Slow β process in simple organic glass formers studied by one and two-dimensional 2H nuclear magnetic resonance.: II.: Discussion of motional models

Applying random walk simulations we analyze results of one- (1D) and two-dimensional (2D) H-2 nuclear magnetic resonance (NMR) experiments carried out to study the slow beta process of toluene-d(5) below the glass transition temperature T-g. In this way, we reveal amplitude and mechanism of rotational jumps involved in the secondary relaxation of a rigid molecule. It is shown that essentially all toluene molecules take part in the beta -process. In the glass, nearly independent of temperature, the attributed reorientation is restricted to small solid angles of typically 4 degrees -5 degrees for the majority of particles. Only 10%-20% of the molecules exhibit amplitudes chi > 10 degrees. Concerning the mechanism of this highly restricted motion we demonstrate that the reorientation takes place step-by-step via several elementary rotational jumps one after the other. Assuming thermally activated jumps within energy landscapes which are, first, attributed to a small section of the unit sphere for each respective molecule and, second, chosen in accordance with the distribution of energy barriers g(E-beta) found in dielectric spectroscopy we succeed in reproducing the results of 1D and 2D H-2 NMR experiments on toluene below T-g. (C) 2001 American Institute of Physics.