Donor-acceptor distance, electron-transfer coupling

Distance The affects of electron donor-acceptor distance on reaction rate arises because electron transfer, like any reaction, requires the wavefunctions of the reactants to mix (i.e. orbital overlap must occur). Unlike atom transfer, the relatively weak overlap which can occur at long distances (> 10 A) may still be sufficient to allow reaction at significant rates. On the basis of work with both proteins and models, it is now generally accepted that donor-acceptor electronic coupling, and thus electron transfer rates, decrease exponentially with distance kji Ve, exp . FCF where v i is the frequency of the mode which promotes reaction (previously estimated between 10 -10 s )FCF is a Franck Condon Factor explained below, and p is empirically estimated to range from 0.8-1.2 with a value of p 0.9 A most common for proteins. 

The distance between the electron donor and acceptor affects the rates and mechanisms of PCET reactions in two different ways. First, an increase in this distance results in a decrease in the coupling between ET states (la/2a, aj2b, bj2a, bj2b). In the limit of electronically non-adiabatic electron transfer, a decrease in this coupling results in a decrease in the rate. Moreover, as the distance between the electron donor and acceptor increases, the interaction between the proton and the electron decreases. Thus, for a symmetric PT system with an initial state of la, EPT is favorable for short electron donor-acceptor distances and ET becomes equally favorable as this distance increases. 

In the simplest case, the R mode is characterized by a low frequency and is not dynamically coupled to the fluctuations of the solvent. The system is assumed to maintain an equilibrium distribution along the R coordinate. In this case, ve can exclude the R mode from the dynamical description and consider an equilibrium ensemble of PCET systems with fixed proton donor-acceptor distances. The electrons and transferring proton are assumed to be adiabatic with respect to the R coordinate and solvent coordinates within the reactant and product states. Thus, the reaction is described in terms of nonadiabatic transitions between two sets of intersecting free energy surfaces ( R, and ej, Zp, corresponding to... 

Harriman and co-workers found a remarkably small attenuation factor (/ = 0.11 A ) for triplet excitation energy transfer (TEET) in a series of binuclear metal complexes (6) linked by oligophenylacetylenes (Harriman et al, 2000), i.e. the electronic coupling decreases only weakly with increasing donor-acceptor distance. Moreover, it was also foimd, by extrapolation, that the rate constant for electron exchange at donor-acceptor orbital contact (A =10 s ) was unusually small. These... 

The influence of the electronic coupling on the electron transfer rate was determined by changing the length of the (A T)n bridge. As expected, the rate decreased as the number n of the A T base pairs between electron donor and electron acceptor increased [4, 7]. But, surprisingly, the exponential correlation of Eq. (1) between the rate kEr and the distance is not valid for short distances. The plots in Fig. 3 and Fig. 4 show that at 6 A the electron transfer rate /cEt is much faster than expected [4, 7]. 

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