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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.  2-Pyridone: monoclinic polymorph 
The asymmetric unit in the title compound, C5H5NO, comprises two independent but virtually identical mol­ecules of 2-pyridone, and represents a monoclinic polymorph of the previously reported ortho­rhom­bic (P212121) form [Penfold (1953 ▶). Acta Cryst. 6, 591–600; Ohms et al. (1984 ▶). Z. Kristallogr. 169, 185–200; Yang & Craven (1998 ▶). Acta Cryst. B54, 912–920]. The independent mol­ecules are linked into supra­molecular dimers via eight-membered {⋯HNC(O)}2 amide synthons in contrast to the helical supra­molecular chains, mediated by {⋯HNC(O)} links, found in the ortho­rhom­bic form.
PMCID: PMC2972078  PMID: 21578899
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.  A second ortho­rhom­bic polymorph of (Z)-3-(9-anthr­yl)-1-(2-thien­yl)prop-2-en-1-one1  
The title heteroaryl chalcone, C21H14OS, is a second ortho­rhom­bic polymorph which crystallizes in the space group P212121. The structure was previously reported [Fun et al. (2009 ▶). Acta Cryst. E65, o2168-o2169] in the space group Pna21. The bond distances and angles are similar in both structures. In contrast, the overall crystal packing is different from that in the first ortho­rhom­bic Pna21 polymorph in which mol­ecules were stacked into columns along the b axis and the thio­phene units of two adjacent columns were stacked in a head to tail fashion. In the present polymorph, mol­ecules are found to dimerize through a weak S⋯S inter­action [3.6513 (7) Å] and these dimers are arranged into sheets parallel to the bc plane. There are no classical hydrogen bonds in the packing which features short C⋯O [3.2832 (2)–3.6251 (9) Å], C⋯S [3.4879 (17)–3.6251 (19) Å] and S⋯O [2.9948 (16) Å] contacts, together with C—H⋯π inter­actions. Similar contacts were found in the other polymorph.
PMCID: PMC2979950  PMID: 21579743
5.  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
6.  An ortho­rhom­bic polymorph of pyrazino­[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile 
The title compound, C16H6N6, is a polymorph of the previously reported structure [Kozlov & Goldberg (2008 ▶). Acta Cryst. C64, o498–o501]. Unlike the previously reported monoclinic polymorph (space group P21/c, Z = 8), the title compound reveals ortho­rhom­bic symmetry (space group Pnma, Z = 4). The mol­ecule shows crystallographic mirror symmetry, while the previously reported structure exhibits two independent mol­ecules per asymmetric unit. In the title compound, adjacent mol­ecules are essentially parallel along the c axis and tend to be vertical along the b axis with dihedral angles of 72.02 (6)°. However, in the reported polymorph, the entire crystal structure shows an anti­parallel arrangement of adjacent columns related by inversion centers and the two independent mol­ecules are nearly parallel with a dihedral angle of 2.48 (6)°.
PMCID: PMC3238907  PMID: 22199760
7.  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
8.  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
9.  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
10.  Dicyclo­hexyl­ammonium thio­cyanate: monoclinic polymorph 
The title salt, C12H24N+·NCS−, represents a monoclinic polymorph of the previously reported ortho­rhom­bic form [Khawar Rauf et al. (2008 ▶). Acta Cryst. E64, o366]. Two independent formula units comprise the asymmetric unit with the major difference in their mol­ecular structures relating to the relative dispositions of the cyclo­hexyl rings [dihedral angles = 79.88 (6) and 67.72 (5)°]. Further, the independent anions form distinctive patterns of hydrogen-bonding inter­actions, i.e. 2 × N—H⋯N versus N—H⋯N and N—H⋯S. The resulting supra­molecular architecture is a supra­molecular chain along the c axis based on a square-wave topology.
PMCID: PMC3247581  PMID: 22219886
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.  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
13.  A second polymorph of bis­(triphenyl-λ5-phosphanyl­idene)ammonium chloride–boric acid adduct 
The title crystal structure is a new triclinic polymorph of [(Ph3P)2N]Cl·(B(OH)3) or C36H30NP2 +·Cl−·BH3O3. The crystal structure of the ortho­rhom­bic polymorph was reported by [Andrews et al. (1983 ▶). Acta Cryst. C39, 880–882]. In the crystal, the [(Ph3P)2N]+ cations have no significant contacts to the chloride ions nor to the boric acid mol­ecules. This is indicated by the P—N—P angle of 137.28 (8)°, which is in the expected range for a free [(Ph3P)2N]+ cation. The boric acid mol­ecules form inversion dimers via pairs of O—H⋯O hydrogen bonds, and each boric acid mol­ecule forms two additional O—H⋯Cl hydrogen bonds to one chloride anion. These entities fill channels, created by the [(Ph3P)2N]+ cations, along the c-axis direction.
PMCID: PMC3884452  PMID: 24427065
14.  An ortho­rhom­bic polymorph of N 1,N 4-diphenyl-3,6-bis­(phenyl­imino)­cyclo­hexa-1,4-diene-1,4-di­amine 
A new ortho­rhom­bic polymorph of the title compound, C30H24N4, with a density of 1.315 Mg m−3, has been obtained. The mol­ecule is centrosymmetric with the centroid of the cyclo­hexa-1,4-diene ring located on an inversion center. The two unique benzene rings are almost perpendicular to each other [dihedral angle = 86.70 (6)°] and are oriented at dihedral angles of 30.79 (5) and 68.07 (5)° with respect to the central cyclo­hexa­diene ring. In the crystal, π–π stacking is observed between the central cyclo­hexa-1,4-diene-1,4-di­amine unit and a phenyl ring of a neighboring mol­ecule [centroid–centroid distance = 3.7043 (7) Å]. The crystal structure of the triclinic polymorph [Ohno et al. (2014 ▶). Acta Cryst. E70, o303–o304] showed chains running along the b-axis direction through weak C—H⋯π inter­actions.
PMCID: PMC3998558  PMID: 24826185
15.  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
16.  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
17.  An ortho­rhom­bic polymorph of 1-[(ferrocen­yl)(hydr­oxy)meth­yl]-1,2-dicarba-closo-dodecaborane 
An ortho­rhom­bic polymorph of the title compound, [Fe(C5H5)(C8H17B10O)] or C13H22B10FeO, is described here in addition to the known monoclinic polymorph [Crundwell et al. (1999 ▶). Acta Cryst. C55, IUC9900087]. The asymmetric unit contains four independent mol­ecules with Ccage–Ccage distances of 1.636 (16)–1.700 (16) Å, and with the methyl­hydr­oxy groups disordered over two positions in each mol­ecule [occupancy ratios 0.80 (2):0.20 (2), 0.59 (3):0.41 (3), 0.60 (2):0.40 (2) and 0.793 (17):0.207 (17)].
PMCID: PMC2980021  PMID: 21579923
18.  5-(Prop-2-yn-1-yl)-5H-dibenzo[b,f]azepine: ortho­rhom­bic polymorph 
In the title ortho­rhom­bic polymorph (space group Iba2), C17H13N, the dihedral angle between the benzene rings is 55.99 (10)° and the azepine ring adopts a boat conformation. In the crystal, mol­ecules are linked by C—H⋯π contacts. The previously-reported polymorph [Yousuf et al. (2012 ▶). Acta Cryst. E68, o1101] crystallizes in the monoclinic system (space group P21/c) with two mol­ecules in the asymmetric unit.
PMCID: PMC3588308  PMID: 23476404
19.  From small structural modifications to adjustment of structurally dependent properties: 1-methyl-3,5-bis­[(E)-2-thienyl­idene]-4-piperidone and 3,5-bis­[(E)-5-bromo-2-thienyl­idene]-1-methyl-4-piperidone 
The mol­ecules of the title compounds, C16H15NOS2, (I), and C16H13Br2NOS2, (II), are E,E-isomers and consist of an extensive conjugated system, which determines their mol­ecular geometries. Compound (I) crystallizes in the monoclinic space group P21/c. It has one thio­phene ring disordered over two positions, with a minor component contribution of 0.100 (3). Compound (II) crystallizes in the noncentrosymmetric ortho­rhom­bic space group Pca21 with two independent mol­ecules in the unit cell. These mol­ecules are related by a noncrystallographic pseudo-inversion center and possess very similar geometries. The crystal packings of (I) and (II) have a topologically common structural motif, viz. stacks along the b axis, in which the mol­ecules are bound by weak C—H⋯O hydrogen bonds. The noncentrosymmetric packing of (II) is governed by attractive inter­molecular Br⋯Br and Br⋯N inter­actions, which are also responsible for the very high density of (II) (1.861 Mg m−3).
PMCID: PMC2665971  PMID: 19346612
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.  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
22.  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
23.  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
24.  Naphthalene-1,8-dicarb­oxy­lic anhydride: a monoclinic polymorph 
A new type of naphthalene-1,8-dicarb­oxy­lic anhydride, C12H6O3, was synthesized hydro­thermally. Unlike the two previously reported polymorphs, which crystallize in the ortho­rhom­bic space groups P212121 [Shok et al. (1971). Kristallografiya, 16, 500–502; Grigor’eva & Chetkina (1975). Kristallografiya, 20, 1289–1290] and Pbca [Shok & Gol’der (1970). Zh. Strukt. Khim. 11, 939–940], this present structure crystallizes in the monoclinic space group P21 /c. In this structure, the planar [total puckering amplitude Q = 0.0362 (15)] mol­ecules lie parallel to each other along the a axis.
PMCID: PMC2983304  PMID: 21587596
25.  A new polymorph of catena-poly[[tri­aqua­cadmium(II)]-μ2-pyrazine-2,3-dicarboxyl­ato] 
The title complex, [Cd(C6H2N2O4)(H2O)3]n, is a new monoclinic polymorph. The ortho­rhom­bic form has previously been reported [Ma et al. (2006 ▶). Acta Cryst. E62, m2528–m2529]. The Cd—N and Cd—O bond lengths range from 2.265 (3) to 2.333 (3) Å; a weak Cd—O inter­action is also present, the inter­atomic distance being 2.658 (4) Å. The CdII ions, which have a distorted penta­gonal-bipyramidal geometry, are bridged by pyrazine-2,3-dicarboxyl­ato ligands, forming a zigzag chain structure. The chains are connected by O—H⋯O hydrogen bonds into a three-dimensional framework.
PMCID: PMC2977175  PMID: 21583415

Results 1-25 (322306)