2,4,6-Trinitrotoluene (TNT) can crystallise in both monoclinic and orthorhombic polymorphic forms characterised by two distinct molecular packing configurations in the solid-state associated with two conformationally unique molecules (A and B) in the asymmetric unit with four asymmetric units in the unit cell. In this the centrosymmetric monoclinic (pseudo b-glide) structure (P21/c) adopts an AABBAABB packing motif and the orthorhombic (pseudo-centrosymmetric structure Pb21a) adopts an ABABABAB packing motif. The close molecular similarity between the conformations of the A and B molecules also gives rise to twinning in the monoclinic phase due to defects in the stacking sequence. The two molecular structures differ in the degree to which the 2,4,6 nitro groups are twisted out of the plane of the benzene ring and there are also subtle differences in the molecular conformation of the two molecular types between the two polymorphs. Lattice energy calculations based on the solved structures reflect the similarity of the packing sequences, suggesting a relative stability of monoclinic > twinned monoclinic > orthorhombic in agreement with experimental studies. The small energy difference between the twinned and non-twinned monoclinic polymorphs explains the abundance of twinning in TNT. Attachment energy calculations predict needle-like crystal morphologies with a plate-like cross-section.
|Number of pages
|Journal of Materials Chemistry
|Published - Feb 1997