Weak electronic coupling

The present formula Eq. (126) is tested in comparison with the Bixon-Jortner perturbation theory in the weak electronic coupling regime [109]. The Arrhenius plot is shown in Fig. 23, where the electronic coupling Had is taken... 

It is important to remember that the Marcus model refers to a weakly electronically coupled model, as embodied in the term outer-sphere ET. Thus it must be assumed that the electronic overlap between the two reactants is so small that no quantum-chemical effects ensue, yet that there must be enough overlap for the transmission coefficient k of the Eyring equation to be equal to 1 (the reaction must be adiabatic). Usually, this minimum overlap requirement is put at a fairly low level, around 0.1 kcal mol-1, which causes no problems for most reactions involving at least one organic species. 

The assumption of weak electronic coupling may not be valid for vibrational levels near the region where the reactant and product surfaces intersect. If the extent of electronic coupling is sufficient (tens of cm ), the timescale for electron transfer for vibrational levels near the intersectional region will approach the vibrational timescale, electronic and nuclear motions are coupled, and the Born-Oppenheimer approximation is no longer valid. 

It is apparent that for sufficiently weak electronic coupling ... 

Singlet excitonic energy transfer between chls is most commonly discussed in terms of the two limiting cases of very strong and very weak electronic coupling (J) between donor (D) and acceptor (A) transition dipoles [159-161]. J(cm" ) may be calculated by the expression given by Pearlstein [162]... 

Figure 7.12 Energy diagrams for surface-molecule interactions in the case of (a) strong electronic coupling, (b) weak electronic coupling, and (c) no electronic coupling. ek stands for an electronic continuum of the metal or semiconductor band, whereas , represents the energy of frontier molecular orbital involved in this interaction...

In the simple ETs just discussed, there are no bond ruptures and usually only a relatively weak electronic coupling of the two reactants, perhaps of the order of kBT, or even much less when the reactants are far apart. 

The weak electronic coupling between distant donor and acceptor sites leads to long-range ET rates (fcCT) that are well-described by a non-adiabatic formulation (23) ... 

Equation (15) is the basis for equation (9) and several other important relationships for electron-transfer systems. For very weak electronic coupling and when high frequency modes do not contribute, the energy of the absorption maximum corresponds to the maximum value of this term (note that the nuclei do not move during the optical transition). 

Observations on the Weak Electronic Coupling Limit Ion-Pair Charge-Transfer Absorption Bands... 

The C +-P-Q state in triads such as 35 eventually recombines to the groimd state, unless it is harvested by subsequent reactions. The simplest possible recombination pathway involves electron transfer from the quinone radical anion directly to the carotenoid radical cation. However, this pathway can be very slow, even if thermodynamics are very favorable, because of the weak electronic coupling between the radical ions. In some cases, charge recombination has been found to fol-... 

The TPQ reduction potential shows a linear variation with pH of -60 mV/pH, indicating a 2e. 211 ET process, in contrast to the 2e, 3H ET observed with model compounds." Presumably the protein matrix or nearby Cu(II) stabilizes the deprotonated topaquinol. No copper electrochemistry was observed. It is not clear whether the copper potential is anomalously low or if it could not be measured due to weak electronic coupling with the phenyl-alkynyl bridge. 

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