(R,S)-3-Carboxy-2-(isoquinolinium-2-yl)propanoate monohydrate

The title compound, C13H11NO4·H2O, is a monohydrate of a betaine exhibiting a positively charged N-substituted isoquinoline group and a deprotonated carboxyl group. In the crystal, molecules are connected via short O—H⋯O hydrogen bonds between protonated and deprotonated carboxyl groups into chains of either R or S enantiomers along [001]. These chains are additionally connected by hydrogen bonding between water molecules and the deprotonated carboxy groups of neighbouring molecules.

The title compound, C 13 H 11 NO 4 ÁH 2 O, is a monohydrate of a betaine exhibiting a positively charged N-substituted isoquinoline group and a deprotonated carboxyl group. In the crystal, molecules are connected via short O-HÁ Á ÁO hydrogen bonds between protonated and deprotonated carboxyl groups into chains of either R or S enantiomers along [001]. These chains are additionally connected by hydrogen bonding between water molecules and the deprotonated carboxy groups of neighbouring molecules.

Experimental
The title compound (I) was prepared according to a method described earlier (Flett & Gardner, 1952). Separate solutions are prepared of isoquinoline (1.17 ml; 10 mmol) and maleic acid (1.16 g; 10 mmol) in anhydrous ether. Upon mixing, isoquinolinium maleate precipitates. This precipitate is separated by filtration, washed, and dried. It is then rapidly heated to its melting point at 103 °C and held at this temperature for a few minutes. Rapid conversion to the betaine takes place.
The betaine is then purified by dissolving it in hot water and treatment with animal charcoal. The solution was set aside for the formation of crystals, yield is 79 %. Crystals suitable for crystallographic study were grown from a solution of (I) in water by slow evaporation at room temperature.

Refinement
The hydrogen atoms of the water molecule were located in the difference Fourier map and refined isotropically with the O-H distance restrained to 0.857 (2) Å. All other H atoms were placed geometrically and included in the refinement in the riding-model approximation with U iso = 1.2 U eq for hydrogen atoms bonded to carbon and U iso = 1.5 U eq for the hydroxyl hydrogen. To the quinolinium subunit rigid bond restraints were applied. Since there are no heavy atoms in the structure the Flack parameter was meaningless due to a large s.u., and the Friedel pairs were merged for the final refinement.
supplementary materials sup-2 Figures   Fig. 1. View of (I) with the atom labeling scheme. Displacement ellipsoids of are shown at 30% probability. Hydrogen atoms are shown as spheres of arbitrary radii.

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 > 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.