ROLE OF ENVIRONMENTAL-FACTORS IN THE DYNAMICS OF INTRAMOLECULAR DISSOCIATIVE ELECTRON-TRANSFER - EFFECT OF SOLVATION AND ION-PAIRING ON CLEAVAGE RATES OF ANION-RADICALS

Solvation and ion-pairing effects on the cleavage reactivity of anion radicals containing frangible bonds strongly depend upon the localization of the negative charge. When this is spread out over the entire molecular framework, as in haloanthracenes, the addition of water results in a small accelerating effect caused by specific solvation of the leaving anion. In anion radicals where the negative charge is concentrated on a small portion of the molecule, as on the oxygen atom of carbonyl or nitro groups, there is a considerable decrease of the cleavage reactivity upon addition of water. Stabilization of the leaving anion is still present, but it is largely overcompensated by a lowering of the energy of the orbital where the unpaired electron is located as revealed by a strong positive shift of the standard potential for the generation of the anion radical, The intramolecular electron transfer to the sigma* of the breaking bond thus possesses a lesser driving force. A change of mechanism ensues where cleavage is eventually replaced by 2e(-) + 2H(+) hydrogenation. For similar reasons, ion pairing agents, such as Li+ or Mg2+ ions, have practically no effect on the cleavage rates and standard potentials of anion radicals with a largely delocalized negative charge, whereas strong effects are observed with anion radicals bearing a charge-localizing group, such as CO or NO2. The strong decrease in cleavage reactivity is again caused by the lowering of the energy well where the transferring electron sits. A change in mechanism also ensues where cleavage is eventually overrun by dimerization of the ion-paired radicals.