A new type of approach to the nonadiabaticity problem of outer-sphere electron-transfer reactions is presented. The approach is based on the analysis of the change in the rate constants of the reactions of the species under consideration with a homogeneous family of redox partners having variable redox potential. In favorable cases such an analysis allows us to disentangle the effects of intrinsic barrier (nuclear term) and nonadiabaticity (electronic term) on the rate constant. The literature data for the reactions of Ru(NH3)62+, Ru(NH3)63+, Fe2+, Fe3+, Eu2+, and Eu3+ with a homogeneous family of partners have been collected and the log kn vs. AG plots have been drawn and examined. For Ru(NH3)6n+ and Fen+ the plots indicate small (Ru(NH3)6n+) and large (Fen+) intrinsic barriers and adiabatic or nearly adiabatic behavior (adiabaticity factor, k> 10-3) in the ∆G range 0 to -1.5 eV. For Eun+, different plots are obtained for Eu2+ oxidation or Eu3+ reduction. These plots suggest a strongly nonadiabatic behavior of europium ions (k ~ 10-6) at moderately negative ∆G values. A theoretical estimate of the adiabaticity factor based on spectroscopic information yields k≤10-5 for electron-transfer reactions between ground-state Eun+ ions and adiabatic-type partners. At large and negative ∆G values more efficient but different channels become available for Eu2+ oxidation or Eu3+ reduction. These channels are tentatively assigned to paths involving different charge-transfer intermediates. The role played by excited states in electron-transfer reactions is discussed. © 1981, American Chemical Society. All rights reserved.
The Nonadiabaticity Problem of Outer-Sphere Electron Transfer Reactions. Reduction and Oxidation of Europium Ions
SCANDOLA, Franco;INDELLI, Maria Teresa
1981
Abstract
A new type of approach to the nonadiabaticity problem of outer-sphere electron-transfer reactions is presented. The approach is based on the analysis of the change in the rate constants of the reactions of the species under consideration with a homogeneous family of redox partners having variable redox potential. In favorable cases such an analysis allows us to disentangle the effects of intrinsic barrier (nuclear term) and nonadiabaticity (electronic term) on the rate constant. The literature data for the reactions of Ru(NH3)62+, Ru(NH3)63+, Fe2+, Fe3+, Eu2+, and Eu3+ with a homogeneous family of partners have been collected and the log kn vs. AG plots have been drawn and examined. For Ru(NH3)6n+ and Fen+ the plots indicate small (Ru(NH3)6n+) and large (Fen+) intrinsic barriers and adiabatic or nearly adiabatic behavior (adiabaticity factor, k> 10-3) in the ∆G range 0 to -1.5 eV. For Eun+, different plots are obtained for Eu2+ oxidation or Eu3+ reduction. These plots suggest a strongly nonadiabatic behavior of europium ions (k ~ 10-6) at moderately negative ∆G values. A theoretical estimate of the adiabaticity factor based on spectroscopic information yields k≤10-5 for electron-transfer reactions between ground-state Eun+ ions and adiabatic-type partners. At large and negative ∆G values more efficient but different channels become available for Eu2+ oxidation or Eu3+ reduction. These channels are tentatively assigned to paths involving different charge-transfer intermediates. The role played by excited states in electron-transfer reactions is discussed. © 1981, American Chemical Society. All rights reserved.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
