Rotationally disordered phase of 1,3-dibromo-5-iodo-2,4,6-trimethylbenzene at 293 K

In the crystal state at room temperature, the molecule of dibromoiodomesitylene (1,3-dibromo-5-iodo-2,4,6-trimethylbenzene), C9H9Br2I, is prone to strong disorder, apparently involving only the three halogen sites (occupied identically by 66.7% Br and 33.3% I). This disorder, of the rotational type according to previously published NMR measurements, corresponds to fast 2pi/3 stochastic in-plane reorientations of the whole molecule between three discernable locations. This kind of rotational disorder can be revealed for the first time by diffractometry thanks to the C2v idealized molecular symmetry of the title compound, although it has been indirectly suspected at room temperature in other trihalogenomesitylenes of similar crystal packing but of D3h molecular symmetry. The average endocyclic angles facing the Br/I sites and the methyl groups are 124.14 (6) and 115.85 (2) degrees, respectively. The angle between the normal to the aromatic ring and the normal to the (100) plane is 4.1 degrees. TLS analysis indicates that only the aromatic ring and the methyl groups behave as a rigid body with respect to the thermal librations.