DYNAMIC PROPERTIES OF P-DIIODOBENZENE INVESTIGATED BY SOLID-STATE H-2 AND C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

p-Diiodobenzene exists in two crystalline phases, both of which are orthorhombic, with space groups Pbca (alpha phase) and Pccn (beta phase) respectively. The alpha phase is the stable form at ambient temperature, and a phase transition to the beta phase occurs at ca. 326 K. In this paper, we present variable-temperature solid-state H-2 NMR studies of perdeuteriated p-diiodobenzene. Over the temperature range 293-320 K, the H-2 NMR spectrum of the alpha phase is a rigid lattice powder pattern, with static quadrupole coupling constant 176 +/- 1 kHz and static asymmetry parameter 0.04 +/- 0.01. Over the temperature range 350-370 K, the H-2 NMR spectrum of the beta phase shows features characteristic of 180-degrees jumps of the aromatic ring about the C(1)-C(4) axis, and the jump frequency (kappa) for this motion has been determined as a function of temperature. On the assumption of Arrhenius behaviour for the temperature dependence Of kappa, the activation energy for this jump motion is estimated to be (90 +/- 10) kJ mol-1. The temperature-dependent structural properties of perdeuteriated p-diiodobenzene have also been investigated by high-resolution solid-state C-13 NMR spectroscopy, and results from these investigations are reported and discussed in the light of the dynamic information determined from the H-2 NMR studies.