4-Amino­pyridinium 2-chloro-4-nitro­benzoate monohydrate

Savig, K.L.; Lemmerer, A.

doi:10.1107/S1600536812046077

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 68| Part 12| December 2012| Page o3361

doi:10.1107/S1600536812046077

Open

access

4-Aminopyridinium 2-chloro-4-nitrobenzoate monohydrate

Kelsey L. Savig ^a and Andreas Lemmerer ^a ^*

^aMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, PO Wits 2050, South Africa
^*Correspondence e-mail: andreas.lemmerer@wits.ac.za

(Received 11 October 2012; accepted 7 November 2012; online 17 November 2012)

In the title hydrated molecular salt, C₅H₇N₂⁺·C₇H₃ClNO₄⁻·H₂O, the ions and water molecules assemble into ribbons of R₆⁵(22) rings along the c axis via O(water)—H⋯O⁻, N⁺—H⋯O(water) and N—H⋯O⁻ hydrogen bonds. N—H⋯O⁻ hydrogen bonds connect adjacent ribbons along the c-axis direction via R₄⁴(12) rings, forming hydrogen-bonded layers. The CO₂ and NO₂ groups make dihedral angles of 81.8 (2) and 1.4 (2)°, respectively, with the ring in the anion.

Related literature

For related structures, see: Lemmerer et al. (2010 ). For graph-set notation, see: Bernstein et al. (1995 ).

Experimental

Crystal data

C₅H₇N₂⁺·C₇H₃ClNO₄⁻·H₂O
M_r = 313.7
Monoclinic, P 2₁ /c
a = 14.4500 (5) Å
b = 14.3300 (5) Å
c = 6.9918 (2) Å
β = 97.804 (2)°
V = 1434.37 (8) Å³
Z = 4
Mo Kα radiation
μ = 0.29 mm⁻¹
T = 173 K
0.78 × 0.31 × 0.09 mm

Data collection

Bruker APEXII CCD area-detector diffractometer
Absorption correction: integration (XPREP; Bruker, 2004 ) T_min = 0.896, T_max = 0.977
14928 measured reflections
3456 independent reflections
2685 reflections with I > 2σ(I)
R_int = 0.061

Refinement

R[F² > 2σ(F²)] = 0.043
wR(F²) = 0.122
S = 1.04
3456 reflections
210 parameters
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.5 e Å⁻³
Δρ_min = −0.36 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N2—H2⋯O1Wⁱ	0.93 (3)	1.72 (3)	2.641 (2)	170 (2)
N3—H3A⋯O1	0.94 (2)	1.99 (2)	2.926 (2)	171.1 (19)
N3—H3B⋯O2ⁱⁱ	0.84 (2)	2.16 (3)	2.982 (2)	165 (2)
O1W—H1W⋯O1ⁱⁱⁱ	0.75 (3)	1.96 (3)	2.698 (2)	166 (3)
O1W—H2W⋯O2	0.78 (4)	1.99 (4)	2.729 (2)	158 (3)
Symmetry codes: (i) $[x, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ ; (ii) -x+2, -y+1, -z+1; (iii) x, y, z+1.

Data collection: APEX2 (Bruker, 2005 ); cell refinement: SAINT-Plus (Bruker, 2004); data reduction: SAINT-Plus and XPREP (Bruker 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012 ) and DIAMOND (Brandenburg, 1999 ); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536812046077/fy2073sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536812046077/fy2073Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536812046077/fy2073Isup3.mol

Supporting information file. DOI: 10.1107/S1600536812046077/fy2073Isup4.cml

checkCIF report

Comment top

The title molecular salt is part of a larger research project looking at the factors that determine if a salt or a co-crystal forms between a specific carboxylic acid and various pyridine ring systems (Lemmerer et al., 2010).

In molecular salt (I) (Fig. 1), formed by dissolving 4-aminopyridine and 2-chloro-4-nitrobenzoic acid in methanol, two kinds of hydrogen-bonded rings are formed. The ring R⁵₆(22) (Bernstein et al., 1995) uses O1W—H···O^-, N⁺—H···O and N—H···O^- hydrogen bonds to connect all three species together into a 1-D ribbon (Fig. 2) along the c-axis. In addition, a R⁴₄(12) ring is formed between two amine groups and two carboxylate groups to connect two ribbons together to form layers (Fig. 3).

Related literature top

For related structures, see: Lemmerer et al. (2010). For graph-set notation, see: Bernstein et al. (1995).

Experimental top

Crystals were grown by slow evaporation of a methanol solution of 2-chloro-4-nitrobenzoic acid (0.100 g; 0.496 mmol) and 4-aminopyridine (0.047 g; 0.50 mmol) in 8 ml of methanol, and afforded colourless plates after three days of slow evaporation at ambient conditions.

Computing details top

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT-Plus (Bruker, 2004); data reduction: SAINT-Plus and XPREP (Bruker 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009).

Figures top

	Fig. 1. The asymmetric unit of (I) showing the atomic numbering scheme. Displacement ellipsoids are shown at the 50% probability level.
	Fig. 2. Hydrogen bonding diagram of (I) showing the R⁵₆(22) rings to form ribbons. Intermolecular hydrogen bonds are shown as dashed red lines forming dimers. Atoms with superscrips (i) and (ii) are at the symmetry positions (x, y, z - 1) and (x, -y + 1/2, z - 1/2) respectively.
	Fig. 3. The layers form by combining the one-dimensional ribbons using R⁴₄(12) rings.

4-Aminopyridinium 2-chloro-4-nitrobenzoate monohydrate top

Crystal data top

C₅H₇N₂⁺·C₇H₃ClNO₄⁻·H₂O	F(000) = 648
M_r = 313.7	D_x = 1.453 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 4262 reflections
a = 14.4500 (5) Å	θ = 2.8–27.8°
b = 14.3300 (5) Å	µ = 0.29 mm⁻¹
c = 6.9918 (2) Å	T = 173 K
β = 97.804 (2)°	Plate, colourless
V = 1434.37 (8) Å³	0.78 × 0.31 × 0.09 mm
Z = 4

Data collection top

Bruker APEXII CCD area-detector diffractometer	2685 reflections with I > 2σ(I)
ω scans	R_int = 0.061
Absorption correction: integration (XPREP; Bruker, 2004)	θ_max = 28°, θ_min = 1.4°
T_min = 0.896, T_max = 0.977	h = −19→19
14928 measured reflections	k = −18→18
3456 independent reflections	l = −8→9

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F² > 2σ(F²)] = 0.043	w = 1/[σ²(F_o²) + (0.0662P)² + 0.1397P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.122	(Δ/σ)_max < 0.001
S = 1.04	Δρ_max = 0.5 e Å⁻³
3456 reflections	Δρ_min = −0.36 e Å⁻³
210 parameters

Crystal data top

C₅H₇N₂⁺·C₇H₃ClNO₄⁻·H₂O	V = 1434.37 (8) Å³
M_r = 313.7	Z = 4
Monoclinic, P2₁/c	Mo Kα radiation
a = 14.4500 (5) Å	µ = 0.29 mm⁻¹
b = 14.3300 (5) Å	T = 173 K
c = 6.9918 (2) Å	0.78 × 0.31 × 0.09 mm
β = 97.804 (2)°

Data collection top

Bruker APEXII CCD area-detector diffractometer	3456 independent reflections
Absorption correction: integration (XPREP; Bruker, 2004)	2685 reflections with I > 2σ(I)
T_min = 0.896, T_max = 0.977	R_int = 0.061
14928 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.043	0 restraints
wR(F²) = 0.122	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	Δρ_max = 0.5 e Å⁻³
3456 reflections	Δρ_min = −0.36 e Å⁻³
210 parameters

Special details top

Experimental. Numerical integration absorption corrections based on indexed crystal faces were applied using the XPREP routine (Bruker, 2004)

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
C1	0.69246 (11)	0.61389 (11)	0.2594 (2)	0.0261 (3)
C2	0.61757 (12)	0.55610 (11)	0.2858 (2)	0.0271 (3)
C3	0.52758 (11)	0.57500 (11)	0.2008 (2)	0.0284 (3)
H3	0.4772	0.5348	0.2188	0.034*
C4	0.51347 (11)	0.65438 (12)	0.0887 (2)	0.0290 (4)
C5	0.58469 (12)	0.71419 (12)	0.0592 (2)	0.0325 (4)
H5	0.5725	0.7687	−0.0176	0.039*
C6	0.67437 (12)	0.69317 (12)	0.1438 (2)	0.0311 (4)
H6	0.7245	0.7332	0.1232	0.037*
C7	0.79130 (11)	0.59307 (11)	0.3515 (2)	0.0291 (3)
N1	0.41775 (10)	0.67644 (11)	0.0004 (2)	0.0349 (3)
O1	0.84325 (9)	0.55490 (9)	0.24589 (19)	0.0412 (3)
O2	0.81313 (9)	0.61801 (10)	0.52270 (17)	0.0424 (3)
O3	0.35519 (9)	0.62315 (10)	0.0282 (2)	0.0462 (3)
O4	0.40575 (10)	0.74821 (11)	−0.0962 (2)	0.0513 (4)
Cl1	0.63660 (3)	0.45679 (3)	0.42817 (7)	0.04119 (15)
C8	0.96351 (14)	0.15836 (13)	0.3659 (2)	0.0361 (4)
H8	1.0019	0.1049	0.3936	0.043*
C9	1.00312 (12)	0.24412 (13)	0.3780 (2)	0.0341 (4)
H9	1.0687	0.2506	0.4123	0.041*
C10	0.94613 (12)	0.32426 (12)	0.3393 (2)	0.0293 (4)
C11	0.84995 (12)	0.30971 (12)	0.2835 (2)	0.0309 (4)
H11	0.8095	0.3614	0.2527	0.037*
C12	0.81509 (13)	0.22161 (13)	0.2739 (2)	0.0352 (4)
H12	0.75	0.2124	0.237	0.042*
N2	0.87086 (12)	0.14714 (11)	0.3155 (2)	0.0357 (3)
N3	0.98294 (12)	0.40973 (11)	0.3540 (2)	0.0368 (4)
H2	0.8439 (17)	0.0887 (18)	0.322 (3)	0.061 (7)*
H3A	0.9432 (16)	0.4610 (15)	0.322 (3)	0.045 (6)*
H3B	1.0408 (17)	0.4127 (15)	0.391 (3)	0.048 (6)*
O1W	0.8108 (2)	0.52435 (14)	0.8618 (3)	0.1094 (11)
H1W	0.817 (2)	0.541 (2)	0.965 (5)	0.080 (10)*
H2W	0.806 (2)	0.562 (2)	0.781 (5)	0.096 (11)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0257 (8)	0.0296 (8)	0.0228 (7)	0.0026 (6)	0.0032 (6)	−0.0001 (6)
C2	0.0310 (8)	0.0261 (8)	0.0245 (7)	0.0017 (6)	0.0048 (6)	0.0017 (6)
C3	0.0268 (8)	0.0306 (8)	0.0281 (8)	−0.0034 (6)	0.0046 (6)	−0.0036 (6)
C4	0.0241 (8)	0.0377 (9)	0.0244 (7)	0.0046 (6)	0.0007 (6)	−0.0018 (6)
C5	0.0319 (9)	0.0339 (9)	0.0312 (8)	0.0036 (7)	0.0030 (7)	0.0082 (7)
C6	0.0265 (9)	0.0340 (9)	0.0325 (8)	−0.0007 (7)	0.0033 (6)	0.0062 (7)
C7	0.0260 (8)	0.0281 (8)	0.0319 (8)	0.0004 (6)	−0.0007 (6)	0.0059 (6)
N1	0.0256 (8)	0.0466 (9)	0.0316 (7)	0.0043 (6)	−0.0001 (6)	−0.0049 (7)
O1	0.0307 (7)	0.0477 (8)	0.0437 (7)	0.0115 (6)	−0.0002 (5)	−0.0040 (6)
O2	0.0367 (7)	0.0580 (8)	0.0302 (6)	−0.0009 (6)	−0.0039 (5)	−0.0001 (6)
O3	0.0254 (7)	0.0589 (9)	0.0530 (8)	−0.0023 (6)	0.0004 (6)	−0.0042 (7)
O4	0.0382 (8)	0.0584 (9)	0.0541 (8)	0.0136 (7)	−0.0054 (6)	0.0154 (7)
Cl1	0.0433 (3)	0.0341 (2)	0.0463 (3)	0.00160 (18)	0.0065 (2)	0.01473 (18)
C8	0.0429 (11)	0.0369 (9)	0.0287 (8)	0.0107 (8)	0.0059 (7)	0.0010 (7)
C9	0.0295 (9)	0.0400 (10)	0.0323 (8)	0.0069 (7)	0.0027 (7)	0.0002 (7)
C10	0.0306 (9)	0.0360 (9)	0.0214 (7)	0.0035 (7)	0.0037 (6)	−0.0002 (6)
C11	0.0282 (9)	0.0381 (9)	0.0262 (8)	0.0056 (7)	0.0033 (6)	0.0018 (7)
C12	0.0316 (9)	0.0472 (10)	0.0266 (8)	−0.0011 (7)	0.0030 (7)	0.0004 (7)
N2	0.0440 (9)	0.0359 (8)	0.0275 (7)	−0.0008 (7)	0.0060 (6)	0.0002 (6)
N3	0.0289 (8)	0.0361 (8)	0.0435 (9)	0.0021 (7)	−0.0018 (7)	−0.0013 (7)
O1W	0.249 (4)	0.0447 (10)	0.0407 (10)	0.0464 (14)	0.0425 (14)	0.0092 (9)

Geometric parameters (Å, º) top

C1—C2	1.395 (2)	C8—N2	1.347 (2)
C1—C6	1.398 (2)	C8—C9	1.353 (3)
C1—C7	1.515 (2)	C8—H8	0.95
C2—C3	1.381 (2)	C9—C10	1.418 (2)
C2—Cl1	1.7369 (16)	C9—H9	0.95
C3—C4	1.381 (2)	C10—N3	1.334 (2)
C3—H3	0.95	C10—C11	1.407 (2)
C4—C5	1.376 (2)	C11—C12	1.358 (2)
C4—N1	1.471 (2)	C11—H11	0.95
C5—C6	1.383 (2)	C12—N2	1.345 (2)
C5—H5	0.95	C12—H12	0.95
C6—H6	0.95	N2—H2	0.93 (3)
C7—O2	1.248 (2)	N3—H3A	0.94 (2)
C7—O1	1.248 (2)	N3—H3B	0.84 (2)
N1—O3	1.219 (2)	O1W—H1W	0.75 (3)
N1—O4	1.230 (2)	O1W—H2W	0.78 (4)

C2—C1—C6	118.13 (15)	O4—N1—C4	117.71 (15)
C2—C1—C7	122.05 (14)	N2—C8—C9	121.43 (16)
C6—C1—C7	119.83 (14)	N2—C8—H8	119.3
C3—C2—C1	121.91 (15)	C9—C8—H8	119.3
C3—C2—Cl1	118.28 (12)	C8—C9—C10	119.60 (17)
C1—C2—Cl1	119.80 (12)	C8—C9—H9	120.2
C4—C3—C2	117.63 (15)	C10—C9—H9	120.2
C4—C3—H3	121.2	N3—C10—C11	121.74 (16)
C2—C3—H3	121.2	N3—C10—C9	120.92 (16)
C5—C4—C3	122.84 (15)	C11—C10—C9	117.33 (16)
C5—C4—N1	118.88 (15)	C12—C11—C10	119.83 (16)
C3—C4—N1	118.27 (15)	C12—C11—H11	120.1
C4—C5—C6	118.46 (15)	C10—C11—H11	120.1
C4—C5—H5	120.8	N2—C12—C11	121.31 (17)
C6—C5—H5	120.8	N2—C12—H12	119.3
C5—C6—C1	121.02 (16)	C11—C12—H12	119.3
C5—C6—H6	119.5	C12—N2—C8	120.46 (16)
C1—C6—H6	119.5	C12—N2—H2	118.9 (15)
O2—C7—O1	126.79 (16)	C8—N2—H2	120.3 (15)
O2—C7—C1	116.89 (15)	C10—N3—H3A	118.3 (13)
O1—C7—C1	116.28 (14)	C10—N3—H3B	116.1 (15)
O3—N1—O4	124.01 (15)	H3A—N3—H3B	126 (2)
O3—N1—C4	118.28 (15)	H1W—O1W—H2W	117 (3)

C6—C1—C2—C3	0.4 (2)	C2—C1—C7—O1	99.31 (18)
C7—C1—C2—C3	−179.64 (14)	C6—C1—C7—O1	−80.7 (2)
C6—C1—C2—Cl1	179.99 (12)	C5—C4—N1—O3	−179.69 (15)
C7—C1—C2—Cl1	0.0 (2)	C3—C4—N1—O3	−0.8 (2)
C1—C2—C3—C4	−0.6 (2)	C5—C4—N1—O4	−0.2 (2)
Cl1—C2—C3—C4	179.75 (12)	C3—C4—N1—O4	178.68 (15)
C2—C3—C4—C5	0.1 (2)	N2—C8—C9—C10	0.8 (3)
C2—C3—C4—N1	−178.79 (14)	C8—C9—C10—N3	178.61 (16)
C3—C4—C5—C6	0.8 (3)	C8—C9—C10—C11	−1.9 (2)
N1—C4—C5—C6	179.59 (14)	N3—C10—C11—C12	−178.78 (16)
C4—C5—C6—C1	−1.0 (3)	C9—C10—C11—C12	1.8 (2)
C2—C1—C6—C5	0.5 (2)	C10—C11—C12—N2	−0.5 (2)
C7—C1—C6—C5	−179.52 (15)	C11—C12—N2—C8	−0.8 (2)
C2—C1—C7—O2	−82.6 (2)	C9—C8—N2—C12	0.6 (2)
C6—C1—C7—O2	97.40 (18)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2···O1Wⁱ	0.93 (3)	1.72 (3)	2.641 (2)	170 (2)
N3—H3A···O1	0.94 (2)	1.99 (2)	2.926 (2)	171.1 (19)
N3—H3B···O2ⁱⁱ	0.84 (2)	2.16 (3)	2.982 (2)	165 (2)
O1W—H1W···O1ⁱⁱⁱ	0.75 (3)	1.96 (3)	2.698 (2)	166 (3)
O1W—H2W···O2	0.78 (4)	1.99 (4)	2.729 (2)	158 (3)

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

Experimental details

Crystal data
Chemical formula	C₅H₇N₂⁺·C₇H₃ClNO₄⁻·H₂O
M_r	313.7
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	173
a, b, c (Å)	14.4500 (5), 14.3300 (5), 6.9918 (2)
β (°)	97.804 (2)
V (Å³)	1434.37 (8)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.29
Crystal size (mm)	0.78 × 0.31 × 0.09

Data collection
Diffractometer	Bruker APEXII CCD area-detector diffractometer
Absorption correction	Integration (XPREP; Bruker, 2004)
T_min, T_max	0.896, 0.977
No. of measured, independent and observed [I > 2σ(I)] reflections	14928, 3456, 2685
R_int	0.061
(sin θ/λ)_max (Å⁻¹)	0.661

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.043, 0.122, 1.04
No. of reflections	3456
No. of parameters	210
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.5, −0.36

Computer programs: APEX2 (Bruker, 2005), SAINT-Plus (Bruker, 2004), SAINT-Plus and XPREP (Bruker 2004), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 1999), WinGX (Farrugia, 2012) and PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2···O1Wⁱ	0.93 (3)	1.72 (3)	2.641 (2)	170 (2)
N3—H3A···O1	0.94 (2)	1.99 (2)	2.926 (2)	171.1 (19)
N3—H3B···O2ⁱⁱ	0.84 (2)	2.16 (3)	2.982 (2)	165 (2)
O1W—H1W···O1ⁱⁱⁱ	0.75 (3)	1.96 (3)	2.698 (2)	166 (3)
O1W—H2W···O2	0.78 (4)	1.99 (4)	2.729 (2)	158 (3)

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

Acknowledgements

This work was supported by the University of the Witwatersrand and the Molecular Sciences Institute, which are thanked for providing the infrastructure required to do this work, and the Friedel Sellshop grant for financial support.

References

Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 35, 1555–1573. CrossRef Web of Science Google Scholar
Brandenburg, K. (1999). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Bruker (2004). SAINT-Plus and XPREP. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Bruker (2005). APEX2. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849–854. Web of Science CrossRef CAS IUCr Journals Google Scholar
Lemmerer, A., Esterhuysen, C. & Bernstein, J. (2010). J. Pharm. Sci. 99, 4054–4071. Web of Science CAS PubMed 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