Transcript ppt - Erice Crystallography 2004

“Polytypical Polymorphs
Occurring in an
Energetic Material”
by Richard Gilardi,
Naval Research Laboratory, Washington, DC
20375 USA.
HMX Polymorphs
a-form
b-form
D = 1.84
D = 1.90
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 conformation 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.
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 pseudocells were derived from a superposition of patterns.
Thus, many other crystals were examined with Xrays (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 dipolepi_cloud interactions.
The six polymorphs of PicrylBr
Acronym
axbxc
α x β x γ
Vol.
SG
Z Z' R1
P-1
HAPBR02Z, alpha
14.8480 x 14.8614 x 15.3318 72.762 x 80.350 x 60.225
2803.8
12
6 0.0397
HAPBR04M, beta
14.8812 x 14.8812 x 22.5449
4323.69 P6(5) 18
3 0.0343
90 x 90 x 120
HAPBR05T, gamma 14.9166 x 14.9424 x 30.3888 94.729 x 93.754 x 119.925 5805.61
HAPBR01M, delta
14.8610 x 14.8610 x 44.0070
90.0 x 90.0 x 120.0
P-1
24 12 0.037
8416.9 P3(1) 36 12 0.0387
HAPBR06T, epsilon 14.9086 x 14.9083 x 22.6784 95.245 x 90.116 x 119.896 4344.89
P1
18 18 0.0284
HAPBR03K, zeta
P1
42 42 0.0919
14.9340 x 14.9368 x 52.8433 94.056 x 90.751 x 119.993 10167.4
Alpha form
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
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.
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 Polymorph
Distribution 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
Delta form
form
Only 1/4 of the
asymmetric unit)
Epsilon
form
Also, 1/4 of the
asymmetric unit)
Zeta
form
Only 1/6 of the
asymmetric unit)
Only 1/14 of the
42 molecule
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’
Three analogs of HMX
TNFX
ADOL3
O 2N
O 2N N
O 2N
NO 2
HNFX
F 2 N NF 2
N NO 2
O 2N
N
N
F 2 N NF 2
NO 2
o rth o rh o m b ic
D = 1 .8 6 4
O 2N N
N
N
NO 2
NO 2
O 2 N NO 2
(H M X )
O 2N
NO 2
N
N NO 2
N
NO 2
o rth o rh o m b ic,
D = 1 .9 0 4 ,
a n d trig o n a l,
D = 1.7 1 2 , fo rm s
F 2 N NF 2
T rig on a l,
D = 1 .7 8 4 ,
is o n ly
fo rm fo u n d .
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).
HNFX conformations -> predicted crystals
Xray structure
Ci symmetry
r = 1.807 g/cc
Lattice energy =
-39.7 kcal/mol
Trigonal, R-3
r = 2.05, LE = -43.2
Monoclinic, P21/c
General posn -> r = 1.96 g/cc; LE = -41.0
Orthorhombic, P212121
On center ->
Ci model
C1 model
No symmetry
r = 1.92, LE = -37.7
Monoclinic, P21/c