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Direct and Inverse Relations between Temperature and Pressure Effects in Crystals: a Case Study on o Xylene J#drzej Marciniak, and Andrzej Katrusiak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03543 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017
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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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The Journal of Physical Chemistry
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Direct and Inverse Relations between Temperature
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and Pressure Effects in Crystals: a Case Study on
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o-Xylene
4
Jędrzej Marciniak, Andrzej Katrusiak*
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Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland.
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ABSTRACT
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The
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(1,2-dimethylbenzene) are inconsistent with the rule of reverse relationship between effects of
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pressure and temperature attributed to most crystals in general. On isobaric cooling at ambient
10
pressure, the o-xylene crystal shrinks, with the strongest contraction of the unit-cell dimensions a
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and c, while during isothermal compression at ambient temperature these are the least
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compressed directions. This direct relationship (as opposed to the 'inverse relationship' rule)
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between the compression and expansion of o-xylene has been associated with weak directional
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CH⋅⋅⋅π interactions arranging the molecules into a 2-dimensional framework and with its distinct
15
mechanisms of distortions occurring at high-pressure and low-temperature. Single crystals of
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o-xylene were grown in situ in isochoric and isothermal conditions in a diamond-anvil cell and
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their structure determined by X-ray diffraction.
isothermal
compression
and
isobaric
expansion
of
crystalline
o-xylene
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INTRODUCTION
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According to the ‘inverse relationship rule’, the structural effects in the compressed and in the
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heated crystal are opposite (Figure 1).1 Recently, this rule primarily formulated for minerals, was
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applied also for other types of crystals, such as metal-organic frameworks (MOFs), organic
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compounds and framework materials.2–7 The only known exceptions to this rule, rutile-type
23
minerals8 and a recently reported MOF [Ag(en)NO3] (where en denotes ethylenediamine),9 both
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contain distinct frameworks adjusting to external stimuli.
25 26
Figure 1. Inverse and direct relations (red and green lines, respectively) of temperature and
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pressure effects schematically illustrated (with exaggerated changes) for a crystal plate heated
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and compressed, as indicated by black arrows. For example, the direct relationship implies the
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similar shape change of the treated sample (at the center), whereas the inverse relationship
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differentiates the shape.
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The flexibility of MOF crystals is often explained by the mechanic distortions of the framework
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of cationic nodes acting as the hinges for anionic linkers.3,9–12 In such a compressed structure the
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separated voids and channel pores usually reduce their volume in high pressure. However, a
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similar mechanism of closing voids was also proposed for molecular crystals without strong
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interactions. For example, weakly interacting benzene molecules form CH⋅⋅⋅π bonded layers with
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small voids.13 Their collapse induces a transition and a new phase of benzene is formed.14 Thus,
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the compression of crystals can change the preferred intermolecular interactions and can cause
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molecular rearrangements, multicomponent crystallization and phase transitions.15–21 Presently
39
we have investigated the effect of pressure and temperature on another molecular crystal of an
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aromatic compound; o-xylene (1,2-dimethylbenzene, melting point, m.p.= 248 K, Figure 2) is
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liquid at normal conditions, due to the absence of cohesion forces stronger than hydrogen bonds
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C-H⋅⋅⋅π.
43 44
Figure 2. o-Xylene molecule with labeled carbon atoms.
45
Weak interactions involving CH groups are usually classified as hydrogen bonds C-H⋅⋅⋅π.22,23
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Under external stimuli they yield considerably easier than electrostatic forces but harder than
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dispersion forces. The C-H⋅⋅⋅π bonds are among the most frequent of all types of contacts in
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organic molecular crystals24 and they play an important role in the molecular aggregation, the
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packing motifs and patterns, molecular conformations, chemical reactions, selectivity and
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resolution of chiral systems, folding of proteins and nucleic acids, structural modifications and
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properties of polymers, mechanochromic and piezochromic properties, luminescence and drug
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design.25–32
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The crystal structures of m-xylene33 (1,3-dimethylbenzene, m.p. 225.5 K, space group Pbca)
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and p-xylene34(1,4-dimethylbenzene, m.p. 286.5 K, space group P21/n) were determined by
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single-crystal X-ray and powder neutron diffraction methods in low temperature; the structure of
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o-xylene was determined only for the perdeuterated C8D10 sample, by powder neutron
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diffraction.35 Presently, we have studied intermolecular interactions in the structure of o-xylene
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as well as the correlation between the isobaric thermal expansion and isothermal compression. In
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this study we found that the strong anisotropic compression and thermal expansion of the
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o-xylene crystal cannot be reconciled with the rule of inverse effects of pressure and temperature.
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This discrepancy is particularly intriguing, given the absence of stronger directional cohesion
62
forces between o-xylene molecules. In order to understand this anomalous behavior we have
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correlated the crystal compression and expansion with the changes in intermolecular contacts as a
64
function of temperature and pressure.
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EXPERIMENTAL
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o-Xylene (analytical grade, from Acros Organics, m. p. 248 K) was used as delivered. All high-
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pressure diffraction experiments were performed by using a Merrill-Bassett diamond-anvil cell
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(DAC) modified by mounting the diamond anvils directly on steel supports.36 The diamond
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anvils had culets 0.7 mm in diameter. Gaskets were made of steel foil with the aperture of 0.4
70
mm. Pressure was determined with an accuracy of 0.03 GPa by the ruby R1 fluorescence line
71
shifts with enhanced resolution measured by a Photon Control spectrometer.37 The DAC chamber
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was filled with o-xylene and sealed; we observed that o-xylene compressed at 296 K freezes into
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a polycrystalline mass at 0.25 GPa. Pressure was further increased to 0.31 GPa and then the
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polycrystalline mass was melted by increasing the temperature to 538K with a heat gun, and a
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single crystal was grown in isochoric conditions by cooling the sample back to room temperature
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at a rate of 60 K/h. The crystal grown in this way completely filled the DAC chamber (Figure 3).
77 78
Figure 3. o-Xylene single crystal at (a) 525K; (b) 515K; (c) 490K; and (d) fully filling the
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chamber at 296K/0.31GPa. Several irregular ruby chips for pressure calibration lie around the
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chamber and most of them have been pushed to the edge of the gasket by the growing crystal.
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The high-pressure diffraction data of o-xylene collected at 0.31 GPa have been refined in the
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monoclinic space group P21/c (Table 1), consistent with the equivalent space group P21/a
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previously chosen for the perdeuterated sample.35
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In order to decrease the temperature of the subsequent crystallizations at still higher pressure we
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have mixed o-xylene with methanol (1:4 vol.); the crystals grown from this mixture in isochoric
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conditions were studied by X-ray diffraction at 0.42, 0.54, 0.80, 0.82 and 1.00 GPa. Each of these
87
crystals was in situ crystallized from a freshly prepared mixture loaded into the DAC.
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We have reduced the melting temperature again by further diluting o-xylene with methanol:
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a 1:9 (vol.) mixture was used for in situ crystallization of crystals at 1.28, 1.63, 1.96, 2.10, 2.54
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and 3.50 GPa. Again, a fresh mixture was prepared for each DAC loading.
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We have noted some systematic discrepancies in the measured unit-cell parameters, as
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exemplified in Table 1 and illustrated in Figure 4 (middle). We have attributed these
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discrepancies to the non-hydrostatic strains generated by cooling the anisotropic crystal in the
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confined rigid space of the DAC chamber.
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Table 1. Selected Crystal Data of o-Xylene at Low Temperature as well as of the High-Pressure
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Hydrostatic and Strained Sample (for details in all 0.31-3.50 GPa range see Table S1 in
97
Supporting Information) low-T
98 hydrostatic strained
P(GPa)
0.0001
0.80
0.82
T(K)
240.2(5)
293(2)
293(2)
SG
P21/c
P21/c
P21/c
a(Å)
9.0350(7)
9.205(12)
9.0256(5)
b (Å)
6.1501(5)
5.714(1)
5.809(1)
c(Å)
12.8242(10) 12.439(5)
12.458(1)
β(°)
109.394(9)
110.32(9)
110.223(5)
V(Å3)
672.16(10)
613.6(9)
612.99(11)
Z
4
4
4
Dx g⋅cm3
1.049
1.149
1.15
359
578
Unique refs 1026 GOF (S)
1.079
1.077
1.089
R
0.0585
0.0639
0.0419
99
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High-pressure single-crystal diffraction data were measured on a Xcalibur Eos diffractometer
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with MoKα X-rays. The data were preliminarily reduced with CrysAlis software.38 The structure
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of o-xylene was solved by direct methods and refined with full-matrix least squares on F2's using
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Shelxs and Shelxl implemented in Olex2.39,40 The reflection intensities were accounted for the
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effects of shadowing of the beams by the gasket and absorption of X-rays by the DAC and the
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sample crystals.41 The arene hydrogen atoms were ideally positioned according to the molecular
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geometry, while in our refinements the methyl groups were free to rotate and assumed positions
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clearly indicated by the difference Fourrier maps. For the structural-data analysis the C-H
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distances in o-xylene were normalized to neutron data:42 arene C-H 1.083 Å and methyl C-H to
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1.059 Å.
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Isobaric crystallizations were performed in situ on a four-circle Oxford Diffraction SuperNova
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diffractometer equipped with a CuKα X-ray source and an Oxford Cryosystems attachment. A
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small drop of liquid o-xylene was placed in a glass capillary, 0.3 mm in diameter, and frozen at
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200 K with the cooling rate 360 K/h. The sample was then heated to 240 K and its diffraction
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data were measured. It was seen from the diffraction pattern that the sample froze in the form of
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several crystals. The data were collected again at 220 K, 200 K and 170 K and the structures
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determined as explained above.
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We have performed the principal strain axis analysis using the PASCal sofware.43 The detailed
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output of this analysis has been listed in section ‘Strain tensor analysis’ in the Supporting
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Information.
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RESULTS AND DISCUSSION
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All presently measured X-ray diffraction data for o-xylene at low temperature and high
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pressure are consistent with the monoclinic structure of perdeuterated o-xylene crystal at 2K.35
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The crystal compression and thermal expansion are monotonic, and no anomalies indicating
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phase transitions have been detected. The compression and thermal expansion of o-xylene crystal
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revealed in this study are clearly inconsistent with the inverse-relationship rule. As shown in
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Figure 4 (the central plot), the least expanding parameter b is most compressed; the expansions of
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a and c are similar, but the compression of parameter c is intermediate between those of
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parameters a and b. In fact, the high-pressure diffraction data of the repeatedly in situ crystallized
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samples indicate that the compression of a is negative to about 0.5 GPa. Likewise, the monoclinic
130
angle β increases with temperature and this trend also continues in high pressure up to about 1.5
131
GPa (Figure 4, the bottom plot).
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Figure 4. Thermal expansion and compression of molecular volume (top); relative changes of the
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unit-cell parameters a; b; and c (middle); as well as changes in the monoclinic β angle (bottom).
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Empty symbols and dashed lines indicate the non-hydrostatically compressed crystals. Dotted
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lines connect the dimensions measured in this work for C8H10 with those of C8D10 at 2 K.35
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Herring-bone schemes of molecular aggregation exaggerate its changes upon heating and
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compression in the insets (bottom). The liquid-sample regions of isothermal compression at 296K
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and isobaric cooling at 0.1 MPa are highlited in blue and scaled to the same width.44 The
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crystallization pressure at 296 K is indicated by pc.
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The principal coefficients of thermal expansion and compressibility of o-xylene (Figure 5),
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determined using the PASCal software,43 reveal that the least thermally expanded direction [010]
143
is the most compressible.
144 145
Figure 5. Calculated (a) expansivity (in MK-1); and (b) compressibility (in TPa-1) indicatrices of
146
the o-xylene crystals viewed along direction [100].
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At the same time the strong thermal expansion of the o-xylene crystal along a is least
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compressible (Table 2).
149
Table 2. Themal Expansivity and Compressibility Related to Crystallographic Axes Calculated
150
for o-Xylene. Thermal expansion
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α (MK-1)
σα (MK-1)
a
Crystal directions b
X1
54.95
1.48
0.00
-1.00
0.00
X2 X3
67.62 124.36
0.76 3.15
0.80 -0.84
0.00 0.00
0.59 0.54
c 0.00 0.90 -0.01
Axes
c
Compression
K (TPa-1)
σK (TPa-1)
a
Crystal directions b
30.50 17.20 3.35
3.41 0.55 3.14
0.00 0.44 1.00
1.00 0.00 0.00
Axes
e11 e22 e33 151 152
Strong anisotropy of crystal expansion and compression is usually connected with the presence of
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‘wine-rack’,4,5,9,11 ‘honeycomb’,3 or ‘spring’3 molecular aggregation patterns through
154
coordination bonds or hydrogen bonds and can even lead to negative linear compressibility
155
(NLC), negative area compressibility (NAC) and negative thermal expansion (NTE). The key
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structural features behind such effects in MOFs are struts and hinges of a mechanical model
157
reproducing the elastic properties of the crystal. These features are absent in the molecular crystal
158
of o-xylene, but nonetheless it is strongly anisotropic and in the pressure region from 0.25 GPa to
159
0.54 GPa the crystal displays even a small NLC effect along [a]. This anisotropy of the o-xylene
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crystal can be due to directional bonds C-H⋅⋅⋅π binding the molecules into a pattern resembling
161
the wine-rack framework, with small voids between the molecules shown in Figure 6.
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Figure 6. (a) Contacts C-H⋅⋅⋅π, blue for methyl and red for arene H-donors in o-xylene at 0.54
164
GPa; (b) a schematic representation of the pseudo wine-rack motif distortion induced by pressure
165
in o-xylene; (c) the structure viewed along the C-H⋅⋅⋅π bonded layers. Each molecule is C-H⋅⋅⋅π
166
bonded to six molecules. Green circles indicate voids 1.28 Å in diameter, and orange circles
167
indicate smaller voids (0.94 Å).
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When pressure increases, the voids close up due to the wine-rack distortion and crystal expands
169
along [a] and shrinks along [b] – hence the strongest anisotropy along these directions. With
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increasing pressure the compression of weak C-H⋅⋅⋅π bonds gradually becomes more significant
171
compared to the wine-rack type distortion, and the strong anisotropic compression is notably
172
reduced around 0.5 GPa. Nonetheless, this pattern of C-H⋅⋅⋅π bonds is preserved and gradually
173
compressed up to 3.50 GPa, at least, where the shortest H⋅⋅⋅C distances are compressed from 2.90
174
Å to 2.57 Å (Figure 7).
175 176
Figure 7. Compression of the shortest C-H⋅⋅⋅π intermolecular contacts in o-xylene, based on the
177
C-H bonds normalized to the neutron data:45 the contacts involving arene H-donors are indicated
178
in red and methyl H-donors in blue. The shortest H⋅⋅⋅H contacts are shown as black squares and
179
black circles. Dotted lines connect our C8H10 data with those of C8D10 at 2 K.35 Empty symbols
180
indicate the structures with non-hydrostatic strain.
181
The layers of o-xylene molecules C-H⋅⋅⋅π bonded into the wine-rack motif extend along crystal
182
plane (001) with the molecules nearly perpendicular to this plane. Between the layers no C-H⋅⋅⋅π
183
bonds are formed and only short H⋅⋅⋅H contacts are present (Figure 6).
184
Anisotropic Stress Generated in the DAC
185
Soft o-xylene crystals become non-hydrostatically strained in the confined space of the DAC
186
chamber (Figure 3). This non-hydrostatic strain affects the unit-cell parameters (Figure 4). The
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effect of nonhydrostatic strain on o-xylene unit-cell parameters appear systematic i.e. repeatedly
188
unit-cell parameter [a] is longer and parameter [b] is shorter compared to these parameters
189
measured in hydrostatic conditions. Thus, this non-hydrostatic compression of the unit-cell
190
parameters can be plotted with smooth curves (Figure 4). The single crystals of o-xylene were
191
grown at isochoric conditions by cooling the DAC and a special care had to be taken to avoid
192
non-hydrostatic strain in the sample occuring due to its anisotropic stress generated by the
193
anisotropic thermal expansion when the temperature is changed. We noticed that when the crystal
194
filled most of the chamber volume, the unit-cell dimensions were different from those obtained
195
for the crystal grown in this way that the sample was surrounded by the hydrostatic fluid and did
196
not collide with the gasket at more than one point. In the latter case, the hydrostatic conditions
197
were secured (Figure 8).
198
199 200
Figure 8. The o-xylene crystal at 0.42 GPa/298K fully immersed in the liquid o-xylene/methanol
201
mixture. Only one side of the crystal is touching the gasket. A ruby chip lies near the upper-left
202
edge of the gasket.
203
It is characteristic that the non-hydrostatic strain enhanced the anisotropy of the crystal
204
compressibility. Thus, due to the non-hydrostatic strain the least compressible direction [100]
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displays a pronounced negative linear compression, and the most compressible direction [010] is
206
even more strongly compressed.
207
In order assess the magnitude of nonhydrostatic pressure in the o-xylene crystals we have
208
repeated the in situ recrystallizations and measured the pressure at various parts of the chamber
209
(Figure 9).
210 211
Figure 9. o-Xylene single crystals fully filling the DAC chamber: (a) crystal grown between
212
340-296 K with the ruby chips immersed in the crystal, and the measured pressure values in GPa;
213
(b) the same crystal after 3 hours annealing at 320 K; (c) another crystal grown between 410-296
214
K (d) this crystal after annealing at 370 K.
215
In the crystal fully filling the DAC chamber at 320 K, the pressure varied between 0.25 and
216
0.36 GPa, whereas after annealing this difference was reduced to 0.27 - 0.33 GPa. For another
217
recrystallization, when the chamber was fully filled by the o-xylene crystal at 390 K, the pressure
218
read-outs were 0.77 - 0.94 GPa, and the annealing at 370 K for three hours reduced the pressure
219
span to 0.77 - 0.85 GPa. The fluorescence of the ruby chips measured before the in situ
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220
crystallization was identical within error. The in situ recrystallization of o-xylene increased the
221
spread of pressure read-outs (Figure 9); non-hydrostatic pressure was reflected in the R1 line
222
wavelength, but no significant widening of the R fluorescence lines was noted. The sample
223
annealing relaxes some of the non-hydrostatic pressure, however, it does not eliminate it for the
224
crystal fully filling the DAC chamber. It appears that the soft crystals built of weakly interacting
225
molecules, such as o-xylene are most affected by the nonhydrostaticity in the lower range of the
226
pressure, when the crystal is still relatively soft. In o-xylene such strong effects were generated
227
about 0.8 GPa. At higher pressure the crystal becomes harder and even though the
228
nonhydrostaticity is stronger, the non-hydrostatic strain in the sample is smaller.
229
CONCLUSIONS
230
We have associated the direct relation between compression and thermal expansion of o-xylene
231
with the weak intermolecular bonds forming a wine-rack pattern and with the presence of small
232
voids in this structure. The increased temperature destbilizes this pattern of weak C-H⋅⋅⋅π bonds,
233
in the manner somewhat similar to the effect of pressure, reducing the directional character of the
234
C-H⋅⋅⋅π bonds, too, through enhanced close packing. Hence the similar effects of increased
235
temperature and pressure for the crystal expansion and compression. The direct relationship
236
observed in o-xylene can be connected in with the framework of the C-H⋅⋅⋅π bonds. It can be
237
stated that at ambient pressure the preferential directions of C-H⋅⋅⋅π bonds support the
238
arrangement of molecules within the framework, whereas at high pressure the framework folds in
239
the direction reducing the crystal volume. On the other hand the decreased temperature increases
240
the directional character of C-H⋅⋅⋅π bonds. Therefore in such structures the strain produced by
241
lowering temperature and increasing pressure can be significantly different. It may appear that
242
the observations of the direct relationships in the o-xylene crystal and in few other cases8,9 are
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243
very unusual because presently there are still very few reports on compression and thermal
244
expansion simultaneously measured for one material.46,47 Such studies are particularly relevant
245
for crystal engineering as the data collected at low-temperature and high-pressure can be used to
246
produce better piezo-responsive materials for three-dimensional pressure detectors, prototype
247
artificial muscle or shock-absorbing composites.48–50 Thus the classes of materials and effects
248
with direct and inverse relationships between changes caused by pressure and temperature can be
249
distinguished. This distinction is also important for geological studies, and provides better
250
understanding of the structural transformations of minerals subjected to high pressure and
251
temperature.
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ASSOCIATED CONTENT
253
Supporting Information
254
Detailed crystal data, Hirshfeld surfaces of o-xylene molecules. The Supporting Information is
255
available free via the Internet at http://pubs.acs.org. Full crystal data have also been deposited in
256
the Cambridge Crystallographic Database Centre as supplementary publication numbers CCDC
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1542572-1542586. Their copies can be obtained free of charge from http://www.ccdc.cam.ac.uk.
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AUTHOR INFORMATION
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Corresponding Author
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*E-mail:
[email protected],
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGEMENTS
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This work was supported by the Polish National Science Center, research grant Preludium
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2013/11/N/ST3/03793. J.M. is a recipient of a scholarship provided by the Adam Mickiewicz
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University Foundation.
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