Cross electron-transfer reaction

Fiomogeneous cross-reaction electron-transfer kinetic studies suggest that many other Cu(II/I) systems obey Scheme 1. However, few Cu(II/I) systems have been subjected to sufficiently low temperature or rapid-scan CV measurements to demonstrate the presence of rate-limiting conformational changes. 

Cross-reaction electron-transfer rates for the cobalt(II) clathrochelates. 

Effect of the anions on the cross-reaction electron-transfer rates for the macrohicychc [Co(sep)]2+ cation. 

In principal, electron transfer reactions with fullerenes could occur via both the singlet- and triplet-excited state. However, due to the short singlet lifetime and the efficient intersystem crossing, intermolecular electron transfer reactions usually occur with the much longer lived triplet-excited state. The result of the electron transfer is a radical ion pair of fullerene and electron donor or acceptor. 

The reaction starts with excitation of the quinone, followed by intersystem crossing and electron transfer from the thiophene to the triplet excited quinone. The ion radical pair collapses to a biradical which loses a chlorine and a hydrogen atom. Yields are high (65-78%) when R1 = halogen and R2 = H, fair (57%) when R1 = R2 = H and poor (2-17%) when R1 = H and R2 = halogen. The regioselectivity has been explained on the basis of calculated electron densities in the cation radicals of thiophenes. 

Figure C3.2.1. A slice tlirough tlie intersecting potential energy curves associated witli tlie K-l-Br2 electron transfer reaction. At tlie crossing point between tlie curves (Afy, electron transfer occurs, tlius Tiarjiooning tlie species,...

A powerful application of outer-sphere electron transfer theory relates the ET rate between D and A to the rates of self exchange for the individual species. Self-exchange rates correspond to electron transfer in D/D (/cjj) and A/A (/c22)- These rates are related through the cross-relation to the D/A electron transfer reaction by the expression... 

One also obtains analogous findings with trace-crossing effects for the electropolymerization of thiophene and pyrrole. This cannot be explained by a simple linear reaction sequence, as presented in Scheme I, because it indicates competing homogeneous and heterogeneous electron transfer processes. Measurements carried out in a diluted solution of JV-phenylcarbazole provide a more accurate insight into the reaction mechanism (Fig. 2). 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

F ure 9.26. Energy profile along (a) the reaction coordinate at an avoided crossing for a photochemical reaction and (h) an electron transfer process. 

In Figure 9.29, we show three surface topologies that we have been able to document in electron transfer processes in radical cations. Two views are presented in each case. At the top, we show a cross-section along the coordinate orthogonal to the reaction path (analogous to that shown for X2 in Fig. 9.28a). Then, at the bottom, we... 

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