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1.  Crystal structure of 8-hy­droxy­quinoline: a new monoclinic polymorph 
In an attempt to grow 8-hy­droxy­quinoline–acetamino­phen co-crystals from equimolar amounts of conformers in a chloro­form–ethanol solvent mixture at room temperature, the title compound, C9H7NO, was obtained. The mol­ecule is planar, with the hy­droxy H atom forming an intra­molecular O—H⋯N hydrogen bond. In the crystal, mol­ecules form centrosymmetric dimers via two O—H⋯N hydrogen bonds. Thus, the hy­droxy H atoms are involved in bifurcated O—H⋯N hydrogen bonds, leading to the formation of a central planar four-membered N2H2 ring. The dimers are bound by inter­molecular π–π stacking [the shortest C⋯C distance is 3.2997 (17) Å] and C—H⋯π inter­actions into a three-dimensional framework. The crystal grown represents a new monoclinic polymorph in the space group P21/n. The mol­ecular structure of the present monoclinic polymorph is very similar to that of the ortho­rhom­bic polymorph (space group Fdd2) studied previously [Roychowdhury et al. (1978 ▶). Acta Cryst. B34, 1047–1048; Banerjee & Saha (1986 ▶). Acta Cryst. C42, 1408–1411]. The structures of the two polymorphs are distinguished by the different geometries of the hydrogen-bonded dimers, which in the crystal of the ortho­rhom­bic polymorph possess twofold axis symmetry, with the central N2H2 ring adopting a butterfly conformation.
PMCID: PMC4186174  PMID: 25309256
8-hy­droxy­quinoline; hydrogen bonds; polymorphism; crystal structure
2.  A second polymorph of aqua­(2,9-di­methyl-1,10-phenanthroline-κ2 N,N′)bis­(formato-κO)copper(II) 
A new monoclinic polymorphic form of the title compound, [Cu(HCO2)2(C14H12N2)(H2O)], is described. It differs from the first ortho­rhom­bic polymorph [Pan, Lin & Zheng (2005 ▶). Z. Kristallogr. New Cryst. Struct. 220, 495–496] in the deviation of the Cu atom relative to the plane of the 2,9-dimethyl-1,10-phenanthroline (dmp) ligand. In the present structure, the Cu atom is shifted from the mean plane of the dmp ligand by only 0.005 (1) Å, compared with 0.318 (6) Å in the ortho­rhom­bic form. Hydrogen-bonding and π–π stacking inter­actions (mean inter­planar distance of 3.59 Å in the title compound) in the two different polymorphs are both essential to the supra­molecular assembly.
PMCID: PMC2961975  PMID: 21203045
3.  An ortho­rhom­bic polymorph of 5-[(4-methyl­phen­yl)diazen­yl]salicylaldehyde 
The title compound, C14H12N2O2, is an ortho­rhom­bic polymorph of the previously reported monoclinic form [Bakir et al. (2005 ▶). Acta Cryst. E61, o1611–o1613]. The dihedral angle between the aromatic rings is 4.32 (13)°. The mol­ecular structures of the two polymorphs, including short intra­molecular O—H⋯O hydrogen bonds between the the hydr­oxy and keto groups, are quite similar but their crystal packings are distinct. Unlike the monoclinic form, in which centrosymmetrically related hydr­oxy and keto groups form {⋯H⋯O}2 synthons via weak O—H⋯O contacts, leading to dimeric aggregates, in the ortho­rhom­bic form, the hydrogen bonding between these groups leads to the formation of supra­molecular chains orientated along the a axis.
PMCID: PMC2971774  PMID: 21578791
4.  1,3-Bis(2-chloro­phen­yl)thio­urea: a monoclinic polymorph 
The title compound, C13H10Cl2N2S, represents a monoclinic polymorph of the previously reported ortho­rhom­bic form [Ramnathan et al. (1996 ▶). Acta Cryst. C52, 134–136]. The mol­ecule is twisted with the dihedral angle between the benzene rings being 55.37 (7)°. The N—H atoms are syn to each other, which contrasts their anti disposition in the ortho­rhom­bic form. In the crystal, mol­ecules assemble into zigzag chains along the c axis via N—H⋯S hydrogen bonds. Chains are connected into layers via C—H⋯Cl inter­actions, and these stack along the a axis.
PMCID: PMC3247371  PMID: 22219989
5.  A novel monoclinic phase of impurity-doped CaGa2S4 as a phosphor with high emission intensity 
In the solid-state synthesis of impurity-doped CaGa2S4, calcium tetra­thio­digallate(III), a novel phosphor material (denominated as the X-phase), with monoclinic symmetry in the space group P21/a, has been discovered. Its emission intensity is higher than that of the known ortho­rhom­bic polymorph of CaGa2S4 crystallizing in the space group Fddd. The asymmetric unit of the monoclinic phase consists of two Ca, four Ga and eight S sites. Each of the Ca and Ga atoms is surrounded by seven and four sulfide ions, respectively, thereby sharing each of the sulfur sites with the nearest neighbours. In contrast, the corresponding sites in the ortho­rhom­bic phase are surrounded by eight and four S atoms, respectively. The photoluminescence peaks from Mn2+ and Ce3+ in the doped X-phase, both of which are supposed to replace Ca2+ ions, have been observed to shift towards the high energy side in comparison with those in the ortho­rhom­bic phase. This suggests that the crystal field around the Mn2+ and Ce3+ ions in the X-phase is weaker than that in the ortho­rhom­bic phase.
PMCID: PMC3379052  PMID: 22719273
6.  A new polymorph of 2,6-dimeth­oxy­benzoic acid 
A new crystalline form of 2,6-dimeth­oxy­benzoic acid, C9H10O4, crystallizing in a tetra­gonal unit cell has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent mol­ecule with a synplanar conformation of the carb­oxy group. The sterically bulky o-meth­oxy substituents force the carb­oxy group to be twisted away from the plane of the benzene ring by 65.72 (15)°. The carb­oxy group is disordered over two sites about the C—C bond [as indicated by the almost equal C—O distances of 1.254 (3) and 1.250 (3) Å], the occupancies of the disordered carboxym H atoms being 0.53 (5) and 0.47 (5). In the known ortho­rhom­bic form reported by Swaminathan et al. [Acta Cryst. (1976), B32, 1897–1900], due to the anti­planar conformation adopted by the OH group, the mol­ecular components are associated in the crystal in chains stabilized by linear O—H⋯O hydrogen bonds. However, in the new tetra­gonal polymorph, mol­ecules form dimeric units via pairs of O—H⋯O hydrogen bonds between the carb­oxy groups.
PMCID: PMC3239035  PMID: 22199883
7.  A second ortho­rhom­bic polymorph of 2-(pyridin-4-ylmeth­oxy)phenol 
The crystal structure of the title compound, C12H11NO2, represents a new ortho­rhom­bic polymorph II of the previously reported ortho­rhom­bic form I [Zhang et al. (2009 ▶) Acta Cryst. E65, o3160]. In polymorph II, the six-membered rings form a dihedral angle of 13.8 (1)° [71.6 (1)° in I], and O—H⋯N hydrogen bonds link mol­ecules into chains along [100], whereas the crystal structure of I features hydrogen-bonded centrosymmetric dimers.
PMCID: PMC3344498  PMID: 22590260
8.  Second monoclinic polymorph of 4-[(1H-benzimidazol-1-yl)meth­yl]benzoic acid 
Recently, we reported the first monoclinic [Kuai & Cheng (2011). Acta Cryst., E67, o2787] and the ortho­rhom­bic polymorph [Kuai & Cheng (2011). Acta Cryst., E67, o3014] of the title compound, C15H12N2O2. Another monoclinic polymorph was obtained accidentally by the hydro­thermal reaction of the title compound with manganese chloride in the presence of potassium hydroxide at 413 K. The asymmetric unit consists of four independent mol­ecules. In the crystal, O—H⋯N hydrogen bonds link the independent mol­ecules into four separate chains parallel to the b axis.
PMCID: PMC3238873  PMID: 22199726
9.  A monoclinic polymorph of N,N′-bis­(2,6-diisopropyl­phen­yl)formamidine 
A new polymorph of N,N′-bis­(2,6-diisopropyl­phen­yl)formamidine, C25H36N2, is reported, which is different from the previously reported ortho­rhom­bic structure. The mol­ecule crystallizes in the E–anti configuration, with tautomeric disorder of the N-bonded H atoms and no clear distinction between imine and amine functionalities. The mol­ecules form hydrogen-bonded dimers with inter­molecular N⋯N distances shorter than those in the ortho­rhom­bic polymorph.
PMCID: PMC2962078  PMID: 21203162
10.  A P212121 polymorph of (+)-clusianone 
The title compound, C33H42O4 [systematic name: (1S,5S,7R)-3-benzoyl-4-hy­droxy-8,8-dimethyl-1,5,7-tris­(3-methyl­but-2-­enyl)bi­cyclo­[3.3.1]nona-3-ene-2,9-dione], has a central bi­cyclo­[3.3.1]nonane-2,4,9-trione surrounded by tetra­prenyl­ated and benzoyl groups. The compound was recrystallized several times in methanol using both a slow evaporation method and with a crystal-seeding technique. This subsequently produced diffraction-quality crystals which crystallize in the ortho­rhom­bic space group P212121, in contrast to a previous report of a structure determination in the Pna21 space group [McCandlish et al. (1976 ▶). Acta Cryst. B32, 1793–1801]. The title compound has a melting point of 365–366 K, and a specific rotation [α]20 value of +51.94°. A strong intra­molecular O—H⋯O hydrogen bond is noted. In the crystal, mol­ecules are assembled in the ab plane by weak C—H⋯O inter­actions.
PMCID: PMC4004436  PMID: 24860292
11.  An ortho­rhom­bic polymorph of 3,4-di­amino­benzo­nitrile 
The title compound, C7H7N3, is an ortho­rhom­bic polymorph that crystallizes in the space group Pca21. The previously reported monoclinic form [Geiger & Parsons (2013 ▶) Acta Cryst. E69, o452] crystallizes in the space group P21/c (Z = 4). In the crystal, two independent HN—H⋯N C hydrogen bonds link the mol­ecules into chains along the a-glide plane. Two further independent HN—H⋯NH2 hydrogen bonds join the chains, forming a three-dimensional network.
PMCID: PMC3647854  PMID: 23723820
12.  Monoclinic polymorph of poly[aqua(μ4-hydrogen tartrato)sodium] 
A monoclinic polymorph of the title compound, [Na(C4H5O6)(H2O)]n, is reported and complements an ortho­rhom­bic form [Kubozono, Hirano, Nagasawa, Maeda & Kashino (1993 ▶). Bull. Chem. Soc. Jpn, 66, 2166–2173]. The asymmetric unit contains a hydrogen tartrate anion, an Na+ cation and a water mol­ecule. The Na+ ion is surrounded by seven O atoms derived from one independent and three symmetry-related hydrogen tartrate anions, and a water mol­ecule, forming a distorted penta­gonal–bipyramidal geometry. Independent units are linked via a pair of inter­molecular bifurcated O—H⋯O acceptor bonds, generating an R 2 1(6) ring motif to form polymeric two-dimensional arrays parallel to the (100) plane. In the crystal packing, the arrays are linked by adjacent ring motifs, together with additional inter­molecular O—H⋯O inter­actions, into a three-dimensional network.
PMCID: PMC2979804  PMID: 21579620
13.  Pseudomerohedrally twinned monoclinic structure of unfolded ‘free’ nona­ctin: comparative analysis of its large conformational change upon encapsulation of alkali metal ions 
The title compound, C40H64O12, crystallizes in a pseudo­merohedrally twinned primitive monoclinic cell with similar contributions of the two twin components. There are two symmetry-independent half-mol­ecules of nona­ctin in the asymmetric unit. Each mol­ecule has a pseudo-S 4 symmetry and resides on a crystallographic twofold axis; the axes pass through the mol­ecular center of mass and are perpendicular to the plane of the macrocycle. The literature description of the room-temperature structure of nona­ctin as an order–disorder structure in an ortho­rhom­bic unit cell is corrected. We report a low-temperature high-precision ordered structure of ‘free’ nona­ctin that allowed for the first time precise determination of its bond distances and angles. It possesses an unfolded and more planar geometry than its complexes with encapsulated Na+, K+, Cs+, Ca2+ or NH4 + cations that exhibit more isometric overall conformations.
PMCID: PMC2816929  PMID: 19805886
14.  A monoclinic polymorph of di-μ-oxido-bis­({2-[2-(methyl­amino)ethyl­imino­methyl]phenolato-κ3 N,N′,O}oxidovanadium(V)) 
A new monoclinic polymorph of the title compound, [V2(C10H13N2O)2O4], which is a centrosymmetric dimer, crystallizes in space group P21/c, whereas the previously known polymorph crystallizes in the ortho­rhom­bic space group Pbca [Mokry & Carrano (1993 ▶). Inorg. Chem. 32, 6119–6121]. Each VV atom is six-coordinated by one oxide group, two N atoms and one O atom from the Schiff base ligand, and by two additional bridging O atoms. The two methyl­ene groups are each disordered over two sites, with occupancy factors of 0.776 (14) and 0.224 (14). In the crystal structure, there are C—H⋯O hydrogen bonds and C—H⋯π inter­actions between the dimers.
PMCID: PMC2959960  PMID: 21581160
15.  Polymorphism and phase transition behavior of 6,6′-bis­(chloro­meth­yl)-1,1′,4,4′-tetra­methyl-3,3′-(p-phenyl­enedimethyl­ene)bis­(piperazine-2,5-dione) 
A crystallographic investigation of the title compound, C22H28Cl2N4O4, using crystals obtained under different crystallization conditions, revealed the presence of two distinct polymorphic forms. The mol­ecular conformation in the two polymorphs is very different: one adopts a ‘C’ shape, whereas the other adopts an ‘S’ shape. In the latter, the molecule lies across a crystallographic twofold axis. The ‘S’-shaped polymorph undergoes a reversible ortho­rhom­bic-to-monoclinic phase transition on cooling, whereas the structure of the ‘C’-shaped polymorph is temperature insensitive.
PMCID: PMC2720150  PMID: 19652319
16.  N-(4-Bromo­phen­yl)acetamide: a new polymorph 
A new polymorph of the title compound, C8H8BrNO, has been determined at 173 K in the space group P21/c. The previous room-temperature structure was reported to crystallize in the ortho­rhom­bic space group Pna21 [Andreetti et al. (1968 ▶). Acta Cryst. B24, 1195–1198]. In the crystal, mol­ecules are linked by N—H⋯O hydrogen bonds forming chains along [010]. Weak C—H⋯π inter­actions are also present.
PMCID: PMC3588444  PMID: 23476627
17.  An orthorhombic polymorph of mulinic acid 
The title compound [systematic name: (3S,3aS,10bR)-3-isopropyl-5a,8-dimethyl-2,3,4,5,5a,6,7,10,10a,10b-deca­hydro-endo-epidioxy­cyclo­hepta­[e]indene-3a(1H)-carboxylic acid], C20H30O4, is a polymorphic form of a previously reported structure [Loyola et al. (1990 ▶). Tetra­hedron, 46, 5413–5420]. The newly found ortho­rhom­bic polymorph crystallizes in P212121 with two mol­ecules in the asymmetric unit. The mol­ecules are linked into discrete D(2) chains by simple O—H⋯O inter­actions. There are only slight variations in the mol­ecular geometry and supra­molecular organization in the crystal structures of the two polymorphs. The densities are 1.145 (monoclinic, P21) and 1.155 Mg m−3 (ortho­rhom­bic, P212121).
PMCID: PMC2979944  PMID: 21579761
18.  A monoclinic polymorph of (R,R)-4,4′-dibromo-2,2′-[cyclo­hexane-1,2-diylbis(nitrilo­methanylyl­idene)]diphenol 
The title compound, C20H20Br2N2O2, a tetra­dentate Schiff base, is the enanti­omerically pure R,R-diastereomer of four possible stereoisomers. The mol­ecular structure reveals two strong intra­molecular O—H⋯N hydrogen bonds between the hy­droxy O atom and the imino N atom, which each generate S(6) rings. In the crystal, mol­ecules are stacked in columns along the a axis; when viewed down the b axis, successive columns are stacked in the opposite direction. The structure reported herein is the monoclinic polymorph of the previously reported ortho­rhom­bic form [Yi & Hu (2009 ▶). Acta Cryst. E65, o2643], in which the complete mol­ecule is generated by a crystallographic twofold axis.
PMCID: PMC3344566  PMID: 22590328
19.  A third polymorph of 1,4-bis­(1H-benzimid­azol-2-yl)benzene 
The title compound, C20H14N4, is a new polymorph of the previously reported structures, which were ortho­rhom­bic, space group Pbca [Bei et al. (2000). Acta Cryst. C56, 718–719] and monoclinic, space group P21/c [Dudd et al. (2003). Green Chem. 5, 187–192]. The asymmetric unit consists of two independent mol­ecules in which the dihedral angels between the central benzene ring and the outer benzimidazole ring systems are 16.81 (10) and 14.23 (10)° in one molecule and 26.09 (10) and 37.29 (10)° in the other. In the crystal, mol­ecules are linked by N—H⋯N and C—H⋯N hydrogen bonds into a tape running along the c-axis direction.
PMCID: PMC4051014  PMID: 24940273
20.  A monoclinic polymorph of N-(3-chloro­phen­yl)benzamide 
The title compound, C13H10ClNO, (I), is a polymorph of the structure, (II), first reported by Gowda et al. [Acta Cryst. (2008), E64, o462]. In the original report, the compound crystallized in the ortho­rhom­bic space group Pbca (Z = 8), whereas the structure reported here is monoclinic P21/c (Z = 4). The principal difference between the two forms lies in the relative orientations of the phenyl and benzene rings [dihedral angle = 8.90 (13)° for (I) and 61.0 (1)° for (II)]. The inclination of the amide –CONH– units to the benzoyl ring is more similar [15.8 (7)° for (I) and 18.2 (2)° for (II)]. In both forms, the N—H bonds are anti to the 3-chloro substituents of the aniline rings. In the crystal, inter­molecular N—H⋯O hydrogen bonds form C(4) chains along c. These chains are bolstered by weak C—H⋯O inter­actions that generate R 2 1(6) and R 2 1(7) ring motifs.
PMCID: PMC3008970  PMID: 21589001
21.  Crystal structure of [Y6(μ6-O)(μ3-OH)8(H2O)24]I8·8H2O 
In the crystal structure of a hexa­nuclear Y3+ compound, the six Y3+ cations are arranged octa­hedrally around an μ6-O atom at the centre of the cationic complex. Each of the eight faces of the Y6 octa­hedron is capped by an μ3-OH group in the form of a distorted cube. The proximity of the cationic complexes and lattice water mol­ecules leads to the formation of a three-dimensional hydrogen-bonded network of medium strength.
The crystal structure of the title compound {systematic name: octa-μ3-hydroxido-μ6-oxido-hexa­kis­[tetra­aqua­yttrium(III)] octa­iodide octa­hydrate}, is characterized by the presence of the centrosymmetric mol­ecular entity [Y6(μ6-O)(μ3-OH)8(H2O)24]8+, in which the six Y3+ cations are arranged octa­hedrally around a μ6-O atom at the centre of the cationic complex. Each of the eight faces of the Y6 octa­hedron is capped by an μ3-OH group in the form of a distorted cube. In the hexa­nuclear entity, the Y3+ cations are coordinated by the central μ6-O atom, the O atoms of four μ3-OH and of four water mol­ecules. The resulting coordination sphere of the metal ions is a capped square-anti­prism. The crystal packing is quite similar to that of the ortho­rhom­bic [Ln 6(μ6-O)(μ3-OH)8(H2O)24]I8·8H2O structures with Ln = La—Nd, Eu—Tb, Dy, except that the title compound exhibits a slight monoclinic distortion. The proximity of the cationic complexes and the lattice water mol­ecules leads to the formation of a three-dimensional hydrogen-bonded network of medium strength.
PMCID: PMC4257455  PMID: 25552996
crystal structure; hexa­nuclear compounds; lanthanide compound; three-dimensional hydrogen-bonded network
22.  Benzene-1,3,5-triol at 105 K 
The structure of the title compound, C6H6O3, has been redetermined at low temperature [room-temperature structure: Maartmann-Moe (1965 ▶). Acta Cryst. 19, 155–157]. The mol­ecule is planar with approximate D 3h point symmetry, yet it crystallizes in the chiral ortho­rhom­bic space group P212121 with a three-dimensional hydrogen-bonding network containing infinite O—H⋯O—H⋯O—H chains.
PMCID: PMC2959261  PMID: 21201217
23.  A monoclinic polymorph of (1E,5E)-1,5-bis­(2-hy­droxy­benzyl­idene)thio­carbono­hydrazide 
The title compound, C15H14N4O2S, is a derivative of thio­ureadihydrazide. In contrast to the previously reported polymorph (ortho­rhom­bic, space group Pbca, Z = 8), the current study revealed monoclinic symmetry (space group P21/n, Z = 4). The mol­ecule shows non-crystallographic C 2 as well as approximate C s symmetry. Intra­molecular bifurcated O—H⋯(N,S) hydrogen bonds, are present. In the crystal, inter­molecular N—H⋯S hydrogen bonds and C—H⋯π contacts connect the mol­ecules into undulating chains along the b axis. The shortest centroid–centroid distance between two aromatic systems is 4.5285 (12) Å.
PMCID: PMC3213636  PMID: 22091213
24.  Tetra­gonal polymorph of 5,5-dichloro­barbituric acid 
The tetra­gonal polymorph of 5,5-dichloro­barbituric acid (m.p. 478 K), C4H2Cl2N2O3, forms an N—H⋯O hydrogen-bonded tape structure along [001]. Two tapes related by a twofold rotation axis are associated via Cl⋯O contacts [3.201 (1) Å], and four such chain pairs are arranged around a fourfold roto-inversion axis. The crystal structures of the monoclinic and ortho­rhom­bic polymorphs have been reported previously [Gelbrich et al. (2011 ▶). CrystEngComm, 13, 5502–5509].
PMCID: PMC3254567  PMID: 22259517
25.  Crystalline Morphology and Polymorphic Phase Transitions in Electrospun Nylon 6 Nanofibers 
Macromolecules  2007;40(17):6283-6290.
Uniform nylon 6 nanofibers with diameters around 200 nm were prepared by electrospinning. Polymorphic phase transitions and crystal orientation of nylon 6 in unconfined (i.e., as-electrospun) and a high Tg (340 °C) polyimide confined nanofibers were studied. Similar to melt-spun nylon 6 fibers, electrospun nylon 6 nanofibers also exhibited predominant, meta-stable γ crystalline form, and the γ-crystal (chain) axes preferentially oriented parallel to the fiber axis. Upon annealing above 150 °C, γ-form crystals gradually melted and recrystallized into the thermodynamically stable α-form crystals, which ultimately melted at 220 °C. Release of surface tension accompanied this melt-recrystallization process, as revealed by differential scanning calorimetry. For confined nanofibers, both the melt-recrystallization and surface tension release processes were substantially depressed; γ-form crystals did not melt and recrystallize into α-form crystals until 210 °C, only 10 °C below the Tm at 220 °C. After complete melting of nano-confined crystals at 240 °C and recrystallization at 100 °C, only α-form crystals oriented perpendicular to the nanofiber axis were obtained. In the polyimide-confined nanofibers, the Brill transition (from the monoclinic α-form to a high temperature monoclinic form) was observed at 180–190 °C, which was at least 20 °C higher than that in unconfined nylon 6 at approximately 160 °C. This, again, was attributed to the confinement effect.
PMCID: PMC2507769  PMID: 18698379

Results 1-25 (800599)