Hydrogen bonding in 1,2-diazine-chloranilic acid (2/1) and 1,4-diazine-chloranilic acid (2/1) determined at 110 K.

The crystal structures of the isomeric title compounds [systematic names: pyridazine-2,5-dichloro-3,6-dihydroxy-p-benzoquinone (2/1), (I), and pyrazine-2,5-dichloro-3,6-dihydroxy-p-benzoquinone (2/1), (II)], 2C(4)H(4)N(2) x C(6)H(2)Cl(2)O(4), have been redetermined at 110 K. The H atom in the intermolecular O...H...N hydrogen bond in each compound was revealed to be disordered; the relative occupancies at the O and N sites are 0.33 (3) and 0.67 (3), respectively, for (I), and 0.56 (4) and 0.44 (4) for (II). The formal charges of the chloranilic acid in (I) and (II) estimated from the occupancy factors are ca -1.3 and -0.8, respectively. The geometries of the centrosymmetric chloranilic acid molecule in (I) and (II) are compared with the neutral, monoanionic and dianionic forms of chloranilic acid optimized by density functional theory (DFT) at the B3LYP/6-311+G(3df,2p) level. The result implies that the chloranilic acid molecule in (I) is close to the monoanionic state, while that in (II) is between neutral and monoanionic, consistent with the result derived from the H-atom occupancies.