An ENDOR study of the hindered internal rotation of the cyclopropyl group in the 2,6-di-t-butyl-4-cyclopropylphenoxyl radical.

概要

論文の詳細を見る
The ESR and ENDOR spectra were observed for the 2,6-di-t-butyl-4-cyclopropylphenoxyl radical. The assignments of the proton hyperfine coupling constants were confirmed with reference to those of the 2,6-di-t-butyl-4-methyl- and 4-(1-deuteriocyclopropyl)phenoxyl radicals. The β-proton hyperfine coupling constant for the cyclopropyl group and its temperature dependence were precisely determined. Of particular interest is the fact that the observed cyclopropyl β-proton splitting is smaller than the γ-proton splittings (A4β:0.72, A4γ1:0.83, A4γ2: 1.44 G at −70 °C). Based on the modified cos2θ rule, A4β=(1.85〈cos2θ〉−0.10)×A4Me, where the conjugative interaction between the π-system and the cyclopropyl pseudo π-orbital was incorporated, as proposed by Hudson and Bauld, the β-proton splitting was evaluated. The magnitude of the β-proton splitting and its temperature dependence were interpreted in terms of the restricted rotation of the cyclopropyl group in a two-fold potential; the stable conformation was the bisected one. The rotational barrier height, which corresponds to the difference in energy between the bisected and perpendicular conformations, was estimated to be 2.6 kcal/mol. The rotational barrier was attributed to the stabilization of the bisected conformation arising from the conjugative interaction of the aromatic and cyclopropyl moieties.