Through-space mechanisms

In summary, electron transfer dynamics, mediated through saturated hydrocarbon bridges and proteins, displays a surprisingly weak distance dependence behaviour (/J = 0.8-12 A 1), compared to that predicted for a pure through-space mechanism (P 3.0 A1). 

The spatial arrangement of the three [FeS] clusters in the molecule and the spectroscopic data suggest that they serve as relays for electron transfer between the active site and the surface, either following a through-space mechanism or through bonded... 

As seen, cyclophane structures shown in Schemes 1.4b through 1.4e have the following unique feature The through-bond distance within the paracyclophane fragment is held constant, whereas the spatial distance between the ion-radicalized and neutral moieties is changed. Therefore, the relative importance of through-bond and through-space mechanisms for intramolecular electron transfer can be learned directly from experimental data on these molecules. 

The possibility of a through-space mechanism, which would be especially sensitive for the a-position for blocked systems such as these, cannot be ruled out. 

For a donor-acceptor pair in the absence of any intervening medium (through-space mechanism), the exponential fall-off of the orbital tails leads to an expected exponential decrease of Hif with distance (Eq. 6)... 

Lone-pair interactions by the through-space mechanism, and the increase of the nucleophilicity of the participating atoms, have been implicated in mechanistic proposals for the oxidation of acetals, and for some reductions of glyculoses. On the other hand, the relative reactivities of the anomers in Sn2 processes has been discussed in terms of through-bond interactions of the lone pairs.Summing up the available evidence, it is clear that both types of lone-pair interaction must be recognized, in order to rationalize the stereoreactivity of saccharides. 

Kinetics of electron tranter. The central problem in investigations of the kinetics of electron transfer is to establish the mechanism and pathways of electron transfer. Is the through space mechanism relevant to this process, or have specific electron conduction pathways (through bond or superexchange) been designed into electron transfer proteins Once this question is resolved, then a quantitative understanding of electron transfer rates may be feasible. 

The production of Si(M) and 7i(M) acetone by the decomposition of tetra-methyl-l,2-dioxetan has been employed to probe the modes of energy transfer in a polystyrene matrix. The most important conclusions reached were as follows (i) diphenylanthracene quenches M by a long-range through-space mechanism (ii) the major mechanism for energy transfer to dibromoanthracene is the spin-forbidden triplet-singlet process et = ca. 1091 mol s-1.122... 

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