Concomitant but disappearing: two polymorphs of 1,4-bis(tribromomethyl)benzene

The title compound, C₈H₄Br₆, (I), initially crystallized from deuterochloroform as the comcomitant polymorphs (Ia) (prisms, space group P2₁/n, Z = 2) and (Ib) (hexagonal plates, space group C2/c, Z = 4). The molecules in both forms display crystallographic inversion symmetry. All further attempts to crystallize the compound led exclusively to (Ib), so that (Ia) may be regarded as a `disappearing polymorph'. Surprisingly, however, the density of (Ia) is greater than that of (Ib). The only significant difference between the molecular structures is the orientation of the CBr<sub>3</sub> groups. The molecular packing of both structures is largely determined by Br...Br interactions, although (Ia) also displays a C—H...Br hydrogen bond and both polymorphs display one Br...π contact. For (Ia), six of the eight contacts combine to form a tube-like substructure parallel to the a axis. For (Ib), the two shortest Br...Br contacts link `half' molecules consisting of C—CBr₃ groups to form double layers parallel to (001) in the regions z≃, .