Bimolecular electron transfer rate constant

Table 2 Bimolecular Electron-Transfer Rate Constants Measured When the Metal Complexes Were Intercalated in BAZrP (0.008% by Weight)...

The rate of enzyme-mediator electron transfer has been described generally to vary exponentially with the electrochemical overpotential in accordance with Marcus theory such that the rate-limiting bimolecular electron transfer rate constant (A et) is given by... 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

The Smoluchowski theory for diffusion-controlled reactions, when combined with the Stokes-Einstein equation for the diffusion coefficient, predicts that the rate constant for a diffusion-controlled reaction will be inversely proportional to the solution viscosity.16 Therefore, the literature values for the bimolecular electron transfer reactions (measured for a solution viscosity of r ) were adjusted by multiplying by the factor r 1/r 2 to obtain the adjusted value of the kinetic constant... 

Tests of the inverted region for bimolecular electron transfer have proven to be more elusive. As mentioned above, a major difficulty is that, for many bimolecular reactions, vetKA > kD and a large portion of the free energy region of experimental interest is lost because the rate constants... 

This is probably the most important radical species in aqueous solution. Oxidative reactions of the hydroxyl radical, ( OH), with inorganic and organic compounds have been well documented [8]. Compilations of bimolecular (second-order) rate constants have been published [3,8]. The OH- can undergo several types of reactions with species in aqueous solution, including addition, hydrogen abstraction, and electron transfer. 

Electron transfer becomes more favorable with decreasing separation of the two reactants, because of the electronic interactions. These are the strongest when the two reactants are in contact. As a consequence, the first step in bimolecular electron transfer reactions is the formation of a precursor complex from the separated reactants. Therefore, in the case of the bimolecular reactions the association constant /fact for the formation of the precursor complex should be included in the expression for the observed overall reaction rate obs- Provided that the formation of the... 

For a simple one-step, bimolecular electron transfer reaction involving ConL and ConlL complexes the rate expression is equation (5.13) where kso is the second-order rate constant. 

Kinetics of Electron-Transfer Quenching Figure 8.6 shows that the excited-state complex [Cr(phen)3]3+ can have either two or three reaction pathways, depending whether or not a quencher Q is present in solution. As the concentration of Q increases, the rate of the bimolecular electron-transfer pathway increases. We can take advantage of this to determine the rate constant of the electron-transfer pathway. 

Simple bimolecular electron transfer reactions can occur between radicals and nonradical species, and they can also occur between radicals and transition metal complexes. The number of reactions in this category is quite large, and they are notable for the wide range of reported rate constants. A sampling of these reactions, selected for their reversibility, is shown in Table 9.8. 

---