Structural disorder in solid p-iodotoluene

Single crystal X-ray diffraction studies have been carried out on p-iodotol uene at two temperatures (293 and 173 K) in order to explore the existence and temperature-dependence of orientational disorder in the crystal structu re. In the average crystal structure at each temperature, the molecules are disordered between two orientations, The two orientations define a "head-t ail" type of disorder in which the intramolecular CH3 --> I vectors are ess entially antiparallel and the molecular planes are essentially coplanar. Th e structures at 293 and 173 K are essentially the same, except that there i s a significant difference in the degree of disorder (the fractional occupa ncy 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 tem peratures, the change in occupancy with temperature suggests that a mechani sm exists for the molecular orientation to relax within the crystal. Solid state C-13 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, inc luding an assessment of energetic aspects concerning the different molecula r orientations within the crystal. (C) 1999 Academic Press.