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Title:Proton-coupled carbon-13 sample spinning NMR studies of macromolecules and ordered systems
Author(s):Chung, John
Doctoral Committee Chair(s):Oldfield, Eric
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Physical
Abstract:We have studied a variety of macromolecular systems such as lipid/water lyotropic phases and elastomeric polymers, and in addition a thermotropic liquid-crystal displaying macroscopic orientational ordering behavior, by Carbon-13 ($\sp{13}$C) nuclear magnetic resonance (NMR) spectroscopy. We find that under "magic angle" spinning (MAS) these systems display high resolution due to extensive motions, and therefore traditional proton-decoupling is not necessary. In the absence of proton decoupling, i.e., proton-coupled MAS (PCMAS), we find that these systems display scalar hyperfine-coupled $\sp{13}$C splittings with radically asymmetric linewidths and shapes. We attribute these effects to cross-correlation terms between two or more different relaxation mechanisms, (dipolar/chemical shift anisotropy (DD/CSA) relaxations) at the high magnetic fields employed.
Since the DD/CSA effects are quite novel and have not been treated in detail experimentally or theoretically, we have extended the previous theories to a more realistic anisotropic motional model and have attempted to explain the proton-coupled $\sp{13}$C longitudinal relaxation data in two elastomers by fitting it to the simple model. The data does not explain the model in an intuitively reasonable fashion; and it is concluded that more work needs to be done in order to study a system with more experimental measurables which would allow for fitting to a more elaborate theory of restricted motions.
We have also tried to extend the usefulness of PC-MAS relaxation cross-correlation study to a thermotropic liquid crystal; and in the process we have determined the effects which sample spinning in a magnetic field can have on an ordered phase such as the nematic phase of a liquid crystal. Our findings indicate that rapid spinning of the sample can lead to destruction of the macroscopic order-director alignment along the sample spinner axis. In order to avoid this difficulty "off-axis" spinning experiments are done to show that the stability of the order director can be maintained. With the added precaution we show that proton-coupled $\sp{13}$C relaxation study is feasible in the nematic phase; and from the measured relaxation time constants we obtain macroscopic cross-correlation spectral densities, although the interpretation of these spectral densities is dependent on the of knowledge of the accurate static tensor elements.
Issue Date:1991
Rights Information:Copyright 1991 Chung, John
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9210769
OCLC Identifier:(UMI)AAI9210769

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