Hydrogen-Bonded Trimers in 4,4'-Dimethylbiphenyl-2,2'-dicarboxylic Acid

The title acid, C₁₆H₁₄O₄, crystallized in the centrosymmetric space group C2/c with 1.5 molecules in the asymmetric unit. Each of the three independent carboxylic H atoms is ordered, and each participates in hydrogen bonding. The O_D...O_A distances in the hydrogen bonds are 2.584 (2), 2.647 (2) and 2.722 (2) Å. Three sets of cyclic dimer hydrogen bonds are formed within and between two asymmetric units, linking them into molecular trimers. The trimers lie on twofold axes which bisect the central C—C bond of one molecule and the COOH dimer connecting the other two. The two biphenyl twist angles are 68.1 (1) and 64.6 (1)°. The dihedral angles between the benzene-ring planes and the planes of the carboxyl groups attached to them are 0.7 (3), 23.3 (2) and 18.4 (2)°. Structural comparisons are made with the `parent' compound, diphenic acid, and with the isomeric compound, 6,6′-dimethylbiphenyl-2,2′-dicarboxylic acid.

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