High-pressure studies on fluorine substituted 2,5-di(phenyl)-1,3,4- oxadiazoles

Article abstract of DOI:10.1016/j.molstruc.2005.03.042

Results are presented from structural and high-pressure investigations on four differently but symmetrically fluorine substituted 2,5-di(phenyl)-1,3,4- oxadiazoles. The substitution pattern includes the para-, meta-, or ortho-substitution and the fully fluorinated 2,5-bis(pentafluorophenyl)-1,3,4- oxadiazole. The crystal structure depends on the molecular structure and results in a different high-pressure behavior. Parameters for the Murnaghan equation of state (EOS) are determined for every compound and the anisotropic pressure response of the crystal lattice is discussed. Although the EOS parameters, bulk modulus K_o and its pressure derivative Ko′ are of the same order of magnitude for all four compounds, the anisotropy of strain is noticeably different.

Full text of DOI:10.1016/j.molstruc.2005.03.042

Journal of Molecular Structure 749 (2005) 144–154
www.elsevier.com/locate/molstruc
High-pressure studies on fluorine substituted
2,5-di(phenyl)-1,3,4-oxadiazoles
a,
b,1
Ingo Orgzall , Olga Franco , Gu¨nter Reck , Burkhard Schulz
c
b
*
aInstitut fu¨r Du¨nnschichttechnologie und Mikrosensorik e. V. Kantstraße 55, D-14513 Teltow, Germany
b
¨ ¨
Universitat Potsdam, Institut fur Physik, Am Neuen Palais 10, D-14469 Potsdam, Germany
¨ ¨ ¨
Bundesanstalt fur Materialforschung und Prufung (BAM), Richard-Willstatter-Straße 11, D-12489 Berlin, Germany
c
Received 6 January 2005; revised 10 March 2005; accepted 11 March 2005
Available online 31 May 2005
Abstract
Results are presented from structural and high-pressure investigations on four differently but symmetrically fluorine substituted 2,5-
di(phenyl)-1,3,4-oxadiazoles. The substitution pattern includes the para-, meta-, or ortho-substitution and the fully fluorinated 2,5-
bis(pentafluorophenyl)-1,3,4-oxadiazole. The crystal structure depends on the molecular structure and results in a different high-pressure
behavior. Parameters for the Murnaghan equation of state (EOS) are determined for every compound and the anisotropic pressure response of
the crystal lattice is discussed. Although the EOS parameters, bulk modulus Ko and its pressure derivative Ko0 are of the same order of
magnitude for all four compounds, the anisotropy of strain is noticeably different.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Oxadiazole; Molecular crystal; Crystal structure; High pressure; Equation of state
1. Introduction—high pressure as a tool
mentioned here that high-pressure investigations like the
determination of an equation of state (EOS) or the study of
possible phase transitions are particularly important for the
pharmaceutical industry [7,9,10]. Besides volume changes,
phase transitions may also occur during pressure application
possibly leading to a different polymorph that exhibits new
properties.
High-pressure investigations belong to indispensable
techniques in many fields of basic as well as applied
sciences, ranging from solid state physics, chemistry,
geophysics and geology up to materials science and even
biochemical and biological problems [1–4].
Principally, most organic crystals show high compressi-
bility and strong anisotropy. Observing the high-pressure
behavior of organic molecular crystals the question
emerges, whether there exist some general rules rather
independent on the individual molecular structure or if this
individual structure and therefore the specific intermolecu-
lar interactions determines the pressure response of the
solid. Such influences result for instance from the structure
of the aromatic system, i.e. from the number of rings and
their mutual arrangement. This determines the conformation
of the molecule, its planarity and thus a possible
conjugation. Further points that deserve some attention are
differences in polarity (dipole moment) or shape anisotropy.
Distinctions in the supramolecular arrangement like the
occurrence of stacks or layers in the crystal structure may
also influence the high-pressure behavior. Additionally,
specific molecular structures or functional groups could
cause steric influences and hindrances or could favor
Although high-pressure studies provide a unique insight
into the complex relations between structure and properties
due to the tuning of the intermolecular interactions by
variation of the distances between the individual building
units, the application in the field of organic solids is still
emerging. Some reviews summarizing the developments
and achievements in this field are found in [4–8]. Such
studies have even a practical impact and it is only shortly
*
Corresponding author. Tel.: C49 3328 3346 0; fax: C49 3328 3346 10.
E-mail addresses: orgzall@rz.uni-potsdam.de (I. Orgzall), franco@
rz.uni-potsdam.de (O. Franco), guenter.reck@bam.de (G. Reck), buschu@
rz.uni-potsdam.de (B. Schulz).
1
´ ´
Present address: Departamento de Quımica Fısica I, Facultad de
´
Ciencias Quımicas, Universidad Complutense, Ciudad Universitaria,
E-28040 Madrid, Spain.
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.03.042

I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
145
the formation of hydrogen bonds. The anisotropic com-
pression due to the molecular arrangement provides insights
into the action of the different forces constituting the organic
molecular crystal as for instance p–p, van der Waals, or
hydrogen bond interactions.
Many investigations always include a group of similar or
related compounds that differ only in their substitutional
scheme to deduce some generalities and trends and to relate
them to the specific chemical structure (see, for instance
[11–14] for studies on biphenyl, oligophenyls, or para-
hexaphenyl, respectively, and [15–17] for anthracene and
disubstituted anthracene derivatives or [18] for further
compounds). Investigating the crystal structures of some
drugs under compression up to 4 GPa either by single
crystal or by powder diffraction techniques usually high
compressibilities resulted. Especially these studies discuss
the strong anisotropic lattice response to pressure appli-
cation, i.e. the lattice strain in relation to the molecular
conformation, the crystal packing, the corresponding
intermolecular interactions, or specific intermolecular
bonds [6–8,19].
In the following, a comprehensive experimental study
will be outlined summarizing results of structural studies
and investigations of the compression behavior of differ-
ently substituted 2,5-di(phenyl)-1,3,4-oxadiazoles with
fluorine as substituent. The aim is to contribute to the
question whether there exist some similarities or general
trends within this class of compounds with changing
substitution scheme (ortho-, meta-, para-, or perfluorinated
compounds) or even in comparison with other organic
molecules based on similar molecular or supramolecular
structures and specifically with other di(phenyl)-oxadia-
zoles [20–22]. The results could also be compared to high-
pressure studies of different halogenated molecules like
fluorinated or chlorinated phenols [23] or chlorinated ethane
[24]. Although the molecular structures are somewhat
different compared to the compounds investigated here the
pressure effects are quite similar as the compression
behavior is considerably determined by the different
intermolecular interactions involving the halogen atoms
like hydrogen bonds or halogen–halogen interactions.
Specific attention is paid to the structural modification
introduced by the changing substituent’s position and the
resulting consequences for the bulk high-pressure behavior
or the compression anisotropy.
Scheme 1.
(84.6% P2O5) at 180 8C for 2 h. After cooling, the clear
solutions were poured into water, filtered and the products
were dried in vacuum. The products were three times
recrystallized from ethanol or hexane. Melting points: para-
substitution, 2,5-bis-(4-fluorophenyl)-1,3,4-oxadiazole
(OXA 1): 201 8C (lit. 213 8C [25]) (for the molecular
structure cf. Scheme 1), meta-substitution, 2,5-bis-(3-
fluorophenyl)-1,3,4-oxadiazole (OXA 2): 134 8C, ortho-
substitution, 2,5-bis-(2-fluorophenyl)-1,3,4-oxadiazole
(OXA 3): 116 8C (commercially available, Lancaster,
112–114 8C), 2,5-bis-(pentafluorophenyl)-1,3,4-oxadiazole
(OXA 4): 151–155 8C (156–158 8C [26,27]).
2.2. Crystal structure determination and results
2. Experimental setup
The crystal structures have been determined by single
crystal structure analysis. The X-ray data were collected on
a Bruker AXS SMART/CCD diffractometer. The structures
were solved by direct methods and refined by full-matrix
least-squares calculations using SHELX-97 [28]. For OXA 1
and OXA 3 all hydrogen atoms were located by difference
Fourier synthesis. The crystals of OXA 2 were twinned.
This was taken into account in the structure refinement
using the twin matrix (1 0 0/0 1 0/0 0 K1). Hydrogen atom
2.1. Sample preparation
The investigated fluorine containing 2,5-di(phenyl)-
1,3,4-oxadiazole compounds were synthesized by direct
condensation of fluorobenzoic acid with hydrazine hydrate
in polyphosphoric acid. The fluorobenzoic acid and
hydrazine hydrate were stirred in polyphosphoric acid

146
I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
positions of OXA 2 were calculated corresponding to their
geometrical conditions and refined using the riding model.
The molecule of OXA 3 contains a two-fold axis coinciding
with that of the space group C2/c. Therefore, only the
atomic coordinates of half of the molecule must be
determined. The crystallographic data have been deposited
at the Cambridge Crystallographic Data Center as sup-
plementary publication Nos. CCDC 255251 for OXA 1,
CCDC 255252 for OXA 2, CCDC 255253 for OXA 3, and
CCDC 255254 for OXA 4. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.
error also depends on the quality of the experimental energy
dispersive pattern and slightly increases with pressure.
This procedure delivers information only about the
variation of the lattice parameters but not on the crystal
symmetry or the individual atomic positions. A deeper
analysis to determine the appropriate molecular confor-
mation is impossible due to the low resolution of the
diffractograms and the disturbing influences from the
gasketing cube. It is supposed that the crystal symmetry
remains always that of the ambient pressure structure.
Nevertheless, the resemblance of the diffraction patterns
under pressure to that pattern under ambient conditions also
in its relative intensities justifies this approach. It was
proven that the diffraction pattern is sensitive to larger
orientational changes and rotations of the individual
molecules as well as to variations of the mutual positions
what would change the whole pattern. However, some slight
changes of both, molecular conformation and mutual
position of the individual molecules cannot fully be ruled
out. In general the main structural elements like stack or
layer arrangement of the di(phenyl)-1,3,4-oxadiazole com-
pounds should be maintained also under pressure. The
evaluation procedure therefore also involves that the
conformation of the molecule remains unaltered. This
approximation describes well the behavior of aromatic
compounds with a partially delocalized p system like the
oxadiazoles. This approach is reasonable since it is usually
expected that the compression of the intramolecular bond
lengths is much smaller compared to that of the inter-
molecular distances.
2.3. High-pressure investigations
All structure investigations under high pressure were
carried out in the multi-anvil press MAX-80 [29] using
synchrotron radiation at HASYLAB, DESY Hamburg. The
sample container, an epoxy cube, serving as pressure
transmitting medium, is filled with the ground sample and
NaCl as pressure standard in separate layers and sealed by a
pyrophyllite disk.
The diffractograms are recorded by an energy dispersive
detector with multi-channel analyzer. Some disadvantages
have to be considered: texture effects due to the powder
technique, a generally lower resolution compared to angle
resolved methods, the occurrence of additional diffraction
lines due to the detector material (escape peaks) or to the
surrounding epoxy material [30]. This was taken into
account during the evaluation of the diffractograms.
The pressure is determined in situ applying the well-
known equation of state for NaCl, i.e. the Decker equation
[31]. The corrections to that scale introduced by the
work of Brown [32] remain small in the investigated
pressure region. Two diffractograms are recorded at every
pressure step, one of the sample and one of NaCl
for pressure determination. Slight gradients of around
0.1 GPa within the sample due to non-hydrostatic effects
cannot be ruled out due to the comparatively large volume
of the pressure cell.
3. Results and discussion
3.1. Crystal structure
Table 1 summarizes the crystallographic data for all
investigated compounds OXA1 - OXA4. All structures
belong to the monoclinic system but show different packing
motifs for the individual compounds.
For the subsequent evaluation of the diffractograms and
the determination of the lattice parameters the program
Powder Cell 2.3 [33] is applied. The experimental patterns
were fitted using the structure data for the ambient pressure
structure that resulted from single crystal investigations as
starting point. The special recourse procedure offered by the
program enables this evaluation independently on texture
influences upon the intensity of the peaks. Here, the
intensity of every peak is treated separately. The shape of
the crystals always introduced texture effects although all
samples were carefully ground. Sometimes an increasing
texture was observed under pressure due to the alignment of
the small crystallites. The relative error of the fit procedure
to determine the lattice parameters using Powder Cell is
estimated to 0.003 for the axis lengths, slightly larger for the
monoclinic angle (for clarity error bars are not given for
every pressure step in the figures). Of course, the resulting
Fig. 1 shows the ORTEP plots of the molecules of OXA
1, OXA 2, OXA 3, and OXA 4. The mean N–C distance of
˚
1.287(8) A of all compounds indicates enlarged double
bond character between these atoms in the oxadiazole ring.
˚
The mean O–C and N–N bonds are 1.364(7) and 1.411(9) A,
respectively, indicating single bonds between the corre-
sponding atoms. In OXA 1, OXA 2, and OXA 3, only one
fluorine atom is bound to each phenyl ring. The mean C–F
˚
bond amounts to 1.362(9) A. In OXA 4, all hydrogen atoms
are substituted by fluorine atoms. In this compound the C–F
distances are significantly shorter than the C–F bonds in
OXA 1, OXA 2, and OXA 3. In OXA 4 the mean C–F bond
˚
amounts to 1.338(5) A. Remarkably, in the case of ortho-
and meta-substitution the bond between phenyl ring and
fluorine substituent points into the direction of the oxygen
atom contrary to symmetric meta- and ortho-amino

I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
147
Table 1
Crystallographic data and structure determination for the investigated compounds OXA1–OXA4 at ambient pressure
OXA 1
OXA 2
OXA 3
OXA 4
Formula
C14H8F2N2O
258.22
C14H8F2N2O
258.22
C14H8F2N2O
258.22
C14F10N2O
402.16
Formula weight
Temperature (K)
293(2)
293(2)
293(2)
293(2)
˚
Wavelength (A)
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
C2/c
0.71073
Monoclinic
P21/n
Crystal system
Space group
˚
˚
˚
˚
Unit cell dimensions
aZ10.44(2) A
aZ3.98(2) A
aZ13.864(6) A
aZ12.32(2) A
˚
˚
˚
˚
bZ11.44(2) A
bZ24.24(7) A
cZ12.44 (4) A
bZ12.397(5) A
bZ4.701(6) A
˚
˚
˚
˚
cZ10.75(2) A
cZ7.079(3) A
cZ23.65(4) A
bZ116.31(9)8
1151(4)
bZ90.12(6)8
1199(7)
bZ110.284(8)8
1141.3(8)
bZ104.96(3)8
1324(3)
3
˚
Volume (A )
Z, calculated density (g cmK3
Absorption coefficient (mmK1
)
4, 1.490
4, 1.430
4, 1.503
4, 2.018
)
0.118
0.114
0.119
0.226
F(000)
528
528
528
784
Crystal size (mm3)
Range for data collection (8)
Limiting indices
0.24!0.16!0.03
2.26–23.0
K15%h%12
K15%k%14
K12%l%15
1602
1.00!0.06, 0.04
1.64–23.0
K5!h!5
K28!k!33
K15!l!12
1674
0.65!0.22!0.18
2.27–25.0
K17!h!19
K14!k!15
K9!l!9
1005
0.30!0.04!0.03
1.71–30.0
K15!h!17
K6!k!6
K33!l!23
3571
Reflection unique
Refinement method
Full-matrix least-squares
on F2
Full-matrix least-squares
on F2
Full-matrix least-squares
on F2
Full-matrix least-squares
on F2
Goodness-of-fit on F2
Final R indices
1.048
1.101
1.018
0.608
R1Z0.076
R2Z0.177
R1Z0.081
R2Z0.177
R1Z0.039
R2Z0.113
R1Z0.038
R2Z0.096
˚
substitution or ortho-nitro substitution, where the bonds
between these groups and the phenyl ring always point into
the direction of the nitrogen atoms [34].
expected to be smaller than 0.006 A and that of the angle a
smaller than 0.28. Table 3 lists these interaction parameters
for OXA 1. These p–p electron interactions between phenyl
and oxadiazole rings of adjacent molecules give rise to the
formation of molecular layers in the crystal structure of
OXA 1 parallel to the a,c-plane as depicted in Figs. 2(a)
and (b). Additionally, the molecules within a layer are held
together by two symmetry independent intermolecular
C–H/F hydrogen bonds, namely C10–H/F19i (i: 1.5C
The conformation of the molecules can be described by
the dihedral angles between the central oxadiazole and the
phenyl rings. The angles between the planes as marked in
Fig. 1 are given in Table 2. The C–C distances between the
˚
oxadiazole and phenyl rings range from 1.451 to 1.474 A
(Table 2). However, no correlations between bond lengths
and deviations from coplanarity were recognized. It is
interesting to note, that OXA 3 with its ortho-substitution
shows the largest deviations from planarity (torsion angle
28.98). These experimental values may be compared to
theoretical Cerius2 calculations of the single molecules
given in [35] for OXA 3 and OXA 4. According to these
calculations OXA 4 should have the largest torsion angle
between oxadiazole and phenyl rings (32.58) compared to
roughly 148 found here, while the OXA 3 molecule should
be completely planar (08).
˚
˚
x, K1.5Ky, 0.5Cz; C–FZ3.32 A, H–FZ2.38 A, C–H–FZ
142.68) and C16–H/F18ii (ii: K1.5Cx, K1.5Ky, K0.5C
˚
˚
z; C–FZ3.44 A, H–FZ2.44 A, C–H–FZ154.88). These
and the symmetry related hydrogen bonds form molecular
chains within the layers (see Fig. 2(b)).
The numerical values describing the p–p electron
interactions for OXA 2 are found in Table 4. These and
the symmetry related p–p electron interactions connect the
molecules of OXA 2 forming stacks extended in a-direction
(see Figs. 3(a) and (b)).
In OXA 3 p–p or dipole–dipole electron interactions
appear only between the central oxadiazole rings. These
interactions are the same for all four interactions between
Cg1 and Cg1 at (Kx, Ky, Kz), (Kx, Ky, 1Kz), (x, Ky,
K1/2Cz), and (x, Ky, 1/2Cz), respectively. The corre-
In the compounds OXA 1, OXA 2, and OXA 3 p–p
electron interactions develop between symmetry related
molecules. These interactions are defined by the distance
between the ring centroids (DC), the perpendicular
distances of the centroids of one ring to the plane of the
other (DP) and the intermolecular angle (a). The corre-
sponding rings are marked in Fig. 1. The interactions are
evaluated using the Platon program [36]. Since this program
does not give standard deviations of the parameters, these
have to be estimated. The maximum error of the distances is
˚
˚
sponding distances are DCZ3.617 A, DPZ3.482 A, DPZ
˚
3.482 A, and aZ08. These interactions give rise to the
formation of stacks extended in c-direction. The stacks and
the corresponding molecular packing are shown in Figs. 4(a)
and (b), respectively.

148
I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
Table 3
Parameters for p–p interactions in OXA 1
˚
DC (A) DP (A)
˚
˚
DP (A)
Interactions between
a (8)
Cg1 and Cg2 at
3.651
3.566
3.651
3.566
4.352
3.537
3.438
3.565
3.517
3.565
3.517
3.537
3.438
3.50
(K1/2Cx, 1/2Ky, K1/2Cz)
Cg1 and Cg3 at
7.63
3.50
7.63
(1/2Cx, 1/2Ky, 1/2Cz)
Cg2 and Cg1 at
(1/2Cx, 1/2Ky, 1/2Cz)
Cg3 and Cg1 at
(K1/2Cx, 1/2Ky, K1/2Cz)
Cg2 and Cg3 at (1Cx, y, z)
3.690
3.488
3.488
3.690
6.15
6.15
Cg3 and Cg2 at (K1Cx, y, z) 4.352
prevented by many short repulsive interactions between
fluorine atoms of adjacent molecules. These range from
˚
2.784 to 2.927 A. The molecular packing of OXA 4 is
shown in Fig. 5.
3.2. Bulk behavior—equation of state
The thermodynamic state of a solid is defined by pressure
p, temperature T, and volume V and described by an
appropriate equation of state (see for instance [37,38]). For
the interpretation of experimental results for complex
substances several semiempirical equations have been
developed. These equations have been proven to be more
successful for this task compared to equations derived from
first principles. The most convenient EOS are such
equations that contain parameters that immediately may
be obtained from the experiments. For instance the
isothermal Murnaghan EOS is usually applied for the
Fig. 1. Molecular structures of OXA 1, OXA 2, OXA 3, and OXA 4,
showing 50% probability displacement ellipsoids.
Surprisingly, there are no p–p electron interactions in
the crystal structure of OXA 4 because the shortest Cg–Cg
˚
distances are larger than 4.7 A. There is no overlap between
parallel and likewise oriented molecules. Probably, this is
Table 2
Dihedral angles (first line) and bond distances (second line) between the
different aromatic rings (cf. Fig. 1).
Cg1–Cg2
Cg1–Cg3
Cg2–Cg3
OXA 1
OXA 2
OXA 3
OXA 4
12.98
1.451 A
16.18
1.451 A
6.28
–
˚
˚
6.08
1.474 A
3.88
1.467 A
8.28
–
˚
˚
28.98
1.458 A
28.98
1.458 A
54.18
–
Fig. 2. (a) Stack interactions in the crystal structure of OXA 1 given by the
distances between the ring centroids. Additionally the hydrogen bonds are
indicated in the molecular layers. (b) Molecular packing in the crystal
structure of OXA 1.
˚
˚
14.18
1.465 A
14.28
1.462 A
8.88
–
˚
˚

I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
149
Table 4
This is used as a quality measure for the whole fit procedure
since it can be compared to the value determined by single
crystal analysis.
p–p interactions in OXA 2
˚
DC (A)
˚
DP (A)
˚
DP (A)
Interactions
between
a (8)
Figs. 6(a) and (b) and Table 5 present the results for the
investigated oxadiazole compounds. Some previous results
for different aromatic compounds from literature [18,20,21]
are supplementary added. From the volume data, it is
difficult to decide how the individual compression behavior
is influenced by the molecular and supramolecular structure.
It is obvious that it is rather similar for these substances
despite of their individual molecular and crystal structures.
The substitution of fluorine in para- and ortho-positions of
the phenyl ring (OXA 1 and OXA 3) seems to have no
significant influence on the compression.
Cg1 and Cg2 at
(1Cx, y, z)
Cg2 and Cg1 at
(K1Cx, y, z)
Cg1 and Cg1 at
(1Cx, y, z)
4.083
4.083
3.976
3.976
3.976
3.438
3.534
3.468
3.509
3.560
3.534
3.438
3.468
3.509
3.560
4.08
4.08
0.0
Cg2 and Cg2 at
(1Cx, y, z)
0.0
Cg3 and Cg3 at
(1Cx, y, z)
0.0
interpretation of experimental data:
For the meta- and per-fluorinated compounds some
deviations occur at increased pressures. In the medium
pressure region around 2 GPa a new diffraction line
originates and variations of other features are observed in
the diffractograms (cf. Fig. 7) leading to the assumption of a
structural phase transition. Other severe indications for a
transition are derived from larger deviations between the fit
of the experimental pattern and the calculated one for OXA
2. Some fitted reflections deviate remarkably from the
experimental pattern resulting in a decreasing fit quality.
Therefore it has to be assumed, that the initial structure does
no longer correctly describe the actual crystal lattice of
OXA 2 under pressure and a structural phase transition
occurs in the pressure region around 2 GPa. The interpret-
ation of the diffraction data with the old structure is
misleading after a phase transition. These changes are fully
reversible upon pressure release, the initial structure is fully
recovered.
"
#
Ko0
ꢀ
ꢁ
Ko
Ko0
Vo
V
p Z
K1
(1)
The index ‘o’ refers to the initial state at zero pressure. Ko is
the bulk modulus and Ko0 its pressure derivative, i.e.
ꢀ
ꢁ
dp
Ko ZK
(2)
dln V
pZ0
and
ꢀ
ꢁ
dk
Ko0 Z
(3)
dp
pZ0
The Murnaghan EOS delivers results for bulk modulus
Ko and its pressure derivative Ko0 with reasonable precision
[37]. Measurements in a relatively small pressure range are
thus sufficient for the extrapolation of the EOS to higher
pressure regions [39] and the equation yields correct moduli
for small compressions [40]. A fit to this equation also
delivers a value for Vo, the volume at ambient pressure.
Unfortunately, the quality of the energy dispersive
powder diffractograms is not sufficient to perform a
structure determination for the new phase. Since no
Fig. 3. (a) Stack interactions in the crystal structure of OXA 2 given by the distances between the ring centroids. (b) Molecular packing in the crystal structure
of OXA 2.

150
I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
mentioned above, the motions may only be small so that
they do not mark sufficiently in the diffractograms. In
addition, in this case additional experiments have to be
carried out in the future. The rather large Ko0 value not
observed for the other oxadiazole compounds could also be
an indication for a phase transition.
In general, besides small differences all fluorine
compounds investigated here show a rather consistent
high-pressure behavior, for those showing a phase transition
at least in the pressure region below this transformation.
Independent on their structure these compounds do not
differ strongly from each other in their V–p relationship.
This also confirms observations of other molecular crystals
as for instance of paracetamol [7], were two polymorphs
with substantial structural differences have the same bulk
compressibility. Only the lattice strain shows characteristic
differences due to the different structural arrangement. The
EOS parameters are in the range that is typical for organic
molecular compounds. These characterize the materials as
rather soft and well compressible (cf., for instance [5,8]).
This bulk behavior is also observed for such different
aromatic compounds like halogenated benzene, benzophe-
none [18], paracetamol, or phenacetine [6,7,19] as well as
for different oligophenylenes [12–14] or other investigated
di(phenyl)-1,3,4-oxadiazole crystals [20–22]. Nevertheless,
the molecular as well as the supramolecular structure, i.e.
the structure of the aromatic system and the packing motif
and therefore the different p–p and van der Waals
interactions between the individual molecules have a
small but distinct influence on the compressional behavior.
While the consideration of the bulk properties delivers
important elastic parameters describing the pressure
response of the solid the description of the lattice distortion
in terms of the volume alone is just insufficient to discuss the
anisotropy of the compression and therefore the different
intermolecular interactions in the crystal. To get some more
insight the behavior of the individual unit cell parameters
has to be discussed together with the structural features of
the compound.
Fig. 4. (a) Stack interactions in the crystal structure of OXA 3. (b)
Molecular packing in OXA 3.
assumptions about the possible high-pressure structure
could be made an evaluation of the diffractograms is
impossible up to now for pressures above 2 GPa. Additional
molecular modeling considerations would be necessary to
obtain more insight into the possible structural changes and
mechanisms.
3.3. Lattice parameters at high pressures
In the pressure range below 2 GPa the behavior of the
initial OXA 2 phase is quite similar to the other fluorine
compounds, the EOS parameters are in the same range as for
the other compounds. It has to be stated that the fit quality
for this compound is generally lower than for the other three
samples. Slightly larger errors occur as seen from the
volume Vo as a measure of the fit quality, showing only fair
agreement with the value determined from single crystal X-
ray investigations.
The compression behavior of one polymorphic form of
the unsubstituted 2,5-di(phenyl)-1,3,4-oxadiazole crystal
(DPO) was described in [20]. Molecular stacks are formed
in a-direction and the p–p interactions along these stacks
prevent a large compression in this direction. Due to weak
van der Waals interactions between adjacent stacks, both
other axes show a larger compressibility. This is in
accordance with observations on paracetamol and phena-
cetine [6,7,19] where it was concluded that the greatest
compression occurs in such directions where the molecules
are only linked by van der Waals forces. Molecular stacks
give rise to a pronounced anisotropy of structural strain as
observed for instance, in p-benzoquinone [6]. The com-
pression behavior in directions with hydrogen bonds
Also the per-fluorine compound OXA 4 deviates at
pressures above 2.0 GPa from the general trend. Again it
cannot be ruled out that a phase transition occurs at higher
pressures above 2.5 GPa to prevent very close fluorine–
fluorine contacts. Then the molecules could be slightly
rearranged to avoid too close contacts. However, as

I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
151
Fig. 5. Molecular packing in OXA 4.
depends on the specific molecular arrangement and there-
fore also on iteractions with the other forces.
between oxadiazole (acceptor) and phenyl (donor) rings of
adjacent molecules (cf. Fig. 2(a)). Additionally, the two
intermolecular hydrogen bonds between C–H and fluorine
in the a,b-plane contribute to the low compressibility in
these directions and therefore also stabilize the structure.
Fig. 8(b) presents results for the ortho-compound OXA
3. The OXA 3 molecules show a larger deviation from
planarity compared to many other investigated di(phenyl)-
1,3,4-oxadiazoles [20–22,34,41]. They show a larger
torsion of the phenyl ring around the interring bond in
opposite directions for both rings (approximatelyG298).
Both phenyl-fluorine bonds point to the oxygen atom of the
The crystal structure changes remarkably if the terminal
hydrogen atom of DPO is substituted by a fluorine atom
(OXA 1). Here, a layered arrangement is observed in the
a,c-plane together with hydrogen bonds in that layer. The
compression of the unit cell is illustrated in Fig. 8(a). Only
one axis—the c-axis—is well compressible (approximately
15% up to 5 GPa), the remaining two axes are less
compressible. They diminish in the pressure range up
to 5 GPa only by 6–7%. The small compressibility of the
a-axis is explained by the strength of the p–p interactions
(a)
(b)
1.00
1.00
0.95
0.90
0.85
0.80
0.95
0.90
0.85
0.80
(a)
(b)
0.0
0.75
0
1
2
3
4
5
0.5
1.0
1.5
2.0
2.5
3.0
Pressure [GPa]
Pressure [GPa]
Fig. 6. Equation of state for (a): OXA 1 [B] and OXA 3 [,] and (b): OXA 2 [B] and OXA 4 [,]. Full symbols represent data after pressure release.
Reference data from 2,5-di(phenyl)-1,3,4-oxadiazole (dotted line) [20], and diphenyl (dashed line) [18] are added for comparison to (a) and data for 2,5-bis(4-
diaminophenyl)-1,3,4-oxadiazole (dotted line) [21] and terphenyl (dashed line) [18] to (b).

152
I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
Table 5
The axes of OXA 2 exhibit a rather analogous
compressional behavior below 2 GPa as Fig. 8(c)
summarizes. The compressibility of the a-axis is slightly
higher compared to that of the b- and c-axes. In general,
the compressibility is in the range of the less compres-
sible axes of the other investigated compounds. OXA 2
is characterized by stacks in a-direction. Again, the
molecular planes are inclined against the stack axis,
therefore the p–p interactions are oriented on a diagonal
of the a,b-plane, explaining the nearly identical behavior
of both axes.
EOS parameters for OXA1–OXA4 together with some values for other 2,5-
di(phenyl)-1,3,4 oxadiazoles and other aromatic compounds for compari-
son from literature
Ko0
Compound
Vo (structure
Vo (EOS—
˚
Eq. (1)) (A )
Ko (GPa)
3
determi-
3
˚
nation) (A )
OXA 1
1151.259
1199.276
1141.316
1323.496
1148.307
2457.033
1156.2G2.7 6.9G0.8 6.2G0.3
1182.3G3.0 6.6G2.3 6.3G1.2
1143.6G2.7 5.1G0.6 9.1G0.4
1323.3G2.3 5.2G0.6 11.2G0.5
OXA 2
OXA 3
OXA 4
DPO [20]
DAPO [21]
Diphenyl [18]#
Terphenyl [18]#
1147.8
2460.3
7.3
5.6
5.1
5.8
6.7
8.2
8.1
8.4
Results for the perfluorinated compound OXA 4 below
2 GPa are given in Fig. 8(d). Interestingly, with
approximately 148 the torsion angles between phenyl
and oxadiazole rings are in the same range as those of
OXA 1 (cf. Table 2). Both phenyl rings are rotated in the
same direction out of the molecular plane. The structure is
characterized by the arrangement of parallel molecules in
b-direction but due to the large intermolecular distance no
overlap between the p systems appears and therefore p–p
interactions are missing. However, a network of fluorine–
fluorine interactions evolves preferentially in the a,c-plane
between neighbored molecules with usual distances
#: Values calculated from pressure-volume data. (DPO—2,5-di(phenyl)-
1,3,4-oxadiazole, monoclinic structure, space group P21/c; DAPO—2,5-
bis(4-aminophenyl)-1,3,4-oxadiazole, orthorhombic structure, space group
Pbca)
oxadiazole ring but due to the ring torsion they are above
and below the molecular plane. The molecules are arranged
in layers in the a,b-plane. Molecular stacks with opposed
molecules (given by the direction of the connection between
the center of the oxadiazole ring and the oxygen atom) are
arranged in c-direction so that the overlap of the p-systems
is rather weak and only given by the oxadiazole ring
(Fig. 4(a)). This is the reason for the large compressibility of
the axis perpendicular to the molecular planes where also a
possible planarization of the molecules could deliver
additional contributions. In a-direction, stronger inter-
actions of the fluorine atoms with carbon atoms of adjacent
phenyl rings and therefore the formation of hydrogen bonds
contribute to the very low compression in the range up to
5 GPa. The weak p–p interactions due to the overlap of
only the oxadizole rings and the less important components
of the fluorine interactions in b-direction allows a good
compressibility of this unit cell axis.
˚
around 2.79–2.93 A. Compared to the other investigated
compounds the axes show a decreased compressibility.
The low compressibility of the a-axis is attributed to the
fluorine–fluorine interactions. The compressibility of the
b- and c-axes of approximately 5% up to a pressure of
2 GPa corresponds to intermediate values compared to the
other compounds. This has also to be ascribed to the
fluorine interactions since the intermolecular p-inter-
actions are not as strong as observed in the perfect
stack arrangement of other oxadiazole compounds, for
instance in DPO [20].
The anisotropy of the compression in the different
compounds is reflected by the anisotropy of strain. The
corresponding strain tensors may be obtained from the
measured changes of the cell parameters under pressure to
obtain additional information on the most and least
compressible directions in the structures. Therefore,
Table 6 is included with selected values of the cell
parameters as function of pressure for all four oxadiazole
compounds, so that the strain tensors may be calculated, for
instance using the program Strain [42].
0.1 GPa
after pressure release
2.4 GPa
2.0 GPa
1.6 GPa
1.0 GPa
All compounds under investigation were also studied
after full pressure release still within the high-pressure
setup. In every case, a full reversibility of the
compression was found. Neither enduring modifications
nor irreversible changes in the structure could be
detected. The determined unit cell parameters were well
in the range of the initial values taking into account
reasonable error limits as for instance slight stress in the
sample and epoxy cube that were accumulated during the
pressure run and vanished completely only after dis-
mounting the sample off the press.
0.4 GPa
0 GPa
* *
*
*
+ cube
powder pattern
5
10
15
20
25
30 3
5
2Θ Cu-Kα [˚]
Fig. 7. X-ray diffractograms of OXA 2 under increasing pressure.
Indications for a structural transition are marked by arrows, the asterisks
denote escape peaks from the detector. Small influences of the gasketing
cube may contribute to line broadening in the range 16–228 (2qCu Ka).

I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
153
1.04
1.00
0.96
0.92
1.00
0.96
0.92
0.88
0.84
(b)
0.88
(a)
0
1
2
3
4
5
0
1
2
3
4
5
Pressure [GPa]
Pressure[GPa]
1.00
0.98
0.96
0.94
1.00
0.96
0.92
(c)
0.0
(d)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.5
1.0
1.5
2.0
2.5
Pressure[GPa]
Pressure[GPa]
Fig. 8. Anisotropic pressure response of the crystal lattice for OXA 1 (a), OXA 3 (b), OXA 2 (c), and OXA 4 (d). (, a/ao; B b/bo; 6 c/co; 7 b/bo; full
symbols represent data after pressure release).
4. Concluding remarks
Additional support may be obtained by further investi-
gations for instance applying improved X-ray methods or by
studying the anisotropy of the compression using the strain
tensor as carried out for instance in [6,7,19]. Also molecular
modeling and theoretical considerations seriously contrib-
ute to a better description of the high-pressure behavior of
such aromatic systems.
As demonstrated here for the example of different
fluorine 2,5-di(phenyl)-1,3,4-oxadiazoles the variation of
the substitution scheme changes the molecular configuration
and the conformation due to varied intramolecular inter-
actions. While the main intermolecular forces are the same
(p–p interactions between the p-systems of the oxadiazole
and phenyl rings and van der Waals forces) a different
crystal structure results with modified packing motifs from
the different three-dimensional network of these intermole-
cular forces that evolves for the differently substituted
molecules. Nevertheless, some motifs like that for the p–p
interactions (nearly parallel and interacting oxadiazole and
phenyl rings) may resemble each other. This results in a
specific anisotropic pressure response of the crystal lattice.
While the EOS parameters are nearly the same for all
investigated compounds the compounds differ in their
individual behavior of the lattice parameters with pressure.
For stack-like arrangements this behavior is fairly well
determined by the orientation of the molecular planes
against the stack and the unit cell axes and the evolving
strong p–p interactions within the stack as well as by the
van der Waals forces between different stacks. In structures
lacking such unambiguous features and showing a more
complex pattern, contributions of various interactions have
to be considered in the different unit cell directions.
Table 6
Cell parameters at selected pressures for compounds OXA1–OXA4
˚
a (A)
˚
b (A)
˚
c (A)
p (GPa)
a (8)
OXA 1
0.9
1.9
3.5
4.6
5.0
10.19
9.92
9.63
9.61
9.55
11.08
10.91
10.71
10.61
10.57
10.05
9.74
9.41
9.30
9.22
112.65
111.12
108.84
109.53
108.38
OXA 2
OXA 3
0.7
1.3
2.0
3.83
3.77
3.71
23.45
23.10
22.86
12.05
11.91
11.83
91.69
91.63
91.54
1.0
2.0
3.1
4.0
4.9
13.86
13.80
13.74
13.70
13.60
11.97
11.76
11.61
11.54
11.44
6.71
6.53
6.41
6.31
6.20
112.30
113.15
113.49
113.74
113.33
OXA 4
0.6
1.0
1.6
2.4
12.03
11.99
11.87
11.80
4.57
4.52
4.46
4.42
23.03
22.84
22.46
22.25
104.47
104.43
103.73
103.29

154
I. Orgzall et al. / Journal of Molecular Structure 749 (2005) 144–154
4: Eigenschaften der Materie bei hohen Dru¨cken, Springer, Berlin,
Acknowledgements
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The authors thank Ch. Lathe and the GeoForschungs-
Zentrum Potsdam for the possibility of using the MAX-80
equipment and the support.
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This work was supported in parts by Deutsche For-
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Ministerium fu¨r Wissenschaft, Forschung und Kultur des
Landes Brandenburg under Grant HWP 24#2597-04/328.
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