Synthesis, structure, and characterisation of a ferromagnetically coupled dinuclear complex containing Co(II) ions in a high spin configuration and thiodiacetate and phenanthroline as ligands and of a series of isomorphous heterodinuclear complexes containing different Co : Zn ratios.

We report the synthesis, crystal structure, and characterisation of a dinuclear Co(II) compound with thiodiacetate (tda) and phenanthroline (phen) as ligands (1), and of a series of metal complexes isomorphous to 1 with different Co : Zn ratios (2, 4 : 1; 3, 1 : 1; 4, 1 : 4; 5, 1 : 10). General characterisation methodologies and X-ray data showed that all the synthesised complexes are isomorphous to Zn(II) and Cu(II) analogues (CSD codes: DUHXEL and BEBQII). 1 consists of centrosymmetric Co(II) ion dimers in which the ions are 3.214 Å apart, linked by two µ-O bridges. Each cobalt atom is in a distorted octahedral environment of the N2O3S type. UV-vis spectra of 1 and 5 are in line with high spin (S = 3/2) Co(II) ions in octahedral coordination and indicate that the electronic structure of both Co(II) ions in the dinuclear unit does not significantly change relative to that of the magnetically isolated Co(II) ion. EPR spectra of powder samples of 5 (Co : Zn ratio of 1 : 10) together with spectral simulation indicated high spin Co(II) ions with high rhombic distortion of the zfs [E/D = 0.31(1), D > 0]. DC magnetic susceptibility experiments on 1 and analysis of the data constraining the E/D value obtained by EPR yielded g = 2.595(7), |D| = 61(1) cm-1, and an intradimer ferromagnetic exchange coupling of J = 1.39(4) cm-1. EPR spectra as a function of Co : Zn ratio for both powder and single crystal samples confirmed that they result from two effective S' = 1/2 spins that interact through dipolar and isotropic exchange interactions to yield magnetically isolated S' = 1 centres and that interdimeric exchange interactions, putatively mediated by hydrophobic interactions between phen moieties, are negligible. The latter observation contrasts with that observed in the Cu(II) analogue, where a transition from S = 1 to S' = 1/2 was observed. Computational calculations indicated that the absence of the interdimeric exchange interaction in 1 is due to a lower Co(II) ion spin density delocalisation towards the metal ligands.

Correction: Effect of non-covalent self-dimerization on the spectroscopic and electrochemical properties of mixed Cu(i) complexes.

[This corrects the article DOI: 10.1039/D2RA05341A.].

Effect of non-covalent self-dimerization on the spectroscopic and electrochemical properties of mixed Cu(i) complexes.

A series of six new Cu(i) complexes with ([Cu(N-{4-R}pyridine-2-yl-methanimine)(PPh3)Br]) formulation, where R corresponds to a donor or acceptor p-substituent, have been synthesized and were used to study self-association effects on their structural and electrochemical properties. X-ray diffraction results showed that in all complexes the packing is organized from a dimer generated by supramolecular π stacking and hydrogen bonding. 1H-NMR experiments at several concentrations showed that all complexes undergo a fast-self-association monomer-dimer equilibrium in solution, while changes in resonance frequency towards the high or low field in specific protons of the imine ligand allow establishing that dimers have similar structures to those found in the crystal. The thermodynamic parameters for this self-association process were calculated from dimerization constants determined by VT-1H-NMR experiments for several concentrations at different temperatures. The values for K D (4.0 to 70.0 M-1 range), ΔH (-1.4 to -2.6 kcal mol-1 range), ΔS (-0.2 to 2.1 cal mol-1 K-1 range), and ΔG 298 (-0.8 to -2.0 kcal mol-1 range) are of the same order and indicate that the self-dimerization process is enthalpically driven for all complexes. The electrochemical profile of the complexes shows two redox Cu(ii)/Cu(i) processes whose relative intensities are sensitive to concentration changes, indicating that both species are in chemical equilibrium, with the monomer and the dimer having different electrochemical characteristics. We associate this behaviour with the structural lability of the Cu(i) centre that allows the monomeric molecules to reorder conformationally to achieve a more adequate assembly in the non-covalent dimer. As expected, structural properties in the solid and in solution, as well as their electrochemical properties, are not correlated with the electronic parameters usually used to evaluate R substituent effects. This confirms that the properties of the Cu(i) complexes are usually more influenced by steric effects than by the inductive effects of substituents of the ligands. In fact, the results obtained showed the importance of non-covalent intermolecular interactions in the structuring of the coordination geometry around the Cu centre and in the coordinative stability to avoid dissociative equilibria.

Synthesis and crystallographic, spectroscopic and computational characterization of 3,3',4,4'-substituted biphenyls: effects of OR substituents on the intra-ring torsion angle.

Presented here are the synthesis, characterization and study (using single crystal X-ray diffraction, Raman scattering, quantum mechanics calculations) of the structures of a series of biphenyls substituted in positions 3, 3', 4 and 4' with a variety of R (R = methyl, acetyl, hexyl) groups connected to the biphenyl core through oxygen atoms. The molecular conformation, particularly the torsion angle between aromatic rings has been extensively studied both in the solid as well as in the liquid state. The results show that the compounds appearing as rigorously planar in the solid present instead a twisted conformation in the melt. The solid versus melt issue strongly suggests that the reasons for planarity are to be found in the packing restraints. A `rule of thumb' is suggested for the design of biphenyls with different molecular conformations, based on the selection of the OR substituent.

Exchange couplings and quantum phases in two dissimilar arrays of similar copper dinuclear units.

To investigate the magnetic properties and the spin entanglement of dinuclear arrays, we prepared compounds [{Cu(pAB)(phen)H2O}2·NO3·pABH·2H2O], 1, and [Cu2(pAB)2(phen)2pz]n, 2 (pABH = p-aminobenzoic acid, phen = 1,10-phenanthroline and pz = pyrazine). The structure of 1 is known and we report here that of 2. They contain similar dinuclear units of CuII ions with 1/2-spins S1 and S2 bridged by pairs of pAB molecules, with similar intradinuclear exchange and fine interactions , but different 3D crystal arrays with weak interdinuclear exchange J', stronger in 2 than in 1. To investigate the magnetic properties and the spin entanglement produced by J', we collected the powder spectra of 1 and 2 at 9.4 GHz and T between 5 and 298 K, and at 34.4 GHz and T = 298 K and single-crystal spectra at room T and 34.4 GHz as a function of magnetic field (B0) orientation in three crystal planes, calculating intradinuclear magnetic parameters J(1)0 = (-75 ± 1) cm-1, J(2)0 = (-78 ± 2) cm-1, |D(1)| = (0.142 ± 0.006) cm-1, |D(2)| = (0.141 ± 0.006) cm-1 and E(1) ∼ E(2) ∼ 0. Single crystal data indicate a quantum entangled phase in 2 around the crossing between two fine structure EPR absorption peaks within the spin triplet. This phase also shows up in powder samples of 2 as a U-peak collecting the signals of the entangled microcrystals, a feature that allows estimating |J'|. Transitions between the two quantum phases are observed in single crystals of 2 changing the orientation of B0. We estimate interdinuclear exchange couplings |J'(1)| < 0.003 cm-1 and |J'(2)| = (0.013 ± 0.005) cm-1, in 1 and 2, respectively. Our analysis indicates that the standard approximation of a spin Hamiltonian with S = 1 for the dinuclear spectra is valid only when the interdinuclear coupling is large enough, as for compound 2 (|J'(2)/J(2)0| ∼ 1.7 × 10-4). When J' is negligible as in 1, the real spin Hamiltonian with two spins 1/2 has to be used. Broken-symmetry DFT predicts correctly the nature and magnitude of the antiferromagnetic exchange coupling in 1 and 2 and ferromagnetic interdinuclear coupling for compound 2.

Playing around with MP, a tool for the analysis of pseudosymmetry: recurrent appearance of local pseudo-space groups in the asymmetric unit of Z' = 4 structures.

A systematic application of the MP procedure [Baggio (2019). Acta Cryst. C75, 837-850] to a subset of Z' = 4 structures in the literature revealed a number of suggestive properties in many of the cases analysed, which allowed us to conclude that: (i) very few of those structures showed no pseudosymmetry (PS) whatsoever linking any of the possible moiety pairs in the asymmetric unit (AU), the vast majority disclosing, instead, at least two (and frequently more) molecules linked by these PS operations; (ii) on occasion, two pairs of molecules were related, each by the same PS element, enabling the resulting pair of dimers to be viewed locally as a Z' = 2 rather than a Z' = 4 case; (iii) sometimes all four moieties in the AU appeared interrelated by PS relationships, closely suggesting a distribution of symmetry operations as found in real space groups; (iv) in many cases, this resemblance was of a superior quality, leading to surprising local replicas of real space groups in the reduced realm of the AU, thus locally reducing the description to a Z' = 1 one. Through a systematic application of the MP procedure, we analyze in this article a number of these latter cases, which proved to be surprisingly common, far beyond any original expectation, and leaving open the question about the eventual reasons for the phenomenon, which would certainly merit appropriate theoretical attention.

Structural characterization of a hybrid terpyridine-pyrazine ligand and its one-dimensional ZnII coordination polymer: a computational approach to conventional and nonconventional intermolecular interactions.

The structures of a new hybrid terpyridine-pyrazine ligand, namely 4'-[4-(pyrazin-2-yl)phenyl]-4,2':6',4''-terpyridine (L2), C25H17N5, and its one-dimensional coordination polymer catena-poly[[bis(acetylacetonato-κ2O,O')zinc]-µ-4'-[4-(pyrazin-2-yl-κN4)phenyl]-4,2':6',4''-terpyridine-κN1], [Zn(C5H7O2)2(C25H17N5)]n or [Zn(acac)2(L2)]n (Hacac is acetylacetone), are reported. Packing interactions in both crystal structures are analyzed using Hirshfeld surface and enrichment ratio techniques. For the simpler structure of the monomeric ligand, further studies on the interaction hierarchy using the energy framework approach were made. The result was a complete picture of the intermolecular interaction landscape, which revealed some subtle details, for example, that some weak (at first sight negligible) C-H...N interactions in the structure of free L2 play a relevant role in the crystal stabilization.

A simple graphical method to pinpoint local pseudosymmetries in Z' > 1 cases.

An intuitive method is presented for detecting pseudosymmetries in Z' > 1 cases as a complement to well-proven strategies already available in the literature. It is based in the simple idea that the mid-points between equivalent atoms in symmetrically related molecules are disposed according to simple well-known patterns, which are easily recognizable by optical inspection. A number of Z' = 4 cases in the literature are analyzed, which allows some of the potentialities of the method to be revealed.

Crystal structure and pseudosymmetry analysis of the triclinic prodrug cloxazolam (Z' = 4).

The prodrug cloxazolam [systematic name: 13-chloro-2-(2-chlorophenyl)-3-oxa-6,9-diazatricyclo[8.4.0.02,6]tetradeca-1(10),11,13-trien-8-one], C17H14Cl2N2O2, crystallizes in the triclinic space group P-1, with four chemically identical independent molecules in the asymmetric unit. However, in order to facilitate the analysis of the striking pseudosymmetry relating the four independent molecules, the structure has been analysed and reported in the nonconventional centred B-1 space-group setting. Pseudosymmetry is an eminently local property, valid only in the realm of the unit-cell boundary and not propagating to the whole crystal structure. It has been analyzed using the MP procedure described in the preceding article [Baggio (2019). Acta Cryst. C75, 837-850]. The molecules consist of a rigid core made up of three rings (five-, six- and seven-membered) and an extra six-membered ring joined to the latter group by a single C-C bond, together with a clamping intramolecular C-H...O interaction preventing free rotation and providing additional rigidity. The four molecules in the asymmetric unit pair into dimers with almost exact twofold pseudosymmetry, further linked into (001) slabs as the building bricks of the structure. Interpenetration of slabs finally leads to a three-dimensional structure of unusual compactness for an organic structure, with a Kitaigorodskii packing index of ca 0.71.

Polymorphic forms of bendamustine hydrochloride: crystal structure, thermal properties and stability at ambient conditions.

Crystallographic, thermal and stability analyses are presented of three different anhydrated forms of bendamustine hydrochloride [(I), (III) and (IV)] and a fourth, monohydrated one (II). Since form (I) presents the higher melting point and the higher heat of fusion, according to the `heat of fusion' rule it should be the most stable in thermodynamic terms [Burger & Ramberger (1979). Mikrochim. Acta, 72, 259-271], though it is unstable in high-humidity conditions. The monohydrate structure (II), in turn, dehydrates by heating and topotactically transform into anhydrate (III). This latter form appears as less stable than anhydrate (I), to which it is linked via a monotropic relationship. For these three different forms, the crystal structure has been determined by single crystal X-ray diffraction. The crystal structures and molecular conformations of forms (II) and (III) are quite similar, as expected from the topotactic transformation linking them; furthermore, under high-humidity conditions, form (III) shows changes compatible with a transformation into form (II) within 24 h. The crystal structure of form (I) is different from the other two. The remaining polymorphic form (IV) could only be obtained as a powder, from which its crystalline structure could not be determined. The relative thermodynamic stability of the different crystalline forms was determined by differential scanning calorimetry and thermogravimetrical studies, and their stability under different humidity conditions analysed.

Thermal, spectroscopic and structural analysis of a thermosalient phase transformation in tapentadol hydrochloride.

Presented herein are detailed optical, thermal, spectroscopic and structural analyses of the phase transformation occurring in tapentadol hydrochloride (C14H24NO+·Cl-), a phenomenon already reported [Fischer et al. (2006); Patent: WO 2006000441 A2]. The thermal behaviour of the compound was studied using single-crystal X-ray diffraction, differential scanning calorimetry and Raman scattering measurements. The compound undergoes a first-order reversible phase transition at Theat = 318.0 (1) K, Tcool = 300.0 (1) K, as assessed by the coexistence of both phases in the vicinity of the transition and the abrupt changes observed in the unit-cell parameters with temperature. The process is accompanied by clear thermosalient behaviour, with a conspicuous movement of the samples. On cooling, the transformation leads from a P212121 symmetry (Z' = 1) to P21, with an abrupt change in ß [90 ↔ 94.78 (1)°] and duplication of the asymmetric unit contents (Z' = 2). The main structural differences observed across the transition are extremely small, with almost no changes in the stronger, non-covalent interaction scheme involving the `conventional' (N-H...Cl, O-H...Cl) hydrogen bonds.

The enrichment ratio of atomic contacts in the crystal structure of isomeric, triply protonated, 4'-functionalized terpyridine cations with [ZnCl4]2- as counter-ion.

We report herein the synthesis, crystallographic analysis and a study of the non-covalent inter-actions observed in the new 4'-substituted terpyridine-based derivative bis-[4'-(isoquinolin-2-ium-4-yl)-4,2':6',4''-terpyridine-1,1''-diium] tris-[tetra-chlorido-zincate(II)], (C24H19N4)2[ZnCl4]3 or (44TPH3)2[ZnCl4]3, where (44TPH3)3+ is the triply protonated cation 4'-(isoquinolinium-4-yl)-4,2':6',4'' terpyridinium. The compound is similar in its formulation to the recently reported 2,2':6',2'' terpyridinium analogue {bis-[4'-(isoquinolin-2-ium-4-yl)-2,2':6',2''-terpyridine-1,1''-diium] tris-[tetra-chlorido-zincate(II)] monohydrate; Granifo et al. (2017 ▸). Acta Cryst. C73, 1121-1130}, although rather different and much simpler in its structural features, mainly in the number and type of non-covalent inter-actions present, as well as in the supra-molecular structure they define.

A crystallographic and thermal study of pridinol mesylate and its monohydrated solvate.

Herein are reported the crystal and molecular structures of the pridinol mesylate salt (C20H25NO+·CH3O3S-) (I) and its monohydrated solvate form (C20H25NO+·CH3O3S-·H2O) (II). A comparison of both with the already reported structure of pure pridinol [1,1-diphenyl-3-piperidino-1-propanol, C20H25NO; Tacke et al. (1980). Chem. Ber. 113, 1962-1980] is made. Molecular structures (I) and (II) are alike in bond distances and bond angles, but differ in their spatial conformation, and, more relevant still, in their hydrogen-bonding motifs. This gives rise to quite different packing schemes, in the form of simple dimers in (I) but water-mediated hydrogen-bonded chains in (II). The dehydration behaviour of form (II) is highly dependent on the heating rate, with slow rates leading to a clear endothermic dehydration step, towards anhydrous (I), with subsequent melting of this latter phase. Increased heating rates result in a more unclear behaviour ending in a structural collapse (melting of the hydrated phase), at temperatures significantly lower than the melting point of the anhydrous phase. The eventual relevance of the water link in the structure of (II) is discussed in regard to this behaviour.

Three new dihydro-ß-agarofuran sesquiterpenes from the seeds of Maytenus boaria.

As part of a project studying the secondary metabolites extracted from the Chilean flora, we report herein three new ß-agarofuran sesquiterpenes, namely (1S,4S,5S,6R,7R,8R,9R,10S)-6-acetoxy-4,9-dihydroxy-2,2,5a,9-tetramethyloctahydro-2H-3,9a-methanobenzo[b]oxepine-5,10-diyl bis(furan-3-carboxylate), C27H32O11, (II), (1S,4S,5S,6R,7R,9S,10S)-6-acetoxy-9-hydroxy-2,2,5a,9-tetramethyloctahydro-2H-3,9a-methanobenzo[b]oxepine-5,10-diyl bis(furan-3-carboxylate), C27H32O10, (III), and (1S,4S,5S,6R,7R,9S,10S)-6-acetoxy-10-(benzoyloxy)-9-hydroxy-2,2,5a,9-tetramethyloctahydro-2H-3,9a-methanobenzo[b]oxepin-5-yl furan-3-carboxylate, C29H34O9, (IV), obtained from the seeds of Maytenus boaria and closely associated with a recently published relative [Paz et al. (2017). Acta Cryst. C73, 451-457]. In the (isomorphic) structures of (II) and (III), the central decalin system is esterified with an acetate group at site 1 and furoate groups at sites 6 and 9, and differ at site 8, with an OH group in (II) and no substituent in (III). This position is also unsubstituted in (IV), with site 6 being occupied by a benzoate group. The chirality of the skeletons is described as 1S,4S,5S,6R,7R,8R,9R,10S in (II) and 1S,4S,5S,6R,7R,9S,10S in (III) and (IV), matching the chirality suggested by NMR studies. This difference in the chirality sequence among the title structures (in spite of the fact that the three skeletons are absolutely isostructural) is due to the differences in the environment of site 8, i.e. OH in (II) and H in (III) and (IV). This diversity in substitution, in turn, is responsible for the differences in the hydrogen-bonding schemes, which is discussed.

Topological study of diverse hydrogen-bonded patterns found in a system of a nickel(II) complex and the sulfate anion.

A nickel(II) coordination complex, bis[2,6-bis(1H-benzimidazol-2-yl-κN3)pyridine-κN]nickel(II) sulfate, [Ni(C19H13N5)2]SO4 or [Ni(H2L)2]SO4, having four peripheral tetrahedrally oriented N-H donor units, combines with sulfate bridges to create hydrogen-bonded structures of varied dimensionality. The three crystal structures reported herein in the space groups P212121, I-4 and Pccn are defined solely by strong charge-assisted N-H...O hydrogen bonds and contain disordered guests (water and dimethylformamide) that vary in size, shape and degree of hydrophilicity. Two of the compounds are channelled solids with three-dimensional structures, while the third is one-dimensional in nature. In spite of their differences, all three present a striking resemblance to the previously reported anhydrous relative [Guo et al. (2011). Chin. J. Inorg. Chem. 27, 1517-1520], which is considered as the reference framework from which all three title compounds are derived. The hydrogen-bonded frameworks are described and compared using crystallographic and topological approaches.

Structure and electrical properties of a one-dimensional polymeric silver thiosaccharinate complex with argentophilic interactions.

Among the potential applications of coordination polymers, electrical conductivity ranks high in technological interest. We report the synthesis, crystal structure and spectroscopic analysis of an AgI-thiosaccharinate one-dimensional coordination polymer {systematic name: catena-poly[[[aquatetrakis(µ3-1,1-dioxo-1,2-benzisothiazole-3-thiolato-κ3N:S3:S3)tetrasilver(I)]-µ2-4,4'-(propane-1,3-diyl)dipyridine-κ2N:N'] dimethyl sulfoxide hemisolvate]}, {[Ag4(C7H4NO2S2)4(C13H14N2)(H2O)]·0.5C2H6OS}n, with the 4,4'-(propane-1,3-diyl)dipyridine ligand acting as a spacer. A relevant feature of the structure is the presence of an unusually short Ag...Ag distance of 2.8306 (9) Å, well within the range of argentophilic interactions, confirmed experimentally as such by a Raman study on the low-frequency spectrum, and corroborated theoretically by an Atoms in Molecules (AIM) analysis of the calculated electron density. Electrical conductivity measurements show that this complex can act as a semiconductor with moderate conductivity.

Synthesis, crystal structure and magnetic properties of diaquabis(2,6-diamino-7H-purin-1-ium-κN9)bis(4,4'-oxydibenzoato-κO)cobalt(II) dihydrate.

The title mononuclear CoII complex, [Co(C5H7N6)2(C14H8O5)2(H2O)2]·2H2O, has been synthesized and its crystal structure determined by X-ray diffraction. The complex crystallizes in the triclinic space group P-1, with one formula unit per cell (Z = 1 and Z' = 1/2). It consists of a mononuclear unit with the CoII ion on an inversion centre coordinated by two 2,6-diamino-7H-purin-1-ium cations, two 4,4'-oxydibenzoate anions (in a nonbridging κO-monodentate coordination mode, which is less common for the anion in its CoII complexes) and two water molecules, defining an octahedral environment around the metal atom. There is a rich assortment of nonbonding interactions, among which a strong N+-H...O- bridge, with a short N...O distance of 2.5272 (18) Å, stands out, with the H atom ostensibly displaced away from its expected position at the donor side, towards the acceptor. The complex molecules assemble into a three-dimensional hydrogen-bonded network. A variable-temperature magnetic study between 2 and 300 K reveals an orbital contribution to the magnetic moment and a weak antiferromagnetic interaction between CoII centres as the temperature decreases. The model leads to the following values: A (crystal field strength) = 1.81, λ (spin-orbit coupling) = -59.9 cm-1, g (Landé factor) = 2.58 and zJ (exchange coupling) = -0.5 cm-1.

Crystallographic and computational study of a network composed of [ZnCl4]2- anions and triply protonated 4'-functionalized terpyridine cations.

We report herein the synthesis, crystallographic analysis and a study of the noncovalent interactions observed in the new 4'-substituted terpyridine-based derivative bis[4'-(isoquinolin-2-ium-4-yl)-2,2':6',2''-terpyridine-1,1''-diium] tris[tetrachloridozincate(II)] monohydrate, (C24H19N4)2[ZnCl4]3·H2O or (ITPH3)2[ZnCl4]3·H2O, where (ITPH3)3+ is the triply protonated cation derived from 4'-(isoquinolin-4-yl)-2,2':6',2''-terpyridine (ITP) [Granifo et al. (2016). Acta Cryst. C72, 932-938]. The (ITPH3)3+ cation presents a number of interesting similarities and differences compared with its neutral ITP relative, mainly in the role fulfilled in the packing arrangement by the profuse set of D-H...A [D (donor) = C, N or O; A (acceptor) = O or Cl], π-π and anion...π noncovalent interactions present. We discuss these interactions in two different complementary ways, viz. using a point-to-point approach in the light of Bader's theory of Atoms In Molecules (AIM), analyzing the individual significance of each interaction, and in a more `global' analysis, making use of the Hirshfeld surfaces and the associated enrichment ratio (ER) approach, evaluating the surprisingly large co-operative effect of the superabundant weaker contacts.

A combined crystallographic, thermal, Raman and computational study on polymorphism and phase transition in 1-(4-hexyloxy-3-hydroxyphenyl)ethanone.

The crystal structure of the triclinic polymorph of 1-(4-hexyloxy-3-hydroxyphenyl)ethanone, C14H20O3, differs markedly from that of the orthorhombic polymorph [Manzano et al. (2015). Acta Cryst. C71, 1022-1027]. The two molecular structures are alike with respect to their bond lengths and angles, but differ in their spatial arrangement. This gives rise to quite different packing schemes, even if built up by similar chains having the hydroxy-ethanone O-H...O hydrogen-bond synthon in common. Both phases were found to be related by a first-order thermally driven phase transformation at 338-340 K, which is discussed in detail. The relative stabilities of both polymorphs are explained on the basis of both the noncovalent interactions operating in each structure and quantum chemical calculations. The polymorphic phase transition has also been studied experimentally by means of differential scanning calorimetry (DSC) experiments, conducted on individual single crystals, Raman spectroscopy and controlled heating under a microscope of individual single crystals, which were further characterized by powder and single-crystal X-ray diffraction.