Adiabaticity effective electron tunneling distance

Relatively little attention has been given in the literature to the electronic transmission coefficient for electrochemical reactions. On the basis of the conventional collisional treatment of the pre-exponential factor for outer-sphere reactions, Kel has commonly been assumed to equal unity, i.e. adiabatic reaction pathways are followed. Nevertheless, as noted above, the dependence of xei upon the spatial position of the transition state is of key significance in the "encounter pre-equilibrium treatment embodied in eqns. (13) and (14). Thus, the manner in which Kel varies with the reactant-electrode separation for outer-sphere reactions will influence the integral of reaction sites that effectively contribute to the overall measured rate constant and hence the effective electron-tunneling distance, Srx, in eqn. (14). 

As noted in Sect. 3.1, the pre-exponential factor for outer-sphere reactions is sensitive to the "effective electron-tunneling distance , SrK%. If the reaction is non-adiabatic at the plane of closest approach (i.e. if 1), given the functional dependence of fcel upon r one deduces that 5r 0.5 A if P 1.5 A 1 (vide supra), and hence drfCe, < 0.5 A. If, however, adiabaticity is maintained for sites beyond the plane of closest approach (i.e. for r > [Pg.43]

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