title: Substituent effects in nitro derivatives of carbazoles investigated by comparison of low-temperature crystallographic studies with density functional theory (DFT) calculations.
abstract: The crystal structure of 9H-carbazole, C12H9N, (I), has been redetermined at low temperature for use as a reference structure in a comparative study with the structures of 1-nitro-9H-carbazole, C12H8N2O2, (II), and 9-nitrocarbazole, C12H8N2O2, (III). The molecule of (I) has crystallographically imposed mirror symmetry (Z' = 0.5). All three solid-state structures are slightly nonplanar, the dihedral angles between the planes of the arene and pyrrole rings ranging from 0.40 (7)° in (III) to 1.82 (18)° in (II). Nevertheless, a density functional theory (DFT) study predicts completely planar conformations for the isolated molecules. To estimate the influence of nitro-group substitution on aromaticity, the HOMA (harmonic oscillator model of aromaticity) descriptor of π-electron delocalization has been calculated in each case. The HOMA indices for the isolated and solid-state molecules are relatively consistent and decrease in value for aromatic rings that are substituted with a π-electron-withdrawing nitro group. Substitution of the arene ring influences the π-electron delocalization in the ring only weakly, showing strong resistance to a perturbation of its geometry, contrary to what is observed for nitro substitution of the five-membered heterocyclic pyrrole ring. In (II), the molecules are arranged in near-planar dimers connected to each other by strong N-H···O hydrogen bonds that stack parallel to the crystallographic b axis. A similar stacking arrangement is observed in (III), although here the stacked structure is formed by stand-alone molecules.
authors.affiliation: Faculty of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Gajda
authors.affiliation: Faculty of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland.
authors.forename: Bartosz
authors.initials: B
authors.lastname: Zarychta
authors.affiliation: Faculty of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Kopka
authors.affiliation: Faculty of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland.
authors.forename: Zdzisław
authors.initials: Z
authors.lastname: Daszkiewicz
authors.affiliation: Faculty of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland.
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Ejsmont
mesh_terms: D002227:Carbazoles / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004583:Electrons; D006860:Hydrogen Bonding; D015394:Molecular Structure; D009574:Nitro Compounds / Q000737:chemistry*; D011789:Quantum Theory; D013696:Temperature
chemical_list: C000596212:1-nitro-9H-carbazole; D002227:Carbazoles; D009574:Nitro Compounds; C041514:carbazole
keywords: DFT calculations; HOMA index; biological activity; carbazoles; crystal structure; electron-withdrawing effects; molecular electronics