Inner-shell activation barrier

Finally, it is worth noting that the nature of the solvent-dependent ET dynamics is also predicted to be affected by the presence of inner-shell (reactant vibrational) contributions to the activation barrier [19]. As might be expected, the presence of higher-frequency vibrational contributions to the activation barrier can yield a marked attenuation in the degree to which overdamped solvent dynamics control the adiabatic barrier-crossing frequency [19]. The experimental exploration of such effects is limited in part by the paucity of redox couples suitable for solvent-dependent studies that exhibit known vibrational barriers, and complicated by the qualitatively similar behavior expected for nonadiabatic pathways. Nevertheless, there is some evidence that vibrational activation can indeed attenuate the role of solvent dynamics, although the theoretical predictions appear to overestimate the magnitude of this effect [10b,20]. 

Model calculations have been employed in elucidating the mechanism of electron transfer reactions in aqueous solution. The contribution of inner shell OH bonds to activation barriers has been estimated from calculation for metal ion hydrates. Calculated electron transfer matrix elements (Hjf) for redox processes of the type, MLe + MLe =... 

For adiabatic reaction pathways (i.e. Kel = 1) the nuclear frequency factor, vn (s 1), represents the rate at which reacting species in the vicinity of the transition state is transformed into products. This frequency will be influenced by a combination of the various motions associated with the passage of the system over the barrier, approximately weighted according to their relative contributions to the activation energy. These motions usually involve bond vibrations and solvent motion, associated with the characteristic inner- and outer-shell frequencies, vis and vos, respectively. A simple formula for vn which has been employed recently is [la, 7]... 

---