Variable-temperature crystal structure studies of m-nitroaniline

The crystal structure of m-nitroaniline has been examined at several temper atures 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 th e principal axes cross-sections of the tensor were drawn. The lattice contr action along the b axis direction has been observed. The rigid-body analysi s including an attached rigid group has provided the values of the translat ion and libration tensors at temperatures studied. The results indicate tha t m-nitroaniline undergoes a glass transition around 130 K arising from fre ezing molecular librations and translations. From above 340 K the growing p lasticity of the m-nitroaniline crystal results in the loss of X-ray diffra ction 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 occ urrence of reorienting aggregates of hydrogen-bonded molecules in the high- temperature plastic phase.