Crystal structure of triclopyr

Cho, S.; Kim, J.; Jeon, Y.; Kim, T.H.

doi:10.1107/S160053681401681X

organic compounds

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ISSN: 2056-9890

Volume 70| Part 9| September 2014| Page o940

doi:10.1107/S160053681401681X

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Crystal structure of triclopyr

Seonghwa Cho,^a Jineun Kim,^a ^* Youngeun Jeon ^a and Tae Ho Kim ^a ^*

^aDepartment of Chemistry and Research Institute of Natural Sciences, Gyeongsang National University, Jinju 660-701, Republic of Korea
^*Correspondence e-mail: thkim@gnu.ac.kr, jekim@gnu.ac.kr

Edited by G. Smith, Queensland University of Technology, Australia (Received 9 July 2014; accepted 21 July 2014; online 1 August 2014)

In the title compound {systematic name: 2-[(3,5,6-trichloropyridin-2-yl)oxy]acetic acid}, the herbicide triclopyr, C₇H₄Cl₃NO₃, the asymmetric unit comprises two independent molecules in which the dihedral angles between the mean plane of the carboxylic acid group and the pyridyl ring plane are 79.3 (6) and 83.8 (5)°. In the crystal, pairs of intermolecular O—H⋯O hydrogen bonds form dimers through an R₂²(8) ring motif and are extended into chains along [100] by weak π–π interactions [ring centroid separations = 3.799 (4) and 3.810 (4) Å]. In addition, short intermolecular Cl⋯Cl contacts [3.458 (2) Å] connect the chains, yielding a two-dimensional architecture extending parallel to (020). The crystal studied was found to be non-merohedrally twinned with the minor component being 0.175 (4).

Keywords: crystal structure; herbicide; triclopyr; hydrogen-bonded dimers; π–π interactions; non-merohedral twinning.

CCDC reference: 1015180

1. Related literature

For information on the toxicity and herbicidal properties of the title compound, see: McMullin et al. (2011 ); Carney et al. (2007 ). For a related crystal structure, see: Smith et al. (1976 ). Non-merohedral twinning in the crystal was identified usinTwinRotMat within PLATON (Spek, 2009 ).

2. Experimental

2.1. Crystal data

C₇H₄Cl₃NO₃
M_r = 256.46
Monoclinic, P 2₁ /c
a = 7.5771 (9) Å
b = 25.409 (3) Å
c = 10.1668 (12) Å
β = 106.261 (8)°
V = 1879.1 (4) Å³
Z = 8
Mo Kα radiation
μ = 0.95 mm⁻¹
T = 173 K
0.50 × 0.09 × 0.06 mm

2.2. Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2009 ) T_min = 0.648, T_max = 0.945
3699 measured reflections
3699 independent reflections
3080 reflections with I > 2σ(I)
R_int = 0.000

2.3. Refinement

R[F² > 2σ(F²)] = 0.044
wR(F²) = 0.096
S = 1.12
3699 reflections
256 parameters
H-atom parameters constrained
Δρ_max = 0.32 e Å⁻³
Δρ_min = −0.28 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O3—H3O⋯O6	0.84	1.85	2.688 (3)	174
O5—H5O⋯O2	0.84	1.84	2.671 (3)	172

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008 ); program(s) used to refine structure: SHELXTL; molecular graphics: DIAMOND (Brandenburg, 2010 ); software used to prepare material for publication: SHELXTL.

Supporting information

CCDC reference: 1015180

Crystal structure: contains datablocks global, I. DOI: 10.1107/S160053681401681X/zs2309sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S160053681401681X/zs2309Isup2.hkl

Supporting information file. DOI: 10.1107/S160053681401681X/zs2309Isup3.cml

checkCIF report

Comment top

Triclopyr, C₇H₄Cl₃NO₃, is a herbicide used extensively in the control of woody plants and broadleaf weeds (McMullin et al., 2011; Carney et al., 2007), and its crystal structure is reported herein. In this compound (Scheme 1, Fig. 1). The asymmetric unit is composed of two independent molecules (Molecule A and Molecule B). The dihedral angles between the mean plane of the carboxyl groups and the pyridyl ring systems are 79.3 (6)° and 83.8 (5)° for Molecule A and Molecule B, respectively. All bond lengths and bond angles are normal and comparable to those observed in the crystal structure of a similar compound (Smith et al., 1976).

In the crystal structure (Fig. 2), two carboxylic acid O—H···O hydrogen bonds are observed (Table 1), forming dimers through an R₂²(8) ring motif. In addition, weak intermolecular π–π interactions between the pyridyl ring systems [Cg1···Cg2ⁱ, 3.799 (4) Å and Cg2···Cg1ⁱⁱ, 3.810 (4) Å], link the dimers into one-dimensional chains extending along (100) (Cg1 and Cg2 are the centeroids of the N1···C5 and N2···C12 rings, respectively). In addition, a short Cl···Cl contact [Cl1···Cl1ⁱⁱⁱ, 3.458 (2) Å] is present [for symmetry codes: (i), -x + 1, -y + 1, -z + 1, (ii), -x + 2, -y+ + 1, -z + 1, and (iii), -x + 1, -y + 1, -z + 2]. The crystal studied was found to be affected by non-merohedral twinning (Spek, 2009) and the data was treated accordingly, giving a final refined BASF parameter of 0.175 (4).

Related literature top

For information on the toxicity and herbicidal properties of the title compound, see: McMullin et al. (2011); Carney et al. (2007). For a related crystal structure, see: Smith et al. (1976). Non-merohedral twinning in the crystal was identified usinTwinRotMat within PLATON (Spek, 2009).

Experimental top

The title compound was purchased from the Dr. Ehrenstorfer GmbH Company. Slow evaporation of a solution in CHCl₃ gave single crystals suitable for X-ray analysis.

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2010); software used to prepare material for publication: SHELXTL (Brandenburg, 2010).

Figures top

	Fig. 1. The asymmetric unit of the title compound with the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as small spheres of arbitrary radius.
	Fig. 2. Crystal packing viewed along the a axis. The intermolecular O—H···O hydrogen bonds, weak π–π interactions, and short Cl···Cl contacts are shown as dashed lines.

2-[(3,5,6-Trichloropyridin-2-yl)oxy]acetic acid top

Crystal data top

C₇H₄Cl₃NO₃	F(000) = 1024
M_r = 256.46	D_x = 1.813 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3761 reflections
a = 7.5771 (9) Å	θ = 2.2–25.5°
b = 25.409 (3) Å	µ = 0.95 mm⁻¹
c = 10.1668 (12) Å	T = 173 K
β = 106.261 (8)°	Needle, colourless
V = 1879.1 (4) Å³	0.50 × 0.09 × 0.06 mm
Z = 8

Data collection top

Bruker APEXII CCD diffractometer	3699 independent reflections
Radiation source: fine-focus sealed tube	3080 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.000
ϕ and ω scans	θ_max = 26.0°, θ_min = 1.6°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −9→8
T_min = 0.648, T_max = 0.945	k = −31→31
3699 measured reflections	l = −5→12

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)] = 0.044	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.096	H-atom parameters constrained
S = 1.12	w = 1/[σ²(F_o²) + (0.0219P)² + 2.0682P] where P = (F_o² + 2F_c²)/3
3699 reflections	(Δ/σ)_max = 0.001
256 parameters	Δρ_max = 0.32 e Å⁻³
0 restraints	Δρ_min = −0.28 e Å⁻³

Crystal data top

C₇H₄Cl₃NO₃	V = 1879.1 (4) Å³
M_r = 256.46	Z = 8
Monoclinic, P2₁/c	Mo Kα radiation
a = 7.5771 (9) Å	µ = 0.95 mm⁻¹
b = 25.409 (3) Å	T = 173 K
c = 10.1668 (12) Å	0.50 × 0.09 × 0.06 mm
β = 106.261 (8)°

Data collection top

Bruker APEXII CCD diffractometer	3699 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2009)	3080 reflections with I > 2σ(I)
T_min = 0.648, T_max = 0.945	R_int = 0.000
3699 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.044	0 restraints
wR(F²) = 0.096	H-atom parameters constrained
S = 1.12	Δρ_max = 0.32 e Å⁻³
3699 reflections	Δρ_min = −0.28 e Å⁻³
256 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

	x	y	z	U_iso*/U_eq
Cl1	0.66278 (13)	0.53311 (3)	0.94222 (8)	0.0336 (2)
Cl2	0.86627 (14)	0.62564 (3)	1.13208 (9)	0.0397 (2)
Cl3	0.67175 (14)	0.75251 (3)	0.69082 (10)	0.0408 (2)
Cl4	1.00995 (14)	0.31553 (3)	0.50221 (9)	0.0380 (2)
Cl5	0.79593 (15)	0.23214 (3)	0.28102 (10)	0.0432 (3)
Cl6	0.66657 (14)	0.38875 (3)	−0.09150 (8)	0.0403 (2)
O1	0.4722 (3)	0.65825 (8)	0.5624 (2)	0.0317 (5)
O2	0.6886 (3)	0.58941 (10)	0.4787 (3)	0.0386 (6)
O3	0.4491 (3)	0.53970 (9)	0.3695 (3)	0.0382 (6)
H3O	0.5290	0.5231	0.3434	0.057*
O4	0.9075 (3)	0.44645 (8)	0.1341 (2)	0.0315 (5)
O5	0.9552 (3)	0.54570 (9)	0.3905 (2)	0.0316 (5)
H5O	0.8784	0.5599	0.4250	0.047*
O6	0.7219 (3)	0.49064 (9)	0.2971 (3)	0.0361 (6)
N1	0.5733 (4)	0.60258 (10)	0.7479 (3)	0.0248 (6)
N2	0.9432 (4)	0.38215 (10)	0.3000 (3)	0.0258 (6)
C1	0.6641 (4)	0.59584 (11)	0.8776 (3)	0.0242 (7)
C2	0.7565 (5)	0.63593 (12)	0.9613 (3)	0.0267 (7)
C3	0.7598 (5)	0.68502 (11)	0.9024 (3)	0.0278 (7)
H3	0.8254	0.7133	0.9552	0.033*
C4	0.6673 (5)	0.69213 (11)	0.7674 (3)	0.0254 (7)
C5	0.5714 (4)	0.64974 (12)	0.6938 (3)	0.0241 (7)
C6	0.3859 (5)	0.61313 (13)	0.4863 (3)	0.0331 (8)
H6A	0.2905	0.6247	0.4032	0.040*
H6B	0.3251	0.5920	0.5430	0.040*
C7	0.5265 (5)	0.57983 (12)	0.4455 (3)	0.0283 (7)
C8	0.9176 (5)	0.33276 (12)	0.3324 (3)	0.0266 (7)
C9	0.8219 (5)	0.29661 (12)	0.2378 (3)	0.0276 (7)
C10	0.7423 (5)	0.31367 (12)	0.1045 (3)	0.0279 (7)
H10	0.6717	0.2901	0.0376	0.033*
C11	0.7668 (5)	0.36496 (12)	0.0706 (3)	0.0266 (7)
C12	0.8738 (4)	0.39780 (11)	0.1722 (3)	0.0255 (7)
C13	1.0054 (5)	0.48192 (12)	0.2385 (3)	0.0301 (8)
H13A	1.0624	0.5100	0.1967	0.036*
H13B	1.1048	0.4627	0.3053	0.036*
C14	0.8768 (5)	0.50619 (12)	0.3117 (3)	0.0269 (7)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.0447 (5)	0.0215 (4)	0.0343 (4)	−0.0055 (4)	0.0106 (4)	0.0039 (3)
Cl2	0.0511 (6)	0.0334 (4)	0.0276 (4)	−0.0038 (4)	−0.0006 (4)	0.0000 (3)
Cl3	0.0532 (6)	0.0246 (4)	0.0453 (5)	−0.0031 (4)	0.0153 (5)	0.0085 (4)
Cl4	0.0483 (6)	0.0327 (4)	0.0293 (4)	0.0081 (4)	0.0048 (4)	0.0055 (3)
Cl5	0.0617 (7)	0.0248 (4)	0.0466 (5)	−0.0052 (4)	0.0210 (5)	0.0019 (4)
Cl6	0.0517 (6)	0.0405 (5)	0.0244 (4)	0.0102 (4)	0.0035 (4)	0.0000 (3)
O1	0.0372 (14)	0.0290 (12)	0.0261 (12)	0.0025 (11)	0.0044 (11)	−0.0002 (9)
O2	0.0295 (15)	0.0436 (14)	0.0423 (15)	−0.0027 (12)	0.0092 (12)	−0.0138 (11)
O3	0.0304 (14)	0.0387 (14)	0.0430 (15)	−0.0022 (12)	0.0064 (12)	−0.0143 (11)
O4	0.0415 (15)	0.0223 (11)	0.0290 (12)	0.0005 (10)	0.0072 (11)	0.0008 (9)
O5	0.0290 (13)	0.0293 (12)	0.0355 (13)	−0.0032 (11)	0.0074 (11)	−0.0063 (10)
O6	0.0315 (15)	0.0296 (12)	0.0471 (15)	−0.0033 (11)	0.0110 (12)	−0.0094 (11)
N1	0.0244 (15)	0.0246 (13)	0.0270 (14)	−0.0031 (12)	0.0098 (12)	−0.0019 (11)
N2	0.0274 (16)	0.0230 (13)	0.0253 (14)	0.0030 (12)	0.0044 (12)	−0.0023 (11)
C1	0.0277 (18)	0.0185 (14)	0.0297 (17)	−0.0006 (13)	0.0136 (15)	0.0012 (12)
C2	0.0278 (19)	0.0281 (16)	0.0238 (16)	−0.0001 (14)	0.0066 (14)	−0.0012 (13)
C3	0.033 (2)	0.0186 (15)	0.0332 (18)	−0.0043 (14)	0.0117 (16)	−0.0043 (13)
C4	0.0314 (19)	0.0181 (14)	0.0301 (17)	0.0001 (14)	0.0141 (15)	0.0033 (12)
C5	0.0248 (18)	0.0252 (16)	0.0244 (16)	0.0021 (13)	0.0105 (14)	−0.0001 (12)
C6	0.031 (2)	0.0383 (19)	0.0274 (18)	0.0025 (16)	0.0038 (16)	−0.0071 (14)
C7	0.034 (2)	0.0296 (17)	0.0194 (16)	0.0003 (15)	0.0038 (15)	0.0009 (13)
C8	0.0269 (19)	0.0256 (16)	0.0279 (17)	0.0085 (14)	0.0089 (15)	0.0038 (13)
C9	0.0301 (19)	0.0227 (15)	0.0331 (18)	0.0020 (14)	0.0138 (16)	0.0015 (13)
C10	0.0268 (19)	0.0272 (16)	0.0305 (18)	−0.0004 (14)	0.0096 (15)	−0.0083 (13)
C11	0.0265 (18)	0.0297 (16)	0.0235 (16)	0.0066 (15)	0.0069 (15)	−0.0018 (13)
C12	0.0252 (18)	0.0210 (15)	0.0322 (18)	0.0063 (14)	0.0113 (15)	−0.0021 (13)
C13	0.032 (2)	0.0223 (15)	0.0353 (19)	−0.0010 (14)	0.0080 (16)	−0.0012 (14)
C14	0.031 (2)	0.0197 (15)	0.0267 (17)	0.0019 (15)	0.0028 (15)	0.0028 (13)

Geometric parameters (Å, º) top

Cl1—C1	1.725 (3)	N2—C12	1.319 (4)
Cl2—C2	1.722 (3)	N2—C8	1.325 (4)
Cl3—C4	1.725 (3)	C1—C2	1.385 (4)
Cl4—C8	1.728 (3)	C2—C3	1.387 (4)
Cl5—C9	1.721 (3)	C3—C4	1.367 (4)
Cl6—C11	1.720 (3)	C3—H3	0.9500
O1—C5	1.354 (4)	C4—C5	1.394 (4)
O1—C6	1.434 (4)	C6—C7	1.507 (5)
O2—C7	1.204 (4)	C6—H6A	0.9900
O3—C7	1.314 (4)	C6—H6B	0.9900
O3—H3O	0.8400	C8—C9	1.379 (4)
O4—C12	1.341 (4)	C9—C10	1.389 (4)
O4—C13	1.431 (4)	C10—C11	1.374 (4)
O5—C14	1.318 (4)	C10—H10	0.9500
O5—H5O	0.8400	C11—C12	1.397 (4)
O6—C14	1.208 (4)	C13—C14	1.513 (5)
N1—C1	1.317 (4)	C13—H13A	0.9900
N1—C5	1.317 (4)	C13—H13B	0.9900

C5—O1—C6	116.6 (2)	O2—C7—O3	125.0 (3)
C7—O3—H3O	109.5	O2—C7—C6	123.6 (3)
C12—O4—C13	117.9 (2)	O3—C7—C6	111.3 (3)
C14—O5—H5O	109.5	N2—C8—C9	122.8 (3)
C1—N1—C5	118.6 (3)	N2—C8—Cl4	116.2 (2)
C12—N2—C8	119.0 (3)	C9—C8—Cl4	120.9 (2)
N1—C1—C2	123.5 (3)	C8—C9—C10	118.1 (3)
N1—C1—Cl1	116.4 (2)	C8—C9—Cl5	122.1 (3)
C2—C1—Cl1	120.1 (2)	C10—C9—Cl5	119.7 (2)
C1—C2—C3	117.6 (3)	C11—C10—C9	119.3 (3)
C1—C2—Cl2	121.7 (2)	C11—C10—H10	120.4
C3—C2—Cl2	120.7 (2)	C9—C10—H10	120.4
C4—C3—C2	119.1 (3)	C10—C11—C12	118.1 (3)
C4—C3—H3	120.5	C10—C11—Cl6	121.3 (3)
C2—C3—H3	120.5	C12—C11—Cl6	120.5 (2)
C3—C4—C5	118.7 (3)	N2—C12—O4	120.4 (3)
C3—C4—Cl3	120.1 (2)	N2—C12—C11	122.5 (3)
C5—C4—Cl3	121.2 (2)	O4—C12—C11	117.0 (3)
N1—C5—O1	119.7 (3)	O4—C13—C14	110.5 (3)
N1—C5—C4	122.4 (3)	O4—C13—H13A	109.6
O1—C5—C4	117.9 (3)	C14—C13—H13A	109.6
O1—C6—C7	110.3 (3)	O4—C13—H13B	109.6
O1—C6—H6A	109.6	C14—C13—H13B	109.6
C7—C6—H6A	109.6	H13A—C13—H13B	108.1
O1—C6—H6B	109.6	O6—C14—O5	125.5 (3)
C7—C6—H6B	109.6	O6—C14—C13	122.9 (3)
H6A—C6—H6B	108.1	O5—C14—C13	111.5 (3)

C5—N1—C1—C2	0.8 (5)	C12—N2—C8—C9	0.3 (5)
C5—N1—C1—Cl1	−179.5 (2)	C12—N2—C8—Cl4	−179.1 (2)
N1—C1—C2—C3	−3.1 (5)	N2—C8—C9—C10	−3.0 (5)
Cl1—C1—C2—C3	177.2 (2)	Cl4—C8—C9—C10	176.4 (2)
N1—C1—C2—Cl2	177.6 (3)	N2—C8—C9—Cl5	178.1 (3)
Cl1—C1—C2—Cl2	−2.1 (4)	Cl4—C8—C9—Cl5	−2.5 (4)
C1—C2—C3—C4	2.3 (5)	C8—C9—C10—C11	2.3 (5)
Cl2—C2—C3—C4	−178.4 (3)	Cl5—C9—C10—C11	−178.8 (3)
C2—C3—C4—C5	0.5 (5)	C9—C10—C11—C12	0.8 (5)
C2—C3—C4—Cl3	−179.1 (3)	C9—C10—C11—Cl6	−178.3 (2)
C1—N1—C5—O1	−177.0 (3)	C8—N2—C12—O4	−175.5 (3)
C1—N1—C5—C4	2.3 (5)	C8—N2—C12—C11	3.1 (5)
C6—O1—C5—N1	−4.9 (4)	C13—O4—C12—N2	−5.8 (4)
C6—O1—C5—C4	175.7 (3)	C13—O4—C12—C11	175.5 (3)
C3—C4—C5—N1	−3.0 (5)	C10—C11—C12—N2	−3.6 (5)
Cl3—C4—C5—N1	176.6 (2)	Cl6—C11—C12—N2	175.5 (2)
C3—C4—C5—O1	176.4 (3)	C10—C11—C12—O4	175.0 (3)
Cl3—C4—C5—O1	−4.1 (4)	Cl6—C11—C12—O4	−5.9 (4)
C5—O1—C6—C7	−75.3 (3)	C12—O4—C13—C14	−80.9 (3)
O1—C6—C7—O2	3.1 (5)	O4—C13—C14—O6	11.0 (4)
O1—C6—C7—O3	−176.7 (3)	O4—C13—C14—O5	−168.6 (2)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3O···O6	0.84	1.85	2.688 (3)	174
O5—H5O···O2	0.84	1.84	2.671 (3)	172

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3O···O6	0.84	1.85	2.688 (3)	174
O5—H5O···O2	0.84	1.84	2.671 (3)	172

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant No. 2014R1A1A4A01009105).

References

Brandenburg, K. (2010). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Bruker (2009). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Carney, E. W., Billington, R. & Barlow, S. M. (2007). Reprod. Toxicol. 23, 165–174. Web of Science CrossRef PubMed CAS Google Scholar
McMullin, R. T., Bell, F. W. & Newmaster, S. G. (2011). For. Ecol. Manage. 264, 90–97. Web of Science CrossRef Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Smith, G., Kennard, C. H. L. & White, A. H. (1976). Aust. J. Chem. 29, 2727–2730. CSD CrossRef CAS Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar