The crystal structure of hexabromobenzene: constrained refinement of neutron powder diffraction data

The structure of hexabromobenzene, C₆Br₆, has been determined at 300 K by neutron powder diffraction. The crystal is monoclinic, a = 15.357 (4), b = 4.007 (1), c = 8.364 (2) Å, β = 92.65 (2)°, space group P2₁/n, Z = 2. The problem is well suited for a constrained refinement with the program EDINP. Within the accuracy of the analysis the orientation of a planar molecule is found and all intramolecular separations correspond to normal van der Waals interactions. No explanation of the multiple-crystal formation can be proposed and, in fact, packing considerations would support twinning on (101) which has not been observed.