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.