SPECIFIC ION-PAIRING EFFECTS IN WEAKLY EXOERGIC INTRAMOLECULAR ELECTRON-TRANSFER

A simple theoretical model of the effect of specific ion-pairing with a chemically inert counterion on the rate of intramolecular charge tra nsfer reactions is presented. The counterion motion is treated as corr esponding to a separate reaction coordinate on the two-dimensional fre e energy surface. For the limiting case of strong ion-pairing and low driving force, i.e. \E(pair)\>\Delta G(0)\> k(B)T, the dependence of t he effect on the donor-acceptor separation, r(DA), and on the Delta G( 0) of reaction are discussed in detail. In this limit the counterion d ynamics effectively controls the rate of the intramolecular electron t ransfer over a broad range of donor-acceptor distances. For the limiti ng case of fast counterion motion the analytical relationship between the reorganization energy and activation energy along the Coulomb coor dinate of the free energy surface has been derived.