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a-IC1 belongs to the space group P21/c with 8 molecules per unit cell. The approximate coordinates of the iodine atoms were found from Patterson syntheses of the three projections and by trial and error. The x and z coordinates of all atoms were refined by successive Fourier syntheses of the [010] projection, the y coordinates by computing the thrde-dimensional electron-density distribution along lines parallel to the b axis through the atoms. The estimated standard deviations in the interatomic distances are rather large: they amount to approximately 0.04 /~ for the I-C1 and to 0.02 /~ for the I-I distances between adjacent atoms.The structure consists of molecules IC1, belonging to two non-equivalent sets. Their bond lengths are 2.37 and 2.44 A respectively.These molecules are arranged in puckered zigzag chains in which the intermolecular distances (I-I----3-08 A and I-C1 ----3.00 /~) indicate a strong interaction between the molecules. Different chains are separated by normal van der Waals distances. Experimentala-ICl was prepared by adding solid, iodine to liquid chlorine (Inorganic Synth., 1939). Since the crystals are very volatile and unstable in moist air, they had to be mounted in glass capillaries. Even then, small crystals tended to move by sublimation during the exposure. Most X-ray photographs were therefore made from crystals obtained in the following way: A narrow capillary was completely filled with liquid IC1. The crystallization was then started at one end of the capillary by cooling with solid CO~ or by adding a small crystal. The crystals thus formed were oriented with their b axis along the axis of the capillary. Unit cell and space groupWeissenberg photographs about the b axis showed the crystals to be monoclinic. The lattice constants were obtained from Weissenberg and oscillation photographs about the b axis and from one Weissenberg and one oscillation photograph about the c axis. Using Cu Kc~ (2 = 1.542 A) and Mo Ks (2 = 0.711 A) radiations, the cell constants were found to be a = 12-60±0.04, b --4-38~-0-02, c = 11.90~=0.04 A; fl = 119.5=~0.5 ° .The density calculated for eight molecules per unit cell is 3-78 g.cm. -3, which is in fair agreement with the experimental value of 3.85 g.cm. -3 at 0 ° C. (Birk, 1928). The systematic absences indicate the space group P21/c. Structure factorsOn account of the volatility of the crystals, no X-ray photographs about the a axis and only one oscillation and one Weissenberg photograph about the c axis could be obtained. The reflexion intensities derived from these photographs were not very reliable as they could be corrected only roughly for absorption. The intensities of most reflexions hkl were measured on Weissenberg photographs about the b axis, using Zrfiltered Mo radiation.Correction for the Lorentz and polarization factors was made in the usual way. An approximate correction for absorption was applied, assuming a cylindrical shape for the crystals. The reflexions were obtained from two crystals for which /~R was 1.1 and about 3.5 respectivel...