research communications
E)-3-[(2,6-dimethylphenyl)diazenyl]-7-methyl-1H-indazole
of (aDepartment of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
*Correspondence e-mail: akitsu@rs.kagu.tus.ac.jp
The title azo compound, C16H16N4, was synthesized from 2,6-dimethylaniline. The diazenyl group adopts a trans (E) conformation, with an N=N bond length of 1.265 (4) Å. The pyrazole ring is approximately planar. In the crystal, zigzag chains along the b-axis direction with a C(3) is graph-set motif are formed by N—H⋯N hydrogen bonds involving the pyrazole moiety.
Keywords: crystal structure; photochromism; azo benzene; pyrazole; indazole; chiral crystallization.
CCDC reference: 1865810
1. Chemical context
Azobenzene derivatives are known to be photochromic compounds and numerous studies have been reported (Aritake et al., 2011; Bobrovsky et al., 2016; Li et al., 2017). As an example of this, our group has reported the crystal structures of several azobenzene derivatives (Moriwaki & Akitsu, 2015; Moriwaki et al., 2017).
Pyrazole is an aromatic compound comprising a five-membered ring with two adjacent N atoms. Pyrazole derivatives are biologically active and have attracted attention for the synthesis of new medicinal products (Ansari et al., 2017).
Here we report the E)-3-[(2,6-dimethylphenyl)diazenyl]-7-methyl-1H-indazole, which has an azobenzene moiety and a pyrazole moiety (Fig. 1).
of (2. Structural commentary
The molecular structure of the title compound is composed of a benzene ring linked to an indazole unit by an N=N bond. In the azobenzene moiety, the azo N=N double bond adopts an E configuration, with an N=N bond length of 1.265 (4) Å and a corresponding C9—N3—N4—C15 torsion angle of 0.7 (4)°.
The molecule is practically flat with a maximum deviation of 0.142 (5) Å (for atom C7) from the mean plane passing through the non-H atoms. The pyrazole ring (N3/N4/C15/C10/C9) is approximately planar with an r.m.s. deviation of 0.0026 Å. The C—C bond lengths of the pyrazole ring are 1.404 (6) and 1.428 (5) Å, the C—N bond lengths are 1.322 (5) and 1.359 (5) Å and the N—N bond length is 1.351 (4) Å, in good agreement with values reported previously for 7-methyl-1H-indazole [1.400 (4), 1.422 (4), 1.320 (4), 1.366 (3) and 1.356 (3) Å, respectively; Foces-Foces, 2005]
3. Supramolecular features
In the crystal, molecules are helically connected along the b-axis direction by N—H⋯N hydrogen bonds (Table 1 and Fig. 2). As a result, chiral crystals of achiral molecules are generated. The angles between the planes of neighbouring molecules in the hydrogen-bonded chains is 82.6 (2)°. Many examples of such achiral molecules forming chiral crystals have been reported, but the prediction of chiral crystallization is still not possible (Koshima & Matsuura, 1998; Matsuura & Koshima, 2005).
In addition, weak supramolecular interactions, such as the C16—H16c⋯Cg1 (2.844 Å) and C16—H16c⋯Cg3 (2.929 Å) C—H⋯π hydrogen bonds, are also found (Table 1 and Fig. 3).
4. Database survey
A similar compound, i.e. 7-methyl-1H-indazole (CCDC refcode 263698; Foces-Foces, 2005), has already been reported and shows a structure comparable with that of the title compound. However, surveys of the Cambridge Structural Database (CSD, Version 5.38; Groom et al., 2016) for the title compound revealed no hits. To our knowledge, this is the first reported for indazole-type azo dyes.
5. Synthesis and crystallization
A mixture of 2,6-dimethylaniline (0.4847 g, 4.000 mmol), concentrated hydrochloric acid (37%, 1 ml) and water was heated and completely dissolved. The mixture was cooled in an ice bath and NaNO2 (0.2967 g, 4.300 mmol) in 4.5 ml water was added. The reaction mixture was stirred at 273 K for 30 min and then salicylaldehyde (0.4885 g, 4.000 mmol) in 10 ml of a 10% NaOH aqueous solution was added dropwise and allowed to stir for an additional 1 h. The obtained orange precipitate was filtered off, washed with water and dried in a desiccator for several days (yield 0.2650 g, 26.06%). This crude orange compound was recrystallized by slow evaporation from acetone to give orange prismatic single crystals. IR (KBr, cm−1): 746 (s), 1147 (s), 1162 (s), 1425 (m), 2923 (s), 3136 (br). 1H NMR (300 MHz, DMSO): δ 2.36 (s, 6H), 2.56 (s, 3H), 7.18–7.23 (m, 5H), 8.02 (d, 1H)
6. Refinement
Crystal data, data collection and structure . All H atoms were located in difference Fourier maps. C-bound H atoms were constrained using a riding model [C—H = 0.95 Å and Uiso(H) = 1.2Ueq(C) for aromatic H atoms, and C—H = 0.98 Å and Uiso(H) = 1.5Ueq(C) for methyl H atoms]. N-bound H atoms were constrained using a riding model [N—H = 0.88 Å and Uiso(H) = 1.2Ueq(N)].
details are summarized in Table 2
|
Supporting information
CCDC reference: 1865810
Data collection: APEX2 (Bruker, 2014); cell
SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and publCIF (Westrip, 2010).C16H16N4 | F(000) = 280 |
Mr = 264.33 | Dx = 1.297 Mg m−3 |
Monoclinic, P21 | Mo Kα radiation, λ = 0.71073 Å |
a = 11.052 (8) Å | Cell parameters from 1267 reflections |
b = 4.565 (4) Å | θ = 2.6–26.0° |
c = 13.541 (10) Å | µ = 0.08 mm−1 |
β = 97.997 (11)° | T = 173 K |
V = 676.5 (9) Å3 | Prism, orange |
Z = 2 | 0.30 × 0.12 × 0.09 mm |
Bruker APEXII CCD diffractometer | 2682 independent reflections |
Radiation source: fine-focus sealed tube | 2055 reflections with I > 2σ(I) |
Detector resolution: 8.3333 pixels mm-1 | Rint = 0.053 |
φ and ω scans | θmax = 27.7°, θmin = 1.5° |
Absorption correction: multi-scan (SADABS; Bruker, 2001) | h = −11→14 |
Tmin = 0.323, Tmax = 0.746 | k = −5→5 |
3767 measured reflections | l = −17→15 |
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)] = 0.067 | H-atom parameters constrained |
wR(F2) = 0.180 | w = 1/[σ2(Fo2) + (0.0993P)2] where P = (Fo2 + 2Fc2)/3 |
S = 1.01 | (Δ/σ)max < 0.001 |
2682 reflections | Δρmax = 0.28 e Å−3 |
184 parameters | Δρmin = −0.34 e Å−3 |
1 restraint | Absolute structure: Flack x determined using 636 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013 |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: −10.0 (10) |
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. |
x | y | z | Uiso*/Ueq | ||
N3 | 0.4743 (3) | 0.9544 (8) | 0.0970 (2) | 0.0333 (8) | |
H3n | 0.4345 | 1.067 | 0.0507 | 0.04* | |
N1 | 0.7288 (3) | 0.3286 (8) | 0.2528 (2) | 0.0334 (8) | |
N4 | 0.5704 (3) | 0.7839 (8) | 0.0832 (2) | 0.0334 (8) | |
N2 | 0.7017 (3) | 0.4534 (8) | 0.1693 (2) | 0.0342 (8) | |
C10 | 0.5295 (3) | 0.7301 (9) | 0.2424 (3) | 0.0302 (9) | |
C15 | 0.6037 (3) | 0.6474 (9) | 0.1687 (3) | 0.0298 (8) | |
C9 | 0.4468 (3) | 0.9302 (9) | 0.1914 (3) | 0.0318 (9) | |
C1 | 0.8281 (3) | 0.1267 (9) | 0.2607 (3) | 0.0327 (9) | |
C14 | 0.3515 (3) | 1.0650 (10) | 0.2343 (3) | 0.0371 (10) | |
C11 | 0.5210 (3) | 0.6599 (10) | 0.3412 (3) | 0.0374 (10) | |
H11 | 0.5759 | 0.5253 | 0.3774 | 0.045* | |
C6 | 0.8553 (3) | 0.0120 (9) | 0.3576 (3) | 0.0359 (10) | |
C2 | 0.8945 (3) | 0.0392 (10) | 0.1838 (3) | 0.0374 (10) | |
C5 | 0.9517 (4) | −0.1836 (10) | 0.3778 (3) | 0.0432 (10) | |
H5 | 0.9722 | −0.2586 | 0.4434 | 0.052* | |
C13 | 0.3463 (4) | 0.9899 (10) | 0.3307 (3) | 0.0423 (11) | |
H13 | 0.2839 | 1.073 | 0.3634 | 0.051* | |
C3 | 0.9897 (4) | −0.1594 (11) | 0.2094 (4) | 0.0450 (11) | |
H3 | 1.0367 | −0.2205 | 0.1595 | 0.054* | |
C8 | 0.7839 (3) | 0.0999 (11) | 0.4390 (3) | 0.0423 (11) | |
H8a | 0.8096 | −0.0192 | 0.4984 | 0.063* | |
H8b | 0.6966 | 0.0693 | 0.4167 | 0.063* | |
H8c | 0.7988 | 0.3073 | 0.455 | 0.063* | |
C4 | 1.0178 (4) | −0.2699 (11) | 0.3043 (4) | 0.0503 (12) | |
H4 | 1.0829 | −0.406 | 0.3189 | 0.06* | |
C12 | 0.4300 (4) | 0.7932 (11) | 0.3843 (3) | 0.0470 (12) | |
H12 | 0.423 | 0.7523 | 0.4521 | 0.056* | |
C7 | 0.8686 (4) | 0.1420 (12) | 0.0775 (3) | 0.0467 (11) | |
H7a | 0.8869 | 0.3516 | 0.0744 | 0.07* | |
H7b | 0.7823 | 0.1085 | 0.0522 | 0.07* | |
H7c | 0.9198 | 0.0329 | 0.0367 | 0.07* | |
C16 | 0.2633 (4) | 1.2688 (12) | 0.1738 (4) | 0.0497 (12) | |
H16a | 0.1983 | 1.3235 | 0.2127 | 0.075* | |
H16b | 0.2275 | 1.1704 | 0.1123 | 0.075* | |
H16c | 0.3067 | 1.4453 | 0.1571 | 0.075* |
U11 | U22 | U33 | U12 | U13 | U23 | |
N3 | 0.0304 (16) | 0.0403 (19) | 0.0275 (16) | 0.0049 (15) | −0.0023 (12) | 0.0025 (15) |
N1 | 0.0317 (16) | 0.036 (2) | 0.0316 (17) | −0.0040 (15) | 0.0026 (12) | 0.0015 (16) |
N4 | 0.0327 (16) | 0.0404 (19) | 0.0264 (16) | −0.0015 (15) | 0.0014 (11) | 0.0010 (15) |
N2 | 0.0321 (16) | 0.041 (2) | 0.0294 (17) | −0.0021 (15) | 0.0036 (12) | −0.0001 (16) |
C10 | 0.0258 (17) | 0.033 (2) | 0.0308 (19) | −0.0040 (15) | 0.0022 (13) | −0.0006 (16) |
C15 | 0.0321 (18) | 0.033 (2) | 0.0246 (18) | −0.0035 (17) | 0.0039 (13) | −0.0007 (16) |
C9 | 0.0298 (18) | 0.033 (2) | 0.032 (2) | −0.0044 (16) | 0.0030 (14) | −0.0029 (18) |
C1 | 0.0253 (18) | 0.033 (2) | 0.040 (2) | −0.0062 (17) | 0.0044 (14) | −0.0061 (19) |
C14 | 0.0299 (19) | 0.036 (2) | 0.046 (2) | −0.0031 (17) | 0.0067 (16) | −0.0062 (19) |
C11 | 0.038 (2) | 0.044 (3) | 0.030 (2) | −0.0034 (19) | 0.0046 (15) | 0.0007 (19) |
C6 | 0.0288 (19) | 0.038 (2) | 0.040 (2) | −0.0060 (18) | 0.0002 (15) | −0.0015 (19) |
C2 | 0.033 (2) | 0.039 (2) | 0.041 (2) | −0.0055 (18) | 0.0074 (16) | −0.0062 (19) |
C5 | 0.034 (2) | 0.045 (3) | 0.049 (3) | 0.002 (2) | −0.0040 (17) | −0.002 (2) |
C13 | 0.037 (2) | 0.045 (3) | 0.047 (2) | 0.002 (2) | 0.0119 (18) | −0.010 (2) |
C3 | 0.033 (2) | 0.048 (3) | 0.055 (3) | 0.0017 (19) | 0.0086 (18) | −0.010 (2) |
C8 | 0.034 (2) | 0.057 (3) | 0.034 (2) | −0.002 (2) | −0.0004 (15) | 0.002 (2) |
C4 | 0.036 (2) | 0.051 (3) | 0.063 (3) | 0.006 (2) | 0.0011 (19) | −0.002 (2) |
C12 | 0.054 (3) | 0.056 (3) | 0.034 (2) | −0.002 (2) | 0.0164 (19) | −0.002 (2) |
C7 | 0.043 (2) | 0.059 (3) | 0.040 (2) | 0.002 (2) | 0.0139 (17) | −0.005 (2) |
C16 | 0.039 (2) | 0.047 (3) | 0.063 (3) | 0.007 (2) | 0.0066 (19) | 0.002 (3) |
N3—N4 | 1.351 (4) | C2—C3 | 1.396 (6) |
N3—C9 | 1.359 (5) | C2—C7 | 1.503 (6) |
N3—H3n | 0.88 | C5—C4 | 1.371 (6) |
N1—N2 | 1.265 (4) | C5—H5 | 0.95 |
N1—C1 | 1.426 (5) | C13—C12 | 1.415 (6) |
N4—C15 | 1.322 (5) | C13—H13 | 0.95 |
N2—C15 | 1.399 (5) | C3—C4 | 1.375 (7) |
C10—C11 | 1.391 (5) | C3—H3 | 0.95 |
C10—C9 | 1.404 (6) | C8—H8a | 0.98 |
C10—C15 | 1.428 (5) | C8—H8b | 0.98 |
C9—C14 | 1.412 (5) | C8—H8c | 0.98 |
C1—C6 | 1.406 (6) | C4—H4 | 0.95 |
C1—C2 | 1.413 (5) | C12—H12 | 0.95 |
C14—C13 | 1.358 (6) | C7—H7a | 0.98 |
C14—C16 | 1.505 (6) | C7—H7b | 0.98 |
C11—C12 | 1.374 (6) | C7—H7c | 0.98 |
C11—H11 | 0.95 | C16—H16a | 0.98 |
C6—C5 | 1.388 (5) | C16—H16b | 0.98 |
C6—C8 | 1.496 (6) | C16—H16c | 0.98 |
N4—N3—C9 | 111.5 (3) | C6—C5—H5 | 119.5 |
N4—N3—H3n | 124.2 | C14—C13—C12 | 122.8 (4) |
C9—N3—H3n | 124.2 | C14—C13—H13 | 118.6 |
N2—N1—C1 | 116.2 (3) | C12—C13—H13 | 118.6 |
C15—N4—N3 | 106.2 (3) | C4—C3—C2 | 122.4 (4) |
N1—N2—C15 | 112.1 (3) | C4—C3—H3 | 118.8 |
C11—C10—C9 | 119.8 (4) | C2—C3—H3 | 118.8 |
C11—C10—C15 | 137.1 (4) | C6—C8—H8a | 109.5 |
C9—C10—C15 | 103.1 (3) | C6—C8—H8b | 109.5 |
N4—C15—N2 | 115.1 (3) | H8a—C8—H8b | 109.5 |
N4—C15—C10 | 111.8 (3) | C6—C8—H8c | 109.5 |
N2—C15—C10 | 133.1 (4) | H8a—C8—H8c | 109.5 |
N3—C9—C10 | 107.5 (3) | H8b—C8—H8c | 109.5 |
N3—C9—C14 | 129.0 (4) | C5—C4—C3 | 119.9 (4) |
C10—C9—C14 | 123.5 (4) | C5—C4—H4 | 120.0 |
C6—C1—C2 | 121.1 (4) | C3—C4—H4 | 120.0 |
C6—C1—N1 | 111.9 (3) | C11—C12—C13 | 121.8 (4) |
C2—C1—N1 | 127.0 (4) | C11—C12—H12 | 119.1 |
C13—C14—C9 | 114.8 (4) | C13—C12—H12 | 119.1 |
C13—C14—C16 | 124.6 (4) | C2—C7—H7a | 109.5 |
C9—C14—C16 | 120.6 (4) | C2—C7—H7b | 109.5 |
C12—C11—C10 | 117.2 (4) | H7a—C7—H7b | 109.5 |
C12—C11—H11 | 121.4 | C2—C7—H7c | 109.5 |
C10—C11—H11 | 121.4 | H7a—C7—H7c | 109.5 |
C5—C6—C1 | 118.8 (4) | H7b—C7—H7c | 109.5 |
C5—C6—C8 | 119.8 (4) | C14—C16—H16a | 109.5 |
C1—C6—C8 | 121.4 (4) | C14—C16—H16b | 109.5 |
C3—C2—C1 | 116.8 (4) | H16a—C16—H16b | 109.5 |
C3—C2—C7 | 118.5 (3) | C14—C16—H16c | 109.5 |
C1—C2—C7 | 124.6 (4) | H16a—C16—H16c | 109.5 |
C4—C5—C6 | 120.9 (4) | H16b—C16—H16c | 109.5 |
C4—C5—H5 | 119.5 | ||
C9—N3—N4—C15 | 0.7 (4) | C10—C9—C14—C16 | −177.7 (4) |
C1—N1—N2—C15 | −179.8 (3) | C9—C10—C11—C12 | 0.1 (6) |
N3—N4—C15—N2 | 178.8 (3) | C15—C10—C11—C12 | −178.5 (5) |
N3—N4—C15—C10 | −0.7 (4) | C2—C1—C6—C5 | 2.0 (6) |
N1—N2—C15—N4 | −179.6 (3) | N1—C1—C6—C5 | −178.7 (3) |
N1—N2—C15—C10 | −0.2 (6) | C2—C1—C6—C8 | −178.9 (4) |
C11—C10—C15—N4 | 179.2 (4) | N1—C1—C6—C8 | 0.4 (5) |
C9—C10—C15—N4 | 0.5 (4) | C6—C1—C2—C3 | −1.6 (6) |
C11—C10—C15—N2 | −0.2 (8) | N1—C1—C2—C3 | 179.2 (3) |
C9—C10—C15—N2 | −178.9 (4) | C6—C1—C2—C7 | 177.6 (4) |
N4—N3—C9—C10 | −0.4 (4) | N1—C1—C2—C7 | −1.6 (7) |
N4—N3—C9—C14 | −178.1 (4) | C1—C6—C5—C4 | −1.6 (6) |
C11—C10—C9—N3 | −179.0 (4) | C8—C6—C5—C4 | 179.3 (4) |
C15—C10—C9—N3 | 0.0 (4) | C9—C14—C13—C12 | 0.3 (6) |
C11—C10—C9—C14 | −1.2 (6) | C16—C14—C13—C12 | 178.9 (4) |
C15—C10—C9—C14 | 177.8 (4) | C1—C2—C3—C4 | 0.8 (7) |
N2—N1—C1—C6 | 176.8 (3) | C7—C2—C3—C4 | −178.5 (4) |
N2—N1—C1—C2 | −3.9 (6) | C6—C5—C4—C3 | 0.8 (7) |
N3—C9—C14—C13 | 178.3 (4) | C2—C3—C4—C5 | −0.5 (7) |
C10—C9—C14—C13 | 1.0 (6) | C10—C11—C12—C13 | 1.2 (6) |
N3—C9—C14—C16 | −0.3 (7) | C14—C13—C12—C11 | −1.4 (7) |
D—H···A | D—H | H···A | D···A | D—H···A |
N3—H3n···N4i | 0.88 | 2.06 | 2.851 (5) | 149 |
C16ii—H16cii···Cg1 | 0.98 | 2.84 | 3.544 | 129 |
C16ii—H16cii···Cg2 | 0.98 | 2.93 | 3.832 | 154 |
Symmetry codes: (i) −x+1, y+1/2, −z; (ii) x, y−1, z. |
Cg1 and Cg2 are the centroids of the C9–C14 and C9–C10/C15/N3–N4 rings, respectively. |
D—H···A | D—H | H···A | D···A | D—H···A |
N3—H3n···N4i | 0.88 | 2.06 | 2.851 (5) | 149 |
C16ii—H16cii···Cg1 | 0.98 | 2.84 | 3.544 | 129 |
C16ii—H16cii···Cg3 | 0.98 | 2.93 | 3.832 | 154 |
Symmetry codes: (i) -x+1, y+1/2, -z; (ii) x, y-1, z. |
References
Ansari, A., Ali, A., Asif, M. & Shamsuzzaman (2017). New J. Chem. 41, 16–41. Google Scholar
Aritake, Y., Takanashi, T., Yamazaki, A. & AKitsu, T. (2011). Polyhedron, 30, 886–894. Web of Science CrossRef Google Scholar
Bobrovsky, A., Shibaev, V., Cigl, M., Hamplová, V., Pociecha, D. & Bubnov, A. (2016). J. Polym. Sci. Part A Polym. Chem. 54, 2962–2970. Web of Science CrossRef Google Scholar
Bruker (2001). SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Bruker (2014). APEX2 and SAINT. 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
Foces-Foces, C. (2005). Acta Cryst. E61, o337–o339. Web of Science CrossRef IUCr Journals Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CSD CrossRef IUCr Journals Google Scholar
Koshima, H. & Matsuura, T. (1998). J. Synth. Org. Chem. Jpn, 56, 466–477. Web of Science CrossRef Google Scholar
Li, S., Feng, Y., Long, P., Qin, C. & Feng, W. (2017). J. Mater. Chem. C, 5, 5068–5075. Web of Science CrossRef Google Scholar
Macrae, C. F., Edgington, P. R., McCabe, P., Pidcock, E., Shields, G. P., Taylor, R., Towler, M. & van de Streek, J. (2006). J. Appl. Cryst. 39, 453–457. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Matsuura, T. & Koshima, H. (2005). J. Photochem. Photobiol. C, 6, 7–24. Web of Science CrossRef Google Scholar
Moriwaki, R. & Akitsu, T. (2015). Acta Cryst. E71, o886–o887. Web of Science CSD CrossRef IUCr Journals Google Scholar
Moriwaki, R., Yagi, S., Haraguchi, T. & Akitsu, T. (2017). IUCrData, 2, x170979. Google Scholar
Parsons, S., Flack, H. D. & Wagner, T. (2013). Acta Cryst. B69, 249–259. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar
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