(S)-Alanine–(S)-2-phen­oxy­propionic acid (1/1)

Amimoto, K.; Nishioka, Y.

doi:10.1107/S1600536812020727

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 68| Part 6| June 2012| Page o1720

doi:10.1107/S1600536812020727

Open

access

(S)-Alanine–(S)-2-phenoxypropionic acid (1/1)

Kiichi Amimoto ^a ^* and Yuma Nishioka ^a

^aDepartment of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, Japan
^*Correspondence e-mail: kamimo@hiroshima-u.ac.jp

(Received 4 April 2012; accepted 8 May 2012; online 16 May 2012)

In the title co-crystal, C₃H₇NO₂·C₉H₁₀O₃, the (S)-alanine molecule exists in the zwitterionic form stabilized by two pairs of N⁺—H⋯O⁻ hydrogen bonds and an electrostatic interaction between the ammonium center and the carboxylate anion, forming a sheet along the ab plane. The carboxyl group of the (S)-2-phenoxypropionic acid molecule is connected to the top and bottom of the sheet via N⁺—H⋯O=C and O—H⋯O⁻ [R₂²(7) graph set] hydrogen bonds, giving an (S,S)-homochiral layer, in which both methyl groups of (S)-alanine and the phenyl rings of (S)-2-phenoxypropionic acid are oriented in the same direction along the b axis.

Related literature

For the use of a chiral resolution agents, see: Hasegawa et al. (1998 ). For the crystal structure of enantiomeric and racemic 2-phenoxypropionic acid, see: Sørensen & Larsen (2003 ). For the crystal structure of (S)-alanine–(R)-2-phenoxypropionic acid, see: Takahashi & Fujii (2004 ).

Experimental

Crystal data

C₃H₇NO₂·C₉H₁₀O₃
M_r = 255.27
Monoclinic, P 2₁
a = 5.227 (5) Å
b = 7.364 (5) Å
c = 17.493 (5) Å
β = 95.232 (5)°
V = 670.5 (8) Å³
Z = 2
Mo Kα radiation
μ = 0.10 mm⁻¹
T = 293 K
0.7 × 0.5 × 0.3 mm

Data collection

Bruker APEXII CCD area-detector diffractometer
3872 measured reflections
2742 independent reflections
2698 reflections with I > 2σ(I)
R_int = 0.021

Refinement

R[F² > 2σ(F²)] = 0.029
wR(F²) = 0.081
S = 1.06
2742 reflections
171 parameters
1 restraint
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.17 e Å⁻³
Δρ_min = −0.13 e Å⁻³
Absolute structure: Flack (1983 ), 930 Friedel pairs
Flack parameter: −0.4 (8)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1A⋯O5ⁱ	0.89	2.05	2.916 (3)	165
N1—H1B⋯O5ⁱⁱ	0.89	1.94	2.822 (3)	170
N1—H1C⋯O2ⁱ	0.89	1.97	2.863 (3)	177
O1—H1O⋯O4ⁱⁱ	1.03 (2)	1.50 (2)	2.521 (3)	169 (2)
Symmetry codes: (i) x+1, y, z; (ii) $[-x+1, y+{\script{1\over 2}}, -z+1]$ .

Data collection: APEX2 (Bruker, 2009 ); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SIR97 (Altomare et al., 1999 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: Mercury (Macrae et al., 2008 ) and ORTEP-3 (Farrugia, 1997 ); software used to prepare material for publication: WinGX (Farrugia, 1999 ) and publCIF (Westrip, 2010 ).

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536812020727/ds2190sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536812020727/ds2190Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536812020727/ds2190Isup3.cml

checkCIF report

Comment top

Chiral 2-phenoxypropionic acid (PPA) has been known as a good and accessible optical resolving agent for amines (Hasegawa et al., 1998). The crystal structure of optical pure and racemic PPA has been reported (Sørensen & Larsen, 2003). And the crystal structure of co-crystal of (R)-PPA with (S)-alanine has been known (Takahashi & Fujii, 2004), but no results have ever reported on the details of the chiral discrimination between PPA and (S)-alanine. In this work, we present the crystal structure of the co-crystal of (S)-PPA with (S)-alanine (I) (Fig. 1). The co-crystal I crystallizes in the monoclinic system of space group P2₁. (S)-Alanine assembles a chiral two-dimensional sheet along the ab plane, in which the ammonium cation is strongly held with the carboxylate anion by two hydrogen bonds and one electrostatic interaction. The N(1)⁺—H···O(5)^- hydrogen bonds are 2.822 (2) Å and 2.916 (2) Å. The interatomic distance between ammonium center N(1)⁺ and carboxylate O(4)^- is 2.960 (2) Å. The carboxyl C=O of PPA is connected to the ammonium N⁺—H of (S)-alanine, and the O—H of PPA to the carboxylate O^- of (S)-alanine. The N(1)⁺—H···O(2) and O(1)—H···O(4)^- distances are 2.863 (2) Å and 2.521 (2) Å, respectively. The phenyl ring of PPA is oriented in the same direction of the methyl group of (S)-alanine of chiral two-dimensional sheet, yielding a (S,S)-homochiral layer (Fig. 2). On the basis of this finding, the development of optical resolution of amino acid using PPA as an optical resolving agent is under investigation.

Related literature top

For the use of a chiral resolution agents, see: Hasegawa et al. (1998). For the crystal structure of enantiomeric and racemic 2-phenoxypropionic acid, see: Sørensen & Larsen (2003). For the crystal structure of (S)-alanine–(R)-2-phenoxypropionic acid, see: Takahashi & Fujii (2004).

Experimental top

All reagents were commercially available from WAKO Co. and used without purification.(S)-2-Phenoxypropionic acid (1.66 g, 10 mmol) and (S)-alanine (0.89 g, 10 mmol) were dissolved in a water/ethanol solution (10 ml, 1:1 v/v). The solution was refluxed for 10 min, cooled to room temperature, and then kept in the refrigerator for three days. Colorless single crystals of I were obtained that were suitable for X-ray diffraction study.

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999) and publCIF (Westrip, 2010).

Figures top

	Fig. 1. The moleculer structure of the title compound I, showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.
	Fig. 2. A view of the crystal packing of the title compound I, viewed down the c axis (anisotropic displacement ellipsoids drawn at 50% probability level). Hydrogen bonds are drawn as dashed lines.

(S)-Alanine–(S)-2-phenoxypropionic acid (1/1) top

Crystal data top

C₃H₇NO₂·C₉H₁₀O₃	F(000) = 272
M_r = 255.27	D_x = 1.264 Mg m⁻³
Monoclinic, P2₁	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: P 2yb	Cell parameters from 3229 reflections
a = 5.227 (5) Å	θ = 2.3–28.5°
b = 7.364 (5) Å	µ = 0.10 mm⁻¹
c = 17.493 (5) Å	T = 293 K
β = 95.232 (5)°	Plate, colourless
V = 670.5 (8) Å³	0.7 × 0.5 × 0.3 mm
Z = 2

Data collection top

Bruker APEXII CCD area-detector diffractometer	2698 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	R_int = 0.021
Graphite monochromator	θ_max = 28.5°, θ_min = 2.3°
Detector resolution: 8.333 pixels mm^-1	h = −6→5
phi and ω scan	k = −9→9
3872 measured reflections	l = −21→23
2742 independent reflections

Refinement top

Refinement on F²	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F² > 2σ(F²)] = 0.029	w = 1/[σ²(F_o²) + (0.0452P)² + 0.0566P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.081	(Δ/σ)_max < 0.001
S = 1.06	Δρ_max = 0.17 e Å⁻³
2742 reflections	Δρ_min = −0.13 e Å⁻³
171 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008)
1 restraint	Extinction coefficient: 0.160 (9)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983), 930 Friedel pairs
Secondary atom site location: difference Fourier map	Absolute structure parameter: −0.4 (8)

Crystal data top

C₃H₇NO₂·C₉H₁₀O₃	V = 670.5 (8) Å³
M_r = 255.27	Z = 2
Monoclinic, P2₁	Mo Kα radiation
a = 5.227 (5) Å	µ = 0.10 mm⁻¹
b = 7.364 (5) Å	T = 293 K
c = 17.493 (5) Å	0.7 × 0.5 × 0.3 mm
β = 95.232 (5)°

Data collection top

Bruker APEXII CCD area-detector diffractometer	2698 reflections with I > 2σ(I)
3872 measured reflections	R_int = 0.021
2742 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.029	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.081	Δρ_max = 0.17 e Å⁻³
S = 1.06	Δρ_min = −0.13 e Å⁻³
2742 reflections	Absolute structure: Flack (1983), 930 Friedel pairs
171 parameters	Absolute structure parameter: −0.4 (8)
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	0.2142 (2)	0.91172 (17)	0.27450 (7)	0.0400 (3)
C2	0.2315 (3)	0.9885 (2)	0.19397 (8)	0.0489 (3)
H2	0.4105	1.0177	0.1867	0.059*
C3	0.0685 (5)	1.1604 (3)	0.18625 (12)	0.0785 (6)
H3A	0.0795	1.2125	0.1363	0.118*
H3B	0.1306	1.2461	0.2249	0.118*
H3C	−0.1071	1.1307	0.1926	0.118*
C4	0.2830 (3)	0.7215 (2)	0.12044 (7)	0.0460 (3)
C5	0.5113 (3)	0.6774 (3)	0.16181 (8)	0.0531 (3)
H5	0.5723	0.7466	0.2041	0.064*
C6	0.6486 (4)	0.5288 (3)	0.13974 (10)	0.0688 (5)
H6	0.8015	0.4971	0.168	0.083*
C7	0.5623 (4)	0.4276 (3)	0.07671 (12)	0.0776 (6)
H7	0.6572	0.3286	0.0622	0.093*
C8	0.3355 (4)	0.4726 (3)	0.03498 (10)	0.0708 (5)
H8	0.2773	0.4045	−0.0079	0.085*
C9	0.1955 (3)	0.6180 (2)	0.05671 (8)	0.0572 (4)
H9	0.0412	0.6477	0.0288	0.069*
C10	0.5537 (2)	0.48177 (15)	0.48456 (7)	0.0332 (2)
C11	0.6485 (2)	0.55376 (16)	0.41011 (6)	0.0337 (2)
H11	0.5119	0.6252	0.3823	0.04*
C12	0.7222 (4)	0.3969 (2)	0.35972 (9)	0.0634 (4)
H12A	0.7836	0.4441	0.3136	0.095*
H12B	0.5746	0.3217	0.3468	0.095*
H12C	0.855	0.3261	0.387	0.095*
N1	0.87704 (17)	0.67096 (13)	0.42842 (5)	0.0322 (2)
H1A	1.0014	0.6064	0.4537	0.048*
H1B	0.835	0.7636	0.4574	0.048*
H1C	0.9317	0.7131	0.3851	0.048*
O1	0.3772 (2)	0.98974 (17)	0.32449 (6)	0.0594 (3)
O2	0.0633 (2)	0.79618 (16)	0.28881 (5)	0.0556 (3)
O3	0.1324 (2)	0.86666 (16)	0.13631 (5)	0.0543 (3)
O4	0.72311 (17)	0.45179 (16)	0.53819 (5)	0.0478 (2)
O5	0.32016 (16)	0.45273 (15)	0.48470 (6)	0.0505 (3)
H1O	0.348 (4)	0.961 (4)	0.3808 (13)	0.090 (7)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0455 (6)	0.0409 (6)	0.0347 (5)	−0.0023 (5)	0.0092 (5)	0.0001 (5)
C2	0.0571 (8)	0.0507 (7)	0.0405 (6)	−0.0004 (6)	0.0123 (5)	0.0086 (6)
C3	0.1025 (15)	0.0610 (10)	0.0741 (11)	0.0222 (10)	0.0188 (11)	0.0239 (9)
C4	0.0487 (7)	0.0597 (8)	0.0306 (5)	−0.0026 (6)	0.0090 (5)	0.0051 (5)
C5	0.0523 (8)	0.0688 (9)	0.0387 (6)	0.0046 (7)	0.0064 (5)	−0.0053 (7)
C6	0.0626 (10)	0.0849 (13)	0.0604 (9)	0.0170 (9)	0.0138 (8)	−0.0034 (9)
C7	0.0906 (14)	0.0761 (13)	0.0697 (11)	0.0092 (11)	0.0269 (10)	−0.0153 (10)
C8	0.0871 (12)	0.0749 (11)	0.0527 (8)	−0.0212 (10)	0.0184 (8)	−0.0165 (8)
C9	0.0613 (9)	0.0745 (10)	0.0355 (6)	−0.0130 (8)	0.0029 (6)	0.0007 (6)
C10	0.0292 (5)	0.0326 (5)	0.0387 (5)	0.0014 (4)	0.0078 (4)	0.0082 (4)
C11	0.0309 (5)	0.0378 (5)	0.0321 (5)	−0.0065 (4)	0.0009 (4)	0.0051 (4)
C12	0.0933 (12)	0.0534 (8)	0.0457 (7)	−0.0229 (8)	0.0179 (8)	−0.0170 (6)
N1	0.0313 (4)	0.0328 (4)	0.0335 (4)	−0.0041 (4)	0.0076 (3)	0.0015 (4)
O1	0.0697 (7)	0.0685 (7)	0.0406 (5)	−0.0264 (6)	0.0089 (5)	−0.0073 (5)
O2	0.0657 (7)	0.0647 (6)	0.0372 (5)	−0.0238 (5)	0.0086 (4)	0.0042 (4)
O3	0.0563 (6)	0.0709 (7)	0.0348 (4)	0.0084 (5)	−0.0010 (4)	0.0028 (4)
O4	0.0364 (4)	0.0718 (7)	0.0358 (4)	0.0054 (4)	0.0060 (3)	0.0173 (5)
O5	0.0288 (4)	0.0571 (5)	0.0663 (6)	−0.0030 (4)	0.0082 (4)	0.0240 (5)

Geometric parameters (Å, º) top

C1—O2	1.2019 (17)	C7—H7	0.93
C1—O1	1.2980 (18)	C8—C9	1.370 (3)
C1—C2	1.5281 (17)	C8—H8	0.93
C2—O3	1.4124 (19)	C9—H9	0.93
C2—C3	1.525 (3)	C10—O5	1.2398 (18)
C2—H2	0.98	C10—O4	1.2494 (16)
C3—H3A	0.96	C10—C11	1.5298 (15)
C3—H3B	0.96	C11—N1	1.4848 (17)
C3—H3C	0.96	C11—C12	1.524 (2)
C4—O3	1.3708 (19)	C11—H11	0.98
C4—C5	1.377 (2)	C12—H12A	0.96
C4—C9	1.393 (2)	C12—H12B	0.96
C5—C6	1.383 (3)	C12—H12C	0.96
C5—H5	0.93	N1—H1A	0.89
C6—C7	1.373 (3)	N1—H1B	0.89
C6—H6	0.93	N1—H1C	0.89
C7—C8	1.375 (3)	O1—H1O	1.03 (2)

O2—C1—O1	125.16 (12)	C9—C8—H8	120.1
O2—C1—C2	123.29 (12)	C7—C8—H8	120.1
O1—C1—C2	111.53 (12)	C8—C9—C4	120.31 (18)
O3—C2—C3	107.36 (15)	C8—C9—H9	119.8
O3—C2—C1	112.01 (12)	C4—C9—H9	119.8
C3—C2—C1	108.02 (12)	O5—C10—O4	126.77 (11)
O3—C2—H2	109.8	O5—C10—C11	117.18 (11)
C3—C2—H2	109.8	O4—C10—C11	115.99 (10)
C1—C2—H2	109.8	N1—C11—C12	108.94 (11)
C2—C3—H3A	109.5	N1—C11—C10	109.53 (9)
C2—C3—H3B	109.5	C12—C11—C10	110.39 (11)
H3A—C3—H3B	109.5	N1—C11—H11	109.3
C2—C3—H3C	109.5	C12—C11—H11	109.3
H3A—C3—H3C	109.5	C10—C11—H11	109.3
H3B—C3—H3C	109.5	C11—C12—H12A	109.5
O3—C4—C5	124.33 (13)	C11—C12—H12B	109.5
O3—C4—C9	115.83 (14)	H12A—C12—H12B	109.5
C5—C4—C9	119.82 (15)	C11—C12—H12C	109.5
C4—C5—C6	119.16 (15)	H12A—C12—H12C	109.5
C4—C5—H5	120.4	H12B—C12—H12C	109.5
C6—C5—H5	120.4	C11—N1—H1A	109.5
C7—C6—C5	120.85 (19)	C11—N1—H1B	109.5
C7—C6—H6	119.6	H1A—N1—H1B	109.5
C5—C6—H6	119.6	C11—N1—H1C	109.5
C6—C7—C8	120.01 (19)	H1A—N1—H1C	109.5
C6—C7—H7	120	H1B—N1—H1C	109.5
C8—C7—H7	120	C1—O1—H1O	114.0 (14)
C9—C8—C7	119.85 (17)	C4—O3—C2	117.36 (12)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O5ⁱ	0.89	2.05	2.916 (3)	165
N1—H1B···O5ⁱⁱ	0.89	1.94	2.822 (3)	170
N1—H1C···O2ⁱ	0.89	1.97	2.863 (3)	177
O1—H1O···O4ⁱⁱ	1.03 (2)	1.50 (2)	2.521 (3)	169 (2)

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

Experimental details

Crystal data
Chemical formula	C₃H₇NO₂·C₉H₁₀O₃
M_r	255.27
Crystal system, space group	Monoclinic, P2₁
Temperature (K)	293
a, b, c (Å)	5.227 (5), 7.364 (5), 17.493 (5)
β (°)	95.232 (5)
V (Å³)	670.5 (8)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	0.10
Crystal size (mm)	0.7 × 0.5 × 0.3

Data collection
Diffractometer	Bruker APEXII CCD area-detector diffractometer
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	3872, 2742, 2698
R_int	0.021
(sin θ/λ)_max (Å⁻¹)	0.672

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.029, 0.081, 1.06
No. of reflections	2742
No. of parameters	171
No. of restraints	1
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.17, −0.13
Absolute structure	Flack (1983), 930 Friedel pairs
Absolute structure parameter	−0.4 (8)

Computer programs: APEX2 (Bruker, 2009), SAINT (Bruker, 2009), SIR97 (Altomare et al., 1999), SHELXL97 (Sheldrick, 2008), Mercury (Macrae et al., 2008) and ORTEP-3 (Farrugia, 1997), WinGX (Farrugia, 1999) and publCIF (Westrip, 2010).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O5ⁱ	0.89	2.05	2.916 (3)	165
N1—H1B···O5ⁱⁱ	0.89	1.94	2.822 (3)	170
N1—H1C···O2ⁱ	0.89	1.97	2.863 (3)	177
O1—H1O···O4ⁱⁱ	1.03 (2)	1.50 (2)	2.521 (3)	169 (2)

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

Acknowledgements

This work was partially supported by a Grant-in-Aid for Young Scientists (B) (23700956) and a Grant-in-Aid for Scientific Research (C) (22300272) from the Japan Society for the Promotion of Science (JSPS). The X-ray crystallographic analysis was performed at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University.

References

Altomare, A., Burla, M. C., Camalli, M., Cascarano, G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori, G. & Spagna, R. (1999). J. Appl. Cryst. 32, 115–119. Web of Science CrossRef CAS IUCr Journals Google Scholar
Bruker (2009). APEX2 and SAINT, Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. CrossRef IUCr Journals Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Hasegawa, G., Miura, T., Watadani, T. & Hong, N. (1998). J. Synth. Org. Chem. Jpn, 56, 773–780. CrossRef CAS Google Scholar
Macrae, C. F., Bruno, I. J., Chisholm, J. A., Edgington, P. R., McCabe, P., Pidcock, E., Rodriguez-Monge, L., Taylor, R., van de Streek, J. & Wood, P. A. (2008). J. Appl. Cryst. 41, 466–470. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sørensen, H. O. & Larsen, S. (2003). Acta Cryst. B59, 132–140. Web of Science CSD CrossRef IUCr Journals Google Scholar
Takahashi, Y. & Fujii, I. (2004). Anal. Sci. 20, x77–x78. CAS Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar