| Résumé | The reactions of a variety of aliphatic selenium compounds with free radicals have been studied by EPR spectroscopy. The methylselenyl radical, CH3Se•, could not be directly observed by EPR, probably because of extreme line broadening by spin-orbit coupling. Sterically hindered β-selenoalkyls, RnMSeĊ(CMe)2, generated by addition of RnM• radicals to di-tert-butyl selenoketone, are readily observed by EPR, but unhindered alkylselenoalkyls, such as CH3SeĊH2, could not be detected directly. This difference is attributed to conformational factors. The hindered radicals adopt a conformation in which the unpaired electron interacts only weakly with the selenium nucleus because the selenium's lone pair lies in the Cα 2pz nodal plane. The unhindered radicals adopt a conformation in which the lone pair is eclipsed by the Cα 2pz orbital and the resultant strong interaction with the selenium nucleus leads to line broadening both by a spin-rotation mechanism and by the spin-orbit mechanism. The kinetics of decay of some RnMSeĊ(CMe3)2 radicals are reported. At low temperatures these radicals exist in equilibrium with a diamagnetic dimer. The (CH3)3COSeĊ(CMe3)2 radical decays by a γ scission of the C-O bond. Dialkyl selenides readily undergo bimolecular homolytic substitution at selenium when attacked by reactive carbon-centered radicals such as C6H5•. Relative and absolute rate constants for some of these reactions have been determined. Some γ- and δ-selenoalkyls have also been identified. |
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