New hydrophobic L‐amino acid salts: maleates of L‐leucine,L‐isoleucine and L‐norvaline

Crystals of maleates of three amino acids with hydrophobic side chains [L‐leucenium hydrogen maleate, C₆H₁₄NO₂⁺·C₄H₃O₄⁻, (I), L‐isoleucenium hydrogen maleate hemihydrate, C₆H₁₄NO₂⁺·C₄H₃O₄⁻·0.5H₂O, (II), and L‐norvalinium hydrogen maleate–L‐norvaline (1/1), C₅H₁₁NO₂⁺·C₄H₃O₄⁻·C₅H₁₂NO₂, (III)], were obtained. The new structures contain C₂²(12) chains, or variants thereof, that are a common feature in the crystal structures of amino acid maleates. The L‐leucenium salt is remarkable due to a large number of symmetrically non‐equivalent units (Z′ = 3). The L‐isoleucenium salt is a hydrate despite the fact that L‐isoleucine is a nonpolar hydrophobic amino acid (previously known amino acid maleates formed hydrates only with lysine and histidine, which are polar and hydrophilic). The L‐norvalinium salt provides the first example where the dimeric cation L‐Nva...L‐NvaH⁺ was observed. All three compounds have layered noncentrosymmetric structures. Preliminary tests have shown the presence of the second harmonic generation (SGH) effect for all three compounds.     

Electronic structure,novel synthesis,and O—H...O and C—H...O interactions in two 6‐oxopiperidine‐2‐carboxylic acid derivatives

Molecules of (S)‐6‐oxo‐1‐(thiophen‐2‐ylmethyl)piperidine‐2‐carboxylic acid, C₁₁H₁₃NO₃S, crystallize as single enantiomers in the space group P2₁ and the thiophene ring is disordered over two positions, while (S)‐6‐oxo‐1‐(thiophen‐3‐ylmethyl)piperidine‐2‐carboxylic acid, C₁₁H₁₃NO₃S, crystallizes as a single enantiomer in the space group P2₁2₁2₁. Their absolute configurations were confirmed by anomalous dispersion effects in diffraction measurements on the crystals. The molecules of each compound are linked by a combination of strong O—H...O hydrogen bonds and weak C—H...O interactions, resulting in two‐ and three‐dimensional networks, respectively, in the crystal structures.     

Synthesis of poly(succinimide) by bulk polycondensation of L‐aspartic acid with an acid catalyst

The bulk polycondensation of L ‐aspartic acid (ASP) with an acid catalyst under batch and continuous conditions was established as a preparative method for producing poly(succinimide) (PSI). Although sulfuric acid, p‐toluenesulfonic acid, and methanesulfonic acid were effective at producing PSI in a high conversion of ASP, o‐phosphoric acid was the most suitable catalyst for yielding PSI with a high weight‐average molecular weight (M_w) in a quantitative conversion; that is, the M_w value was 24,000. For the continuous process using a twin‐screw extruder at 3.0 kg · h⁻¹ of the ASP feed rate, the conversion was greater than 99%, and the M_w value was 23,000 for the polycondensation with 10 wt % o‐phosphoric acid at 260°C. Sodium polyaspartate (PASP‐Na) originating from the acid‐catalyzed polycondensation exhibited high biodegradability and calcium‐ion‐chelating ability. The total organic carbon value was 86 ∼ 88%, and 100 g of PASP‐Na chelated with 5.5 ∼ 5.6 g of calcium ion, which was similar to the value for PASP‐Na from the acid‐catalyzed polycondensation with a mixed solvent © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 117–122, 2000     

Two polymorphs of 5‐cyclohexyl‐5‐ethylbarbituric acid and their packing relationships with other barbiturates

Polymorph (Ia) (m.p. 474 K) of the title compound, C₁₂H₁₈N₂O₃, displays an N—H...O=C hydrogen‐bonded layer structure which contains R₆⁶(28) rings connecting six molecules, as well as R₂²(8) rings linking two molecules. The 3‐connected hydrogen‐bonded net resulting from these interactions has the hcb topology. Form (Ib) (m.p. 471 K) displays N—H...O=C hydrogen‐bonded looped chains in which neighbouring molecules are linked to one another by two different R₂²(8) rings. Polymorph (Ia) is isostructural with the previously reported form II of 5‐(2‐bromoallyl)‐5‐isopropylbarbituric acid (noctal) and polymorph (Ib) is isostructural with the known crystal structures of four other barbiturates.     

Selective transport of D,L‐tryptophan through poly(L‐glutamic acid) membranes

Permeability coefficients of D ‐ and L ‐tryptophan (D ‐, L ‐Trp) were estimated for poly(L ‐glutamic acid) (PLG) membranes immersed in aqueous ethanol. D ‐tryptophan was selectively transported (the maximum permeability ratio was 2.6) depending on the amount and the species of crosslinking agent, and on the composition of immersing solvent. It is suggested that hydrogen bonding between uncharged permeates and carboxyl and/or amide groups of PLG is an essential factor for the selective transport. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1035–1041, 1999     

Three polymorphs of an inclusion compound of 2,2′‐(disulfanediyl)dibenzoic acid and trimethylamine

Polymorphism is the ability of a solid material to exist in more than one form or crystal structure and this is of interest in the fields of crystal engineering and solid‐state chemistry. 2,2′‐(Disulfanediyl)dibenzoic acid (also called 2,2′‐dithiosalicylic acid, DTSA) is able to form different hydrogen bonds using its carboxyl groups. The central bridging S atoms allow the two terminal arene rings to rotate freely to generate various hydrogen‐bonded linking modes. DTSA can act as a potential host molecule with suitable guest molecules to develop new inclusion compounds. We report here the crystal structures of three new polymorphs of the inclusion compound of DTSA and trimethylamine, namely trimethylazanium 2‐[(2‐carboxyphenyl)disulfanyl]benzoate 2,2′‐(disulfanediyl)dibenzoic acid monosolvate, C₃H₁₀N⁺·C₁₄H₉O₄S₂⁻·C₁₄H₁₀O₄S₂, (1), tetrakis(trimethylazanium) bis{2‐[(2‐carboxyphenyl)disulfanyl]benzoate} 2,2′‐(disulfanediyl)dibenzoate 2,2′‐(disulfanediyl)dibenzoic acid monosolvate, 4C₃H₁₀N⁺·2C₁₄H₉O₄S₂⁻·C₁₄H₈O₄S₂²⁻·C₁₄H₁₀O₄S₂, (2), and trimethylazanium 2‐[(2‐carboxyphenyl)disulfanyl]benzoate, C₃H₁₀N⁺·C₁₄H₉O₄S₂⁻, (3). In the three polymorphs, DTSA utilizes its carboxyl groups to form conventional O—H…O hydrogen bonds to generate different host lattices. The central N atoms of the guest amine molecules accept H atoms from DTSA molecules to give the corresponding cations, which act as counter‐ions to produce the stable crystal structures via N—H…O hydrogen bonding between the host acid and the guest molecule. It is noticeable that although these three compounds are composed of the same components, the final crystal structures are totally different due to the various configurations of the host acid, the number of guest molecules and the inducer (i.e. ancillary experimental acid).     

Two different products from the reaction of 1‐aryl‐5‐chloro‐3‐methyl‐1H‐pyrazole‐4‐carbaldehyde with cyclohexylamine when the aryl substituent is phenyl or pyridin‐2‐yl: hydrogen‐bonded sheets versus dimers

Cyclohexylamine reacts with 5‐chloro‐3‐methyl‐1‐(pyridin‐2‐yl)‐1H‐pyrazole‐4‐carbaldehyde to give 5‐cyclohexylamino‐3‐methyl‐1‐(pyridin‐2‐yl)‐1H‐pyrazole‐4‐carbaldehyde, C₁₆H₂₀N₄O, (I), formed by nucleophilic substitution, but with 5‐chloro‐3‐methyl‐1‐phenyl‐1H‐pyrazole‐4‐carbaldehyde the product is (Z)‐4‐[(cyclohexylamino)methylidene]‐3‐methyl‐1‐phenyl‐1H‐pyrazol‐5(4H)‐one, C₁₇H₂₁N₃O, (II), formed by condensation followed by hydrolysis. Compound (II) crystallizes with Z′ = 2, and in one of the two independent molecular types the cyclohexylamine unit is disordered over two sets of atomic sites having occupancies of 0.65 (3) and 0.35 (3). The vinylogous amide portion in each compound shows evidence of electronic polarization, such that in each the O atom carries a partial negative charge and the N atom of the cyclohexylamine portion carries a partial positive charge. The molecules of (I) contain an intramolecular N—H...N hydrogen bond, and they are linked by C—H...O hydrogen bonds to form sheets. Each of the two independent molecules of (II) contains an intramolecular N—H...O hydrogen bond and each molecular type forms a centrosymmetric dimer containing one R₂²(4) ring and two inversion‐related S(6) rings.     

The Polymorphs of L‐Phenylalanine

The solid‐state structure of the amino acid phenylalanine (Phe) offers a potential key to understanding the behavior of a large class of important aromatic compounds. Obtaining good single crystals is, however, notoriously difficult. The structure of the common polymorph of Phe, form I, was first reported by Weissbuch et al. (as D ‐Phe) in 1990, but the correctness of the published C2 unit cell with two disordered molecules in the asymmetric unit was later questioned and other space groups suggested. The identity of form I of L ‐Phe is here established to be P2₁ with Z′=4, based on data from a well‐diffracting single crystal grown from an acetic acid solution of the amino acid. A second new polymorph, form IV, together with the two recently described forms II and III provide unprecedented information on the structural complexity of this essential amino acid. It is furthermore documented that the racemate, dl ‐Phe, does not grow proper single crystals.     

Hydrogen‐bonded bilayers in 7‐benzyl‐3‐tert‐butyl‐1‐phenyl‐4,5,6,7‐tetrahydro‐1H‐spiro[pyrazolo[3,4‐b]pyridine‐5,2′‐indan]‐1′,3′‐dione

In the title compound, C₃₁H₂₉N₃O₂, the reduced pyridine ring adopts a conformation intermediate between the envelope and half‐chair forms. The aryl rings of the benzyl and phenyl substituents are nearly parallel and overlap, indicative of an intramolecular π–π stacking interaction. A combination of two C—H...O hydrogen bonds and one C—H...N hydrogen bond links the molecules into a bilayer having tert‐butyl groups on both faces.<!?tpb=19.5pt>     

Cyclic heterotetrameric and low‐dimensional hydrogen‐bonded polymeric structures in the morpholinium salts of ring‐substituted benzoic acid analogues

The morpholinium (tetrahydro‐2H‐1,4‐oxazin‐4‐ium) cation has been used as a counter‐ion in both inorganic and organic salt formation and particularly in metal complex stabilization. To examine the influence of interactive substituent groups in the aromatic rings of benzoic acids upon secondary structure generation, the anhydrous salts of morpholine with salicylic acid, C₄H₁₀NO⁺·C₇H₅O₃⁻, (I), 3,5‐dinitrosalicylic acid, C₄H₁₀NO⁺·C₇H₃N₂O₇⁻, (II), 3,5‐dinitrobenzoic acid, C₄H₁₀NO⁺·C₇H₃N₂O₆⁻, (III), and 4‐nitroanthranilic acid, C₄H₁₀NO⁺·C₇H₅N₂O₄⁻, (IV), have been prepared and their hydrogen‐bonded crystal structures are described. In the crystal structures of (I), (III) and (IV), the cations and anions are linked by moderately strong N—H…O_carboxyl hydrogen bonds, but the secondary structure propagation differs among the three, viz. one‐dimensional chains extending along [010] in (I), a discrete cyclic heterotetramer in (III), and in (IV), a heterotetramer with amine N—H…O hydrogen‐bond extensions along b, giving a two‐layered ribbon structure. With the heterotetramers in both (III) and (IV), the ion pairs are linked though inversion‐related N—H…O_carboxylate hydrogen bonds, giving cyclic R₄⁴(12) motifs. With (II), in which the anion is a phenolate rather than a carboxylate, the stronger assocation is through a symmetric lateral three‐centre cyclic R₁²(6) N—H…(O,O′) hydrogen‐bonding linkage involving the phenolate and nitro O‐atom acceptors of the anion, with extension through a weaker O—H…O_carboxyl hydrogen bond. This results in a one‐dimensional chain structure extending along [100]. In the structures of two of the salts [i.e. (II) and (IV)], there are also π–π ring interactions, with ring‐centroid separations of 3.5516 (9) and 3.7700 (9) Å in (II), and 3.7340 (9) Å in (IV).     

Heterocyclic tautomerism: reassignment of two crystal structures of 2‐amino‐1,3‐thiazolidin‐4‐one derivatives

The structures of 5‐(2‐hydroxyethyl)‐2‐[(pyridin‐2‐yl)amino]‐1,3‐thiazolidin‐4‐one, C₁₀H₁₁N₃O₂S, (I), and ethyl 4‐[(4‐oxo‐1,3‐thiazolidin‐2‐yl)amino]benzoate, C₁₂H₁₂N₂O₃S, (II), which are identical to the entries with refcodes GACXOZ [Váňa et al. (2009). J. Heterocycl. Chem. 46 , 635–639] and HEGLUC [Behbehani & Ibrahim (2012). Molecules, 17 , 6362–6385], respectively, in the Cambridge Structural Database [Allen (2002). Acta Cryst. B 58 , 380–388], have been redetermined at 130 K. This structural study shows that both investigated compounds exist in their crystal structures as the tautomer with the carbonyl–imine group in the five‐membered heterocyclic ring and an exocyclic amine N atom, rather than the previously reported tautomer with a secondary amide group and an exocyclic imine N atom. The physicochemical and spectroscopic data of the two investigated compounds are the same as those of GACXOZ and HEGLUC, respectively. In the thiazolidin‐4‐one system of (I), the S and chiral C atoms, along with the hydroxyethyl group, are disordered. The thiazolidin‐4‐one fragment takes up two alternative locations in the crystal structure, which allows the molecule to adopt R and S configurations. The occupancy factors of the disordered atoms are 0.883 (2) (for the R configuration) and 0.117 (2) (for the S configuration). In (I), the main factor that determines the crystal packing is a system of hydrogen bonds, involving both strong N—H...N and O—H...O and weak C—H...O hydrogen bonds, linking the molecules into a three‐dimensional hydrogen‐bond network. On the other hand, in (II), the molecules are linked via N—H...O hydrogen bonds into chains.     

Unusual hydrogen bonding in L‐cysteine hydrogen fluoride

L‐Cysteine hydrogen fluoride, or bis(L‐cysteinium) difluoride–L‐cysteine–hydrogen fluoride (1/1/1), 2C₃H₈NO₂S⁺·2F⁻·C₃H₇NO₂S·HF or L‐Cys⁺(L‐Cys...L‐Cys⁺)F⁻(F⁻...H—F), provides the first example of a structure with cations of the `triglycine sulfate' type, i.e.A⁺(A...A⁺) (where A and A⁺ are the zwitterionic and cationic states of an amino acid, respectively), without a doubly charged counter‐ion. The salt crystallizes in the monoclinic system with the space group P2₁. The dimeric (L‐Cys...L‐Cys⁺) cation and the dimeric (F⁻...H—F) anion are formed via strong O—H...O or F—H...F hydrogen bonds, respectively, with very short O...O [2.4438 (19) Å] and F...F distances [2.2676 (17) Å]. The F...F distance is significantly shorter than in solid hydrogen fluoride. Additionally, there is another very short hydrogen bond, of O—H...F type, formed by a L‐cysteinium cation and a fluoride ion. The corresponding O...F distance of 2.3412 (19) Å seems to be the shortest among O—H...F and F—H...O hydrogen bonds known to date. The single‐crystal X‐ray diffraction study was complemented by IR spectroscopy. Of special interest was the spectral region of vibrations related to the above‐mentioned hydrogen bonds.     

The three‐dimensional hydrogen‐bonded structures in the ammonium and sodium salt hydrates of 4‐aminophenylarsonic acid

The structures of two hydrated salts of 4‐aminophenylarsonic acid (p‐arsanilic acid), namely ammonium 4‐aminophenylarsonate monohydrate, NH₄⁺·C₆H₇AsNO₃⁻·H₂O, (I), and the one‐dimensional coordination polymer catena‐poly[[(4‐aminophenylarsonato‐κO)diaquasodium]‐μ‐aqua], [Na(C₆H₇AsNO₃)(H₂O)₃]_n, (II), have been determined. In the structure of the ammonium salt, (I), the ammonium cations, arsonate anions and water molecules interact through inter‐species N—H...O and arsonate and water O—H...O hydrogen bonds, giving the common two‐dimensional layers lying parallel to (010). These layers are extended into three dimensions through bridging hydrogen‐bonding interactions involving the para‐amine group acting both as a donor and an acceptor. In the structure of the sodium salt, (II), the Na⁺ cation is coordinated by five O‐atom donors, one from a single monodentate arsonate ligand, two from monodentate water molecules and two from bridging water molecules, giving a very distorted square‐pyramidal coordination environment. The water bridges generate one‐dimensional chains extending along c and extensive interchain O—H...O and N—H...O hydrogen‐bonding interactions link these chains, giving an overall three‐dimensional structure. The two structures reported here are the first reported examples of salts of p‐arsanilic acid.     

Crystal structures and properties of two hydrated conglomerate forms of the heart‐rate‐lowering agent ivabradine hydrochloride

Ivabradine hydrochloride (IVA‐HCl) (systematic name: {[3,4‐dimethoxybicyclo[4.2.0]octa‐1(6),2,4‐trien‐7‐yl]methyl}[3‐(7,8‐dimethoxy‐2‐oxo‐2,3,4,5‐tetrahydro‐1H‐3‐benzazepin‐3‐yl)propyl]methylazanium), is a novel medication used for the symptomatic management of stable angina pectoris. In many recent patents, it has been claimed to exist in a very large number of polymorphic, hydrated and solvated phases, although no detailed analysis of the structural features of these forms has been published to date. Here, we have successfully crystallized the tetrahydrate form of IVA‐HCl (form β), C₂₇H₃₇N₂O₅⁺·Cl^?·4H₂O, and elucidated its structure for the first time. Simultaneously, a new crystal form of IVA‐HCl, i.e. the hemihydrate (form II), C₂₇H₃₇N₂O₅⁺·Cl^?·0.5H₂O, was discovered. Its crystal structure was also accurately determined and compared to that of the tetrahydrate form. While the tetrahydrate form of IVA‐HCl crystallized in the orthorhombic space group P2₁2₁2₁, the new form (hemihydrate) was solved in the monoclinic space group P2₁. Detailed conformational and packing comparisons between the two forms have allowed us to understand the role of water in the crystal assembly of this hydrochloride salt. The stabilities of the two forms were compared theoretically by calculating the binding energy of the water in the crystal lattice using differential scanning calorimetry (DSC). The stability experiments show that the tetrahydrate is stable under high‐humidity conditions, while the hemihydrate is stable under high‐temperature conditions.     

Two polymorphs of N‐(2‐methoxyphenyl)‐4‐oxo‐4H‐chromone‐3‐carboxamide

The title compound, C₁₇H₁₃NO₄, crystallizes in two polymorphic forms, each with two molecules in the asymmetric unit and in the monoclinic space group P2₁/c. All of the molecules have intramolecular hydrogen bonds involving the amide group. The amide N atoms act as donors to the carbonyl group of the pyrone and also to the methoxy group of the benzene ring. The carbonyl O atom of the amide group acts as an acceptor of the β and β′ C atoms belonging to the aromatic rings. These intramolecular hydrogen bonds have a profound effect on the molecular conformation. In one polymorph, the molecules in the asymmetric unit are linked to form dimers by weak C—H...O interactions. In the other, the molecules in the asymmetric unit are linked by a single weak C—H...O hydrogen bond. Two of these units are linked to form centrosymmetric tetramers by a second weak C—H...O interaction. Further interactions of this type link the molecules into chains, so forming a three‐dimensional network. These interactions in both polymorphs are supplemented by π–π interactions between the chromone rings and between the chromone and methoxyphenyl rings.     

Synthesis and characterization of two one‐dimensional CdII coordination polymers (CPs) with 5‐amino‐2,4,6‐tribromoisophthalic acid and flexible N‐donor bipyridyl ligands

The bromo‐substituted aromatic dicarboxylic acid 5‐amino‐2,4,6‐tribromoisophthalic acid (H₂ATBIP) was used to assemble with Cd^II ions in the presence of the N‐donor flexible bipyridyl ligands 3,3′‐(diazene‐1,2‐diyl)dipyridine (mzpy) and 1,3‐bis(pyridin‐3‐ylmethyl)urea (3bpmu), leading to the formation of two chain coordination polymers by adopting solution methods, namely, catena‐poly[[[triaqua(5‐amino‐2,4,6‐tribromoisophthalato‐κO)cadmium(II)]‐μ‐3,3′‐(diazene‐1,2‐diyl)dipyridine‐κ²N¹:N^1′] dihydrate], {[Cd(C₈H₂Br₃NO₄)(C₁₀H₈N₄)(H₂O)₃]·2H₂O}_n or {[Cd(ATBIP)(mzpy)(H₂O)₃]·2H₂O}_n, ( 1 ), and catena‐poly[[[tetraaquacadmium(II)]‐μ‐1,3‐bis(pyridin‐3‐ylmethyl)urea‐κ²N¹:N^1′‐[diaquabis(5‐amino‐2,4,6‐tribromoisophthalato‐κO)cadmium(II)]‐μ‐1,3‐bis(pyridin‐3‐ylmethyl)urea‐κ²N¹:N^1′] octahydrate], {[Cd(C₈H₂Br₃NO₄)(C₁₂H₁₂N₄O)(H₂O)₃]·4H₂O}_n or {[Cd(ATBIP)(3bpmu)(H₂O)₃]·4H₂O}_n, ( 2 ). Both complexes were characterized by FT–IR spectroscopic analysis, thermogravimetric analysis (TGA), solid‐state diffuse reflectance UV–Vis spectroscopic analysis, and single‐crystal and powder X‐ray diffraction analysis (PXRD). The mzpy and 3bpmu ligands bridge the Cd^II metal centres in ( 1 ) and ( 2 ) into one‐dimensional chains, and the ATBIP²⁻ ligands show a monodentate coordination to the Cd^II centres in both coordination polymers. A discrete water tetramer exists in ( 1 ). Within the chains of ( 1 ) and ( 2 ), there are halogen bonds between adjacent ATBIP²⁻ and mzpy or 3bpmu ligands, as well as hydrogen bonds between the ATBIP²⁻ ligands and the coordinated water molecules. With the aid of weak interactions, the structures of ( 1 ) and ( 2 ) are further extended into three‐dimensional supramolecular networks. An analysis of the solid‐state diffuse reflectance UV–Vis spectra of ( 1 ) and ( 2 ) indicates that a wide indirect band gap exists in both complexes. Complexes ( 1 ) and ( 2 ) exhibit irreversible and reversible dehydration–rehydration behaviours, respectively, and the solid‐state fluorescence properties of both complexes have been studied.     

The effect of the coordination orientation of N atoms on polymeric structures: synthesis and characterization of one‐ and two‐dimensional CuII coordination polymers based on 4‐amino‐3‐(pyridin‐2‐yl)‐5‐[(pyridin‐3‐ylmethyl)sulfanyl]‐1,2,4‐triazole

Three new one‐ (1D) and two‐dimensional (2D) Cu^II coordination polymers, namely poly[[bis{μ₂‐4‐amino‐3‐(pyridin‐2‐yl)‐5‐[(pyridin‐3‐ylmethyl)sulfanyl]‐1,2,4‐triazole}copper(II)] bis(methanesulfonate) tetrahydrate], {[Cu(C₁₃H₁₂N₅S)₂](CH₃SO₃)₂·4H₂O}_n ( 1 ), catena‐poly[[copper(II)‐bis{μ₂‐4‐amino‐3‐(pyridin‐2‐yl)‐5‐[(pyridin‐4‐ylmethyl)sulfanyl]‐1,2,4‐triazole}] dinitrate methanol disolvate], {[Cu(C₁₃H₁₂N₅S)₂](NO₃)₂·2CH₃OH}_n ( 2 ), and catena‐poly[[copper(II)‐bis{μ₂‐4‐amino‐3‐(pyridin‐2‐yl)‐5‐[(pyridin‐4‐ylmethyl)sulfanyl]‐1,2,4‐triazole}] bis(perchlorate) monohydrate], {[Cu(C₁₃H₁₂N₅S)₂](ClO₄)₂·H₂O}_n ( 3 ), were obtained from 4‐amino‐3‐(pyridin‐2‐yl)‐5‐[(pyridin‐3‐ylmethyl)sulfanyl]‐1,2,4‐triazole with pyridin‐3‐yl terminal groups and from 4‐amino‐3‐(pyridin‐2‐yl)‐5‐[(pyridin‐4‐ylmethyl)sulfanyl]‐1,2,4‐triazole with pyridin‐4‐yl terminal groups. Compound 1 displays a 2D net‐like structure. The 2D layers are further linked through hydrogen bonds between methanesulfonate anions and amino groups on the framework and guest H₂O molecules in the lattice to form a three‐dimensional (3D) structure. Compound 2 and 3 exhibit 1D chain structures, in which the complicated hydrogen‐bonding interactions play an important role in the formation of the 3D network. These experimental results indicate that the coordination orientation of the heteroatoms on the ligands has a great influence on the polymeric structures. Moreover, the selection of different counter‐anions, together with the inclusion of different guest solvent molecules, would also have a great effect on the hydrogen‐bonding systems in the crystal structures.     

The structural properties of a noncentrosymmetric polymorph of 4‐aminobenzoic acid

The crystal structure of a polymorph of 4‐aminobenzoic acid (PABA), C₇H₇NO₂, at 100 K is noncentrosymmetric, as opposed to centrosymmetric in the structures of the other known polymorphs. The two crystallographically independent PABA molecules form pseudocentrosymmetric O—H...O hydrogen‐bonded dimers that are further linked by N—H...O hydrogen bonds into a three‐dimensional network. The benzene rings stack in the b direction. The CO₂ moieties are bent out slightly from the benzene ring plane.     

2‐Amino‐5‐iodopyridinium bromide hemihydrate and 2‐amino‐5‐iodopyridinium chloride monohydrate

The hydrobromide and hydrochloride salts of 2‐amino‐5‐iodopyridine were prepared from aqueous solutions. The hydrobromide salt, C₅H₆IN₂⁺·Br⁻·0.5H₂O, crystallizes as a hemihydrate, and exhibits hydrogen bonding and π‐stacking which stabilize the crystal structure. The hydrochloride salt, C₅H₆IN₂⁺·Cl⁻·H₂O·0.375HCl, crystallized as the hydrate and exhibits similar hydrogen bonding and π‐stacking in the lattice. The most interesting feature of the hydrochloride salt is the presence of an additional fractional HCl molecule which introduces disorder in the location of the water molecule. The additional proton from the fractional HCl molecule is accounted for by the presence of a partial hydronium ion on one of the water sites.     

Concomitant polymorphs of 1,3‐bis(3‐fluorophenyl)urea

Hydrogen bonding between urea functionalities is a common structural motif employed in crystal‐engineering studies. Crystallization of 1,3‐bis(3‐fluorophenyl)urea, C₁₃H₁₀F₂N₂O, from many solvents yielded concomitant mixtures of at least two polymorphs. In the monoclinic form, one‐dimensional chains of hydrogen‐bonded urea molecules align in an antiparallel orientation, as is typical of many diphenylureas. In the orthorhombic form, one‐dimensional chains of hydrogen‐bonded urea molecules have a parallel orientation rarely observed in symmetrically substituted diphenylureas.