Molecular dynamics in supercooled glycerol: results from 13C NMR spectroscopy.
(13)C NMR spectra of glycerol are collected over the entire temperature range of supercooling: T(g)(185 K) < or = T < or = T(m)(293 K). The temperature dependent evolution of the (13)C NMR line shape indicates dynamical averaging of the chemical shift anisotropy at the carbon sites with increasing temperature, resulting from isotropic tumbling of the constituent molecules. This isotropic reorientation dynamics involves random molecular jumps over all possible angles, and its time scale is in excellent agreement with the alpha-relaxation time scale of the supercooled liquid. The increasing activation energy of such molecular jumps with decreasing temperature and hence the fragility of supercooled glycerol are likely to be related to the corresponding temperature dependence of the average number of hydrogen bonds per molecule. The absence of any beta peak in the dielectric relaxation spectra of supercooled glycerol is possibly related to a strong coupling between intramolecular dynamics and the tumbling of the entire molecule as a whole.
Jain
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Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, California 95616, USA.
Levchenko
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A
Yu
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P
Sen
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S