title: Structural phase transitions, and Br.N and Br.Br interactions in 1-phenyl-2-methyl-4-nitro-5-bromoimidazole.
abstract: Crystals of C(10)H(8)N(3)O(2)Br 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 alpha-phase) to P2(1)/c, Z' = 1 (beta-phase, 200-120 K). During this transition the molecule loses the C(s) symmetry of the alpha-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 P1; (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.
authors.affiliation: Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland. mkubicki@amu.edu.pl
authors.forename: Maciej
authors.initials: M
authors.lastname: Kubicki
mesh_terms: D001966:Bromine / Q000737:chemistry; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry; D009593:Nitroimidazoles / Q000737:chemistry*; D044367:Phase Transition; D044366:Transition Temperature
chemical_list: C488182:1-phenyl-2-methyl-4-nitro-5-bromoimidazole; D009593:Nitroimidazoles; D009584:Nitrogen; D001966:Bromine