Ammonium 2-aminopyrazine-3-carboxylate

The title compound NH4 +·C5H4N3O2 − crystallizes with two formula units in the asymmetric unit. In each anion, the carboxylate is deprotonated and the planar amino group [angle sums of 359 (3) and 355 (3)° at N] remains protonated. In the crystal, the cations and anions are bridged by N—H⋯O and N—H⋯N hydrogen bonds, forming a three-dimensional network.

The title compound NH 4 + ÁC 5 H 4 N 3 O 2 À crystallizes with two formula units in the asymmetric unit. In each anion, the carboxylate is deprotonated and the planar amino group [angle sums of 359 (3) and 355 (3) at N] remains protonated. In the crystal, the cations and anions are bridged by N-HÁ Á ÁO and N-HÁ Á ÁN hydrogen bonds, forming a threedimensional network.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: ZL2357). Comment 2-Pyrazinecarboxylic acid is a complexation reagent in transition metal chemistry (Ptasiewicz-Bak et al., 1995) with a large variety of coordination modes (Ellsworth & zur Loye, 2008). The corresponding 3-aminopyrazine-2-carboxylic has been used in a similar way for the complexation of nickel (Ptasiewicz-Bak & Leciejewicz, 1999). The crystal structure of the free acid has been determined by Dobson & Gerkin (1996) and Ptasiewicz-Bak & Leciejewicz (1997). The two independent molecules are very similar, as can be seen in a quaternion fit (Fig. 2). This allows the generation of a half-normal probability plot (Fig. 3). The largest differences between the two molecules are in the C-O distances (Δ = 3.5σ).
A possible explanation is the different hydrogen bonding situation of the four O atoms. If the anions in (I) are compared with the neutral molecule of the free acid (Dobson & Gerkin, 1996) the geometries are again very similar. As expected, the only difference is in the carboxylate, which is deprotonated in (I) and protonated in the free acid. The distances C51-O11 and C52-O12 in (I) are 1.266 (2) and 1.256 (2)Å compared to the C-OH distance of 1.328 (2)Å in the free acid. This is accompanied by a change of the corresponding C-C-O angles, which are 116.01 (14) and 116.79 (14)° in (I) compared to 118.20 (10)° in the free acid.
The two independent molecules in (I) can be modelled by rigid body model using the program THMA11 (Schomaker & Trueblood, 1998). The fit of this TLS model is good, as indicated by R-values (R={[Σ(wΔU) 2 ]/[Σ(wU obs ) 2 ]} 1/2 ) of 0.080 and 0.085 for the two molecules. The two molecules can thus be appropriately described as rigid bodies. The T tensor has eigenvalues of 0.01925, 0.01329, and 0.01076 Å 2 for the first independent molecule in (I), and 0.02060, 0.01368, and 0.01227 Å 2 for the second molecule. The L tensor has eigenvalues of 13.62, 6.42, and 4.28 deg. 2 for the first molecule, and 12.96, 7.20, and 4.22 deg. 2 for the second molecule.
The amino moieties of the anions act as donors of two hydrogen bonds, respecively. One is intramolecular to the carboxylate [graph set S 1 1 (6)], and one is intermolecular to a pyrazine N atom [graph set D 1 1 (2)]. Overall, this results in one-dimensional hydrogen-bonded chains along the b axis. These chains are interconnected by the ammonium cations to form a three-dimensional network. Hydrogen atoms H3C and H4A of the ammonium cations are involved in bifurcated hydrogen bonds (Table 1, Fig. 4).
supplementary materials sup-2 Experimental 212 mg of 2-aminopyrazine-3-carboxylic acid were suspended in 20 ml water. A concentrated solution of ammonium hydroxide was added dropwise until the suspension became clear. Slow evaporation at room temperature gave crystals of (I) suitable for the diffraction experiment.

Refinement
During the intensity integration, a small second crystal fragment has been ignored (less than 5% occupancy). Friedel pairs have been averaged prior to the refinement.
Hydrogen atoms were located in difference Fourier maps. N-H hydrogen atoms were refined freely with isotropic displacement parameters. C-H hydrogen atoms were refined using a riding model with C-H = 0.95 Å and with U iso (H) = 1.2 times U eq (C). Fig. 1. : Displacement ellipsoid plot of (I). View along the b axis. Non-hydrogen atoms are drawn at the 50% probability level; H atoms are drawn as spheres with arbitrary radii.  (Mackay, 1984) of the two independent anions in (I). One of the molecules is inverted. The r.m.s. deviation of the fit is 0.041 Å. Fig. 3. : Half-normal probability plot (Abrahams & Keve, 1971) of the bond lengths of the two independent molecules of (I). On the vertical axis are the experimental Δ/σ data, on the horizontal axis the theoretical expectation values. Linear regression results in a slope of 2.2 and an intercept of 0.08.   (2)