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

Applying random walk simulations we analyze results of one- (1D) and two-di mensional (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 rotation al jumps involved in the secondary relaxation of a rigid molecule. It is sh own that essentially all toluene molecules take part in the beta -process. In the glass, nearly independent of temperature, the attributed reorientati on is restricted to small solid angles of typically 4 degrees -5 degrees fo r the majority of particles. Only 10%-20% of the molecules exhibit amplitud es chi > 10 degrees. Concerning the mechanism of this highly restricted mot ion we demonstrate that the reorientation takes place step-by-step via seve ral elementary rotational jumps one after the other. Assuming thermally act ivated jumps within energy landscapes which are, first, attributed to a sma ll section of the unit sphere for each respective molecule and, second, cho sen 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.