Temperature-Dependent Structural Properties, Phase Transition Behavior, and Dynamic Properties of a Benzene Derivative in the Solid State

Article abstract of DOI:10.1021/acs.cgd.8b01775

We report the solid-state structural properties and phase transition behavior of 1,4-dibromo-2,3,5,6-tetramethylbenzene, demonstrating that this material undergoes an order-disorder phase transition below ambient temperature (at ca. 154 K on cooling and c

Full text of DOI:10.1021/acs.cgd.8b01775

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Article
Temperature-Dependent Structural Properties, Phase Transition Behaviour
and Dynamic Properties of a Benzene Derivative in the Solid State
Yating Zhou, Rhian Patterson, P Andrew Williams, Benson M. Kariuki, Colan E. Hughes, Ranita
Samanta, Ramesh Devarapalli, C. Malla Reddy, David C. Apperley, and Kenneth D.M. Harris
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01775 • Publication Date (Web): 07 Mar 2019
Downloaded from http://pubs.acs.org on March 7, 2019
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Temperature-Dependent Structural Properties, Phase
Transition Behaviour and Dynamic Properties of a Benzene
Derivative in the Solid State
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Yating Zhou, Rhian Patterson, P. Andrew Williams, Benson M. Kariuki, Colan E. Hughes,
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Ranita Samanta, Ramesh Devarapalli, C. Malla Reddy, David C. Apperley,
Kenneth D. M. Harris1*
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School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, Wales.
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata,
Mohanpur 741246, West Bengal, India.
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Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, England.
Author for correspondence: HarrisKDM@cardiff.ac.uk
Abstract
We report the solid-state structural properties and phase transition behaviour of 1,4-dibromo-2,3,5,6-
tetramethylbenzene, demonstrating that this material undergoes an order-disorder phase transition
below ambient temperature (at ca. 154 K on cooling and ca. 160 K on heating). In both the high-
temperature and low-temperature phases, the crystal structure is based on π-stacking of the molecules.
In the crystal structure of the high-temperature phase, the bromine occupancy in each substituent site
is ca. 1/3 and the methyl group occupancy in each substituent site is ca. 2/3, consistent with statistical
orientational disorder of the molecule between six distinct orientations. Natural-abundance solid-state
2H NMR spectroscopy confirms that, at ambient temperature, this disorder is dynamic via rapid
molecular reorientation about an axis perpendicular to the aromatic ring. In the low-temperature
phase, the bromine and methyl substituents occupy preferred sites within the crystal structure, with
the distribution of site occupancies becoming progressively more ordered on decreasing temperature.
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Introduction
The phenomenon of polymorphism, the existence of multiple crystalline forms with identical
chemical composition but different crystal structures, is a widely studied topic.1,2 Under a given set
of physical conditions, only one polymorph is thermodynamically stable, and the other polymorphs
are meta-stable. On changing the physical conditions (for example, by variation of temperature), the
relative stabilities of the different solid phases may change, and solid-solid phase transformations
may be thermodynamically favoured. In many cases, the transformation between two solid phases is
reversible, although hysteresis is often observed (i.e., the observed phase transition temperature may
differ depending on whether the phase transition occurs on cooling or heating).
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Order-disorder phase transitions3-5 are a class of solid-solid phase transformation in which a
disordered structure undergoes a structural change on cooling across the phase transition temperature
into a more ordered structure. In many cases, the disorder in the high-temperature phase is dynamic
disorder, with the motion becoming "frozen" upon entering the ordered low-temperature phase (which
is often associated with a reduction in crystal symmetry). In the case of organic solids, dynamic
disorder in the high-temperature phase often takes the form of molecular reorientational processes,
which cease to occur (or at least become significantly restricted) in the low-temperature phase. In
some cases, the low-temperature phase may still exhibit some degree of static orientational disorder
as a remnant of the dynamic disorder that existed in the high-temperature phase. Examples of organic
materials that undergo such order-disorder phase transitions include rotator phase solids, in which
highly symmetrical molecules [for example, hexamethylbenzene,6-9 adamantane10-16 and
tetrakis(trimethylsilyl)silane17-19] undergo facile reorientational motions within their crystal
structures in the high-temperature phase and transform to structures of lower symmetry in the low-
temperature phase. Solid inclusion compounds (for example, urea20-23 and thiourea24-26 inclusion
compounds) also exhibit order-disorder phase transitions in which dynamic disorder in the high-
temperature phase concerns the reorientational motions of guest molecules inside a solid host
structure, and the phase transition to the ordered phase is associated with "freezing" of the motion of
the guest molecules and lowering of the symmetry of the host structure.
A common type of behaviour associated with order-disorder phase transitions3-5,27 is for certain
atomic sites in the crystal structure of the disordered high-temperature phase to be occupied by two
(or more) different types of atom with essentially random occupancies as a consequence of a dynamic
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process, recalling that crystal structures determined from diffraction data are a time-averaged
representation of the true structure, and hence represent a time-average over any dynamic processes
that occur in the material. In the low-temperature phase, the site occupancies become non-random,
representing an increased degree of ordering (for example, as a consequence of "freezing" the
dynamic process and the molecule adopting a specific preferred orientation in the crystal structure).
This type of phase transition is exemplified by the material discussed in the present paper.
In this paper, we explore the temperature-dependent structural properties of 1,4-dibromo-
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,3,5,6-tetramethylbenzene (DBTMB; Figure 1), demonstrating that this material undergoes a
reversible order-disorder phase transition below ambient temperature. We report the temperature
dependence of the structural properties, determined from single-crystal X-ray diffraction (XRD). The
dynamic properties in the disordered high-temperature phase, established from natural-abundance
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solid-state H NMR spectroscopy, are also reported.
It is relevant to note that polymorphism has been reported previously within the families of
bromobenzenes and bromomethylbenzenes. For 1,2,4,5-tetrabromobenzene, a reversible
thermosalient single-crystal to single-crystal phase transition has been reported28-30 to occur at 46.5
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°
C. Polymorphism has also been reported for 1-bromo-2,3,5,6-tetramethylbenzene, 1,4-dibromo-
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,5-dimethylbenzene32,33 and 1,3-dibromo-2,4,6-trimethylbenzene.34
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Experimental
3
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Synthesis of DBTMB was carried out following a literature procedure with slight modifications.
Chloroform (25 mL), 1,2,4,5-tetramethylbenzene (6.7 g, 0.05 mol) and aluminium chips (20 mg)
were placed in a 250 mL flask and cooled to 0 ºC while stirring. Bromine (5.13 mL, 0.1 mol) was
then added slowly to the flask and the solution was stirred for about 2 hr, yielding a crystalline
precipitate. Chloroform (40 mL) was added to dissolve the product. The solution was filtered and
cooled to 0 ºC to recover the colourless crystalline product (ca. 4.5 g). The product was identified as
1
DBTMB [ H NMR (400 MHz, CDCl ): d 2.5 (s, 12H)].
3
Crystals of DBTMB suitable for single-crystal XRD were grown by slow evaporation of solvent
(at ambient temperature over ca. 1 – 2 days) from a solution of DBTMB in methanol. The
characteristic crystal morphology is needle-like, with an approximately rectangular cross section (in
general, one side of the approximate rectangle is significantly longer than the other side).
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Differential scanning calorimetry (DSC) data were recorded on a TA Instruments Q100
differential scanning calorimeter. The powder sample was ground lightly and loaded into a
hermetically sealed aluminium pan. The DSC data were recorded with cooling/heating rates of 5 °C
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Single-crystal XRD data were recorded (Table 1) for a single crystal of DBTMB at ambient
temperature (296 K) and at the following temperatures on cooling the crystal (the same crystal was
studied at each temperature): 160 K, 150 K, 140 K, 130 K, 115 K, 100 K. Data were collected on an
Agilent SuperNova Dual Atlas diffractometer (CuKα; λ = 1.5418 Å) equipped with a mirror
monochromator and an Oxford Cryosystems cooling apparatus. Structure solution and refinement
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were carried out using SHELXS-86 and SHELXL respectively. In the refinement, disorder of the
bromine and methyl substituents of the DBTMB molecule was modelled by constraining the total
occupancy of bromine and methyl groups on each substituent site to be equal to 1. In addition, the
total occupancy of the bromine atoms in all six substituent sites was fixed at 2 and the total occupancy
of the methyl groups in all six substituent sites was fixed at 4, consistent with the molecular formula
C Br (CH ) . For non-hydrogen atoms, anisotropic displacement parameters were refined. Hydrogen
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atoms were inserted according to standard geometries and a riding model was used for all hydrogen
atoms with U set at 1.2 or 1.5 times the value of U for the carbon atom to which the hydrogen
iso
eq
atom is bonded. Standard protocol was used for refinement of methyl groups. Methyl groups were
initially free to rotate about the C–CH bond but restraints on the orientation of the methyl groups
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relative to the aromatic ring were subsequently applied in cases for which convergence was not
achieved.
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Natural-abundance solid-state H NMR data were recorded at ambient temperature (25 °C) on
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a Varian VNMRS solid-state NMR spectrometer ( H Larmor frequency, 61.383 MHz). The data were
recorded using magic-angle sample spinning (MAS) at two different MAS frequencies of 2 kHz
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120,000 scans; total time, 33.3 hr) and 6 kHz (6,000 scans; total time, 1.7 hr), with high-power H
decoupling (TPPM) applied at 62.5 kHz (recycle delay, 1 s).
3
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Results and Discussion
3
.1 Structural Properties
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Differential scanning calorimetry (DSC) data recorded for a powder sample of DBTMB (Figure 2)
show that, on cooling from ambient temperature, an exothermic event occurs at ca. 152 – 154 K
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[
–119.3 °C (153.8 K) at 5 °C min ; –121.5 °C (151.6 K) at 15 °C min ], and on subsequent heating,
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an endothermic event occurs at ca. 160 K [–113.6 °C (159.6 K) at both 5 °C min and 15 °C min ].
These observations indicate the occurrence of a reversible phase transformation with slight hysteresis.
The crystal structure of DBTMB was determined from single-crystal XRD data recorded at
several temperatures in the high-temperature and low-temperature phases (i.e., above and below ca.
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54 K, respectively, on slow cooling) for the same single crystal. Structural data and refinement
parameters at each temperature studied are given in Table 1, and cif files have been deposited at the
Cambridge Crystallographic Data Centre (see Associated Content below).
In the high-temperature phase of DBTMB, the crystal structure (Figure 3) has monoclinic
metric symmetry with space group P2 /c (Z = 2) and the following refined lattice parameters at 296
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K: a = 8.3646(11) Å, b = 4.0140(4) Å, c = 17.382(3) Å, β = 117.518(15)°, V = 517.58(14) Å . The
molecular arrangement in the crystal structure is similar to those found for other polyhalogenated
3
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benzenes. The planar DBTMB molecules form stacks along the b-axis, which corresponds to the
.
..
long axis of the needle-like crystal morphology. Along the stacks, strong π π interactions exist
between adjacent molecules with a distance of 4.01 Å (at 296 K) between the centres of adjacent
molecules, corresponding to the periodic repeat distance along the b-axis. The normal to the aromatic
.
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ring is tilted by ca. 23° from the stacking axis to optimize the geometry of the π π interactions. The
perpendicular distance between aromatic planes is 3.70 Å.
In the high-temperature phase, the DBTMB molecule is located on a crystallographic inversion
centre, and the asymmetric unit comprises half the molecule (Z' = ½). As a consequence, there are
only three crystallographically distinct substituent sites. Each substituent site exhibits disorder in
terms of occupancy by the bromine and methyl substituents. At 296 K, the refined occupancies of the
bromine and methyl substituents at each site are: Br4/C4, 0.313(3)/0.687(3); Br5/C5,
0
.359(3)/0.641(3); Br6/C6, 0.328(3)/0.672(3). Thus, for each of the three substituent sites, the
occupancy of bromine is approximately 1/3 and the occupancy of the methyl group is approximately
/3, consistent with statistical orientational disorder of the DBTMB molecule between six distinct
2
orientations (we note that similar orientational disorder is observed in the crystal structures of other
hexa-substituted benzenes, for example pentachloronitrobenzene39,40). For each molecular
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orientation, the carbon atoms of the aromatic ring occupy an identical set of positions. The question
of whether the orientational disorder in the high-temperature phase is static or dynamic is discussed
below.
At the phase transition temperature (ca. 154 K on slow cooling), the crystal undergoes a single-
crystal-to-single-crystal phase transition. The crystal structure of the low-temperature phase (Figure
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) remains monoclinic with space group P2 /c, but the unit cell volume is increased by a factor of
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about 6 compared to the unit cell of the high-temperature phase. Compared to the unit cell of the high-
temperature phase, the length of the a-axis of the low-temperature phase is increased by a factor of
about 2, the length of the b-axis is increased by a factor of about 3, while the length of the c-axis and
value of the β-angle are approximately unchanged.
At 150 K, the unit cell parameters are: a = 16.5482(8) Å, b = 11.8252(4) Å, c = 17.1772(9) Å,
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β = 117.259(7)°, V = 2988.0(3) Å . The number of DBTMB molecules in the unit cell is Z = 12, and
the number of crystallographically independent molecules in the asymmetric unit is Z' = 3. As in the
high-temperature phase, the DBTMB molecules form stacks along the b-axis, with π···π interactions
between adjacent molecules in a given stack. For each molecule, the normal to the molecular plane
is tilted by ca. 23° from the stacking axis to optimize π···π interactions. The distance between the
centres of adjacent molecules along the stack (at 150 K) are 3.926 Å, 3.929 Å and 3.972 Å for the
three independent distances along the stack. Interestingly, at all temperatures studied in the low-
temperature phase, one intermolecular distance along the stack is significantly longer (by ca. 0.4 –
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.6 Å) than the other two distances. In contrast, in the high-temperature phase, all intermolecular
(centre-to-centre) distances along the stack are equal (4.014 Å at 296 K), with neighbouring
molecules related by translation along the stacking axis (b-axis).
For the three crystallographically independent molecules in the low-temperature phase, there
are a total of 18 substituent sites. The transformation from the high-temperature phase to the low-
temperature phase is accompanied by a substantial increase in ordering of the bromine and methyl
substituents among the substituent sites, which become significantly more localized in the low-
temperature phase leading to almost full occupancy (greater than ca. 0.90) of one type of substituent
at each site (Figure 5). Subsequently, we refer to the substituent sites by the type of substituent with
highest occupancy on the site. Within a given stack along the b-axis, the two sites with bromine
substituents as the major component "rotate" in the manner of an approximate 3-fold helix on moving
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along the b-axis; half of the stacks represent a right-handed helical arrangement and the other stacks
represent a left-handed helical arrangement.
To assess the temperature dependence of the substituent site occupancies in the low-temperature
phase, the crystal structure was determined at five different temperatures from 150 K to 100 K (on
cooling the same single crystal). The occupancies of the major component for all 18 substituent sites
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(i.e., six substituent sites for each of the three independent molecules in the asymmetric unit)
determined at each temperature are shown in Figure 5. At 150 K, the occupancy of the major
component is greater than 0.90 for all substituent sites, ranging from 0.908(3) to 0.986(3). As
temperature is decreased, there is a monotonic increase in the occupancy of the major component at
each substituent site, and the range of occupancies becomes significantly narrower. Thus, at 100 K,
the occupancy of the major component for all substituent sites is in the range from 0.969(3) to
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.997(1).
Molecules in adjacent stacks interact via van der Waals interactions involving the methyl and
bromine substituents. In addition, favourable end-to-side Br···Br interactions exist between
molecules in adjacent stacks. Within the ac-plane, groups of three molecules establish favourable
.
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...
end-to-side Br Br interactions (Figure 6) giving rise to a triangular arrangement of three short Br Br
contacts with the following distances (at 150 K): Br3···Br1, 3.52 Å; Br1···Br5, 3.55 Å; Br5···Br3,
3
.74 Å. This triangular motif involving three bromine substituents is well recognized in the context
of halogen-bonding.38,41-43 The other three bromine sites in the asymmetric unit are not engaged in
..
.
Br Br interactions of this type.
3
.2 Dynamic Properties
2
Solid-state H NMR is a powerful technique for studying reorientational motions of molecules in
solids.44-48 For H nuclei in organic solids, the dominant anisotropic NMR interaction is the
quadrupolar interaction, and a powder sample containing a random distribution of crystal orientations
2
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2
gives a characteristic broad-line H NMR "powder pattern". Analysis of the H NMR powder pattern
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allows the parameters that define the quadrupole interaction of the H nucleus to be established,
specifically the quadrupole coupling constant () and asymmetry parameter (). When reorientational
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motion involving the H nucleus occurs at a rate greater than ca. 10 s , the quadrupole interaction
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is influenced significantly by the reorientational motion, and the H NMR powder pattern changes in
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a well-defined manner. When the rate of motion is in the range 10 s to 10 s , analysis of the
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powder pattern, but the specific rate of motion cannot be established (as discussed below, the
dynamics of DBTMB in the high-temperature phase are within this so-called "rapid motion regime").
2H NMR lineshape analysis is generally carried out by calculating the lineshapes for proposed
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dynamic models, and finding the dynamic model for which the calculated H NMR lineshape gives
2
the best agreement with the experimental H NMR lineshape.
2
In the present work, we have recorded the H NMR spectrum for a sample of DBTMB with
natural isotopic abundances. As described elsewhere,49-57 given the very low natural abundance of
2
2
the H isotope (0.015%), it is advantageous to record the H NMR spectrum for natural abundance
samples under conditions of magic-angle sample spinning (MAS; the frequency of MAS is denoted
2
ν ). Under such circumstances, the broad-line H NMR powder pattern is split into a set of narrow
r
peaks separated by the spinning frequency ν , with the overall intensity envelope reflecting, at least
r
2
approximately, the intensity distribution of the H NMR powder pattern. To determine the dynamic
2
model, the simulated H NMR spectrum is calculated assuming a given dynamic model and at the
2
same MAS frequency ν as the experimental H NMR spectrum.
r
2
The natural-abundance solid-state H NMR spectrum of DBTMB recorded at ambient
temperature (25 °C) with MAS at ν = 2 kHz is shown in Figure 7a. The intensity distribution of the
r
2
spinning sidebands is very similar to that observed previously in the same type of H NMR
measurement at the same MAS frequency for a sample of hexamethylbenzene with natural isotopic
4
9
abundances. From this observation, we deduce that the DBTMB molecules in the high-temperature
phase may undergo similar dynamics to hexamethylbenzene, which has been studied extensively by
solid-state NMR techniques.9,49,50,58-60 In particular, the dynamic process involves rapid rotation of
each methyl group about the C–CH bond and rapid reorientation of the whole molecule by 60 jumps
3
2
about an axis perpendicular to the plane of the aromatic ring (Figure 8). The simulated H NMR
spectrum for this dynamic model (for ν = 2 kHz) has been calculated assuming that both components
r
2
8
–1
of motion are in the rapid motion regime for H NMR (i.e., rate of motion greater than ca. 10 s ),
using motionally averaged values of the quadrupole interaction parameters consistent with those
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reported in previous literature for hexamethylbenzene9,50,58 (the best-fit simulated spectra shown in
Figure 7 were obtained for the following values of the motionally averaged quadrupole interaction
2
parameters:  = 18.0 kHz,  = 0). The excellent agreement between the simulated H NMR spectrum
2
(
Figure 7b) and the experimental H NMR spectrum (Figure 7a) recorded at 25 °C with ν = 2 kHz
9
1
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confirms the validity of the proposed dynamic model at this temperature. The natural-abundance
2
solid-state H NMR spectrum was also recorded at 25 °C using a higher MAS frequency (ν = 6 kHz;
r
2
Figure 7c). Again, the simulated H NMR spectrum (Figure 7d) calculated (for ν = 6 kHz) on the
r
assumption of the dynamic model discussed above is in excellent agreement with the experimental
2H NMR spectrum (Figure 7c), giving further support to the veracity of the dynamic model.
4
.
Concluding Remarks
2
Although our H NMR measurements were restricted only to ambient temperature, it is reasonable to
suggest that the same dynamic model may be applicable throughout the high-temperature phase,
although the rate of the 60 jump motion of the whole molecule is expected to decrease as temperature
is decreased within this phase. We emphasize that the occurrence of the 60 jump motion of the whole
molecule is fully consistent with the observation from our single-crystal XRD data that the
occupancies of all substituent sites correspond to statistical occupancies (bromine occupancy ca. 1/3;
methyl occupancy ca. 2/3) at both temperatures (296 K and 160 K) studied in the high-temperature
phase.
For the low-temperature phase, the fact that the occupancies of the substituent sites differ
significantly from the statistical occupancies suggests that there is now a significant energetic
advantage for each of the three molecules in the asymmetric unit to adopt a specific orientation,
although with a small residual population of each molecule in the other two (higher energy)
orientations. Our observation that the population of the preferred orientation increases progressively
as temperature is decreased within the low-temperature phase (see Figure 5) suggests that a dynamic
process still exists in this phase, allowing a minor redistribution of site occupancies to occur as
temperature is changed via reorientation of the molecule about an axis perpendicular to the plane of
the aromatic ring. At this stage, we have not explored the dynamic properties of the low-temperature
phase of DBTMB, but it is anticipated that the dynamic process allowing redistribution of the site
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occupancies as a function of temperature is likely to be too slow to be probed by H NMR lineshape
analysis.
ASSOCIATED CONTENT
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Accession Codes
CCDC 1583004 – 1583008 and CCDC 1583183 contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*
E-mail: HarrisKDM@cardiff.ac.uk
ORCID
Kenneth D.M. Harris: 0000-0001-7855-8598
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
We are grateful to Cardiff University for general support and to Diamond Light Source for a Ph.D.
studentship (to R. P.). R. S. thanks UGC (India) for a senior research fellowship and C. M. R. is
grateful to Royal Society of Chemistry for a Researcher Mobility Grant (2015).
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Table 1 Crystal structure and refinement data for DBTMB at several temperatures in the high-
temperature (HT) and low-temperature (LT) phases.
10 2
C H12Br
FW
292.02
Monoclinic
P2 /c
System
Space group
 / Å
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1.54184
Size / mm3
0.599  0.074  0.061
T / K
100(2)
115(2)
130(2)
140(2)
150(2)
LT
160(2)
HT
296(2)
HT
Phase
a / Å
LT
LT
LT
LT
16.5037(5)
11.7773(3)
17.1364(5)
90
16.5234(5)
11.7905(3)
17.1484(5)
90
16.5416(5)
11.8140(3)
17.1647(5)
90
16.5498(6)
11.8204(3)
17.1725(6)
90
16.5482(8) 8.2901(12)
11.8252(4) 3.9439(5)
17.1772(9) 17.197(2)
8.3646(11)
4.0140(4)
17.382(3)
90
b / Å
c / Å
α / °
90
90
β / °
117.204(4)
90
117.234(4)
90
117.255(4)
90
117.258(4)
90
117.259(7) 117.258(13) 117.518(15)
 / °
90
90
90
V / Å3
2962.35(17) 2970.49(17) 2981.96(17) 2986.33(19) 2988.0(3)
499.82(13)
517.58(14)
Z
12
12
12
12
12
2
2

cal / Mg m–3
1.964
1.959
1.951
1.948
1.947
1.940
1.874
Number of
Reflections
1
1150
10247
10248
11100
11483
1509
1578
Unique Data
R(int)
5859
5848
5855
5891
5881
959
997
0.0323
1.064
0.0402
0.0274
1.048
0.0288
1.038
0.0294
1.036
0.0441
1.018
0.0657
1.049
0.0710
1.046
Gof
R1 [I>2(I)]
0.0433
0.1157
0.0499
0.1243
0.0438
0.1185
0.0511
0.1276
0.0437
0.1174
0.0515
0.1264
0.0479
0.1262
0.0573
0.1396
0.0630
0.1729
0.0671
0.1782
0.0607
0.1818
0.0689
0.1959
wR2 [I>2(I)] 0.1088
R1 (all data)
wR2(all data)
0.0456
0.1156
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Figure 1 Molecular structure of DBTMB.
–
1
Figure 2 DSC data recorded for DBTMB (cooling/heating rate, 15 °C min ). Heat flow is positive
for exothermic processes and negative for endothermic processes.
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Figure 3 Crystal structure of DBTMB at 296 K viewed: (a) along the b-axis (parallel to the stacking
axis), and (b) along the a-axis (perpendicular to the stacking axis, which runs vertically).
Each substituent position on the aromatic ring is disordered and occupied by both bromine
(occupancy ca. 1/3) and methyl (occupancy ca. 2/3) substituents.
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Figure 4 Crystal structure of the low-temperature phase of DBTMB at 150 K, viewed: (a) along
the b-axis (parallel to the stacking axis), and (b) along the a-axis (perpendicular to the
stacking axis, which runs vertically). For each substituent site, only the substituent of
highest occupancy (greater than ca. 0.90) is shown. In (a), the full crystal structure is
shown, while in (b), only a single layer of stacks parallel to the bc-plane is shown. It is
clear from (b) that the three independent molecules along the stack have different
molecular orientations.
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Figure 5 Occupancy of the major component in each of the 18 substituent positions in the crystal
structure of the low-temperature phase of DBTMB determined as a function of
temperature on cooling from 150 K to 100 K.
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Figure 6 A group of three molecules linked by intermolecular Br···Br contacts in the crystal
structure of the low-temperature phase of DBTMB at 150 K. Only the molecular
orientations of higher occupancy are shown.
2
Figure 7 Experimental solid-state H NMR spectra recorded at 25 °C for a powder sample of
DBTMB with natural isotopic abundances at MAS frequencies of (a) 2 kHz and (c) 6
2
kHz. Simulated solid-state H NMR spectra calculated for the dynamic model of the
DBTMB molecule shown in Figure 7 at MAS frequencies of (b) 2 kHz and (d) 6 kHz.
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Figure 8 Dynamic model for DBTMB in the high-temperature phase, involving rapid rotation of
each methyl group about the C–CH bond (green arrows) and rapid reorientation of the
3
whole molecule by 60 jumps about an axis perpendicular to the plane of the aromatic
ring (red arrow).
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For Table of Contents Use Only
Temperature-Dependent Structural Properties, Phase Transition Behaviour and
Dynamic Properties of a Benzene Derivative in the Solid State
9
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Yating Zhou, Rhian Patterson, P. Andrew Williams, Benson M. Kariuki, Colan E. Hughes, Ranita
Samanta, Ramesh Devarapalli, C. Malla Reddy, David C. Apperley,
Kenneth D. M. Harris*
TOC Graphic
Synopsis
Solid-state structural studies show that 1,4-dibromo-2,3,5,6-tetramethylbenzene undergoes an order-
disorder phase transition at ca. 154 K. In the high-temperature phase, bromine and methyl groups are
2
distributed among the six substituent sites with statistical occupancies; solid-state H NMR shows
that this disorder is dynamic. In the low-temperature phase, bromine and methyl groups occupy
preferred sites, becoming progressively more ordered as temperature decreases.
2
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