Radicals appear as reactive intermediates in various chemical reactions and biological processes. Their stability and reactivity strongly depends on effects such as hyperconjugation and resonance. While stabilization by hyperconjugation originates from the donation of<br/>electrons from neighboring C-H bonds into the partially filled p orbital, resonance is based on delocalization of the electron(s), which allows them to move more freely and lower the energy of the overall molecule. Therefore, it is a common rule in Organic Chemistry that molecules with more resonance structures result in higher stability. However, experimental and theoretical studies show that the most basic resonance stabilized system, the allyl radical, is more stable than the benzyl radical. The aim of this work is to understand the origin behind the underlying phenomena in the means of electronic structure theory. Density Functional Theory is used to obtain rotational energy barriers (REB) and bond dissociation enthalpies (BDE) as well as enthalpy of formations as a quantitative measure of resonance stability. It is found that the allyl radical is more stable than the benzyl radical by around 3 kcal/mol, which is in agreement with previous calculations. Natural Bond Orbital (NBO), Spin Distributions and Molecular Orbital analysis show that the electron in the benzyl radical is only partially delocalized, while in the allyl radical a equal delocalization can be found.

Stability and electronic structure of resonance stabilized radicals

ACS Spring Meeting 2021

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