Hydrogen‐bonding interactions in (3,4‐dimethoxyphenyl)acetic acid monohydrate

The crystal structure of the title compound, C₁₀H₁₂O₄·H₂O, consists of (3,4‐dimethoxyphenyl)acetic acid and water molecules linked by O—H...O hydrogen bonds to form cyclic structures with graph‐set motifs R₁²(5) and R₄⁴(12). These hydrogen‐bond patterns result in a three‐dimensional network with graph‐set motifs R₄⁴(20) and R₄⁴(22), and the formation of larger macrocycles, respectively. The C—C bond lengths and the endocyclic angles of the benzene ring show a noticeable asymmetry, which is connected with the charge‐transfer interaction of the carboxyl or methoxy groups and the benzene ring. The title compound is one of the simple carboxylic acid systems that form hydrates. Thus, the significance of this study lies in the analysis of the interactions in this structure and the aggregations occurring via hydrogen bonds in two crystalline forms of (3,4‐dimethoxyphenyl)acetic acid, namely the present hydrate and the anhydrous form [Chopra, Choudhury & Guru Row (2003). Acta Cryst. E 59 , o433–o434]. The correlation between the IR spectrum of this compound and its structural data are also discussed.     

Hydrogen‐bonding features in the 1:2 adduct of 4‐aminobenzoic acid and l‐proline

The title adduct, 4‐aminobenzoic acid–l ‐proline–water (1/2/1), C₇H₇NO₂·2C₅H₉NO₂·H₂O, contains two independent proline chains with a C(5) motif, each of the head‐to‐tail type and each held together by N—H...O hydrogen bonds, propagated parallel to the b and c axes of the unit cell. Thus, the proline residues aggregate parallel to the ac plane. 4‐Aminobenzoic acid (PABA) residues are arranged on both sides of the proline aggregate and are connected through water O atoms, which act as acceptors for PABA and as hydrogen‐bond donors to the amino acids. The characteristic features of PABA, viz. twisting of the carboxyl plane from the aromatic ring and the formation of a head‐to‐tail chain motif [C(8)] along the b axis, are observed. A distinct feature of the structure is that no proton transfer occurs between proline and PABA.     

Hydrogen bonding in the bromide salts of 4‐aminobenzoic acid and 4‐aminoacetophenone

In the title compounds, 4‐carboxyanilinium bromide, C₇H₈NO₂⁺·Br⁻, (I), and 4‐acetylanilinium bromide, C₈H₁₀NO⁺·Br⁻, (II), each asymmetric unit contains a discrete cation with a protonated amino group and a halide anion. Both crystal structures are characterized by two‐dimensional hydrogen‐bonded networks. The ions in (I) are connected via N—H...Br, N—H...O and O—H...Br hydrogen bonds, with three characteristic graph‐set motifs, viz. C(8), C₂¹(4) and R₃²(8). The centrosymmetric hydrogen‐bonded R²₂(8) dimer motif characteristic of carboxylic acids is absent. The ions in (II) are connected via N—H...Br and N—H...O hydrogen bonds, with two characteristic graph‐set motifs, viz. C(8) and R₄²(8). The significance of this study lies in its illustration of the differences between the supramolecular aggregations in two similar compounds. The presence of the methyl group in (II) at the site corresponding to the hydroxyl group in (I) results in a significantly different hydrogen‐bonding arrangement.     

4‐Oxo­cyclo­hexane­carboxyl­ic acid: hydrogen bonding in the monohydrate of a δ‐keto acid

The title monohydrate, C₇H₁₀O₃·H₂O, aggregates as a complex hydrogen‐bonding network, in which the water mol­ecule accepts a hydrogen bond from the carboxyl group of one mol­ecule and donates hydrogen bonds to ketone and carboxyl Czdbnd;O functions in two additional mol­ecules, yielding a sheet‐like structure of parallel ribbons. The keto acid adopts a chiral conformation through rotation of the carboxyl group by 62.50 (15)° relative to the plane defined by its point of attachment and the ketone C and O atoms. Two C—H⋯O close contacts exist in the structure.     

Hydrogen‐bonding motifs in 4‐carboxy­phenyl­ammonium nitrate and perchlorate monohydrate,and in bis(4‐carboxy­phenyl­ammonium) sulfate

In the title compounds, C₇H₈NO₂⁺·NO₃⁻, (I), C₇H₈NO₂⁺·ClO₄⁻·H₂O, (II), and 2C₇H₈NO₂⁺·SO₄²⁻, (III), the carboxyl planes of the 4‐carboxy­phenyl­ammonium cations are twisted from the aromatic plane. A homonuclear C(8) hydrogen‐bonding motif of 4‐carboxy­phenyl­ammonium cations is observed in both (I) and (II), leading to `head‐to‐tail' layers. The cations in (III) form carboxyl group dimers, making a graph‐set motif of R₂²(8). In all the structures, anions connect the cationic layers and an infinite chain running along the c axis is observed, having the C₂²(6) graph‐set motif. Inter­estingly, in (II), the anions are connected through weak hydrogen bonds involving the water mol­ecules, leading to a graph‐set motif of R₄⁴(12). Alternate hydro­phobic and hydro­philic layers are observed in all three compounds as a result of the column‐like arrangement of the aromatic rings of the cations and the anions. Furthermore, in (I), head‐to‐tail N—H⋯O inter­actions and inter­actions linking the cations and anions form an R₆⁴(16) hydrogen‐bonding motif, resulting in a pseudo‐inversion centre at (, , 0).     

The 1:1 adduct of 4‐aminobenzoic acid with 4‐aminobenzonitrile

The 1:1 adduct of 4‐amino­benzoic acid (PABA) with 4‐am‐inobenzonitrile (PABN), C₇H₇NO₂·C₇H₆N₂, consists of a primary centrosymmetric cyclic hydrogen‐bonded PABA dimer interaction [O?O 2.640 (3) Å] peripherally linked into chains by weaker hydrogen bonds via a head‐to‐tail PABN interaction [N?N 3.179 (4) and N?O 3.062 (4) Å], and is linked between the chains by amine‐N (PABN) to amine‐N (PABA) interactions [N?N 3.233 (5) Å]. No proton transfer occurs.     

Hydrogen‐bonding and C—H⋯π interactions in ethyl 4‐acetyl‐5‐methyl‐3‐phenyl‐1H‐pyrrole‐2‐carboxylate monohydrate

In the title compound, C₁₆H₁₇NO₃·H₂O, the pyrrole ring is distorted slightly from ideal C_2v symmetry. Three strong hydrogen bonds link the substituted pyrrole and water mol­ecules to form infinite chains, in which the hydrogen bonds form rings and chain patterns. Two intermolecular C—H?π interactions maintain the internal cohesion between these chains. The molecular structure differs slightly from that of the isolated mol­ecule calculated by ab initio quantum‐mechanical calculations. In the latter model, the non‐H substituent atoms share the plane of the pyrrole ring, except for the phenyl group, which lies almost perpendicular to this plane.     

Zwitterionic 4‐piperidine­carboxyl­ic acid monohydrate

The title compound, 4‐piperidinio­carboxyl­ate (isonipecotic acid), crystallizes as a zwitterion and incorporates one water mol­ecule, i.e. C₆H₁₁NO₂·H₂O. The piperidine ring adopts a chair conformation and the α‐carboxyl­ate group is oriented in the equatorial position. The combination of the interactions between the α‐amino and α‐carboxyl­ate groups and the water mol­ecules builds a three‐dimensional assembly of hydrogen bonds.     

Hydrogen‐bonding motifs in 3‐carboxyanilinium bromide and iodide

In the title compounds, C₇H₈NO₂⁺·Br⁻, (I), and C₇H₈NO₂⁺·I⁻, (II), the asymmetric unit contains a discrete 3‐carboxyanilinium cation, with a protonated amine group, and a halide anion. The compounds are not isostructural, and the crystal structures of (I) and (II) are characterized by different two‐dimensional hydrogen‐bonded networks. The ions in (I) are connected into ladder‐like ribbons via N—H...Br hydrogen bonds, while classic cyclic O—H...O hydrogen bonds between adjacent carboxylic acid functions link adjacent ribbons to give three characteristic graph‐set motifs, viz. C₂¹(4), R₄²(8) and R₂²(8). The ions in (II) are connected via N—H...I, N—H...O and O—H...I hydrogen bonds, also with three characteristic graph‐set motifs, viz. C(7), C₂¹(4) and R₄²(18), but an O—H...O interaction is not present.     

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.     

Hydrogen‐bonding patterns in rac‐1‐acetyl‐5‐methyl‐2‐thioxoimidazolidin‐4‐one

In the title compound, C₆H₈N₂O₂S, also known as N‐acetyl‐2‐thiohydantoin–alanine, the molecules are joined by N—H...O hydrogen bonds, forming centrosymmetric R₂²(8) dimers; these dimers are linked by C—H...O interactions to form R₂²(10) rings, thus forming C₂²(10) chains that run along the [101] direction.     

Hydrogen bonding in two ammonium salts of 5‐sulfosalicylic acid: ammonium 3‐carboxy‐4‐hydroxybenzenesulfonate monohydrate and triammonium 3‐carboxy‐4‐hydroxybenzenesulfonate 3‐carboxylato‐4‐hydroxybenzenesulfonate

The structures of two ammonium salts of 3‐carboxy‐4‐hydroxybenzenesulfonic acid (5‐sulfosalicylic acid, 5‐SSA) have been determined at 200 K. In the 1:1 hydrated salt, ammonium 3‐carboxy‐4‐hydroxybenzenesulfonate monohydrate, NH₄⁺·C₇H₅O₆S⁻·H₂O, (I), the 5‐SSA⁻ monoanions give two types of head‐to‐tail laterally linked cyclic hydrogen‐bonding associations, both with graph‐set R₄⁴(20). The first involves both carboxylic acid O—H...O_water and water O—H...O_sulfonate hydrogen bonds at one end, and ammonium N—H...O_sulfonate and N—H...O_carboxy hydrogen bonds at the other. The second association is centrosymmetric, with end linkages through water O—H...O_sulfonate hydrogen bonds. These conjoined units form stacks down c and are extended into a three‐dimensional framework structure through N—H...O and water O—H...O hydrogen bonds to sulfonate O‐atom acceptors. Anhydrous triammonium 3‐carboxy‐4‐hydroxybenzenesulfonate 3‐carboxylato‐4‐hydroxybenzenesulfonate, 3NH₄⁺·C₇H₄O₆S²⁻·C₇H₅O₆S⁻, (II), is unusual, having both dianionic 5‐SSA²⁻ and monoanionic 5‐SSA⁻ species. These are linked by a carboxylic acid O—H...O hydrogen bond and, together with the three ammonium cations (two on general sites and the third comprising two independent half‐cations lying on crystallographic twofold rotation axes), give a pseudo‐centrosymmetric asymmetric unit. Cation–anion hydrogen bonding within this layered unit involves a cyclic R₃³(8) association which, together with extensive peripheral N—H...O hydrogen bonding involving both sulfonate and carboxy/carboxylate acceptors, gives a three‐dimensional framework structure. This work further demonstrates the utility of the 5‐SSA⁻ monoanion for the generation of stable hydrogen‐bonded crystalline materials, and provides the structure of a dianionic 5‐SSA²⁻ species of which there are only a few examples in the crystallographic literature.     

Hydrogen‐bonding patterns in six derivatives of 2,4‐dimethyl­pyrrole

The crystal and mol­ecular structures of 4‐ethyl‐3,5‐dimethyl­pyrrole‐2‐carbaldehyde, C₁₀H₁₅NO, (I), benzyl 3,5‐dimethyl­pyrrole‐2‐carboxyl­ate, C₁₄H₁₅NO₂, (II), benzyl 4‐acetyl‐3,5‐dimethyl­pyrrole‐2‐carboxyl­ate, C₁₆H₁₇NO₃, (III), dimethyl 3,5‐dimethyl­pyrrole‐2,4‐dicarboxyl­ate, C₁₀H₁₃NO₄, (IV), 4‐ethyl‐3,5‐dimethyl‐2‐(p‐tos­ylacet­yl)pyrrole, C₁₇H₂₁NO₃S, (V), and ethyl 4‐(2‐ethoxy­carbonyl‐2‐hydroxy­acrylo­yl)‐3,5‐dimethyl­pyrrole‐2‐carboxyl­ate, C₁₅H₁₉NO₆, (VI), were determined at 130 K. Compounds (I), (II), (IV), (V) and (VI) form hydrogen‐bonded dimers [N—H⋯O=C = 1.97 (2)–2.03 (3) Å]. Four dimers, viz. (I) and (IV)–(VI), have inversion symmetry, while the dimer of (II) has twofold symmetry. Only (III) forms polymeric chains involving hydrogen bonds between the pyrrole H atom and the acetyl carbonyl group [H⋯O = 1.97 (2) Å] and is further stabilized by CH₃⋯O inter­actions (C—H⋯O = 2.28–2.49 Å). Compound (VI) was found to occur as the enol ether in the crystal.     

Hydrogen‐bonding patterns in 3‐alkyl‐3‐hydroxyindolin‐2‐ones

The molecules of racemic 3‐benzoylmethyl‐3‐hydroxyindolin‐2‐one, C₁₆H₁₃NO₃, (I), are linked by a combination of N—H...O and O—H...O hydrogen bonds into a chain of centrosymmetric edge‐fused R₂²(10) and R₄⁴(12) rings. Five monosubstituted analogues of (I), namely racemic 3‐hydroxy‐3‐[(4‐methylbenzoyl)methyl]indolin‐2‐one, C₁₇H₁₅NO₃, (II), racemic 3‐[(4‐fluorobenzoyl)methyl]‐3‐hydroxyindolin‐2‐one, C₁₆H₁₂FNO₃, (III), racemic 3‐[(4‐chlorobenzoyl)methyl]‐3‐hydroxyindolin‐2‐one, C₁₆H₁₂ClNO₃, (IV), racemic 3‐[(4‐bromobenzoyl)methyl]‐3‐hydroxyindolin‐2‐one, C₁₆H₁₂BrNO₃, (V), and racemic 3‐hydroxy‐3‐[(4‐nitrobenzoyl)methyl]indolin‐2‐one, C₁₆H₁₂N₂O₅, (VI), are isomorphous in space group P. In each of compounds (II)–(VI), a combination of N—H...O and O—H...O hydrogen bonds generates a chain of centrosymmetric edge‐fused R₂²(8) and R₂²(10) rings, and these chains are linked into sheets by an aromatic π–π stacking interaction. No two of the structures of (II)–(VI) exhibit the same combination of weak hydrogen bonds of C—H...O and C—H...π(arene) types. The molecules of racemic 3‐hydroxy‐3‐(2‐thienylcarbonylmethyl)indolin‐2‐one, C₁₄H₁₁NO₃S, (VII), form hydrogen‐bonded chains very similar to those in (II)–(VI), but here the sheet formation depends upon a weak π–π stacking interaction between thienyl rings. Comparisons are drawn between the crystal structures of compounds (I)–(VII) and those of some recently reported analogues having no aromatic group in the side chain.     

Hydrogen‐bonded 1,2‐bis(4‐pyridyl)ethylene and maleic acid

The title compound (systematic name: 4,4′‐ethyl­ene­dipyridinium dimaleate), C₁₂H₁₂N₂²⁺·2C₄H₃O₄^?, is a 1:2 adduct of 1,2‐bis(4‐pyridyl)­ethyl­ene (BPE) and maleic acid (MA). The interaction between the two components in the molecular complex is due to intermolecular hydrogen bonding via an N⁺—H?O^? hydrogen bond.     

(–)‐Gibberic acid: hydrogen‐bonding pattern of the monohydrate of a non‐­racemic tetracyclic δ‐keto acid derivative of gibberellin A3

In the the title compound, 1,7‐di­methyl‐8‐oxo‐4bα,7α‐gibba‐1,3,4a(10a)‐triene‐10β‐carboxyl­ic acid monohydrate, C₁₈H₂₀O₃·H₂O, the water of hydration accepts a hydrogen bond from the carboxyl and donates hydrogen bonds to the carboxyl carbonyl and the ketone in two different screw‐related neighbors, which are mutually translational, yielding a complex three‐dimensional hydrogen‐bonding array.     

Hydrogen‐bonded molecular ladders in S‐(4‐nitrophenyl)thioglycolic acid

Molecules of the title compound, [(4‐nitro­phenyl)­sulfanyl]­acetic acid, C₈H₇NO₄S, are linked by paired O—H?O hydrogen bonds [H?O 1.81 Å, O?O 2.6456 (15) Å and O—H?O 178°] into centrosymmetric dimers containing an R(8) motif. A single C—H?O hydrogen bond having a nitro O atom as acceptor [H?O 2.47 Å, 3.3018 (19) Å and C—H?O 147°] links the dimers into a molecular ladder, and neighbouring ladders are weakly linked into sheets by aromatic π–π‐stacking interactions.     

Hydrogen‐bonding interactions in the active site of a low molecular weight protein‐tyrosine phosphatase

Molecular dynamics simulation of the Michaelis complex, phospho‐enzyme intermediate, and the wild‐type and C12S mutant have been carried out to examine hydrogen‐bonding interactions in the active site of the bovine low molecular weight protein‐tyrosine phosphatase (BPTP). It was found that the S_γ atom of the nucleophilic residue Cys‐12 is ideally located at a position opposite from the phenylphosphate dianion for an inline nucleophilic substitution reaction. In addition, electrostatic and hydrogen‐bonding interactions from the backbone amide groups of the phosphate‐binding loop strongly stabilize the thiolate anion, making Cys‐12 ionized in the active site. In the phospho‐enzyme intermediate, three water molecules are found to form strong hydrogen bonds with the phosphate group. In addition, another water molecule can be identified to form bridging hydrogen bonds between the phosphate group and Asp‐129, which may act as the nucleophile in the subsequent phosphate hydrolysis reaction, with Asp‐129 serving as a general base. The structural difference at the active site between the wild‐type and C12S mutant has been examined. It was found that the alkoxide anion is significantly shifted toward one side of the phosphate binding loop, away from the optimal position enjoyed by the thiolate anion of the wild‐type enzyme in an S_N2 process. This, coupled with the high pK_a value of an alcoholic residue, makes the C12S mutant catalytically inactive. These molecular dynamics simulations provided details of hydrogen bonding interactions in the active site of BPTP, and a structural basis for further studies using combined quantum mechanical and molecular mechanical potential to model the entire dephosphorylation reaction by BPTP. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1192–1203, 2000     

Hydrogen‐bonding patterns in pyrimethaminium dinitrate

The title compound, 2,4‐diamino‐5‐(4‐chloro­phen­yl)‐6‐ethyl­pyrimidine‐1,3‐diium dinitrate, C₁₂H₁₅ClN₄²⁺·2NO₃⁻, contains two crystallographically independent pyrimethamine (PMN) mol­ecules, which differ in the relative orientations of the pyrimidine and benzene rings and of the eth­yl substitutents. In both pyrimethamine mol­ecules, all the pyrimidine N atoms are protonated, unlike most related compounds, in which only one pyrimidine N atom is protonated. The two pyrimethamine moieties are bridged by a variety of N—H⋯O(nitrate) inter­actions, including some three‐centre hydrogen bonds.     

4‐Carboxyanilinium (2R,3R)‐tartrate and a redetermination of the α‐polymorph of 4‐aminobenzoic acid

In the title compounds, 4‐carboxyanilinium (2R,3R)‐tartrate, C₇H₈NO₂⁺·C₄H₅O₆⁻, (I), and 4‐aminobenzoic acid, C₇H₇NO₂, (II), the carboxyl planes of the 4‐carboxyanilinium cations/4‐aminobenzoic acid are twisted from the aromatic plane. In (I), the characteristic head‐to‐tail interactions are observed through the tartrate anions, forming two C₂²(7) chain motifs propagating parallel to the a and c axes of the unit cell. Also, the tartrate anions are connected through two primary C₁¹(6) and C₁¹(7) chain motifs, leading to a secondary R₄⁴(22) ring motif. In (II), head‐to‐tail interaction is seen through a discrete D₁¹(2) motif and carboxyl group dimerization is observed through centrosymmetrically related R₂²(8) motifs around the inversion centres of the unit cell. The crystal structures of both compounds are stabilized by intricate three‐dimensional hydrogen‐bonding networks. Alternate hydrophobic and hydrophilic layers are observed in (I) as a result of a column‐like arrangement of the anions and the aromatic rings of the cations.