Kinetics of Electron Transfer from Nitroaromatic Radical Anions in Aqueous solutions. Effects of Temperature and Steric Configuration

Article abstract of DOI:10.1021/j100410a036

Rate constants for electron transfer from various nitroaromatic radical anions to other nitroaromatic compounds in aqueous solutions have been determined by kinetic spectrophotometric pulse radiolysis.The rate constants were found to increase from ca. 10⁵ to ca. 10⁸ M^-1 s^-1 upon increase in driving force (ΔE) from 0 to ca. 300mV, while the activation energies decrease from ca. 6 to ca 3 kcal/mol.Nitro compounds with ortho substituents exhibit lower reduction potentials due to rotation of the nitro group away from the ring plane.Anion radicals of such compounds transfer their electrons much more slowly (k down to ca. 10⁶ M^-1 s^-1) despite the apparent increase in ΔE (to ca. 550 mV).This behavior is rationalized by an increase in solvent reorganization energies around these radical anions caused by localization of the charge.In general, nitroaromatic radical anions donate electrons much more slowly than other radical anions, in reactions with similar driving forces, due to low self-exchange rates for ArNO2/ArNO^-..The kinetics show no anomalies in supercooled solutions.