A partially known structure, which may be a fragment of a molecule found by direct methods, can be solved completely by applying a weighted tangent refinement procedure on difference structure factors, similar to the authors' procedure for heavy-atom structures. Often the molecular fragment is found to be misplaced with respect to the symmetry elements: then, the space-group symmetry is reduced to P 1, and the procedure is used to locate the symmetry elements.

Abstract. K2(CsHs).(CH3OCH2CH2)20, (K2COT.diglyme), orthorhombic, Pnma, a = 7.689 (3), b = 15.376 (5), c= 13.703 (5) A (from diffractometer data), Z=4, Dx= 1.30 g cm -3. The packing of the K + and COT 2-ions may be described as a zigzag chain of K2COT units. This chain is in the direction of the a axis and is embedded in a diglyme environment. Only one potassium ion of each K2COT unit is coordinated by the complexing ether, diglyme.Introduction. The crystals were prepared under exclusion of air and water by slowly adding a mixture of diglyme and tetrahydrofuran (THF) (1:2) to a 0.8 mol K2COT solution in THF. After one day single crystals separated out. The results of an elementary analysis are K, 24"60 (24"71), C, 52"82 (53.12) and H, 7.07 (7-01)%. (Values in parentheses are the calculated ones.) The crystals are very sensitive to air. Therefore they were mounted in paraffin (m.p. 42-44°C) in thin-walled glass capillaries under a He atmosphere. Unit-cell dimensions and intensities were obtained from a crystal of approximate dimensions 0.4 × 0-3 × 0.2 ram, with a computer-controlled Nonius CAD-3 diffractometer (Cu Ks radiation, Ni-filtered, 0-20 scan). Systematic

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Abstract[Na (CsH 1804) The biphenyl anion is significantly non-planar. The local symmetry of the sodium-triglyme part of the structure is close to twofold and nearly 222.

813 |040|[~oI] the proposed structure for the high temperature form will be about 1.78 A for the shorter and 2.83 A for the longer bond. The important transformation vectors can be seen in Fig. 5. This mechanism will lead to smaller and more regular IO3 pyramids compared to the room-temperature form. As the contracting I-O bond is nearly parallel to b, it must cause a small shrinkage parallel to b. (Received 12 May 1975; accepted23 June 1975) Direct methods are applied to the difference structure factors for a structure containing one or more heavy atoms in known positions. For those reflexions whose sign is determined by the heavy atoms, the known heavy-atom contribution is subtracted from the observed structure factor to obtain the magnitude and the sign of the light-atom contribution. The signs of the reflexions that do not have an appreciable heavy-atom contribution are found using the Y.2 sign relationship. For reflexions with an intermediate heavy-atom contribution the sign and magnitude ambiguity is also solved by the Y.2 formula. Thus it is possible to maximize the number of correct signs, and correct some of the amplitudes, before a difference Fourier map is calculated.