Electron transfer model systems

Sessler. J.L. Brown, C.T. O Connor, D. Springs. S.L. Wang. R. Sathiosatham, M. Hirose, T. A rigid chlorin-naphthalene diimide conjugate. A possible new noncova-lent electron transfer model system. J. Org. Chem. 1998. 63 (21). 7370-7374. 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

Figure 3.22 Generic electron transfer (ET) system composed of local donor (D) and acceptor (A) sites, the intervening bridge (B), and the surrounding medium (or solvent). In the two-state approximation (TSA), the ET kinetics (e.g., for charge separation (CS) DBA-D+BA ) may be modeled in terms of initial (i) and final (f) states, in which the transferring charge is localized primarily on the D and A sites, respectively.

A central preoccupation of electron-transfer models is to provide estimates of the activation free energy of a single electron-transfer step, AG, from the structural and thermodynamic properties of the system [31]. Two key features of these models should be noted at the outset. Firstly, as noted above, it is advantageous to separate AG into intrinsic and thermodynamic components according to [cf. eqn. (6)]... 

In the following sections, selected comparisons between experimental kinetics for single-step electrochemical reactions and the foregoing electron-transfer models will be presented in order to characterize the physical features of the experimental systems as well as to scrutinize the applicability of the theoretical models themselves. 

This chapter will focus on the predictions of the traditional two-state electron transfer model. Only the ground and lowest excited state of the system are considered and contributions from higher electronic states are ignored. Thermal and optical electron transfers in both weakly and strongly interacting systems are discussed. The treatment is not intended to be exhaustive but instead will focus on certain features of the model that may be less familiar but which nevertheless have important implications. 

The electron transfer model presented here recalls the process of charge transport in semiconductors that is, a conduction band is populated by a thermalized electron, which then moves freely through the semiconductor via wavelike k states. While the possibility of semiconductorlike electron transfer in biological systems was first raised many years ago by DeVault and Chance [93], it has never been found experimentally in fact, there was reasonable skepticism that nature would choose such a mechanism in natural biological systems [84]. The density matrix method allows one to construct a model in which the conditions for such a process can be clarified and investigated in a detailed way. 

Finally, the conductance of the DBA bridged system can also be estimated from Equation (46a) using a traditional electron transfer model. Within this model the transmission coefficient... 

Levanon H and Mobius K 1997 Advanced EPR spectroscopy on electron transfer processes in photosynthesis and biomimetic model systems Ann. Rev. Biophys. Biomol. Struct. 26 495-540... 

Minimal END has also been applied to a model system for intramolecular electron transfer. The small triatomic system LiHLi is bent C2v structure. But the linear structure presents an unrestricted Haiti ee-Fock (TJHF) broken symmetry solution with the two charge localized stmctures... 

Several titanium(IV) complexes are efficient and reliable Lewis acid catalysts and they have been applied to numerous reactions, especially in combination with the so-called TADDOL (a, a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol) (22) ligands [53-55]. In the first study on normal electron-demand 1,3-dipolar cycloaddition reactions between nitrones and alkenes, which appeared in 1994, the catalytic reaction of a series of chiral TiCl2-TADDOLates on the reaction of nitrones 1 with al-kenoyloxazolidinones 19 was developed (Scheme 6.18) [56]. These substrates have turned out be the model system of choice for most studies on metal-catalyzed normal electron-demand 1,3-dipolar cycloaddition reactions of nitrones as it will appear from this chapter. When 10 mol% of the catalyst 23a was applied in the reaction depicted in Scheme 6.18 the reaction proceeded to give a yield of up to 94% ee after 20 h. The reaction led primarily to exo-21 and in the best case an endo/ exo ratio of 10 90 was obtained. The chiral information of the catalyst was transferred with a fair efficiency to the substrates as up to 60% ee of one of the isomers of exo3 was obtained [56]. 

Willner, I and Willner, B. Artifical Photosynthetic Model Systems Using Light-Induced Electron Transfer Reactions in Catalytic and Biocatalytic Assemblies. 159, 153-218... 

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