Rotational motion in the molecular crystals meta- and ortho-carborane studied by deuteron nuclear magnetic resonance.

Spin-lattice and spin-spin-relaxation times, one- and two-dimensional spectra as well as two- and four-time correlation functions were measured for the molecular crystals ortho- and meta-carborane using deuteron nuclear magnetic resonance. It is found that in their noncubic phases these crystals exhibit highly anisotropic motions. In order to allow for a quantitative description of the motional geometry of the carboranes several stochastic models are formulated. By comparison of the model calculations with the experimental results it is found that the dynamics of these quasi-icosahedrally shaped molecules is governed by a composite reorientation process. Here the molecules perform threefold jumps around a molecule-fixed axis which itself can be tilted in four different directions with respect to a crystal-fixed axis. The tilt angle increases significantly with increasing temperature. On the basis of measurements of four-time stimulated-echo functions, implications for dynamic heterogeneity also in comparison with that of supercooled liquids are discussed.