Effects of Kinetic Coupling on Experimentally Determined Electron Transfer Parameters

Abstract

Coupled electron transfer (ET) occurs when a relatively slow nonadiabatic ET reaction is preceded by a rapid but unfavorable adiabatic reaction that is required to activate the system for ET. As a consequence of this, the observed ET rate constant (k(ET)) is an apparent value equal to the product of the true k(ET) and the equilibrium constant for the preceding reaction step. Analysis of such reactions by ET theory may yield erroneous values for the reorganizational energy (lambda), electronic coupling (H(AB)), and ET distance that are associated with the true k(ET). If the DeltaG degrees dependence of the rate of a coupled ET reaction is analyzed, an accurate value of lambda will be obtained but the experimentally determined H(AB) will be less than the true H(AB) and the ET distance will be greater than the true distance. If the temperature dependence of the rate of a coupled ET reaction is analyzed, the experimentally determined value of lambda will be greater than the true lambda. The magnitude of this apparent lambda will depend on the magnitude of DeltaH degrees for the unfavorable reaction step that precedes ET. The experimentally determined values of H(AB) and distance will be accurate if DeltaS degrees for the preceding reaction is zero. If DeltaS degrees is positive, then H(AB) will be greater than the true value and the distance will be less than the true value. If DeltaS degrees is negative, then H(AB) will be less than the true value and the distance will be greater than the true value. Data sets for coupled ET reactions have been simulated and analyzed by ET theory to illustrate these points.