dl-Asparaginium nitrate

In the title compound, C4H9N2O3 +·NO3 −, alternatively called (1RS)-2-carbamoyl-1-carboxyethanaminium nitrate, the asymmetric unit comprises one asparaginium cation and one nitrate anion. The strongest cation–cation O—H⋯O hydrogen bond in the structure, together with other strong cation–cation N—H⋯O hydrogen bonds, generates a succession of infinite chains of R 2 2(8) rings along the b axis. Additional cation–cation C—H⋯O hydrogen bonds link these chains into two-dimensional layers formed by alternating R 4 4(24) and R 4 2(12) rings. Connections between these layers are provided by the strong cation–anion N—H⋯O hydrogen bonds, as well as by one weak C—H⋯O interaction, thus forming a three-dimensional network. Some of the cation–anion N—H⋯O hydrogen bonds are bifurcated of the type D—H⋯(A 1,A 2).

In the title compound, C 4 H 9 N 2 O 3 + ÁNO 3 À , alternatively called (1RS)-2-carbamoyl-1-carboxyethanaminium nitrate, the asymmetric unit comprises one asparaginium cation and one nitrate anion. The strongest cation-cation O-HÁ Á ÁO hydrogen bond in the structure, together with other strong cation-cation N-HÁ Á ÁO hydrogen bonds, generates a succession of infinite chains of R 2 2 (8) rings along the b axis. Additional cation-cation C-HÁ Á ÁO hydrogen bonds link these chains into twodimensional layers formed by alternating R 4 4 (24) and R 4 2 (12) rings. Connections between these layers are provided by the strong cation-anion N-HÁ Á ÁO hydrogen bonds, as well as by one weak C-HÁ Á ÁO interaction, thus forming a threedimensional network. Some of the cation-anion N-HÁ Á ÁO hydrogen bonds are bifurcated of the type D-HÁ Á Á(A 1 ,A 2 ).
The crystal structures of the L-enantiomer compounds are reported most often, indeed, L-asparagine monohydrate, determined by X-ray or neutron diffraction, has been reported for the first time by Kartha & de Vries (1961) and then reported, i.a., by Verbist et al. (1972); Ramanadham et al. (1972); Wang et al. (1985); Recently, Yamada et al. (2007) reported the crystal structure of the anhydrous L-asparagine and Aarthy et al. (2005) reported the crystal structure of L-asparaginium nitrate.
In the present study, the single-crystal structure determination of anhydrous DL-asparaginium nitrate as a part of the work of our team is reported. The asymmetric unit is formed by the monoprotonated asparaginium cation (C 4 H 9 O 3 N 2 ) + and the nitrate anion (NO 3 ) - (Fig. 1) The crystal structure is stabilized by O-H···O, N-H···O and C-H···O, cation-cation (Fig. 2) and cation-anion (Fig. 3) hydrogen bonds. The O1-H1···O3 cation-cation hydrogen bond is the strongest one observed in the title structure at all (Tab. 1).
The backbone conformation of the cation asparaginium is stabilized by the intramolecular N2-H2N···O3 interaction, with the S(6) motif (Bernstein et al., 1995), between the O atom of the amide group as an acceptor and one of the H atoms of the -NH 3 group. The pertinent angle N2-H2N···O3 is quite acute (Tab. 1). A similiar intramolecular interaction is observed in L-asparaginium picrate  and L-asparaginium nitrate (Aarthy et al., 2005).

Experimental
The title compound was prepared by heating of a mixture of DL-asparagine monohydrate of purity 98 % (Alfa Aesar) and nitric acid. This mixture was obtained by dissolution and agitation for 20 minutes of 0.75 g of the DL-asparagine monohydrate in 15 ml of water at 25°C followed by addition of 15 ml of 1 M nitric acid. Colourless needle crystals with approximate dimensions 0.30 x 0.20 x 0.10 mm were obtained by evaporation of the solution at room temperature in the course of a few weeks.

Refinement
All the H atoms were located in the difference electron density maps. All the H atoms attached to C were treated as riding with C-H = 1.00 Å (methine) or 0.99 Å (methylene) with U iso H = 1.2U eq C. The coordinate parameters of the H atoms attached to N or O were freely refined with U iso H = 1.2U eq N and U iso H = 1.5U eq O. Fig. 1. The title molecule with the atom-numbering scheme. The displacement ellipsoids are drawn at the 50% probability level