Quantifying the Electrostatic Driving Force behind SmI2 Reductions

The equilibrium constant for the electron transfer between SmI2 and substituted benzophenones was determined. The electron transfer reactions are exothermic with DeltaG(eq) ranging from -5.1 to -1.6 kcal/mol. Redox potentials suggest that the electron transfer reactions should be endothermic by ca. 25 kcal/mol, contrary to the experimental observation. It is suggested that the change from endothermicity to exothermicity stems from the electrostatic attraction between the negatively charged radical anion and Sm(3+).