No abstract

Two crystal structures of 2-4-6 trinitrotoluene (TNT) are given, the monoclinic form (a 0 = 1.49113 (1) nm, b 0 = 0.60340 (1) nm, c 0 = 2.08815(3) nm, β = 110.365 (1)°, V = 1.76137 (4) nm3, space group = P21/a, T = 100 K) and the orthorhombic form (a 0 = 1.4910 (2) nm, b 0 = 0.6031 (2) nm, c 0 = 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.

High-resolution X-ray powder diffraction using synchrotron radiation has been used to determine the unit-cell parameters of some of the n-alkanes within the range C, , H2, -C, , H, 22.These have, in general, been found to conform to the structural patterns predicted by Nyburg and Potworowski (Acta Crystallogr., Sect. B, 1973, 29, 347). Polymorphic behaviour has been observed for the n =even alkanes associated with increasing symmetry from a triclinic structure (space group Pl), to a monoclinic structure (space group P2,/a) and to an orthorhombic structure (mostly space group Pca2,) as the chain length increases. The n-alkanes C38H78, C4,,Hs2 and C44H90 were found to crystallize in the polytypic orthorhombic polymorph (space group Pbca) proposed by Boistelle et a/.

The morphology of monoclinic paracetamol crystals has been investigated both theoretically and experimentally. Calculations using the computer program HABIT 95 with both DREIDING II and MOMANY force fields predict prismatic forms in which {100}, {001}, {110}, and {201̄} show approximately equivalent morphological importance. Whereas all of these faces are observed experimentally, the real crystals showed a {110} dominance at low supersaturations which gave way to an increasing {001} dominance as the supersaturation increased. This variation was accompanied by a change from a columnar to a platelike habit. Surface examinations using phase contrast microscopy showed the habit changes to be due principally to changes in the growth mechanism of the {110} faces. A slow growth process involving two-dimensional nucleation at a few growth sources occurred at low supersaturations. This gave way to dislocation growth and finally at high supersaturations, to a fast growing mixed mechanism combining two-dimensional growth from the edges and vertexes of the {110} faces with the operation of dislocation sources at the face center. The increasing dominance of the two-dimensional growth contribution at the highest supersaturations coupled with an increase in macrostep formation resulted in the development of inclusions in the {110} sectors. This phenomenon will result in significant increases in the solvent impurity content of crystals at the high supersaturations normally used in the production of this material. The results of this study show well the dominant part that the growth mechanism can play in the definition of the morphology of crystals and hence the care which must be taken in the interpretation of modeling calculations.

Crystals of acentric R-resorcinol (space group Pna2 1 , point group mm2), grown from the vapor phase, present a wide range of morphological forms. Increase in supersaturation (σ) yields, successively, prismatic (σ ∼ 0.001), columnar (σ ∼ 0.003), needlelike (σ ∼ 0.005), and, finally, hexagonal platelike (σ > 0.007) morphologies. This change in habit is achieved principally by extension in the [001 h] polar direction. No observable continuous growth occurs on the (011) and (01 h1) faces presenting in the [001] direction. Similar platelike morphologies and absence of growth in the [001] direction are reported for crystals grown from aqueous solutions at the highest supersaturations (σ > 0.01). Growth from aqueous solution at lower supersaturations could not be studied with the growth method used due to the low growth rates of all crystal faces under these conditions. Previous reports of apparently faster rates of growth along the [001] direction in R-resorcinol crystals grown from aqueous solution are shown to have resulted from the observation of facetting in the early stages of development of the nucleus or of damaged seeds. Once this facetting is completed on the faces propagating with a growth vector in the [001] direction [e.g., the ( 011), (01 h1), and contiguous {111} faces], the growth rates of these faces, and hence in the [001] direction, fall to zero or immeasurably low values. At the same time, the growth of faces presenting in the 〈hk0〉 and [001 h] directions continues normally. These close similarities in the development of crystals from both the vapor and the aqueous phases lead to the conclusion that the polar morphology of this and, potentially, of all similar acentric materials, is an intrinsic consequence of differences in normal growth (or lack of growth) of the faces at the opposite ends of the polar axis. It does not result primarily from solvent inhibition of growth, as is the current proposal.