Reorientational dynamics in simple supercooled liquids

The geometry of the reorientational dynamics in the van der Waals liquid, toluene, and the hydrogen bond network, glycerol, are compared. Both systems have contributions from small angle fluctuations. In glycerol the fraction of these small angle fluctuations is much larger than in toluene, due to the stronger anisotropic interactions in the former substance. The average reorientational angle in both systems is similar and on the order of 10 degrees. In addition we analyze the stretching of the rotational correlation functions of rank one and two. In both cases we find that the second rank correlation function has a more pronounced stretching than the corresponding first rank correlation function. This effect can be understood in the framework of a recently introduced free-energy landscape model for the primary relaxation in supercooled liquids. (C) 1998 Elsevier Science B.V. All rights reserved.