Molecular Factors Influencing Electron-Transfer

A wide range of optically interesting bronze materials such as LiFe Fe F6 have been described.Although architecturally and magnetically of interest, typical resistivity is high at room temperature, 10 2 cm. The factors that influence the rates of intramolecular electron transfer in solids have been discussed by Hendrickson. Intramolecular electron transfer in this area is being intensely examined for possible application in molecular electronics. 

One of the main factors influencing the activation barrier in fast electron-transfer reactions is the change in the polarization of the immediate space surrounding the activated complex in solution. The more-well-known salt effects as well as the relatively new field of micellar effects can be used as mechanistic probes in this context. Since micelles have a hydrophobic as well as a hydrophilic part, this creates two different kinds of interfaces where electron transfer can occur if one of either the oxidant or reductant is contained or associated with these molecular aggregates. A futuristic approach could be that studies of this kind may serve as models for enzymatic reactions with complex bioaggregates such as membranes. 

Solubility enhancement by use of cyclodextrins is achieved for a number of drug substances, an approach of interest in formulation of drugs for topical, parenteral, and oral use (Stella and Rajewski, 1997 Loftsson and Masson, 2001 Qi and Sikorski, 2001). The solubilizing effect can be extensive even in low concentrations of cyclodextrin. The use of 0.1 M sulfobutyl-ether-P-cyclodextrin increases the solubility of prednisolone acetate and testosterone by a factor of 426 and 2020, respectively (Myrdal and Yalkowsky, 2002). Cyclodextrin encapsulation of a molecule will affect many of its physicochemical properties (Loftsson, 1995). As a result of complexation, solubility, pKa value, spectral properties, and the chemical reactivity of the included substance will change. The cyclodextrins are known to affect molecular orientation and to have an influence on rates and efficiency of electron transfer, excited state proton transfer, and rate of decomposition (Chattopadhyay, 1991 Fox, 1991 Sur et al., 2000). Cyclodextrins can also be used in combination with liposomes a cyclodextrin-liposome entity represents an even more complex environment to the drug molecule (Loukas et al., 1995). 

Lower activities were attributed to reduced stability of the active species under polymerization conditions. The lower molecular weights of their products were explained to be the result of increased hydrogen transfer rates. Variations within the heterocyclic components of the ligand showed that both steric and electronic factors influence polymerization behavior of such catalysts. 

Linear regression of one-electron reduction potentials (which are proportional to free energies for electron transfer reactions) with log kohs values for the transformation of 8 halogenated aliphatic compoimds by FeS produced only a weak LFER (R =0.48) (7 7). The poor R value and the small slope of the LFER were attributed to several factors, including the influence of thermodynamic or molecular parameters other than one-electron reduction potentials on reaction rates, different mechanisms of adsorption to the FeS surface for different pollutants, different reaction mechanisms at the FeS surface, and/or significant adsorption of certain pollutants to non-reactive FeS surface sites (77). 

These studies indicate that the charge transfer at the metal-oxide interface alters the electronic structure of the metal thin film, which in turn affects the adsorption of molecules to these surfaces. Understanding the effect that an oxide support has on molecular adsorption can give insight into how local environmental factors control the reactivity at the metal surface, presenting new avenues for tuning the properties of metal thin films and nanoparticles. Coupled with the knowledge of how particle size and shape modify the metal s electronic properties, these results can be used to predict how local structure and environment influence the reactivity at the metal surface. 

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