Redox properties solvent effects

The combination of theoretical and experimental results strongly suggests that conformational variations observed for porphyrin derivatives can provide a mechanism for altering optical and redox properties. Such effects, in combination with additional modulations induced by protein residues (or solvents), thus provide an attractive mechanism for fine-tuning the electronic properties of the chromophores in vitro and in vivo. 

The treatment of electrostatics and dielectric effects in molecular mechanics calculations necessary for redox property calculations can be divided into two issues electronic polarization contributions to the dielectric response and reorientational polarization contributions to the dielectric response. Without reorientation, the electronic polarization contribution to e is 2 for the types of atoms found in biological systems. The reorientational contribution is due to the reorientation of polar groups by charges. In the protein, the reorientation is restricted by the bonding between the polar groups, whereas in water the reorientation is enhanced owing to cooperative effects of the freely rotating solvent molecules. 

Deviation includes, in fact, the summation of steric and electronic effects, and basicity is somewhat a useful predictor for properties of complex dyes (solvent sensitivity, isomeric forms of trinuclear dyes) and gives also semiquantitative data for color structure relation charge density, redox properties. 

The redox potentials found for given concentrations of acids are related to the Ka values, which are indicative of their EPA properties. The effects become more pronounced at extremely high acid contents and increase from propionic acid to formic add. This is a bulk solvent effect (formic acid is more highly structured than propionic acid). 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

This book was written to provide readers with some knowledge of electrochemistry in non-aqueous solutions, from its fundamentals to the latest developments, including the current situation concerning hazardous solvents. The book is divided into two parts. Part I (Chapters 1 to 4) contains a discussion of solvent properties and then deals with solvent effects on chemical processes such as ion solvation, ion complexation, electrolyte dissociation, acid-base reactions and redox reactions. Such solvent effects are of fundamental importance in understanding chem-... 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

An approach to quantifying the interaction between solute and solvent and hence to solvent effects on redox potentials is that developed by Gutmann.41 Interactions between solvent and solute are treated as donor-acceptor interactions, with each solvent being characterized by two independent parameters which attempt to quantify the electron pair donor properties (donor number)... 

The effect of complexation on redox properties was studied by cyclic voltammetry. Unbound flavin, dissolved in an aprotic solvent (dichloromethane), undergoes a two electron reduction perfectly explained by the ECE mechanism. Upon addition of cyclene ligand and coordination of flavin to the zinc ion complex, the flavohydroquinone redox state was stabilised. 

Taras-Goslinska K, Jonsson M. (2006) Solvent effects on the redox properties of thioethers./Pfryr Chem A 110 9513-9517. 

As a final note concerning solvent effects, amphiphilic alcohols added to hydro-phobic electroactive n-alkanethiol SAMs in aqueous solution appear to aggregate on the monolayer surfaces, decreasing the capacitive envelope and enhancing the barrier properties [109, 110]. However, the formal potentials of the redox couples are shifted positively, and the electrochemical reversibility is decreased. This effect had previously been used by Becka and Miller to determine the pinhole current in the presence of a freely diffusing redox probe (see above) [96]. 

The exact nature of the reaction (oxidative vs. reductive) will depend on the redox properties of I ) and Q. The electron transfer process is a special case of exciplex formation favored in the strongly polar solvents, such as water. The involvement of an exciplex in a photochemical reaction is generally established by studying the effects of known exciplex quenchers such as amines on the exciplex fluorescence and the product formation. The heavy atom effect, due to the presence of substituents such as bromine or iodine intra- or intermolecularly, causes an exciplex to move to the triplet state preferentially, with a quenching of fluorescence. 

Reviews of interest include a general review of anation reactions of cobalt(III) complexes and a discussion of the solvation of transition metal complexes/ The nature of the solvent can have a very large effect on such properties as solubilities, reactivities, redox potentials, formation constants, and various types of spectra. Such solvent effects reflect changes in the solvation of ions, complexes, initial states, transition states, and excited states. 

Research workers investigating the solvent effect selected model systems with some well measurable property (e.g., light absorption in the UV, visible or IR spectrum, heat of formation, an NMR, Mossbauer or NQR parameter, the redox potential, reaction rate, etc.) which changes appreciably due to the effect of the solvent. Hence, these experimentally measurable data, characteristic of the extent of the interaction between the solvent and the solute, may serve to categorize the solvating powers of solvents. Of course, solvent scales obtained in this way can be compared with one another only if the solvation process in the different model systems is governed by analogous factors. 

In this section, the effect of solvent and the nature of the peptide substituent on the electrochemical properties of the Fc redox probe are discussed. The redox properties of Fc-peptides esters 17-19 and 23-25 were investigated in methanol, acetonitrile and dichloromethane, while the acid analog 20-22 and 26-28 were studied in... 

The properties (the effective viscosity and density) of the liquid layer in close vicinity to the interface can differ from their bulk values. There are various reasons for these phenomena. For example, the properties of a thin liquid layer confined between solid walls are determined by interactions with the solid walls [58,59]. In electrochemical system the structuring of a solvent induced by the substrate and a nonuniform ion distribution in the diffuse double layer can significantly influence the properties of the solution at the interface. The nonuniform distribution of species, which influences the properties of the liquid near the electrode, also occurs in the case of diffusion kinetics. The latter was considered in Ref. 60, where the ferro/ferri redox system was studied by the EQCM. This was the case where the velocity decay length ( >25 pm) was much less than the thickness of the diffusion layer ( >100 pm), in which the composition of the solution is different from the bulk composition. 

Taraszewska, J.,and Roslonek,G., "Effect of solvent on the kinetic and thermodynamic redox properties of Ni(III)/ Ni(II) in tetraazamacrocyclic complexes with pendant amino group and cyclam studied on the glassy carbon electrodes", J.Electroanal.Chem.,(in press). 

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