A redetermination of 2-nitrobenzoic acid

The crystal structure of the title compound, C7H5NO4, was first reported by Kurahashi, Fukuyo & Shimada [(1967). Bull. Chem. Soc. Jpn, 40, 1296]. It has been re-examined, improving the precision of the derived geometric parameters. The asymmetric unit comprises a non-planar independent molecule, as the nitro and the carboxy substituents force each other to be twisted away from the plane of the aromatic ring by 54.9 (2) and 24.0 (2)°, respectively. The molecules form a conventional dimeric unit via centrosymmetric intermolecular hydrogen bonds.

The crystal structure of the title compound, C 7 H 5 NO 4 , was first reported by Kurahashi, Fukuyo & Shimada [(1967). Bull. Chem. Soc. Jpn, 40, 1296]. It has been re-examined, improving the precision of the derived geometric parameters. The asymmetric unit comprises a non-planar independent molecule, as the nitro and the carboxy substituents force each other to be twisted away from the plane of the aromatic ring by 54.9 (2) and 24.0 (2) , respectively. The molecules form a conventional dimeric unit via centrosymmetric intermolecular hydrogen bonds.
Data were corrected for Lp effects as well as for the effect of spot extension, but not for absorption [µ(Cu Kα)= 135 mm -1 ]. 694 visually estimated reflections having values significantly above background were used in the isotropic least-squares refinement. The final calculations led to R = 0.142 for 49 refined parameters, as the H atoms were not localized. A further anisotropic refinement of the structure was eventually carried out (Tavale & Pant, 1973). In this calculation, based on the same data set but with the inclusion of H atoms. the R factor decreased to 0.104, with a data-to-parameter ratio of 5.6, and average standard deviations of 0.013Å in C-C bond lengths and 0.9° in bond angles.
The asymmetric unit of (I) comprises a non-planar independent molecule, as the nitro and carboxy substituents force each other to be twisted away from the plane of the aromatic ring by 54.9 (2) and 24.0 (2)°, respectively (Fig. 1). The pattern of bond lengths and bond angles is consistent with those reported in previous structural investigations concerning the effect of the nitro and the carboxy groups on the geometry of polysubstituted benzene rings (Colapietro et al., 1984;Domenicano et al., 1989). Analysis of the crystal packing of (I), (Fig. 2), shows that the molecular components form the conventional dimeric units observed in monocarboylic acids (Leiserowitz, 1976). The structure is stabilized by very short [2.660 (3) Å] intermolecular O-H···O interactions of descriptor R 2 2 (8) (Etter et al., 1990;Bernstein et al., 1995;Motherwell et al., 1999) ( Experimental o-Nitrobenzoic acid (0.1 mmol, Sigma Aldrich at 95% purity) was dissolved in water (5 ml) and gently heated under reflux for 1 h. After cooling the solution to an ambient temperature, crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of the solvent after few days.

Refinement
All H atoms were found in a difference map and then treated as riding atoms, with C-H = 0.93Å and U iso values equal to 1.2U eq (C, phenyl). The remaining H atom of the carboxy group was freely refined.
supplementary materials sup-2 Figures   Fig. 1. The molecular structure of (I), showing the atom-labelling scheme. Displacements ellipsoids are at the 50% probability level.

2-nitrobenzoic acid
Crystal data  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.