Nitroxydes

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Abstract
The synthesis of three nitroxide radicals labelled with 13C in the α position makes possible the unambiguous determination of the corresponding 13C nuclear-electron contact coupling constant: di-t-butyl nitroxide (1) (quaternary 13C), N-methyl 13C,N-t-octyl nitroxide (2), and N-methyl 13C,N-phenyl (d 5) nitroxide (3). The order of magnitude of this interaction is consistent with a polarization of the N-Cα bond by the nitrogen spin density ρn π. Variations of nitrogen and carbon hyperfine splittings a n, a c α and a cβ have been studied through the solvent effect on radical (1). For radicals (2) and (3), a c α has the same value. In order to interpret these results (and the similar variation of a h α in H-NO-R nitroxides) we propose to take into account the influence of oxygen spin density ρo π and of the non-planarity of the CNOC system, at least in non-aromatic nitroxides. Study of selective broadening of particular lines in the hyperfine spectra shows that α-carbon interactions are probably negative.
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