Crystal Structure of the Potassium Salt of Chloranil

The crystal structures of K+(chloranil)− have been studied by X-ray diffraction. Several polymorphs were recognized, two of which (α- and β-forms) grew into comparatively good crystals. The α-form is orthorhombic, space group P212121 with unit cell dimensions: a=13.49, b=17.02, c=4.03 Å. The β-form is orthorhombic, space group P2212 with unit cell dimensions: a=14.13, b=17.35, c=3.71 Å. The structure of the α-form was deduced from the Patterson synthesis and refined by the block-diagonal least-squares method. The final R value was 0.13 for 146 observed reflections. Chloranil anions are stacked with equal intervals in the crystal, forming columns parallel to the c axis, the interplanar spacing being 3.47 Å. The electrostatic energy calculated by Evjen’s method was about −148 kcal/mol, indicating that the structure is essentially ionic.