High- and low-temperature phases in isostructural 4-chloro-3-nitroaniline and 4-iodo-3-nitroaniline

摘要

The structures of 4-chloro-3-nitroaniline, C_6H_5ClN_2O_2, (I), and 4-iodo-3-nitroaniline, C_6H_5IN_2O_2, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single-crystal X-ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high-temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene-ring planes at two different orientations. In the low-temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, the b axis doubles with respect to the room-temperature cell. Each of the low-temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallo-graphic inversion centres. The molecules within each pair have the same absolute value of the inclination angle.