PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of actaeInternational Union of Crystallographysearchopen accessarticle submissionjournal home pagethis article
 
Acta Crystallogr Sect E Struct Rep Online. 2008 July 1; 64(Pt 7): o1343.
Published online 2008 June 28. doi:  10.1107/S1600536808018746
PMCID: PMC2961773

12-Nitro­methyl-14-deoxy­andro­graph­olide

Abstract

In the mol­ecule of the title compound {systematic name: 3-[2-(6-hydr­oxy-5-hydroxy­methyl-5,8a-dimethyl-2-methyl­ene­per­hydro-1-napth­yl)-1-(nitro­meth­yl)eth­yl]-2(4H)-furan­one}, C21H31NO6, the cyclo­hexane rings have chair conformations. Intra­molecular O—H(...)O hydrogen bonding results in the formation of a six-membered non-planar ring with a twist conformation. In the crystal structure, inter­molecular O—H(...)O hydrogen bonds link the mol­ecules into infinite chains along the c axis.

Related literature

For bond-length data, see: Allen et al. (1987 [triangle]). For ring puckering parameters, see: Cremer & Pople (1975 [triangle]).

An external file that holds a picture, illustration, etc.
Object name is e-64-o1343-scheme1.jpg

Experimental

Crystal data

  • C21H31NO6
  • M r = 393.47
  • Orthorhombic, An external file that holds a picture, illustration, etc.
Object name is e-64-o1343-efi1.jpg
  • a = 11.503 (2) Å
  • b = 13.151 (3) Å
  • c = 13.434 (3) Å
  • V = 2032.2 (7) Å3
  • Z = 4
  • Mo Kα radiation
  • μ = 0.09 mm−1
  • T = 294 (2) K
  • 0.40 × 0.20 × 0.20 mm

Data collection

  • Enraf–Nonius CAD-4 diffractometer
  • Absorption correction: ψ scan (North et al., 1968 [triangle]) T min = 0.964, T max = 0.982
  • 3993 measured reflections
  • 3643 independent reflections
  • 2711 reflections with I > 2σ(I)
  • R int = 0.032
  • 3 standard reflections frequency: 120 min intensity decay: 1%

Refinement

  • R[F 2 > 2σ(F 2)] = 0.054
  • wR(F 2) = 0.188
  • S = 0.98
  • 3643 reflections
  • 256 parameters
  • 1 restraint
  • H atoms treated by a mixture of independent and constrained refinement
  • Δρmax = 0.19 e Å−3
  • Δρmin = −0.20 e Å−3

Data collection: CAD-4 Software (Enraf–Nonius, 1989 [triangle]); cell refinement: CAD-4 Software; data reduction: XCAD4 (Harms & Wocadlo, 1995 [triangle]); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 [triangle]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 [triangle]); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997 [triangle]); software used to prepare material for publication: SHELXTL (Sheldrick, 2008 [triangle]).

Table 1
Hydrogen-bond geometry (Å, °)

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808018746/hk2476sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S1600536808018746/hk2476Isup2.hkl

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

The authors thank the Center for Testing and Analysis, Nanjing University, for support.

supplementary crystallographic information

Comment

Some derivatives of andrographolide are important chemical materials. We report herein the crystal structure of the title compound, (I).

In the molecule of (I), (Fig. 1), the bond lengths (Allen et al., 1987) and angles are within normal ranges. Rings A (C2-C7) and B (C5/C6/C8-C11) adopt chair [[var phi] = -86.32 (2)° and θ = 4.69 (3)° (for ring A) and [var phi] = -148.49 (3)° and θ = 86.21 (3)° (for ring B)] conformations, having total puckering amplitudes, QT, of 0.606 (3) Å and 0.642 (3) Å, respectively (Cremer & Pople, 1975). Ring C (O4/C18-C21) is, of course, planar. The intramolecular O-H···O hydrogen bond (Table 1) results in the formation of a six-membered non-planar ring: D (O1/H1A/O2/C8/C9/C13), in which it adopts twisted conformation, having total puckering amplitude, QT, of 1.200 (3) Å (Cremer & Pople, 1975).

In the crystal structure, intermolecular O-H···O hydrogen bonds (Table 1) link the molecules into infinite chains along the c axis (Fig. 2), in which they may be effective in the stabilization of the structure.

Experimental

For the preparation of the title compound, andrographolide (10 g) was dissolved in methanol (40 ml), and then nitromethane (16 ml), methanol (32 ml) and sodium methoxide (4.2 g) were added by stirring at room temperature. The reaction mixture was poured into ice salt water (120 ml). After the reaction finished, it was extracted with ethyl acetate, washed with saturated salt water and dryed with sodium sulfate. The product was filtrated and the organic layer was concentrated. Crystals suitable for X-ray analysis were obtained by slow evaporation of an ethyl acetate solution.

Refinement

H2A atom was located in difference map and refined [O2-H2A = 0.843 (10) Å; Uiso(H) = 0.080 Å2]. The remaining H atoms were positioned geometrically, with O-H = 0.82 Å (for OH) and C-H= 0.93 and 0.98 Å (for aromatic and methine H), 0.93 and 0.97 Å (for methylene H) and 0.96 Å (for methyl H), and constrained to ride on their parent atoms, with Uiso(H) = xUeq(C,O), where x = 1.5 for OH and methyl H, and x = 1.2 for all other H atoms.

Figures

Fig. 1.
The molecular structure of the title molecule, with the atom-numbering scheme. Hydrogen bond is shown as dashed line.
Fig. 2.
A partial packing diagram of (I). Hydrogen bonds are shown as dashed lines. H atoms not involved in hydrogen bonding have been omitted for clarity.

Crystal data

C21H31NO6F000 = 848
Mr = 393.47Dx = 1.289 Mg m3
Orthorhombic, P212121Mo Kα radiation λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 25 reflections
a = 11.503 (2) Åθ = 10–13º
b = 13.151 (3) ŵ = 0.09 mm1
c = 13.434 (3) ÅT = 294 (2) K
V = 2032.2 (7) Å3Block, colorless
Z = 40.40 × 0.20 × 0.20 mm

Data collection

Enraf–Nonius CAD-4 diffractometerRint = 0.032
Radiation source: fine-focus sealed tubeθmax = 25.2º
Monochromator: graphiteθmin = 2.2º
T = 294(2) Kh = 0→13
ω/2θ scansk = 0→15
Absorption correction: ψ scan(North et al., 1968)l = −16→16
Tmin = 0.964, Tmax = 0.9823 standard reflections
3993 measured reflections every 120 min
3643 independent reflections intensity decay: 1%
2711 reflections with I > 2σ(I)

Refinement

Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.055H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.188  w = 1/[σ2(Fo2) + (0.1P)2 + 1.5P] where P = (Fo2 + 2Fc2)/3
S = 0.98(Δ/σ)max < 0.001
3643 reflectionsΔρmax = 0.19 e Å3
256 parametersΔρmin = −0.20 e Å3
1 restraintExtinction correction: none
Primary atom site location: structure-invariant direct methods

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 > 2sigma(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)

xyzUiso*/Ueq
O10.3022 (3)0.2945 (3)0.3635 (3)0.0757 (11)
H1A0.32100.28680.42190.114*
O20.4711 (3)0.2805 (3)0.5078 (2)0.0664 (9)
H2A0.477 (5)0.325 (3)0.553 (3)0.080*
O30.9911 (4)0.6195 (2)0.1980 (3)0.0767 (11)
O41.0518 (4)0.5831 (2)0.3513 (2)0.0705 (10)
O51.2868 (3)0.3264 (4)0.0613 (3)0.0923 (14)
O61.2176 (3)0.4675 (3)0.1109 (3)0.0743 (11)
N1.2092 (3)0.3771 (4)0.0954 (3)0.0557 (10)
C10.7547 (4)0.4347 (4)0.0088 (3)0.0629 (13)
H1B0.71620.4713−0.04040.075*
H1C0.80970.3861−0.00880.075*
C20.7313 (4)0.4515 (3)0.1031 (3)0.0448 (10)
C30.6437 (4)0.5286 (3)0.1353 (4)0.0522 (11)
H3A0.61290.56360.07750.063*
H3B0.68060.57860.17800.063*
C40.5440 (4)0.4763 (3)0.1922 (3)0.0488 (11)
H4A0.49090.52740.21750.059*
H4B0.50110.43260.14700.059*
C50.5913 (3)0.4130 (3)0.2788 (3)0.0344 (8)
H5A0.63780.46150.31720.041*
C60.6812 (3)0.3322 (3)0.2437 (3)0.0331 (8)
C70.7821 (3)0.3923 (3)0.1901 (3)0.0359 (8)
H7A0.80880.44330.23800.043*
C80.4951 (3)0.3781 (3)0.3532 (3)0.0407 (9)
C90.5559 (4)0.3237 (4)0.4396 (3)0.0500 (10)
H9A0.60210.37400.47610.060*
C100.6359 (4)0.2395 (3)0.4063 (3)0.0496 (11)
H10A0.59080.18760.37250.059*
H10B0.67170.20850.46420.059*
C110.7308 (3)0.2784 (3)0.3364 (3)0.0445 (10)
H11A0.77860.22160.31530.053*
H11B0.78020.32550.37240.053*
C120.4332 (5)0.4722 (4)0.3948 (4)0.0646 (13)
H12A0.37360.45130.44050.097*
H12B0.39880.50980.34110.097*
H12C0.48840.51440.42890.097*
C130.4032 (3)0.3085 (4)0.3047 (4)0.0519 (11)
H13A0.38060.33750.24120.062*
H13B0.43800.24270.29170.062*
C140.6321 (4)0.2524 (3)0.1714 (3)0.0465 (10)
H14A0.57020.21570.20320.070*
H14B0.69250.20580.15270.070*
H14C0.60270.28570.11300.070*
C150.8890 (3)0.3280 (3)0.1645 (3)0.0422 (9)
H15A0.87420.29140.10310.051*
H15B0.90110.27830.21670.051*
C161.0006 (3)0.3920 (3)0.1522 (3)0.0392 (9)
H16A0.98760.44250.09980.047*
C171.0987 (4)0.3222 (4)0.1201 (4)0.0555 (12)
H17A1.11410.27390.17310.067*
H17B1.07380.28390.06210.067*
C181.0289 (3)0.4467 (3)0.2473 (3)0.0400 (9)
C191.0200 (4)0.5570 (4)0.2574 (3)0.0534 (11)
C201.0817 (5)0.4937 (4)0.4047 (4)0.0668 (13)
H20A1.03200.48550.46250.080*
H20B1.16200.49650.42680.080*
C211.0646 (4)0.4099 (4)0.3342 (3)0.0523 (11)
H21A1.07690.34150.34810.063*

Atomic displacement parameters (Å2)

U11U22U33U12U13U23
O10.0430 (18)0.083 (2)0.101 (3)−0.0086 (17)0.0210 (19)0.002 (2)
O20.069 (2)0.081 (2)0.0489 (18)−0.007 (2)0.0189 (17)0.0045 (16)
O30.112 (3)0.0437 (17)0.075 (2)0.001 (2)−0.020 (2)0.0099 (17)
O40.094 (3)0.061 (2)0.0572 (19)−0.0127 (19)−0.0167 (19)−0.0096 (17)
O50.055 (2)0.131 (4)0.091 (3)0.000 (2)0.018 (2)−0.038 (3)
O60.050 (2)0.078 (3)0.095 (3)−0.0172 (18)0.0090 (19)0.015 (2)
N0.038 (2)0.089 (3)0.0404 (19)0.000 (2)0.0067 (16)−0.003 (2)
C10.054 (3)0.080 (3)0.055 (3)−0.012 (3)0.001 (2)0.014 (3)
C20.041 (2)0.047 (2)0.047 (2)−0.0133 (19)0.0018 (18)0.0099 (19)
C30.052 (3)0.044 (2)0.061 (3)0.002 (2)−0.005 (2)0.015 (2)
C40.044 (2)0.048 (2)0.055 (3)0.0084 (19)−0.001 (2)0.016 (2)
C50.0328 (19)0.0331 (19)0.0373 (19)−0.0008 (15)−0.0054 (15)−0.0029 (15)
C60.0341 (19)0.0299 (18)0.0353 (19)−0.0043 (16)−0.0006 (16)−0.0001 (16)
C70.0347 (19)0.0321 (18)0.041 (2)−0.0026 (17)−0.0040 (17)−0.0023 (16)
C80.037 (2)0.0405 (19)0.045 (2)0.0007 (18)0.0087 (18)−0.0034 (17)
C90.054 (2)0.058 (3)0.038 (2)−0.012 (2)0.0076 (19)−0.0002 (19)
C100.051 (3)0.056 (2)0.042 (2)0.004 (2)−0.004 (2)0.018 (2)
C110.041 (2)0.043 (2)0.049 (2)0.0014 (18)0.0008 (19)0.0079 (18)
C120.064 (3)0.058 (3)0.072 (3)0.006 (2)0.017 (3)−0.006 (2)
C130.036 (2)0.056 (3)0.064 (3)−0.003 (2)0.004 (2)0.008 (2)
C140.051 (2)0.039 (2)0.049 (2)−0.0112 (19)0.007 (2)−0.0069 (19)
C150.040 (2)0.0364 (19)0.050 (2)−0.0072 (17)0.0065 (18)−0.0022 (17)
C160.038 (2)0.0405 (19)0.039 (2)−0.0032 (18)0.0041 (17)0.0026 (17)
C170.041 (2)0.065 (3)0.061 (3)−0.005 (2)0.011 (2)−0.009 (2)
C180.033 (2)0.044 (2)0.043 (2)−0.0048 (17)−0.0007 (18)0.0054 (18)
C190.058 (3)0.049 (2)0.053 (2)−0.008 (2)−0.005 (2)−0.001 (2)
C200.065 (3)0.087 (4)0.048 (3)−0.009 (3)−0.007 (2)0.004 (3)
C210.048 (2)0.058 (3)0.050 (2)−0.001 (2)−0.004 (2)0.011 (2)

Geometric parameters (Å, °)

O1—C131.417 (5)C8—C131.542 (6)
O1—H1A0.8200C9—C101.509 (6)
O2—C91.453 (5)C9—H9A0.9800
O2—H2A0.85 (4)C10—C111.528 (6)
O3—C191.194 (5)C10—H10A0.9700
C3—C41.540 (6)C10—H10B0.9700
C3—H3A0.9700C11—H11A0.9700
C3—H3B0.9700C11—H11B0.9700
O4—C191.358 (5)C12—H12A0.9600
O4—C201.420 (6)C12—H12B0.9600
N—O51.205 (5)C12—H12C0.9600
N—O61.211 (5)C13—H13A0.9700
N—C171.499 (6)C13—H13B0.9700
C1—C21.314 (6)C14—H14A0.9600
C1—H1B0.9300C14—H14B0.9600
C1—H1C0.9300C14—H14C0.9600
C2—C31.494 (6)C15—C161.544 (5)
C2—C71.521 (6)C15—H15A0.9700
C4—C51.530 (5)C15—H15B0.9700
C4—H4A0.9700C16—C181.502 (6)
C4—H4B0.9700C16—C171.517 (6)
C5—C61.556 (5)C16—H16A0.9800
C5—C81.561 (5)C17—H17A0.9700
C5—H5A0.9800C17—H17B0.9700
C6—C141.538 (5)C18—C211.329 (6)
C6—C111.542 (5)C18—C191.460 (6)
C6—C71.578 (5)C20—C211.467 (7)
C7—C151.531 (5)C20—H20A0.9700
C7—H7A0.9800C20—H20B0.9700
C8—C91.532 (6)C21—H21A0.9300
C8—C121.533 (6)
C13—O1—H1A109.5H10A—C10—H10B107.9
O5—N—O6123.3 (4)C10—C11—C6112.7 (3)
O5—N—C17116.5 (4)C10—C11—H11A109.1
O6—N—C17120.2 (4)C6—C11—H11A109.1
C2—C1—H1B120.0C10—C11—H11B109.1
C2—C1—H1C120.0C6—C11—H11B109.1
H1B—C1—H1C120.0H11A—C11—H11B107.8
C1—C2—C3122.1 (4)C8—C12—H12A109.5
C1—C2—C7125.2 (4)C8—C12—H12B109.5
C3—C2—C7112.6 (3)H12A—C12—H12B109.5
C9—O2—H2A97 (4)C8—C12—H12C109.5
C2—C3—C4110.1 (3)H12A—C12—H12C109.5
C2—C3—H3A109.6H12B—C12—H12C109.5
C4—C3—H3A109.6O1—C13—C8113.8 (4)
C2—C3—H3B109.6O1—C13—H13A108.8
C4—C3—H3B109.7C8—C13—H13A108.8
H3A—C3—H3B108.2O1—C13—H13B108.8
C19—O4—C20109.0 (4)C8—C13—H13B108.8
C5—C4—C3110.8 (3)H13A—C13—H13B107.7
C5—C4—H4A109.5C6—C14—H14A109.5
C3—C4—H4A109.5C6—C14—H14B109.5
C5—C4—H4B109.5H14A—C14—H14B109.5
C3—C4—H4B109.5C6—C14—H14C109.5
H4A—C4—H4B108.1H14A—C14—H14C109.5
C4—C5—C6112.2 (3)H14B—C14—H14C109.5
C4—C5—C8113.3 (3)C7—C15—C16113.1 (3)
C6—C5—C8117.7 (3)C7—C15—H15A109.0
C4—C5—H5A103.9C16—C15—H15A109.0
C6—C5—H5A103.9C7—C15—H15B109.0
C8—C5—H5A103.9C16—C15—H15B109.0
C14—C6—C11109.5 (3)H15A—C15—H15B107.8
C14—C6—C5114.4 (3)C18—C16—C17111.8 (4)
C11—C6—C5108.3 (3)C18—C16—C15110.6 (3)
C14—C6—C7108.9 (3)C17—C16—C15108.6 (3)
C11—C6—C7109.0 (3)C18—C16—H16A108.6
C5—C6—C7106.6 (3)C17—C16—H16A108.6
C2—C7—C15114.7 (3)C15—C16—H16A108.6
C2—C7—C6108.9 (3)N—C17—C16113.7 (4)
C15—C7—C6114.7 (3)N—C17—H17A108.8
C2—C7—H7A105.9C16—C17—H17A108.8
C15—C7—H7A105.9N—C17—H17B108.8
C6—C7—H7A105.9C16—C17—H17B108.8
C9—C8—C12108.2 (4)H17A—C17—H17B107.7
C9—C8—C13110.9 (3)C21—C18—C19107.6 (4)
C12—C8—C13108.3 (4)C21—C18—C16129.7 (4)
C9—C8—C5107.4 (3)C19—C18—C16122.7 (4)
C12—C8—C5109.0 (3)O3—C19—O4121.5 (4)
C13—C8—C5112.9 (3)O3—C19—C18129.9 (4)
O2—C9—C10108.0 (4)O4—C19—C18108.6 (4)
O2—C9—C8110.7 (4)O4—C20—C21105.2 (4)
C10—C9—C8113.3 (3)O4—C20—H20A110.7
O2—C9—H9A108.2C21—C20—H20A110.7
C10—C9—H9A108.2O4—C20—H20B110.7
C8—C9—H9A108.2C21—C20—H20B110.7
C9—C10—C11111.9 (3)H20A—C20—H20B108.8
C9—C10—H10A109.2C18—C21—C20109.6 (4)
C11—C10—H10A109.2C18—C21—H21A125.2
C9—C10—H10B109.2C20—C21—H21A125.2
C11—C10—H10B109.2
C1—C2—C3—C4117.9 (5)C5—C8—C9—C10−53.2 (4)
C7—C2—C3—C4−58.4 (5)O2—C9—C10—C11−178.3 (3)
C2—C3—C4—C554.5 (5)C8—C9—C10—C1158.7 (5)
C3—C4—C5—C6−56.8 (4)C9—C10—C11—C6−57.0 (5)
C3—C4—C5—C8167.0 (3)C14—C6—C11—C10−74.5 (4)
C4—C5—C6—C14−62.0 (4)C5—C6—C11—C1050.8 (4)
C8—C5—C6—C1472.1 (4)C7—C6—C11—C10166.4 (3)
C4—C5—C6—C11175.6 (3)C9—C8—C13—O172.8 (4)
C8—C5—C6—C11−50.3 (4)C12—C8—C13—O1−45.8 (5)
C4—C5—C6—C758.4 (4)C5—C8—C13—O1−166.6 (4)
C8—C5—C6—C7−167.5 (3)C2—C7—C15—C1675.8 (4)
C1—C2—C7—C1515.6 (6)C6—C7—C15—C16−157.0 (3)
C3—C2—C7—C15−168.2 (3)C7—C15—C16—C1860.9 (4)
C1—C2—C7—C6−114.4 (5)C7—C15—C16—C17−176.2 (4)
C3—C2—C7—C661.8 (4)O5—N—C17—C16−173.8 (4)
C14—C6—C7—C264.7 (4)O6—N—C17—C166.9 (6)
C11—C6—C7—C2−175.9 (3)C18—C16—C17—N−63.8 (5)
C5—C6—C7—C2−59.2 (4)C15—C16—C17—N174.0 (4)
C14—C6—C7—C15−65.3 (4)C17—C16—C18—C21−52.3 (6)
C11—C6—C7—C1554.1 (4)C15—C16—C18—C2168.9 (5)
C5—C6—C7—C15170.8 (3)C17—C16—C18—C19127.8 (4)
C4—C5—C8—C9−175.4 (3)C15—C16—C18—C19−111.0 (4)
C6—C5—C8—C951.0 (4)C20—O4—C19—O3−179.9 (5)
C4—C5—C8—C12−58.4 (5)C20—O4—C19—C18−0.1 (5)
C6—C5—C8—C12168.0 (4)C21—C18—C19—O3−179.8 (5)
C4—C5—C8—C1362.1 (4)C16—C18—C19—O30.1 (8)
C6—C5—C8—C13−71.5 (4)C21—C18—C19—O40.4 (5)
C12—C8—C9—O267.8 (4)C16—C18—C19—O4−179.7 (3)
C13—C8—C9—O2−50.9 (4)C19—O4—C20—C21−0.2 (5)
C5—C8—C9—O2−174.7 (3)C19—C18—C21—C20−0.5 (5)
C12—C8—C9—C10−170.7 (4)C16—C18—C21—C20179.6 (4)
C13—C8—C9—C1070.6 (4)O4—C20—C21—C180.5 (5)

Hydrogen-bond geometry (Å, °)

D—H···AD—HH···AD···AD—H···A
O1—H1A···O20.822.082.751 (5)139
O2—H2A···O3i0.85 (4)2.11 (4)2.906 (5)156 (4)

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

Footnotes

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HK2476).

References

  • 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.
  • Cremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc.97, 1354–1358.
  • Enraf–Nonius (1989). CAD-4 Software Enraf–Nonius, Delft, The Netherlands.
  • Farrugia, L. J. (1997). J. Appl. Cryst.30, 565.
  • Harms, K. & Wocadlo, S. (1995). XCAD4 University of Marburg, Germany.
  • North, A. C. T., Phillips, D. C. & Mathews, F. S. (1968). Acta Cryst. A24, 351–359.
  • Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. [PubMed]

Articles from Acta Crystallographica Section E: Structure Reports Online are provided here courtesy of International Union of Crystallography