Homogeneous reduction of haloacetonitriles by electrogenerated aromatic radical anions: Determination of the reduction potential of (CH2CN)-C-center dot

The mechanism of homogeneous reduction of XCH2CN (X) Cl, Br, I) by organic radical anions (D•-) has been investigated in DMF. All three haloacetonitriles undergo a concerted dissociative electron transfer with formation of a fragment cluster in the solvent cage. The interaction energy Dp of the fragment cluster has been determined by applying the “sticky” dissociative electron-transfer model to the kinetic data obtained for the reaction between each XCH2CN and a series of donors. The interaction energies lie in the range from 0.19 to 1.67 kcal/mol and decrease from Cl to Br and to I. Both the smallness of Dp values and their dependence on the bulkiness of X- confirm the electrostatic character of these interactions. The intermediate radical stemming from the dissociative electron transfer to XCH2CN reacts with D•- either by radical coupling (kc) or by electron transfer(ket). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q) ket/(kc+ ket), as a function of E°D/D•- allows determination of the standard reduction potential of •CH2CN (E° = -0.69 V vs SCE) as well as the reorganization energy λ of the redox process. A significant contribution of internal reorganization to λ has been found, indicating a change of structure from •CH2CN to -CH2CN.