The crystal structure of tetrachloro-p-benzoquinone (chloranil)

Abstract

A three-dimensional structure analysis of chloranil, C6C1,O 2, has been carried out to determine the positional parameters and the thermal motion of the mulecules. A contemporary two-dimensional analysis has been reported by Ueda (1961) and the results are compared. The molecule has a quinoid structure and the quinoid ring is planar, with the oxygen and chlorine atoms displaced slightly (0.05 A) out of the plane. The bond lengths and valence angles are normal, agreeing well with the data from 1245 tetrachlorobenzene and p-benzoquinone. The one unusual feature of the structure is a carbon to oxygen intermolecular separation of 2.85/~, an observation which may be relevant to the well-known facility of chloranil to form charge transfer complexes. Because of this anomalous distance, a confirmation of the solution to the phase problem was obtained by an independent physical method. The orientation of the C-C1 bonds with respect to each other and the symmetry axis and, less precisely, the direction of the normal to the quinoid ring were measured by nuclear quadrupole resonance methods. These results completely confirmed the X-ray analysis.