1,3-Dihydroxy-2-(hydroxymethyl)propan-2-aminium 2,2-dichloroacetate

The title compound, C4H12NO3 +·C2HCl2O2 −, was obtained from dichloroacetic acid and 2-amino-2-(hydroxymethyl)propane-1,3-diol. In the crystal structure, the cations and anions are connected by intermolecular N—H⋯O and O—H⋯O hydrogen bonding, forming a two-dimensional array parallel to (001). The crystal used for analysis was a merohedral twin, as indicated by the Flack parameter of 0.67 (6).

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: DN2442).

Comment
During the past 15 years, organic crystals for quadratic nonlinear optics have been intensely engineered (Etter & Frankenbach, 1989;Yaghi et al., 1997). Arising from the complexation of organic and inorganic molecules based on acid-base interactions, highly polarisable cations, responsible for NLO properties, are linked to inorganic anions through hydrogen bond networks which generate a noncentrosymmetric structural organization (Etter, 1990). In this paper, a novel nonlinear hybrid molecular crystal, NH 2 C(CH 2 OH) 3 , has been prepared by complexation between dichloroacetic and tris(hydroxymethyl)amino methane.
The structure is built up from cations and anions ( Fig. 1) connected through strong intermolecular hydrogen bonds (Table   1, Fig. 2) to form a two-dimensional layer developing parallel to the (001) plane. As suggested by the value of the Flack parameter (Flack, 1983), 0.67 (6), based on 920 Friedel's pairs, the particular crystal is twinned by inversion.

Experimental
The crystals were grown by slow evaporation at ambient temperature of the solution prepared by adding dichloroacetic acid to the aqueous solution of tris(hydroxymethyl)aminomethane in a stoichiometric ratio. For the X-ray diffraction analysis, suitable single crystals of compound (I) were obtained after one night by slow evaporation from an filtration water solution.

Refinement
All H atoms were found from a difference Fourier map but they were treated as riding on their parent atoms with C-H

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.