Geometry and time scale of the rotational dynamics in supercooled toluene

Abstract

Multidimensional deuteron NMR provides powerful tools for studying molecular reorientation in supercooled liquids. We present results on selectively deuterated toluene-$d_5,$ which may be one of the molecularly most simple van der Waals glass formers. From two-time correlation functions the time scale of reorientation was obtained slightly above the calorimetric glass transition temperature. The applied stimulated echo method provides a geometry parameter that, in analogy to $q$-dependent scattering experiments, allows one to investigate the geometry of the elementary rotational process. Continuous time random walk computer simulations were used for the interpretation of the data. It is shown that an isotropic single jump angle model does not describe the toluene rotation, rather the existence of several jump angles is required. Assuming mainly small jump angles $<6\mathrm{}°$ but also some larger angles up to 30° an acceptable fit to the experimental data was obtained. Using four-time correlation functions further properties of molecular reorientation are elucidated. Slow reorienting subensembles can be selected. Their return to the full ensemble occurs on the same time scale as the elementary rotational jump process. In accord with previous investigations in other supercooled liquids, a heterogeneous scenario is found for the rotational dynamics of toluene.