Electron-exchange process

If a system is not at equilibrium, which is common for natural systems, each reaction has its own Eh value and the observed electrode potential is a mixed potential depending on the kinetics of several reactions. A redox pair with relatively high ion activity and whose electron exchange process is fast tends to dominate the registered Eh. Thus, measurements in a natural environment may not reveal information about all redox reactions but only from those reactions that are active enough to create a measurable potential difference on the electrode surface. 

For the fate of the excited states in condensed media, we must add to this list energy transfer processes. These are broadly classified as radiative (or trivial ), coulombic (mainly dipole-dipole interaction), or electron-exchange processes. 

Upon further contact with a redox reagent or at higher redox potentials, additional electrons can be transferred. After a two-electron transfer, each redox unit can accept one charge with formation of a singlet or triplet dianion, or (less favourably from an electrostatic point of view) both charges can enter one redox unit. Here again, an intramolecular electron-exchange process is possible. 

Of course, the degree of non-adiabatic character in the electron-exchange process is an intrinsic property, independent of the wavefunction representation used to describe it. For example, in terms of t f+ and t f, an electronically adiabatic process is one which can be analyzed in terms of a single adiabatic... 

For this double electron exchange process to operate, there should be a molecular orbital overlap between the excited donor and the acceptor molecular orbital. For a bimolecular process, intermolecular collisions are required as well. This mechanism involves short-range interactions ( 6—20 A and shorter). Because it relies on tunneling, it is attenuated exponentially with the intermolecular distance between the donor and the acceptor.17 The rate constant can be expressed by the following equation. 

Even without the complexities introduced via electron-exchange processes of the type outlined above, the fundamental nature of the interaction between two solvated electrons leading to spin pairing has yet to be firmly established. A treatment of the possible bonding in... 

Usanovic acid-base theory — A general theory of acids and bases taking into consideration electron exchange processes (redox reactions) was proposed in 1939 by Usanovic. His definition is a symmetrical one and includes all concepts discussed above, i.e., an acid is defined as a substance which is able to liberate protons or other cations (cation donator) or to take up anions or electrons (anion acceptor, electron acceptor). A base is defined as a substance which is able to release anions or electrons (anion donator, electron donator) or to take up protons or other cations (cation acceptor). According to that theory, all chemical reactions (excluding reactions between radicals resulting in covalent bindings) can be considered as acid-base reactions. 

Dahms-Ruff theory — For fast electron exchange processes coupled to isothermal diffusion in solution, the theoretical description and its experimental verification were given by Dahms [i] and by Ruff and co-workers [ii—v]. Ruff and co-workers studied the displacement of the centers of mass particles, which is brought about by both common migrational motion and chemical exchange reaction of the type... 

Ultrasound assistance can be provided before and (or) during analysis. In those techniques which do not involve any electron exchange at the solution-electrode interface, US is normally applied prior to analysis in order to activate the electrode surface and hence in the absence of sample in electron-exchange processes, US can be applied as a pretreatment, but also during analysis. In the latter case, US can be applied during the measurement step and, in stripping techniques, during deposition of the analyte of interest on the electrode, either electrochemically or by physisorption. 

From the addition ofca. 1 %offerricytochrome c-551 to a solution of ferrocytochrome c-551 appreciable broadening of the heme meso, heme methyl, Met-61, and His-16 signals occurs, from which a bimolecular rate constant for the electron exchange process of 1-2 X is calculated. (667) Such electron exchange is en-... 

Obviously a different density of states is measured with electron impact, compared to photoionisation. This is not surprising in view of the different selection rules for the two processes. For photoionisation the dipole selection rule is valid, while for electron-impact ionisation electron-exchange processes may dominate at threshold. 

Configurational and conformational effects on the rates of electron exchange process have been determined for some stereo- and dia-stereoisomers of 7V-methyl derivatives of 21 [44]. 

Chapter 17 of this text focuses on the interface between molecular systems and metals or semiconductors and in particular on electron exchange processes at such interfaces. Electron injection or removal processes into/from metals and semiconductors underline many other important phenomena such as contact potentials (the... 

Most likely the earliest solution work reported was a study of linewidth measurements of the Cu resonance signal in concentrated hydrochloric acid solution containing cuprous and cupric chloride. These studies afforded reasonable calculations for the bimolecular rate constant for the equilibrium electron-exchange process (Eq. 1) ... 

When (37) and (41) are thermolyzed in the presence of rubrene, luminescence is enhanced as fluorescence. It is deduced that the acceptor in the electron-exchange process for (37) is the trioxane ring, and (42) in the case of (41). 

These can be divided into two main classes (1) those in which the electron transfer effects no net chemical change and (2) those in which there is a chemical change. The former, called electron-exchange processes, can be followed only indirectly, as by isotopic labelling or by nmr. The latter are the usual oxidation-reduction reactions and can be followed by many standard chemical and physical methods. The electron-exchange processes are of interest because of their particular suitability for theoretical study. 

In electron-transfer reactions between two dissimilar ions, in which there is a net decrease in free energy, the rates are generally higher than in comparable electron-exchange processes. In other words, one factor favoring rapid electron transfer is the thermodynamic favorability of the overall reaction. This generalization seems to apply not only to the outer-sphere processes now under discussion but also to the inner-sphere mechanism to be discussed shortly. 

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