João M. Campos

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 °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 °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.

Molecular Motions in the Solid Rotator Phase of 2-Adamantone: A Thermally Stimulated Currents Study

Abstract A Thermally Stimulated Discharge Currents (t.s.d.c.) study of 2-adamantanone has been performed in the temperature range from -160 °C to + 100 °C in order to analyse the phase transitions and the molecular motions present in this solid. A relaxation was observed at - 130°C which was attributed to the transition between an ordered brittle phase and a biphasic system composed by this ordered phase and an orientationaly disordered phase. An higher temperature relaxation was also observed which probably corresponds to the onset of the molecular self-diffusion in the solid. It was suggested that the t.s.d.c. technique is potentially a powerfull technique to study molecular motions in solid rotator phases: