Marcus nonadiabatic behavior

Beginning in the 1950s, Marcus developed and refined a classical theoretical description of redox reactions occurring in homogeneous solution [79-84]. He also extended the treatment to include electrode processes [81-86]. Related treatments are the quantum models of Levich and Doganadze and the works of Gerisher. More recently, Chidsey [29] presented a derivation of Marcus nonadiabatic (MNA) behavior, characterized by the quadratic free energy relationship for electrode processes ... 

The redox potentials of organic cofactors are directly responsible for controlling the equilibrium behavior of the corresponding cofactor-mediated electron transfer processes. The relative redox potentials of the cofactor and its redox partner are also intimately related to the rate of adiabatic electron transfer Ret through the classical Marcus equation [13, 14], and nonadiabatic electron transfer through the semi-classical Marcus equation [15, 16]. The direct dependence of both the kinetics and thermodynamics of electron transfer processes on the cofactor redox potential makes the control of these potentials a key determinant of the activity of redox proteins. 

The reversible reaction between [Ru(NH3)5(nicotinamide)] and [Ru(NH3)5(isonicotinamide)] has been investigated to yield a self-ex-change rate for all [Ru(NH3)5py] complexes of 4.7 x 10 mol" liter s Electron transfer reactions of [Ru(NH3)5py] complexes appear for the most part to follow Marcus behavior, and reasonable agreement with the thermal electron transfer rate calculated from the IT bond of [(NH3)5Ru(4,4 -bipy)-Ru(NH3)5] was found. However, in the oxidations by Fe , deviations from Marcus behavior which were found to be entropic in nature are considered to indicate a degree of nonadiabaticity in the electron transfer and a transmission coefficient of 5 X 10 was estimated. 

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