Role of Hyperconjugation in Determining Carbon−Oxygen Bond Dissociation Enthalpies in Alkylperoxyl Radicals

[reaction: see text] Theoretical calculations of carbon-oxygen bond dissociation enthalpies in substituted methylperoxyl radicals (YCH(2)OO*) reveal that bond strengths are not governed solely by the stability of YCH2* radicals but are largely affected by hyperconjugation when Y is electron-donating or conjugating. In many cases, this hyperconjugative effect is greater than stabilization of the methyl radical by Y. All electron-withdrawing Y exert small destabilizing effects via inductive withdrawal of electrons from the polarized C-OO* bond.