Dynamic properties of p-diiodobenzene investigated by solid-state 2H and 13C nuclear magnetic resonance spectroscopy

Abstract

p-Diiodobenzene exists in two crystalline phases, both of which are orthorhombic, with space groups Pbca(α phase) and Pccn(β phase) respectively. The α phase is the stable form at ambient temperature, and a phase transition to the β phase occurs at ca. 326 K. In this paper, we present variable-temperature solid-state ²H NMR studies of perdeuteriated p-diiodobenzene. Over the temperature range 293–320 K, the ²H NMR spectrum of the α 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 ²H NMR spectrum of the β phase shows features characteristic of 180° jumps of the aromatic ring about the C(1)–C(4) axis, and the jump frequency (κ) for this motion has been determined as a function of temperature. On the assumption of Arrhenius behaviour for the temperature dependence of κ, 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 ¹³C NMR spectroscopy, and results from these investigations are reported and discussed in the light of the dynamic information determined from the ²H NMR studies.