Molecular motion and phase transitions in solid hexafluorobenzene + benzene complex by nuclear magnetic resonance and heat capacity measurements

Abstract

Heat capacity measurements on the hexafluorobenzene + benzene complex between 60 and 320 K have revealed the presence of solid–solid transitions at 199, 247.5 and 272.0 K. N.m.r. and heat capacity measurements showed that the phase normally stable between 199 and 247.5 K could be supercooled below 199 K and kept without transforming for a considerable period of time. ¹⁹F and D spin–lattice relaxation times, measured between 100 and 300 K on the hexafluorobenzene +[²H₆]benzene complex, were used to characterize the in-plane reorientations of the hexafluorobenzene and [²H₆]benzene molecules. HFB reorientations are shown to occur independently of the [²H₆]benzene motions. In the low temperature phases activation energies for these motions are comparable with thoses obtained for the corresponding pure solids. The deuterium quadrupole coupling constant for [²H₆] benzene in the complex was measured to be 185 kHz.