| Résumé | At 140 K the EPR spectra of cyclohexylmethyl and (4-alkylcyclohexyl)methyl radicals in which the CH2· group adopts the axial conformation have aHβ ∼ 42-43 G, while those in which the CH2· group adopts the equatorial conformation have aHβ = 30-31 G. Both conformers can be observed over a range of temperatures for cyclohexylmethyl and for (cis-4-methylcyclohexyl)methyl. For the former radical, the relative conformer concentrations under conditions where ring inversion is rapid relative to radical lifetime can be described by log ([axial]/[equatorial]) = 0.79 - 1.79/θ (where θ = 2.3RT kcal/mol). At 300 K, therefore, the conformational free energy difference of the CH2· group in the axial and equatorial positions is -ΔG° = 0.71 kcal/mol. The greater aHβ value for the axial conformers is attributed to a higher barrier to rotation about the Cα-Cβ bond. |
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