l-Argininium ethyl sulfate

Karapetyan, H.A.

doi:10.1107/S1600536808029954

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 64| Part 10| October 2008| Page o1982

doi:10.1107/S1600536808029954

Open

access

L-Argininium ethyl sulfate

Harutyun A. Karapetyan ^a ^*

^aMolecule Structure Research Center, National Academy of Sciences RA, Azatutyan ave. 26, 375014 Yerevan, Republic of Armenia
^*Correspondence e-mail: harkar@nfsat.am

(Received 29 August 2008; accepted 17 September 2008; online 20 September 2008)

The title compound, C₆H₁₅N₄O₂⁺·C₂H₅O₄S⁻, exhibits nonlinear optical properties. An extensive hydrogen-bonding network [N⋯O = 2.786 (4)–3.196 (5) Å] links cations and anions into a three-dimensional structure.

Related literature

For crystal structures and nonlinear optical properties of related compounds, see: Monaco et al. (1987 ); Petrosyan et al. (2000 ). For details of the synthesis, see: Petrosyan (2005 ).

Experimental

Crystal data

C₆H₁₅N₄O₂⁺·C₂H₅O₄S⁻
M_r = 300.34
Orthorhombic, P 2₁ 2₁ 2₁
a = 9.1504 (18) Å
b = 12.519 (3) Å
c = 12.551 (3) Å
V = 1437.8 (5) Å³
Z = 4
Mo Kα radiation
μ = 0.25 mm⁻¹
T = 293 (2) K
0.26 × 0.22 × 0.14 mm

Data collection

Enraf–Nonius CAD-4 diffractometer
Absorption correction: none
4566 measured reflections
4171 independent reflections
3091 reflections with I > 2σ(I)
R_int = 0.061
3 standard reflections every 400 reflections intensity decay: none

Refinement

R[F² > 2σ(F²)] = 0.071
wR(F²) = 0.202
S = 1.03
4171 reflections
175 parameters
H-atom parameters constrained
Δρ_max = 0.74 e Å⁻³
Δρ_min = −0.59 e Å⁻³
Absolute structure: Flack (1983 ), 1775 Friedel pairs
Flack parameter: 0.05 (16)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1B⋯O5	0.89	1.94	2.823 (5)	173
N1—H1C⋯O6ⁱ	0.89	2.01	2.896 (4)	172
N1—H1A⋯O1ⁱⁱ	0.89	1.97	2.786 (4)	152
N2—H2⋯O3ⁱⁱⁱ	0.86	2.25	3.098 (6)	170
N3—H3B⋯O6ⁱⁱⁱ	0.86	2.35	3.196 (5)	167
N3—H3A⋯O2^iv	0.86	1.93	2.771 (4)	165
N4—H4B⋯O1^iv	0.86	2.00	2.847 (4)	170
N4—H4A⋯O4ⁱⁱ	0.86	2.11	2.945 (5)	165
Symmetry codes: (i) $[-x+{\script{1\over 2}}, -y+2, z-{\script{1\over 2}}]$ ; (ii) $[x+{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1]$ ; (iii) $[x-{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1]$ ; (iv) $[-x+{\script{1\over 2}}, -y+1, z-{\script{1\over 2}}]$ .

Data collection: DATCOL in CAD-4 Manual (Enraf–Nonius, 1988 ); cell refinement: LS in CAD-4 Manual; data reduction: HELENA (Spek, 1997 ); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808029954/cv2443sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808029954/cv2443Isup2.hkl

checkCIF report

Comment top

In a search of analogs of the L-arginine phosphate (LAP) a large number of new materials [Monaco et al., 1987, Petrosyan et al., 2000] have been obtained by the interaction of L-arginine with various acids by choosing appropriate conditions. The crystals from the interaction of L-arginine with H₂SO₄ could not be obtained due to extremely high solubility of reaction product (Petrosyan, 2005). Nevertheless, the conditions for obtaining the crystals of L-arginine salt with ethylsulforic acid were found (Petrosyan, 2005).

We present herein a structural study of the L-argininium ethylsulfate, C₆H₁₅N₄O₂⁺.C₂H₅O₄S^-, (I). A view of the asymmetric unit is shown in Fig. 1. The geometric parameters found in (I) are in a good agreement with the common accepted values. In the crystal, all eight active H atoms are involved in hydrogen bonding (Table 1), which link the kations and anions into three-dimensional structure.

Related literature top

For crystal structures and nonlinear optical properties of related compounds, see: Monaco et al. (1987); Petrosyan et al. (2000). For details of the synthesis, see: Petrosyan (2005).

Experimental top

The single crystals of (I) were obtained by slow evaporation of the aqueous solution of exchange reaction product described by Petrosyan (2005):

L-Arg × HBF₄ + KC₂H₅SO₄ → L-Arg × HC₂H₅SO₄ + KBF₄.

Computing details top

Data collection: DATCOL in CAD-4 Manual (Enraf–Nonius, 1988); cell refinement: LS in CAD-4 Manual (Enraf–Nonius, 1988); data reduction: HELENA (Spek, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top

Fig. 1. A perspective view of the asymmetric unit of (I) showing the atomic numbering and displacement ellipsoids at the 50% probability level.

L-Argininium ethyl sulfate top

Crystal data top

C₆H₁₅N₄O₂⁺·C₂H₅O₄S⁻	F(000) = 640
M_r = 300.34	D_x = 1.387 Mg m⁻³
Orthorhombic, P2₁2₁2₁	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 24 reflections
a = 9.1504 (18) Å	θ = 14–16°
b = 12.519 (3) Å	µ = 0.25 mm⁻¹
c = 12.551 (3) Å	T = 293 K
V = 1437.8 (5) Å³	Block, colourless
Z = 4	0.26 × 0.22 × 0.14 mm

Data collection top

Enraf–Nonius CAD-4 diffractometer	R_int = 0.062
Radiation source: fine-focus sealed tube	θ_max = 30.0°, θ_min = 2.3°
Graphite monochromator	h = 0→12
ω/2θ scans	k = 0→17
4566 measured reflections	l = −17→17
4171 independent reflections	3 standard reflections every 400 reflections
3091 reflections with I > 2σ(I)	intensity decay: none

Refinement top

Refinement on F²	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F² > 2σ(F²)] = 0.072	w = 1/[σ²(F_o²) + (0.0927P)² + 1.2922P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.202	(Δ/σ)_max < 0.001
S = 1.03	Δρ_max = 0.74 e Å⁻³
4171 reflections	Δρ_min = −0.59 e Å⁻³
175 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.007 (2)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983), 1775 Friedel pairs
Secondary atom site location: difference Fourier map	Absolute structure parameter: 0.05 (16)

Crystal data top

C₆H₁₅N₄O₂⁺·C₂H₅O₄S⁻	V = 1437.8 (5) Å³
M_r = 300.34	Z = 4
Orthorhombic, P2₁2₁2₁	Mo Kα radiation
a = 9.1504 (18) Å	µ = 0.25 mm⁻¹
b = 12.519 (3) Å	T = 293 K
c = 12.551 (3) Å	0.26 × 0.22 × 0.14 mm

Data collection top

Enraf–Nonius CAD-4 diffractometer	R_int = 0.062
4566 measured reflections	3 standard reflections every 400 reflections
4171 independent reflections	intensity decay: none
3091 reflections with I > 2σ(I)

Refinement top

R[F² > 2σ(F²)] = 0.072	H-atom parameters constrained
wR(F²) = 0.202	Δρ_max = 0.74 e Å⁻³
S = 1.03	Δρ_min = −0.59 e Å⁻³
4171 reflections	Absolute structure: Flack (1983), 1775 Friedel pairs
175 parameters	Absolute structure parameter: 0.05 (16)
0 restraints

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
S1	0.23564 (11)	0.86276 (9)	0.82316 (10)	0.0575 (3)
O1	0.1060 (2)	0.6386 (2)	0.5342 (2)	0.0447 (6)
O2	0.3427 (3)	0.6749 (2)	0.5555 (3)	0.0541 (8)
O3	0.3324 (5)	0.7594 (3)	0.8318 (5)	0.126 (2)
O4	0.0977 (4)	0.8395 (4)	0.7810 (5)	0.114 (2)
O5	0.3241 (6)	0.9208 (6)	0.7447 (3)	0.126 (2)
O6	0.2501 (5)	0.9140 (2)	0.9236 (3)	0.0755 (11)
N1	0.3133 (3)	0.8825 (2)	0.5232 (2)	0.0331 (5)
H1A	0.3969	0.8533	0.5017	0.050*
H1B	0.3140	0.8890	0.5938	0.050*
H1C	0.3032	0.9467	0.4937	0.050*
N2	0.1657 (4)	0.7002 (3)	0.1525 (2)	0.0450 (7)
H2	0.0751	0.7186	0.1518	0.054*
N3	0.0934 (4)	0.5395 (3)	0.0841 (3)	0.0531 (9)
H3A	0.1139	0.4759	0.0629	0.064*
H3B	0.0051	0.5627	0.0803	0.064*
N4	0.3325 (4)	0.5638 (3)	0.1274 (3)	0.0519 (9)
H4A	0.4024	0.6035	0.1505	0.062*
H4B	0.3503	0.4994	0.1074	0.062*
C1	0.2151 (3)	0.6992 (3)	0.5313 (3)	0.0340 (6)
C2	0.1889 (3)	0.8130 (2)	0.4905 (2)	0.0296 (6)
H1	0.0993	0.8406	0.5233	0.036*
C3	0.1696 (3)	0.8141 (3)	0.3697 (2)	0.0332 (6)
H3C	0.0883	0.7678	0.3515	0.040*
H3D	0.1437	0.8860	0.3479	0.040*
C4	0.3030 (4)	0.7788 (3)	0.3057 (3)	0.0392 (7)
H4C	0.3842	0.8262	0.3209	0.047*
H4D	0.3309	0.7072	0.3272	0.047*
C5	0.2713 (4)	0.7799 (3)	0.1865 (3)	0.0408 (7)
H5A	0.2353	0.8501	0.1670	0.049*
H5B	0.3621	0.7683	0.1483	0.049*
C6	0.1980 (4)	0.6018 (3)	0.1226 (3)	0.0400 (8)
C7	0.2636 (9)	0.6578 (5)	0.8418 (6)	0.100 (2)
H7A	0.2037	0.6434	0.7797	0.120*
H7B	0.2012	0.6567	0.9043	0.120*
C8	0.3784 (9)	0.5765 (5)	0.8517 (5)	0.098 (2)
H8A	0.4249	0.5665	0.7838	0.147*
H8B	0.3359	0.5103	0.8747	0.147*
H8C	0.4496	0.5996	0.9029	0.147*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1	0.0378 (5)	0.0573 (6)	0.0773 (7)	0.0002 (4)	0.0002 (4)	−0.0298 (5)
O1	0.0284 (11)	0.0372 (12)	0.0686 (17)	−0.0036 (10)	−0.0039 (11)	0.0095 (12)
O2	0.0285 (12)	0.0425 (14)	0.091 (2)	−0.0015 (10)	−0.0121 (13)	0.0221 (14)
O3	0.067 (2)	0.070 (3)	0.241 (7)	0.014 (2)	−0.018 (3)	−0.067 (4)
O4	0.0435 (17)	0.094 (3)	0.205 (6)	0.0008 (19)	−0.016 (3)	−0.055 (3)
O5	0.103 (3)	0.213 (6)	0.061 (2)	−0.050 (4)	0.017 (2)	−0.028 (3)
O6	0.121 (3)	0.0484 (15)	0.0577 (17)	0.006 (2)	0.017 (2)	−0.0105 (14)
N1	0.0310 (12)	0.0337 (13)	0.0347 (12)	0.0004 (10)	−0.0024 (10)	−0.0021 (10)
N2	0.0364 (15)	0.0459 (16)	0.0528 (17)	0.0022 (13)	0.0005 (13)	−0.0137 (14)
N3	0.0346 (15)	0.0485 (17)	0.076 (2)	−0.0008 (13)	−0.0013 (16)	−0.0241 (17)
N4	0.0336 (15)	0.0494 (18)	0.073 (2)	0.0035 (13)	−0.0012 (15)	−0.0241 (17)
C1	0.0281 (14)	0.0341 (14)	0.0396 (15)	0.0023 (12)	0.0025 (12)	0.0055 (12)
C2	0.0214 (11)	0.0297 (13)	0.0377 (15)	0.0021 (10)	0.0025 (11)	0.0017 (12)
C3	0.0285 (13)	0.0346 (15)	0.0364 (14)	0.0024 (12)	0.0006 (12)	0.0001 (12)
C4	0.0299 (15)	0.0458 (17)	0.0421 (17)	−0.0007 (13)	0.0029 (13)	−0.0074 (14)
C5	0.0439 (18)	0.0385 (16)	0.0401 (17)	−0.0027 (14)	0.0093 (15)	−0.0078 (13)
C6	0.0355 (17)	0.0437 (18)	0.0406 (17)	−0.0023 (14)	0.0059 (14)	−0.0087 (14)
C7	0.118 (6)	0.072 (4)	0.111 (5)	−0.010 (4)	0.000 (4)	0.032 (3)
C8	0.153 (7)	0.057 (3)	0.084 (4)	0.005 (4)	0.022 (4)	0.002 (3)

Geometric parameters (Å, º) top

S1—O4	1.399 (4)	N4—H4B	0.8600
S1—O6	1.421 (3)	C1—C2	1.533 (4)
S1—O5	1.468 (5)	C2—C3	1.527 (4)
S1—O3	1.571 (4)	C2—H1	0.9800
O1—C1	1.255 (4)	C3—C4	1.526 (4)
O2—C1	1.244 (4)	C3—H3C	0.9700
O3—C7	1.426 (7)	C3—H3D	0.9700
N1—C2	1.490 (4)	C4—C5	1.524 (5)
N1—H1A	0.8900	C4—H4C	0.9700
N1—H1B	0.8900	C4—H4D	0.9700
N1—H1C	0.8900	C5—H5A	0.9700
N2—C6	1.322 (5)	C5—H5B	0.9700
N2—C5	1.453 (5)	C7—C8	1.467 (9)
N2—H2	0.8600	C7—H7A	0.9700
N3—C6	1.326 (5)	C7—H7B	0.9700
N3—H3A	0.8600	C8—H8A	0.9600
N3—H3B	0.8600	C8—H8B	0.9600
N4—C6	1.321 (5)	C8—H8C	0.9600
N4—H4A	0.8600

O4—S1—O6	120.9 (3)	C4—C3—H3C	108.4
O4—S1—O5	110.3 (3)	C2—C3—H3C	108.4
O6—S1—O5	108.7 (3)	C4—C3—H3D	108.4
O4—S1—O3	111.3 (3)	C2—C3—H3D	108.4
O6—S1—O3	105.0 (3)	H3C—C3—H3D	107.5
O5—S1—O3	98.2 (4)	C5—C4—C3	111.2 (3)
C7—O3—S1	119.5 (4)	C5—C4—H4C	109.4
C2—N1—H1A	109.5	C3—C4—H4C	109.4
C2—N1—H1B	109.5	C5—C4—H4D	109.4
H1A—N1—H1B	109.5	C3—C4—H4D	109.4
C2—N1—H1C	109.5	H4C—C4—H4D	108.0
H1A—N1—H1C	109.5	N2—C5—C4	114.1 (3)
H1B—N1—H1C	109.5	N2—C5—H5A	108.7
C6—N2—C5	125.1 (3)	C4—C5—H5A	108.7
C6—N2—H2	117.5	N2—C5—H5B	108.7
C5—N2—H2	117.5	C4—C5—H5B	108.7
C6—N3—H3A	120.0	H5A—C5—H5B	107.6
C6—N3—H3B	120.0	N4—C6—N2	122.1 (3)
H3A—N3—H3B	120.0	N4—C6—N3	118.5 (3)
C6—N4—H4A	120.0	N2—C6—N3	119.4 (3)
C6—N4—H4B	120.0	O3—C7—C8	108.1 (6)
H4A—N4—H4B	120.0	O3—C7—H7A	110.1
O2—C1—O1	126.3 (3)	C8—C7—H7A	110.1
O2—C1—C2	117.1 (3)	O3—C7—H7B	110.1
O1—C1—C2	116.6 (3)	C8—C7—H7B	110.1
N1—C2—C3	110.9 (2)	H7A—C7—H7B	108.4
N1—C2—C1	109.3 (2)	C7—C8—H8A	109.5
C3—C2—C1	111.0 (3)	C7—C8—H8B	109.5
N1—C2—H1	108.5	H8A—C8—H8B	109.5
C3—C2—H1	108.5	C7—C8—H8C	109.5
C1—C2—H1	108.5	H8A—C8—H8C	109.5
C4—C3—C2	115.3 (3)	H8B—C8—H8C	109.5

O4—S1—O3—C7	−25.9 (8)	C1—C2—C3—C4	−63.9 (3)
O6—S1—O3—C7	106.5 (6)	C2—C3—C4—C5	178.5 (3)
O5—S1—O3—C7	−141.5 (6)	C6—N2—C5—C4	−89.4 (4)
O2—C1—C2—N1	−19.4 (4)	C3—C4—C5—N2	−67.5 (4)
O1—C1—C2—N1	162.5 (3)	C5—N2—C6—N4	4.4 (6)
O2—C1—C2—C3	103.2 (4)	C5—N2—C6—N3	−173.9 (3)
O1—C1—C2—C3	−74.9 (4)	S1—O3—C7—C8	−178.3 (5)
N1—C2—C3—C4	57.9 (4)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1B···O5	0.89	1.94	2.823 (5)	173
N1—H1C···O6ⁱ	0.89	2.01	2.896 (4)	172
N1—H1A···O1ⁱⁱ	0.89	1.97	2.786 (4)	152
N2—H2···O3ⁱⁱⁱ	0.86	2.25	3.098 (6)	170
N3—H3B···O6ⁱⁱⁱ	0.86	2.35	3.196 (5)	167
N3—H3A···O2^iv	0.86	1.93	2.771 (4)	165
N4—H4B···O1^iv	0.86	2.00	2.847 (4)	170
N4—H4A···O4ⁱⁱ	0.86	2.11	2.945 (5)	165

Symmetry codes: (i) −x+1/2, −y+2, z−1/2; (ii) x+1/2, −y+3/2, −z+1; (iii) x−1/2, −y+3/2, −z+1; (iv) −x+1/2, −y+1, z−1/2.

Experimental details

Crystal data
Chemical formula	C₆H₁₅N₄O₂⁺·C₂H₅O₄S⁻
M_r	300.34
Crystal system, space group	Orthorhombic, P2₁2₁2₁
Temperature (K)	293
a, b, c (Å)	9.1504 (18), 12.519 (3), 12.551 (3)
V (Å³)	1437.8 (5)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.25
Crystal size (mm)	0.26 × 0.22 × 0.14

Data collection
Diffractometer	Enraf–Nonius CAD-4 diffractometer
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	4566, 4171, 3091
R_int	0.062
(sin θ/λ)_max (Å⁻¹)	0.703

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.072, 0.202, 1.03
No. of reflections	4171
No. of parameters	175
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.74, −0.59
Absolute structure	Flack (1983), 1775 Friedel pairs
Absolute structure parameter	0.05 (16)

Computer programs: DATCOL in CAD-4 Manual (Enraf–Nonius, 1988), LS in CAD-4 Manual (Enraf–Nonius, 1988), HELENA (Spek, 1997), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1B···O5	0.89	1.94	2.823 (5)	172.9
N1—H1C···O6ⁱ	0.89	2.01	2.896 (4)	171.7
N1—H1A···O1ⁱⁱ	0.89	1.97	2.786 (4)	152.1
N2—H2···O3ⁱⁱⁱ	0.86	2.25	3.098 (6)	169.7
N3—H3B···O6ⁱⁱⁱ	0.86	2.35	3.196 (5)	167.2
N3—H3A···O2^iv	0.86	1.93	2.771 (4)	164.7
N4—H4B···O1^iv	0.86	2.00	2.847 (4)	169.5
N4—H4A···O4ⁱⁱ	0.86	2.11	2.945 (5)	164.5

Symmetry codes: (i) −x+1/2, −y+2, z−1/2; (ii) x+1/2, −y+3/2, −z+1; (iii) x−1/2, −y+3/2, −z+1; (iv) −x+1/2, −y+1, z−1/2.

Acknowledgements

The author expresses his thanks to Dr A. M. Petrosyan for providing the crystals and for valuable discussion of the results. This work was supported by US CRDF grant No. AE2-2533-YE-03.

References

Enraf–Nonius (1988). CAD-4 Manual. Enraf–Nonius, Delft, The Netherlands. Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Monaco, S. B., Davis, L. E., Velsko, S. P., Wang, F. T., Eimerl, D. & Zalkin, A. (1987). J. Cryst. Growth, 85, 252–257. CSD CrossRef CAS Web of Science Google Scholar
Petrosyan, A. M. (2005). Proceedings of Conference on Laser Physics 2005, 11–14 October 2005, Ashtarak, Armenia, pp. 123–126. Google Scholar
Petrosyan, A. M., Sukiasyan, R. P., Karapetyan, H. A., Terzyan, S. S. & Feigelson, R. S. (2000). J. Cryst. Growth, 213, 103–257. Web of Science CrossRef CAS Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (1997). HELENA. University of Utrecht, The Netherlands. Google Scholar