6-Methyl-4-{[4-(tri­methyl­sil­yl)-1H-1,2,3-triazol-1-yl]meth­yl}-2H-chromen-2-one

Carmel Y.S.; Prasad, N.L.; Begum, N.S.; Suresh, H.P.

doi:10.1107/S2414314620004277

organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 5| Part 4| April 2020| x200427

https://doi.org/10.1107/S2414314620004277

Open

access

6-Methyl-4-{[4-(trimethylsilyl)-1H-1,2,3-triazol-1-yl]methyl}-2H-chromen-2-one

Sowmiya Carmel Y.,^a N. L. Prasad,^a Noor Shahina Begum ^a ^* and Hari Prasad Suresh ^b

^aDepartment of Studies in Chemistry, Bangalore University, Jnana Bharathi Campus, Bangalore-560 056, Karnataka, India, and ^bDepartment of Studies in Chemistry, Bengaluru Central University, Central College Campus, Bengaluru-560 001, Karnataka, India
^*Correspondence e-mail: noorsb05@gmail.com

Edited by W. T. A. Harrison, University of Aberdeen, Scotland (Received 24 March 2020; accepted 28 March 2020; online 3 April 2020)

In the title compound, C₁₆H₁₉N₃O₂Si, the dihedral angle between the coumarin ring system (r.m.s. deviation = 0.031 Å) and the triazole ring is 73.81 (8)°. In the crystal, molecules are linked into [010] chains by weak C—H⋯O interactions.

Keywords: crystal structure; coumarin; weak C—H⋯O interactions.

CCDC reference: 1993489

Structure description

Coumarins are a family of benzopyrones and are widely distributed in nature (Venugopala et al., 2013 ). They have been extensively studied as a result of their broad array of biological activities, low toxicity and low drug resistance properties (Lipeeva et al., 2019 ). As part of our work in this area, we now describe the synthesis and crystal structure of the title compound in which the coumarin ring system bears a trimethylsilyl triazole substituent.

The title compound crystallizes in the monoclinic crystal system in space group C2/c with one molecule in the asymmetric unit (Fig. 1). The dihedral angle between the C1–C9/O1 chromen-2-one fused ring system (r.m.s. deviation = 0.031 Å) and the N1–N3/C11/C12 1,2,3-triazole ring is 73.81 (8)°. In the crystal, weak C—H⋯O hydrogen bonds (Table 1) link the molecules into [010] chains, with atom O2 accepting two such bonds from the adjacent molecule (Fig. 2) related by simple translation.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C10—H10A⋯O2ⁱ	0.99	2.61	3.427 (1)	140
C11—H11⋯O2ⁱ	0.95	2.45	3.242 (1)	141
Symmetry code: (i) x, y+1, z.

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. labels on the small side and displaced a long way from their respective atoms

Figure 2
Part of a [010] chain in the crystal of the title compound showing C—H⋯O interactions as dotted lines. H-atoms not involved in hydrogen bonding have been excluded.

Synthesis and crystallization

Trimethylsilyl acetylene (2.00 mmol) was added dropwise over a period of 30 min to an ice-cold suspension of bromomethylcoumarin (2.00 mmol), sodium azide (1.50 mmol) and copper iodide (1 µmol) in 10 ml (1:1 v/v) water/acetone. The resulting mixture was allowed to warm to room temperature and stirred for 8 h: progress of the reaction was monitored by TLC and GC through micro-workup of aliquots. After the completion of the reaction as indicated by the chromatograms, the excess acetone was removed under rotary evaporation and the crude product was purified by column chromatography, using silica gel (100–200 mesh) and 2:5 ethyl acetate–petroleum benzine (60–74°C fraction) eluent to obtain the title compound as a buff-coloured solid (91%); melting point: 110–112°C; (KBr disk, cm⁻¹): 3126, 2920, 2850, 1705, 1573, 1492, 1382, 1247, 1193, 1116, 1056, 950, 825, 756, 630, 557, 509; ¹H NMR (400 MHz, CDCl₃): δ 0.34 (s, 9 H), 2.42 (s, 3 H), 5.74 (s, 2 H), 5.92 (s, 1H), 7.26–7.57 (m, 4 H, 3 H of coumarinyl aromatic protons and 1 H of triazoyl aromatic proton); ¹³C NMR (100 MHz, CDCl₃): δ −1.0, 21.2, 49.5, 114.9, 116.9, 117.4, 123.4, 129.6, 133.8, 134.8, 148.2, 148.5, 151.9, 160.3; MS: calculated 313.12, found m/z (relative abundance) 313.22 (9.75%), 314.26 (M + 1 = 3.45%), 73.13 (100%); CHNS: Calculated C: 61.31%, H: 6.11%, N:13.41% Found C: 60.92%, H: 6.04%, N: 13.33%. Colourless blocks of the title compound were recrystallized from ethanol solution.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	C₁₆H₁₉N₃O₂Si
M_r	313.43
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	20.869 (2), 6.5971 (6), 24.561 (2)
β (°)	103.419 (4)
V (Å³)	3289.1 (5)
Z	8
Radiation type	Mo Kα
μ (mm⁻¹)	0.15
Crystal size (mm)	0.14 × 0.14 × 0.12

Data collection
Diffractometer	Bruker SMART APEX CCD
Absorption correction	Multi-scan (SADABS; Bruker, 1998 )
T_min, T_max	0.979, 0.982
No. of measured, independent and observed [I > 2σ(I)] reflections	21620, 3589, 2644
R_int	0.071
(sin θ/λ)_max (Å⁻¹)	0.639

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.054, 0.125, 0.99
No. of reflections	3589
No. of parameters	203
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.42, −0.31
Computer programs: SMART (Bruker, 1998), SAINT-Plus (Bruker, 1998), SHELXS97 and SHELXL97 (Sheldrick, 2008 ) and CAMERON (Watkin et al., 1996 ).

Structural data

CCDC reference: 1993489

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S2414314620004277/hb4344sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314620004277/hb4344Isup2.hkl

Infrared spectrum. DOI: https://doi.org/10.1107/S2414314620004277/hb4344sup3.tif

1H NMR Spectrum. DOI: https://doi.org/10.1107/S2414314620004277/hb4344sup4.tif

13C NMR spectrum. DOI: https://doi.org/10.1107/S2414314620004277/hb4344sup5.tif

MS. DOI: https://doi.org/10.1107/S2414314620004277/hb4344sup6.tif

Supporting information file. DOI: https://doi.org/10.1107/S2414314620004277/hb4344Isup7.cml

checkCIF report

Computing details top

Data collection: SMART (Bruker, 1998); cell refinement: SAINT-Plus (Bruker, 1998); data reduction: SAINT-Plus (Bruker, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

6-Methyl-4-{[4-(trimethylsilyl)-1H-1,2,3-triazol-1-yl]methyl}-2H-chromen-2-one top

Crystal data top

C₁₆H₁₉N₃O₂Si	F(000) = 1328
M_r = 313.43	D_x = 1.266 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 20.869 (2) Å	Cell parameters from 3589 reflections
b = 6.5971 (6) Å	θ = 2.9–27.0°
c = 24.561 (2) Å	µ = 0.15 mm⁻¹
β = 103.419 (4)°	T = 100 K
V = 3289.1 (5) Å³	Block, colorless
Z = 8	0.14 × 0.14 × 0.12 mm

Data collection top

Bruker SMART APEX CCD diffractometer	3589 independent reflections
Radiation source: fine-focus sealed tube	2644 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.071
ω scans	θ_max = 27.0°, θ_min = 2.9°
Absorption correction: multi-scan (SADABS; Bruker, 1998)	h = −26→26
T_min = 0.979, T_max = 0.982	k = −8→8
21620 measured reflections	l = −31→31

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)] = 0.054	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.125	H atoms treated by a mixture of independent and constrained refinement
S = 0.99	w = 1/[σ²(F_o²) + (0.0517P)² + 6.1123P] where P = (F_o² + 2F_c²)/3
3589 reflections	(Δ/σ)_max < 0.001
203 parameters	Δρ_max = 0.42 e Å⁻³
0 restraints	Δρ_min = −0.31 e Å⁻³

Special details top

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.

The H atoms were placed at calculated positions in the riding-model approximation with C—H = 0.95 Å, 1.00 Å and 0.96 Å for aromatic, methyne and methyl H-atoms respectively, with U_iso(H) = 1.5U_eq(C) for methyl H atoms and U_iso(H) = 1.2U_eq(C) for other hydrogen atoms.

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

	x	y	z	U_iso*/U_eq
Si1	0.03716 (3)	0.5449 (10)	0.15123 (2)	0.02394 (18)
O1	0.23596 (8)	−0.0948 (2)	0.43023 (6)	0.0249 (4)
O2	0.15849 (8)	−0.2426 (2)	0.36664 (6)	0.0326 (4)
N1	0.20048 (8)	0.3969 (3)	0.26552 (7)	0.0191 (4)
N2	0.20697 (9)	0.2741 (3)	0.22317 (7)	0.0254 (4)
N3	0.15636 (9)	0.3094 (3)	0.18156 (7)	0.0250 (4)
C1	0.35583 (11)	0.2153 (4)	0.52305 (8)	0.0252 (5)
H1	0.3807	0.2112	0.5607	0.030*
C2	0.31413 (11)	0.0572 (3)	0.50291 (9)	0.0247 (5)
H2	0.3101	−0.0547	0.5262	0.030*
C3	0.27813 (10)	0.0645 (3)	0.44800 (8)	0.0202 (5)
C4	0.28295 (10)	0.2265 (3)	0.41304 (8)	0.0187 (4)
C5	0.32612 (10)	0.3849 (3)	0.43475 (8)	0.0210 (5)
H5	0.3304	0.4963	0.4114	0.025*
C6	0.36260 (10)	0.3823 (3)	0.48958 (9)	0.0234 (5)
C7	0.19476 (11)	−0.0985 (3)	0.37792 (9)	0.0247 (5)
C8	0.19909 (11)	0.0696 (3)	0.34119 (8)	0.0223 (5)
H8	0.1708	0.0714	0.3048	0.027*
C9	0.24152 (10)	0.2231 (3)	0.35664 (8)	0.0195 (4)
C10	0.24919 (10)	0.3949 (3)	0.31845 (8)	0.0207 (5)
H10A	0.2464	0.5245	0.3381	0.025*
H10B	0.2936	0.3870	0.3107	0.025*
C11	0.14617 (10)	0.5105 (3)	0.25053 (8)	0.0190 (4)
H11	0.1309	0.6089	0.2728	0.023*
C12	0.11697 (10)	0.4563 (3)	0.19648 (8)	0.0191 (4)
C13	0.03782 (12)	0.5050 (5)	0.07685 (9)	0.0384 (7)
H13A	0.0740	0.5828	0.0678	0.058*
H13B	−0.0042	0.5505	0.0531	0.058*
H13C	0.0440	0.3606	0.0703	0.058*
C14	−0.03101 (12)	0.3935 (4)	0.16733 (9)	0.0368 (6)
H14A	−0.0733	0.4511	0.1476	0.055*
H14B	−0.0280	0.3963	0.2077	0.055*
H14C	−0.0278	0.2531	0.1552	0.055*
C15	0.02778 (15)	0.8163 (4)	0.16805 (14)	0.0603 (10)
H15A	0.0610	0.8968	0.1554	0.091*
H15B	0.0337	0.8320	0.2086	0.091*
H15C	−0.0163	0.8320	0.1490	0.091*
C16	0.40799 (11)	0.5535 (4)	0.51336 (9)	0.0297 (5)
H16A	0.4063	0.6577	0.4846	0.045*
H16B	0.4531	0.5021	0.5255	0.045*
H16C	0.3942	0.6124	0.5454	0.045*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Si1	0.0243 (3)	0.0291 (4)	0.0149 (3)	0.0056 (3)	−0.0026 (2)	−0.0034 (3)
O1	0.0367 (9)	0.0208 (8)	0.0168 (7)	−0.0033 (7)	0.0049 (7)	0.0003 (6)
O2	0.0487 (11)	0.0256 (9)	0.0241 (8)	−0.0140 (8)	0.0100 (7)	−0.0068 (7)
N1	0.0222 (9)	0.0219 (9)	0.0119 (8)	0.0000 (8)	0.0012 (7)	−0.0007 (7)
N2	0.0279 (10)	0.0316 (11)	0.0148 (9)	0.0071 (9)	0.0012 (7)	−0.0039 (8)
N3	0.0261 (10)	0.0320 (11)	0.0147 (9)	0.0060 (8)	−0.0001 (7)	−0.0019 (8)
C1	0.0281 (12)	0.0331 (13)	0.0122 (10)	0.0068 (10)	0.0006 (9)	0.0015 (9)
C2	0.0313 (12)	0.0252 (12)	0.0178 (10)	0.0067 (10)	0.0059 (9)	0.0058 (9)
C3	0.0250 (11)	0.0187 (11)	0.0169 (10)	0.0023 (9)	0.0051 (8)	−0.0010 (8)
C4	0.0217 (11)	0.0215 (11)	0.0125 (9)	0.0030 (9)	0.0030 (8)	−0.0011 (8)
C5	0.0225 (11)	0.0233 (11)	0.0169 (10)	0.0010 (9)	0.0041 (8)	0.0024 (8)
C6	0.0215 (11)	0.0299 (12)	0.0184 (11)	0.0022 (10)	0.0038 (9)	−0.0022 (9)
C7	0.0346 (13)	0.0217 (12)	0.0191 (11)	−0.0010 (10)	0.0087 (9)	−0.0034 (9)
C8	0.0264 (11)	0.0246 (12)	0.0142 (10)	−0.0007 (10)	0.0013 (8)	−0.0017 (9)
C9	0.0225 (11)	0.0211 (11)	0.0146 (10)	0.0035 (9)	0.0035 (8)	−0.0004 (8)
C10	0.0224 (11)	0.0235 (11)	0.0129 (10)	−0.0006 (9)	−0.0024 (8)	0.0011 (8)
C11	0.0218 (11)	0.0206 (11)	0.0148 (10)	0.0009 (9)	0.0047 (8)	−0.0004 (8)
C12	0.0206 (10)	0.0228 (11)	0.0134 (9)	0.0013 (9)	0.0029 (8)	−0.0003 (8)
C13	0.0306 (13)	0.0660 (19)	0.0164 (11)	−0.0041 (13)	0.0008 (10)	0.0070 (11)
C14	0.0266 (12)	0.0652 (19)	0.0174 (11)	0.0048 (13)	0.0028 (9)	−0.0006 (11)
C15	0.0529 (19)	0.0388 (17)	0.070 (2)	0.0193 (15)	−0.0244 (16)	−0.0181 (15)
C16	0.0290 (12)	0.0383 (14)	0.0198 (11)	−0.0060 (11)	0.0014 (9)	−0.0023 (10)

Geometric parameters (Å, º) top

Si1—C13	1.849 (2)	C6—C16	1.502 (3)
Si1—C14	1.854 (3)	C7—C8	1.446 (3)
Si1—C15	1.858 (3)	C8—C9	1.341 (3)
Si1—C12	1.869 (2)	C8—H8	0.9500
O1—C7	1.370 (3)	C9—C10	1.503 (3)
O1—C3	1.375 (3)	C10—H10A	0.9900
O2—C7	1.207 (3)	C10—H10B	0.9900
N1—C11	1.337 (3)	C11—C12	1.373 (3)
N1—N2	1.350 (2)	C11—H11	0.9500
N1—C10	1.453 (2)	C13—H13A	0.9800
N2—N3	1.309 (2)	C13—H13B	0.9800
N3—C12	1.375 (3)	C13—H13C	0.9800
C1—C2	1.375 (3)	C14—H14A	0.9800
C1—C6	1.401 (3)	C14—H14B	0.9800
C1—H1	0.9500	C14—H14C	0.9800
C2—C3	1.384 (3)	C15—H15A	0.9800
C2—H2	0.9500	C15—H15B	0.9800
C3—C4	1.389 (3)	C15—H15C	0.9800
C4—C5	1.402 (3)	C16—H16A	0.9800
C4—C9	1.453 (3)	C16—H16B	0.9800
C5—C6	1.385 (3)	C16—H16C	0.9800
C5—H5	0.9500

C13—Si1—C14	108.43 (11)	C4—C9—C10	117.21 (18)
C13—Si1—C15	112.47 (15)	N1—C10—C9	114.30 (17)
C14—Si1—C15	110.24 (15)	N1—C10—H10A	108.7
C13—Si1—C12	109.47 (10)	C9—C10—H10A	108.7
C14—Si1—C12	109.08 (10)	N1—C10—H10B	108.7
C15—Si1—C12	107.11 (11)	C9—C10—H10B	108.7
C7—O1—C3	121.84 (16)	H10A—C10—H10B	107.6
C11—N1—N2	110.77 (16)	N1—C11—C12	106.13 (18)
C11—N1—C10	128.60 (17)	N1—C11—H11	126.9
N2—N1—C10	120.63 (17)	C12—C11—H11	126.9
N3—N2—N1	106.71 (16)	C11—C12—N3	106.42 (17)
N2—N3—C12	109.96 (16)	C11—C12—Si1	128.90 (16)
C2—C1—C6	121.8 (2)	N3—C12—Si1	124.57 (14)
C2—C1—H1	119.1	Si1—C13—H13A	109.5
C6—C1—H1	119.1	Si1—C13—H13B	109.5
C1—C2—C3	118.6 (2)	H13A—C13—H13B	109.5
C1—C2—H2	120.7	Si1—C13—H13C	109.5
C3—C2—H2	120.7	H13A—C13—H13C	109.5
O1—C3—C2	116.45 (19)	H13B—C13—H13C	109.5
O1—C3—C4	121.63 (18)	Si1—C14—H14A	109.5
C2—C3—C4	121.9 (2)	Si1—C14—H14B	109.5
C3—C4—C5	118.08 (18)	H14A—C14—H14B	109.5
C3—C4—C9	117.72 (19)	Si1—C14—H14C	109.5
C5—C4—C9	124.18 (19)	H14A—C14—H14C	109.5
C6—C5—C4	121.4 (2)	H14B—C14—H14C	109.5
C6—C5—H5	119.3	Si1—C15—H15A	109.5
C4—C5—H5	119.3	Si1—C15—H15B	109.5
C5—C6—C1	118.2 (2)	H15A—C15—H15B	109.5
C5—C6—C16	121.6 (2)	Si1—C15—H15C	109.5
C1—C6—C16	120.21 (19)	H15A—C15—H15C	109.5
O2—C7—O1	116.9 (2)	H15B—C15—H15C	109.5
O2—C7—C8	126.0 (2)	C6—C16—H16A	109.5
O1—C7—C8	117.04 (19)	C6—C16—H16B	109.5
C9—C8—C7	122.4 (2)	H16A—C16—H16B	109.5
C9—C8—H8	118.8	C6—C16—H16C	109.5
C7—C8—H8	118.8	H16A—C16—H16C	109.5
C8—C9—C4	119.28 (19)	H16B—C16—H16C	109.5
C8—C9—C10	123.50 (19)

C11—N1—N2—N3	−0.5 (2)	C7—C8—C9—C4	−2.7 (3)
C10—N1—N2—N3	−179.99 (18)	C7—C8—C9—C10	177.1 (2)
N1—N2—N3—C12	0.4 (2)	C3—C4—C9—C8	2.3 (3)
C6—C1—C2—C3	0.1 (3)	C5—C4—C9—C8	−176.1 (2)
C7—O1—C3—C2	175.88 (19)	C3—C4—C9—C10	−177.55 (18)
C7—O1—C3—C4	−2.5 (3)	C5—C4—C9—C10	4.1 (3)
C1—C2—C3—O1	−178.51 (19)	C11—N1—C10—C9	100.6 (2)
C1—C2—C3—C4	−0.1 (3)	N2—N1—C10—C9	−79.9 (2)
O1—C3—C4—C5	178.70 (18)	C8—C9—C10—N1	8.1 (3)
C2—C3—C4—C5	0.4 (3)	C4—C9—C10—N1	−172.13 (18)
O1—C3—C4—C9	0.3 (3)	N2—N1—C11—C12	0.3 (2)
C2—C3—C4—C9	−178.0 (2)	C10—N1—C11—C12	179.78 (19)
C3—C4—C5—C6	−0.7 (3)	N1—C11—C12—N3	0.0 (2)
C9—C4—C5—C6	177.7 (2)	N1—C11—C12—Si1	176.29 (16)
C4—C5—C6—C1	0.6 (3)	N2—N3—C12—C11	−0.3 (2)
C4—C5—C6—C16	−178.9 (2)	N2—N3—C12—Si1	−176.77 (16)
C2—C1—C6—C5	−0.3 (3)	C13—Si1—C12—C11	157.8 (2)
C2—C1—C6—C16	179.2 (2)	C14—Si1—C12—C11	−83.7 (2)
C3—O1—C7—O2	−178.32 (19)	C15—Si1—C12—C11	35.6 (3)
C3—O1—C7—C8	2.1 (3)	C13—Si1—C12—N3	−26.5 (2)
O2—C7—C8—C9	−179.0 (2)	C14—Si1—C12—N3	92.0 (2)
O1—C7—C8—C9	0.5 (3)	C15—Si1—C12—N3	−148.7 (2)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C10—H10A···O2ⁱ	0.99	2.61	3.427 (1)	140
C11—H11···O2ⁱ	0.95	2.45	3.242 (1)	141

Symmetry code: (i) x, y+1, z.

References

Bruker. (1998). SMART, SAINT-Plus and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Lipeeva, A. V., Zakharov, D. O., Burova, L. G., Frolova, T. S., Baev, D. S., Shirokikh, I. V., Evstropov, A. N., Sinitsyna, O. I., Tolsikova, T. G. & Shults, E. E. (2019). Molecules, 24, 21–26. Web of Science CrossRef Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Venugopala, K. N., Rashmi, V. & Odhav, B. (2013). BioMed Res. Int., 1–14. Google Scholar
Watkin, D. J., Prout, C. K. & Pearce, L. J. (1996). CAMERON. Chemical Crystallography Laboratory, University of Oxford, England. Google Scholar

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.