Different mol-ecular conformations in the crystal structures of three 5-nitro-imidazolyl derivatives

The crystal structures of (E)-1-methyl-5-nitro-1H-imidazole-2-carbaldehyde O-benzyl-oxime, C₁₂H₁₂N₄O₃, (I), (E)-1-methyl-5-nitro-1H-imidazole-2-carb-alde-hyde O-(4-fluoro-benz-yl) oxime, C₁₂H₁₁FN₄O₃, (II), and (E)-1-methyl-5-nitro-1H-imidazole-2-carbaldehyde O-(4-bromo-benz-yl) oxime, C₁₂H₁₁BrN₄O₃, (III), are described. The dihedral angle between the ring systems in (I) is 49.66 (5)° and the linking N_m-C-C=N (m = methyl-ated) bond shows an anti conformation [torsion angle = 175.00 (15)°]. Compounds (II) and (III) are isostructural [dihedral angle between the aromatic rings = 8.31 (5)° in (II) and 5.34 (15)° in (III)] and differ from (I) in showing a near-syn conformation for the N_m-C-C=N linker [torsion angles for (II) and (III) = 17.64 (18) and 8.7 (5)°, respectively], which allows for the occurrence of a short intra-molecular C-H⋯N contact. In the crystal of (I), C-H⋯N hydrogen bonds link the mol-ecules into [010] chains, which are cross-linked by very weak C-H⋯O bonds into (100) sheets. Weak aromatic π-π stacking inter-actions occur between the sheets. The extended structures of (II) and (III) feature several C-H⋯N and C-H⋯O hydrogen bonds, which link the mol-ecules into three-dimensional networks, which are consolidated by aromatic π-π stacking inter-actions. Conformational energy calculations and Hirshfeld fingerprint analyses for (I), (II) and (III) are presented and discussed.