Diffusion effects, electron-transfer

DNA mediated photoelectron transfer reactions have been demonstrated60 . Binding to DNA assists the electron transfer between the metal-centered donor-acceptor pairs. The increase in rate in the presence of DNA illustrates that reactions at a macromolecular surface may be faster than those in bulk homogeneous phase. These systems can provide models for the diffusion of molecules bound on biological macromolecular surfaces, for protein diffusion along DNA helices, and in considering the effect of medium, orientation and diffusion on electron transfer on macromolecular surfaces. 

Figure 3.9. The dependence on diffusion of the effective electron transfer radius Rq. The experimental data, indicated by circles, are approximated by thin and dashed lines, representing the contact and the generalized Collins-Kimball relationships, respectively. The thick line depicts the same dependence, but for the exponential transfer rate with / = 0.85 A and Wc = 180 ns-1. The dashed-dotted line at the bottom represents the high-viscosity asymptote of such dependence but for the Marcus transfer rate. (From. Ref. 18.)...

Much attention has been paid to the chemistry and biological chemistry of singlet dioxygen and superoxide ion. The lifetime of the former species, known to be solvent-dependent, is much increased for deuterated acetone, acetonitrile, benzene and chloroform compared to the undeuterated solvent. This is a remarkable solvent deuterium isotope effect. Electron transfer to 02 from tetramethylphenylenediamine to give superoxide has been confirmed, the rate of reaction being close to the diffusion-controlled limit. Kinetic studies on the photochemical formation of superoxide from jU-superoxodecacyanodicobalt(III) ions and oxygenated ethanol solutions have been reported. 

The investigation by Becker et al. (1977 b) also included work on the effect of pyrene added as electron donor. Pyrene has an absorption maximum at 335 nm (e = 55000 M-1cm-1, in petroleum). Much more hydro-de-diazoniation takes place in the presence of pyrene with irradiation at 365 nm, and even more on irradiation with light of wavelength <313 nm. Photoexcited pyrene has a half-life of 300 ns and is able to transfer an electron to the diazonium ion. This electron transfer is diffusion-controlled (k= (2-3) X 1010 m 1s 1, Becker et al., 1977a). The radical pairs formed (ArN2 S +) can be detected by 13C- and 15N-CIDNP experiments (Becker et al., 1983, and papers cited there). 

Pai Vemeker and Kannan [1273] observe that data for the decomposition of BaN6 single crystals fit the Avrami—Erofe ev equation [eqn. (6), n = 3] for 0.05 < a < 0.90. Arrhenius plots (393—463 K) showed a discontinuous rise in E value from 96 to 154 kJ mole-1 at a temperature that varied with type and concentration of dopant present Na+ and CO2-impurities increased the transition temperature and sensitized the rate, whereas Al3+ caused the opposite effects. It is concluded, on the basis of these and other observations, that the rate-determining step in BaN6 decomposition is diffusion of Ba2+ interstitial ions rather than a process involving electron transfer. 

Theoretical models available in the literature consider the electron loss, the counter-ion diffusion, or the nucleation process as the rate-limiting steps they follow traditional electrochemical models and avoid any structural treatment of the electrode. Our approach relies on the electro-chemically stimulated conformational relaxation control of the process. Although these conformational movements179 are present at any moment of the oxidation process (as proved by the experimental determination of the volume change or the continuous movements of artificial muscles), in order to be able to quantify them, we need to isolate them from either the electrons transfers, the counter-ion diffusion, or the solvent interchange we need electrochemical experiments in which the kinetics are under conformational relaxation control. Once the electrochemistry of these structural effects is quantified, we can again include the other components of the electrochemical reaction to obtain a complete description of electrochemical oxidation. 

It has been seen from the above simple examples that the concentration of the substrate has a profound effect on the rate of the electrode process. It must be remembered, however, that the process may show different reaction orders in the different potential regions of the i-E curve. Thus, electron transfer is commonly the slow step in the Tafel region and diffusion control in the plateau region and these processes may have different reaction orders. Even at one potential the reaction order may vary with the substrate concentration as, for example, in the case discussed above where the electrode reaction requires adsorption of the starting material. 

Consider first the diffusion-limited regime. The simplest experiment to perform is a chronoamperometric measurement, i.e. to monitor the current after a potential step to a value where an electroactive species will undergo electron transfer. This effectively allows us to monitor the rate of reaction, v, as a function of time, through the relationship ... 

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