A THEORETICAL-EXAMINATION OF THE MOLECULAR PACKING, INTERMOLECULAR BONDING AND CRYSTAL MORPHOLOGY OF 2,4,6-TRINITROTOLUENE IN RELATION TO POLYMORPHIC STRUCTURAL STABILITY

2,4,6-Trinitrotoluene (TNT) can crystallise in both monoclinic and ort horhombic polymorphic forms characterised by two distinct molecular pa cking configurations in the solid-state associated with two conformati onally unique molecules (A and B) in the asymmetric unit with four asy mmetric units in the unit cell. In this the centrosymmetric monoclinic (pseudo b-glide) structure (P2(1)/c) adopts an AABBAABB packing moth and the orthorhombic (pseudo-centrosymmetric structure Pb2(1)a) 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 gr oups are twisted out of the plane of the benzene ring and there are al so subtle differences in the molecular conformation of the two molecul ar types between the two polymorphs. Lattice energy calculations based on the solved structures reflect the similarity of the packing sequen ces, suggesting a relative stability of monoclinic > twinned monoclini c > orthorhombic in agreement with experimental studies. The small ene rgy difference between the twinned and: non-twinned monoclinic polymor phs explains the abundance of twinning in TNT. Attachment energy calcu lations predict needle-like crystal morphologies with a platelike cros s-section.