Crystal structure and Hirshfeld surface analysis of a third polymorph of 2,6-dimethoxybenzoic acid

A third monoclinic polymorph of 2,6-dimethoxybnzoic acid is reported. The acidic O—H bond of the carboxyl group adopts a synplanar conformation.

A third crystalline form of the title compound, C 9 H 10 O 4 , crystallizing in the centrosymmetric monoclinic space group P2 1 /c, has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent molecule with a synplanar conformation of the OH group. The sterically bulky o-methoxy substituents force the carboxy group to be twisted away from the plane of the benzene ring by 74.10 (6) . The carboxy group exhibits the acidic H atom disordered over two sites between two O atoms. A similar situation has been found for the second tetragonal polymorph reported [Portalone (2011). Acta Cryst. E67, o3394-o3395], in which molecules with the OH group in a synplanar conformation form dimeric units via strong O-HÁ Á ÁO hydrogen bonds. In contrast, in the first orthorhombic form reported [Swaminathan et al. (1976). Acta Cryst. B32, 1897-1900; Bryan & White (1982). Acta Cryst. B38, 1014-1016; Portalone (2009). Acta Cryst. E65, o327-o328], the molecular components do not form conventional dimeric units, as an antiplanar conformation adopted by the OH group favors the association of molecules in chains stabilized by linear O-HÁ Á ÁO hydrogen bonds.

Chemical context
Until now, two polymorphs are known for 2,6-dimethoxybenzoic acid. Polymorph (I) crystallizes in the orthorhombic space group P2 1 2 1 2 1 with one molecule in the asymmetric unit (Swaminathan et al., 1976;Bryan & White, 1982;Portalone, 2009). As a result of the antiplanar conformation adopted by the OH group, the molecular components are associated in the crystal in chains stabilized by linear O-HÁ Á ÁO hydrogen bonds. Polymorph (I) crystallizes in the tetragonal space group P4 1 2 1 2 with one molecule in the asymmetric unit (Portalone, 2011). In the crystal of the second polymorph, the synplanar conformation of the OH group favours the formation of dimers through O-HÁ Á ÁO hydrogen bonds. In this article, it is reported the crystal structure of a third polymorph, (I), of 2,6-dimethoxybenzoic acid produced unexpectedly during an attempt to synthesize co-crystals of 5-fluorouracil with the title compound.

Structural commentary
The title compound (I) crystallizes in the monoclinic centrosymmetric space group P2 1 /c, and the asymmetric unit comprises a non-planar independent molecule. The carboxy group is twisted away from the plane of the benzene ring by 74.10 (6) because of a significant steric hindrance of the two o-methoxy substituents (Fig. 1). The above angle between the planes is comparable with that found for the orthorhombic form, 56.12 (9) , and for the tetragonal form, 65.72 (15) . The carboxy group, in which OH adopts a synplanar conformation similar to that observed for the tetragonal form, exhibits the carboxy H atom disordered over two sites between two O atoms. The pattern of bond lengths and bond angles of the phenyl ring is consistent with that reported in the structure determination of the two previously determined polymorphs, and a comparison of the present results with those obtained for similar benzene derivatives (Colapietro et al., 1984;Irrera et al., 2012;Portalone, 2012) shows no appreciable effects of the crystal environment on the ring deformation induced by substituents.

Figure 1
The molecular structure of (I), showing the atom-labeling scheme. Displacement ellipsoids are at the 50% probability level.
R 2 2 (6) between the methoxy and the carboxy groups of adjacent molecules to form a two-dimensional network parallel to the bc plane.
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) was carried out using CrystalExplorer (Turner et al., 2017). The surface enables the visualization of intermolecular contacts over the surface by different colors and color intensity, and shorter and longer contacts are indicated as red and blue spots, respectively. In Fig. 3 are shown the 3D Hirshfeld surface, modeled by choosing one of the two equally disordered components and mapped over d norm , and the twodimensional fingerprint plots, which give the contribution of the interatomic contacts to the Hirshfeld surface. The most prominent interactions, due to strong O-HÁ Á ÁO hydrogen bonds, are shown by large and deep red spots on the surface. Small red spots on the surface indicate the areas where closecontact interactions due to weak C-HÁ Á ÁO hydrogen bonds take place. The HÁ Á ÁH contacts, representing van der Waals interactions, and the OÁ Á ÁH/HÁ Á ÁO contacts, representing intermolecular hydrogen bonds, are the most populated contacts and contribute 39.2 and 39.1% of the total intermolecular contacts, respectively. Other important contacts, such as CÁ Á ÁH/HÁ Á ÁC (19.1%), also supplement the overall crystal packing. The contributions of the OÁ Á ÁC/CÁ Á ÁO (2.5%) contacts are less significant.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )