Electron paramagnetic resonance (EPR) lineshapes of nitroxide spin probes in a variety of organic supercooled liquids and glasses were analyzed within the framework of the fast librational motion model. Computer simulation confirmed that the EPR lineshape may be fitted well by the librational model. For all systems studied, a mean-squared amplitude of this motion, 〈α2〉, was found to depend linearly on temperature in the low-temperature region, which is typical of harmonic solids. The slope of the linear dependence varies from glass to glass and does not change for different nitroxides in the same glass. This slope seems to correlate with the strength of the intermolecular bonds and with a degree of the fragility of the glass. Steep rises in 〈α2〉 were observed at high temperature.

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