University of Illinois Urbana-Champaign

Multi-Frequency Electronic Magnetic Resonance Studies of Polycrystalline Systems

Shi, Wenjun

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Description

Title
Multi-Frequency Electronic Magnetic Resonance Studies of Polycrystalline Systems
Author(s)
Shi, Wenjun
Issue Date
1993
Doctoral Committee Chair(s)
Belford, R. Linn
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Abstract
  • The conventional electronic magnetic resonance (EMR) spectra of disordered samples often suffer poor resolution due to line-broadening. This thesis describes how a multi-frequency approach applied to the EMR studies of two distinct polycrystalline paramagnetic systems resulted in dramatic resolution improvements.
  • The EMR techniques used include continuous wave electron paramagnetic resonance (cw-EPR) and electron spin echo envelope modulation (ESEEM). Performing the cw-EPR experiments at tenfold higher microwave frequency than the conventional X-band (ca. 9 GHz), greatly improved resolution in the field-dependent electronic Zeeman interaction of the nitro-aromatic radicals adsorbed on metal oxide surfaces. In applying the multi-frequency method to ESEEM to reveal weak superhyperfine (shfs, a) and nuclear quadrupole (NQR) interactions, we have established by model calculations a criterion to identify the "exact cancellation" condition, by which the NQR parameters can be determined directly from the multi-frequency ESEEM spectra. This criterion, taking into account the g and a anisotropies, has been verified by ESEEM experiments performed on the remote nitrogen atom of the meta-dinitrobenzene radical on surfaces. The magnetic parameters obtained this way are in excellent agreement with simulations.
  • Low-frequency (2-4 GHz) ESEEM spectra of adducts of a copper chelate to an axially coordinated nitrogen-containing Lewis base showed pronounced angle selection patterns. The copper-base systems also provide a large g anisotropy, in which a close approach to cancellation is demonstrated. The nitrogen quadrupole parameters obtained by the pseudo-cancellation method are in reasonably good agreement with those obtained by the orientation-selected ESEEM; the residual discrepancy is discussed.
  • Low-frequency ESEEM spectra of oxygen-17, magnesium-25, and aluminum-27 in the metal oxides adsorbed with nitro-aromatic species are also reported and discussed.
Graduation Semester
1993
Type of Resource
text
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http://hdl.handle.net/2142/72285

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