Abstract
A new crystalline form of 2,6-dimeth-oxy-benzoic acid, C(9)H(10)O(4), crystallizing in a tetra-gonal unit cell has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent mol-ecule with a synplanar conformation of the carb-oxy group. The sterically bulky o-meth-oxy substituents force the carb-oxy group to be twisted away from the plane of the benzene ring by 65.72 (15)°. The carb-oxy group is disordered over two sites about the C-C bond [as indicated by the almost equal C-O distances of 1.254 (3) and 1.250 (3) Å], the occupancies of the disordered carboxym H atoms being 0.53 (5) and 0.47 (5). In the known ortho-rhom-bic form reported by Swaminathan et al. [Acta Cryst. (1976), B32, 1897-1900], due to the anti-planar conformation adopted by the OH group, the mol-ecular components are associated in the crystal in chains stabilized by linear O-H⋯O hydrogen bonds. However, in the new tetra-gonal polymorph, mol-ecules form dimeric units via pairs of O-H⋯O hydrogen bonds between the carb-oxy groups.