Activationless electron transfer

The total reorganization energy is equal to the free energy change for an activationless electron transfer reaction (40-48). In Marcus theory (48,53) this condition is expressed as AG -X, where... 

Figure 2.2 Reaction coordinate parabolas for diabatic electron transfer, illustrating the relationship between AG° and k for (a) the normal region, where AG° < k (b) the activationless electron transfer region, where AG° = k (c) the inverted region, where AG° > k...

Jasaitis A, Rappaport F, Pilet E, Liebl U, Vos MH. Activationless electron transfer through the hydrophobic core of cytochrome oxidase. Proc Natl Acad Sci USA 2005 102 10882-6. 

Here r is the separation distance, ro the sum of the van der Waals radii of the donor and acceptor sites. Ho the electronic coupling strength at van der Waals contact, and P the distance attenuation factor, which can be used to parameterize the efficiency of the donor-acceptor coupling. Thus, under conditions of activationless electron transfer (-AG = A,), the maximum ET rate constant, is expected to obey Eq. (4) ... 

FIGURE 34.4 Dependence of electrochemical rate constant on the electrode potential for outer-sphere electron transfer. An exponential increase in the normal region changes for the plateau in the activationless region. 

In the activationless region (so called because AG = 0), a change in the thermodynamic driving force has a negligible effect on the rate of electron transfer. 

Figure 2. Theoretical prediction for the temperature dependence of the electron transfer rate for activated and for activationless processes. Solid lines are calculated for a continuum of vibrational modes dotted lines represent the single-mode approximation (6, 8). Upper curve AE, —2000 cm 1 P, 20 and S, 20. Lower curves AE, —800 cm"1 P, 8 and S, 20.

Solvent water + methanol Xi = 580 or 600 nm very fast charge recombination (k 10M010 s"1) the charge separation is adiabatic, activationless and solvent-controlled electron transfer... 

Figure 13 The different Marcus regimes of electron transfer normal activationless inverted.

The transition from the contact to distant electron transfer causes the inverted branch of the FEG curve to slope more gently than normal [32,110,111], This branch is extended to an even greater extent if the transfer is multichannel (see Fig. 3.2). In this case the inverted branch is composed from the tops of the partial FEG curves, where the transfer is activationless. Therefore, their sum is also temperature independent and smooth. But the best way of stretching the diffusional plateau is by taking into account the space dependence of X(r). Since maximum W/(r) moves away with increasing AG the effective X... 

A third and provisionally accepted explanation is that electron transfer can take place to vibrationally excited states of the products, i.e. nuclear tunnelling of the reactants to vibrationally excited states of the products takes place (Efrima and Bixon, 1974, 1976). The potential surfaces depicted in Fig. 10 show the rationale behind this mechanism. For AG° > — X (Fig. 10a) we have the normal situation with an activation barrier for electron transfer. At AG0 = —X (Fig. 106) the maximum rate for an activationless process has been reached, whereas for AG° < —X an activation barrier appears again (Fig. 10c, representing the inverted region). With electron transfer allowed to an excited vibrational level (dotted line in Fig. 10d) we have once again an activationless reaction proceeding at the maximum rate. For large molecules there is a... 

In the high-resolution ESR (326 GHz) study of the biradical state Qa - Qb - in the Rb. Spheroids, RC determines the exchange integral in the biradical (Jo = 109 s 1) (Calvo et al., 2001). Because the rate constant of electron transfer from Qa to Qb is essentially less (kET 104 s 1) (Feher et al., 1992 Xu et al, 2000) than expected for an nonadiabatic activationless ET and the kET values considerably deviate from the dependence of the supperexchange attenuation parameter (yET) on the distance between donor and acceptor centers in RCs (Fig. XXX), we can conclude that the ET is adiabatic and requires thermal activation. 

The super-exchange electronic coupling term describes the coupling of the P Ba" state to the P and P Ha" states at the position of the intersection between the potential surfaces of the P and P Ha" states. When the P and P Ha" potential surfaces intersect at the minimum of the P potential surface (i.e. assuming that electron transfer from P to P Ha" is activationless), the super-exchange coupling (Vsuper) is given by ... 

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