The low-temperature phase of 1,3-dibromo-2,4,6-trimethylbenzene: a single-crystal neutron diffraction study at 120 and 14 K.

In the low-temperature phase of dibromomesitylene (1,3-dibromo-2,4,6-trimethylbenzene), C(9)H(10)Br(2), the molecule deviates significantly from the C(3h) molecular symmetry encountered in tribromomesitylene (1,3,5-tribromo-2,4,6-trimethylbenzene), even for the endocyclic bond angles. An apparent C(2v) molecular symmetry is observed. The angle between the normal to the molecular plane and the normal to the (100) plane is approximately 20 degrees. The overall displacement was analysed at 120 K with rigid-body-motion tensor analysis. The methyl group located intermediate between the two Br atoms is rotationally disordered at both temperatures. This disorder was treated using two different approaches at 14 K, viz. the conventional split-atom model and a model using the special annular shapes of the atomic displacement parameters that are available in CRYSTALS [Watkin, Prout, Carruthers & Betteridge (1999). Issue 11. Chemical Crystallography Laboratory, Oxford, England], but only through the latter approach at 120 K. The disorder locally breaks the C(2v) molecular symmetry at 14 K only. Intra- and intermolecular contacts are described and discussed in relation to this methyl-group disorder. The bidimensional pseudo-hexagonal structural topology of trihalogenomesitylenes is altered in dibromomesitylene insofar as the (100) molecular layers are undulated and are not coplanar as a result of an alternating tilt angle of approximately 34 degrees propagating along the [011] and [0-11] directions between successive antiferroelectric molecular columns oriented roughly along the a axis.