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The crystal structure of the potassium salt of 2,2,5,5-tetramethyl-3-carboxypyrroline-1-oxyl (I), which is a free radical of the nitroxide type, has been determined by three-dimensional X-ray methods. The unit cell contains two crystallographically different types of molecule, one of which lies with its five-membered ring entirely in a mirror plane and is therefore perfectly flat. Molecules of the second type lie in general positions and are also flat to within experimental error. Materials and Methods.-The preparation of I was first described by Rozantzev and Krinitzkaya.1 Our sample was synthesized by Dr. Carole L. Hamilton of the Stauffer Laboratory, Stanford University, and recrystallized from water. The initial X-ray photographs of the needle-shaped crystals indicated that they were hexagonal. Nonspace group absences on higher layer-line Weissenberg photographs as well as a slight doubling of spots suggested, however, that each crystal consisted of three orthorhombic individuals, trilled to give apparent hexagonal symmetry.2 This requires an axial ratio of a/3:1, which was indeed found on examination of a single crystal fragment obtained by splitting a crystal along its needle axis. The crystals are slightly hygroscopic, and therefore a single needle was sealed with some calcium chloride into a thin-walled glass capillary for the X-ray work. The cell data were established as follows from a set of oscillation and Weissenberg photographs:

THE CRYSTAL AND MOLECULAR STRUCTURE OF THE POTASSIUM SALT OF 2,2,5,5-TETRAMETHYL-3-CARBOXYPYRROLINE-1-OXYL