(2S)-1,1-Dichloro-2-(2-chloro­phen­yl)-2-(4-chloro­phen­yl)ethane

Cantillana, T.; Eriksson, L.

doi:10.1107/S160053680804405X

organic compounds

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ISSN: 2056-9890

Volume 65| Part 2| February 2009| Page o297

doi:10.1107/S160053680804405X

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(2S)-1,1-Dichloro-2-(2-chlorophenyl)-2-(4-chlorophenyl)ethane

Tatiana Cantillana ^a and Lars Eriksson ^b ^*

^aDepartment of Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden, and ^bDepartment of Physical, Inorganic and Structural Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
^*Correspondence e-mail: lerik@struc.su.se

(Received 15 December 2008; accepted 27 December 2008; online 14 January 2009)

The title compound, C₁₄H₁₀Cl₄, is easily crystallized while the other enantiomorph only forms an oil upon crystallization attempts. The title compound has a considerably higher density, ρ ≃ 1.562 Mg m⁻³ compared to the racemic substance, ρ ≃ 1.514 Mg m⁻³. This is supported by the fact there are two intermolecular halogen–halogen contacts in the title compound compared with only one the racemic compound. The dihedral angle between the two phenyl rings is 76.83 (5)°

Related literature

For related literature regarding the structure of the racemic compound, see: Arora & Bates (1976 ). For related literature on the toxicological effects, see: Allolio & Fassnacht (2006 ), Benecke et al. (1991 ), Bergenstal et al. (1960 ); Cantillana et al. (2009 ).

Experimental

Crystal data

C₁₄H₁₀Cl₄
M_r = 320.02
Monoclinic, P 2₁
a = 6.13530 (10) Å
b = 12.0715 (2) Å
c = 9.4525 (2) Å
β = 103.5490 (18)°
V = 680.59 (2) Å³
Z = 2
Mo Kα radiation
μ = 0.85 mm⁻¹
T = 100 (2) K
0.34 × 0.24 × 0.04 mm

Data collection

Oxford Diffraction Xcalibur-3 κ-diffractometer with Sapphire-III CCD
Absorption correction: gaussian (CrysAlis RED; Oxford Diffraction, 2008 $[Oxford Diffraction. (2008). CrysAlis CCD and CrysAlis RED. Oxford Diffraction Ltd, Abingdon, Oxfordshire, England.]$ ) T_min = 0.814, T_max = 0.968
18569 measured reflections
4258 independent reflections
3935 reflections with I > 2σ(I)
R_int = 0.032

Refinement

R[F² > 2σ(F²)] = 0.026
wR(F²) = 0.060
S = 1.01
4258 reflections
164 parameters
1 restraint
H-atom parameters constrained
Δρ_max = 0.40 e Å⁻³
Δρ_min = −0.31 e Å⁻³
Absolute structure: Flack (1983 ), 1755 Friedel pairs
Flack parameter: 0.00 (4)

Table 1
Selected interatomic distances (Å)

Cl1⋯Cl4ⁱ	3.4370 (5)
Cl2⋯Cl3ⁱⁱ	3.4888 (5)
Symmetry codes: (i) $[-x+2, y-{\script{1\over 2}}, -z+1]$ ; (ii) $[-x+1, y-{\script{1\over 2}}, -z]$ .

Data collection: CrysAlis CCD (Oxford Diffraction, 2008 $[Oxford Diffraction. (2008). CrysAlis CCD and CrysAlis RED. Oxford Diffraction Ltd, Abingdon, Oxfordshire, England.]$ ); cell refinement: CrysAlis RED (Oxford Diffraction, 2008 $[Oxford Diffraction. (2008). CrysAlis CCD and CrysAlis RED. Oxford Diffraction Ltd, Abingdon, Oxfordshire, England.]$ ); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Bergerhoff, 1996 ); software used to prepare material for publication: PLATON (Spek, 2003 ) and SHELXL97.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S160053680804405X/bq2116sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S160053680804405X/bq2116Isup2.hkl

checkCIF report

Comment top

The title compound is commercially available as a racemate which has been structurally characterized earlier (Arora & Bates, 1976). When purifying and separating the two enantiomers of the racemate, one of the enantiomers, the title compound easily formed crystals while the other enantiomer only formed an oil upon crystallization attempts. A salient feature of the racemic compound o,p'-DDD (Mitotane) is its selective toxicity to the adrenal cortex. It has been used for 40 years for treatment of adrenocortical carcinoma (ACC) (Bergenstal et al., 1960) and Cushing's syndrome (Benecke et al., 1991). The efficacy and potency is however low, and o,p'-DDD treatment is frequently associated with severe side effects (Allolio & Fassnacht, 2006). The differences in toxicity of the two enantiomers of o,p'-DDD and the pharmacokinetics connected with these two compounds has recently been examined in Göttingen mini pigs and will be reported elsewhere (Cantillana et al., 2009).

The crystal structure of (I) shown in Fig. 1 show normal bond distances and angles. The dihedral angle between the two phenyl rings is 76.83 (5)°. Both phenyl rings are planar within 0.01 Å with the Cl3 deviating 0.103 (2) Å from the least square plane calculated from C3→C8 and the Cl4 deviating 0.048 (2) from the least square plane of C9→C14. All four chlorines are involved in the intermolecular Cl···Cl contacts between the different molecules building up a corrugated layer extending in the [010] and [101] directions. The title compound has a considerably higher density, ρ≈ 1.562 g/cm³ compared to the racemate, ρ≈ 1.514 g/cm³ (Arora & Bates, 1976). A tentative model for the higher density of the pure enantiomer is that it may be a result of the more numerous intermolecular short halogen-halogen contacts.

Related literature top

For related literature, regarding the structure of the racemic compound, see: Arora & Bates (1976). For related literature on the toxicological effects, see: Allolio & Fassnacht (2006), Benecke et al. (1991), Bergenstal et al. (1960).

For related literature, see: Cantillana et al. (2009).

Experimental top

The title compound was purified from a racemic mixture present in the commercially available product, 1,1-Dichloro-2-(2-chlorophenyl)-2-(4-chlorophenyl)ethane (o,p'-DDD) using high performance liquid chromatography (HPLC), Shimadzu LC-9 A (Kyoto, Japan) equipped with an UV detector, UV100 from Spectra-Physics (Fremont, USA) and a permethylated γ-cyclodextrin column, Nucleodex gamma-PM (250 x 10 mm, 5µm, Macherey-Nagel GmbH & Co, Düren, Germany). The detection wavelength was 240 nm and the flow rate was 4 ml/min and injection volume of 200µl. The mobile phase was methanol:water (80:20) and 1% triethylamine:acetic acid (1:2 v/v). Thin plate-like crystals suitable for X-ray analysis were obtained upon recrystallization from methanol.

Computing details top

Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis RED (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Bergerhoff, 1996); software used to prepare material for publication: PLATON (Spek, 2003) and SHELXL97 (Sheldrick, 2008).

Figures top

Fig. 1. The title compound (I) with displacement ellipsoids at 50% probability with the unique atoms labeled.

(2S)-1,1-Dichloro-2-(2-chlorophenyl)-2-(4-chlorophenyl)ethane top

Crystal data top

C₁₄H₁₀Cl₄	F(000) = 324
M_r = 320.02	D_x = 1.562 Mg m⁻³
Monoclinic, P2₁	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2yb	Cell parameters from 11963 reflections
a = 6.1353 (1) Å	θ = 3.8–32.1°
b = 12.0715 (2) Å	µ = 0.85 mm⁻¹
c = 9.4525 (2) Å	T = 100 K
β = 103.5490 (18)°	Plate, colourless
V = 680.59 (2) Å³	0.34 × 0.24 × 0.04 mm
Z = 2

Data collection top

Oxford Diffraction Xcalibur-3 κ- diffractometer with Sapphire-III CCD	4258 independent reflections
Radiation source: Enhance (Mo) X-ray Source	3935 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.032
Detector resolution: 16.54 pixels mm^-1	θ_max = 32.2°, θ_min = 3.8°
ω scans at different ϕ	h = −9→9
Absorption correction: gaussian (CrysAlis RED; Oxford Diffraction, 2008)	k = −17→15
T_min = 0.814, T_max = 0.968	l = −13→14
18569 measured reflections

Refinement top

Refinement on F²	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F² > 2σ(F²)] = 0.026	w = 1/[σ²(F_o²) + (0.0358P)²] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.060	(Δ/σ)_max = 0.001
S = 1.01	Δρ_max = 0.40 e Å⁻³
4258 reflections	Δρ_min = −0.31 e Å⁻³
164 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
1 restraint	Extinction coefficient: 0.009 (2)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983), 1755 Friedel pairs
Secondary atom site location: difference Fourier map	Absolute structure parameter: 0.00 (4)

Crystal data top

C₁₄H₁₀Cl₄	V = 680.59 (2) Å³
M_r = 320.02	Z = 2
Monoclinic, P2₁	Mo Kα radiation
a = 6.1353 (1) Å	µ = 0.85 mm⁻¹
b = 12.0715 (2) Å	T = 100 K
c = 9.4525 (2) Å	0.34 × 0.24 × 0.04 mm
β = 103.5490 (18)°

Data collection top

Oxford Diffraction Xcalibur-3 κ- diffractometer with Sapphire-III CCD	4258 independent reflections
Absorption correction: gaussian (CrysAlis RED; Oxford Diffraction, 2008)	3935 reflections with I > 2σ(I)
T_min = 0.814, T_max = 0.968	R_int = 0.032
18569 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.026	H-atom parameters constrained
wR(F²) = 0.060	Δρ_max = 0.40 e Å⁻³
S = 1.01	Δρ_min = −0.31 e Å⁻³
4258 reflections	Absolute structure: Flack (1983), 1755 Friedel pairs
164 parameters	Absolute structure parameter: 0.00 (4)
1 restraint

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

	x	y	z	U_iso*/U_eq
C1	0.5723 (2)	0.58327 (13)	0.35281 (15)	0.0153 (3)
H1	0.4179	0.5513	0.3360	0.018*
C2	0.5682 (2)	0.70231 (12)	0.40708 (16)	0.0136 (3)
H2	0.7233	0.7330	0.4219	0.016*
Cl1	0.75951 (6)	0.50047 (3)	0.48348 (4)	0.02031 (8)
Cl2	0.66068 (6)	0.58092 (3)	0.18581 (4)	0.02172 (9)
C3	0.4118 (2)	0.77539 (12)	0.29597 (15)	0.0143 (3)
C4	0.4896 (3)	0.87540 (13)	0.25332 (16)	0.0178 (3)
H4	0.6434	0.8943	0.2871	0.021*
C5	0.3459 (3)	0.94815 (14)	0.16209 (16)	0.0196 (3)
H5	0.4006	1.0162	0.1335	0.024*
C6	0.1222 (3)	0.92006 (14)	0.11361 (15)	0.0174 (3)
C7	0.0412 (3)	0.81928 (14)	0.15077 (17)	0.0197 (3)
H7	−0.1116	0.7996	0.1141	0.024*
C8	0.1859 (2)	0.74824 (14)	0.24185 (17)	0.0185 (3)
H8	0.1310	0.6796	0.2683	0.022*
Cl3	−0.06289 (6)	1.01373 (3)	0.00690 (4)	0.02230 (9)
C9	0.5012 (2)	0.70772 (12)	0.55256 (16)	0.0149 (3)
C10	0.3199 (3)	0.64714 (13)	0.57858 (16)	0.0177 (3)
H10	0.2424	0.5974	0.5062	0.021*
C11	0.2503 (3)	0.65801 (14)	0.70765 (17)	0.0192 (3)
H11	0.1263	0.6162	0.7223	0.023*
C12	0.3609 (3)	0.72933 (15)	0.81459 (17)	0.0212 (3)
H12	0.3129	0.7366	0.9026	0.025*
C13	0.5417 (3)	0.79019 (14)	0.79373 (17)	0.0207 (3)
H13	0.6192	0.8389	0.8674	0.025*
C14	0.6088 (2)	0.77922 (13)	0.66334 (16)	0.0157 (3)
Cl4	0.83309 (6)	0.86142 (3)	0.64058 (4)	0.02012 (8)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0144 (6)	0.0158 (7)	0.0172 (6)	0.0000 (5)	0.0071 (5)	−0.0006 (6)
C2	0.0121 (6)	0.0131 (7)	0.0159 (6)	−0.0025 (5)	0.0041 (5)	−0.0019 (5)
Cl1	0.01984 (16)	0.01761 (17)	0.02400 (17)	0.00417 (14)	0.00622 (12)	0.00057 (14)
Cl2	0.02671 (18)	0.02192 (19)	0.02051 (16)	−0.00424 (15)	0.01356 (14)	−0.00507 (14)
C3	0.0158 (6)	0.0140 (7)	0.0139 (6)	−0.0016 (5)	0.0054 (5)	−0.0012 (5)
C4	0.0171 (6)	0.0189 (8)	0.0175 (6)	−0.0054 (6)	0.0042 (5)	−0.0007 (6)
C5	0.0261 (8)	0.0167 (7)	0.0162 (7)	−0.0061 (6)	0.0054 (6)	0.0018 (5)
C6	0.0210 (7)	0.0189 (7)	0.0129 (6)	0.0025 (6)	0.0052 (5)	0.0020 (5)
C7	0.0173 (7)	0.0212 (8)	0.0202 (7)	−0.0024 (6)	0.0039 (6)	0.0019 (6)
C8	0.0171 (7)	0.0174 (7)	0.0206 (7)	−0.0043 (6)	0.0037 (5)	0.0021 (6)
Cl3	0.02784 (18)	0.02156 (19)	0.01717 (15)	0.00582 (15)	0.00459 (13)	0.00309 (14)
C9	0.0156 (6)	0.0138 (7)	0.0154 (6)	0.0041 (5)	0.0034 (5)	0.0005 (5)
C10	0.0205 (7)	0.0166 (7)	0.0172 (6)	0.0015 (6)	0.0070 (5)	0.0021 (6)
C11	0.0186 (7)	0.0203 (8)	0.0205 (7)	0.0034 (6)	0.0081 (6)	0.0048 (6)
C12	0.0263 (8)	0.0224 (8)	0.0163 (7)	0.0089 (7)	0.0077 (6)	0.0029 (6)
C13	0.0258 (8)	0.0189 (8)	0.0158 (7)	0.0070 (6)	0.0020 (6)	−0.0012 (6)
C14	0.0146 (6)	0.0126 (7)	0.0185 (6)	0.0029 (5)	0.0013 (5)	−0.0002 (5)
Cl4	0.01702 (16)	0.01679 (17)	0.02510 (18)	−0.00235 (13)	0.00201 (13)	−0.00440 (14)

Geometric parameters (Å, º) top

C1—C2	1.528 (2)	C7—C8	1.381 (2)
C1—Cl1	1.7831 (15)	C7—H7	0.9500
C1—Cl2	1.7855 (14)	C8—H8	0.9500
C1—H1	1.0000	C9—C14	1.398 (2)
C2—C9	1.526 (2)	C9—C10	1.400 (2)
C2—C3	1.526 (2)	C10—C11	1.390 (2)
C2—H2	1.0000	C10—H10	0.9500
C3—C4	1.392 (2)	C11—C12	1.379 (2)
C3—C8	1.399 (2)	C11—H11	0.9500
C4—C5	1.391 (2)	C12—C13	1.382 (2)
C4—H4	0.9500	C12—H12	0.9500
C5—C6	1.383 (2)	C13—C14	1.394 (2)
C5—H5	0.9500	C13—H13	0.9500
C6—C7	1.390 (2)	C14—Cl4	1.7503 (16)
C6—Cl3	1.7462 (16)

Cl1···Cl4ⁱ	3.4370 (5)	Cl2···Cl3ⁱⁱ	3.4888 (5)

C2—C1—Cl1	110.68 (10)	C8—C7—C6	119.01 (14)
C2—C1—Cl2	110.12 (10)	C8—C7—H7	120.5
Cl1—C1—Cl2	108.88 (8)	C6—C7—H7	120.5
C2—C1—H1	109.0	C7—C8—C3	121.31 (14)
Cl1—C1—H1	109.0	C7—C8—H8	119.3
Cl2—C1—H1	109.0	C3—C8—H8	119.3
C9—C2—C3	109.67 (11)	C14—C9—C10	116.49 (13)
C9—C2—C1	111.82 (12)	C14—C9—C2	121.46 (13)
C3—C2—C1	111.69 (12)	C10—C9—C2	121.92 (13)
C9—C2—H2	107.8	C11—C10—C9	121.65 (15)
C3—C2—H2	107.8	C11—C10—H10	119.2
C1—C2—H2	107.8	C9—C10—H10	119.2
C4—C3—C8	118.35 (14)	C12—C11—C10	120.19 (15)
C4—C3—C2	119.83 (13)	C12—C11—H11	119.9
C8—C3—C2	121.69 (13)	C10—C11—H11	119.9
C5—C4—C3	121.07 (14)	C11—C12—C13	120.04 (14)
C5—C4—H4	119.5	C11—C12—H12	120.0
C3—C4—H4	119.5	C13—C12—H12	120.0
C6—C5—C4	119.09 (15)	C12—C13—C14	119.25 (15)
C6—C5—H5	120.5	C12—C13—H13	120.4
C4—C5—H5	120.5	C14—C13—H13	120.4
C5—C6—C7	121.11 (15)	C13—C14—C9	122.37 (15)
C5—C6—Cl3	119.54 (13)	C13—C14—Cl4	117.29 (12)
C7—C6—Cl3	119.33 (12)	C9—C14—Cl4	120.33 (12)

Cl1—C1—C2—C9	−57.58 (13)	C2—C3—C8—C7	174.53 (14)
Cl2—C1—C2—C9	−178.02 (9)	C3—C2—C9—C14	−96.67 (15)
Cl1—C1—C2—C3	179.08 (9)	C1—C2—C9—C14	138.85 (14)
Cl2—C1—C2—C3	58.64 (13)	C3—C2—C9—C10	78.99 (17)
C9—C2—C3—C4	107.05 (15)	C1—C2—C9—C10	−45.48 (18)
C1—C2—C3—C4	−128.40 (14)	C14—C9—C10—C11	0.2 (2)
C9—C2—C3—C8	−68.77 (17)	C2—C9—C10—C11	−175.63 (14)
C1—C2—C3—C8	55.78 (18)	C9—C10—C11—C12	−0.3 (2)
C8—C3—C4—C5	1.6 (2)	C10—C11—C12—C13	−0.1 (2)
C2—C3—C4—C5	−174.36 (14)	C11—C12—C13—C14	0.6 (2)
C3—C4—C5—C6	0.1 (2)	C12—C13—C14—C9	−0.7 (2)
C4—C5—C6—C7	−2.1 (2)	C12—C13—C14—Cl4	178.08 (12)
C4—C5—C6—Cl3	176.66 (12)	C10—C9—C14—C13	0.3 (2)
C5—C6—C7—C8	2.3 (2)	C2—C9—C14—C13	176.15 (14)
Cl3—C6—C7—C8	−176.42 (12)	C10—C9—C14—Cl4	−178.44 (11)
C6—C7—C8—C3	−0.6 (2)	C2—C9—C14—Cl4	−2.55 (19)
C4—C3—C8—C7	−1.3 (2)

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

Experimental details

Crystal data
Chemical formula	C₁₄H₁₀Cl₄
M_r	320.02
Crystal system, space group	Monoclinic, P2₁
Temperature (K)	100
a, b, c (Å)	6.1353 (1), 12.0715 (2), 9.4525 (2)
β (°)	103.5490 (18)
V (Å³)	680.59 (2)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	0.85
Crystal size (mm)	0.34 × 0.24 × 0.04

Data collection
Diffractometer	Oxford Diffraction Xcalibur-3 κ- diffractometer with Sapphire-III CCD
Absorption correction	Gaussian (CrysAlis RED; Oxford Diffraction, 2008)
T_min, T_max	0.814, 0.968
No. of measured, independent and observed [I > 2σ(I)] reflections	18569, 4258, 3935
R_int	0.032
(sin θ/λ)_max (Å⁻¹)	0.751

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.026, 0.060, 1.01
No. of reflections	4258
No. of parameters	164
No. of restraints	1
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.40, −0.31
Absolute structure	Flack (1983), 1755 Friedel pairs
Absolute structure parameter	0.00 (4)

Computer programs: CrysAlis CCD (Oxford Diffraction, 2008), CrysAlis RED (Oxford Diffraction, 2008), SHELXS97 (Sheldrick, 2008), DIAMOND (Bergerhoff, 1996), PLATON (Spek, 2003) and SHELXL97 (Sheldrick, 2008).

Selected interatomic distances (Å) top

Cl1···Cl4ⁱ

3.4370 (5)

Cl2···Cl3ⁱⁱ

3.4888 (5)

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

Acknowledgements

This work was supported by a grant from the Swedish Research Council and by the Faculty of Natural Sciences at Stockholm University.

References

Allolio, B. & Fassnacht, M. (2006). J. Clin. Endocrinol. Metab. 91, 2027–2037. Web of Science CrossRef PubMed CAS Google Scholar
Arora, S. K. & Bates, R. B. (1976). J. Org. Chem. 41, 554–556. CSD CrossRef PubMed CAS Web of Science Google Scholar
Benecke, R., Keller, E., Vetter, B. & de Zeeuw, R. A. (1991). Eur. J. Clin. Pharmacol. 41, 259–261. CrossRef PubMed CAS Web of Science Google Scholar
Bergenstal, D. M., Hertz, R., Lipsett, M. B. & Moy, R. H. (1960). Ann. Intern. Med. 53, 672–682. CAS Google Scholar
Bergerhoff, G. (1996). DIAMOND. Gerhard-Domagk Strasse 1, D-53121 Bonn, Germany. Google Scholar
Cantillana, T., Lindström, V., Eriksson, L., Brandt, I. & Bergman, Å. (2009). In preparation. Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Oxford Diffraction. (2008). CrysAlis CCD and CrysAlis RED. Oxford Diffraction Ltd, Abingdon, Oxfordshire, England. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2003). J. Appl. Cryst. 36, 7–13. Web of Science CrossRef CAS IUCr Journals Google Scholar