Electron charge transfer process rate variation

In studies of molecular charge transfer systems a transient method is generally used to determine an electron transfer rate constant from the variation of the concentration of a reactant or product as a function of time. By contrast, in characterizing charge transfer processes in electrochemical cells or at metal-molecule- metal junctions the parameter of interest is usually the resistance or conductance (reciprocal of resistance) of the cell or junction. The conductance of the cell or junction is generally determined from the variation of the current through the system as a function of applied voltage. Here we briefly consider the relationship between electron transfer rate constants and conductances. 

By analyzing the variation of peak position as a function of a scan rate, it is possible to gain an estimate for the electron transfer rate constants. Adsorption processes on electrode surface can be distinguished from charge transfer processes, as in former case cyclic voltammogram is symmetrical around potential axes. 

The variations in the exogenous ligand L in the mixed-metal hemoglobin hybrids, [MP, Fe(L)P] where M = Zn or Mg and L = H2O, imidazole, CN , F", and Nf, have a considerable effect on both the photoinitiated [ MFe(III)] and thermally activated [M <-Fe(II)] electron transfer processes. Electron transfer between zinc and iron porphyrins across a variety of aromatic spacers has been examined. The rate constants for charge separation displayed an inverse dependence on the center-center distance but no correlation with the orientation of the porphyrin rings. The charge recombination rate constants were independent of the separation distance up to 23 A. 

Adsorption is the formation of some type of bond between the adsorbate and the electrode surface. The interaction may be merely electrostatic (e.g. the adsorption of cations or anions on a surface of opposite charge) or charge-dipole in nature (e.g. the adsorption of amines, thiourea, or benzene) or due to the formation of a covalent bond. Moreover, one sees great variations in the strength of the bonding and the reversibility of the adsorption process. Electrode reactions are most strongly affected when the adsorbate is the electroactive species, a reaction intermediate, or the product, but the adsorption of species apparently not directly involved in the electrode process can also change the rate of electron transfer and indeed the final product. 

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