Guanidinium 3-carboxy-2,3-dihydroxypropanoate monohydrate

In the title hydrated salt, CH6N3 +·C4H5O6 −·H2O, the deprotonated carboxyl group is disordered over two positions with a site-occupancy ratio of 0.945 (3):0.055 (3). The bond lengths in the guanidinium cation are intermediate between normal C—N and C=N bond lengths, indicating significant delocalization in this species. In the crystal structure, anions and water molecules are linked into sheets parallel to the ab plane by intermolecular O—H⋯O hydrogen bonds. The linking of the anions and water molecules with the cations by intermolecular N—H⋯O hydrogen bonds creates a three-dimensional network.

In the title hydrated salt, CH 6 N 3 + ÁC 4 H 5 O 6 À ÁH 2 O, the deprotonated carboxyl group is disordered over two positions with a site-occupancy ratio of 0.945 (3):0.055 (3). The bond lengths in the guanidinium cation are intermediate between normal C-N and C N bond lengths, indicating significant delocalization in this species. In the crystal structure, anions and water molecules are linked into sheets parallel to the ab plane by intermolecular O-HÁ Á ÁO hydrogen bonds. The linking of the anions and water molecules with the cations by intermolecular N-HÁ Á ÁO hydrogen bonds creates a three-dimensional network.
The guanidine solution then was added drop-wise to the flask of tartaric acid with stirring. The reactant mixture was left stirring for 3 h at room temperature. The colourless single crystals formed were washed with THF and dried at 353 K.

Refinement
Atom H1O2 was placed in a calculated position, with O-H = 0.82 Å and U iso = 1.5U eq (O), and was refined using a freely  Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids for non-H atoms and the atom-numbering scheme. Open bonds indicate the minor disordered component.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1)K.
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.