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

Abstract

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.

Mechanically Flexible Crystals of Benzene Derivatives with Halogen or Methyl Groups

“…On the other hand, the crystal structures of Group B molecules ( Br2 , Br4 , Br6 , and I2 ) are completely different from those of Group A molecules; all Group B molecules formed column structures by π–π stacking interaction (Figure ). In the Br2 crystal, intercolumnar interactions are absent on the (001) plane . In I2 crystals, intermolecular distances between iodine in I2 were 3.8447(10) and 3.8239(9) Å, smaller than twice the van der Waals radius (∼1.98 Å) of iodine.…”

“…Br•••Br interactions were supposed to be formed temporarily since Br4 was disordered in the crystal. Given that rotation of the halogen-substituted benzene derivatives in the crystal has been reported, 48,57 the Br4 columnar structure would undergo a similar rotation. In addition, the column structure of Br4 is arranged in a zigzag along the c-axis but lined along the a-axis.…”

“…In the Br2 crystal, intercolumnar interactions are absent on the (001) plane. 48 In I2 crystals, 49 intermolecular distances between iodine in I2 were 3.8447(10) and 3.8239(9) Å, smaller than twice the van der Waals radius (∼1.98 Å) 53 7) Å, thus the strength of all Br•••Br interactions is considered to be about the same. Br•••Br interactions were supposed to be formed temporarily since Br4 was disordered in the crystal.…”

“…47 Copies of the data can be obtained free of charge via the CCDC website. Crystallographic data for Br2, Br3, and I2 deposited at the CCDC were used for density functional theory (DFT) calculation and Hirshfeld surface analysis (CCDC 1583004, 48 1057994, 11 and 219572, 49 respectively). Hirshfeld Surface Analysis.…”

Temperature Dependence of the Entropy and the Heat Capacity Calculated from the Raman Frequency Shifts for Solid Benzene, Naphthalene and Anthracene

“…More recently, structural properties of solid benzene have been studied experimentally in neutron and x-ray diffraction under high pressure-high temperature conditions and the phase diagram of benzene has been obtained [29]. Also, the solid state structural properties and phase transition (order-disorder) behaviour of benzene derivatives have been studied [30]. Phase diagrams of mixtures of benzene and acetonitrite have been obtained [31].…”