| Abstract | Rate constants for the isomerization of 2,4,6-tri-tert-butylphenyl to 3,5-di-tert-butylneophyl have been measured from -26 to -160 °C, and for the corresponding isomerization of 2,4,6-tri-tert-(perdeuteriobutyl)phenyl from 20 to -150 °C. This pair of reactions has an exceptionally large deuterium kinetic isotope effect at all temperatures. Arrhenius plots for both reactions are nonlinear and over any range of temperature the activation energy and Arrhenius preexponential factor are much larger for deuterium than for hydrogen transfer. The experimental results can be quantitatively accounted for by quantummechanical tunneling through a potential barrier. The analogous isomerization of 2,4,6-tri(1′-adamantyl)phenyl, which was studied from -28 to -167 °C, also occurs by quantum-mechanical tunneling. Attempts to detect other aryl radicals by EPR spectroscopy are described. |
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