Electron transfer within charge-localized dinitroaromatic radical anions.

Rate constants for the intramolecular electron-transfer reaction in the 2,7-dinitronaphthalene (2(-)), 4,4'-dinitrotolane (3(-)), and 2,2'-dimethyl-4,4'-dinitrobiphenyl (4(-)) radical anions in several polar aprotic solvents were estimated by simulating their ESR spectra at different temperatures. At 298 K, the rate constants are in the 2.0-8.0 x 10(9) s(-1) range for 2(-) and 3(-) and in the 0.4-2.6 x 10(9) s(-1) range for 4(-). The rate constants of 3(-) and 4(-), when corrected for changes in the activation energy (taken as the changes in lambda, the transition energy of the mixed valence band), correlate with the inverse of the solvent relaxation time, showing that the reaction is controlled by solvent dynamics. Solvent effects are only found for 2(-) in benzonitrile (PhCN), the most viscous solvent studied. Calculations of the rate constants using the Kramers-based theory adapted to the adiabatic limit fit the Eyring plots of 2(-) in PhCN and of 3(-) and 4(-) both in MeCN and PhCN rather well.