(±)-3-Carboxy-2-(imidazol-3-ium-1-yl)propanoate

The title compound, C7H8N2O4, crystallizes as a zwitterion, with molecules organized into molecular sheets via carboxyl–carboxylate and N+—H⋯carboxylate contacts. These sheets are constructed from translationally related molecules that further link to neighboring motifs via π-stacking [centroid–centroid distance 3.504 (3) Å] and weak C—H⋯O contacts.


Comment
Our recent reinvestigation of Pasteur's 1853 quasiracemates (Pasteur, 1853;Wheeler et al., 2008) has motivated us to explore other examples of these unusual materials of historical and supramolecular importance. In 1899, Centnerzwer reported that mixtures of (+)-chlorosuccinic acid and (-)-bromosuccinic acid formed a binary compound that also exhibited quasiracemic behavior (Centnerzwer, 1899). Our initial attempts to grow crystals of this quasiracemic phase were unsuccessful. This result was somewhat anticipated given that Centnerzwer's melting point phase diagrams showed the crystal stabilities of the homochiral phases more stable than the quasiracemate. We then turned our attention to investigating the effects of co-crystalline additives to crystal growth of this quasiracemate and the corresponding racemic and homochiral compounds.
During the course of these co-crystal screening investigations, we observed the formation of crystals of the title compound from slow evaporation of a methanol:CH 2 Cl 2 (1:1) solution of (±)-2-chlorosuccinic acid and imidazole.
The title compound, (I), formed from the substitution reaction of imidazole and 2-chlorosuccinic acid, crystallizes in space group P2 1 /c as the imidazolium carboxylate zwitterion (Fig. 1). Inspection of the molecular structure reveals a resonance The crystal structure of (I) is organized by a complex blend of strong and weak intermolecular contacts (Table 1). Neighboring molecules are linked by carboxyl···carboxylate interactions to give a catemeric motif that propagates along the a-axis ( Fig. 2). This motif is extended by N2 + -H···carboxylate contacts to produce a molecular sheet in the ab plane. The participation of the imidazolium N + -H group in hydrogen bonding is also a common feature in the 44 structures retrieved from the CSD. Each of these structures show N + -H···A contacts with a diverse set of acceptors [A = oxygen(52), nitrogen(3), halogen(15), or π(2); 72 contacts]. Interestingly, each molecular sheet in (I) consists of translationally related molecules with imidazolium groups exposed on one side of the motif and carboxyl O4 atoms on the other side. The crystal structure of (I) is characterized by the stacking of these molecular sheets with adjacent motifs related by inversion symmetry and linked by either interdigitated imadazolium···imidazolium stacks [3.504 (3) Å] or weak C5-H5···O4 interactions (Fig. 3).
supplementary materials sup-2 Refinement H atoms (for OH and NH) were located in difference Fourier synthesis and refined isotropically. The remaining H atoms were positioned geometrically with C-H = 0.93, 0.98 and 0.97 Å, for aromatic, methine and metnylene H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C). Fig. 1. The molecular structure of the title molecule, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.   (±)-3-Carboxy-2-(imidazol-3-ium-1-yl)propanoate

Special details
Experimental. The instrument used for data collection was a Bruker P4 with a APEXII CCD detector upgrade. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 1.00100 (17