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

Abstract

From 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.

Investigations of the Degenerate Intramolecular Charge Exchange in Symmetric Organic Mixed Valence Compounds: Solvent Dynamics of Bis(triarylamine)paracyclophane Redox Systems

“…Since the pioneering work of Harriman and Maki on several bis­( p -nitrophenyl) radical anions, it is known that the asymmetrical charge distribution undergoing thermal intramolecular electron transfer (IET) reactions is observable by ESR line broadening effects. More recently, the degenerate electron exchange rate constants in the radical anions of several 1,3-dinitrobenzenes, − benzene-1,3-dicarbaldehyde, , and 2,7-dinitronaphthalenes have been measured by dynamic ESR spectroscopy. In a series of impressive papers, Nelsen et al have investigated the intramolecular electron transfer within numerous σ-bond linked tetraalkylhydrazines and related compounds. − Since the pioneering work of R. A. Marcus, − it is well-established that electron transfer reactions in solution are energetically and dynamically influenced by the solvents.…”

Theoretical Study of Temperature and Solvent Dependence of the Free-Energy Surface of the Intramolecular Electron-Transfer Based on the RISM-SCF Theory: Application to the 1,3-Dinitrobenzene Radical Anion in Acetonitrile and Methanol

“…The system seems to be the best test example of the approaches since there are many experimental reports with which to be compared. [35][36][37][38][39][40][41] In this report, we put special attention on the temperature dependence of the free-energy profiles of the intramolecular electron-transfer reaction. This paper is organized as follows.…”

Radical-anions of dinitrophenols in aqueous solution: intramolecular electron exchange and acid–base equilibria

“…The activation parameters were calculated by using the Marcus equation 11- 13 and the Rip-Jortner approach 14 for a uniform and adiabatic reaction. As discussed in previous work 5, 6 the expression of the rate constant for this type of reaction can be reduced to eqn. (1), with ∆G* being…”

“…The rate constants determined by the analysis of the line broadening effects of the EPR spectra are in the fast region range (k ~10 9 s Ϫ1 ) in aprotic solvents while in alcohols the exchange is slow (k ~10 6 s Ϫ1 ) on the EPR timescale. [5][6][7] With the aim of investigating the effects of the solvent and of the intramolecular hydrogen bonding on the rates of electron exchange, we generated in aqueous solution and studied by EPR spectroscopy the radical-anions of three symmetric dinitrophenols: 3,5-dinitrophenol 1, 2,6-dinitrophenol 2 and 4,6-dinitrobenzene-1,3-diol 3. The present work reports the distribution of the spin densities and the kinetics of the intramolecular electron exchange reaction, together with the pK a values of the radicals.…”