Polymorphism in 2-4-6 Trinitrotoluene

Two crystal structures of 2-4-6 trinitrotoluene (TNT) are given, the monoclinic form (a0 = 1.49113 (1)
nm, b0 = 0.60340 (1) nm, c0 = 2.08815(3) nm, β = 110.365 (1)°, V = 1.76137 (4) nm3, space group = P21/a, T = 100
K) and the orthorhombic form (a0 = 1.4910 (2) nm, b0 = 0.6031 (2) nm, c0 = 1.9680 (4) nm, V = 1.7706 (7) nm3, space
group = Pca21, T = 123 K). Of these two forms, the most stable is the monoclinic and the less stable is the
orthorhombic form. These two polymorphs are shown to be orientational, rather than configurational in character.
Due to their restricted molecular motifs, no strong hydrogen bonding exists and the crystalline form is dominated
by van der Waals type forces. The two structures are shown to be closely related and an analysis of the two structures
shows that they are effectively large scale polytypes. Calorimetric studies show that the two polymorphs are
monotropic and that the enthalpy of transformation is very low, concurring with the similarity shown by the diffraction
data and calculated lattice energies. The thermal expansion coefficients are defined, and it is shown that both
polymorphs have similar thermal expansions.