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The crystal structures of the two related compounds 1,3,4,6,7,9-hexachloroxanthene (C13H4C160) and 1,3,4,6,7,9-hexachloroxanthone (C13H2C1602) have been determined by X-ray diffraction techniques.Both structures are monoclinic, space group P21/n with a= 16"114 (4), b= 11.950 (2), c=7.254 (1)/~, ,8=92.29 (2) °, Z=4, Dx= 1-85 g cm -3 and a= 15.960 (3), b= 11.833 (2), c= 7.348 (2) ,~, ,8=93.11 (2) °, Z= 4, Dx = 1.93 g cm -a, for the xanthene and xanthone derivatives respectively. Both structures were refined to R values of 0.06. The molecules of both compounds were found to be almost planar, with the largest deviation of the nonhydrogen atoms from planarity being shown by the pyranoid ring of the hexachloroxanthene molecule. The crystal structures are built up by successive layers (separation 3"4 A) of nearly coplanar molecules, where the centroids of the molecules are approximately hexagonally distributed within one layer.

The c/s and trans isomers of 1,2-bis(4-chlorophenyl)-1,2-dichloroethylene, C14HsC14, are monoclinic, space group P21/c. The 107 parameters describing the trans isomer [a = 6.002 (5), b = 14.500 (12), c = 7.630 (5) A,/~ = 94.55 (2) °, Z = 2, Dx = 1. 596 g cm-31 were refined versus the 854 most significant single-crystal Xray reflexions to an R of 0.043. The 195 parameters of the cis isomer [a = 8.062 (8), b = 28.560 (12), c = 6.118 (6) A,/3 = 97.60 (2) °, Z = 4, Dx = 1.513 g cm-31 were refined versus the 1017 most significant reflexions to an R of 0.048. The molecules of both isomers were found to be nonplanar. The C-C=C bond angles at the ethylene bridge were significantly greater than 120 (125) for the cis isomer and less than 120 ° ( 118 o) for the trans isomer.

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One C-CI bond adopts a parallel, the other a perpendicular orientation with respect to the plane containing the bridgehead chlorines.