The influence of gew-substitution on the stability of open-shell and closed-shell systems. The captodative and anomeric effects re-examined

Abstract

In this paper we examine the influence of gem-substitution on the stability of carbon-centred radicals and their parent molecules, using the unifying concept of thermodynamic stabilization energy. We deduce this quantity either from available experimental data or from theoretical enthalpies of formation calculated at various theoretical levels. It is found that captodative substitution significantly stabilizes carbon-icentred radicals and generally destabilizes methane. Moreover, most captodative (cd) radicals studied in this work possess a sizable extra stabilization which demonstrates a synergetic effect of the captor and donor substituents at the radical centre. Didative (dd)-substituted methyl radicals and parent molecules are most often stabilized by a non-negligible anomeric effect. Finally most cc-substituted species are destabilized. On the whole, cdCH 2 compounds have systematically lower C-H bond strengths than ccCH 2 and ddCH 2 species due to the joint action of the captodative and the generalized anomeric effects. We have also shown that enthalpies of reactions can not be used to analyse the influence of substituents on the stability of chemical species. More particularly, the energy changes of the processes R . + CH 4 → RH + ·H 3 and those of group separation reactions are far from being equal to radical and anomeric stabilization energies, respectively.