1975 Volume 48 Issue 6 Pages 1732-1739
The electron spin resonance (ESR) and the electron nuclear double resonance (ENDOR) spectra of tetraaryl allyl radicals have been observed. A comparison of the ENDOR spectra of α,α,γ,γ-tetraphenyl allyl, α,α-diphenyl-γ,γ-dianisyl allyl, and α,α,γ,γ-tetraanisyl allyl revealed that the four aryl groups are not equivalent, but are apparently to be classified into two types with different conformations in their molecular frameworks. The molecular orbital calculations on the basis of this model have shown a fairly good agreement with the observed spectra. The temperature dependence of the ENDOR spectra at −60∼−110 °C were analyzed, assuming mutually different twisting angles for the allyl skeleton. The activation energy of the torsional vibration of the allyl skeleton is estimated to be about 2.5 kcal/mol. The ESR spectra of α,α,γ,γ-tetraphenyl allyl and α,α-diphenyl-γ,γ-diphenylene allyl change drastically with the temperature at around +115°C and show a marked line-width alternation effect. It is concluded from the results that the torsional vibration becomes vigorous with an increase in the temperature and that the allyl skeleton begins to rotate internally at high temperatures. The activation energy of the internal rotation of the allyl skeleton has been estimated to be about 15∼20 kcal/mol. The temperature-dependent hyperfine splitting of the methine proton has also been explained by assuming a model of the internal rotation of the allyl skeleton.
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