Polytypical Polymorphs Occurring in an Energetic Material by Richard Gilardi, Naval Research Laborat

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Polytypical PolymorphsOccurring in an
Energetic Material by Richard Gilardi, Naval
Research Laboratory, Washington, DC 20375 USA.
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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.
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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 a form and g form (shown
above) are essentially identical in molecular
conformation, but their crystal packings and
densities are different.
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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.
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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.

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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.

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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.

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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 CHO2N hydrogen bonds and BrO2N
  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.

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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 CHON
H-bonds. Lattice translations extend the planar
triad into a planar (but bumpy) sheet, linked by
additional BrON induced-dipole, dipole
interactions. Different stackings of these
infinite sheets are the only appreciable
differences between the several polymorphs.
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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.

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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 NOC approaches are
observed that are less than van der Waals
contacts, indicating probable dipole-pi_cloud
interactions.
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The six polymorphs of PicrylBr
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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 torsions61.0, 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 torsions83.0, 60.1 88.9, 89.5 82.7, 67.4
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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)

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Picryl Bromide tri-molecular packing unit (from b
polymorph)
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Picryl bromide beta PolymorphDistribution of
distances on packing surfaces (fingerprints) of
each of the three molecules in the asymmetric
unit
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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)
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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.
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Fingerprint distance distributions for the two
triads comprising the asymmetric unit of a-Pbr
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Fingerprints for the two a-Pbr triads compared to
the b-Pbr triad
Triad a-abc Triad a-def
Triad b-abc
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The asymmetric unit of a-Pbr (the Crystal
Explorer shape function is displayed on the
surface of one triad)
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The asymmetric unit of a-Pbr (the C.E. shape
function is now displayed on surfaces of both
triads)
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The previous image rotated 180 about the
horizontal axis note that the trigonal pattern
is absent in the left triad
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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.
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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.
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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.
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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).
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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 (R0.092), it is considered
tentative, and may be due to some confusing or
interfering twin effect.
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2.028
1.979
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Packing in ONC - closest approaches in a
close-packed layer - all are O...O
D 1.979
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Packing in an ONC/nitrobenzene xtal
Triclinic, P1bar
Nitro manages to penetrate the ONC nitro shield
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Three analogs of HMX
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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 Es 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).
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HNFX conformations -gt predicted crystals