Deuterated hexamethylbenzene (HMB) is used as a probe molecule for2H NMR studies of the crystalline state of hexachlorobenzene and of several organic glasses. By measuring the spinndash;lattice relaxation and the line shape in the temperature range of 4ndash;300 K the dynamical parameters of the molecular reorientation are investigated. For the system HMB/hexachlorobenzene, we find exponential relaxation and for the correspondingT1an increase of its activation energy by a factor of 2 in comparison to the neat HMB. A homogeneous mixing of the guest and host molecules is found at least for guest concentrations up to 7percnt;. In contrast, nonexponential spinndash;lattice relaxation is characteristic for all glass matrices, indicating motional heterogeneities. A logndash;Gauss distribution for the corresponding motional correlation times gives a good fit of the data. Its width parameter decreases linearly with temperature, while the mean correlation times are described by an Arrhenius law. The mean activation energy is reduced by a factor of about 3.5 as compared to neat HMB, demonstrating a loose packing of the molecules in the glass matrices.
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