Most equimolecular mixtures of enantiomers, i.e., tively few (there are ca. 250 inventoried cases1) are conracemates, crystallize as racemic compounds, while reíaglomerates, i.e., cases which permit spontaneous resolution.

Since nitroxide free radicals are spin labelled molecules, we use their resonance spectra to get information about their crystallographic structure. ESR spectra of nitroxide single crystals (pyrroline, piperidine, and pyrrolidine) show only a single line. From the evolution of line position with crystal orientation we try to obtain information about the orientations of g tensors. Generally, we calculate the angles defining individual cc > NO groups orientations with an accuracy better than 5 deg. Principal g values are given for several nitroxides.

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(Recu le 29 avril 1971)Biradical tetramethyl-2,2,5,5-aza-l-cyclopentanone-3-azine-3-oxyl is a nitroxide radical which crystallizes in the monoclinic system, space group P2~/c. There are two molecules in the unit cell, each at a centre of symmetry. The molecule is nearly planar.Le biradical t6tram6thyl-2,2,5,5-aza-l-cyclopentanone-3-azine-3-oxyle fait partie des radicaux nitroxydes synth6tis6s au laboratoire de Chimie Organique Physique du C.E.N.G. (Dupeyre, Lemaire & Rassat, 1965). Ces radicaux pr6sentent un grand int6r& du fait de leur stabilit6 et font l'objet de nombreuses ~tudes: propri6t6s chimiques, susceptibilit6 magn6tique, chaleur sp6-cifique, r6sonance paramagn6tique 61ectronique (r.p.e.) et r6sonance magn6tique nucl6aire (r.m.n.). Donn6es cristallographiques

(Regu le 6 janvier 1975(Regu le 6 janvier , acceptO le 16 janvier 1975 2,2,5,5-Tetramethylpyrrolidine-3-carboxamide-l-oxyl is a chiral molecule and crystallizes simultaneously into a racemic form (space group P2dc; a=7.983, b= 10.840, c=25"349 ,~, fl=99-66°; Z=8) and an optically active form (space group P21; a= 6.551, b= 10.156, c= 8"098 A,, B= 107.87°; Z= 2). The two structures have been solved by direct methods. The pyrrolidine ring has a half-chair configuration and the ~NO group is nearly planar. Molecules are held together by hydrogen bonds, stronger in the structure of the racemic form than in the optically active one.