There is longstanding evidence that a donor and an acceptor conjugated to the same radical center can stabilize it more than the combined stability of the donor's and acceptor's acting independently.  This synergism, known as the captodative effect, has not been explored systematically for systems in which the acceptor is hyperconjugated to the radical center.  We have characterized the hyperconjugative captodative effect by calculating C-H and N-H bond dissociation energies for molecules containing substituents with second-, third-, and fourth-period heteroatoms.  We used a variety of density functional theory and composite methods.  We found that the B3LYP/6-31G(d), M06-2X/6-31+G(d,p), and CBS-QB3 model chemistries agreed best with the limited experimental data available.  Our calculations indicate that many carbon-centered radicals demonstrate a hyperconjugative captodative effect.  In contrast, for nitrogen-centered radicals, the combined effect of a donor and a hyperconjugated acceptor is usually less stabilizing than if the two substituents acted independently.

Quantum chemical studies of the hyperconjugative captodative effect

243rd National Meeting (2012)

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