Variable-temperature crystal structure studies of m-nitroaniline.

The crystal structure of m-nitroaniline has been examined at several temperatures over the 90-350 K range. Thermal evolution of the lattice parameters reveals a weak anomaly at 110 K and an important one at 300 K. The thermal expansion coefficients have been calculated at several temperatures and the principal axes cross-sections of the tensor were drawn. The lattice contraction along the b axis direction has been observed. The rigid-body analysis including an attached rigid group has provided the values of the translation and libration tensors at temperatures studied. The results indicate that m-nitroaniline undergoes a glass transition around 130 K arising from freezing molecular librations and translations. From above 340 K the growing plasticity of the m-nitroaniline crystal results in the loss of X-ray diffraction reflections. This is probably a second-order phase transition. It is coupled with a considerable increase in the nitro group torsion amplitude, but the NH.O hydrogen bonds are preserved. Analysis of the temperature evolution of short intermolecular distances enabled us to consider the occurrence of reorienting aggregates of hydrogen-bonded molecules in the high-temperature plastic phase.