1,1′,2,2′-Tetra­methyl-3,3′-(p-phenyl­enedimethyl­ene)diimidazol-1-ium bis­(hexa­fluoridophosphate)

Puvaneswary, S.; Alias, Y.; Ng, S.W.

doi:10.1107/S1600536809026336

organic compounds

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ISSN: 2056-9890

Volume 65| Part 8| August 2009| Page o1830

doi:10.1107/S1600536809026336

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1,1′,2,2′-Tetramethyl-3,3′-(p-phenylenedimethylene)diimidazol-1-ium bis(hexafluoridophosphate)

Subramaniam Puvaneswary,^a Yatimah Alias ^a and Seik Weng Ng ^a ^*

^aDepartment of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
^*Correspondence e-mail: seikweng@um.edu.my

(Received 4 July 2009; accepted 6 July 2009; online 11 July 2009)

The title imidazolium-based ionic-liquid salt, C₁₈H₂₄N₄²⁺·2PF₆⁻, has the cation lying about a center of inversion. The five-membered imidazole ring is disordered over two positions (the methyl substituents are ordered). This imidazole ring is approximately perpendicular to the six-membered phenylene ring [dihedral angle = 81.3 (8)° for one disorder component and 83.8 (8)° for the other; the two components are off-set by 2.7 (8)°]. The crystal is a non-merohedral twin with a twin component of 23%.

Related literature

For background to imidazolium-based ionic liquid salts, see: Ganesan et al. (2008 ). For the procedure to manipulate twinned diffraction data, see: Spek (2003).

Experimental

Crystal data

C₁₈H₂₄N₄²⁺·2PF₆⁻
M_r = 586.35
Triclinic, $[P \overline 1]$
a = 7.3808 (3) Å
b = 8.2169 (4) Å
c = 11.0553 (5) Å
α = 73.435 (3)°
β = 71.173 (3)°
γ = 73.897 (3)°
V = 595.43 (5) Å³
Z = 1
Mo Kα radiation
μ = 0.29 mm⁻¹
T = 140 K
0.35 × 0.03 × 0.03 mm

Data collection

Bruker SMART APEX diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ) T_min = 0.905, T_max = 0.991
4724 measured reflections
2654 independent reflections
1654 reflections with I > 2σ(I)
R_int = 0.037

Refinement

R[F² > 2σ(F²)] = 0.071
wR(F²) = 0.224
S = 1.08
2654 reflections
158 parameters
41 restraints
H-atom parameters constrained
Δρ_max = 0.53 e Å⁻³
Δρ_min = −0.48 e Å⁻³

Data collection: APEX2 (Bruker, 2008 ); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: X-SEED (Barbour, 2001 ); software used to prepare material for publication: publCIF (Westrip, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809026336/tk2496sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809026336/tk2496Isup2.hkl

checkCIF report

Related literature top

For background to imidazolium-based ionic liquid salts, see: Ganesan et al. (2008). For the procedure to manipulate twinned diffraction data, see: Spek (2003).

Experimental top

α,α-Dibromo-p-xylene (0.78 g, 3 mmol) and 1,2-dimethylimidazole (0.58 g, 7.6 mmol) were refluxed in DMF (50 ml) for 3 h. The product that separated from solution was collected and washed with ether. Crystals were grown from its solution in water.

The bromide salt (0.46 g, 1 mmol) and sodium hexafluorophosphate (0.17 g, 1 mol) were stirred in water (100 ml) for 24 h. The product that separated from solution was collected and washed with ethanol. Crystals were grown from its solution in DMF.

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: publCIF (Westrip, 2009).

Figures top

Fig. 1. Thermal ellipsoid plot (Barbour, 2001) of [C₁₈H₂₄N₄]²⁺ 2[PF₆]^- at the 70% probability level; hydrogen atoms are drawn as spheres of arbitrary radius. The imidazolyl ring is disordered; only one component (unprimed) of the disorder is shown. The non-H atoms comprising the asymmetric unit are labelled and the unlabelled atoms are related by 2-x, 2-y, -z.

1,1',2,2'-Tetramethyl-3,3'-(p-phenylenedimethylene)diimidazol-1-ium bis(hexafluoridophosphate) top

Crystal data top

C₁₈H₂₄N₄²⁺·2PF₆⁻	Z = 1
M_r = 586.35	F(000) = 298
Triclinic, P1	D_x = 1.635 Mg m⁻³
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 7.3808 (3) Å	Cell parameters from 951 reflections
b = 8.2169 (4) Å	θ = 2.6–23.6°
c = 11.0553 (5) Å	µ = 0.29 mm⁻¹
α = 73.435 (3)°	T = 140 K
β = 71.173 (3)°	Prism, colorless
γ = 73.897 (3)°	0.35 × 0.03 × 0.03 mm
V = 595.43 (5) Å³

Data collection top

Bruker SMART APEX diffractometer	2654 independent reflections
Radiation source: fine-focus sealed tube	1654 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.037
ω scans	θ_max = 27.5°, θ_min = 2.0°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −9→9
T_min = 0.905, T_max = 0.991	k = −10→10
4724 measured reflections	l = −14→14

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.071	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.224	H-atom parameters constrained
S = 1.08	w = 1/[σ²(F_o²) + (0.0937P)² + 1.0627P] where P = (F_o² + 2F_c²)/3
2654 reflections	(Δ/σ)_max = 0.001
158 parameters	Δρ_max = 0.53 e Å⁻³
41 restraints	Δρ_min = −0.48 e Å⁻³

Crystal data top

C₁₈H₂₄N₄²⁺·2PF₆⁻	γ = 73.897 (3)°
M_r = 586.35	V = 595.43 (5) Å³
Triclinic, P1	Z = 1
a = 7.3808 (3) Å	Mo Kα radiation
b = 8.2169 (4) Å	µ = 0.29 mm⁻¹
c = 11.0553 (5) Å	T = 140 K
α = 73.435 (3)°	0.35 × 0.03 × 0.03 mm
β = 71.173 (3)°

Data collection top

Bruker SMART APEX diffractometer	2654 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	1654 reflections with I > 2σ(I)
T_min = 0.905, T_max = 0.991	R_int = 0.037
4724 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.071	41 restraints
wR(F²) = 0.224	H-atom parameters constrained
S = 1.08	Δρ_max = 0.53 e Å⁻³
2654 reflections	Δρ_min = −0.48 e Å⁻³
158 parameters

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

	x	y	z	U_iso*/U_eq	Occ. (<1)
P1	0.27195 (18)	0.30311 (17)	0.76433 (12)	0.0235 (3)
F6	0.2767 (5)	0.4311 (4)	0.6246 (3)	0.0364 (8)
F4	0.3686 (5)	0.1439 (4)	0.6946 (3)	0.0428 (8)
F5	0.2653 (5)	0.1760 (4)	0.9036 (3)	0.0400 (8)
F3	0.0593 (4)	0.2794 (5)	0.7741 (3)	0.0417 (8)
F2	0.1736 (5)	0.4638 (4)	0.8340 (3)	0.0494 (10)
F1	0.4829 (4)	0.3294 (5)	0.7547 (3)	0.0440 (9)
C1	1.1492 (7)	1.0037 (6)	0.3848 (4)	0.0233 (10)
H1	1.2518	1.0052	0.3063	0.028*
C2	1.0666 (7)	0.8581 (7)	0.4421 (5)	0.0259 (10)
H2	1.1122	0.7612	0.4018	0.031*
C3	0.9177 (6)	0.8525 (6)	0.5581 (5)	0.0218 (10)
C4	0.8353 (7)	0.6892 (6)	0.6166 (5)	0.0263 (11)
H4A	0.7815	0.6670	0.5530	0.032*	0.50
H4B	0.9412	0.5891	0.6352	0.032*	0.50
H4C	0.7946	0.6625	0.5486	0.032*	0.50
H4D	0.9413	0.5921	0.6398	0.032*	0.50
N1	0.683 (2)	0.706 (4)	0.7365 (15)	0.0231 (17)	0.50
C5	0.493 (3)	0.753 (2)	0.7349 (14)	0.0216 (14)	0.50
N2	0.3833 (7)	0.7643 (18)	0.858 (2)	0.0250 (13)	0.50
C6	0.505 (3)	0.724 (4)	0.9349 (6)	0.034 (2)	0.50
H6	0.4677	0.7219	1.0259	0.040*	0.50
C7	0.690 (2)	0.688 (5)	0.860 (2)	0.031 (3)	0.50
H7	0.8057	0.6562	0.8892	0.038*	0.50
N1'	0.670 (2)	0.695 (4)	0.7318 (14)	0.0231 (17)	0.50
C5'	0.477 (3)	0.749 (2)	0.7430 (16)	0.0216 (14)	0.50
N2'	0.3852 (8)	0.7552 (19)	0.869 (2)	0.0250 (13)	0.50
C6'	0.523 (3)	0.705 (4)	0.9363 (7)	0.034 (2)	0.50
H6'	0.4993	0.6975	1.0272	0.040*	0.50
C7'	0.6988 (18)	0.668 (5)	0.851 (2)	0.031 (3)	0.50
H7'	0.8213	0.6300	0.8718	0.038*	0.50
C8	0.3963 (5)	0.7903 (4)	0.6307 (4)	0.0303 (11)
H8A	0.4926	0.7588	0.5517	0.046*	0.50
H8B	0.3380	0.9143	0.6118	0.046*	0.50
H8C	0.2937	0.7228	0.6587	0.046*	0.50
H8D	0.5010	0.7607	0.5544	0.046*	0.50
H8E	0.3388	0.9145	0.6118	0.046*	0.50
H8F	0.2953	0.7233	0.6506	0.046*	0.50
C9	0.1775 (5)	0.8077 (4)	0.9104 (4)	0.0407 (14)
H9A	0.1131	0.8479	0.8392	0.061*	0.50
H9B	0.1463	0.9001	0.9582	0.061*	0.50
H9C	0.1311	0.7052	0.9703	0.061*	0.50
H9D	0.1199	0.7958	0.8458	0.061*	0.50
H9E	0.1428	0.9291	0.9183	0.061*	0.50
H9F	0.1271	0.7344	0.9955	0.061*	0.50

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
P1	0.0225 (6)	0.0259 (7)	0.0221 (6)	−0.0038 (5)	−0.0050 (5)	−0.0074 (5)
F6	0.0438 (18)	0.0321 (17)	0.0305 (16)	−0.0089 (14)	−0.0136 (14)	0.0030 (13)
F4	0.051 (2)	0.0294 (17)	0.0420 (19)	0.0005 (14)	−0.0048 (15)	−0.0157 (15)
F5	0.0462 (19)	0.046 (2)	0.0278 (16)	−0.0157 (15)	−0.0126 (14)	0.0012 (14)
F3	0.0283 (17)	0.060 (2)	0.0410 (19)	−0.0154 (15)	−0.0101 (14)	−0.0102 (16)
F2	0.056 (2)	0.044 (2)	0.050 (2)	−0.0074 (17)	−0.0001 (17)	−0.0294 (17)
F1	0.0302 (17)	0.064 (2)	0.0415 (19)	−0.0187 (16)	−0.0123 (14)	−0.0048 (17)
C1	0.020 (2)	0.030 (3)	0.019 (2)	−0.0075 (19)	−0.0014 (17)	−0.0062 (19)
C2	0.022 (2)	0.027 (3)	0.028 (3)	−0.0026 (19)	−0.0044 (19)	−0.010 (2)
C3	0.017 (2)	0.024 (2)	0.025 (2)	0.0003 (18)	−0.0121 (18)	−0.0026 (19)
C4	0.020 (2)	0.027 (3)	0.031 (3)	−0.005 (2)	−0.0047 (19)	−0.006 (2)
N1	0.020 (2)	0.026 (3)	0.026 (2)	−0.010 (2)	−0.0078 (19)	−0.0027 (18)
C5	0.022 (3)	0.022 (2)	0.021 (3)	−0.009 (2)	−0.003 (2)	−0.0026 (19)
N2	0.027 (2)	0.025 (2)	0.024 (3)	−0.0109 (17)	−0.0049 (17)	−0.003 (2)
C6	0.046 (4)	0.042 (5)	0.017 (2)	−0.019 (4)	−0.010 (2)	−0.001 (2)
C7	0.035 (3)	0.036 (7)	0.032 (4)	−0.014 (3)	−0.017 (3)	−0.003 (3)
N1'	0.020 (2)	0.026 (3)	0.026 (2)	−0.010 (2)	−0.0078 (19)	−0.0027 (18)
C5'	0.022 (3)	0.022 (2)	0.021 (3)	−0.009 (2)	−0.003 (2)	−0.0026 (19)
N2'	0.027 (2)	0.025 (2)	0.024 (3)	−0.0109 (17)	−0.0049 (17)	−0.003 (2)
C6'	0.046 (4)	0.042 (5)	0.017 (2)	−0.019 (4)	−0.010 (2)	−0.001 (2)
C7'	0.035 (3)	0.036 (7)	0.032 (4)	−0.014 (3)	−0.017 (3)	−0.003 (3)
C8	0.033 (3)	0.027 (3)	0.034 (3)	−0.006 (2)	−0.015 (2)	−0.004 (2)
C9	0.032 (3)	0.042 (3)	0.041 (3)	−0.011 (2)	0.008 (2)	−0.016 (3)

Geometric parameters (Å, º) top

P1—F4	1.586 (3)	C6—C7	1.3500
P1—F5	1.590 (3)	C6—H6	0.9500
P1—F1	1.597 (3)	C7—H7	0.9500
P1—F2	1.597 (3)	N1'—C5'	1.3500
P1—F6	1.598 (3)	N1'—C7'	1.3500
P1—F3	1.600 (3)	C5'—N2'	1.3500
C1—C2	1.388 (7)	C5'—C8	1.459 (7)
C1—C3ⁱ	1.393 (6)	N2'—C6'	1.3500
C1—H1	0.9500	N2'—C9	1.428 (6)
C2—C3	1.392 (6)	C6'—C7'	1.3500
C2—H2	0.9500	C6'—H6'	0.9500
C3—C1ⁱ	1.393 (6)	C7'—H7'	0.9500
C3—C4	1.514 (7)	C8—H8A	0.9800
C4—N1	1.449 (7)	C8—H8B	0.9800
C4—N1'	1.452 (7)	C8—H8C	0.9800
C4—H4A	0.9900	C8—H8D	0.9800
C4—H4B	0.9900	C8—H8E	0.9800
C4—H4C	0.9900	C8—H8F	0.9800
C4—H4D	0.9900	C9—H9A	0.9800
N1—C5	1.3500	C9—H9B	0.9800
N1—C7	1.3500	C9—H9C	0.9800
C5—N2	1.3500	C9—H9D	0.9800
C5—C8	1.462 (7)	C9—H9E	0.9800
N2—C6	1.3500	C9—H9F	0.9800
N2—C9	1.422 (6)

F4—P1—F5	90.52 (18)	C5'—N1'—C4	131 (2)
F4—P1—F1	90.71 (19)	C7'—N1'—C4	121 (2)
F5—P1—F1	90.42 (18)	N1'—C5'—N2'	108.0
F4—P1—F2	179.6 (2)	N1'—C5'—C8	122 (2)
F5—P1—F2	89.74 (19)	N2'—C5'—C8	130 (2)
F1—P1—F2	89.58 (19)	C6'—N2'—C5'	108.0
F4—P1—F6	89.62 (18)	C6'—N2'—C9	132 (2)
F5—P1—F6	179.52 (18)	C5'—N2'—C9	120 (2)
F1—P1—F6	90.04 (18)	N2'—C6'—C7'	108.0
F2—P1—F6	90.11 (18)	N2'—C6'—H6'	126.0
F4—P1—F3	89.98 (19)	C7'—C6'—H6'	126.0
F5—P1—F3	89.90 (18)	C6'—C7'—N1'	108.0
F1—P1—F3	179.2 (2)	C6'—C7'—H7'	126.0
F2—P1—F3	89.73 (19)	N1'—C7'—H7'	126.0
F6—P1—F3	89.64 (17)	C5'—C8—H8A	113.7
C2—C1—C3ⁱ	120.3 (4)	C5—C8—H8A	109.5
C2—C1—H1	119.8	C5'—C8—H8B	109.7
C3ⁱ—C1—H1	119.8	C5—C8—H8B	109.5
C1—C2—C3	120.7 (4)	H8A—C8—H8B	109.5
C1—C2—H2	119.7	C5'—C8—H8C	104.9
C3—C2—H2	119.7	C5—C8—H8C	109.5
C2—C3—C1ⁱ	119.0 (4)	H8A—C8—H8C	109.5
C2—C3—C4	118.2 (4)	H8B—C8—H8C	109.5
C1ⁱ—C3—C4	122.8 (4)	C5'—C8—H8D	109.5
N1—C4—C3	110.7 (13)	C5—C8—H8D	105.1
N1'—C4—C3	115.8 (13)	C5'—C8—H8E	109.5
N1—C4—H4A	109.5	C5—C8—H8E	109.2
N1'—C4—H4A	103.2	H8A—C8—H8E	109.4
C3—C4—H4A	109.5	H8C—C8—H8E	109.9
N1—C4—H4B	109.5	H8D—C8—H8E	109.5
N1'—C4—H4B	110.3	C5'—C8—H8F	109.5
C3—C4—H4B	109.5	C5—C8—H8F	114.0
H4A—C4—H4B	108.1	H8A—C8—H8F	105.3
N1—C4—H4C	114.7	H8B—C8—H8F	109.1
N1'—C4—H4C	108.3	H8D—C8—H8F	109.5
C3—C4—H4C	108.3	H8E—C8—H8F	109.5
N1—C4—H4D	107.2	N2—C9—H9A	109.5
N1'—C4—H4D	108.3	N2'—C9—H9A	114.6
C3—C4—H4D	108.3	N2—C9—H9B	109.5
C5—N1—C7	108.0	N2'—C9—H9B	108.2
C5—N1—C4	120 (2)	H9A—C9—H9B	109.5
C7—N1—C4	132 (2)	N2—C9—H9C	109.5
N2—C5—N1	108.0	N2'—C9—H9C	105.5
N2—C5—C8	119 (2)	H9A—C9—H9C	109.5
N1—C5—C8	133 (2)	H9B—C9—H9C	109.5
C6—N2—C5	108.0	N2—C9—H9D	105.5
C6—N2—C9	121 (2)	N2'—C9—H9D	109.5
C5—N2—C9	131 (2)	N2—C9—H9E	108.2
N2—C6—C7	108.0	N2'—C9—H9E	109.5
N2—C6—H6	126.0	H9D—C9—H9E	109.5
C7—C6—H6	126.0	N2—C9—H9F	114.6
C6—C7—N1	108.0	N2'—C9—H9F	109.5
C6—C7—H7	126.0	H9D—C9—H9F	109.5
N1—C7—H7	126.0	H9E—C9—H9F	109.5
C5'—N1'—C7'	108.0

C3ⁱ—C1—C2—C3	−0.8 (8)	C9—N2—C6—C7	179.8 (3)
C1—C2—C3—C1ⁱ	0.8 (8)	N2—C6—C7—N1	0.0
C1—C2—C3—C4	−179.3 (4)	C5—N1—C7—C6	0.0
C2—C3—C4—N1	180.0 (11)	C4—N1—C7—C6	178 (2)
C1ⁱ—C3—C4—N1	−0.1 (12)	C3—C4—N1'—C5'	87.8 (19)
C2—C3—C4—N1'	−175.4 (11)	C3—C4—N1'—C7'	−81.7 (9)
C1ⁱ—C3—C4—N1'	4.5 (12)	C7'—N1'—C5'—N2'	0.0
N1'—C4—N1—C5	−44 (19)	C4—N1'—C5'—N2'	−170.5 (19)
C3—C4—N1—C5	97.1 (15)	C7'—N1'—C5'—C8	179.9 (3)
N1'—C4—N1—C7	139 (21)	C4—N1'—C5'—C8	9.4 (19)
C3—C4—N1—C7	−80.4 (10)	N1'—C5'—N2'—C6'	0.0
C7—N1—C5—N2	0.0	C8—C5'—N2'—C6'	−179.9 (3)
C4—N1—C5—N2	−178.0 (17)	N1'—C5'—N2'—C9	−179.8 (3)
C7—N1—C5—C8	179.9 (3)	C8—C5'—N2'—C9	0.3 (4)
C4—N1—C5—C8	1.9 (17)	C5'—N2'—C6'—C7'	0.0
N1—C5—N2—C6	0.0	C9—N2'—C6'—C7'	179.8 (3)
C8—C5—N2—C6	−179.9 (2)	N2'—C6'—C7'—N1'	0.0
N1—C5—N2—C9	−179.8 (3)	C5'—N1'—C7'—C6'	0.0
C8—C5—N2—C9	0.2 (4)	C4—N1'—C7'—C6'	171.6 (18)
C5—N2—C6—C7	0.0

Symmetry code: (i) −x+2, −y+2, −z+1.

Experimental details

Crystal data
Chemical formula	C₁₈H₂₄N₄²⁺·2PF₆⁻
M_r	586.35
Crystal system, space group	Triclinic, P1
Temperature (K)	140
a, b, c (Å)	7.3808 (3), 8.2169 (4), 11.0553 (5)
α, β, γ (°)	73.435 (3), 71.173 (3), 73.897 (3)
V (Å³)	595.43 (5)
Z	1
Radiation type	Mo Kα
µ (mm⁻¹)	0.29
Crystal size (mm)	0.35 × 0.03 × 0.03

Data collection
Diffractometer	Bruker SMART APEX diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.905, 0.991
No. of measured, independent and observed [I > 2σ(I)] reflections	4724, 2654, 1654
R_int	0.037
(sin θ/λ)_max (Å⁻¹)	0.650

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.071, 0.224, 1.08
No. of reflections	2654
No. of parameters	158
No. of restraints	41
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.53, −0.48

Computer programs: APEX2 (Bruker, 2008), SAINT (Bruker, 2008), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), X-SEED (Barbour, 2001), publCIF (Westrip, 2009).

Acknowledgements

We thank the University of Malaya (grant Nos. TA 0009/2008 A and FS343/2008 A) for supporting this study.

References

Barbour, L. J. (2001). J. Supramol. Chem. 1, 189–191. CrossRef CAS Google Scholar
Bruker (2008). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Ganesan, K., Alias, Y. & Ng, S. W. (2008). Acta Cryst. C64, o478–o480. Web of Science CSD CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany. Google Scholar
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Spek, A. L. (2003). J. Appl. Cryst. 36, 7–13. Web of Science CrossRef CAS IUCr Journals Google Scholar
Westrip, S. P. (2009). publCIF. In preparation. Google Scholar

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