Homogeneous reduction of haloacetonitriles by electrogenerated aromatic radical anions:: Determination of the reduction potential of •CH2CN

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 D-p 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(-1) and decrease from Cl to Br and to I. Both the smallness of D-p 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 (k(c)) or by electron transfer (k(et)). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q = k(et)/(k(c) + k(et)), as a function of E-D/D.(-)degrees allows determination of the standard reduction potential of (CH2CN)-C-. (Edegrees = -0.69 V vs SCE) as well as the reorganization energy A, of the redox process. A significant contribution of internal reorganization to A has been found, indicating a change of structure from (CH2CN)-C-. to -CH2CN.