Crystal structures and properties of isomers of 3,5-dinitro-(4-acetylphenyl)aminobenzoyl (p-bromophenyl)amide

The para and ortho isomers of 3,5-dinitro-(4-acetylphenyl)aminobenzoyl (p-bromophenyl)amide (I and II, respectively) are synthesized, and their physicochemical properties and structure are investigated. The para isomer I has a higher melting temperature and is less soluble in organic solvents as compared to the ortho isomer II. The electronic absorption spectra indicate that absorption for molecule I occurs at longer wavelengths than for molecule II. A correlation between the physicochemical properties and the crystal structures of compounds I and II is revealed. Crystals I · 0.5C₆H₆ are triclinic; the unit cell parameters are a = 11.760(2) Å, b = 13.958(3) Å, c = 15.012(3) Å, α = 108.01(2)°, β = 103.95(1)°, γ = 92.00(2)°, V = 2258.3(8) ų, space group $P\bar 1$ , and Z = 4. Crystals II are monoclinic; the unit cell parameters are a = 9.302(2) Å, b = 16.380(3) Å, c = 13.480(3) Å, β = 100.09(3)°, V = 2022.1(7) ų, space group P2₁/c, and Z = 4. Structures I · 0.5C₆H₆ and II are characterized by intramolecular and intermolecular hydrogen bonds.     

Synthesis and structure of two crystalline modifications of Bis(1,10-Phenanthroline)trinitratoeuropium(III) Eu(NO3)3(Phen)2

Two crystalline modifications (I and II) of the phenanthroline complex of europium nitrate with the same chemical composition, Eu(NO₃)₃(Phen)₂, are synthesized under different conditions by varying the solvents, temperatures, and crystallization rates. The crystal structures of these modifications are determined using X-ray diffraction. Crystalline modifications I and II differ in the unit cell parameters and the positions of the complexes in the unit cell. The geometric characteristics of the complexes in the structures of compounds I and II differ insignificantly. Crystals of compound I belong to the isostructural family Ln(NO₃)₃(Phen)₂ (Ln = La-Lu). Crystals of compound II (new phase) are studied for the first time. Crystals of I and II are monoclinic, space group C2/c, and Z = 4. The unit cell parameters are as follows: a = 11.1555(10) Å, b = 17.9698(10) Å, c = 13.0569(10) Å, β = 100.507(10)°, and V = 2572.1(3) ų for modification I and a = 9.5153(10) Å, b = 15.4546(10) Å, c = 17.1763(10) Å, β = 93.451(10)°, and V = 2521.3(3) ų for modification II. The difference between the molecular complexes in the structures of compounds I and II is revealed by the superposition method. Complexes II are arranged along the C ₂ axis and are statistically disordered with respect to this axis.     

Synthesis and the crystal and molecular structure of two modifications of bis(1,10-phenanthroline)trinitratoerbium(III) Er(NO3)3(Phen)2

Two crystalline modifications (I and II) of the phenanthroline complex of erbium nitrate with the same chemical composition, Er(NO₃)₃(Phen)₂, are synthesized by a procedure similar to that used for preparing the phenanthroline complexes of europium nitrate. The crystal structures of these modifications are determined using X-ray diffraction. Crystals of compound I belong to the isostructural family Ln(NO₃)₃(Phen)₂ (Ln = La-Lu). Crystals of compound II are isostructural to those of modification II (new phase) of the Eu(NO₃)₃(Phen)₂ compound. Crystals of I and II are monoclinic, space group C2/c, and Z = 4. The unit cell parameters are as follows: a = 11.126 Å, b = 17.815 Å, c = 12.976 Å, β = 100.45°, and V = 2529 ų for modification I and a = 9.459 Å, b = 15.463 Å, c = 17.076 Å, β = 93.52°, and V = 2493 ų for modification II. The molecular complexes in the structures of compounds I and II are nearly identical. The mean lengths of the Er-N and Er-O bonds are equal to 2.500 and 2.466 Å in compound I and 2.508 and 2.457 Å in compound II, respectively. The difference between the structures of compounds I and II is associated with the difference between intermolecular interactions in the unit cell.     

Crystal structures of mixed compounds Cu(2)Gly(D-Ser)(L-Ser)2 and Cu(2)Gly 3(L-Ser)

-The crystal structures of mixed coordination compounds, Cu(2)Gly(D-Ser)(L-Ser)₂(I) and Cu(2)Gly ₃(L-Ser)(II), which contain the amino acid residues of glycine (Gly) and serine (Ser) in the 1: 3 and 3: 1 ratio, respectively, are studied by electron diffraction. Crystals I and II are triclinic, Z = 1, and space group P1. For I, a = 8.96(2) Å, b = 9.66(2) Å, c = 5.07(2) Å, α = β = 90°, and γ = 92.8(3)°. For II, a = 8.37(2) Å, b = 9.65(2) Å, c = 5.06(2) Å, α = β = 90°, and γ = 92.8(3)°. Compounds I and II have layered structures that are based on the CuGly(L-Ser) fragment. Structures I and II differ mainly in their interlayer spacing and configuration of the interlayer space.     

Crystal structure and vibrational spectra of two cadmium halide complexes with 1,2-Bis[2-(Diphenylphosphinylmethyl)phenoxy]ethane (L 1) and 1,3-bis[2-diphenylphosphinylmethyl)phenoxy]propane (L), CdBr2 ⋅ L 1 and CdI2 ⋅ L

Two cadmium halide complexes with 1,2-bis[2-(diphenylphosphinylmethyl)phenoxy]ethane (L ¹) and 1,3-bis[2-(diphenylphosphinylmethyl)phenoxy]propane (L), namely, CdBr₂ ? L ¹ (I) and CdI₂ ? L(II), have been synthesized. An analysis of their vibrational spectra is carried out. The structures of I and II are determined by X-ray diffraction. Crystals I are monoclinic, a = 31.562(6) Å, b = 13.548(3) Å, c = 18.733(4) Å, β = 91.28(3)°, space group C2/c, Z = 8, and R = 0.051 for 3776 reflections. Crystals II are triclinic, a = 11.803(2) Å, b = 12.554(3) Å, c = 14.686(3) Å, α = 90.30(3)°, β = 90.29(3)°, γ = 106.08(3)°, space group $P\bar 1$ , Z = 2, and R = 0.043 for 4916 reflections. Compounds I and II exhibit a polymeric chain structure. The potentially tetradentate ligands L ¹ and L are coordinated to the metal atoms only through two phosphoryl oxygen atoms and fulfill the bidentate bridging function. The environment of the Cd atom is completed to the tetrahedral coordination by two Br atoms in complex I and two I atoms in complex II. The mean distances are as follows: Cd-Br, 2.526(2) Å; Cd-I, 2.695 Å; and Cd-O, 2.243(8) Å in I and 2.210(4) Å in II. The L ¹ and L ligands in complexes I and II adopt an S-shaped conformation.     

Synthesis and crystal structure of new double indium phosphates M 3 I In(PO4)2 (M I = K and Rb)

Double potassium indium and rubidium indium phosphates K₃In(PO₄)₂ (I) and Rb₃In(PO₄)₂ (II) are synthesized by solid-phase sintering at T = 900°C. The compounds prepared are characterized by X-ray powder diffraction (I and II), X-ray single-crystal diffraction (II), and laser-radiation second harmonic generation. Structure I is solved using the Patterson function and refined by the Rietveld method. Both compounds crystallize in the monoclinic crystal system. For crystals I, the unit cell parameters are as follows: a = 15.6411(1) Å, b = 11.1909(1) Å, c = 9.6981(1) Å, β = 90.119(1)°, space group C2/c, R _p = 4.02%, and R _wp = 5.25%. For crystals II, the unit cell parameters are as follows: a = 9.965(2) Å, b = 11.612(2) Å, c = 15.902(3) Å, β = 90.30(3)°, space group P2₁/n, R ₁ = 4.43%, and wR ₂ = 10.76%. Structures I and II exhibit a similar topology of the networks which are built up of { In[PO₄]₂} (I) and { In₂[PO₄]₄} (II) structural units.     

X-ray mapping in heterocyclic design: XI. X-ray diffraction study of 1-benzoyl-3-(pyridin-2-yl)-thiocarbamide and the product of its reaction with oxidants

The structures of 1-benzoyl-3-(pyridin-2-yl)-thiocarbamide C₁₃H₁₁N₃O₁S₁ (I) and 2-benzoylimino-1,2,4-thiadiazole[2,3-a]pyridine C₁₃H₉N₃O₁S₁ (II) are studied by X-ray diffraction. Structures I [a = 5.342(4) Å, b = 20.428(5) Å, c = 11.784(4) Å, β = 90.55(2)°, Z = 4, space group P2₁/n) and II [a = 6.258(6) Å, b = 18.068(14) Å, c = 10.185(10) tA, β = 95.45(8)°, Z = 4, space group P2₁/n) are determined by direct methods and refined to R ₁ = 0.0673 and 0.0802, respectively. In structure I, both intramolecular (involving the O atom) and intermolecular (involving the N and S atoms) hydrogen bonds are observed. The latter bonds are responsible for the formation of centrosymmetric molecular dimers. In structure II, a short intramolecular contact (2.168 Å) is observed between the S and O atoms.     

Five-coordinate Co(II) complexes with nitrilotriacetic acid: Crystal structures of Ca[Co(Nta)X] · nH2O (X − = Cl, Br, or NCS)

The synthesis and X-ray diffraction study of three Ca[Co(Nta)X] · nH₂O complexes [X ^? = Cl, n = 2.3 (I); X ^? = Br, n = 2 (II); and X ^? = NCS, n = 2 (III)] are performed. The main structural units of crystals I–III are the [CoX(Nta)]^2? anionic complexes and hydrated Ca²⁺ cations. The anionic complexes have similar structures. The coordination of the Co²⁺ atom in the shape of a trigonal bipyramid is formed by N + 3O atoms of the Nta ^3? ligand and the X ^? anion in the trans position with respect to N. In structures I–III, the Co-O and Co-N bond lengths lie in the ranges 1.998–2.032 and 2.186–2.201 Å, respectively. The Co-X bond lengths are 2.294 (I), 2.436 and 2.445 (II), and 1.982 Å (III). The environments of the Ca²⁺ cations include oxygen atoms of one or two water molecules and six or seven O(Nta) atoms with the coordination number of 9 in I or 8 in II and III. The Ca-O(Nta) bonds form a three-dimensional framework in I or layers in II and III. Water molecules are involved in the hydrogen bonds O(w)-H···O(Nta), O(w)-H···X, and O(w)-H···O(w). Structural data for crystals I–III are deposited with the Cambridge Structural Database (CCDC nos. 287 814–287 816).     

Crystal structures of benzo-18-crown-6 complexes with oxonium hexafluorotantalate and oxonium hexafluoroniobate

The crystal structures of [(benzo-18-crown-6) · H₃O]TaF₆ and [(benzo-18-crown-6) · H₃O]NbF₆ complex compounds are determined by X-ray diffraction. It is revealed that the tantalum complex has two polymorphic modifications, namely, the monoclinic (I) and orthorhombic (II) modifications. The unit cell parameters of these compounds are as follows: a = 14.784(2) Å, b = 8.390(4) Å, c = 18.005(6) Å, β = 95.78(2)°, Z = 4, and space group P2₁/n for modification I; and a = 8.456(2) Å, b = 21.967(5) Å, c = 24.122(5) Å, Z = 8, and space group Pbca for modification II. The orthorhombic niobium complex [(benzo-18-crown-6) · H₃O]NbF₆ with the unit cell parameters a = 8.454(1) Å, b = 21.943(3) Å, c = 24.127(4) Å, Z = 8, and space group Pbca (III) is isostructural with tantalum complex II. The oxonium cation in complexes I–III is encapsulated by the crown ether and thus forms one ordinary and two bifurcate hydrogen bonds with the oxygen atoms of the crown ether. This macrocyclic cation is bound to the anions through the C-H ...F contacts (H...F, 2.48–2.58 Å).     

X-ray structure investigation of 1-acetoxy-1-cyano-2-naphthylethylene and 1,1-dicyano-2-naphthylethylene

The crystal structures of 1-acetoxy-1-cyano-2-naphthylethylene (I) and 1,1-dicyano-2-naphthylethylene (II) are determined by X-ray structure analysis. Crystals I are monoclinic; at 25° C, the unit cell parameters are as follows: a = 17.308(6) Å, b = 4.507(1) Å, c = 17.845(5) Å, β = 107.90(2)°, V = 1324.7(7) ų, d _calcd = 1.260 g/cm³, Z = 4, and space group P2₁/n. Crystals II are monoclinic; at 25°C, the unit cell parameters are a = 3.827(1) Å, b = 15.784(4) Å, c = 17.226(2) Å, β = 91.22(2)°, V = 1040.3(4) ų, d _calcd = 1.304 g/cm³, Z = 4, and space group P2₁/n. It is revealed that, in crystal structures of I and II, the molecular stacks characteristic of compounds of this series are formed through stacking contacts along the direction of the smallest lattice parameter.     

Crystal structures of double vanadates, Ca9 R(VO4)7 ⋅ III. R = Nd, Sm, Gd, or Ce

The crystal structures of Ca₉ R(VO₄)₇ (R = Nd (I), Sm (II), or Gd (III)) were studied by the Rietveld method. The compounds are isostructural to Ca₃(VO₄)₂ and are crystallized in the trigonal system (sp. gr. R3c, Z = 6). The unit-cell parameters are as follows: for I, a = 10.8720(5) Å, c = 38.121(1) Å; for II, a = 10.8652(5) Å, c = 38.098(1) Å; and for III, a = 10.8631(5) Å, c = 38.072(1) Å. In the structures of I and II, the M(1), M(2), and M(3) positions are statistically occupied by the rare-earth cations and calcium anions. In the structure of III, the Gd³⁺ cations occupy the _M(1) and _M(2) positions. The distributions of the R ³⁺ cations over the positions are characteristic of each structure. The composition of the cerium-ontaining compound Ca_9.81Ce_0.42(VO₄)₇ (a = 10.8552(5) Å, c = 38.037(1) Å) was refined and its crystal structure was solved from the X-ray powder data. In this compound, cerium atoms are in the oxidation states +3 and +4.     

Stereochemical activity of a lone electron pair in antimony(III) and bismuth(III) chelates: Crystal structures of Ca[Sb(Edta)]2 ⋅ 8H2O and Ba{[Bi(Edta)] 2H2O ⋅ H2O

The crystal structures of Ca[Sb(Edta)]₂ ? 8H₂O (I) and Ba{ [Bi(Edta)]₂H₂O} ? H₂O (II) are determined by X-ray diffraction. Crystals I are monoclinic, a = 7.132 Å, b = 21.906 Å, c = 10.896 Å, β = 91.13°, Z = 2, and space group P2₁/n. Crystals II are triclinic, a = 8.995 Å, b = 12.750 Å, c = 13.577 Å, α = 77.42°, β = 73.90°, γ = 86.53°, Z = 2, and space group $P\bar 1$ . In structure I, the coordination number of the antimony atom is 6 + LEP (lone electron pair), and the polyhedron is a ψ-pentagonal bipyramid with the lone electron pair at an equatorial position. In structure II, two crystallographically independent complexes Bi(Edta)^? and the coordination water molecule form tetranuclear associates. The environments of two independent bismuth atoms (the coordination number is eight) are similar, and their polyhedra can be described as distorted dodecahedra. The effect of the lone electron pair on the structures of polyhedra of antimony and bismuth is discussed.     

Crystal structure of the supramolecular system of indole-2,3-dione 1-(2-oxopropyl)-3-ethylene ketal and its thiosemicarbazone

The crystal structure of a supramolecular system consisting of indole-2,3-dione 1-(2-oxopropyl)-3-ethylene ketal (I) and indole-2,3-dione 1-(2-oxopropyl)-3-ethylene ketal thiosemicarbazone (II) molecules that are linked together by hydrogen bonds is determined using X-ray diffraction. The crystal is monoclinic, and the unit cell parameters are as follows: a = 12.8360(3) Å, b = 10.7330(3) Å, c = 19.4610(3) Å, β = 99.566(1)°, space group P2₁/c, and Z = 4 (C₂₇H₂₉N₅O₇S). In molecules I and II, the indole-2,3-dione 3-ethylene ketal fragments have a virtually identical structure. The pyrrole and dioxolane fragments are spiro-linked through the carbon atom with a dihedral angle close to 90°. The adjacent pyrrole and benzene rings are coplanar to within 4.4°. In molecule II, the oxygen atom of the dioxolane fragment and the terminal nitrogen atom of the thiosemicarbazide fragment are involved in the N-HïO intramolecular hydrogen bond [3.294(2) Å]. The key role in the formation of the crystal structure is played by intermolecular hydrogen bonds of the N-H?dO, C-H?O, C-H?N, and N-H?S types.     

Molecular and crystal structures of 1R,4R-cis-2-(4-hydroxybenzylidene)-p-menthan-3-one

The molecular and crystal structures of chiral 1R, 4R-cis-2-(4-hydroxybenzylidene)-p-menthan-3-one (I) are determined by X-ray diffraction analysis. Single crystals of I are orthorhombic, a = 8.997(2) Å, b = 11.314(2) Å, c = 14.847(3) Å, V = 1511.3(5) ų, Z = 4, and space group P2₁2₁2₁. The cyclohexanone ring in molecules of compound I has a chair-type conformation with the axial methyl and equatorial isopropyl groups. The enone and benzylidene groupings are nonplanar. The considerable distortion of bond angles at the sp ² carbon atoms of the benzylidene grouping and the puckering parameters of the cyclohexanone ring in the structure of I are close to those observed for the previously studied compound with the p-methoxy substituent. In the crystal, molecules I are linked by very short intermolecular hydrogen bonds .     

Synthesis and crystal structure of (CN3H6)2[UO2(OH)2(NCS)]NO3 and β-Cs3[UO2(NCS)5]

Compounds (CN₃H₆)₂[UO₂(OH)₂(NCS)]NO₃ (I) and β-Cs₃[UO₂(NCS)₅] (II) are synthesized and studied by IR spectroscopy and single-crystal X-ray diffraction. I and II crystallize in the orthorhombic system. For I, a = 12.2015(13) Å, b = 7.3295(8) Å, c = 16.310(2) Å, space group Pnma, Z = 4, and R = 0.0327; for II, a = 21.7891(6) Å, b = 13.5120(3) Å, c = 6.8522(2) Å, space group Pnma, Z = 4, and R = 0.0268. In structure I, complex groups form infinite chains [UO₂(OH)₂(NCS)] _n ^n? belonging to the AM ₂ ² M ¹ crystal chemical group of uranyl complexes (A = UO ₂ ²⁺ , M ² = OH^?, and M ¹ = NCS^?). The main structural elements of crystals II are mononuclear [UO₂(NCS)₅]^3? groups belonging to the AM ₅ ¹ group of uranyl complexes (A = UO ₂ ²⁺ and M ¹ = NCS^?). In I and II, uranium-containing complexes are connected with outer-sphere cations by electrostatic interactions, and in I a system of hydrogen bonds also contributes to their binding. Specific features of the packing of complex [UO₂(NCS)₅]^3? groups in the structures of two modifications of Cs₃[UO₂(NCS)₅] are discussed.     

Effect of the acid-base interactions in a solution on the composition of the coordination sphere of aluminum and gallium complexonates

Hydroxocomplexonate K₂[GaEdta(OH)] · 6H₂O (I) and nitronium salts BH⁺GaEdta · 4H₂O (II) and BH⁺AlEdta · 4H₂O (IV) are synthesized from aqueous solutions at pH 8, 6, and 7, respectively. AlHEdta(H₂O) (III) is isolated from an acid solution (pH 1.5). Structures I, II, and IV are determined by single-crystal X-ray diffraction. The X-ray powder diffraction analysis of III has revealed that its crystals are not isostructural with those of similar complexes of other metals. Crystals I–IV are monoclinic. The unit cell parameters are a = 10.482(1), 15.735(4), 5.768(4), and 15.756(4) Å; b = 10.442(2), 12.511(2), 14.884(11), and 12.453(3) Å; c = 19.590(4), 17.330(5), 19.113(12), and 17.328(6) Å; β = 101.30(2)°, 104.54(2)°, 90.74(5)°, and 104.75(2)° for I–IV, respectively.     

Structure determination of 3β-acetoxy-cholest-5-ene-7-one—A steroid

The crystal structure of 3β-acetoxy-cholest-5-ene-7-one (C₂₉H₄₆O₃) has been determined by X-ray diffraction methods. The compound crystallizes in the monoclinic crystal system (space group P2₁) with the unit cell parameters a = 9.632(1) Å, b = 12.280(1) Å, c = 23.099(2) Å, β = 99.52(1)°, and Z = 4. The structure has been solved by direct methods and refined to an R-value of 0.065 for 3927 observed reflections [F ₀ > 4σ(F ₀)]. Two crystallographically independent molecules (I and II) in the asymmetric unit have been observed. In both molecules, rings A and C of the steroid nucleus exist in a chair conformation. Ring B of molecule I adopts a 5α,6β half-chair conformation, and ring B of molecule II shows a 6α sofa conformation. Ring D adopts a 13α,14β half-chair conformation in molecule I and a 13α,14β half-chair conformation in molecule II. The crystal structure is stabilized by the intramolecular and intermolecular C-H?O interactions.     

Crystal structures of salicylideneguanylhydrazinium chloride and its copper(II) and cobalt(III) chloride complexes

The crystal structures of salicylideneguanylhydrazinium chloride hydrate hemiethanol solvate (I), salicylideneguanylhydrazinium trichloroaquacuprate(II) (II), and bis(salicylideneguanylhydrazino)cobalt(III) chloride trihydrate (III) are determined using X-ray diffraction. The structures of compounds I, II, and III are solved by direct methods and refined using the least-squares procedure in the anisotropic approximation for the non-hydrogen atoms to the final factors R = 0.0597, 0.0212, and 0.0283, respectively. In the structure of compound I, the monoprotonated molecules and chlorine ions linked by hydrogen bonds form layers aligned parallel to the (010) plane. In the structure of compound II, the salicylaldehyde guanylhydrazone cations and polymer chains consisting of trichloroaquacuprate(II) anions are joined by an extended three-dimensional network of hydrogen bonds. In the structure of compound III, the [Co(LH)₂]⁺ cations, chloride ions, and molecules of crystallization water are linked together by a similar network.     

Structure studies of solid solutions of oxygen in electrolytic niobium

The crystal structures (cubic system, sp. gr. Im3m) of solid solutions of oxygen in niobium of the Compositions NbO _x<0.01 (I), NbO_0.150(6) (II) and NbO_0.107(5) (III) were studied for the first time by single-crystal X-ray diffraction analysis (Syntex P1 diffractometer, λMoKα radiation). Crystals I (a = 3.2969(8) Å) and II (a = 3.3082(6) Å) were prepared by the electrolysis of salt melts. Crystal III (a = 3.3114(4) Å) was obtained after partial dissolution of crystal I in a solution of HNO₃, HCl, and HF resulting in its saturation with oxygen. The residual electron density maps for crystal I revealed no O atoms. Localization of the O atoms in the tetrahedral cavities of crystal II and in the tetrahedral and octahedral cavities of crystal III indicates that the arrangement of the interstitial positions of oxygen atoms depends on the procedure of preparing solid solutions. In crystals II and III, the amplitudes of thermal vibrations of the Nb atoms are smaller and the anharmonicity of these vibrations is higher than those in crystal I virtually devoid of oxygen. The reasons for the substantially higher oxygen content (up to 17 at. %) in cubic crystals of the solid solutions of O in Nb prepared by the electrolytic method than in the analogous phase obtained upon heating (lower than 7 at. %) are discussed.     

Low-temperature study of the molecular and crystal structures of 2-(2′-tosylamino-5′-nitrophenyl)-4H-3,1-benzoxazin-4-one, an organic luminophore

The crystal structure of 2-(2′-tosylamino-5′-nitrophenyl)-4H-3,1-benzoxazin-4-one (I) is studied by X-ray diffraction at 100 K (C₂₁H₁₅N₃O₆S, a = 20.899(2) Å, b = 10.948(1) Å, c = 8.260(1) Å, V = 1889.3(1) ų, Z = 4, and space group Pbn2₁). The compound exhibits an anomalous Stokes shift. Upon cooling, the oxazineaminophenyl fragment of compound I acquires a quinoid structure and the linear parameters of the intramolecular N-H?N hydrogen bond increase (the distance between the heterocyclic nitrogen atom and the hydrogen atom of the tosylamino group becomes 1.92 Å). The complete optimization of the geometry of molecules in compound I and unsubstituted 2-(2′-tosylaminophenyl)-4H-3,1-benzoxazin-4-one in the ground singlet electronic state is performed by the semiempirical method with the MOPAC program. It is shown that the oxygen atoms in the sulfo group of molecule I are nonequivalent, because one of them is involved in the intermolecular C-H?O hydrogen bond.