Stepwise and concerted electron-transfer/bond breaking reactions. solvent control of the existence of unstable pi ion radicals and of the activation barriers of their heterolytic cleavage

Available data from various sources seem to indicate an important role of solvation in the cleavage rates of intermediate pi ion radicals, in the passage from concerted to stepwise electron-transfer/bond breaking reaction pathways and even in the very existence of pi ion radicals. After preliminary computations treating the solvent as dielectric continuum, these expectations are examined with the help of a simple model system involving the anion radical of ONCH(2)Cl and two molecules of water, which allows the application of advanced computational techniques and a treatment of these solvent effects that emphasizes the role of solvent molecules that sit close to the charge centers of the molecule. A pi ion radical minimum indeed appears upon introduction of the two water molecules, and cleavage is accompanied by their displacement toward the leaving anion, thus offering a qualitative mimicry of the experimental observations.