Stability of Carbon-Centered Radicals: Effect of Functional Groups on the Energetics of Addition of Molecular Oxygen

摘要

In this paper we examine a series of hydrocarbons with structural features which cause a weakening of the C-H bond. We use theoretical calculations to explore whether the carbon-centered radicals R-center dot which are created after breaking the bond can be stabilized enough so that they resist the addition of molecular oxygen, i.e. where the reaction R-center dot + O-2 ROO center dot becomes energetically unfavorable. Calculations using a B3LYP-based method provide accurate bond dissociation enthalpies (BDEs) for R-H and R-OO center dot bonds, as well as Gibbs free energy changes for the addition reaction. The data show strong correlations between R-OO center dot and R-H BDEs for a wide variety of structures. They also show an equally strong correlation between the R-OO center dot BDE and the unpaired spin density at the site of addition. Using these data we examine the major functional group categories proposed in several experimental studies, and assess their relative importance. Finally, we combine effects to try to optimize resistance to the addition of molecular oxygen, an important factor in designing carbon-based antioxidants.