Bimolecular redox reactions

The participation of 5 molecules in homogeneous redox processes is mainly controlled by their short lifetimes. The lifetime of 5 is shortened by a bimolecular redox reaction (5.10.3) or (5.10.4) as follows ... 

Fig. 7. Free energy scheme for a bimolecular redox reaction...

E. Intramolecular Redox Assistance of Bimolecular Redox Reactions Conclusions... 

Unlike conventional electrochemical techniques, in SECM measurements the ITIES is poised by concentrations of the potential-determining ions, providing a constant driving force for the ET process. This eliminates some of the problems mentioned above. In a typical SECM/ITIES experiment, a tip ultramicroelectrode (UME) with a radius a is placed in the upper liquid phase containing reduced form of redox species, Rj. The tip is held at a positive potential, and Ri reacts at the tip surface to produce the oxidized form of the species, Oi. When the tip approaches the ITIES, the mediator can be regenerated at the interface via bimolecular redox reactions between Oi in the upper phase and R2 in the bottom phase ... 

Consequently, a wealth of information on the energetics of electron transfer for individual redox couples ("half-reactions") can be extracted from measurements of reversible cell potentials and electrochemical rate constant-overpotential relationships, both studied as a function of temperature. Such electrochemical measurements can, therefore, provide information on the contributions of each redox couple to the energetics of the bimolecular homogeneous reactions which is unobtainable from ordinary chemical thermodynamic and kinetic measurements. 

In biological systems, electron transfer kinetics are determined by many factors of different physical origin. This is especially true in the case of a bimolecular reaction, since the rate expression then involves the formation constant Kf of the transient bimolecular complex as well as the rate of the intracomplex transfer [4]. The elucidation of the factors that influence the value of Kf in redox reactions between two proteins, or between a protein and organic or inorganic complexes, has been the subject of many experimental studies, and some of them are presented in this volume. The complexation step is essential in ensuring specific recognition between physiological partners. However, it is not considered in the present chapter, which deals with the intramolecular or intracomplex steps which are the direct concern of electron transfer theories. 

Estimation of rates for redox reactions in environmental systems requires that the problem be formulated in terms of specific oxidation and reduction half-reactions. In addition, we assume that the rate-limiting step of the transformation mechanism is bimolecular—that is, the slow step requires an encounter (collision) between the electron donor and electron acceptor. Under most conditions found in environmental systems, such reactions exhibit rate laws for the disappearance of a pollutant, P, that are first-order in concentration of P and first-order in the concentration of environmental oxidant or reductant, E,... 

As discussed in section 3.3, there is no reason to expect that measures of "overall" redox conditions (such as Pt electrode potentials or concentrations of dissolved H2) will ever provide an improved basis for quantitatively predicting rates of environmental redox reactions. However, extensions and refinements to the simplified bimolecular model can be made when sufficient data are available. 

The mechanism of the fourth category of bimolecular surface steps is peculiar to redox reactions catalysed by metals and semiconductors. Here both reactants sit on the surface, not necessarily on adjacent sites, and the electrons are transferred from the reducing to the oxidising species through the solid catalyst. The rate therefore depends not only on the concentrations at the surface but also on the potential taken up by the catalyst, and this potential in turn is a function of the concentrations of the electroactive species present. Equations (28) and (29) fail to represent the kinetics in these cases because khel is no longer independent of concentration. These kinetics must accordingly be treated by an electrochemical method of analysis and this is done in Sect. 4.1. 

Such redox reactions are frequently catalysed by platinum [3], other noble metals [232], silver [126-128], and carbons [233] which are all electronconducting solids. This fact points to a simple catalytic mechanism whereby the electron is transferred from Redi to Ox2 through the solid phase, as depicted in Fig. 19. In contrast to other bimolecular catalytic mechanisms (Sect. 1.5.3), the two reactants do not need to occupy neighbouring sites. Since the catalytic rate depends upon the coupled transfers of an electron from Red] to the solid and from the solid to Ox2, the kinetics are best treated in electrochemical terms. 

Another evident mechanism for energy transfer to activated ions may be by bimolecular collisions between water molecules and solvated ion reactants, for which the collision number is n(ri+ r2)2(87tkT/p )l/2> where n is the water molecule concentration, ri and r2 are the radii of the solvated ion and water molecule of reduced mass p. With ri, r2 = 3.4 and 1.4 A, this is 1.5 x 1013 s"1. The Soviet theoreticians believed that the appropriate frequency should be for water dipole librations, which they took to be equal 10n s 1. This in fact corresponds to a frequency much lower than that of the classical continuum in water.78 Under FC conditions, the net rate of formation of activated molecules (the rate of formation minus rate of deactivation) multiplied by the electron transmission coefficient under nonadiabatic transfer conditions, will determine the preexponential factor. If a one-electron redox reaction has an exchange current of 10 3 A/cm2 at 1.0 M concentration, the extreme values of the frequency factors (106 and 4.9 x 103 cm 2 s 1) correspond to activation energies of 62.6 and 49.4 kJ/mole respectively under equilibrium conditions for adiabatic FC electron transfer. 

The photochemical reactions " of Cr(III) include both ligand substitutions and isomerization in solution and solids. Intramolecular redox reactions also are noted when charge-transfer bands are excited as well as intermolecular ones in cases where a long-lived state is quenched by bimolecular electron transfer to another species in solution. ... 

Quenching of the ( CT)[Ru(bipy)3] by [Cr(bipy)]3 has been studied. This is via electron transfer to the Cr complex and a rapid back reaction. The ruthenium complex will also quench the 727 nm emission of the metal-centred doublet excited state of the chromium species, by a similar mechanism. Evidently both ligand- and metal-centred excited states can be quenched by bimolecular redox processes. A number of Ru complexes, e.g. [Ru(bipy)3] and [Ru(phen)3] also have their luminescence quenched by electron transfer to Fe or paraquat. Both the initial quenching reactions and back reactions are close to the diffusion-controlled limit. These mechanisms involve initial oxidation of Ru to Ru [equation (1)]. However, the triplet excited state is more active than the ground state towards reductants as well as... 

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