Exchange processes in disordered systems studied by solid-state 2D NMR

1D and 2D H-2 NMR spectra of binary liquids were measured above and below the glass transition. We show that the molecular axes of the smaller molecules in these systems reorientate isotropically and that this dynamics is described by a broad distribution of correlation times. Further it is demonstrated that exchange processes within this distribution, i.e., exchange between fast and slow reorientational behavior of the molecules, are directly probed by measuring 2D NMR spectra. Exchange and reorientation occur on the same time scale.