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

Abstract

A third crystalline form of the title compound, C9H10O4, crystallizing in the centrosymmetric monoclinic space group P21/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—H...O 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—H...O hydrogen bonds.