research communications\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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ISSN: 2056-9890

(R)-Baclofen [(R)-4-amino-3-(4-chloro­phen­yl)butanoic acid]

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aUniversity of Rostock, Institute of Chemistry, Biocatalytic synthesis group, Albert-Einstein-Str. 3A, 18059 Rostock, Germany, and bUniversity of Rostock, Institute of Chemistry, X-ray structure analytics, Albert-Einstein-Str. 3A, 18059 Rostock, Germany
*Correspondence e-mail: jan.langermann@uni-rostock.de

Edited by C. Schulzke, Universität Greifswald, Germany (Received 3 November 2021; accepted 1 December 2021; online 1 January 2022)

This article provides the first single-crystal XRD-based structure of enanti­opure (R)-baclofen (form C), C10H12ClNO2, without any co-crystallized substances. In the enanti­opure title compound, the mol­ecules arrange themselves in an ortho­rhom­bic crystal structure (space group P212121). In the crystal, strong hydrogen bonds and C—H⋯Cl bonds inter­connect the zwitterionic mol­ecules.

1. Chemical context

(R)-Baclofen, an unnatural β-amino acid and artificial GABA receptor agonist, is a frequently used non-addictive drug to treat muscle spasticity (Dario & Tomei, 2004[Dario, A. & Tomei, G. (2004). Drug Saf. 27, 799-818.]). Although baclofen is conventionally applied as a racemic mixture, only the (R)-enanti­omer actually mediates a therapeutic effect (Olpe et al., 1978[Olpe, H.-R., Demiéville, H., Baltzer, V., Bencze, W. L., Koella, W. P., Wolf, P. & Haas, H. L. (1978). Eur. J. Pharmacol. 52, 133-136.]). In addition, baclofen has been recently approved in France as an alternative medication to treat alcohol dependence (Reade, 2021[Reade, M. C. (2021). JAMA, 325, 727-729.]). Considering those new developments, the establishment of synthetic routes towards enanti­opure (R)-baclofen were discussed recently (Córdova-Villanueva et al., 2018[Córdova-Villanueva, E. N., Rodríguez-Ruiz, C., Sánchez-Guadarrama, O., Rivera-Islas, J., Herrera-Ruiz, D., Morales-Rojas, H. & Höpfl, H. (2018). Cryst. Growth Des. 18, 7356-7367.]; Gendron et al., 2019[Gendron, F.-X., Mahieux, J., Sanselme, M. & Coquerel, G. (2019). Cryst. Growth Des. 19, 4793-4801.]).

[Scheme 1]

2. Structural commentary

The mol­ecular structure of the title compound is shown in Fig. 1[link]. A partial packing diagram is shown in Fig. 2[link].

[Figure 1]
Figure 1
The mol­ecular structure of (R)-baclofen with displacement ellipsoids shown at the 50% probability level.
[Figure 2]
Figure 2
Partial packing diagram of (R)-baclofen form C.

A prediction of crystal forms of the title compound was previously presented by Couvrat et al. (2021[Couvrat, N., Sanselme, M., Poupard, M., Bensakoun, C., Drouin, S. H., Schneider, J.-M. & Coquerel, G. (2021). J. Pharm. Sci. 110, 3457-3463.]), which is based on detailed XRPD-studies and Rietveld refinement. Based on the available XRPD-data, three forms, A, B and C, were observed, of which form C is considered to be the most stable form at higher temperatures. The (R)-baclofen crystal analyzed in this work corresponds to the newly predicted polymorphic form C presented by Couvrat et al. (2021[Couvrat, N., Sanselme, M., Poupard, M., Bensakoun, C., Drouin, S. H., Schneider, J.-M. & Coquerel, G. (2021). J. Pharm. Sci. 110, 3457-3463.]).

The mol­ecules crystallize in a zwitterionic configuration, forming an ammonium and a carboxyl­ate residue. The N-bound hydrogen atoms were located and refined freely. Bond lengths and angles fall into the typically observed ranges for organic mol­ecules without any strain.

3. Supra­molecular features

In the crystal of enanti­opure (R)-baclofen form C, short N—H⋯O hydrogen bonds occur between the carboxyl­ate and the ammonium group of the neighboring baclofen mol­ecule. In parallel, additional hydrogen bonding occurs with neighboring baclofen mol­ecules, resulting in a two-dimensional network parallel to (001), which yields a layered formation of baclofen mol­ecules. Parallel to the hydrogen bonding, T-shaped C—H⋯π inter­actions occur along the layers of aromatic rings within the mol­ecules [C9—H9 ⋯ Cg1viii= 2.74 Å; C6—H6 ⋯ Cg1ix= 3.24 Å; symmetry codes: (viii) −x + 2, y − [{1\over 2}], −z + [{3\over 2}]; (ix) −x + 1, y + [{1\over 2}], −z + [{3\over 2}]; Cg1 is the centroid of the C5–C10 benzene ring] . The inter­action planes both form angles of 67° with the plane of the corresponding benzene ring (C5–C10).

The combination of both effects yields the observed structure of form C of (R)-baclofen. In contrast, the cohesion of the apparently less stable form A is ensured by ππ inter­actions.

Hydrogen-bond geometry data as well as non-classical C—H⋯Cl inter­action data are summarized in Table 1[link].

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
N1—H1A⋯O2i 0.91 (3) 1.91 (3) 2.820 (2) 176 (3)
N1—H1B⋯O2ii 0.93 (3) 1.80 (3) 2.7149 (19) 168 (2)
N1—H1C⋯O1iii 0.85 (3) 1.93 (3) 2.775 (2) 174 (3)
N1—H1B⋯Cl1iv 0.93 (3) 2.95 (2) 3.3192 (14) 105.3 (16)
C4—H4A⋯Cl1v 0.99 2.73 3.6306 (19) 152
C4—H4B⋯Cl1vi 0.99 2.81 3.5668 (19) 134
Symmetry codes: (i) [x+{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+1]; (ii) x, y+1, z; (iii) [x-{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+1]; (iv) [-x+{\script{3\over 2}}, -y+1, z-{\script{1\over 2}}]; (v) [-x+1, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]; (vi) [-x+2, y+{\script{1\over 2}}, -z+{\script{3\over 2}}].

4. Database survey

Using the CSD database (version 5.42 updates 2 and 3; Groom et al., 2016[Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171-179.]), a search for the title compound's structure and names used in this article was conducted with CONQUEST (version 2021.2.0; Bruno et al., 2002[Bruno, I. J., Cole, J. C., Edgington, P. R., Kessler, M., Macrae, C. F., McCabe, P., Pearson, J. & Taylor, R. (2002). Acta Cryst. B58, 389-397.]).

While the crystal structures of (R)- and (S)-baclofenium hydro­chloride were reported in the early 1980s (Chang et al., 1981[Chang, C.-H., Yang, D. S. C., Yoo, C. S., Wang, B.-c., Pletcher, J. & Sax, M. (1981). Acta Cryst. A37, C71.], 1982[Chang, C.-H., Yang, D. S. C., Yoo, C. S., Wang, B.-C., Pletcher, J., Sax, M. & Terrence, C. F. (1982). Acta Cryst. B38, 2065-2067.]; refcodes: CRBMZB, CRBMZC10), studies on the phase behavior of pure baclofen have gained attention just recently. This is particularly relevant for the crystal structure of enanti­omerically pure (R)-baclofen since X-ray powder diffraction studies were recently described by Couvrat et al. (2021[Couvrat, N., Sanselme, M., Poupard, M., Bensakoun, C., Drouin, S. H., Schneider, J.-M. & Coquerel, G. (2021). J. Pharm. Sci. 110, 3457-3463.]). A total of three polymorphic forms (A, B and C) of (R)-baclofen were analyzed by X-ray powder diffraction, form C being identified as previously unknown. Based on this nomenclature, the crystal structure of form C is reported in this study. For the crystal structure of racemic baclofen, see Maniukiewicz et al. (2016[Maniukiewicz, W., Oracz, M. & Sieroń, L. (2016). J. Mol. Struct. 1123, 271-275.]; refcode: AQEKUE). A further array of racemic baclofenium co-crystal structures with various carb­oxy­lic acids were published by Báthori & Kilinkissa (2015[Báthori, N. B. & Kilinkissa, O. E. Y. (2015). CrystEngComm, 17, 8264-8272.]; refcodes: LUSXAA, LUSXEE, LUSXII, LUSXUU, LUSXOO, LUSYAB) and Malapile et al. (2021[Malapile, R. J., Nyamayaro, K., Nassimbeni, L. R. & Báthori, N. B. (2021). CrystEngComm, 23, 91-99.]; refcodes: LABJIL, LABJOR, LABJUX, LABKAE, LABKEI, LABKIM, LABKOS). Additionally, Gendron et al. (2019[Gendron, F.-X., Mahieux, J., Sanselme, M. & Coquerel, G. (2019). Cryst. Growth Des. 19, 4793-4801.]; refcode: WONSIE01) presented the crystal structure of (R)-baclofen hydrogenium maleate.

5. Synthesis and crystallization

Crystals of the title compound were grown from a saturated aqueous solution containing enanti­opure (R)-baclofen, which was evaporated slowly by a stream of dry argon at 313 K. The purity of the (R)-baclofen was verified via 1H NMR. Enanti­opure (R)-baclofen was purchased from abcr GmbH (Karlsruhe, Germany) under the name (R)-4-amino-3-(4-chloro­phen­yl)butanoic acid.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2[link]. The N-bound hydrogen atoms were found in difference syntheses, and refined freely. All C-bound H atoms were positioned geometrically and refined using a riding model, with C—H = 0.99 Å (methyl­ene groups), 1.00 Å (methine groups) or 0.95 Å (aryl CH) and with Uiso(H) = 1.2Ueq(C) (methyl­ene groups, aryl CH, methine groups). The structure was refined as a two-component inversion twin (BASF 0.04470).

Table 2
Experimental details

Crystal data
Chemical formula C10H12ClNO2
Mr 213.66
Crystal system, space group Orthorhombic, P212121
Temperature (K) 123
a, b, c (Å) 6.8913 (5), 7.6898 (5), 19.7527 (14)
V3) 1046.75 (13)
Z 4
Radiation type Mo Kα
μ (mm−1) 0.34
Crystal size (mm) 0.27 × 0.19 × 0.16
 
Data collection
Diffractometer Bruker D8 QUEST diffractometer
Absorption correction Multi-scan (SADABS2016/2; Krause et al., 2015[Krause, L., Herbst-Irmer, R., Sheldrick, G. M. & Stalke, D. (2015). J. Appl. Cryst. 48, 3-10.])
Tmin, Tmax 0.662, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections 23624, 3796, 3447
Rint 0.042
(sin θ/λ)max−1) 0.756
 
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.033, 0.082, 1.07
No. of reflections 3796
No. of parameters 140
H-atom treatment H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3) 0.36, −0.30
Absolute structure Refined as an inversion twin
Absolute structure parameter 0.04 (6)
Computer programs: APEX2 and APEX2 (Bruker, 2003[Bruker (2003). Apex, V7, 51A SADABS, SAINT, SHELXTL and SMART. Bruker AXS Inc., Madison, Wisconsin, USA.]), SAINT (Bruker, 2003[Bruker (2003). Apex, V7, 51A SADABS, SAINT, SHELXTL and SMART. Bruker AXS Inc., Madison, Wisconsin, USA.]), SHELXT (Sheldrick, 2015a[Sheldrick, G. M. (2015a). Acta Cryst. A71, 3-8.]), SHELXL2014/7 (Sheldrick, 2015b[Sheldrick, G. M. (2015b). Acta Cryst. C71, 3-8.]) and ORTEP-3 for Windows (Farrugia, 2012[Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849-854.]).

Supporting information


Computing details top

Data collection: APEX2 (Bruker, 2003); cell refinement: APEX2 (Bruker, 2003); data reduction: SAINT (Bruker, 2003); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL2014/7 (Sheldrick, 2015b).

(R)-4-amino-3-(4-chlorophenyl)butanoic acid top
Crystal data top
C10H12ClNO2Dx = 1.356 Mg m3
Mr = 213.66Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P212121Cell parameters from 9871 reflections
a = 6.8913 (5) Åθ = 2.8–33.0°
b = 7.6898 (5) ŵ = 0.34 mm1
c = 19.7527 (14) ÅT = 123 K
V = 1046.75 (13) Å3Block, colourless
Z = 40.27 × 0.19 × 0.16 mm
F(000) = 448
Data collection top
Bruker D8 QUEST
diffractometer
3796 independent reflections
Radiation source: microfocus sealed tube3447 reflections with I > 2σ(I)
Detector resolution: 10.4167 pixels mm-1Rint = 0.042
phi and ω scansθmax = 32.5°, θmin = 2.1°
Absorption correction: multi-scan
(SADABS2016/2; Krause et al., 2015)
h = 1010
Tmin = 0.662, Tmax = 0.747k = 1111
23624 measured reflectionsl = 2929
Refinement top
Refinement on F2Hydrogen site location: mixed
Least-squares matrix: fullH atoms treated by a mixture of independent and constrained refinement
R[F2 > 2σ(F2)] = 0.033 w = 1/[σ2(Fo2) + (0.0372P)2 + 0.2629P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.082(Δ/σ)max = 0.001
S = 1.07Δρmax = 0.36 e Å3
3796 reflectionsΔρmin = 0.30 e Å3
140 parametersAbsolute structure: Refined as an inversion twin
0 restraintsAbsolute structure parameter: 0.04 (6)
Primary atom site location: dual
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.

Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane)

3.7985 (0.0046) x + 6.4064 (0.0034) y + 0.9085 (0.0146) z = 5.2962 (0.0108)

* 0.0030 (0.0012) C5 * -0.0067 (0.0012) C6 * 0.0039 (0.0013) C7 * 0.0025 (0.0013) C8 * -0.0062 (0.0014) C9 * 0.0035 (0.0014) C10 2.7365 (0.0020) H9_$8 3.2353 (0.0036) H6_$9

Rms deviation of fitted atoms = 0.0046

3.7985 (0.0045) x - 6.4064 (0.0034) y + 0.9085 (0.0145) z = 0.4604 (0.0136)

Angle to previous plane (with approximate esd) = 67.162 ( 0.045 )

* -0.0030 (0.0012) C5_$8 * 0.0067 (0.0012) C6_$8 * -0.0039 (0.0013) C7_$8 * -0.0025 (0.0014) C8_$8 * 0.0062 (0.0014) C9_$8 * -0.0035 (0.0014) C10_$8

Rms deviation of fitted atoms = 0.0046

3.7985 (0.0045) x + 6.4064 (0.0034) y - 0.9085 (0.0145) z = 7.2622 (0.0039)

Angle to previous plane (with approximate esd) = 66.899 ( 0.045 )

* -0.0030 (0.0012) C5_$6 * 0.0067 (0.0012) C6_$6 * -0.0039 (0.0013) C7_$6 * -0.0025 (0.0013) C8_$6 * 0.0062 (0.0014) C9_$6 * -0.0035 (0.0014) C10_$6

Rms deviation of fitted atoms = 0.0046

Refinement. Refined as a 2-component inversion twin (BASF 0.04470).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Cl10.90826 (7)0.16066 (7)0.88304 (2)0.02370 (11)
N10.5842 (2)0.60260 (18)0.52352 (7)0.0134 (2)
O10.76064 (19)0.02092 (17)0.54492 (7)0.0194 (3)
O20.45342 (19)0.06602 (16)0.53636 (7)0.0178 (3)
C10.5825 (3)0.0466 (2)0.54937 (7)0.0123 (3)
C20.5085 (2)0.2255 (2)0.57066 (9)0.0147 (3)
H2A0.40810.20980.60600.018*
H2B0.44540.28090.53110.018*
C30.6643 (2)0.3497 (2)0.59801 (8)0.0124 (3)
H30.78180.33780.56870.015*
C40.5966 (3)0.5388 (2)0.59429 (8)0.0139 (3)
H4A0.46740.54900.61590.017*
H4B0.68820.61300.61990.017*
C50.7231 (2)0.3034 (2)0.67012 (8)0.0134 (3)
C60.6148 (2)0.3582 (2)0.72594 (8)0.0160 (3)
H60.50050.42480.71900.019*
C70.6719 (3)0.3166 (2)0.79192 (8)0.0173 (3)
H70.59860.35600.82970.021*
C80.8370 (3)0.2172 (2)0.80119 (8)0.0176 (3)
C90.9459 (3)0.1590 (3)0.74684 (9)0.0223 (4)
H91.05800.08960.75400.027*
C100.8886 (3)0.2037 (2)0.68165 (8)0.0200 (3)
H100.96390.16550.64420.024*
H1A0.701 (5)0.586 (4)0.5031 (16)0.044 (8)*
H1B0.555 (4)0.721 (3)0.5245 (11)0.022 (6)*
H1C0.490 (4)0.557 (4)0.5021 (14)0.029 (7)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Cl10.02181 (19)0.0335 (2)0.01578 (16)0.0041 (2)0.00328 (16)0.00057 (16)
N10.0134 (6)0.0094 (6)0.0174 (6)0.0003 (5)0.0012 (5)0.0000 (4)
O10.0152 (6)0.0173 (6)0.0258 (6)0.0016 (5)0.0031 (5)0.0055 (5)
O20.0167 (6)0.0096 (5)0.0272 (6)0.0011 (4)0.0027 (5)0.0011 (5)
C10.0156 (7)0.0097 (6)0.0117 (6)0.0006 (6)0.0003 (6)0.0009 (5)
C20.0147 (7)0.0091 (6)0.0202 (7)0.0006 (6)0.0025 (6)0.0018 (6)
C30.0130 (6)0.0099 (6)0.0144 (6)0.0013 (6)0.0013 (5)0.0010 (5)
C40.0159 (7)0.0104 (6)0.0155 (6)0.0015 (6)0.0012 (6)0.0012 (5)
C50.0133 (7)0.0100 (7)0.0169 (7)0.0006 (5)0.0013 (5)0.0010 (5)
C60.0146 (7)0.0146 (7)0.0187 (7)0.0028 (6)0.0015 (5)0.0012 (6)
C70.0186 (7)0.0171 (8)0.0163 (7)0.0010 (7)0.0029 (6)0.0004 (6)
C80.0188 (8)0.0188 (8)0.0151 (7)0.0001 (6)0.0025 (6)0.0002 (6)
C90.0200 (9)0.0276 (9)0.0194 (7)0.0115 (8)0.0034 (6)0.0002 (7)
C100.0188 (8)0.0243 (9)0.0169 (7)0.0090 (7)0.0015 (6)0.0030 (6)
Geometric parameters (Å, º) top
Cl1—C81.7445 (17)C3—H31.0000
N1—C41.484 (2)C4—H4A0.9900
N1—H1A0.91 (3)C4—H4B0.9900
N1—H1B0.93 (3)C5—C101.394 (2)
N1—H1C0.85 (3)C5—C61.396 (2)
O1—C11.247 (2)C6—C71.398 (2)
O2—C11.268 (2)C6—H60.9500
C1—C21.526 (2)C7—C81.383 (3)
C2—C31.535 (2)C7—H70.9500
C2—H2A0.9900C8—C91.384 (2)
C2—H2B0.9900C9—C101.390 (2)
C3—C51.523 (2)C9—H90.9500
C3—C41.529 (2)C10—H100.9500
C4—N1—H1A109 (2)C3—C4—H4A109.2
C4—N1—H1B108.4 (14)N1—C4—H4B109.2
H1A—N1—H1B110 (3)C3—C4—H4B109.2
C4—N1—H1C112.1 (19)H4A—C4—H4B107.9
H1A—N1—H1C114 (2)C10—C5—C6118.31 (15)
H1B—N1—H1C104 (2)C10—C5—C3119.94 (14)
O1—C1—O2124.61 (16)C6—C5—C3121.76 (14)
O1—C1—C2119.43 (15)C5—C6—C7121.13 (15)
O2—C1—C2115.95 (15)C5—C6—H6119.4
C1—C2—C3115.09 (14)C7—C6—H6119.4
C1—C2—H2A108.5C8—C7—C6118.76 (15)
C3—C2—H2A108.5C8—C7—H7120.6
C1—C2—H2B108.5C6—C7—H7120.6
C3—C2—H2B108.5C7—C8—C9121.46 (16)
H2A—C2—H2B107.5C7—C8—Cl1119.47 (13)
C5—C3—C4110.37 (13)C9—C8—Cl1119.07 (14)
C5—C3—C2111.71 (14)C8—C9—C10119.01 (16)
C4—C3—C2111.18 (13)C8—C9—H9120.5
C5—C3—H3107.8C10—C9—H9120.5
C4—C3—H3107.8C9—C10—C5121.32 (16)
C2—C3—H3107.8C9—C10—H10119.3
N1—C4—C3112.15 (12)C5—C10—H10119.3
N1—C4—H4A109.2
O1—C1—C2—C311.2 (2)C10—C5—C6—C70.9 (3)
O2—C1—C2—C3169.78 (14)C3—C5—C6—C7179.16 (16)
C1—C2—C3—C576.36 (17)C5—C6—C7—C81.0 (3)
C1—C2—C3—C4159.86 (13)C6—C7—C8—C90.1 (3)
C5—C3—C4—N1165.95 (14)C6—C7—C8—Cl1178.86 (14)
C2—C3—C4—N169.51 (18)C7—C8—C9—C100.8 (3)
C4—C3—C5—C10138.39 (16)Cl1—C8—C9—C10179.81 (16)
C2—C3—C5—C1097.37 (19)C8—C9—C10—C50.9 (3)
C4—C3—C5—C641.7 (2)C6—C5—C10—C90.0 (3)
C2—C3—C5—C682.53 (19)C3—C5—C10—C9179.86 (18)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···O2i0.91 (3)1.91 (3)2.820 (2)176 (3)
N1—H1B···O2ii0.93 (3)1.80 (3)2.7149 (19)168 (2)
N1—H1C···O1iii0.85 (3)1.93 (3)2.775 (2)174 (3)
N1—H1B···Cl1iv0.93 (3)2.95 (2)3.3192 (14)105.3 (16)
C4—H4A···Cl1v0.992.733.6306 (19)152
C4—H4B···Cl1vi0.992.813.5668 (19)134
Symmetry codes: (i) x+1/2, y+1/2, z+1; (ii) x, y+1, z; (iii) x1/2, y+1/2, z+1; (iv) x+3/2, y+1, z1/2; (v) x+1, y+1/2, z+3/2; (vi) x+2, y+1/2, z+3/2.
 

Acknowledgements

Funding by the Central SME Innovation Programme (ZIM, project No. ZF4402103CR9) is gratefully acknowledged. The authors thank Isabel Schicht and Sandra Diederich for experimental and technical support.

Funding information

Funding for this research was provided by: Bundesministerium für Wirtschaft und Energie (grant No. ZF4402103CR9 to Dr Jan von Langermann).

References

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