Radical stability - A theoretical perspective

The thermodynamic stability of radicals as defined through isodesmic hydrogen transfer reactions has been explored at a variety of theoretical levels. Radical stabilization energies (RSEs) derived from single point calculations at the ROMP2/6-311+G(3df,2p)//UBecke3LYP/6-31G(d)level of theory in combination with scaled zero point vibrational energies calculated at the UBecke3LYP/6-31G(d) level have been determined for a broad variety of systems. For the three radical types considered in this study (carbon-, nitrogen-, and oxygen-centered radicals) the radical stabilization energy (RSE) depends on the same fundamental effects such as resonance stabilization of the unpaired spin, electron donation through adjacent alkyl groups or lone pairs, and through inductive electron donation/electron withdrawal. The influence of ring strain effects as well as the synergistic combination of individual substituent effects have also been explored.