Multi-Frequency Electronic Magnetic Resonance Studies of Polycrystalline Systems

237 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1993.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.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD