometer described in chapter 2 of this work.
Another advantage of performing S-band ESEEM experiments is having the ability to extend the range of Zeeman fields employed, allowing for extended adjustment of field dependent interactions. For instance, for quadrupolar nuclei (1 $>$ 1/2) which sustain a hyperfine interaction dominated by the isotropic component, an external magnetic field can be selected such that the nuclear Zeeman interaction approximately cancels the hyperfine interaction in one of the electron spin states on resonance. The ensuing nuclear energy levels, in the electron spin state in which the cancellation occurs, correspond (approximately) to the zero-field nuclear quadrupole states. This permits observation of the NQR transitions, the frequencies of which can be used to calculate the quadrupole parameters K = e$\sp2$qQ/4 and $\eta$. This cancellation technique was used to record the N14 NQR spectrum from the nitro groups on the picryl ring in the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical. The values of the quadrupole parameters were determined to be K = 0.26 $\pm$ 0.02 MHz and $\eta$ = 0.35 $\pm$ 0.02.
In chapter 4 of this thesis, the results of a multi-frequency EPR investigation of a binuclear copper(II) complex are presented. The two macrocycles in which the metal centers reside are bridged by four methylene groups. A magnetic exchange interaction, determined to be propagated through the aliphatic bridge, was evident from the appearance of the EPR spectra. It was determined that the magnitude of the exchange parameter J $>>$ 542 MHz.
|Rights Information:||Copyright 1990 Brown, David R.|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9114184|