Impact of Conjugation and Hyperconjugation on the Radical Stability of Allylic and Benzylic Systems: A Theoretical Study.

Resonantly stabilized radicals are some of the most investigated chemical species due to their preferential formation in a wide variety of chemical environments. Density functional theory and post-Hartree-Fock calculations were utilized to elucidate the chemical interactions that contribute to the stability of two ubiquitous, resonantly stabilized radicals, allyl and benzyl radicals. The relative stability of these radical species was quantified through bond dissociation energies and relative rotational energy barriers, with a difference of only 0.1 kcal/mol. To clarify and contextualize the energetic results, natural bond orbitals were used to evaluate the atomic spin density distribution in the given molecules. The benzyl radical was found to be ∼3 kcal/mol less stable than the allyl radical, which was attributed to the inability to efficiently delocalize the spin on a phenyl unit, starkly contrary to general chemistry knowledge. Increasing the degree of π-conjugation and hyperconjugation was shown to benefit allyl radicals to a greater degree than benzyl radicals, again due to more efficient radical delocalization in allyl radicals. This work highlights that more resonance structures do not always lead to a more stabilized radical species, and provides fundamental knowledge about how conjugation and hyperconjugation impact the stabilization of nonbonding electrons in these systems.