title: Crystal structure of carnidazole form II from synchrotron X-ray powder diffraction: structural comparison with form I, the hydrated form and the low energy conformations in vacuo.
abstract: The crystal structure of carnidazole form II, O-methyl [2-(2-methyl-5-nitro-1H-imidazole-1-yl)ethyl]thiocarbamate, has been determined using synchrotron X-ray powder diffraction in combination with simulated annealing and whole profile pattern matching, and refined by the Rietveld method. For structure solution, 12 degrees of freedom were defined: one motion group and six torsions. Form II crystallizes in space group P2(1)/n, Z=4, with unit cell parameters after Rietveld refinement: a=13.915(4), b=8.095(2), c=10.649(3) A, beta=110.83(1) degrees, and V=1121.1(5) A3. The two polymorphic forms, as well as the hydrate, crystallize in the monoclinic space group P2(1)/n having four molecules in the cell. In form II, the molecules are held together by forming two infinite zig-zag chains via hydrogen bonds of the type N--H...N, the same pattern as in form I. A conformational study of carnidazole, at semiempirical PM3 level, was performed using stochastic approaches based on modification of the flexible torsion angles. The values of the torsion angles for the molecules of the two polymorphic forms and the hydrate of carnidazole are compared to those obtained from the conformational search. Form I and form II are enantiotropic polymorphic pairs this agrees with the fact that the two forms are conformational polymorphs.
authors.affiliation: Laboratorium voor Biokristallografie, Katholieke Universiteit Leuven, Faculteit Farmaceutische Wetenschappen, O & N2 Campus Gasthuisberg, Herestraat 49 Bus 822, B-3000 Leuven, Belgium. hector.novoa@pharm.kuleuven.be
authors.forename: Héctor Novoa
authors.initials: HN
authors.lastname: de Armas
authors.forename: Oswald M
authors.initials: OM
authors.lastname: Peeters
authors.forename: Norbert
authors.initials: N
authors.lastname: Blaton
authors.forename: Guy
authors.initials: G
authors.lastname: Van den Mooter
authors.forename: Dirk J A
authors.initials: DJ
authors.lastname: De Ridder
authors.forename: Henk
authors.initials: H
authors.lastname: Schenk
mesh_terms: D002152:Calorimetry, Differential Scanning; D003460:Crystallization; D008958:Models, Molecular; D008968:Molecular Conformation; D009593:Nitroimidazoles / Q000737:chemistry*; D017356:Synchrotrons; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D009593:Nitroimidazoles; D014867:Water; C014394:carnidazole