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 Pna2(1) [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 center dot center dot center dot N hydrogen bonds into a chain and further via C-H center dot center dot center dot O interactions into a sheet parallel to (010). No face-to-face stacking interactions are observed in this polymorph.