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“ Polytypical Polymorphs Occurring in an Energetic Material ” by Richard Gilardi, Naval Research Laboratory, Washington, DC 20375 USA. HMX Polymorphs. a -form. b -form. D = 1.90. D = 1.84.
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“Polytypical PolymorphsOccurring in an Energetic Material”by Richard Gilardi, Naval Research Laboratory, Washington, DC 20375 USA.
HMX Polymorphs a-form b-form D = 1.90 D = 1.84 In HMX, two polymorphs make sense - there are two distinct low-energy conformations, a with 2-fold symmetry and b with a center. But a difference is not necessary for polymorphism. The uncommon d form has the same shape as the a form.
CL-20 Polymorphs a g In CL20, the polyaza cage backbone is rigid, but the nitramine appendages are flexible – the amine nitrogen can convert from a planar to a pyramidal conforma-tion with little energy cost. However, the four polymorphs display only three conformations - the aform and g form (shown above) are essentially identical in molecular conformation, but their crystal packings and densities are different.
CL-20 Polymorphs b e Here are the two polymorphs which differ in conformation from the a form. The densities of the CL20 polymorphs cover quite a range. The least (1.916) and most (2.044) dense forms are the g and the e. Obviously, reliable polymorph preparation is essential for reproducible material properties.
Picryl Bromide - Background In recent years, Picryl Bromide has been re-investigated at NRL. This material dates back to the very early days of crystallography and energetic materials research, and is a useful chemical precursor to other energetic materials. Two polymorphs were reported in 1933, but no X-ray structure was then, or has since been reported, although Herbstein and Kaftory, in 1975, reported two co-crystals of Picryl Br with large aromatic ring compounds.
More Background At NRL, the two forms reported in 1933 - one triclinic (a) and one trigonal (b) - were found. First attempts at solution were unsuccessful. Because the asymmetric units seemed “too large”, it was thought that perhaps the crystals were twinned, and pseudo-cells were derived from a superposition of patterns. Thus, many other crystals were examined with X-rays (seeking for the true single unit cell!). Five, and possibly six, polymorphs were found, but none were simpler than the b polymorph, with Z’ = 3.
Picryl Bromide – Complex Polymorphism Once they were accepted (by us) as single crystals and not twinned crystals, the Picryl Bromide polymorphs could be analyzed (with care) by usual direct computer methods. Each analysis was somewhat slow because these crystals have heavy Br atoms, and pack with complicated schemes, involving the determination of large “asymmetric units” – clusters of 3 to 18 molecules. [Most other molecules, more than 99%, pack in crystals with two molecules or less in the asymmetric unit.]
Picryl Bromide – the Bumpy Sheet Motif All of our picryl bromide polymorphs are built from the same type of “bumpy planes”, seemingly held together by CH…O2N hydrogen bonds and Br…O2N dipole-induced-dipole close interactions. The planes are built from triads of PBr molecules that are virtually identical in all polymorphs. A detailed view of one sheet is shown in the next slide.
Formation of planar sheets from triads of Picryl Br In all PBr polymorphs, a basic triad unit is found to be parallel to the a and b axes of the cell. Three molecules are linked by weak CH…ON H-bonds. Lattice translations extend the planar triad into a planar (but bumpy) sheet, linked by additional Br…ON induced-dipole, dipole interactions. Different stackings of these infinite sheets are the only appreciable differences between the several polymorphs.
Picryl Bromide – Stacking of planes The bumpy sheets formed from triads of molecules stack roughly parallel to one another, but not directly above one another. They are offset in various ways in the polymorphs. In the simplest (beta) polymorph, the offset is such that each benzene ring is closely approached by a nitro group from an adjacent layer.
Interactions between sheets in beta [hexagonal, P6(5)] polymorph of Picryl Br Three molecules from adjacent sheets related by the six-fold screw axis. The out-of-plane nitro groups are situated directly above or beneath benzene rings. Several NO…C approaches are observed that are less than van der Waals contacts, indicating probable dipole-pi_cloud interactions.
Alpha form All of the PBr crystal forms have large asymmetric units, which means that many molecules must be determined, and that they may have different shapes. However, in PBr polymorphs the asymmetric units were made up of almost identical molecules, and these molecules are always arranged in the motif shown here – a triad, flat except for two twisted nitro groups. Nitro torsions:61.0, 49.4 84.0, 62.2 88.4, 68.5 (A basic triad – in this case, 1/2 of the asymmetric unit) Beta form (The whole asymmetric unit) Nitro torsions:83.0, 60.1 88.9, 89.5 82.7, 67.4
Packing Interactions in b-PBr (Using Crystal Explorer to generate Hirshfeld surfaces, as demonstrated at this conference in workshop II and the lecture by Spackman)
Picryl Bromide tri-molecular packing unit (from b polymorph)
Picryl bromide – beta PolymorphDistribution of distances on packing surfaces (fingerprints) of each of the three molecules in the asymmetric unit
Triad motifs occur in all polymorphs, with very slight nitro torsion differences Gamma form Delta form Only 1/4 of the asymmetric unit) Also, 1/4 of the asymmetric unit) Epsilonform Zeta form Only 1/14 of the 42 molecule asymmetric unit) Only 1/6 of the asymmetric unit)
The alpha polymorph – triclinic, P-1 The asymmetric unit of the crystal contains 6 molecules(one red, blue, green, yellow, pink, and turquoise in the above plot). The other half of the unit cell is related by a center of symmetry at the center of the unit cell. This is the smallest polymorph, in terms of molecules per cell (12), and also seems to be the most common in our experience with regrown batches.
Fingerprint distance distributions for the two triads comprising the asymmetric unit of a-Pbr
Fingerprints for the two a-Pbr triads compared to the b-Pbr triad Triad a-abc Triad a-def Triad b-abc
The asymmetric unit of a-Pbr (the Crystal Explorer ‘shape function’ is displayed on the surface of one triad)
The asymmetric unit of a-Pbr (the C.E. ‘shape function’ is now displayed on surfaces of both triads)
The previous image rotated 180° about the horizontal axis – note that the trigonal pattern is absent in the left triad
The beta polymorph of Picryl Br – hexagonal, P6(5) The asymmetric unit of the crystal contains 3 molecules (one blue, red, green in above plot), which is the simplest seen in the whole series of polymorphs. Successive triads of molecules, generated by 6-fold crystal symmetry, fill the cell with 18 molecules.
The gamma polymorph of Picryl Br – triclinic, P-1. The asymmetric unit of the crystal contains 12 molecules which are each represented by a different color in the above plot. This unit, made up of four triads, fills half of the unit cell. This cell, at first glance, appears to be an exact double of the cell of the alpha polymorph, but the inter-axial angles are distinctly different.
The delta [trigonal, P3(1)] polymorph of Picryl Br The asymmetric unit contains 12 molecules (each represented by a different color above), or four PBr triads, filling one-third of the unit cell. The 3-fold screw axis fills the rest of the cell.
The epsilon polymorph of Picryl Br – triclinic, P1 The asymmetric unit of the crystal contains 18 molecules – equal to the contents of the whole unit cell. The confluence of this space group, which has NO symmetry except lattice translations, with such a large asymmetric unit, is very unusual, but the refinement seems unequivocal (R = 0.028).
The zeta (sixth) polymorph of Picryl Br The asymmetric unit contains 42 molecules and is equal to the contents of the whole unit cell. Since a really good set of data does not exist for this polymorph (R=0.092), it is considered tentative, and may be due to some confusing or interfering twin effect.
2.028 1.979
Packing in ONC - closest approaches in a close-packed layer - all are O...O D = 1.979
Packing in an ONC/nitrobenzene xtal Triclinic, P1bar Nitro manages to penetrate the ONC ‘nitro shield’
HNFX Polymorphs An Orthorhombic Form of HNFX, Dens = 1.945, computed and optimized by Ammon.This form is isostructural with TNFX, a known hybrid NO2/NF2 molecule. Note: other computed polymorphs have Dens up to 2.045, lattice E’s as low as –43.4kcal/mol Trigonal Form, from X-ray analyses at NRL (Gilardi). Note two large holes in diagram of molecular packing, which are disordered (fluid) solvent channels. Dens = 1.807g/cc, with channels empty. Lattice Energy = -39.7 kcal/mol (computed, Ammon, 2002).
