Solvent Induced Intramolecular Electron Exchange Kinetics in 1,3‐Isodisubstituted Aromatic Radical Anions II. Activation Parameters

Abstract

AbstractFrom the alternant line broadening effect in the ESR‐spectra of the radical anions of 1,3‐dinitrobenzene and benzene‐1,3‐dicarbaldehyde in various protic and aprotic solvents the rates of intramolecular electron exchange were determined in the temperature range 260–340 K. Using Marcus theory and the Rips‐Jortner approach for the uniform adiabatic limit of electron transfer it is shown that the amount of transferred charge ze0 decreases with increasing temperature. This result is explained on the basis of the effect of temperature on the specific asymmetric solvation of the anions. For the 1,3‐dinitrobenzene radical anion in protic solvents the charge transfer number z changes slightly from 0.96 to 0.90 within 250–340 K and in aprotic solvents from 0.55 to 0.32 within 280–340 K. For benzene‐1,3‐dicarbaldehyde radical anion in protic solvents z changes from 0.40 to 0.29 within 260–320 K. From the measured activation parameters for the intramolecular electron exchange the values for the inner sphere reorganization energies λi were estimated, using values for the outer sphere reorganization λ0 as obtained by applying an ellipsoidal cavity model to the measured solvent dependence of the rates. For the 1,3‐dinitrobenzene radical anion λ0 increases with the degree of solvent association (from 18.4 to 106.4 kJ/mol), whereas λi varies much less (82.4–116.0 kJ/mol) for all protic and aprotic solvents investigated. From the difference in λ0 for protic and aprotic solvents, the value for the energy of solvation of the anions by hydrogen bonding is obtained as ≈ 10 kJ/mol.