4-Amino-2-chloro­benzoic acid

Khan, M.H.; Khan, I.U.; Akkurt, M.

doi:10.1107/S1600536811030728

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 67| Part 9| September 2011| Page o2247

doi:10.1107/S1600536811030728

Open

access

4-Amino-2-chlorobenzoic acid

Muneeb Hayat Khan,^a Islam Ullah Khan ^a and Mehmet Akkurt ^b ^*

^aMaterials Chemistry Laboratory, Department of Chemistry, GC University, Lahore 54000, Pakistan, and ^bDepartment of Physics, Faculty of Sciences, Erciyes University, 38039 Kayseri, Turkey
^*Correspondence e-mail: akkurt@erciyes.edu.tr

(Received 25 July 2011; accepted 30 July 2011; online 6 August 2011)

The title compound, C₇H₆ClNO₂, crystallizes with two roughly planar molecules in the asymmetric unit (r.m.s. deviations = 0.073 and 0.074 Å). The amine H atoms of the two molecules have opposite orientations. In the crystal, molecules are linked into dimers by pairs of O—H⋯O hydrogen bonds, generating R₂²(8) loops. N—H⋯N and N—H⋯Cl hydrogen bonds link the dimers into a three-dimensional network. The crystal studied was found to be a racemic twin.

Related literature

For an isomer (2-amino-4-chlorobenzoic acid) of the title compound, see: Farag et al. (2011 ). For hydrogen-bond motifs, see: Bernstein et al. (1995 ).

Experimental

Crystal data

C₇H₆ClNO₂
M_r = 171.58
Monoclinic, P 2₁
a = 3.9595 (2) Å
b = 22.6656 (11) Å
c = 8.0285 (4) Å
β = 104.257 (2)°
V = 698.32 (6) Å³
Z = 4
Mo Kα radiation
μ = 0.49 mm⁻¹
T = 296 K
0.28 × 0.13 × 0.12 mm

Data collection

Bruker APEXII CCD diffractometer
3009 measured reflections
2295 independent reflections
2197 reflections with I > 2σ(I)
R_int = 0.011

Refinement

R[F² > 2σ(F²)] = 0.029
wR(F²) = 0.077
S = 1.07
2295 reflections
218 parameters
7 restraints
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.21 e Å⁻³
Δρ_min = −0.23 e Å⁻³
Absolute structure: Flack (1983 ), 514 Freidel pairs
Flack parameter: 0.50 (6)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—HO1⋯O4ⁱ	0.82 (3)	1.84 (3)	2.650 (3)	171 (3)
N1—HN2⋯N2	0.86 (3)	2.60 (3)	3.375 (4)	150 (3)
O3—HO3⋯O2ⁱⁱ	0.81 (3)	1.84 (3)	2.650 (3)	173 (3)
N2—HN3⋯Cl2ⁱⁱⁱ	0.86 (3)	2.81 (3)	3.374 (2)	125 (2)
N2—HN4⋯N1^iv	0.86 (3)	2.47 (3)	3.302 (4)	163 (2)
Symmetry codes: (i) $[-x, y-{\script{1\over 2}}, -z+1]$ ; (ii) $[-x, y+{\script{1\over 2}}, -z+1]$ ; (iii) x, y, z+1; (iv) x+1, y, z.

Data collection: APEX2 (Bruker, 2007 ); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT; program(s) used to solve structure: SIR97 (Altomare et al., 1999 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997 ); software used to prepare material for publication: WinGX (Farrugia, 1999 ) and PLATON (Spek, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536811030728/hb6334sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536811030728/hb6334Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536811030728/hb6334Isup3.cml

checkCIF report

Comment top

The title compound, (I), is used as a starting material for the synthesis of various sulphonamides.

As shown in Fig. 1, the asymmetric unit of the title compound contains two independent molecules. Both have the bond lengths and angles as expected for a molecule of this kind (Farag et al., 2011).

The amine H atoms of the two molecules have opposite orientations. In the crystal, the molecules form dimers via intermolecular O—H···O hydrogen bonds, forming a graph-set motif R²₂(8) (Bernstein et al., 1995; Table 1, Fig. 2). Furthermore, C—H···O, N—H···N and N—H···Cl interactions stabilize the crystal structure.

Related literature top

For an isomer (2-amino-4-chlorobenzoic acid) of the title compound, see: Farag et al. (2011). For hydrogen-bond motifs, see: Bernstein et al. (1995).

Experimental top

To a 100-ml round bottom flask equipped with a reflux condenser, was placed 0.5 g (2.486 mmol) of 2-chloro-4-amino benzoic acid and 0.447 g m of granulated tin. Then, 30 ml of concentrated HCl in was added in six intervals (5 ml each time with cooling in ice). The reaction is highly exothermic and the reaction mixture is keept under control by keeping it in ice water. When all the HCl was added and the temprature of the reaction mixture was stable, the round bottom flask was placed on water bath for 90 min under reflux.

TLC check after 90 min showed completion of reaction. Reaction mixtures was treated with 60% NaOH solution followed by the addition of NaCl solution and extration with di-ethyl ether.

Diethyl ether was evaporated on rotary evaporator and reddish brown precipates of the required product were obtained. This was recrystallized in methanol to yield reddish brown prisms of (I).

Computing details top

Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999) and PLATON (Spek, 2009).

Figures top

	Fig. 1. The molecule of the title compound, with displacement ellipsoids for non-H atoms drawn at the 50% probability level.
	Fig. 2. Partial view of the dimers by two O—H···O hydrogen bonds and the packing in the crystal. Hydrogen atoms that not involved in the hydrogen-bonding (dashed lines) have been omitted for clarity.

4-Amino-2-chlorobenzoic acid top

Crystal data top

C₇H₆ClNO₂	F(000) = 352
M_r = 171.58	D_x = 1.632 Mg m⁻³
Monoclinic, P2₁	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2yb	Cell parameters from 3211 reflections
a = 3.9595 (2) Å	θ = 2.8–28.3°
b = 22.6656 (11) Å	µ = 0.49 mm⁻¹
c = 8.0285 (4) Å	T = 296 K
β = 104.257 (2)°	Prism, reddish brown
V = 698.32 (6) Å³	0.28 × 0.13 × 0.12 mm
Z = 4

Data collection top

Bruker APEXII CCD diffractometer	2197 reflections with I > 2σ(I)
Radiation source: sealed tube	R_int = 0.011
Graphite monochromator	θ_max = 28.3°, θ_min = 2.8°
ϕ and ω scans	h = −5→5
3009 measured reflections	k = −18→30
2295 independent reflections	l = −10→10

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.029	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.077	w = 1/[σ²(F_o²) + (0.0454P)² + 0.092P] where P = (F_o² + 2F_c²)/3
S = 1.07	(Δ/σ)_max < 0.001
2295 reflections	Δρ_max = 0.21 e Å⁻³
218 parameters	Δρ_min = −0.23 e Å⁻³
7 restraints	Absolute structure: Flack (1983), 514 Freidel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.50 (6)

Crystal data top

C₇H₆ClNO₂	V = 698.32 (6) Å³
M_r = 171.58	Z = 4
Monoclinic, P2₁	Mo Kα radiation
a = 3.9595 (2) Å	µ = 0.49 mm⁻¹
b = 22.6656 (11) Å	T = 296 K
c = 8.0285 (4) Å	0.28 × 0.13 × 0.12 mm
β = 104.257 (2)°

Data collection top

Bruker APEXII CCD diffractometer	2197 reflections with I > 2σ(I)
3009 measured reflections	R_int = 0.011
2295 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.029	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.077	Δρ_max = 0.21 e Å⁻³
S = 1.07	Δρ_min = −0.23 e Å⁻³
2295 reflections	Absolute structure: Flack (1983), 514 Freidel pairs
218 parameters	Absolute structure parameter: 0.50 (6)
7 restraints

Special details top

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles

Refinement. Refinement on F² for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The observed criterion of F² > σ(F²) is used only for calculating -R-factor-obs 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 (Å²) top

	x	y	z	U_iso*/U_eq
Cl1	0.23950 (15)	0.81998 (3)	0.56801 (6)	0.0427 (2)
O1	−0.2726 (5)	0.65233 (9)	0.6710 (2)	0.0502 (6)
O2	−0.0913 (6)	0.70711 (10)	0.4820 (2)	0.0577 (7)
N1	0.5447 (5)	0.83421 (11)	1.2139 (3)	0.0431 (7)
C1	0.2193 (5)	0.78948 (10)	0.7631 (3)	0.0277 (5)
C2	0.3803 (5)	0.82200 (12)	0.9065 (3)	0.0316 (5)
C3	0.3834 (5)	0.80158 (10)	1.0704 (3)	0.0306 (6)
C4	0.2106 (6)	0.74934 (11)	1.0871 (3)	0.0349 (6)
C5	0.0515 (6)	0.71758 (11)	0.9432 (3)	0.0338 (6)
C6	0.0508 (5)	0.73617 (10)	0.7767 (3)	0.0294 (6)
C7	−0.1085 (5)	0.69776 (11)	0.6301 (3)	0.0341 (6)
Cl2	0.80948 (15)	0.96180 (3)	0.66956 (7)	0.0442 (2)
O3	0.3027 (6)	1.13060 (8)	0.7718 (2)	0.0511 (6)
O4	0.4834 (7)	1.07555 (10)	0.5823 (2)	0.0581 (7)
N2	1.1098 (6)	0.94780 (12)	1.3131 (3)	0.0491 (8)
C8	0.7883 (5)	0.99282 (10)	0.8643 (3)	0.0293 (5)
C9	0.9466 (5)	0.96044 (12)	1.0074 (3)	0.0321 (5)
C10	0.9516 (5)	0.98080 (11)	1.1714 (3)	0.0349 (6)
C11	0.7808 (6)	1.03355 (12)	1.1881 (3)	0.0356 (6)
C12	0.6218 (6)	1.06497 (11)	1.0437 (3)	0.0347 (6)
C13	0.6238 (5)	1.04643 (10)	0.8776 (3)	0.0295 (6)
C14	0.4640 (5)	1.08498 (11)	0.7309 (3)	0.0336 (6)
HO1	−0.335 (8)	0.6315 (13)	0.585 (3)	0.0640*
H2	0.48700	0.85770	0.89330	0.0380*
HN1	0.607 (7)	0.8126 (13)	1.303 (3)	0.0510*
HN2	0.707 (6)	0.8583 (11)	1.203 (4)	0.0510*
H4	0.20270	0.73590	1.19550	0.0420*
H5	−0.05990	0.68240	0.95690	0.0410*
HO3	0.220 (8)	1.1532 (14)	0.694 (4)	0.0660*
HN3	1.163 (7)	0.9675 (13)	1.407 (3)	0.0530*
HN4	1.262 (6)	0.9232 (11)	1.296 (4)	0.0530*
H9	1.05120	0.92460	0.99420	0.0390*
H11	0.77480	1.04740	1.29640	0.0430*
H12	0.50820	1.09990	1.05690	0.0420*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.0554 (3)	0.0449 (3)	0.0274 (2)	−0.0095 (3)	0.0097 (2)	0.0044 (2)
O1	0.0723 (11)	0.0413 (11)	0.0365 (9)	−0.0205 (9)	0.0123 (8)	−0.0066 (8)
O2	0.0901 (14)	0.0528 (12)	0.0312 (8)	−0.0299 (11)	0.0170 (9)	−0.0109 (9)
N1	0.0473 (10)	0.0514 (15)	0.0276 (9)	−0.0042 (9)	0.0033 (8)	−0.0061 (9)
C1	0.0280 (8)	0.0302 (11)	0.0252 (9)	0.0023 (8)	0.0073 (7)	0.0022 (8)
C2	0.0313 (8)	0.0335 (11)	0.0290 (9)	0.0005 (9)	0.0058 (7)	−0.0018 (10)
C3	0.0292 (8)	0.0340 (12)	0.0281 (9)	0.0038 (8)	0.0061 (7)	−0.0020 (8)
C4	0.0407 (10)	0.0384 (13)	0.0267 (10)	0.0060 (9)	0.0103 (9)	0.0025 (9)
C5	0.0384 (10)	0.0301 (12)	0.0332 (11)	−0.0001 (9)	0.0097 (8)	0.0019 (9)
C6	0.0316 (9)	0.0288 (11)	0.0274 (10)	0.0026 (8)	0.0068 (7)	0.0004 (8)
C7	0.0355 (9)	0.0323 (12)	0.0328 (11)	−0.0013 (8)	0.0053 (8)	−0.0034 (9)
Cl2	0.0540 (3)	0.0475 (3)	0.0305 (3)	0.0080 (3)	0.0091 (2)	−0.0060 (3)
O3	0.0750 (12)	0.0388 (11)	0.0384 (9)	0.0208 (10)	0.0121 (8)	0.0072 (9)
O4	0.0890 (14)	0.0535 (12)	0.0324 (9)	0.0303 (11)	0.0163 (9)	0.0112 (9)
N2	0.0545 (11)	0.0603 (17)	0.0320 (10)	0.0120 (11)	0.0095 (9)	0.0132 (11)
C8	0.0304 (8)	0.0325 (11)	0.0253 (9)	−0.0041 (8)	0.0073 (7)	−0.0026 (9)
C9	0.0319 (8)	0.0304 (10)	0.0335 (9)	0.0030 (9)	0.0072 (7)	0.0026 (10)
C10	0.0322 (9)	0.0419 (13)	0.0298 (10)	−0.0042 (9)	0.0062 (8)	0.0083 (9)
C11	0.0419 (10)	0.0379 (13)	0.0266 (10)	−0.0022 (9)	0.0077 (9)	−0.0037 (9)
C12	0.0421 (10)	0.0308 (11)	0.0314 (10)	−0.0001 (9)	0.0094 (8)	−0.0029 (9)
C13	0.0310 (9)	0.0290 (11)	0.0278 (10)	−0.0019 (8)	0.0057 (8)	0.0033 (8)
C14	0.0391 (10)	0.0322 (12)	0.0291 (10)	0.0004 (9)	0.0078 (8)	0.0039 (9)

Geometric parameters (Å, º) top

Cl1—C1	1.732 (2)	C3—C4	1.390 (3)
Cl2—C8	1.735 (2)	C4—C5	1.375 (3)
O1—C7	1.302 (3)	C5—C6	1.401 (3)
O2—C7	1.226 (3)	C6—C7	1.475 (3)
O1—HO1	0.82 (3)	C2—H2	0.9300
O3—C14	1.299 (3)	C4—H4	0.9300
O4—C14	1.233 (3)	C5—H5	0.9300
O3—HO3	0.81 (3)	C8—C9	1.377 (3)
N1—C3	1.386 (3)	C8—C13	1.395 (3)
N1—HN1	0.85 (3)	C9—C10	1.391 (3)
N1—HN2	0.86 (3)	C10—C11	1.396 (4)
N2—C10	1.377 (3)	C11—C12	1.374 (3)
N2—HN4	0.86 (3)	C12—C13	1.400 (3)
N2—HN3	0.86 (3)	C13—C14	1.478 (3)
C1—C6	1.398 (3)	C9—H9	0.9300
C1—C2	1.383 (3)	C11—H11	0.9300
C2—C3	1.392 (3)	C12—H12	0.9300

C7—O1—HO1	108 (2)	C5—C4—H4	120.00
C14—O3—HO3	116 (2)	C3—C4—H4	120.00
C3—N1—HN1	111.7 (19)	C4—C5—H5	119.00
HN1—N1—HN2	112 (3)	C6—C5—H5	119.00
C3—N1—HN2	117 (2)	C9—C8—C13	121.7 (2)
C10—N2—HN4	115 (2)	Cl2—C8—C9	115.01 (18)
HN3—N2—HN4	117 (3)	Cl2—C8—C13	123.30 (18)
C10—N2—HN3	114.0 (19)	C8—C9—C10	120.7 (2)
Cl1—C1—C2	115.19 (18)	N2—C10—C11	121.2 (2)
Cl1—C1—C6	123.03 (18)	C9—C10—C11	118.7 (2)
C2—C1—C6	121.8 (2)	N2—C10—C9	120.0 (2)
C1—C2—C3	120.3 (2)	C10—C11—C12	119.7 (2)
C2—C3—C4	119.0 (2)	C11—C12—C13	122.6 (2)
N1—C3—C4	120.8 (2)	C8—C13—C12	116.5 (2)
N1—C3—C2	120.2 (2)	C8—C13—C14	124.8 (2)
C3—C4—C5	120.0 (2)	C12—C13—C14	118.7 (2)
C4—C5—C6	122.5 (2)	O3—C14—C13	114.2 (2)
C5—C6—C7	118.9 (2)	O4—C14—C13	123.5 (2)
C1—C6—C7	124.5 (2)	O3—C14—O4	122.3 (2)
C1—C6—C5	116.5 (2)	C8—C9—H9	120.00
O2—C7—C6	123.8 (2)	C10—C9—H9	120.00
O1—C7—C6	114.0 (2)	C10—C11—H11	120.00
O1—C7—O2	122.1 (2)	C12—C11—H11	120.00
C1—C2—H2	120.00	C11—C12—H12	119.00
C3—C2—H2	120.00	C13—C12—H12	119.00

Cl1—C1—C2—C3	−179.44 (17)	Cl2—C8—C9—C10	179.22 (17)
C6—C1—C2—C3	1.1 (3)	C13—C8—C9—C10	−0.9 (3)
Cl1—C1—C6—C5	−179.00 (17)	Cl2—C8—C13—C12	178.57 (17)
Cl1—C1—C6—C7	4.2 (3)	Cl2—C8—C13—C14	−3.3 (3)
C2—C1—C6—C5	0.4 (3)	C9—C8—C13—C12	−1.3 (3)
C2—C1—C6—C7	−176.4 (2)	C9—C8—C13—C14	176.8 (2)
C1—C2—C3—N1	−180.0 (2)	C8—C9—C10—N2	179.8 (2)
C1—C2—C3—C4	−2.5 (3)	C8—C9—C10—C11	2.6 (3)
N1—C3—C4—C5	179.9 (2)	N2—C10—C11—C12	−179.2 (2)
C2—C3—C4—C5	2.5 (3)	C9—C10—C11—C12	−2.0 (3)
C3—C4—C5—C6	−1.0 (4)	C10—C11—C12—C13	−0.2 (4)
C4—C5—C6—C1	−0.5 (3)	C11—C12—C13—C8	1.9 (3)
C4—C5—C6—C7	176.5 (2)	C11—C12—C13—C14	−176.4 (2)
C1—C6—C7—O1	−175.4 (2)	C8—C13—C14—O3	175.1 (2)
C1—C6—C7—O2	4.4 (4)	C8—C13—C14—O4	−5.7 (4)
C5—C6—C7—O1	7.8 (3)	C12—C13—C14—O3	−6.8 (3)
C5—C6—C7—O2	−172.3 (2)	C12—C13—C14—O4	172.4 (2)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—HO1···O4ⁱ	0.82 (3)	1.84 (3)	2.650 (3)	171 (3)
N1—HN2···N2	0.86 (3)	2.60 (3)	3.375 (4)	150 (3)
O3—HO3···O2ⁱⁱ	0.81 (3)	1.84 (3)	2.650 (3)	173 (3)
N2—HN3···Cl2ⁱⁱⁱ	0.86 (3)	2.81 (3)	3.374 (2)	125 (2)
N2—HN4···N1^iv	0.86 (3)	2.47 (3)	3.302 (4)	163 (2)

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

Experimental details

Crystal data
Chemical formula	C₇H₆ClNO₂
M_r	171.58
Crystal system, space group	Monoclinic, P2₁
Temperature (K)	296
a, b, c (Å)	3.9595 (2), 22.6656 (11), 8.0285 (4)
β (°)	104.257 (2)
V (Å³)	698.32 (6)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.49
Crystal size (mm)	0.28 × 0.13 × 0.12

Data collection
Diffractometer	Bruker APEXII CCD diffractometer
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	3009, 2295, 2197
R_int	0.011
(sin θ/λ)_max (Å⁻¹)	0.668

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.029, 0.077, 1.07
No. of reflections	2295
No. of parameters	218
No. of restraints	7
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.21, −0.23
Absolute structure	Flack (1983), 514 Freidel pairs
Absolute structure parameter	0.50 (6)

Computer programs: APEX2 (Bruker, 2007), SAINT (Bruker, 2007), SIR97 (Altomare et al., 1999), SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 1997), WinGX (Farrugia, 1999) and PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—HO1···O4ⁱ	0.82 (3)	1.84 (3)	2.650 (3)	171 (3)
N1—HN2···N2	0.86 (3)	2.60 (3)	3.375 (4)	150 (3)
O3—HO3···O2ⁱⁱ	0.81 (3)	1.84 (3)	2.650 (3)	173 (3)
N2—HN3···Cl2ⁱⁱⁱ	0.86 (3)	2.81 (3)	3.374 (2)	125 (2)
N2—HN4···N1^iv	0.86 (3)	2.47 (3)	3.302 (4)	163 (2)

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

Acknowledgements

The authors are grateful to the Higher Education Commission (HEC), Pakistan, for providing funds for the single-crystal XRD facilities at GC University, Lahore.

References

Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J. Appl. Cryst. 32, 115–119. Web of Science CrossRef CAS IUCr Journals Google Scholar
Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555–1573. CrossRef CAS Web of Science Google Scholar
Bruker (2007). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Farag, A. M., Teoh, S. G., Osman, H., Yeap, C. S. & Fun, H.-K. (2011). Acta Cryst. E67, o37. CrossRef IUCr Journals Google Scholar
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. CrossRef IUCr Journals Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar