Structural phase transitions, and Br•••N and Br•••Br interactions in 1-phenyl-2-methyl-4-nitro-5-bromoimidazole

Crystals of C10H8N3O2Br undergo two reversible phase transitions between 295 and 100 K. The first, of an order-disorder nature, is a second-order transition and takes place continuously over a wide temperature range. This transition is connected with the doubling of the length of the c axis of the unit cell and with the change of the space group from P2(1)/m with Z' = 1/2 (room-temperature a-phase) to P2(1)/c, Z' = 1 (beta-phase, 200-120 K). During this transition the molecule loses the C-s symmetry of the a-phase. The second transition takes place between 118 and 115 K, and is accompanied by a change of the crystal symmetry to the triclinic space group P(1) over bar (low-temperature gamma-phase). This second phase transition is accompanied by the twinning of the crystal. Neither the molecular geometry nor the crystal packing shows any dramatic changes during these phase transitions. Halogen bonds C-Br...N and dihalogen interactions Br...Br play a crucial role in determining the crystal packing and compete successfully with other kinds of weak intermolecular interactions.