Parameters of Electron-Transfer Kinetics

Abstract

Over the last two decades, there has been a considerable increase in experimental information about the rates of electron transfer processes, particularly in solution in ionizing solvents. The main features (enthalpies, free energies and entropies of activation) of a large class of these reactions have been broadly understood for about the same length of time. It is interesting to note that the results of very early calculations of outer-sphere reaction rates and their tempera ture dependence frequently agree with recent more detailed estimates. In order to assess the appli cability of different approaches, and thus to discriminate between them, it is desirable to have as much detailed information as possible about the relevant basic parameters. These include, for example, electron coupling or overlap energies, electron-phonon coupling strengths, vibrational frequencies, and spin coupling data. A brief survey is given of the types of experiment which may provide this information. With such data, discussions on matters such as the applicability of Golden Rule perturbation approaches, electronic adiabaticity, the role of nuclear tunnelling, etc., can be put on firmer ground, and some of the outstanding theoretical issues awaiting reso lution are outlined.The theory of homogeneous electron transfer processes, as well as of the closely-related electron exchanges with metallic electrodes, has been the subject of considerable study.The proposal by Hush and by Marcus that these processes are, for simple systems, either usually electronically adiabatic or 0097-6156/82/0198-0301$09.00/0

A molecularly based theory for electron transfer reorganization energy

“…For the Co 2+ /Co 3+ exchange, the DSCFT-calculated activation energy, as well as the activation energy calculated in earlier literature, is much higher than the experimentally obtained activation energy. The reason for this may be the presence of an alternative pathway for electron transfer, as discussed in several earlier papers 78,79 and reviewed by Sutin in Ref.…”

Shallow traps for thermally induced hole hopping in DNA

“…For both (B.1) and (B.2) structures, charge hopping in DNA is described in terms of the quantum mechanical non-adiabatic theory [25,[28][29][30][32][33][34][35][36], a field to which important contributions were made by Noel Hush [37][38][39][40][41][42][43][44][45]. The hopping rates between the G resting sites, between the resting and the bridge sites G and A, and between the A bridge sites, are each described in terms of electronic (direct exchange or superexchange) coupling [32][33][34][35][36], nuclear coupling with a low-frequency medium, and intermolecular modes together with high-frequency intramolecular modes [25,[28][29][30][32][33][34][35][36].…”

“…We advanced and applied a kinetic-quantum mechanical model [25,[28][29][30][31][32][33][34][35][36][37][38][39][40][44][45][46]49] for hole transport in DNA duplexes, which rests on quantum mechanical non-adiabatic intersite hopping rates in conjunction with kinetic schemes, to describe the compound mechanism of superexchange TIH transport in G + (A-T) n GGG duplexes (Fig. 1).…”

A Critical Assessment of Classical and Semi-Classical Models for Electron Transfer Reactions in Solution

“…The calculated exchange rate for Co(OH 2 ) 6 2+/3+ is 8 orders of magnitude lower than the experimental rate. Detailed experimental studies on the possibility of alternative mechanisms for this reaction have failed to explain the origin of this anomaly [10,11,[57][58][59][60][61]. Controversy exists concerning the adiabatic nature of exchanges involving cobalt complexes like Co(NH 3 ) 6 2+/3+ and Co(phen) 3 2+/3+ .…”

“…In this work we will make the approximation that ∆E ‡ = ∆G ‡ [16]. The reduced mass of the system is determined on the assumption that the oxidized and reduced oscillators have common ∆x and ∆E ‡ values [161] √µ = √µ ox + √µ red (61) This is consistent with the "equal bond-distortion" pathway employed by ISM. The normal- Figure 23.…”