Methyl Substitution Destabilizes Alkyl Radicals

We have quantum chemically investigated how methyl substituents affect the stability of alkyl radicals Me_m H_3-m C⋅ and the corresponding Me_m H_3-m C-X bonds (X = H, CH₃ , OH; m = 0 - 3) using density functional theory at M06-2X/TZ2P. The state-of-the-art in physical organic chemistry is that alkyl radicals are stabilized upon an increase in their degree of substitution from methylm H_3-m C⋅ radical is destabilized with increasing substitution. The reason that the corresponding C-H bond nevertheless becomes weaker is that substitution destabilizes the sterically more congested Me_m H_3-m C-H molecule even more.