| Résumé | A large number of 1,1-di-tert-butyl-2-substituted ethyl radicals, B2CCH2MRn have been prepared, principally by radical addition to 1,1-di-tert-butylethylene. The epr spectral parameters for these sterically hindered radicals are compared with those of the analogous, but unhindered, 2-substituted ethyl radicals, CH2CH2MRn. In B2CCH2MRn all MRn groups eclipse the orbital occupied by the unpaired electron (i.e., the Cα pz orbital). This same conformation (2) is adopted by CH2CH2MRn radicals when M is from rows 2, 3, or 4 of the periodic table, but when M is from row 1 the preferred conformation (1) is that with the MRn group lying in the nodal plane of the Cα pz orbital. Since the calculated extents of hyperconjugative delocalization of unpaired spin density into the Cg-M bond are about the same in B2CCH2MRn whether M is carbon or silicon, it is concluded that the difference in conformation produced by these two elements in CH2CH2MRn radicals cannot be due to hyperconjugation. Some other interaction, such as pπ-dπ bonding, must be invoked to account for the preference for conformation 2 with row 2 elements. Although β-hydrogen splittings serve to distinguish between conformation 1 and 2, it is suggested that they may not be a reliable guide to possible small distortions of the β carbon from its normal tetrahedral geometry. Other conclusions are that B2CCH2C6H5 and analogous radicals have the phenyl ring arranged with one edge towards the unpaired electron and that all B2CCH5MRn radicals have planar ligand geometry at the α carbon. |
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