(3R,4Z)-1,3-Diethyl-4-(2-oxo­propyl­idene)-2,3,4,5-tetra­hydro-1H-1,5-benzodiazepin-2-one

El Foujji, L.; Sebhaoui, J.; El Bakri, Y.; El Ghayati, L.'; Essassi, E.M.; Mague, J.T.

doi:10.1107/S2414314618005151

organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 3| Part 4| April 2018| x180515

https://doi.org/10.1107/S2414314618005151

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(3R,4Z)-1,3-Diethyl-4-(2-oxopropylidene)-2,3,4,5-tetrahydro-1H-1,5-benzodiazepin-2-one

Laila El Foujji,^a ^* Jihad Sebhaoui,^a Youness El Bakri,^a L'houssaine El Ghayati,^a El Mokhtar Essassi ^a and Joel T. Mague ^b

^aLaboratoire de Chimie Organique Hétérocyclique, Centre de Recherche des Sciences des Médicaments, URAC 21, Pôle de Compétence Pharmacochimie, Av Ibn Battouta, BP 1014, Faculté des Sciences, Université Mohammed V, Rabat, Morocco, and ^bDepartment of Chemistry, Tulane University, New Orleans, LA 70118, USA
^*Correspondence e-mail: elfoujji.laila18@gmail.com

Edited by W. T. A. Harrison, University of Aberdeen, Scotland (Received 30 March 2018; accepted 31 March 2018; online 17 April 2018)

In the title compound, C₁₆H₂₀N₂O₂, the seven-membered ring adopts a bowl-shaped conformation while the orientation of the 2-oxopropylidene substituent is determined by an intramolecular N—H⋯O hydrogen bond, which generates an S(6) ring. In the crystal, inversion dimers linked by pairs of very weak C—H⋯O interactions occur, which generate R₂²(8) loops.

Keywords: crystal structure; benzodiazepine; hydrogen bond.

CCDC reference: 1833967

Structure description

1,4- and 1,5-Benzodiazepines are commonly used as anxiolytic and anticonvulsive drugs: these effects are primarily mediated via the benzodiazepine receptors located in the central nervous system (Tallman et al., 1980 ). In a continuation on our studies (Sebhaoui et al., 2017 ; Samba et al., 2018 ) of benzodiazepine derivatives, we report here the alkylation of (4Z)-4-(2-oxopropylidene)-2,3,4,5-tetrahydro-1,5-benzodiazepin-2-one with ethyl iodide under solid–liquid phase-transfer catalytic conditions leading to the title compound.

The seven-membered ring adopts a bowl-shaped conformation with puckering parameters Q(2) = 0.8767 (13) Å, Q(3) = 0.2633 (13) Å, φ(2) = 203.90 (8)° and φ(3) = 309.7 (3)°. The total puckering is 0.9154 (13) Å. The orientation of the 2-oxopropylidene substituent is largely determined by the intramolecular N1—H1⋯O2 hydrogen bond (Table 1 and Fig. 1). Apart from a possible weak C12—H12A⋯O2 hydrogen bond (H⋯O is 0.09 Å less than the sum of the van der Walls' radii), the packing is determined by van der Waals contacts (Fig. 2).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1⋯O2	0.887 (19)	1.897 (19)	2.6326 (13)	139.2 (16)
C12—H12A⋯O2ⁱ	0.98 (2)	2.63 (2)	3.5334 (17)	153.4 (17)
Symmetry code: (i) -x+1, -y+2, -z+2.

Figure 1
The title molecule with 50% probability ellipsoids.

Figure 2
Packing viewed along the c-axis direction.

Synthesis and crystallization

To a solution of (4Z)-4-(2-oxopropylidene)-2,3,4,5-tetrahydro-1,5-benzodiazepin-2-one (1 mmol) and potassium carbonate K₂CO₃ (1.5 mmol) in 30 ml of N,N-dimethylformamide, were added ethyl iodide (0.02 mol) and tetra-n-butylammonium bromide as a phase-transfer catalyst. The reaction mixture was stirred at room temperature for 10 h. The residue obtained, after evaporation of the solvent, was chromatographed on a silica gel column using a hexane/ethyl acetate 4:1 mixture as eluent. The solid obtained was recrystallized from ethanol solution to afford colourless plates of the title compound.

Refinement

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

Table 2
Experimental details

Crystal data
Chemical formula	C₁₆H₂₀N₂O₂
M_r	272.34
Crystal system, space group	Triclinic, P $[\overline{1}]$
Temperature (K)	150
a, b, c (Å)	8.8546 (2), 8.9841 (2), 9.5523 (2)
α, β, γ (°)	98.481 (1), 96.412 (1), 106.772 (1)
V (Å³)	710.00 (3)
Z	2
Radiation type	Cu Kα
μ (mm⁻¹)	0.68
Crystal size (mm)	0.31 × 0.15 × 0.07

Data collection
Diffractometer	Bruker D8 VENTURE PHOTON 100 CMOS
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.88, 0.96
No. of measured, independent and observed [I > 2σ(I)] reflections	5416, 2616, 2387
R_int	0.023
(sin θ/λ)_max (Å⁻¹)	0.618

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.037, 0.091, 1.05
No. of reflections	2616
No. of parameters	262
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.21, −0.22
Computer programs: APEX3 and SAINT (Bruker, 2016 ), SHELXT (Sheldrick, 2015a ), SHELXL2018 (Sheldrick, 2015b ), DIAMOND (Brandenburg & Putz, 2012 ) and SHELXTL (Sheldrick, 2008 ).

Structural data

CCDC reference: 1833967

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S2414314618005151/hb4224sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314618005151/hb4224Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2414314618005151/hb4224Isup3.cdx

Supporting information file. DOI: https://doi.org/10.1107/S2414314618005151/hb4224Isup4.cml

checkCIF report

Computing details top

Data collection: APEX3 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg & Putz, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

(3R,4Z)-1,3-Diethyl-4-(2-oxopropylidene)-2,3,4,5-tetrahydro-1H-1,5-benzodiazepin-2-one top

Crystal data top

C₁₆H₂₀N₂O₂	Z = 2
M_r = 272.34	F(000) = 292
Triclinic, P1	D_x = 1.274 Mg m⁻³
a = 8.8546 (2) Å	Cu Kα radiation, λ = 1.54178 Å
b = 8.9841 (2) Å	Cell parameters from 4659 reflections
c = 9.5523 (2) Å	θ = 5.2–72.4°
α = 98.481 (1)°	µ = 0.68 mm⁻¹
β = 96.412 (1)°	T = 150 K
γ = 106.772 (1)°	Plate, colourless
V = 710.00 (3) Å³	0.31 × 0.15 × 0.07 mm

Data collection top

Bruker D8 VENTURE PHOTON 100 CMOS diffractometer	2616 independent reflections
Radiation source: INCOATEC IµS micro-focus source	2387 reflections with I > 2σ(I)
Mirror monochromator	R_int = 0.023
Detector resolution: 10.4167 pixels mm^-1	θ_max = 72.4°, θ_min = 5.2°
ω scans	h = −10→10
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	k = −11→10
T_min = 0.88, T_max = 0.96	l = −11→11
5416 measured reflections

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F² > 2σ(F²)] = 0.037	All H-atom parameters refined
wR(F²) = 0.091	w = 1/[σ²(F_o²) + (0.0377P)² + 0.2722P] where P = (F_o² + 2F_c²)/3
S = 1.05	(Δ/σ)_max < 0.001
2616 reflections	Δρ_max = 0.21 e Å⁻³
262 parameters	Δρ_min = −0.22 e Å⁻³
0 restraints	Extinction correction: SHELXL2018 (Sheldrick, 2015b), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0107 (11)

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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
O1	0.20343 (11)	0.41685 (11)	0.35492 (10)	0.0288 (2)
O2	0.41944 (11)	0.99801 (11)	0.76940 (9)	0.0286 (2)
N1	0.26316 (12)	0.87183 (13)	0.50781 (11)	0.0207 (2)
H1	0.282 (2)	0.940 (2)	0.590 (2)	0.040 (5)*
N2	0.06087 (12)	0.58264 (12)	0.31111 (11)	0.0216 (2)
C1	0.05091 (14)	0.73584 (15)	0.29983 (13)	0.0206 (3)
C2	−0.06584 (15)	0.74986 (17)	0.19536 (14)	0.0272 (3)
H2	−0.131 (2)	0.655 (2)	0.1266 (18)	0.036 (4)*
C3	−0.09068 (16)	0.89355 (17)	0.18786 (15)	0.0301 (3)
H3	−0.175 (2)	0.902 (2)	0.1165 (19)	0.042 (5)*
C4	0.00296 (16)	1.02877 (16)	0.28445 (15)	0.0272 (3)
H4	−0.0131 (19)	1.1329 (19)	0.2803 (17)	0.031 (4)*
C5	0.12020 (15)	1.01790 (15)	0.38658 (14)	0.0231 (3)
H5	0.1877 (19)	1.1120 (19)	0.4583 (17)	0.031 (4)*
C6	0.14635 (14)	0.87331 (15)	0.39597 (12)	0.0196 (3)
C7	0.36913 (13)	0.78969 (14)	0.49951 (13)	0.0192 (3)
C8	0.35391 (14)	0.68791 (15)	0.35474 (13)	0.0203 (3)
H8	0.3357 (17)	0.7520 (17)	0.2802 (16)	0.022 (3)*
C9	0.20103 (14)	0.54911 (15)	0.33985 (13)	0.0211 (3)
C10	0.48033 (14)	0.79991 (15)	0.61585 (13)	0.0213 (3)
H10	0.5525 (18)	0.7358 (17)	0.6064 (16)	0.023 (4)*
C11	0.50091 (14)	0.90620 (15)	0.74916 (13)	0.0225 (3)
C12	0.62834 (17)	0.9093 (2)	0.86900 (15)	0.0311 (3)
H12A	0.585 (3)	0.902 (2)	0.958 (2)	0.059 (6)*
H12B	0.713 (3)	1.014 (3)	0.883 (2)	0.071 (7)*
H12C	0.680 (2)	0.828 (2)	0.8474 (19)	0.042 (5)*
C13	−0.08886 (15)	0.44886 (16)	0.28977 (15)	0.0261 (3)
H13A	−0.175 (2)	0.4960 (19)	0.3099 (17)	0.031 (4)*
H13B	−0.0722 (18)	0.3840 (19)	0.3624 (17)	0.028 (4)*
C14	−0.12777 (18)	0.34639 (18)	0.14073 (16)	0.0338 (3)
H14A	−0.221 (2)	0.253 (2)	0.136 (2)	0.050 (5)*
H14B	−0.155 (2)	0.406 (2)	0.067 (2)	0.051 (5)*
H14C	−0.034 (2)	0.3101 (19)	0.1212 (17)	0.034 (4)*
C15	0.49743 (15)	0.63217 (16)	0.32949 (13)	0.0245 (3)
H15A	0.5090 (17)	0.5527 (18)	0.3921 (16)	0.024 (4)*
H15B	0.598 (2)	0.7274 (19)	0.3576 (17)	0.030 (4)*
C16	0.48269 (18)	0.5586 (2)	0.17249 (15)	0.0331 (3)
H16A	0.380 (2)	0.468 (2)	0.1413 (19)	0.044 (5)*
H16B	0.574 (2)	0.512 (2)	0.157 (2)	0.046 (5)*
H16C	0.479 (2)	0.638 (2)	0.111 (2)	0.050 (5)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0290 (5)	0.0216 (5)	0.0362 (5)	0.0095 (4)	0.0027 (4)	0.0058 (4)
O2	0.0321 (5)	0.0323 (5)	0.0228 (4)	0.0156 (4)	0.0017 (4)	0.0006 (4)
N1	0.0210 (5)	0.0230 (5)	0.0181 (5)	0.0095 (4)	0.0002 (4)	0.0011 (4)
N2	0.0179 (5)	0.0203 (5)	0.0246 (5)	0.0040 (4)	0.0015 (4)	0.0037 (4)
C1	0.0188 (5)	0.0227 (6)	0.0217 (6)	0.0081 (5)	0.0039 (4)	0.0049 (5)
C2	0.0234 (6)	0.0307 (7)	0.0260 (6)	0.0093 (5)	−0.0015 (5)	0.0028 (6)
C3	0.0271 (6)	0.0376 (8)	0.0289 (7)	0.0159 (6)	−0.0005 (5)	0.0085 (6)
C4	0.0279 (6)	0.0289 (7)	0.0309 (7)	0.0152 (5)	0.0073 (5)	0.0099 (6)
C5	0.0218 (6)	0.0240 (6)	0.0251 (6)	0.0089 (5)	0.0058 (5)	0.0044 (5)
C6	0.0179 (5)	0.0238 (6)	0.0189 (6)	0.0081 (5)	0.0044 (4)	0.0048 (5)
C7	0.0178 (5)	0.0187 (6)	0.0210 (6)	0.0047 (4)	0.0046 (4)	0.0044 (5)
C8	0.0190 (6)	0.0224 (6)	0.0192 (6)	0.0067 (5)	0.0025 (4)	0.0031 (5)
C9	0.0212 (6)	0.0224 (6)	0.0192 (6)	0.0075 (5)	0.0017 (4)	0.0019 (5)
C10	0.0194 (6)	0.0238 (6)	0.0211 (6)	0.0077 (5)	0.0027 (5)	0.0040 (5)
C11	0.0215 (6)	0.0259 (6)	0.0202 (6)	0.0065 (5)	0.0039 (5)	0.0063 (5)
C12	0.0305 (7)	0.0427 (8)	0.0207 (6)	0.0151 (6)	−0.0002 (5)	0.0032 (6)
C13	0.0197 (6)	0.0242 (6)	0.0314 (7)	0.0020 (5)	0.0039 (5)	0.0057 (6)
C14	0.0318 (7)	0.0271 (7)	0.0341 (8)	0.0014 (6)	−0.0035 (6)	0.0022 (6)
C15	0.0221 (6)	0.0300 (7)	0.0225 (6)	0.0114 (5)	0.0031 (5)	0.0017 (6)
C16	0.0311 (7)	0.0428 (9)	0.0259 (7)	0.0154 (7)	0.0066 (6)	−0.0016 (6)

Geometric parameters (Å, º) top

O1—C9	1.2237 (15)	C8—C9	1.5291 (17)
O2—C11	1.2513 (15)	C8—H8	1.007 (15)
N1—C7	1.3529 (15)	C10—C11	1.4338 (17)
N1—C6	1.4059 (15)	C10—H10	0.978 (15)
N1—H1	0.887 (19)	C11—C12	1.5061 (17)
N2—C9	1.3676 (15)	C12—H12A	0.98 (2)
N2—C1	1.4232 (15)	C12—H12B	1.00 (3)
N2—C13	1.4804 (15)	C12—H12C	0.980 (19)
C1—C2	1.4008 (17)	C13—C14	1.5212 (19)
C1—C6	1.4045 (17)	C13—H13A	0.993 (16)
C2—C3	1.3811 (19)	C13—H13B	0.994 (16)
C2—H2	0.978 (18)	C14—H14A	0.99 (2)
C3—C4	1.390 (2)	C14—H14B	1.00 (2)
C3—H3	0.979 (18)	C14—H14C	0.999 (17)
C4—C5	1.3781 (18)	C15—C16	1.5222 (18)
C4—H4	0.991 (16)	C15—H15A	1.016 (15)
C5—C6	1.3973 (17)	C15—H15B	1.017 (17)
C5—H5	0.993 (17)	C16—H16A	1.01 (2)
C7—C10	1.3734 (17)	C16—H16B	1.025 (19)
C7—C8	1.5103 (16)	C16—H16C	0.99 (2)
C8—C15	1.5245 (16)

C7—N1—C6	126.56 (10)	C7—C10—C11	122.91 (11)
C7—N1—H1	113.3 (12)	C7—C10—H10	118.9 (9)
C6—N1—H1	119.5 (11)	C11—C10—H10	118.2 (9)
C9—N2—C1	124.31 (10)	O2—C11—C10	122.89 (11)
C9—N2—C13	116.88 (10)	O2—C11—C12	118.70 (11)
C1—N2—C13	118.80 (10)	C10—C11—C12	118.40 (11)
C2—C1—C6	118.27 (12)	C11—C12—H12A	110.3 (13)
C2—C1—N2	119.06 (11)	C11—C12—H12B	107.1 (13)
C6—C1—N2	122.52 (10)	H12A—C12—H12B	107.8 (18)
C3—C2—C1	121.58 (12)	C11—C12—H12C	113.4 (11)
C3—C2—H2	119.7 (10)	H12A—C12—H12C	110.9 (16)
C1—C2—H2	118.8 (10)	H12B—C12—H12C	107.0 (17)
C2—C3—C4	119.82 (12)	N2—C13—C14	113.09 (11)
C2—C3—H3	121.0 (10)	N2—C13—H13A	106.4 (9)
C4—C3—H3	119.2 (10)	C14—C13—H13A	111.7 (9)
C5—C4—C3	119.49 (12)	N2—C13—H13B	105.9 (9)
C5—C4—H4	119.7 (9)	C14—C13—H13B	109.3 (9)
C3—C4—H4	120.8 (9)	H13A—C13—H13B	110.3 (13)
C4—C5—C6	121.39 (12)	C13—C14—H14A	109.5 (11)
C4—C5—H5	121.4 (9)	C13—C14—H14B	110.6 (11)
C6—C5—H5	117.2 (9)	H14A—C14—H14B	107.8 (16)
C5—C6—C1	119.44 (11)	C13—C14—H14C	109.2 (10)
C5—C6—N1	117.53 (11)	H14A—C14—H14C	108.8 (14)
C1—C6—N1	122.94 (11)	H14B—C14—H14C	111.0 (15)
N1—C7—C10	121.37 (11)	C16—C15—C8	110.84 (10)
N1—C7—C8	114.87 (10)	C16—C15—H15A	109.6 (8)
C10—C7—C8	123.76 (11)	C8—C15—H15A	111.1 (8)
C7—C8—C15	115.57 (10)	C16—C15—H15B	109.4 (9)
C7—C8—C9	105.80 (9)	C8—C15—H15B	108.3 (9)
C15—C8—C9	111.94 (10)	H15A—C15—H15B	107.5 (12)
C7—C8—H8	107.0 (8)	C15—C16—H16A	110.4 (10)
C15—C8—H8	108.7 (8)	C15—C16—H16B	111.0 (10)
C9—C8—H8	107.4 (8)	H16A—C16—H16B	106.7 (14)
O1—C9—N2	121.94 (11)	C15—C16—H16C	110.0 (11)
O1—C9—C8	122.29 (11)	H16A—C16—H16C	106.9 (15)
N2—C9—C8	115.77 (10)	H16B—C16—H16C	111.7 (15)

C9—N2—C1—C2	141.65 (12)	C10—C7—C8—C15	−16.89 (18)
C13—N2—C1—C2	−37.92 (16)	N1—C7—C8—C9	−72.16 (13)
C9—N2—C1—C6	−42.92 (17)	C10—C7—C8—C9	107.58 (13)
C13—N2—C1—C6	137.50 (12)	C1—N2—C9—O1	176.65 (11)
C6—C1—C2—C3	−1.59 (19)	C13—N2—C9—O1	−3.77 (17)
N2—C1—C2—C3	174.03 (12)	C1—N2—C9—C8	−2.22 (16)
C1—C2—C3—C4	0.8 (2)	C13—N2—C9—C8	177.36 (10)
C2—C3—C4—C5	0.2 (2)	C7—C8—C9—O1	−103.76 (13)
C3—C4—C5—C6	−0.50 (19)	C15—C8—C9—O1	22.94 (16)
C4—C5—C6—C1	−0.26 (18)	C7—C8—C9—N2	75.10 (13)
C4—C5—C6—N1	−176.96 (11)	C15—C8—C9—N2	−158.19 (10)
C2—C1—C6—C5	1.28 (17)	N1—C7—C10—C11	−5.14 (19)
N2—C1—C6—C5	−174.19 (11)	C8—C7—C10—C11	175.13 (11)
C2—C1—C6—N1	177.79 (11)	C7—C10—C11—O2	−0.2 (2)
N2—C1—C6—N1	2.32 (18)	C7—C10—C11—C12	−179.09 (12)
C7—N1—C6—C5	−140.34 (12)	C9—N2—C13—C14	−79.55 (14)
C7—N1—C6—C1	43.09 (18)	C1—N2—C13—C14	100.05 (13)
C6—N1—C7—C10	178.73 (12)	C7—C8—C15—C16	−169.08 (11)
C6—N1—C7—C8	−1.52 (17)	C9—C8—C15—C16	69.70 (14)
N1—C7—C8—C15	163.37 (11)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O2	0.887 (19)	1.897 (19)	2.6326 (13)	139.2 (16)
C12—H12A···O2ⁱ	0.98 (2)	2.63 (2)	3.5334 (17)	153.4 (17)

Symmetry code: (i) −x+1, −y+2, −z+2.

Funding information

The support of NSF–MRI Grant No. 1228232 for the purchase of the diffractometer and Tulane University for support of the Tulane Crystallography Laboratory are gratefully acknowledged.

References

Brandenburg, K. & Putz, H. (2012). DIAMOND, Crystal Impact GbR, Bonn, Germany. Google Scholar
Bruker (2016). APEX3, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Krause, L., Herbst-Irmer, R., Sheldrick, G. M. & Stalke, D. (2015). J. Appl. Cryst. 48, 3–10. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Samba, M., Minnih, M. S., El Bakri, Y., Essassi, E. M. & &Mague, J. T. (2018). IUCrData, 3, x180326. Google Scholar
Sebhaoui, J., El Bakri, Y., Rayni, I., El Bourakadi, K., Essassi, E. M. & Mague, J. T. (2017). IUCrData, 2, x170493. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Tallman, J. F., Paul, S. M., Skolnick, P. & Gallager, D. W. (1980). Science, 207, 274–281. CrossRef CAS Google Scholar

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