l-Asparagine–l-tartaric acid (1/1)

In the title compound, C4H8N2O3·C4H6O6, the amino acid molecule exists as a zwitterion and the carboxylic acid in an un-ionized state. The tartaric acid molecules are linked into layers parallel to the ab plane by O—H⋯O hydrogen bonds. The amino acid molecules are also linked into layers parallel to the ab plane by N—H⋯O and C—H⋯O hydrogen bonds. The alternating tartaric acid and amino acid layers are linked into a three-dimensional framework by N—H⋯O and O—H⋯O hydrogen bonds.

In the title compound, C 4 H 8 N 2 O 3 ÁC 4 H 6 O 6 , the amino acid molecule exists as a zwitterion and the carboxylic acid in an un-ionized state. The tartaric acid molecules are linked into layers parallel to the ab plane by O-HÁ Á ÁO hydrogen bonds. The amino acid molecules are also linked into layers parallel to the ab plane by N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds. The alternating tartaric acid and amino acid layers are linked into a three-dimensional framework by N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds.

Related literature
Our interest in the determination of the structure of the title compound is due to recent advances in organic non-linear optical (NLO) materials on account of their widespread potential industrial applications. For studies on organic nonlinear optical materials, see: Cole et al. (2000); Ravi et al. (1998); Sarma et al. (1997).
Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL97. Results have shown that an inherent relationship exists between the structure of these materials and their observed properties. On the molecular scale, the extent of charge transfer is assumed to dominate the SHG output while on the supramolecular scale, a high SHG output requires non-centrosymmetry, strong intermolecular interactions and good phase-matching ability (Sarma et al., 1997: Ravi et al., 1998: Cole et al., 2000 Fig.1 illustrates the molecular structure of the title compound, (I), and the atomic numbering scheme adopted. The amino acid molecule exists as a zwitterion, an uncommon ionization state in the crystal structures of amino-carboxylic acid complexes. Usually, a proton transfer is favoured from the carboxylic acid to the amino acid in these complexes, the former exists in the anionic state and the latter in the cationic state. Similar zwitterionic state for the amino acid molecule is observed in L-phenylalanine fumaric acid and L-phenylalanine benzoic acid. The asparagine carboxylate skeleton, which includes O2, O3, C1 and C2 is nearly planar. The deviation of the amine N atom from the plane of the carboxylate group is 0.516 (2) Å. The twist of the carboxylate group of the asparagine molecule is described by ψ 1 = 160.4 (6)° and ψ 2 = -24.9 (9)°, corresponding to trans and cis arrangements. The side-chain conformations are observed as χ 1 = 63.9 (8)°, χ 21 = -80.7 (9)°a nd χ 22 = 95.9 (8)° for the asparagine molecule.

Structure Reports Online
The tartaric acid molecule is in the unionized state. The angle between the planes of the half molecules, O9/O8/C8/C7/O7 and O4/O5/C5/C6/O6 is 57.6 (3)°, which is closer to the value of 54.6° found in the structure of tartaric acid. The carbon skeleton of the tartrate molecule is non-planar, with a C5-C6-C7-C8 torsion angle of -168.5 (6)°.

Experimental
Colourless, prismatic single crystals of (I) were grown from a saturated solution of water containing L-asparagine and tartaric acid in a 1:1 stoichiometric ratio.   Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids and the atom-numbering scheme.