Structures cristallographiques de formes racémique et optiquement active du tétraméthyl-2,2,5,5 pyrrolidine-3 carboxamide oxyle-1

Abstract

(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.

Solid‐state ESR differentiation between racemate versus enantiomer

“…The densities of the enantiopure and racemic crystals are markedly different: 1.20 and 1.14 g/cm 3 , respectively. 26 Thus, the rigid binding of the nitroxide radicals by means of the Hbonding network in 1b leads to the less dense packing of the molecules in comparison with that of 1a. In addition, as one can see from Figure 6, the minimal distances between the >N-O groups are smaller in 1a (5.41 and 5.11 Å ) than in 1b (5.69 and 5.55 Å ).…”

“…Therefore, we can conclude that the direction of the rotation axis was close to the Y axis of the nitroxide radicals, whereas all radicals were aligned coaxially with their canonic axes. It should be noted that, according to the X-ray data 26 ( Figs. 5 and 7), orientation of the radicals in 1a indeed leaves molecular axes parallel.…”

“…27 Crystal structures of both enantiopure 1a (space group P2 1 ) and racemic 1b (space group P2 1 /c) single crystals were solved in 1975. 26 Typical single crystals of 1a and 1b used for the ESR investigation are depicted in Figure 1.…”

“…26 We employed this peculiarity to study the crystalline diastereomorphs without tedious resolution of the racemate into the enantiomers. A mixture of wellformed crystals (each up to 20 mg) of 1a (mp ¼ 174-1768C) and 1b (mp ¼ 1808C) was obtained by crystallization of 1 from an acetone/benzene (1:1) mixture or from pure acetone.…”

“…Comparison of the experimental PXRD spectra with the calculated spectra unambiguously showed correspondence of the single crystals either to 1a or to 1b (see Supplementary Material). PXRD spectra were calculated on the basis of the single-crystal X-ray data 26 using Endeavour 1 1.2c software. The single crystals of 1a in contrast to those of 1b possessed optical activity ([a] D ¼ À288, c 0.92, acetone).…”