Redox solvent effects

The donor-acceptor approach to solvent effects on the rates of redox reactions between different metal complexes, R. Schmid, Rev. Inorg. Chem., 1979,1,117-131 (48). 

Noviandri I, Brown KN, Fleming DS, Gulyas PT, Lay PA, Masters AF, Phillips L (1999) The decamethylferrocenium/decamethylferrocene redox couple a superior redox standard to the ferrocenium/ferrocene redox couple for studying solvent effects on the thermodynamics of electron transfer. J Phys Chem B 103 6713-6722... 

Azo-bridged ferrocene oligomers also show a marked dependence on the redox potentials and IT-band characteristics of the solvent, as is usual for class II mixed valence complexes 21,22). As for the conjugated ferrocene dimers, 2 and 241 the effects of solvents on the electron-exchange rates were analyzed on the basis of the Marcus-Hush theory, in which the t/max of the IT band depends on (l/Dop — 1 /Ds), where Dop and Ds are the solvent s optical and static dielectric constants, respectively (155-157). However, a detailed analysis of the solvent effect on z/max of the IT band of the azo-bridged ferrocene oligomers, 252,64+, and 642+, indicates that the i/max shift is dependent not only on the parameters in the Marcus-Hush theory but also on the nature of the solvent as donor or acceptor (92). 

Consequently, it is also apparent that the solvent effect can be described on the basis of mathematical relationships between parameters which fall within the relationships defined as free energy correlations. In fact, the more parameters that are included in the mathematical treatment (multi-parameter equations), the better the description of the solvent effect that results. However, we will consider here only those parameters which take into account the solvent effect on redox potentials. 

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). 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

PKa = 4.4, in water), less than O2 that the potential of 0 2 /H02 becomes higher than that of 02/0 2 . As a consequence, the superoxide disproportionates into O2 and HO2 , in the presence of proton sources. An evaluation of the solvent effect on the redox potential of the 02/0 2 system is not easy because of the difficulty in comparing the potential scales in various media but, obviously, assuming that the junction potential between the aqueous SCE and every solvent does not exist is far from correct [12] adopting any extrathermodynamic hypothesis would be better. The important shift in the one-electron reduction of O2 to 0 2 , almost 0.5 V, has been attributed to the solvation of 0 2 , which is much more strongly solvated by water than by the aprotic media hexamethylphosphorotriamide (HMPT) is the solvent where the 2/0 2 potential is... 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

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. 

Solvent Effects on Various Types of Redox Reactions... 

Solvent Effects on Redox Potentials and Redox Reaction Mechanisms... 

In this section, solvent effects are considered for each of the above reactions, focusing on the standard redox potentials and the reaction mechanisms. It should be noted that the potentials here are based on a scale common to all solvents, so... 

The developments in Os pentaammine chemistry described in this review have enabled an extensive library off 0 values to be obtained, spanning a range of 2 V (—200 kJ mol ) (Fig. 7). Table XIV compares the redox potentials of the Os(III/II) complexes with Ru(III/II) analogs versus the appropriate [M(NH3>6]3+/2+ couples. The data are presented in this manner in order to correct for solvent effects, which can be comparable with those induced by the v effects (574). The correction of... 

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)... 

While solvent effects are small for the ring redox ( 0.2 V), the effect of donor solvents which stabilize the higher oxidation state is marked for the metal redox, especially so for those of low-spin d6/d7 and d1jd%. There is a linear relationship between E° for the Feu/I couple and the Gutmann donicity number. The Com u and Cou/I couples shift positively as the pof substituted pyridine, as solvent, decreases and the ring oxidation precedes the CoIII/n couple in noncoordinating solvents such as CH2C12. 

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, firstly by ion-dipole or ion-induced dipole interactions and secondly by more direct interactions, such as hydrogen bonding to anions or electron pair donation to cations. The latter interactions, thus, depend on the Lewis acidity and basicity, respectively, of the solvents (Table 4.3). The redox properties of the ions at an electrode therefore depend on their being solvated, and the solvent effects on electrode potentials or polarographic half wave potentials, or similar quantities in voltammetry are manifested through the different solvation abilities of the solvents. 

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