Electron-coupled transport

The dynamics of inter- vs intrastrand hole transport has also been the subject of several theoretical investigations. Bixon and Jortner [38] initially estimated a penalty factor of ca. 1/30 for interstrand vs intrastrand G to G hole transport via a single intervening A T base pair, based on the matrix elements computed by Voityuk et al. [56]. A more recent analysis by Jortner et al. [50] of strand cleavage results reported by Barton et al. [45] led to the proposal that the penalty factor depends on strand polarity, with a factor of 1/3 found for a 5 -GAC(G) sequence and 1/40 for a 3 -GAC(G) sequence (interstrand hole acceptor in parentheses). The origin of this penalty is the reduced electronic coupling between bases in complementary strands. 

Unlike solid state -stacks, however, double helical DNA is a molecular structure. Here CT processes are considered in terms of electron or hole transfer and transport, rather than in terms of material conductivity. Moreover, the 7r-stack of DNA is constructed of four distinct bases and is therefore heterogeneous and generally non-periodic. This establishes differences in redox energetics and electronic coupling along the w-stack. The intimate association of DNA with the water and counterions of its environment further defines its structure and contributes to inhomogeneity along the mole-... 

The thermodynamics of insertion electrodes is discussed in detail in Chapter 7. In the present chapter attention is focused mainly on the general kinetic aspects of electrode reactions and on the techniques by which the transport of species within electrodes may be determined. The electrodes are treated in a general fashion as exhibiting mixed ionic and electronic transport, and attention is concentrated on the description of the coupled transport of these species. In this context it is useful to consider that an electronically conducting lead provides the electrons at the electrodes and compensates the charges of the ions transferred by the electrolyte. 

Lovric M, Scholz F (1999) A model for the coupled transport of ions and electrons in redox conductive microcrystals, J Solid State Electrochem 3 172-175. 

Current densities in the cathode are mainly determined by the respective value of oxide anion conductivity compared to the electronic conductivity (/Co" and ice", coupled to each other in Wagner diffusion). Equation (34) describes the current density limit for coupled transport of oxygen anions and electrons (777) ... 

If the cations of variable valency (e.g., Fe2+/Fe3 + ) are present in not too low concentrations, the crystals will be semiconductors. In non-equilibrium vermiculites, the internal electric field is then strongly influenced by their electronic conductivity, as explained in Section 4.4.2. If we start with an equilibrium crystal and change either pH, ae, aor a, (where i designates any other component), coupled transport processes are induced. The coupling is enforced firstly by the condition of electroneutrality, secondly by the site conservation requirements in the T-O-T blocks (Fig. 15-3), and thirdly by the available free volume in the (van der Waals) interlayer. It is in this interlayer that the cations and the molecules are the more mobile species. However, local ion exchange between the interlayer and the relatively rigid T-O-T blocks is also possible. 

Fig. 12. Electron-cation coupled transport a redox-driven electron-cation symport consisting of an electron carrier (nickel complex) and a selective cation carrier (macrocyclic polyether). RED, potassium dithionite OX, Na3[Fe(CNft)].

Fox, M. A., Fundamentals in the design of molecular electronic devices Long-range charge carrier transport and electronic coupling. Acc. Chem. Res. 1999, 32, 201-207. 

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