Two conformational polymorphs of 4-methylhippuric acid.

4-Methylhippuric acid {systematic name: 2-[(4-methylbenzoyl)amino]ethanoic acid}, a p-xylene excreted metabolite with a backbone containing three rotatable bonds (R-bonds), is likely to produce more than one stable molecular structure in the solid state. In this work, we prepared polymorph I by slow solvent evaporation (plates with Z' = 1) and polymorph II by mechanical grinding (plates with Z' = 2). Potential energy surface (PES) analysis, rotating the molecule about the C-C-N-C torsion angle, shows four conformational energy basins. The second basin, with torsion angles near -73°, agree with the conformations adopted by polymorph I and molecules A of polymorph II, and the third basin at 57° matched molecules B of polymorph II. The energy barrier between these basins is 27.5 kJ mol-1. Superposition of the molecules of polymorphs I and II rendered a maximum r.m.s. deviation of 0.398 Å. Polymorphs I and II are therefore true conformational polymorphs. The crystal packing of polymorph I consists of C(5) chains linked by N-H...O interactions along the a axis and C(7) chains linked by O-H...O interactions along the b axis. In polymorph II, two molecules (A with A or B with B) are connected by two acid-amide O-H...O interactions rendering R22(14) centrosymmetric dimers. These dimers alternate to pile up along the b axis linked by N-H...O interactions. A Hirshfeld surface analysis localized weaker noncovalent interactions, C-H...O and C-H...π, with contact distances close to the sum of the van der Waals radii. Electron density at a local level using the Quantum Theory of Atoms in Molecules (QTAIM) and the Electron Localization Function (ELF), or a semi-local level using noncovalent interactions, was used to rank interactions. Strong closed shell interactions in classical O-H...O and N-H...O hydrogen bonds have electron density highly localized on bond critical points. Weaker delocalized electron density is seen around the p-methylphenyl rings associated with dispersive C-H...π and H...H interactions.

Non-covalent interactions in the multicomponent crystal of 1-aminocyclopentane carboxylic acid, oxalic acid and water: a crystallographic and a theoretical approach.

Single-crystal X-ray diffraction and quantum mechanical theories were used to examine in detail the subtle nature of non-covalent interactions in the [2:1:1] multicomponent crystal of 1,1-aminocyclopentanecarboxylic acid:oxalic acid:water. The crystal, which is a hydrate salt of the amino acid with the hydrogen-oxalate ion, also contains the zwitterion of the amino acid in equal proportions. It was found that a dimeric cation [Acc5(Z)...Acc5(C)]+ bonded by an O-H...O hydrogen bond exists due to a charge transfer between acid and carboxylate groups. The three-dimensional crystal is built by blocks stacked along the [101] direction by dispersion interactions, with each block growing along two directions: a hydrogen oxalate HOX-...HOX- catameric supramolecular structure along the [010] direction; and double ...HOX--W-[Acc5(Z)... Acc5(C)]+... chains related by inversion centers along the [1 0 {\bar 1}] direction. A PBE-DFT optimization, under periodic boundary conditions, was carried out. The fully optimized structure obtained was used to extract the coordinates to calculate the stabilization energy between the dimers under the crystal field, employing the M062X/aug-cc-pVTZ level of theory. The non-covalent index isosurfaces employed here allow the visualization of where the hydrogen bond and dispersion interactions contribute within the crystal. The crystal atomic arrangements are analyzed by employing the Atoms in Molecules and electron localization function theories. Within this context, the presence of density bond critical points is employed as a criterion for proving the existence of the hydrogen bond and it was found that these results agree with those rendered by the crystallographic geometrical analysis, with only three exceptions, for which bond critical points were not found.

4-Carboxypiperidinium 1-carboxycyclobutane-1-carboxylate.

The title salt, C(6)H(12)NO(2)(+)·C(6)H(7)O(4)(-) or ISO(+)·CBDC(-), is an ionic ensemble assisted by hydrogen bonds. The amino acid moiety (ISO or piperidine-4-carboxylic acid) has a protonated ring N atom (ISO(+) or 4-carboxypiperidinium), while the semi-protonated acid (CBDC(-) or 1-carboxycyclobutane-1-carboxylate) has the negative charge residing on one carboxylate group, leaving the other as a neutral -COOH group. The -(+)NH(2)- state of protonation allows the formation of a two-dimensional crystal packing consisting of zigzag layers stacked along a separated by van der Waals distances. The layers extend in the bc plane connected by a complex network of N-H···O and O-H···O hydrogen bonds. Wave-like ribbons, constructed from ISO(+) and CBDC(-) units and described by the graph-set symbols C(3)(3)(10) and R(3)(3)(14), run alternately in opposite directions along c. Intercalated between the ribbons are ISO(+) cations linked by hydrogen bonds, forming rings described by the graph-set symbols R(6)(6)(30) and R(4)(2)(18). A detailed analysis of the structures of the individual components and the intricate hydrogen-bond network of the crystal structure is given.

Redetermination of 1-carboxy-cyclo-hexan-1-aminium chloride.

The crystal structure of the title compound, C(7)H(14)NO(2) (+)·Cl(-), was reported previously [Chacko, Srinivasan & Zand (1975 ▶). J. Cryst. Mol. Struct.5, 353-357] from Weissenberg photographic data with R = 0.113. It has now been redetermined, providing a significant increase in the precision of the derived geometric parameters, viz. mean σ(C-C) = 0.003 Šin the present work compared with 0.021 Šfor the previous work. The complete cation is generated by crystallographic mirrror symmetry, with three C atoms, two O atoms and the N atom lying on the reflecting plane; the chloride anion also has m site symmetry. The crystal structure is established by a two-dimensional network of O-H⋯Cl and N-H⋯Cl hydrogen bonds, generating C(1) (2)(4) and C(1) (2)(7) chains, and R(2) (4)(8) and R(2) (4)(14) rings.