Hydrogen bonding in 4,4',6,6'-tetramethyl-biphenyl-2,2'-dicarboxylic acid.

The title compound, C18H18O4, crystallizes in the centrosymmetric space group P2(1)/c, with two molecules in the asymmetric unit. Each of the four independent carboxylic H atoms is ordered, and each participates in hydrogen bonding. The OD...OA distances in the hydrogen bonds are 2.598 (3), 2.608 (3), 2.652 (3) and 2.641 (3) A. Two cyclic hydrogen-bonded dimers are found, neither occurring about a center of symmetry; each, however, occurs about a pseudo-center. Numerous hydrogen-bonded chains, all propagating along [010], are also present. The biphenyl twist angles are 88.4 (2) and 87.5 (2) degrees. Structural comparisons are made with the structures of the 'parent' compound, diphenic acid, and its 4,4'- and 6,6'-dimethyl derivatives.