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

• Published in: Crystal growth & design 2019-04, Vol.19 (4), p.2155-2162
• Main Authors: Zhou, Yating, Patterson, Rhian, Williams, P. Andrew, Kariuki, Benson M, Hughes, Colan E, Samanta, Ranita, Devarapalli, Ramesh, Reddy, C. Malla, Apperley, David C, Harris, Kenneth D. M
• Format: Article
• Language: English
• ISSN: 1528-7483; 1528-7505
• 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 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.