4-Nitro­anilinium nitrate

The crystal structure of the title compound, C₆H₇N₂O₂⁺·NO₃^-, is built up from 4-nitro­anilinium cations and nitrate anions. The NO₂ group is coplanar with the aryl ring, which shows significant distortion from the ideal hexagonal form. The NO₃^- anion is planar but shows distortion from the C_3h symmetry that is predicted by molecular orbital calculations. Two of the three O atoms of the NO₃ group are involved in hydrogen bonds as acceptors.

S1. Comment

The present study is a continuation of our investigations characterizing the hydrogen-bonding system formed by self-assembly of components containing complementary arrays of hydrogen-bonding sites (Janczak & Perpétuo, 2004, and references therein; Desiraju, 1990; Krische & Lehn, 2000; Sherington & Taskinen, 2001). To expand our understanding of the solid-state physical-organic chemistry of compounds containing N—H···O hydrogen-bonding systems, we present here the solid-state structure of p-nitroanilinium mitrate, (I), and compare the results with that predicted for isolated oppositely charged parts of the crystal of (I), i.e. the 4-nitroanilinium cation and the nitrate anion, by ab-initio fully optimized geometry calculation at the HF/6–31 G(d,p) level (Frisch et al., 1998). The molecular orbital calculations were carried out on isolated ions corresponding to the gas phase, and the results are shown in Fig. 1.

As revealed by X-ray structure analysis, the nitro group is coplanar with the aryl ring, which shows significant distortions from the ideal C_{6 h} symmetry (Table 1). The C—C bond lengths within the ring are consistent with those found in other molecules of this type (Allen, 2002). The two internal C—C—C angles in the ring at the substituted atoms C1 and C4 are greater than 120°, while the other four C—C—C angles within the ring are smaller than 120°. These angular differences have been attributed to the substitution effect of the NH₃⁺ and NO₂ groups in the ring, respectively, at the 1- and 4-positions. The optimized geometry of the 4-nitroanilinium cation shows values similar to those found in the crystal, but the variation of the internal C—C—C angles is more pronounced. The calculated C—N_amine bond length is greater than the X-ray value, while the calculated C—N_nitro bond is slightly less than the X-ray value. The O—N—O angle in the NO₂ group is significantly greater than 120° as a result of the steric effect of the lone-pair of electrons on both O atoms that is predicted by the valence-shell electron-pair repulsion theory (Gillespie, 1963, 1992). The steric effect of the lone-pair of electrons is further evidenced in the ab-initio fully optimized geometry. The X-ray geometry of the nitrate anion shows significant distortion from the D_{3 h} symmetry obtained by molecular orbital calculations (the three N—O bonds in the isolated NO₃⁻ ion are equivalent, with a distance of 1.226 Å).

The N—O bonds in the nitro group are slightly shorter than those in the anion. These values indicate a bond order of 2 in the nitro group, while in the NO₃⁻ anion the bond order is smaller than 2 because of the delocalization of the two π bonds over three N—O bonds. The experimental and calculated N—O bond lengths in the nitro group are comparable, since in the crystal the nitro O atoms do not form any hydrogen bonds. Two of the N—O bonds of the anion (to atoms O1 and O3) are longer in the crystal than calculated for the gas phase, since they are involved as acceptors in two hydrogen bonds. The third N—O bond length (to atom O2) is similar as calculated from both X-ray and ab-inito methods, since atom O2 does not form hydrogen bonds (Fig. 3).

In the crystal of (I), the oppositely charged 4-nitroanilinum cations and nitrate anions related by a 2₁ screw axis interact via N—H···O hydrogen bonds, forming chains parallel to [010] (Fig. 4) in which the 4-nitroanilinium cations resemble the branches of fir trees. The cations of adjacent chains related by inversion are interdigitated, with parallel rings. The hydrogen-bonding interactions are responsible for the relatively high density of the crystal.

S2. Experimental

4-Nitroaniline was dissolved in 10% aqueous nitric acid; after several days, colourless single crystals formed, which proved to be suitable for single-crystal X-ray diffraction.

S3. Refinement

All H atoms were treated as riding atoms, with C—H distances of 0.93 Å and N—H distances of 0.89 Å, and with U_iso(H) = 1.2 U_eq(C) and 1.3 U_eq(N).

Computing details

Data collection: KM-4 Software (Kuma, 2001; cell refinement: KM-4 Sfotware; data reduction: KM-4 Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Sheldrick, 1990); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The results of the optimized molecular orbital calculations (Å, °) for the p-nitroanilinium cation.
	Fig. 2. A view of the molecular structure of (I), showing displacement ellipsoids at the 50% probability level and H atoms as spheres of arbitrary radii.
	Fig. 3. A view of one [010] chain showing the cation–anion hydrogen bonds. For the sake of clarity, H atoms bonded to C atoms have been omitted, along with the unit-cell box. The symmetry codes are as given in Table 2.
	Fig. 4. A view of the crystal packing, showing the interdigited cation in adjacent chains. For the sake of clarity, the H atoms bonded to C atoms have been omitted.

p-nitroanilinium nitrate

Crystal data

C6H7N2O2^+·NO3^−	F(000) = 416
Mr = 201.15	Dx = 1.568 Mg m^−3 Dm = 1.56 Mg m^−3 Dm measured by floatation in a mixture of CHCl3/CHBr3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 1950 reflections
a = 5.644 (1) Å	θ = 2.9–29°
b = 9.682 (2) Å	µ = 0.14 mm^−1
c = 15.662 (3) Å	T = 293 K
β = 95.23 (2)°	Parallelepiped, pink
V = 852.3 (3) Å^3	0.37 × 0.28 × 0.22 mm
Z = 4

Data collection

Kuma KM-4 CCD area-detector diffractometer	2254 independent reflections
Radiation source: fine-focus sealed tube	1950 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.014
Detector resolution: 1024x1024 with blocks 2x2 pixels mm^-1	θmax = 29.0°, θmin = 3.4°
ω–scan	h = −7→7
Absorption correction: analytical face-indexed (SHELXTL; Sheldrick, 1990)	k = −13→13
Tmin = 0.941, Tmax = 0.958	l = −20→20
11065 measured 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)] = 0.041	H-atom parameters constrained
wR(F^2) = 0.113	w = 1/[σ^2(Fo^2) + (0.0691P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.00	(Δ/σ)max < 0.001
2254 reflections	Δρmax = 0.18 e Å^−3
129 parameters	Δρmin = −0.16 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.032 (5)

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
N1	0.57973 (17)	0.07943 (10)	0.18706 (6)	0.0483 (3)
H1N	0.6949	0.0163	0.1909	0.063*
H2N	0.4390	0.0375	0.1849	0.063*
H3N	0.5965	0.1345	0.2327	0.063*
C1	0.59408 (18)	0.16131 (11)	0.10951 (7)	0.0417 (3)
C2	0.4091 (2)	0.15860 (13)	0.04584 (7)	0.0524 (3)
H2	0.2780	0.1021	0.0511	0.063*
C3	0.4197 (2)	0.24044 (14)	−0.02582 (7)	0.0549 (3)
H3	0.2957	0.2405	−0.0692	0.066*
C4	0.6166 (2)	0.32171 (11)	−0.03186 (7)	0.0453 (3)
C5	0.8028 (2)	0.32354 (13)	0.03094 (8)	0.0539 (3)
H5	0.9358	0.3782	0.0251	0.065*
C6	0.7892 (2)	0.24293 (13)	0.10276 (8)	0.0523 (3)
H6	0.9122	0.2440	0.1465	0.063*
N2	0.6261 (2)	0.41095 (11)	−0.10746 (7)	0.0574 (3)
O21	0.4425 (2)	0.43157 (11)	−0.15285 (6)	0.0750 (3)
O22	0.8194 (2)	0.46025 (14)	−0.12143 (7)	0.0914 (4)
N3	0.41754 (17)	0.36708 (10)	0.29761 (6)	0.0460 (3)
O1	0.62221 (14)	0.32257 (9)	0.28949 (5)	0.0580 (3)
O2	0.24146 (16)	0.30365 (10)	0.26838 (7)	0.0599 (3)
O3	0.39947 (18)	0.47839 (10)	0.33559 (7)	0.0577 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0503 (5)	0.0495 (6)	0.0440 (5)	0.0070 (4)	−0.0009 (4)	0.0007 (4)
C1	0.0398 (5)	0.0440 (6)	0.0408 (6)	0.0040 (4)	0.0015 (4)	−0.0053 (4)
C2	0.0426 (6)	0.0671 (8)	0.0463 (6)	−0.0107 (5)	−0.0028 (5)	0.0043 (5)
C3	0.0464 (6)	0.0733 (8)	0.0428 (6)	−0.0071 (6)	−0.0069 (5)	0.0025 (6)
C4	0.0497 (6)	0.0453 (6)	0.0407 (6)	−0.0009 (5)	0.0028 (5)	−0.0038 (4)
C5	0.0496 (6)	0.0516 (7)	0.0593 (7)	−0.0128 (5)	−0.0020 (5)	−0.0007 (5)
C6	0.0437 (6)	0.0577 (7)	0.0529 (7)	−0.0041 (5)	−0.0088 (5)	−0.0015 (5)
N2	0.0782 (8)	0.0494 (6)	0.0447 (6)	−0.0094 (5)	0.0059 (5)	−0.0016 (4)
O21	0.0889 (7)	0.0741 (7)	0.0600 (6)	0.0085 (5)	−0.0042 (5)	0.0112 (5)
O22	0.1047 (9)	0.1016 (9)	0.0669 (7)	−0.0538 (7)	0.0034 (6)	0.0118 (6)
N3	0.0485 (5)	0.0435 (5)	0.0454 (5)	−0.0014 (4)	0.0005 (4)	0.0035 (4)
O1	0.0499 (5)	0.0647 (6)	0.0581 (5)	0.0124 (4)	−0.0013 (4)	−0.0060 (4)
O2	0.0523 (6)	0.0535 (6)	0.0698 (8)	−0.0190 (4)	−0.0161 (5)	0.0105 (5)
O3	0.0603 (7)	0.0484 (6)	0.0651 (7)	0.0082 (5)	0.0111 (5)	−0.0234 (5)

Geometric parameters (Å, º)

N1—C1	1.4590 (14)	C4—C5	1.3726 (16)
N1—H1N	0.8900	C4—N2	1.4707 (15)
N1—H2N	0.8900	C5—C6	1.3770 (17)
N1—H3N	0.8900	C5—H5	0.9300
C1—C6	1.3672 (16)	C6—H6	0.9300
C1—C2	1.3766 (15)	N2—O21	1.2182 (14)
C2—C3	1.3797 (16)	N2—O22	1.2287 (14)
C2—H2	0.9300	N3—O2	1.2212 (12)
C3—C4	1.3719 (16)	N3—O3	1.2396 (13)
C3—H3	0.9300	N3—O1	1.2502 (12)
C1—N1—H1N	109.5	C3—C4—C5	122.02 (10)
C1—N1—H2N	109.5	C3—C4—N2	118.78 (10)
H1N—N1—H2N	109.5	C5—C4—N2	119.19 (10)
C1—N1—H3N	109.5	C4—C5—C6	118.81 (10)
H1N—N1—H3N	109.5	C4—C5—H5	120.6
H2N—N1—H3N	109.5	C6—C5—H5	120.6
C6—C1—C2	121.19 (10)	C1—C6—C5	119.75 (10)
C6—C1—N1	118.99 (10)	C1—C6—H6	120.1
C2—C1—N1	119.78 (10)	C5—C6—H6	120.1
C1—C2—C3	119.48 (11)	O21—N2—O22	123.65 (11)
C1—C2—H2	120.3	O21—N2—C4	118.35 (11)
C3—C2—H2	120.3	O22—N2—C4	118.00 (12)
C4—C3—C2	118.74 (10)	O2—N3—O3	121.15 (11)
C4—C3—H3	120.6	O2—N3—O1	121.09 (10)
C2—C3—H3	120.6	O3—N3—O1	117.75 (10)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H3N···O1	0.89	2.03	2.8465 (13)	153
N1—H1N···O1^i	0.89	2.15	3.0061 (13)	161
N1—H1N···O3^i	0.89	2.39	3.1485 (15)	143
N1—H2N···O3^ii	0.89	1.99	2.8676 (14)	168

Symmetry codes: (i) −x+3/2, y−1/2, −z+1/2; (ii) −x+1/2, y−1/2, −z+1/2.