Structural disorder in solid p-Iodotoluene

Single crystal X-ray diffraction studies have been carried out on p-iodotoluene at two temperatures (293 and 173 K) in order to explore the existence and temperature-dependence of orientational disorder in the crystal structure. In the average crystal structure at each temperature, the molecules are disordered between two orientations. The two orientations define a head-tail type of disorder in which the intramolecular CH{sub 3} {r_arrow} I vectors are essentially antiparallel and the molecular planes are essentially coplanar. The structures at 293 and 173 K are essentially the same, except that there is a significant difference in the degree of disorder (the fractional occupancy of the minor molecular orientation is 0.17 at 293 K and 0.08 at 173 K). As the same crystal was used for the X-ray diffraction studies at both temperatures, the change in occupancy with temperature suggests that a mechanism exists for the molecular orientation to relax within the crystal. Solid state {sup 13}C NMR results are also consistent with (although do not prove) the suggested dynamic interconversion between the two molecular orientations. Several aspects of the disorder in this system are discussed in detail, including an assessment of energetic aspects concerning the different molecular orientations within the crystal.