S. Athimoolam

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, C(7)H(8)NO(2)+.C(4)H(5)O(6)-, (I), and 4-aminobenzoic acid, C(7)H(7)NO(2), (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 C2(2)(7) chain motifs propagating parallel to the a and c axes of the unit cell. Also, the tartrate anions are connected through two primary C1(1)(6) and C1(1)(7) chain motifs, leading to a secondary R4(4)(22) ring motif. In (II), head-to-tail interaction is seen through a discrete D1(1)(2) motif and carboxyl group dimerization is observed through centrosymmetrically related R2(2)(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.

L-Prolinium picrate and 2-methyl-pyridinium picrate

In the structure of L-prolinium picrate, C5H10NO2+.C6H2N3O7-, the Cgamma atom of the pyrrolidine ring has conformational disorder. Both the major and minor conformers of the pyrrolidine ring adopt conformations intermediate between a half-chair and an envelope. Both the cation and anion are packed through chelated three-centred N-H...O hydrogen bonds. The prolinium cation connects two different picrate anions, leading to an infinite chain running along the b axis. In 2-methylpyridinium picrate, C6H8N+.C6H2N3O7-, the cations and anions are packed separately along the a axis and are interconnected by N-H...O hydrogen bonds. Intramolecular contacts between phenolate O atoms and adjacent nitro groups are identified in both structures. A graph-set motif of R1(2)(6) is observed in both structures.

l‐Leucine l‐leucinium picrate

The asymmetric unit of the title compound, C6H13NO2·C6H14NO2+·C6H2N3O7−, contains two unprotonated leucine residues, two protonated leucinium cations and two picrate anions. The leucine residues show a class II hydrogen-bonding scheme and the leucinium residues show a class I hydrogen-bonding scheme. The leucine and leucinium residues form infinite hydrogen-bonded chains running along the a axis.

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), C(7)H(7)NO(2).2C(5)H(9)NO(2).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.

dl‐Methio­nine dl‐methio­ninium picrate

The asymmetric unit of the title compound, (C5H11NO2SC5H12NO2S+.C6H2N3O7-)-C-., is composed of two crystallographically independent methionine residues and a picrate anion. The methionine and protonated methioninium residues are bonded through a strong O-(HO)-O-... hydrogen bond, and an inversion-related DL1 head-to-tail sequence along the a axis is observed in the zwitterion. The amino groups of two residues connect to the lone oxygen of two inversion-related picrate anions. Hydrophobic layers are observed in the y = 0 plane.

Lamellar aggregation and hydrogen-bonding motifs in 3-(aminocarbonyl)pyridinium perchlorate and hydrogen oxalate.

In the title compounds, C(6)H(7)N(2)O(+).ClO(4)(-), (I), and C(6)H(7)N(2)O(+).C(2)HO(4)(-), (II), the carboxamide plane is twisted from the plane of the protonated pyridine ring. Lamellar or sheet-like structural features are observed through N-H...O and O-H...O hydrogen-bonded motifs of cations and anions in (I) and (II), respectively. These sheets are aggregated through C(4) and C(5) chain motifs in (I) and (II), respectively. R(1)(2)(4) ring motifs in (I) and R(1)(2)(5) motifs in (II) are formed via pyridine-anion bifurcated N-H...O interactions. In (II), carboxamide groups form N-H...O dimers around the inversion centres of the unit cell, with R(2)(2)(8) ring motifs. A 2(1) screw-related helical or ribbon-like structure along the b axis is formed in (II) through carboxamide and pyridinium N-H...O hydrogen bonds with the oxalate anions.

3-(Aminocarbonyl)pyridinium trifluoroacetate

In the title compound, C6H7N2O+ center dot C2O2F3-, the carboxamide group is twisted by 16.3 (8) degrees from the pyridine ring plane. The protonated cations are linked to the anions by N-H center dot center dot center dot O hydrogen bonds to form a distorted lamellar structure.

Bis(nicotinic acid) hydrogen perchlorate

The title compound, 2C6H5NO2·H+·ClO4−, consists of [(nicotinic acid)2H]+ cations and ClO4− anions. Two nicotinic acid zwitterions are linked by a strong symmetric O—H⋯O hydrogen bond, with the H atom on an inversion center and an O⋯·O distance of 2.464 (4) A, forming a cation. The nicotinic acid cation forms hydrogen bonds to adjacent cations, giving two-dimensional layers. The Cl atom of the perchlorate anion lies on a twofold axis.