Molecular motions in solid chloropentamethylbenzene: A thermally stimulated depolarisation currents study

The dipolar relaxation mechanisms present in solid chloropentamethylbenzene have been studied using the technique of thermally stimulated discharge currents (TSDC). A relaxation peak observed at ca. -150 degrees C has been attributed to the onset of the reorientational freedom in the crystal and is believed to correspond to the transition between an ordered phase and a rotationally disordered phase. At higher temperatures, ca. 10 degrees C, another relaxation peak appears in the TSDC spectrum of chloropentamethylbenzene, whose assignment at the molecular level is not yet completely elucidated. Taking into account other results from the literature, it is suggested that this peak could arise from self-diffusional motions in the solid, which probably involve out-of-plane molecular tumbling as well as in-plane molecular motions. The results obtained in the present work are compared with those obtained, by the same technique, on other hexasubstituted benzenes. In this context, some general features of the TSDC technique are discussed, namely the existence of a general relationship between the activation parameters associated with the different individual components of the relaxations in the TSDC spectrum, independent of the materials under study and of the nature of the observed relaxations; A reformulation of the so-called 'zero entropy condition' is also presented.