Effects of kinetic coupling on experimentally determined electron transferparameters

Coupled electron transfer (ET) occurs when a relatively slow nonadiabatic E T reaction is preceded by a rapid but unfavorable adiabatic reaction that i s required to activate the system for ET, As a consequence of this, the obs erved 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 ste p. Analysis of such reactions by ET theory may yield erroneous values for t he reorganizational energy (lambda), electronic coupling (H-AB), and ET dis tance that are associated with the true k(ET) If the Delta G degrees depend ence 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 les s than the true HAB and the ET distance will be greater than the true dista nce. If the temperature dependence of the rate of a coupled ET reaction is analyzed, the experimentally determined value of lambda will be greater tha n the true lambda. The magnitude of this apparent lambda will depend on the magnitude of Delta H degrees for the unfavorable reaction step that preced es ET. The experimentally determined values of H-AB and distance will be ac curate if Delta S degrees for the preceding reaction is zero. If Delta S de grees is positive, then H-AB will be greater than the true Value and the di stance will be less than the true value. If Delta S degrees is negative, th en H-AB will be less than the true value and the distance will be greater t han the true value. Data sets for coupled ET reactions have been simulated and analyzed by ET theory to illustrate these points.