We report the solid-state structural properties and phase transition behaviour of 1,4-dibromo-2,3,5,6tetramethylbenzene, 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 hightemperature 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 2 H 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.
In a recent paper (Couzi
et al.
2018
R. Soc. open sci.
5
, 180058. (
doi:10.1098/rsos.180058
)), we proposed a new phenomenological model to account for the I↔II↔“III” phase sequence in incommensurate
n
-alkane/urea inclusion compounds, which represents an alternative interpretation to that proposed in work of Toudic
et al.
In a Comment (Toudic
et al.
2019
R. Soc. open sci.
6
, 182073. (
doi:10.1098/rsos.182073
)), Toudic
et al.
have questioned our assignment of the superspace group of phase II of
n
-nonadecane/urea, which they have previously assigned, based on a (3 + 2)-dimensional superspace, as C222
1
(00
γ
)(10
δ
). In this Reply, we present new results from a comprehensive synchrotron single-crystal X-ray diffraction study of
n
-nonadecane/urea, involving measurements as a detailed function of temperature across the I↔II↔“III” phase transition sequence. Our results demonstrate conclusively that “main reflections” (
h, k, l
, 0) with
h+k
odd are observed in phase II of
n
-nonadecane/urea (including temperatures in phase II that are just below the transition from phase I to phase II), in full support of our assignment of the (3+1)-dimensional superspace group P2
1
2
1
2
1
(00
γ
) to phase II. As our phenomenological model is based on phase II and phase “III” of this incommensurate material having the
same
(3+1)-dimensional superspace group P2
1
2
1
2
1
(00
γ
), it follows that the new X-ray diffraction results are in full support of our phenomenological model.
X-ray Birefringence Imaging is shown to be a sensitive method for spatially resolved mapping of molecular orientations in liquid-crystalline materials.
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