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van Remoortere, et al.148 mm, 20X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals

The crystal and molecular structure of 2,3,7,8-tetrachlorodibenzo-p-dioxin has been determined by a three-dimensional single-crystal X-ray diffraction study. The lattice parameters of the triclinic unit cell: a=3.783+0.003, b=9.975+0.009, c=15-639+0.015 /~, ~=94.14+0.02, fl=95.20+0.04, 7=92.77+ 0.04 °, give a calculated density of 1.827 g.cm -a for ClzH402C14 and Z=2. The intensities of 2666 independent reflections were measured using the 0-20 scan mode of a Picker four-circle diffractometer and Mo K~ radiation. The space group P1 was assigned on a statistical basis and subsequently confirmed by the excellent behavior of metric and thermal parameters upon refinement. The structure was solved from the Patterson function, and the coordinates of all atoms in the molecule, the isotropic temperature factors of the hydrogens, and the anisotropic temperature factors for the other atoms were refined by full-matrix least squares. The final R value was 3.6% for the 2381 reflections above background. The structure consists of two independent molecules situated on inversion centers at (0,0,0) and ~7,tl ~-,1 7J1~ and stacked along [100]. The molecules are essentially planar; no non-hydrogen atom deviates more than 0.018 A from its least-squares molecular plane. Thus the approximate molecular symmetry is mmm (D2h). The four unique C-CI distances range from 1.

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