Pyrrolidinium-2-carboxylate–4-nitrophenol (1/2)

In the title compound, C5H9NO2·2C6H5NO3, the pyrrolidine ring of the pyrrolidinium-2-carboxylate zwitterion adopts a twisted conformation on the –CH2—CH2– bond adjacent to the N atom. The mean plane of this pyrrolidine ring forms dihedral angles of 25.3 (3) and 32.1 (3)° with the two nitrophenol rings. An intramolecular N—H⋯O hydrogen bond occurs in the pyrrolidinium-2-carboxylate molecule. In the crystal, molecules are linked via O—H⋯O and N—H⋯O hydrogen bonds, enclosing R 3 2(8) ring motifs, forming chains running parallel to the a axis. These chains are further cross-linked by O—H⋯O and C—H⋯O hydrogen bonds, forming undulating two-dimensional networks lying parallel to (001).

In the title compound, C 5 H 9 NO 2 Á2C 6 H 5 NO 3 , the pyrrolidine ring of the pyrrolidinium-2-carboxylate zwitterion adopts a twisted conformation on the -CH 2 -CH 2 -bond adjacent to the N atom. The mean plane of this pyrrolidine ring forms dihedral angles of 25.3 (3) and 32.1 (3) with the two nitrophenol rings. An intramolecular N-HÁ Á ÁO hydrogen bond occurs in the pyrrolidinium-2-carboxylate molecule. In the crystal, molecules are linked via O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds, enclosing R 3 2 (8) ring motifs, forming chains running parallel to the a axis. These chains are further crosslinked by O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, forming undulating two-dimensional networks lying parallel to (001).

Comment
Nitro-aromatics are widely used either as materials or as intermediates in explosives, dyestuffs, pesticides and organic synthesis (Yan et al., 2006). They also occur as industrial wastes and direct pollutants in the environment. They are relatively soluble in water and detectable in rivers, ponds and soil (Yan et al., 2006;Soojhawon et al., 2005).
The title compound was synthesized by mixing eqimolar amounts of pyrrolidine carboxylic acid and para-nitrophenol in water. The crystals obtained were found to be composed of one molecule of pyrrolidinium-2-carboxylate, in the zwitterion form, and two molecules of para-nitrophenol, Fig In the crystal, molecules are linked via O-H···O and N-H···O intra-and inter-molecular hydrogen bonds (Table 1 and   Fig. 2), with R 3 2 (8) ring motifs, forming chains running parallel to the a axis. These chains are further cross-linked by O -H···O and C-H···O hydrogen bond forming undulating two-dimensional networks lying parallel to the ab plane (Table   1 and Fig. 2).

Experimental
An equimolar (1:1:1) ratio of pyrrolidine carboxylic acid and para-nitrophenol were added to distilled water as solvent and the mixture stirred for 1 h, giving a clear solution. The solution was filtered into a clean beaker and sealed with parafilm and kept at room temperature for three days, after which block-like colourless crystals suitable for X-ray diffraction analysis were obtained.

Refinement
The NH H-atoms were located in difference electron-density maps and refined with distance restraints: N-H = 0.92 (2) Å.
The OH and C-bound H-atoms were included in calculated positions and treated as riding atoms: O-H = 0.82 Å, C-H = 0.93, 0.97 and 0.98 Å for CH(aromatic), CH 2 , and CH(methine) H-atoms, respectively, with U iso (H) = 1.5U eq (C-methyl and O), and = 1.2U eq (C) for other H-atoms. In the final cycles of refinement, in the absence of significant anomalous scattering effects, 1490 Friedel pairs were merged and Δf ′′ set to zero.

Figure 1
The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The crystal packing of the title compound viewed along the c axis. The hydrogen bonds are shown as dashed lines (see Table 1 for details; H atoms not involved in hydrogen bonding have been excluded for clarity).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. 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 > 2sigma(F 2 ) is used only for calculating R-factors(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.