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ISSN: 2056-9890

5,6-Di­methyl-1,2,9,10-tetra­hydro­pyrano[3,2-f]chromene-3,8-dione

aDepartment of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand
*Correspondence e-mail: jsimpson@alkali.otago.ac.nz

(Received 18 June 2012; accepted 18 June 2012; online 27 June 2012)

The title mol­ecule, C14H14O4, lies on a twofold rotation axis that bis­ects the central benzene ring, with only one half-mol­ecule in the asymmetric unit. The pyran­one systems adopt distorted twist- boat conformations, with the two methyl­ene C atoms displaced by 0.537 (1) and 0.163 (2) Å from the best-fit plane through the remaining five C and O atoms (r.m.s. deviation = 0.073 Å). In the crystal, bifurcated C—H⋯(O,O) hydrogen bonds link pairs of adjacent mol­ecules in an obverse fashion, stacking mol­ecules along c. These contacts are further stabilized by very weak ππ inter­actions between adjacent benzene rings with centroid–centroid distances of 4.1951 (4) Å. Additional C—H⋯O contacts link these stacks, giving a three-dimensional network.

Related literature

For the synthesis, see: Lecea et al. (2010[Lecea, M., Hernández-Torres, G., Urbano, A., Carreño, M. C. & Colobert, F. (2010). Org. Lett. 12, 580-583.]). For details of the Cambridge Structural Database, see: Allen (2002[Allen, F. H. (2002). Acta Cryst. B58, 380-388.]) and for related structures, see: Cameron et al. (2011[Cameron, S. A., Goswami, S. K., Hanton, L. R., McAdam, C. J., Moratti, S. C. & Simpson, J. (2011). Acta Cryst. E67, o2141-o2142.]); Goswami et al. (2011[Goswami, S. K., Hanton, L. R., McAdam, C. J., Moratti, S. C. & Simpson, J. (2011). Acta Cryst. E67, o1566-o1567.]). For standard bond lengths, see: Allen et al. (1987[Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1-19.]).

[Scheme 1]

Experimental

Crystal data
  • C14H14O4

  • Mr = 246.25

  • Monoclinic, C 2/c

  • a = 16.0726 (2) Å

  • b = 8.7982 (1) Å

  • c = 8.0555 (1) Å

  • β = 96.1134 (7)°

  • V = 1132.65 (2) Å3

  • Z = 4

  • Mo Kα radiation

  • μ = 0.11 mm−1

  • T = 92 K

  • 0.53 × 0.50 × 0.22 mm

Data collection
  • Bruker APEXII CCD area-detector diffractometer

  • Absorption correction: multi-scan (SADABS; Bruker, 2011[Bruker (2011). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.570, Tmax = 0.748

  • 9744 measured reflections

  • 2989 independent reflections

  • 2358 reflections with I > 2σ(I)

  • Rint = 0.038

Refinement
  • R[F2 > 2σ(F2)] = 0.050

  • wR(F2) = 0.148

  • S = 1.08

  • 2989 reflections

  • 83 parameters

  • H-atom parameters constrained

  • Δρmax = 0.47 e Å−3

  • Δρmin = −0.38 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
C61—H61A⋯O1i 0.98 2.54 3.3907 (11) 145
C2—H2A⋯O1ii 0.99 2.60 3.4301 (12) 142
C2—H2A⋯O2ii 0.99 2.64 3.4813 (10) 143
C2—H2B⋯O1iii 0.99 2.44 3.3528 (11) 154
Symmetry codes: (i) [-x+{\script{3\over 2}}, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]; (ii) [x, -y, z+{\script{1\over 2}}]; (iii) [-x+{\script{3\over 2}}, -y-{\script{1\over 2}}, -z+1].

Data collection: APEX2 (Bruker, 2011[Bruker (2011). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: APEX2 (Bruker, 2011[Bruker (2011). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]) and SAINT (Bruker, 2011[Bruker (2011). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]) and TITAN2000 (Hunter & Simpson, 1999[Hunter, K. A. & Simpson, J. (1999). TITAN2000. University of Otago, New Zealand.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]) and TITAN2000; molecular graphics: SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]) and Mercury (Macrae et al., 2008[Macrae, C. F., Bruno, I. J., Chisholm, J. A., Edgington, P. R., McCabe, P., Pidcock, E., Rodriguez-Monge, L., Taylor, R., van de Streek, J. & Wood, P. A. (2008). J. Appl. Cryst. 41, 466-470.]); software used to prepare material for publication: SHELXL97, enCIFer (Allen et al., 2004[Allen, F. H., Johnson, O., Shields, G. P., Smith, B. R. & Towler, M. (2004). J. Appl. Cryst. 37, 335-338.]), PLATON (Spek, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]) and publCIF (Westrip 2010[Westrip, S. P. (2010). J. Appl. Cryst. 43, 920-925.]).

Supporting information


Comment top

Our current research interests involve the preparation of redox monomers for the synthesis of electroactive gels. The title compound (I), a chromene-dione was obtained as a by-product of the synthesis of the desired 6-hydroxy-7,8-dimethylchroman-2-one as previously reported by Lecea et al. (2010). The chromene-dione (I) provides access to an exciting new redox-active cross-linker in three steps through ring opening, oxidation and condensation reactions.

The asymmetric unit of the title compound contains only one half of the molecule, which lies on a twofold rotation axis that bisects the central benzene ring (Fig 1) The pyranone ring systems adopt distorted twist boat conformations, with the C2 and C3 methylene carbon atoms displaced by 0.537 (1) and 0.163 (2) Å respectively from the best fit plane through C1/(O1)/O2/C5/C4 which has an r.m.s. deviation 0.073 Å. A search of the Cambridge Database (Allen, 2002) revealed no comparable compounds, with or without substitution on the benzene ring. However we have previously reported closely related chroman-2-one derivatives without a second pyranone ring system (Goswami et al., 2011, Cameron et al., 2011). Bond lengths in the structure are not unusual (Allen et al., 1987) and are comparable to those in the chroman-2-one compounds mentioned previously.

In the crystal structure, bifurcated C2–H2A···O1 hydrogen bonds link pairs of adjacent molecules in an obverse fashion stacking molecules along c. Very weak ππ interactions, Cg···Cg = 4.1951 (4) Å, between adjacent benzene rings bolster these contacts further, Fig. 2. Additional C–H···O contacts, Table 1, generate layers of molecules in planes parallel to (1,0,1), Fig 3 while the overall result of this series of contacts is an extended three dimensional network, Fig. 4.

Related literature top

For the synthesis, see: Lecea et al. (2010). For details of the Cambridge Structural Database, see: Allen (2002) and for related structures, see: Cameron et al. (2011); Goswami et al. (2011). For standard bond lengths, see: Allen et al. (1987).

Experimental top

The title compound was obtained as a by-product from a Friedel-Crafts type addition reaction of 2,3-dimethylhydroquinone with acrylic acid during the synthesis of 6-hydroxy-7,8-dimethylchroman-2-one (Lecea et al., 2010). Following work-up according to the literature, X-ray quality crystals of (I) were obtained from dichloromethane solution.

Refinement top

All H-atoms bound to carbon were refined using a riding model with d(C—H) = 0.99 Å, Uiso=1.2Ueq (C) for methylene and 0.98 Å, Uiso = 1.5Ueq (C) for CH3 H atoms.

Structure description top

Our current research interests involve the preparation of redox monomers for the synthesis of electroactive gels. The title compound (I), a chromene-dione was obtained as a by-product of the synthesis of the desired 6-hydroxy-7,8-dimethylchroman-2-one as previously reported by Lecea et al. (2010). The chromene-dione (I) provides access to an exciting new redox-active cross-linker in three steps through ring opening, oxidation and condensation reactions.

The asymmetric unit of the title compound contains only one half of the molecule, which lies on a twofold rotation axis that bisects the central benzene ring (Fig 1) The pyranone ring systems adopt distorted twist boat conformations, with the C2 and C3 methylene carbon atoms displaced by 0.537 (1) and 0.163 (2) Å respectively from the best fit plane through C1/(O1)/O2/C5/C4 which has an r.m.s. deviation 0.073 Å. A search of the Cambridge Database (Allen, 2002) revealed no comparable compounds, with or without substitution on the benzene ring. However we have previously reported closely related chroman-2-one derivatives without a second pyranone ring system (Goswami et al., 2011, Cameron et al., 2011). Bond lengths in the structure are not unusual (Allen et al., 1987) and are comparable to those in the chroman-2-one compounds mentioned previously.

In the crystal structure, bifurcated C2–H2A···O1 hydrogen bonds link pairs of adjacent molecules in an obverse fashion stacking molecules along c. Very weak ππ interactions, Cg···Cg = 4.1951 (4) Å, between adjacent benzene rings bolster these contacts further, Fig. 2. Additional C–H···O contacts, Table 1, generate layers of molecules in planes parallel to (1,0,1), Fig 3 while the overall result of this series of contacts is an extended three dimensional network, Fig. 4.

For the synthesis, see: Lecea et al. (2010). For details of the Cambridge Structural Database, see: Allen (2002) and for related structures, see: Cameron et al. (2011); Goswami et al. (2011). For standard bond lengths, see: Allen et al. (1987).

Computing details top

Data collection: APEX2 (Bruker, 2011); cell refinement: APEX2 (Bruker, 2011) and SAINT (Bruker, 2011); data reduction: SAINT (Bruker, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008) and TITAN2000 (Hunter & Simpson, 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) and TITAN2000 (Hunter & Simpson, 1999); molecular graphics: SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008), enCIFer (Allen et al., 2004), PLATON (Spek, 2009) and publCIF (Westrip 2010).

Figures top
[Figure 1] Fig. 1. The structure of (I) with ellipsoids drawn at the 50% probability level.
[Figure 2] Fig. 2. Bifurcated C–H···O hydrogen bonds (dashed lines), augmented by ππ stacking interactions (dotted lines) stacking molecules along c.
[Figure 3] Fig. 3. Layers of molecules in planes parallel to (101). Hydrogen bonds are drawn as dashed lines.
[Figure 4] Fig. 4. Overall packing of (I) with hydrogen bonds drawn as dashed lines.
5,6-dimethyl-1,2,9,10-tetrahydropyrano[3,2-f]chromene-3,8-dione top
Crystal data top
C14H14O4F(000) = 520
Mr = 246.25Dx = 1.444 Mg m3
Monoclinic, C2/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2ycCell parameters from 3955 reflections
a = 16.0726 (2) Åθ = 2.6–38.3°
b = 8.7982 (1) ŵ = 0.11 mm1
c = 8.0555 (1) ÅT = 92 K
β = 96.1134 (7)°Rectangular block, yellow
V = 1132.65 (2) Å30.53 × 0.50 × 0.22 mm
Z = 4
Data collection top
Bruker APEXII CCD area-detector
diffractometer
2989 independent reflections
Radiation source: fine-focus sealed tube2358 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.038
φ & ω scansθmax = 39.3°, θmin = 3.6°
Absorption correction: multi-scan
(SADABS; Bruker, 2011)
h = 2726
Tmin = 0.570, Tmax = 0.748k = 147
9744 measured reflectionsl = 1314
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.050Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.148H-atom parameters constrained
S = 1.08 w = 1/[σ2(Fo2) + (0.0702P)2 + 0.4368P]
where P = (Fo2 + 2Fc2)/3
2989 reflections(Δ/σ)max < 0.001
83 parametersΔρmax = 0.47 e Å3
0 restraintsΔρmin = 0.38 e Å3
Crystal data top
C14H14O4V = 1132.65 (2) Å3
Mr = 246.25Z = 4
Monoclinic, C2/cMo Kα radiation
a = 16.0726 (2) ŵ = 0.11 mm1
b = 8.7982 (1) ÅT = 92 K
c = 8.0555 (1) Å0.53 × 0.50 × 0.22 mm
β = 96.1134 (7)°
Data collection top
Bruker APEXII CCD area-detector
diffractometer
2989 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2011)
2358 reflections with I > 2σ(I)
Tmin = 0.570, Tmax = 0.748Rint = 0.038
9744 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0500 restraints
wR(F2) = 0.148H-atom parameters constrained
S = 1.08Δρmax = 0.47 e Å3
2989 reflectionsΔρmin = 0.38 e Å3
83 parameters
Special details top

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 (Å2) top
xyzUiso*/Ueq
O10.76307 (4)0.03359 (9)0.53742 (9)0.02826 (17)
C10.70230 (5)0.04789 (10)0.61199 (10)0.01983 (16)
O20.64973 (4)0.07377 (7)0.61147 (8)0.01902 (14)
C20.68037 (5)0.18492 (10)0.70770 (10)0.02081 (16)
H2A0.69970.17000.82740.025*
H2B0.70990.27450.66820.025*
C30.58596 (5)0.21578 (9)0.68795 (10)0.01919 (15)
H3A0.56840.25330.57370.023*
H3B0.57280.29520.76810.023*
C40.53894 (4)0.07275 (8)0.71898 (9)0.01485 (14)
C50.57500 (4)0.06647 (8)0.68637 (9)0.01470 (14)
C60.53839 (4)0.20612 (8)0.71558 (9)0.01519 (14)
C610.57956 (5)0.35308 (10)0.67541 (12)0.02167 (17)
H61A0.60300.40220.77920.033*
H61B0.53800.42040.61560.033*
H61C0.62450.33250.60530.033*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0216 (3)0.0340 (4)0.0307 (3)0.0048 (2)0.0097 (2)0.0013 (3)
C10.0175 (3)0.0226 (4)0.0194 (3)0.0039 (2)0.0021 (2)0.0023 (3)
O20.0158 (2)0.0186 (3)0.0232 (3)0.00139 (18)0.0047 (2)0.0013 (2)
C20.0214 (3)0.0204 (4)0.0207 (3)0.0070 (3)0.0026 (3)0.0000 (3)
C30.0220 (3)0.0143 (3)0.0212 (3)0.0028 (2)0.0018 (2)0.0018 (2)
C40.0165 (3)0.0124 (3)0.0152 (3)0.0007 (2)0.0003 (2)0.0005 (2)
C50.0140 (3)0.0145 (3)0.0154 (3)0.0003 (2)0.0009 (2)0.0001 (2)
C60.0150 (3)0.0122 (3)0.0178 (3)0.0004 (2)0.0007 (2)0.0006 (2)
C610.0196 (3)0.0149 (3)0.0302 (4)0.0028 (2)0.0012 (3)0.0024 (3)
Geometric parameters (Å, º) top
O1—C11.2065 (10)C3—H3B0.9900
C1—O21.3634 (10)C4—C51.3920 (10)
C1—C21.4934 (13)C4—C4i1.3962 (14)
O2—C51.4018 (9)C5—C61.3931 (10)
C2—C31.5329 (11)C6—C6i1.4058 (14)
C2—H2A0.9900C6—C611.5033 (11)
C2—H2B0.9900C61—H61A0.9800
C3—C41.5025 (11)C61—H61B0.9800
C3—H3A0.9900C61—H61C0.9800
O1—C1—O2116.80 (8)C5—C4—C4i118.34 (4)
O1—C1—C2126.11 (8)C5—C4—C3118.59 (7)
O2—C1—C2117.07 (7)C4i—C4—C3123.06 (4)
C1—O2—C5121.45 (6)C4—C5—C6123.53 (7)
C1—C2—C3112.01 (7)C4—C5—O2120.97 (6)
C1—C2—H2A109.2C6—C5—O2115.40 (6)
C3—C2—H2A109.2C5—C6—C6i118.10 (4)
C1—C2—H2B109.2C5—C6—C61121.25 (7)
C3—C2—H2B109.2C6i—C6—C61120.66 (4)
H2A—C2—H2B107.9C6—C61—H61A109.5
C4—C3—C2110.14 (7)C6—C61—H61B109.5
C4—C3—H3A109.6H61A—C61—H61B109.5
C2—C3—H3A109.6C6—C61—H61C109.5
C4—C3—H3B109.6H61A—C61—H61C109.5
C2—C3—H3B109.6H61B—C61—H61C109.5
H3A—C3—H3B108.1
O1—C1—O2—C5176.44 (7)C4i—C4—C5—O2174.81 (8)
C2—C1—O2—C54.87 (11)C3—C4—C5—O25.95 (11)
O1—C1—C2—C3142.07 (9)C1—O2—C5—C419.48 (11)
O2—C1—C2—C339.38 (10)C1—O2—C5—C6164.02 (7)
C1—C2—C3—C449.20 (9)C4—C5—C6—C6i1.15 (14)
C2—C3—C4—C527.91 (10)O2—C5—C6—C6i177.55 (8)
C2—C3—C4—C4i151.29 (9)C4—C5—C6—C61179.02 (7)
C4i—C4—C5—C61.40 (14)O2—C5—C6—C612.62 (11)
C3—C4—C5—C6177.84 (7)
Symmetry code: (i) x+1, y, z+3/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C61—H61A···O1ii0.982.543.3907 (11)145
C2—H2A···O1iii0.992.603.4301 (12)142
C2—H2A···O2iii0.992.643.4813 (10)143
C2—H2B···O1iv0.992.443.3528 (11)154
Symmetry codes: (ii) x+3/2, y+1/2, z+3/2; (iii) x, y, z+1/2; (iv) x+3/2, y1/2, z+1.

Experimental details

Crystal data
Chemical formulaC14H14O4
Mr246.25
Crystal system, space groupMonoclinic, C2/c
Temperature (K)92
a, b, c (Å)16.0726 (2), 8.7982 (1), 8.0555 (1)
β (°) 96.1134 (7)
V3)1132.65 (2)
Z4
Radiation typeMo Kα
µ (mm1)0.11
Crystal size (mm)0.53 × 0.50 × 0.22
Data collection
DiffractometerBruker APEXII CCD area-detector
Absorption correctionMulti-scan
(SADABS; Bruker, 2011)
Tmin, Tmax0.570, 0.748
No. of measured, independent and
observed [I > 2σ(I)] reflections
9744, 2989, 2358
Rint0.038
(sin θ/λ)max1)0.891
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.050, 0.148, 1.08
No. of reflections2989
No. of parameters83
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.47, 0.38

Computer programs: APEX2 (Bruker, 2011) and SAINT (Bruker, 2011), SAINT (Bruker, 2011), SHELXS97 (Sheldrick, 2008) and TITAN2000 (Hunter & Simpson, 1999), SHELXL97 (Sheldrick, 2008) and TITAN2000 (Hunter & Simpson, 1999), SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008), SHELXL97 (Sheldrick, 2008), enCIFer (Allen et al., 2004), PLATON (Spek, 2009) and publCIF (Westrip 2010).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C61—H61A···O1i0.982.543.3907 (11)145.0
C2—H2A···O1ii0.992.603.4301 (12)141.9
C2—H2A···O2ii0.992.643.4813 (10)142.5
C2—H2B···O1iii0.992.443.3528 (11)153.7
Symmetry codes: (i) x+3/2, y+1/2, z+3/2; (ii) x, y, z+1/2; (iii) x+3/2, y1/2, z+1.
 

Acknowledgements

We thank the New Economy Research Fund (grant No. UOO-X0808) for support of this work and the University of Otago for the purchase of the diffractometer.

References

First citationAllen, F. H. (2002). Acta Cryst. B58, 380–388.  Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
First citationAllen, F. H., Johnson, O., Shields, G. P., Smith, B. R. & Towler, M. (2004). J. Appl. Cryst. 37, 335–338.  Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
First citationAllen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1–19.  CSD CrossRef Web of Science Google Scholar
First citationBruker (2011). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
First citationCameron, S. A., Goswami, S. K., Hanton, L. R., McAdam, C. J., Moratti, S. C. & Simpson, J. (2011). Acta Cryst. E67, o2141–o2142.  Web of Science CSD CrossRef IUCr Journals Google Scholar
First citationGoswami, S. K., Hanton, L. R., McAdam, C. J., Moratti, S. C. & Simpson, J. (2011). Acta Cryst. E67, o1566–o1567.  Web of Science CSD CrossRef IUCr Journals Google Scholar
First citationHunter, K. A. & Simpson, J. (1999). TITAN2000. University of Otago, New Zealand.  Google Scholar
First citationLecea, M., Hernández-Torres, G., Urbano, A., Carreño, M. C. & Colobert, F. (2010). Org. Lett. 12, 580–583.  Web of Science CrossRef CAS PubMed Google Scholar
First citationMacrae, C. F., Bruno, I. J., Chisholm, J. A., Edgington, P. R., McCabe, P., Pidcock, E., Rodriguez-Monge, L., Taylor, R., van de Streek, J. & Wood, P. A. (2008). J. Appl. Cryst. 41, 466–470.  Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
First citationSheldrick, G. M. (2008). Acta Cryst. A64, 112–122.  Web of Science CrossRef CAS IUCr Journals Google Scholar
First citationSpek, A. L. (2009). Acta Cryst. D65, 148–155.  Web of Science CrossRef CAS IUCr Journals Google Scholar
First citationWestrip, S. P. (2010). J. Appl. Cryst. 43, 920–925.  Web of Science CrossRef CAS IUCr Journals Google Scholar

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