Substituent effects in nitro derivatives of carbazoles investigated by comparison of low-temperature crystallographic studies with density functional theory (DFT) calculations

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.

S1. Introduction

Carbazole and its derivatives have attracted significant attention owing to their applications in pharmacy and molecular electronics. These compounds exhibit various biological activities, such as anti­tumour, anti­oxidative (Itoigawa et al., 2000), anti-inflammatory and anti­mutagenic behaviour (Ramsewak et al., 1999). The compounds are also considered to be potential candidates for electronic applications, such as colour displays, organic semiconductors, lasers and solar cells, as they demonstrate electroactivity and luminescence (Friend et al., 1999; Zhang et al., 2004). In recent years, crosslinked polycarbazole materials have also been widely employed as electron donors in materials for organic light-emitting diodes (OLEDs) (Shirota & Kageyama, 2007; Tao et al., 2011; Yook & Lee, 2012 and Zhang et al., 2009). Herein, we present the crystal and molecular structures of 9H-carbazole, (I), 1-nitro-9H-carbazole, (II), and 9-nitro-carbazole, (III), at 100 K. The structure of (I) has been previously reported by several authors at room temperature [Cambridge Structural database (CSD), Version? (Allen, 2002), How many hits?] as well as at 168 K by Gerkin & Reppart (1986). We have remeasured this structure at 100 K to obtain comparable geometric data for structural and electronic analysis of the compounds. The structure of (II) was reported earlier, but no detailed comparitive analysis of this structure was given (Kautny & Stöger, 2014).

The nitro group is an outstanding substituent for the study of substituent effects (Exner & Krygowski, 1996). The formally positive charge on the N atom explains the strong electron-attracting power of the whole group. This is manifested by the high values of electronegativity and dipole moments. The most sensitive structural parameter of the group is the length of the C—N bond, which may serve as an approximate measure of the resonance effect (Exner & Krygowski, 1996). The presence of such an electron-withdrawing group, along with strong inter­molecular inter­actions, may influence the geometry and electronic structure of aromatic systems and, in consequence, π-electron delocalization effects.

A convenient and easily accessible measure of π-electron delocalization (aromaticity) in chemical compounds is the harmonic oscillator model of aromaticity (HOMA). This geometry-based index stipulates that bond lengths in aromatic systems are between the values that are typical for single and double bonds (Kruszewski & Krygowski, 1973; Krygowski, 1993; Krygowski & Cyrański, 1996). The aromaticity of a series of carbazole derivatives has been analysed by means of different local aromaticity criteria (Poater et al., 2004).

The aim of this article is to analyse how the substituent effect of nitro groups located at the C1 and N9 positions works in carbazole derivatives in both the gas and the solid state. The geometries of isolated molecules (gas state) of the studied compounds have been obtained by quantum-mechanical calculation using density functional theory (DFT).

S2. Experimental

S2.1. Synthesis and crystallization

Compound (I) was purchased from Sigma–Aldrich. Compounds (II) and (III) were prepared according to literature procedures, viz. Kyzioł et al. (1987) for (II) and Kyzioł & Daszkiewicz (1985) for (III). Crystals of (I), (II) and (III) suitable for single-crystal X-ray diffraction were grown by slow cooling of ethanol solutions. Based on the solid-state geometry, the molecular structures of (I) and (II) and (III) were optimized using the B3LYP hybrid functional (Becke, 1988; 1993; Lee et al., 1988) at the 6-311++G(d,p) level of theory. All species correspond to the minima at the B3LYP/6-311++G(d,p) level with no imaginary frequencies. All calculations were performed using the GAUSSIAN09 program package (Frisch et al., 2010) employing four CPUs [AMD Opteron 6274 (6200 series, 32 nm) class] and 7.2 GB memory.

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were generated in idealized positions and refined in riding mode, with C—H = 0.93 Å and N—H = 0.86 Å, and with U_iso (H) = 1.2U_eq(C,N).

S3. Results and discussion

The solid-state molecular structures of the three compounds, along with the atom-numbering schemes used, are presented in Fig. 1.

The crystal structure of (I) has been reported previously at 168 K (Gerkin & Reppart, 1986). The molecule of (I) has crystallographically imposed mirror symmetry, Z' = 1/2, with the mirror plane running through atoms N9 and H9. At 100 K an N—H···Cgⁱ short contact is revealed, with H···Cgⁱ = 2.88 Å and N—H···Cgⁱ = 118.7 (2)° [where Cg is defined as the centroid of the pyrrole ring; symmetry code: (i) -1/2 + x, y, 1/2 - z].

In (II), the nitro group participates in a strong inter­molecular N—H···O bond (Table 2) which arranges the molecules into near-planar dimers. These dimers stack parallel to the crystallographic b direction, with neighbouring stacks oriented to give a herringbone structure (Fig. 2). The parallel separation between dimers within a stack is 3.167 (3) Å, and the angle between planes of adjacent dimers is 73.2 (2)°.

In the crystal of (III), the molecules stack into columns parallel to the crystallographic b direction, with adjacent molecules in each column twisted with respect to their neighbours (Fig. 2). Adjacent columns are connected to each other by weak C—H···O bonds (Table 3). The average distance between stacked molecules is 3.384 (2) Å and the angle between adjacent stacks is 42.8 (1)°.

In all three compounds, three planar fragments may be distinguished: two benzene rings [C1–C9a (A) and C5a–C8a (B)] and a pyrrole ring (C). The values of the dihedral angles between A/C and B/C for each compound are given in Table 4. All molecules of the studied compounds are almost planar. The largest value for an angle between planes is observed in structure (II) [1.82 (18)°], while the smallest is found in (III) [0.40 (7)°]. A search of the CSD gave an average value of the dihedral angle between adjacent planes in other N- and C1-substituted carbazoles of 1.83° (median = 1.512), while the maximum values did not exceed 8°.

In (II) and (III), the nitro group is slightly twisted with respect to the plane of the aromatic system [dihedral angles of 3.73 (36) and 6.38 (13)°, respectively]. In the geometries of the isolated molecules, the nitro groups in both cases are perfectly coplanar with the rings. This discrepancy can be attributed to the presence of inter­molecular hydrogen bonds of different strengths and stereochemistries. As pointed out above, the molecules of (II) form near linear hydrogen bonds between dimers, while the C—H···O hydrogen bond in (III) lies in the direction of the nitro-group twist. Furthermore, both nitro groups take part in intra­molecular N—H···O [for (II)] and C—H···O [for (III)] hydrogen bonds, forming six-membered rings [O–N–C–C–N–H in (II) and two pseudo-symmetric O–N–N–C–C–H contacts in (III)].

In all three compounds, both the isolated molecules and the solid-state structures show geometric deviations connected with the pyrrole ring. The differences correspond to the different contributions of the canonical structures presented in Fig. 3 to the mesomeric hybrid. In (II), the two N—C bond distances of the pyrrole ring are slightly different, with the bond nearest to the substituted benzene ring being the shorter. In the case of (III), the N—N bond length is shorter than a single N—N bond (1.42 Å; Allen et al., 2006) but longer than a double N═N bond (1.24 Å; Allen et al., 2006), indicating its partially double-bonded character. Such delocalization of π-electrons forms a rigid and planar four-centred –NNO₂ group.

As mentioned above, the most reliable and convenient measure of changes in the electronic structure of aromatic rings is the HOMA index (Kruszewski & Krygowski, 1973; Krygowski, 1993; Krygowski & Cyrański, 1996). Compound (I) is perfectly suitable as a reference structure for analysis of the influence of the substituent on the π-electron delocalization in (II) and (III). The HOMA indices for the isolated and solid-state molecules correspond well, although the calculated geometries are characterized by lower values. The molecular environment in the solid state may cause this effect. In (II) and (III), the largest decreases in aromaticity are observed for the nitro-substituted aromatic systems. The HOMA value for the nitro-substituted benzene ring in (II) is reduced by 0.021 for the isolated molecule (0.022 for the solid-state structure), and in (III) the nitro-substituted pyrrole ring differs by 0.162 (0.193 for the solid-state structure). This indicates a strong resistance of the benzene ring to the perturbation caused by a substituent (Krygowski et al., 2004), whilst the five-membered heterocyclic ring is far more sensitive, as expected. Compared with the previous results of Poater et al. (2004), the values of the HOMA indices are slightly different. This could be attributed to the use of different basis sets, although the tendencies between substituted, non-substituted and pyrrole rings are coherent.

Related literature

For related literature, see: Allen (2002); Allen et al. (2006); Becke (1988, 1993); Exner & Krygowski (1996); Friend et al. (1999); Frisch et al. (2010); Gerkin & Reppart (1986); Itoigawa et al. (2000); Kautny & Stöger (2014); Kruszewski & Krygowski (1973); Krygowski (1993); Krygowski & Cyrański (1996); Krygowski et al. (2004); Kyzioł & Daszkiewicz (1985); Kyzioł et al. (1987); Poater et al. (2004); Ramsewak et al. (1999); Shirota & Kageyama (2007); Tao et al. (2011); Yook & Lee (2012); Zhang et al. (2004, 2009).

Computing details

For all compounds, data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis RED (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structures in the solid state of (a) (I), (b) (II) and (c) (III), showing the atom-numbering schemes. Displacement ellipsoids are drawn at the 50% probability level. Dashed lines in (II) and (III) indicate intramolecular hydrogen bonds.
	Fig. 2. Packing diagrams for (a) (II) and (b) (III). Dashed lines indicate intramolecular N9—H9···O11^ii [in (II)] and C3—H3···O12^iii [in (III)] hydrogen bonds. [Symmetry codes: (ii) -x + 1, -y + 1, -z; (iii) x - 1/2, y, -z + 3/2.
	Fig. 3. The cannonical forms of (a) (II) and (b) (III).

(I) 9H-carbazole

Crystal data

C12H9N	F(000) = 352
Mr = 167.20	Dx = 1.348 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 831 reflections
a = 7.6371 (2) Å	θ = 3.8–26.0°
b = 19.0042 (6) Å	µ = 0.08 mm^−1
c = 5.67758 (14) Å	T = 100 K
V = 824.03 (4) Å^3	Plate, colourless
Z = 4	0.38 × 0.25 × 0.12 mm

Data collection

Oxford Xcalibur diffractometer	717 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.017
Graphite monochromator	θmax = 26.0°, θmin = 3.8°
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm^-1	h = −9→8
ω scans	k = −21→23
5112 measured reflections	l = −6→7
831 independent reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.033	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.093	H-atom parameters not refined
S = 1.08	w = 1/[σ^2(Fo^2) + (0.0572P)^2 + 0.1429P] where P = (Fo^2 + 2Fc^2)/3
831 reflections	(Δ/σ)max < 0.001
61 parameters	Δρmax = 0.19 e Å^−3
0 restraints	Δρmin = −0.22 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.26385 (15)	0.61974 (6)	0.3038 (2)	0.0241 (3)
H1	0.2075	0.6062	0.1658	0.029*
C2	0.32391 (14)	0.57044 (6)	0.4642 (2)	0.0261 (3)
H2	0.3076	0.5229	0.4329	0.031*
C3	0.40869 (14)	0.59049 (6)	0.6724 (2)	0.0246 (3)
H3	0.4481	0.5561	0.7761	0.030*
C4	0.43466 (14)	0.66072 (6)	0.72582 (19)	0.0215 (3)
H4	0.4910	0.6737	0.8644	0.026*
C4a	0.37483 (12)	0.71176 (6)	0.56807 (18)	0.0189 (3)
N9	0.24348 (17)	0.7500	0.2323 (2)	0.0224 (3)
H9	0.1921	0.7500	0.0974	0.027*
C9a	0.29118 (13)	0.69050 (6)	0.35748 (19)	0.0202 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0196 (6)	0.0323 (7)	0.0205 (6)	−0.0048 (5)	0.0019 (4)	−0.0056 (5)
C2	0.0243 (6)	0.0234 (6)	0.0306 (7)	−0.0040 (5)	0.0073 (5)	−0.0031 (5)
C3	0.0224 (6)	0.0261 (6)	0.0253 (6)	0.0013 (4)	0.0042 (4)	0.0057 (5)
C4	0.0170 (6)	0.0296 (7)	0.0179 (5)	−0.0005 (5)	0.0014 (4)	0.0008 (4)
C4a	0.0136 (5)	0.0254 (6)	0.0177 (6)	−0.0009 (4)	0.0033 (4)	−0.0013 (4)
N9	0.0224 (7)	0.0291 (8)	0.0156 (7)	0.000	−0.0041 (5)	0.000
C9a	0.0151 (5)	0.0279 (6)	0.0175 (5)	−0.0002 (4)	0.0032 (4)	0.0017 (5)

Geometric parameters (Å, º)

C1—C2	1.3847 (17)	C4—C4a	1.3970 (16)
C1—C9a	1.3946 (16)	C4—H4	0.9300
C1—H1	0.9300	C4a—C9a	1.4145 (15)
C2—C3	1.4006 (17)	C4a—C4a^i	1.453 (2)
C2—H2	0.9300	N9—C9a^i	1.3845 (13)
C3—C4	1.3830 (17)	N9—C9a	1.3845 (13)
C3—H3	0.9300	N9—H9	0.8600
C2—C1—C9a	117.34 (11)	C4a—C4—H4	120.6
C2—C1—H1	121.3	C4—C4a—C9a	119.43 (10)
C9a—C1—H1	121.3	C4—C4a—C4a^i	133.97 (7)
C1—C2—C3	121.60 (11)	C9a—C4a—C4a^i	106.60 (6)
C1—C2—H2	119.2	C9a^i—N9—C9a	109.52 (13)
C3—C2—H2	119.2	C9a^i—N9—H9	125.2
C4—C3—C2	120.93 (11)	C9a—N9—H9	125.2
C4—C3—H3	119.5	N9—C9a—C1	129.49 (10)
C2—C3—H3	119.5	N9—C9a—C4a	108.64 (10)
C3—C4—C4a	118.84 (11)	C1—C9a—C4a	121.85 (10)
C3—C4—H4	120.6
C9a—C1—C2—C3	0.04 (16)	C2—C1—C9a—N9	−179.08 (12)
C1—C2—C3—C4	0.35 (16)	C2—C1—C9a—C4a	−0.75 (16)
C2—C3—C4—C4a	−0.04 (16)	C4—C4a—C9a—N9	179.71 (10)
C3—C4—C4a—C9a	−0.64 (15)	C4a^i—C4a—C9a—N9	0.60 (9)
C3—C4—C4a—C4a^i	178.17 (6)	C4—C4a—C9a—C1	1.06 (15)
C9a^i—N9—C9a—C1	177.51 (8)	C4a^i—C4a—C9a—C1	−178.05 (8)
C9a^i—N9—C9a—C4a	−1.00 (16)

Symmetry code: (i) x, −y+3/2, z.

(II) 1-nitro-9H-carbazole

Crystal data

C12H8N2O2	F(000) = 440
Mr = 212.20	Dx = 1.497 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 1649 reflections
a = 10.4341 (9) Å	θ = 3.8–25.0°
b = 5.3101 (4) Å	µ = 0.11 mm^−1
c = 17.2566 (14) Å	T = 100 K
β = 99.951 (8)°	Plate, yellow
V = 941.74 (13) Å^3	0.38 × 0.25 × 0.07 mm
Z = 4

Data collection

Oxford Xcalibur diffractometer	1173 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.054
Graphite monochromator	θmax = 26.0°, θmin = 3.8°
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm^-1	h = −12→12
ω scans	k = −6→6
5978 measured reflections	l = −21→18
1860 independent reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.053	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.132	H-atom parameters not refined
S = 0.94	w = 1/[σ^2(Fo^2) + (0.0739P)^2] where P = (Fo^2 + 2Fc^2)/3
1860 reflections	(Δ/σ)max < 0.001
145 parameters	Δρmax = 0.32 e Å^−3
0 restraints	Δρmin = −0.21 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.7341 (2)	0.0698 (4)	0.01740 (14)	0.0238 (6)
C2	0.8384 (2)	−0.0852 (5)	0.01118 (14)	0.0278 (6)
H2	0.8875	−0.0564	−0.0281	0.033*
C3	0.8699 (2)	−0.2831 (5)	0.06321 (14)	0.0299 (6)
H3	0.9417	−0.3837	0.0598	0.036*
C4	0.7948 (2)	−0.3320 (5)	0.12047 (14)	0.0280 (6)
H4	0.8158	−0.4666	0.1548	0.034*
C4a	0.6891 (2)	−0.1816 (4)	0.12669 (13)	0.0231 (6)
C5a	0.5949 (2)	−0.1797 (4)	0.17898 (13)	0.0230 (6)
C5	0.5699 (2)	−0.3344 (5)	0.24004 (14)	0.0285 (6)
H5	0.6211	−0.4756	0.2546	0.034*
C6	0.4688 (3)	−0.2761 (5)	0.27848 (15)	0.0317 (7)
H6	0.4512	−0.3798	0.3188	0.038*
C7	0.3924 (3)	−0.0627 (5)	0.25766 (15)	0.0325 (7)
H7	0.3260	−0.0241	0.2854	0.039*
C8	0.4133 (2)	0.0919 (5)	0.19678 (14)	0.0282 (6)
H8	0.3612	0.2321	0.1823	0.034*
C8a	0.5140 (2)	0.0311 (4)	0.15823 (13)	0.0231 (6)
N9	0.55472 (18)	0.1503 (4)	0.09503 (11)	0.0237 (5)
H9	0.5194	0.2829	0.0720	0.028*
C9a	0.6585 (2)	0.0272 (4)	0.07485 (14)	0.0216 (5)
N10	0.7050 (2)	0.2799 (4)	−0.03676 (12)	0.0271 (5)
O11	0.60697 (16)	0.4037 (3)	−0.03166 (10)	0.0309 (5)
O12	0.77789 (18)	0.3239 (3)	−0.08412 (10)	0.0355 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0276 (13)	0.0182 (13)	0.0243 (12)	−0.0025 (11)	0.0004 (10)	−0.0016 (10)
C2	0.0291 (14)	0.0254 (14)	0.0289 (13)	−0.0040 (12)	0.0056 (11)	−0.0073 (11)
C3	0.0294 (15)	0.0195 (13)	0.0394 (15)	0.0035 (11)	0.0023 (12)	−0.0076 (11)
C4	0.0338 (15)	0.0161 (13)	0.0307 (13)	0.0013 (11)	−0.0040 (11)	−0.0038 (11)
C4a	0.0278 (14)	0.0130 (12)	0.0258 (12)	−0.0021 (10)	−0.0027 (10)	−0.0022 (10)
C5a	0.0307 (14)	0.0118 (11)	0.0244 (12)	−0.0025 (10)	−0.0014 (10)	−0.0038 (10)
C5	0.0392 (16)	0.0146 (13)	0.0280 (13)	−0.0051 (11)	−0.0046 (11)	0.0000 (10)
C6	0.0458 (17)	0.0219 (14)	0.0271 (13)	−0.0121 (12)	0.0056 (12)	0.0003 (11)
C7	0.0382 (16)	0.0281 (15)	0.0324 (14)	−0.0105 (12)	0.0094 (12)	−0.0097 (12)
C8	0.0328 (15)	0.0172 (13)	0.0343 (14)	−0.0044 (11)	0.0050 (11)	−0.0040 (11)
C8a	0.0287 (14)	0.0133 (12)	0.0262 (13)	−0.0049 (10)	0.0015 (10)	−0.0016 (10)
N9	0.0301 (12)	0.0144 (10)	0.0262 (10)	0.0010 (9)	0.0033 (9)	0.0029 (8)
C9a	0.0217 (12)	0.0161 (12)	0.0259 (12)	−0.0019 (10)	0.0006 (10)	−0.0028 (10)
N10	0.0304 (13)	0.0235 (12)	0.0267 (11)	−0.0066 (10)	0.0027 (10)	−0.0028 (9)
O11	0.0345 (11)	0.0220 (10)	0.0349 (10)	0.0025 (8)	0.0023 (8)	0.0037 (8)
O12	0.0466 (12)	0.0313 (11)	0.0306 (10)	−0.0080 (9)	0.0124 (9)	0.0008 (8)

Geometric parameters (Å, º)

C1—C2	1.384 (3)	C5—C6	1.374 (3)
C1—C9a	1.388 (3)	C5—H5	0.9300
C1—N10	1.453 (3)	C6—C7	1.396 (4)
C2—C3	1.384 (3)	C6—H6	0.9300
C2—H2	0.9300	C7—C8	1.380 (3)
C3—C4	1.387 (3)	C7—H7	0.9300
C3—H3	0.9300	C8—C8a	1.376 (3)
C4—C4a	1.380 (3)	C8—H8	0.9300
C4—H4	0.9300	C8a—N9	1.389 (3)
C4a—C9a	1.426 (3)	N9—C9a	1.361 (3)
C4a—C5a	1.445 (3)	N9—H9	0.8600
C5a—C5	1.396 (3)	N10—O12	1.231 (2)
C5a—C8a	1.411 (3)	N10—O11	1.232 (2)
C2—C1—C9a	120.7 (2)	C5—C6—C7	120.7 (2)
C2—C1—N10	119.1 (2)	C5—C6—H6	119.7
C9a—C1—N10	120.2 (2)	C7—C6—H6	119.7
C1—C2—C3	120.2 (2)	C8—C7—C6	121.3 (2)
C1—C2—H2	119.9	C8—C7—H7	119.3
C3—C2—H2	119.9	C6—C7—H7	119.3
C2—C3—C4	120.3 (2)	C8a—C8—C7	117.6 (2)
C2—C3—H3	119.9	C8a—C8—H8	121.2
C4—C3—H3	119.9	C7—C8—H8	121.2
C4a—C4—C3	120.3 (2)	C8—C8a—N9	129.4 (2)
C4a—C4—H4	119.9	C8—C8a—C5a	122.5 (2)
C3—C4—H4	119.9	N9—C8a—C5a	108.1 (2)
C4—C4a—C9a	119.7 (2)	C9a—N9—C8a	110.29 (19)
C4—C4a—C5a	133.8 (2)	C9a—N9—H9	124.9
C9a—C4a—C5a	106.5 (2)	C8a—N9—H9	124.9
C5—C5a—C8a	118.5 (2)	N9—C9a—C1	132.9 (2)
C5—C5a—C4a	134.7 (2)	N9—C9a—C4a	108.4 (2)
C8a—C5a—C4a	106.8 (2)	C1—C9a—C4a	118.8 (2)
C6—C5—C5a	119.4 (2)	O12—N10—O11	124.2 (2)
C6—C5—H5	120.3	O12—N10—C1	119.2 (2)
C5a—C5—H5	120.3	O11—N10—C1	116.6 (2)
C9a—C1—C2—C3	−1.0 (4)	C5—C5a—C8a—N9	−178.1 (2)
N10—C1—C2—C3	178.5 (2)	C4a—C5a—C8a—N9	1.2 (2)
C1—C2—C3—C4	1.9 (4)	C8—C8a—N9—C9a	−179.8 (2)
C2—C3—C4—C4a	−0.9 (4)	C5a—C8a—N9—C9a	−0.6 (2)
C3—C4—C4a—C9a	−1.1 (3)	C8a—N9—C9a—C1	−179.6 (2)
C3—C4—C4a—C5a	−179.0 (2)	C8a—N9—C9a—C4a	−0.3 (2)
C4—C4a—C5a—C5	−4.1 (5)	C2—C1—C9a—N9	178.2 (2)
C9a—C4a—C5a—C5	177.8 (3)	N10—C1—C9a—N9	−1.3 (4)
C4—C4a—C5a—C8a	176.8 (2)	C2—C1—C9a—C4a	−0.9 (3)
C9a—C4a—C5a—C8a	−1.4 (2)	N10—C1—C9a—C4a	179.6 (2)
C8a—C5a—C5—C6	−0.7 (3)	C4—C4a—C9a—N9	−177.4 (2)
C4a—C5a—C5—C6	−179.8 (2)	C5a—C4a—C9a—N9	1.1 (2)
C5a—C5—C6—C7	−0.8 (4)	C4—C4a—C9a—C1	2.0 (3)
C5—C6—C7—C8	1.8 (4)	C5a—C4a—C9a—C1	−179.6 (2)
C6—C7—C8—C8a	−1.3 (4)	C2—C1—N10—O12	−4.0 (3)
C7—C8—C8a—N9	178.9 (2)	C9a—C1—N10—O12	175.5 (2)
C7—C8—C8a—C5a	−0.2 (3)	C2—C1—N10—O11	176.1 (2)
C5—C5a—C8a—C8	1.2 (3)	C9a—C1—N10—O11	−4.4 (3)
C4a—C5a—C8a—C8	−179.5 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N9—H9···O11	0.86	2.24	2.702 (2)	114
N9—H9···O11^i	0.86	2.16	3.002 (3)	166

Symmetry code: (i) −x+1, −y+1, −z.

(III) 9-nitro-carbazole

Crystal data

C12H8N2O2	F(000) = 880
Mr = 212.20	Dx = 1.519 Mg m^−3
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 1637 reflections
a = 14.7742 (7) Å	θ = 3.3–25.0°
b = 7.2681 (3) Å	µ = 0.11 mm^−1
c = 17.2792 (8) Å	T = 100 K
V = 1855.44 (14) Å^3	Plate, yellow
Z = 8	0.40 × 0.12 × 0.07 mm

Data collection

Oxford Xcalibur diffractometer	1120 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.053
Graphite monochromator	θmax = 26.0°, θmin = 3.3°
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm^-1	h = −18→18
ω scans	k = −5→8
11599 measured reflections	l = −21→21
1822 independent reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.034	H-atom parameters not refined
wR(F^2) = 0.080	w = 1/[σ^2(Fo^2) + (0.0421P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.86	(Δ/σ)max < 0.001
1822 reflections	Δρmax = 0.24 e Å^−3
146 parameters	Δρmin = −0.18 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0035 (6)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.20019 (12)	0.2788 (2)	0.69948 (10)	0.0220 (4)
H1	0.2432	0.3145	0.7358	0.026*
C2	0.10767 (12)	0.2906 (2)	0.71459 (10)	0.0262 (4)
H2	0.0885	0.3336	0.7625	0.031*
C3	0.04371 (11)	0.2400 (2)	0.66019 (11)	0.0253 (4)
H3	−0.0175	0.2493	0.6721	0.030*
C4	0.06968 (11)	0.1759 (2)	0.58872 (10)	0.0215 (4)
H4	0.0265	0.1435	0.5520	0.026*
C4a	0.16127 (11)	0.1604 (2)	0.57218 (9)	0.0172 (4)
C5a	0.20890 (11)	0.0991 (2)	0.50338 (9)	0.0171 (4)
C5	0.17843 (11)	0.0336 (2)	0.43260 (9)	0.0204 (4)
H5	0.1167	0.0249	0.4225	0.024*
C6	0.24072 (11)	−0.0183 (2)	0.37750 (10)	0.0223 (4)
H6	0.2210	−0.0623	0.3299	0.027*
C7	0.33310 (12)	−0.0052 (2)	0.39268 (10)	0.0233 (4)
H7	0.3740	−0.0418	0.3548	0.028*
C8	0.36570 (11)	0.0606 (2)	0.46245 (10)	0.0214 (4)
H8	0.4275	0.0692	0.4722	0.026*
C8a	0.30204 (11)	0.1130 (2)	0.51729 (9)	0.0173 (4)
N9	0.31168 (9)	0.18282 (18)	0.59387 (8)	0.0187 (3)
C9a	0.22460 (10)	0.2116 (2)	0.62784 (10)	0.0179 (4)
N10	0.39241 (10)	0.22425 (19)	0.62893 (9)	0.0243 (4)
O11	0.46097 (7)	0.21016 (17)	0.58992 (8)	0.0329 (4)
O12	0.38957 (9)	0.27288 (18)	0.69655 (7)	0.0356 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0288 (10)	0.0184 (10)	0.0186 (9)	0.0014 (8)	−0.0022 (9)	0.0005 (8)
C2	0.0320 (11)	0.0275 (10)	0.0191 (9)	0.0057 (9)	0.0070 (9)	0.0014 (8)
C3	0.0206 (9)	0.0295 (10)	0.0257 (10)	0.0051 (8)	0.0067 (9)	0.0052 (9)
C4	0.0171 (9)	0.0254 (10)	0.0220 (10)	−0.0021 (7)	−0.0013 (8)	0.0028 (9)
C4a	0.0187 (9)	0.0154 (8)	0.0177 (9)	0.0005 (7)	−0.0005 (7)	0.0034 (7)
C5a	0.0170 (9)	0.0148 (9)	0.0194 (9)	0.0003 (7)	0.0003 (8)	0.0036 (7)
C5	0.0210 (10)	0.0188 (9)	0.0215 (10)	−0.0015 (7)	−0.0014 (8)	0.0034 (7)
C6	0.0313 (10)	0.0180 (9)	0.0176 (9)	−0.0009 (8)	−0.0015 (9)	0.0006 (7)
C7	0.0263 (10)	0.0202 (9)	0.0234 (10)	0.0046 (8)	0.0072 (8)	0.0019 (8)
C8	0.0181 (9)	0.0197 (9)	0.0265 (10)	0.0017 (7)	0.0039 (8)	0.0033 (8)
C8a	0.0189 (9)	0.0143 (9)	0.0188 (9)	−0.0003 (7)	0.0006 (8)	0.0024 (7)
N9	0.0148 (7)	0.0221 (8)	0.0191 (8)	−0.0001 (6)	−0.0028 (7)	0.0001 (7)
C9a	0.0173 (9)	0.0159 (9)	0.0204 (9)	0.0007 (7)	0.0009 (7)	0.0041 (8)
N10	0.0201 (8)	0.0238 (8)	0.0291 (9)	−0.0006 (7)	−0.0055 (8)	0.0020 (7)
O11	0.0146 (7)	0.0414 (8)	0.0427 (8)	0.0018 (6)	0.0016 (6)	−0.0042 (7)
O12	0.0331 (8)	0.0438 (9)	0.0298 (8)	−0.0013 (7)	−0.0109 (7)	−0.0044 (7)

Geometric parameters (Å, º)

C1—C9a	1.379 (2)	C5—C6	1.377 (2)
C1—C2	1.394 (2)	C5—H5	0.9300
C1—H1	0.9300	C6—C7	1.393 (2)
C2—C3	1.383 (2)	C6—H6	0.9300
C2—H2	0.9300	C7—C8	1.384 (2)
C3—C4	1.374 (2)	C7—H7	0.9300
C3—H3	0.9300	C8—C8a	1.389 (2)
C4—C4a	1.388 (2)	C8—H8	0.9300
C4—H4	0.9300	C8a—N9	1.424 (2)
C4a—C9a	1.392 (2)	N9—N10	1.3712 (18)
C4a—C5a	1.452 (2)	N9—C9a	1.430 (2)
C5a—C5	1.387 (2)	N10—O11	1.2209 (17)
C5a—C8a	1.401 (2)	N10—O12	1.2216 (17)
C9a—C1—C2	116.51 (16)	C5—C6—C7	120.41 (17)
C9a—C1—H1	121.7	C5—C6—H6	119.8
C2—C1—H1	121.7	C7—C6—H6	119.8
C3—C2—C1	121.76 (16)	C8—C7—C6	121.91 (16)
C3—C2—H2	119.1	C8—C7—H7	119.0
C1—C2—H2	119.1	C6—C7—H7	119.0
C4—C3—C2	120.67 (16)	C7—C8—C8a	116.99 (15)
C4—C3—H3	119.7	C7—C8—H8	121.5
C2—C3—H3	119.7	C8a—C8—H8	121.5
C3—C4—C4a	119.01 (16)	C8—C8a—C5a	121.90 (15)
C3—C4—H4	120.5	C8—C8a—N9	131.61 (15)
C4a—C4—H4	120.5	C5a—C8a—N9	106.47 (14)
C4—C4a—C9a	119.42 (15)	N10—N9—C8a	125.15 (13)
C4—C4a—C5a	131.79 (16)	N10—N9—C9a	124.70 (13)
C9a—C4a—C5a	108.78 (14)	C8a—N9—C9a	110.09 (13)
C5—C5a—C8a	119.67 (15)	C1—C9a—C4a	122.61 (15)
C5—C5a—C4a	132.06 (15)	C1—C9a—N9	130.99 (15)
C8a—C5a—C4a	108.27 (14)	C4a—C9a—N9	106.39 (14)
C6—C5—C5a	119.12 (16)	O11—N10—O12	125.51 (15)
C6—C5—H5	120.4	O11—N10—N9	117.34 (14)
C5a—C5—H5	120.4	O12—N10—N9	117.15 (14)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1···O12	0.93	2.29	2.799 (2)	114
C8—H8···O11	0.93	2.33	2.831 (2)	113
C3—H3···O12^i	0.93	2.66	3.372 (2)	134

Symmetry code: (i) x−1/2, y, −z+3/2.