research communications
7,12]docosane dinitrate dihydrate from synchrotron X-ray data
of 3,14-diethyl-2,13-diaza-6,17-diazoniatricyclo[16.4.0.0aBeamline Department, Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea, and bDepartment of Chemistry, Andong National University, Andong 36729, Republic of Korea
*Correspondence e-mail: jhchoi@anu.ac.kr
The 22H46N42+·2NO3−·2H2O, has been determined using synchrotron radiation at 220 K. The reveals that protonation has occurred at diagonally opposite amine N atoms. The contains half a centrosymmetric dication, one nitrate anion and one water molecule. The molecular dication, C22H46N42+, together with the nitrate anion and hydrate water molecule are involved in an extensive range of hydrogen bonds. The molecule is stabilized, as is the conformation of the dication, by forming intermolecular N—H⋯O, O—H⋯O, together with intramolecular N—H⋯N hydrogen bonds.
of title salt, CKeywords: crystal structure; protonated macrocycle; nitrate; hydrate; hydrogen bonds; synchrotron radiation.
CCDC reference: 1918729
1. Chemical context
The 3,14-diethyl-2,6,13,17-tetraazatricyclo(16.4.0.07,12)docosane macrocycle (C22H44N4, L) contains a cyclam backbone with two cyclohexane subunits. Ethyl groups are also attached to the 3 and 14 carbon atoms of the propyl chains that bridge opposite pairs of N atoms in the structure. The macrocyclic ligand L is a strongly basic amine capable of forming the dication, [C22H46N4]2+ or the tetracation [C22H48N4]4+ in which all of the N—H bonds are generally available for hydrogen-bond formation. These di- or tetra-ammonium cations may be suitable for the removal of toxic heavy metal ions from water. The crystal structures of [Cu(L)](ClO4)2 (Lim et al., 2006), [Cu(L)](NO3)2, [Cu(L)(H2O)2](SCN)2 (Choi et al., 2012), [Ni(L)(NO3)2] (Subhan & Choi, 2014), [Ni(L)(N3)2] (Lim et al., 2015) and [Ni(L)(NCS)2] (Lim & Choi, 2017) have been reported. In these complexes, CuII or NiII cations have tetragonally distorted octahedral environments with the four N atoms of the macrocyclic ligand in equatorial positions and the O/N atoms of anions or water molecules in the axial positions, while [Ni(L)](ClO4)2·2H2O (Subhan & Choi, 2014) has a square-planar geometry around the NiII atom that binds to the four nitrogen atoms of the macrocyclic ligand. The macrocyclic ligands in the CuII and NiII complexes adopt the most stable trans-III conformation. Recently, we also reported the crystal structures of [C22H46N4](ClO4)2 (Aree et al., 2018), [C22H46N4]Cl2·4H2O (Moon et al., 2013) and (C22H44N4)2·2NaClO4 (Aree et al., 2018). To further investigate the hydrogen-bonding behavior, we report here on the synthesis of a new hydrated nitrate salt, [C22H46N4](NO3)2·2H2O, (I), and its structural characterization by synchrotron single-crystal X-ray diffraction.
2. Structural commentary
The title compound has a positively charged macrocyclic dication, two nitrate anions and two solvent water molecules and was prepared during a study of the macrocyclic ligand and its silver(II) complex. An ellipsoid plot of the molecular components in compound (I) is shown in Fig. 1 along with the atom-numbering scheme. The consists of one half of the macrocycle, which lies about a center of inversion, one nitrate anion and one solvent water molecule. The four N atoms are coplanar, and the two ethyl substituents are anti with respect to the macrocyclic plane as a result of the molecular inversion symmetry. The six-membered cyclohexane ring is in a stable chair conformation. Within the centrosymmetric diprotonated amine unit [C22H46N4]2+, the C—C and N—C bond lengths vary from 1.517 (2) to 1.533 (2) Å and from 1.485 (2) to 1.501 (2) Å, respectively. The macrocycle is protonated at the N2 atom, which is similar to the situation found for [C22H46N4](ClO4)2 (Aree et al., 2018), but differs from the protonation of the N1 atom in [C22H46N4]Cl2·4H2O (Moon et al., 2013). The protonation on the N atom might depend on the location of the acceptor atoms of the counter-anion involved in hydrogen bonding. The ranges of N—C—C and C—N—C angles are 108.07 (11) to 111.14 (12)° and 115.09 (11) to 115.19 (10)°, respectively. The bond lengths and angles within the [C22H46N4]2+ dication are comparable to those found in [C22H46N4](ClO4)2 (Aree et al., 2018) and [C22H46N4]Cl2·4H2O (Moon et al., 2013). The nitrate counter-anion has a distorted trigonal-planar geometry as a result of the influence of hydrogen bonding on the N—O bond lengths and the O—N—O angles. The N—O bond distances range from 1.204 (3) to 1.214 (2) Å and the O—N—O angles from 117.4 (2) to 123.1 (3)°.
3. Supramolecular features
Extensive N—H⋯O, O—H⋯O and N—H⋯N hydrogen-bonding interactions occur in the ). The crystal packing viewed along the a axis is shown in Fig. 2. The O—H⋯O hydrogen bonds link the water molecules to neighboring nitrate anions, while N—H⋯O hydrogen bonds interconnect the [C22H46N4]2+ cations with both the nitrate anions and the water molecules. The is stabilized by molecular hydrogen bonds involving the macrocycle N—H groups and water O—H groups as donors, and the O atoms of the water molecules and nitrate anions as acceptors, giving rise to a three-dimensional framework (Figs. 1 and 2).
(Table 14. Database survey
A search of the Cambridge Structural (Version 5.40, Feb 2019 with 1 update; Groom et al., 2016) gave just three hits for organic compounds containing the macrocycles [C22H48N4]4+, [C22H46N4]2+ or (C22H44N4). The crystal structures of [C22H46N4](ClO4)2 (Aree et al., 2018), [C22H46N4]Cl2·4H2O (Moon et al., 2013) and (C22H44N4)2·2NaClO4 (Aree et al., 2018) were reported by us previously. Until now, no crystal structures of any [C22H46N4]2+ or [C22H48N4]4+ compounds with a nitrate anion have been deposited.
5. Synthesis and crystallization
Commercially available trans-1,2-cyclohexanediamine, ethyl vinyl ketone and silver nitrate (Sigma–Aldrich) were used as provided. All other chemicals were reagent grade and used without further purification. As a starting material, 3,14-diethyl-2,6,13,17-tetraazatricyclo(16.4.0.07,12)docosane, L was prepared according to a published procedure (Lim et al., 2006). A solution of the macrocyclic ligand, L (0.33 g, 1.0 mmol) in methanol 10 mL was added dropwise to a stirred solution of AgNO3 (0.34 g, 2.0 mmol) in water 10 mL. The solution turned an orange color and the metallic silver that formed was filtered off. The orange filtrate was kept in an open beaker, protected from light, at room temperature. Block-like colorless crystals of suitable for X-ray analysis were obtained unexpectedly from the solution over a period of a few weeks.
6. Refinement
Crystal data, data collection and structure . All C and N-bound H atoms in the complex were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C—H distances of 0.97–0.99 Å, and with an N—H distance of 0.90 Å with Uiso(H) values of 1.2 and 1.5 times the Ueq of the parent atoms, respectively. The N-bound H atoms of the [C22H46N4]2+ cation and the O-bound H atoms of the water molecules were located in a difference-Fourier map and refined isotropically, with the N—H distance restrained using DFIX [0.9 (2) Å] and the O—H distances and H—O—H angles restrained using DFIX and DANG constraints [0.94 (2) and 1.55 (2) Å], respectively.
details are summarized in Table 2
|
Supporting information
CCDC reference: 1918729
https://doi.org/10.1107/S2056989019007655/sj5572sup1.cif
contains datablock I. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989019007655/sj5572Isup2.hkl
Data collection: PAL BL2D-SMDC Program (Shin et al., 2016); cell
HKL3000sm (Otwinowski & Minor, 1997); data reduction: HKL3000sm (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXT2018 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: DIAMOND 4 (Putz & Brandenburg, 2014); software used to prepare material for publication: publCIF (Westrip, 2010).C22H46N42+·2NO3−·2H2O | F(000) = 576 |
Mr = 526.68 | Dx = 1.254 Mg m−3 |
Monoclinic, P21/c | Synchrotron radiation, λ = 0.610 Å |
a = 8.6420 (17) Å | Cell parameters from 46866 reflections |
b = 16.687 (3) Å | θ = 0.4–33.7° |
c = 9.7340 (19) Å | µ = 0.07 mm−1 |
β = 96.46 (3)° | T = 220 K |
V = 1394.8 (5) Å3 | Block, colorless |
Z = 2 | 0.13 × 0.09 × 0.05 mm |
Rayonix MX225HS CCD area detector diffractometer | 2968 reflections with I > 2σ(I) |
Radiation source: PLSII 2D bending magnet | Rint = 0.027 |
ω scan | θmax = 25.0°, θmin = 2.0° |
Absorption correction: empirical (using intensity measurements) (HKL3000sm SCALEPACK; Otwinowski & Minor, 1997) | h = −11→11 |
Tmin = 0.919, Tmax = 1.000 | k = −21→21 |
14284 measured reflections | l = −13→13 |
3736 independent reflections |
Refinement on F2 | 4 restraints |
Least-squares matrix: full | Hydrogen site location: mixed |
R[F2 > 2σ(F2)] = 0.062 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.211 | w = 1/[σ2(Fo2) + (0.1295P)2 + 0.2454P] where P = (Fo2 + 2Fc2)/3 |
S = 1.10 | (Δ/σ)max < 0.001 |
3736 reflections | Δρmax = 0.73 e Å−3 |
170 parameters | Δρmin = −0.56 e Å−3 |
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. |
x | y | z | Uiso*/Ueq | ||
N1 | 0.51219 (14) | 0.59626 (8) | 0.36407 (11) | 0.0308 (3) | |
H1N | 0.473300 | 0.634200 | 0.422800 | 0.037* | |
N2 | 0.28123 (13) | 0.47313 (7) | 0.42499 (11) | 0.0278 (3) | |
H2AN | 0.385711 | 0.476834 | 0.431710 | 0.033* | |
H2BN | 0.246719 | 0.513235 | 0.475356 | 0.033* | |
C1 | 0.76020 (15) | 0.60393 (9) | 0.51374 (13) | 0.0286 (3) | |
H1 | 0.716885 | 0.647557 | 0.566781 | 0.034* | |
C2 | 0.93544 (17) | 0.61843 (10) | 0.51640 (16) | 0.0365 (3) | |
H2A | 0.983763 | 0.617356 | 0.612413 | 0.044* | |
H2B | 0.981148 | 0.575002 | 0.466503 | 0.044* | |
C3 | 0.97155 (18) | 0.69835 (10) | 0.45121 (17) | 0.0386 (4) | |
H3A | 0.936038 | 0.742285 | 0.506664 | 0.046* | |
H3B | 1.084368 | 0.703693 | 0.450342 | 0.046* | |
C4 | 0.89135 (18) | 0.70410 (10) | 0.30389 (15) | 0.0373 (3) | |
H4A | 0.935358 | 0.663904 | 0.245983 | 0.045* | |
H4B | 0.910323 | 0.757161 | 0.266013 | 0.045* | |
C5 | 0.71640 (17) | 0.69051 (9) | 0.30041 (15) | 0.0342 (3) | |
H5A | 0.667680 | 0.692382 | 0.204553 | 0.041* | |
H5B | 0.670830 | 0.733224 | 0.352056 | 0.041* | |
C6 | 0.68403 (16) | 0.60924 (9) | 0.36398 (13) | 0.0296 (3) | |
H6 | 0.726833 | 0.566354 | 0.308993 | 0.035* | |
C7 | 0.42136 (18) | 0.59147 (10) | 0.22379 (14) | 0.0353 (3) | |
H7A | 0.467359 | 0.550519 | 0.168718 | 0.042* | |
H7B | 0.427202 | 0.643030 | 0.176434 | 0.042* | |
C8 | 0.25211 (17) | 0.57083 (9) | 0.23440 (14) | 0.0337 (3) | |
H8A | 0.192762 | 0.580870 | 0.144215 | 0.040* | |
H8B | 0.212715 | 0.607652 | 0.300777 | 0.040* | |
C9 | 0.21823 (17) | 0.48504 (9) | 0.27811 (13) | 0.0314 (3) | |
H9 | 0.103745 | 0.478672 | 0.271539 | 0.038* | |
C10 | 0.2802 (3) | 0.42191 (11) | 0.18588 (16) | 0.0471 (4) | |
H10A | 0.393272 | 0.428242 | 0.189202 | 0.056* | |
H10B | 0.259978 | 0.368749 | 0.222744 | 0.056* | |
C11 | 0.2090 (4) | 0.42595 (16) | 0.0363 (2) | 0.0792 (8) | |
H11A | 0.096417 | 0.427628 | 0.032597 | 0.119* | |
H11B | 0.239639 | 0.378996 | −0.012820 | 0.119* | |
H11C | 0.245453 | 0.473817 | −0.006600 | 0.119* | |
N3 | 0.2912 (2) | 0.63006 (12) | 0.76865 (18) | 0.0581 (5) | |
O1 | 0.3086 (2) | 0.58294 (10) | 0.6746 (2) | 0.0812 (6) | |
O2 | 0.2872 (2) | 0.70307 (10) | 0.74420 (17) | 0.0696 (5) | |
O3 | 0.2875 (5) | 0.6094 (2) | 0.8868 (3) | 0.1636 (16) | |
O4 | 0.42576 (16) | 0.72874 (9) | 0.50503 (14) | 0.0501 (3) | |
H1O | 0.373 (3) | 0.7221 (16) | 0.5829 (17) | 0.075* | |
H2O | 0.366 (3) | 0.7605 (14) | 0.439 (2) | 0.075* |
U11 | U22 | U33 | U12 | U13 | U23 | |
N1 | 0.0327 (6) | 0.0418 (7) | 0.0171 (5) | −0.0017 (5) | −0.0003 (4) | 0.0035 (4) |
N2 | 0.0303 (6) | 0.0363 (6) | 0.0163 (5) | −0.0001 (4) | −0.0003 (4) | 0.0006 (4) |
C1 | 0.0300 (6) | 0.0370 (7) | 0.0185 (6) | −0.0006 (5) | 0.0012 (5) | 0.0007 (5) |
C2 | 0.0303 (7) | 0.0486 (9) | 0.0302 (7) | −0.0013 (6) | 0.0012 (5) | 0.0050 (6) |
C3 | 0.0361 (7) | 0.0496 (9) | 0.0305 (7) | −0.0078 (6) | 0.0055 (6) | 0.0016 (6) |
C4 | 0.0380 (7) | 0.0482 (9) | 0.0269 (7) | −0.0029 (6) | 0.0090 (6) | 0.0038 (6) |
C5 | 0.0368 (7) | 0.0423 (8) | 0.0240 (6) | −0.0006 (5) | 0.0048 (5) | 0.0065 (5) |
C6 | 0.0317 (6) | 0.0389 (7) | 0.0180 (6) | 0.0007 (5) | 0.0024 (5) | 0.0015 (5) |
C7 | 0.0393 (8) | 0.0476 (8) | 0.0176 (6) | −0.0062 (6) | −0.0027 (5) | 0.0057 (5) |
C8 | 0.0344 (7) | 0.0437 (8) | 0.0213 (6) | 0.0024 (5) | −0.0040 (5) | 0.0046 (5) |
C9 | 0.0329 (7) | 0.0430 (8) | 0.0172 (6) | −0.0033 (5) | −0.0013 (5) | 0.0010 (5) |
C10 | 0.0721 (12) | 0.0461 (10) | 0.0240 (7) | −0.0041 (8) | 0.0097 (7) | −0.0044 (6) |
C11 | 0.141 (3) | 0.0743 (15) | 0.0214 (8) | −0.0135 (15) | 0.0046 (11) | −0.0093 (9) |
N3 | 0.0674 (11) | 0.0633 (11) | 0.0418 (9) | −0.0045 (8) | −0.0025 (8) | 0.0016 (7) |
O1 | 0.1056 (14) | 0.0553 (10) | 0.0739 (12) | 0.0074 (8) | −0.0284 (10) | −0.0228 (8) |
O2 | 0.1060 (14) | 0.0545 (9) | 0.0509 (9) | 0.0048 (8) | 0.0204 (9) | −0.0098 (6) |
O3 | 0.285 (5) | 0.140 (3) | 0.0760 (18) | 0.010 (3) | 0.063 (2) | 0.0501 (17) |
O4 | 0.0555 (8) | 0.0574 (8) | 0.0384 (7) | 0.0082 (6) | 0.0088 (6) | −0.0009 (5) |
N1—C7 | 1.4987 (17) | C5—H5B | 0.9800 |
N1—C6 | 1.5009 (18) | C6—H6 | 0.9900 |
N1—H1N | 0.94 | C7—C8 | 1.517 (2) |
N2—C1i | 1.4784 (18) | C7—H7A | 0.9800 |
N2—C9 | 1.4850 (16) | C7—H7B | 0.9800 |
N2—H2AN | 0.9000 | C8—C9 | 1.531 (2) |
N2—H2BN | 0.9000 | C8—H8A | 0.9800 |
C1—C2 | 1.5309 (19) | C8—H8B | 0.9800 |
C1—C6 | 1.5330 (18) | C9—C10 | 1.520 (2) |
C1—H1 | 0.9900 | C9—H9 | 0.9900 |
C2—C3 | 1.524 (2) | C10—C11 | 1.517 (3) |
C2—H2A | 0.9800 | C10—H10A | 0.9800 |
C2—H2B | 0.9800 | C10—H10B | 0.9800 |
C3—C4 | 1.524 (2) | C11—H11A | 0.9700 |
C3—H3A | 0.9800 | C11—H11B | 0.9700 |
C3—H3B | 0.9800 | C11—H11C | 0.9700 |
C4—C5 | 1.525 (2) | N3—O3 | 1.204 (3) |
C4—H4A | 0.9800 | N3—O1 | 1.229 (3) |
C4—H4B | 0.9800 | N3—O2 | 1.241 (2) |
C5—C6 | 1.529 (2) | O4—H1O | 0.937 (10) |
C5—H5A | 0.9800 | O4—H2O | 0.941 (9) |
C7—N1—C6 | 115.09 (11) | N1—C6—C1 | 108.07 (11) |
C7—N1—H1N | 114 | C5—C6—C1 | 110.95 (12) |
C6—N1—H1N | 109 | N1—C6—H6 | 109.0 |
C1i—N2—C9 | 115.19 (10) | C5—C6—H6 | 109.0 |
C1i—N2—H2AN | 108.5 | C1—C6—H6 | 109.0 |
C9—N2—H2AN | 108.5 | N1—C7—C8 | 111.11 (11) |
C1i—N2—H2BN | 108.5 | N1—C7—H7A | 109.4 |
C9—N2—H2BN | 108.5 | C8—C7—H7A | 109.4 |
H2AN—N2—H2BN | 107.5 | N1—C7—H7B | 109.4 |
N2i—C1—C2 | 114.50 (12) | C8—C7—H7B | 109.4 |
N2i—C1—C6 | 109.65 (11) | H7A—C7—H7B | 108.0 |
C2—C1—C6 | 108.97 (11) | C7—C8—C9 | 116.55 (12) |
N2i—C1—H1 | 107.8 | C7—C8—H8A | 108.2 |
C2—C1—H1 | 107.8 | C9—C8—H8A | 108.2 |
C6—C1—H1 | 107.8 | C7—C8—H8B | 108.2 |
C3—C2—C1 | 112.32 (12) | C9—C8—H8B | 108.2 |
C3—C2—H2A | 109.1 | H8A—C8—H8B | 107.3 |
C1—C2—H2A | 109.1 | N2—C9—C10 | 111.14 (12) |
C3—C2—H2B | 109.1 | N2—C9—C8 | 109.38 (11) |
C1—C2—H2B | 109.1 | C10—C9—C8 | 113.13 (13) |
H2A—C2—H2B | 107.9 | N2—C9—H9 | 107.7 |
C4—C3—C2 | 110.74 (13) | C10—C9—H9 | 107.7 |
C4—C3—H3A | 109.5 | C8—C9—H9 | 107.7 |
C2—C3—H3A | 109.5 | C11—C10—C9 | 113.86 (17) |
C4—C3—H3B | 109.5 | C11—C10—H10A | 108.8 |
C2—C3—H3B | 109.5 | C9—C10—H10A | 108.8 |
H3A—C3—H3B | 108.1 | C11—C10—H10B | 108.8 |
C3—C4—C5 | 110.87 (12) | C9—C10—H10B | 108.8 |
C3—C4—H4A | 109.5 | H10A—C10—H10B | 107.7 |
C5—C4—H4A | 109.5 | C10—C11—H11A | 109.5 |
C3—C4—H4B | 109.5 | C10—C11—H11B | 109.5 |
C5—C4—H4B | 109.5 | H11A—C11—H11B | 109.5 |
H4A—C4—H4B | 108.1 | C10—C11—H11C | 109.5 |
C4—C5—C6 | 110.41 (12) | H11A—C11—H11C | 109.5 |
C4—C5—H5A | 109.6 | H11B—C11—H11C | 109.5 |
C6—C5—H5A | 109.6 | O3—N3—O1 | 123.1 (3) |
C4—C5—H5B | 109.6 | O3—N3—O2 | 117.4 (2) |
C6—C5—H5B | 109.6 | O1—N3—O2 | 119.23 (19) |
H5A—C5—H5B | 108.1 | H1O—O4—H2O | 109.8 (18) |
N1—C6—C5 | 110.75 (11) | ||
N2i—C1—C2—C3 | 179.62 (12) | N2i—C1—C6—C5 | 176.50 (11) |
C6—C1—C2—C3 | 56.43 (16) | C2—C1—C6—C5 | −57.46 (15) |
C1—C2—C3—C4 | −55.92 (17) | C6—N1—C7—C8 | 174.85 (12) |
C2—C3—C4—C5 | 55.43 (18) | N1—C7—C8—C9 | −71.31 (16) |
C3—C4—C5—C6 | −57.05 (17) | C1i—N2—C9—C10 | −61.66 (16) |
C7—N1—C6—C5 | 64.03 (15) | C1i—N2—C9—C8 | 172.72 (11) |
C7—N1—C6—C1 | −174.25 (12) | C7—C8—C9—N2 | 68.87 (16) |
C4—C5—C6—N1 | 178.64 (11) | C7—C8—C9—C10 | −55.60 (17) |
C4—C5—C6—C1 | 58.63 (16) | N2—C9—C10—C11 | 174.71 (16) |
N2i—C1—C6—N1 | 54.91 (14) | C8—C9—C10—C11 | −61.8 (2) |
C2—C1—C6—N1 | −179.05 (12) |
Symmetry code: (i) −x+1, −y+1, −z+1. |
D—H···A | D—H | H···A | D···A | D—H···A |
N2—H2AN···N1 | 0.90 | 2.40 | 2.9703 (18) | 121 |
N2—H2AN···N1i | 0.90 | 2.41 | 2.8141 (17) | 107 |
N1—H1N···O4 | 0.94 | 1.84 | 2.7493 (19) | 163 |
N2—H2AN···N1 | 0.90 | 2.40 | 2.9703 (18) | 121 |
N2—H2BN···O1 | 0.90 | 2.27 | 3.031 (2) | 142 |
O4—H1O···O1 | 0.94 (1) | 2.57 (2) | 3.169 (3) | 122 (2) |
O4—H1O···O2 | 0.94 (1) | 1.84 (1) | 2.768 (2) | 174 (2) |
O4—H2O···O2ii | 0.94 (1) | 2.04 (1) | 2.914 (2) | 155 (2) |
O4—H2O···O3ii | 0.94 (1) | 2.31 (2) | 3.120 (4) | 144 (2) |
Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x, −y+3/2, z−1/2. |
Funding information
This work was supported by a Research Grant of Andong National University. The X-ray crystallography experiment at the PLS-II BL2D-SMC beamline was supported in part by MSICT and POSTECH.
References
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