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

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.