Libration motion of guest spin probe molecules in organic glasses: CW EPR and electron spin echo study.

Echo-detected (ED) EPR spectra of nitroxide spin probes dissolved in glassy materials provide evidence that guest molecules in these media undergo fast librational motion. Theory of spin relaxation of a librating molecule is presented. The mean squared amplitude, <alpha2>, of this motion which can be derived from continuous wave (CW) EPR spectral splitting is found to depend linearly on temperature in the low temperature region. This may be ascribed to thermal harmonic vibrations. The slope of the linear dependence varies from glass to glass and seems to correlate with the strength of the intermolecular bonds and with a degree of the fragility of the glass. Above the glass transition temperature <alpha2> increases sharply. Different applications are discussed: study of molecular properties of glass, intracellular glass formation in plant tissues, structural investigations.