The ortho­rhom­bic polymorph of pyrazine-2-carboxylic acid revisited

The crystal structure of the ortho­rhom­bic polymorph of the title compound, C₅H₄N₂O₂, was previously reported in the space group Pna2₁ [Takusagawa, Higuchi, Shimada, Tamura & Sasada (1974). Bull. Chem. Soc. Jpn, 47, 1409–1413]. Redetermination of this structure shows that the space group is centrosymmetric, Pnma, with the mol­ecule located at a special position of m symmetry. The mol­ecules are joined via O—H...N hydrogen bonds into a chain and further via C—H...O inter­actions into a sheet parallel to (010). No face-to-face stacking inter­actions are observed in this polymorph.

Key indicators

• Single-crystal X-ray study
• T = 150 K
• Mean [sigma](C-C)= 0.003 Å
• R factor = 0.030
• wR factor = 0.083
• Data-to-parameter ratio = 10.2

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S1. Comment

The pyrazine-2-carboxylic acid is a one of the most common substances used for synthesis of pyrazine derivatives, which are important due to their bacteriostatic activity (Shi et al., 2006), antituberculous activity (Wieser et al., 1997) and selective oxidizing properties toward alkanes, benzene and alcohols (Shul'pin & Suss-Fink, 1995).

The title compound was reported in two polymorphic forms: orthorhombic, space group Pna2₁, (Ia) (Takusagawa et al., 1974), and monoclinic, space group P2₁, (Ib) (Shi et al., 2006). The structure of the polymorph (Ia) was determined by photographic methods and, based on solution derived from a sharpened Patterson map, the Pna2₁ space group was chosen as the correct one. However closer inspection of the atomic coordinates shows that the entire molecule is located on a mirror plane perpendicular to the z axis.

We have redetermined the crystal structure of the polymorph (Ia) in the Pnma space group at 150 K. Our results show that for the earlier reported crystal structure the choice of non-centrosymmetric space group was incorrect.

In (Ia) all atoms lie on a symmetry plane. The molecules are connected via O1—H1O···N2 hydrogen bonds (Table 1) into a zigzag chain extending along the [100] direction and further through C—H···O interactions (Table 1) into a sheet parallel to the (010) plane. This sheet is nearly identical to that observed in the polymorph (Ib). The main structural difference between the two polymorphs is in stacking of the adjacent sheets: in (Ib) they are related by a translation along the a axis (3.7249 (14) Å) whereas in (Ia) they are related by a 2₁ screw axis parallel to b. In the first case the sheet packing leads to face-to-face stacking interactions of the aromatic rings whereas in (Ia) no stacking interactions are observed as the closest distance between the ring centroids is 5.1427 (10) Å.

Related literature

For previously reported polymorphs of the title compound, see: Takusagawa et al. (1974), Shi et al. (2006). For applications of pyrazine-2-carboxylic acid, see: Wieser et al. (1997), Shul'pin & Suss-Fink (1995).

S2. Experimental

Commercially available 2-pyrazinic acid (CAS: 98–97-5) was recrystallized from saturated water solution.

S3. Refinement

The C bonded hydrogen atoms were placed in calculated positions after four cycles of anisotropic refinement and were refined as riding on adjacent carbon atom with U_iso(H) = 1.2U_eq(C). The O bonded H atom was found in a difference Fourier synthesis after four cycles of anisotropic refinement and was refined as riding on adjacent O atom with U_iso(H) = 1.5U_eq(O).

Computing details

Data collection: CrysAlis CCD (UNILIC & Kuma, 2000); cell refinement: CrysAlis RED (UNILIC & Kuma, 2000); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in SHELXTL/PC (Sheldrick, 1990) and ORTEP-3 (Version 1.062; Farrugia, 1997); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 1990).

Figures

	Fig. 1. Molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. Sheets parallel to (010) formed via O—H···O and C—H···O interactions (dashed lines).

pyrazine-2-carboxylic acid

Crystal data

C5H4N2O2	F(000) = 256
Mr = 124.10	Dx = 1.570 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 984 reflections
a = 11.3261 (18) Å	θ = 3–20°
b = 6.3180 (12) Å	µ = 0.13 mm^−1
c = 7.3389 (11) Å	T = 150 K
V = 525.16 (15) Å^3	Prism, colourless
Z = 4	0.20 × 0.08 × 0.05 mm

Data collection

Kuma KM-4-CCD diffractometer	561 independent reflections
Radiation source: fine-focus sealed tube	399 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.028
Detector resolution: 1048576 pixels mm^-1	θmax = 26.0°, θmin = 3.3°
ω scans	h = −13→12
Absorption correction: numerical (X-RED; Stoe & Cie, 1999)	k = −7→6
Tmin = 0.990, Tmax = 0.999	l = −9→9
3291 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.030	Hydrogen site location: mixed
wR(F^2) = 0.083	H-atom parameters constrained
S = 0.96	w = 1/[σ^2(Fo^2) + (0.0564P)^2] where P = (Fo^2 + 2Fc^2)/3
561 reflections	(Δ/σ)max = 0.003
55 parameters	Δρmax = 0.12 e Å^−3
0 restraints	Δρmin = −0.30 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O2	−0.13159 (11)	0.2500	0.57951 (18)	0.0269 (4)
O1	−0.00947 (12)	0.2500	0.82097 (18)	0.0286 (4)
H1O	−0.0764	0.2500	0.8797	0.043*
N1	0.05965 (14)	0.2500	0.3485 (2)	0.0239 (4)
N2	0.28508 (14)	0.2500	0.5024 (2)	0.0249 (4)
C1	0.07595 (16)	0.2500	0.5291 (3)	0.0209 (5)
C2	0.18758 (17)	0.2500	0.6053 (3)	0.0227 (5)
H2	0.1951	0.2500	0.7315	0.027*
C3	0.26968 (18)	0.2500	0.3227 (3)	0.0247 (5)
H3	0.3353	0.2500	0.2465	0.030*
C4	0.15758 (17)	0.2500	0.2468 (3)	0.0254 (5)
H4	0.1502	0.2500	0.1206	0.031*
C5	−0.03326 (18)	0.2500	0.6448 (3)	0.0223 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O2	0.0166 (8)	0.0410 (9)	0.0232 (8)	0.000	−0.0006 (6)	0.000
O1	0.0170 (7)	0.0515 (9)	0.0174 (8)	0.000	0.0006 (6)	0.000
N1	0.0215 (10)	0.0315 (9)	0.0185 (9)	0.000	−0.0003 (6)	0.000
N2	0.0190 (9)	0.0377 (10)	0.0181 (9)	0.000	0.0019 (7)	0.000
C1	0.0192 (10)	0.0254 (10)	0.0181 (10)	0.000	0.0000 (8)	0.000
C2	0.0192 (10)	0.0333 (11)	0.0156 (10)	0.000	−0.0003 (8)	0.000
C3	0.0206 (10)	0.0340 (11)	0.0194 (11)	0.000	0.0033 (8)	0.000
C4	0.0254 (12)	0.0341 (11)	0.0168 (10)	0.000	0.0008 (8)	0.000
C5	0.0200 (10)	0.0272 (11)	0.0196 (10)	0.000	−0.0006 (8)	0.000

Geometric parameters (Å, º)

O2—C5	1.212 (2)	C1—C2	1.382 (3)
O1—C5	1.321 (2)	C1—C5	1.500 (3)
O1—H1O	0.8718	C2—H2	0.9300
N1—C4	1.337 (3)	C3—C4	1.386 (3)
N1—C1	1.338 (2)	C3—H3	0.9300
N2—C3	1.330 (3)	C4—H4	0.9300
N2—C2	1.338 (2)
C5—O1—H1O	107.9	N2—C3—C4	121.22 (19)
C4—N1—C1	116.02 (16)	N2—C3—H3	119.4
C3—N2—C2	116.84 (17)	C4—C3—H3	119.4
N1—C1—C2	121.79 (17)	N1—C4—C3	122.36 (19)
N1—C1—C5	116.53 (17)	N1—C4—H4	118.8
C2—C1—C5	121.68 (18)	C3—C4—H4	118.8
N2—C2—C1	121.77 (18)	O2—C5—O1	125.05 (19)
N2—C2—H2	119.1	O2—C5—C1	122.27 (17)
C1—C2—H2	119.1	O1—C5—C1	112.69 (17)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O···N2^i	0.87	1.79	2.664 (2)	179
C2—H2···O1	0.93	2.41	2.736 (2)	101
C2—H2···O2^ii	0.93	2.40	3.089 (2)	131
C3—H3···O2^iii	0.93	2.42	3.156 (2)	136

Symmetry codes: (i) x−1/2, y, −z+3/2; (ii) x+1/2, y, −z+3/2; (iii) x+1/2, y, −z+1/2.