Electronic distance dependence

The second contribution to the energy arises from the electrostatic repulsion between pairs III electrons. This interaction depends on the electron-electron distance and, as we have seen, is calculated from infegrals such as ... 

Composilion has a marked effect on p. Dilution can cause (i to drop by orders of magnitude. The functional dependence is often expressed in terms of an exponential dependence on intersite distance R=at m as suggested by the homogeneous lattice gas model, p c)exponential distance dependence of intersite coupling precludes observing a percolation threshold for transport. 

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

The occurrence of energy transfer requires electronic interactions and therefore its rate decreases with increasing distance. Depending on the interaction mechanism, the distance dependence may follow a 1/r (resonance (Forster) mechanism) or e (exchange (Dexter) mechanisms) [ 1 ]. In both cases, energy transfer is favored by overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. 

Fig. 3 Distance dependence of the relative electron transfer rate from a single guanine to the enol ether radical cation... 

Fig. 2b. Both kcs and kcr are seen to decrease as the distance R between Sa and G C increases. In accord with a superexchange mechanism for photoin-duced electron transfer, the distance dependence can be described by Eq. (2) ... 

Distance dependence of the hole transfer process from the G-region (5 -GTGTGTG-3 ) to the Py moiety was studied via pulse radiolysis of 5 -Py-conjugated ODNs with a different number of intervening A-T base pairs between the G-region and Py moiety (PyODNn (n= 1 5)) (Scheme 3). Transient absorption with a maximum peak at 470 nm assigned to Py + was observed after the electron pulse during the pulse radiolysis (Fig. 2). This initial for-... 

In the absence of an endogenous centre, the specificity of relaxation data can be improved by introducing in known positions paramagnetic probes. The unpaired electron spin enhances the relaxation rate of the surrounding nuclei in a distance-dependent fashion. In this way, it is possible to evaluate specific interaction sites between a marked polymer and the small molecules. 

The foregoing treatment can be extended to cases where the electron-ion recombination is only partially diffusion-controlled and where the electron scattering mean free path is greater than the intermolecular separation. Both modifications are necessary when the electron mobility is - 100 cm2v is-1 or greater (Mozumder, 1990). It has been shown that the complicated random trajectory of a diffusing particle with a finite mean free path can have a simple representation in fractal diffusivity (Takayasu, 1982). In practice, this means the diffusion coefficient becomes distance-dependent of the form... 

The distance dependence of the electron-transfer rate can also be demonstrated for the non-benzenoid systems [14] and [15], which... 

Eng MP, Albinsson B (2006) Non-exponential distance dependence of bridge-mediated electronic coupling. Angew Chem Int Ed 45 5626—5629... 

Helms A, Heiler D, McLendon G (1992) Electron transfer in bis-porphyrin donor-acceptor compounds with polyphenylene spacers shows a weak distance dependence. J Am Chem Soc 114 6227-6238... 

Wold DV, Haag R, Rampi MA, Frisbie CD (2002) Distance dependence of electron tunneling through self-assembled monolayers measured by conducting probe atomic force microscopy unsaturated versus saturated molecular junctions. J Phys Chem B 106 2813-2816... 

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