The causes of high activation energies for anion-radical association processes

Abstract

In contrast to simple cation-radical association processes, which characteristically have negligible activation energies, some simple anion-radical association processes have recently been shown to have exceptionally high activation energies. The causes of this effect have been investigated using the MINDO/3 MO method and by applying ideas derived from intermolecular perturbation theory within the context of unrestricted Hartree-Fock theory. The results indicate that high activation energies occur when the repulsive interactions between the frontier orbitals greatly exceed the attractive interaction, which arises from overlap between the highest occupied and lowest unoccupied orbitals in the anion and radical, respectively. It is shown that these terms are related to the electronegativities of the atoms across the incipient bond and the nature of the substituents on these atoms. The results enable some conclusions to be drawn concerning the types of bond scission processes in molecular anions that would be expected to have high reverse activation energies and therefore whether the negative ion mass spectrum for a molecule may be predicted using thermochemical data for the products.