Electron migration

FIGURE 16.33 In a ligand-to-metal charge-transfer transition, an energetically excited electron migrates from a ligand to the central metal ion. This type of transition is responsible for the intense purple of the permanganate ion, MnCF,. ... 

Chlorine would have to lose seven electrons to reach an electron configuration like that of neon. But if it gained one, it would have the same stable electron configuration as argon. So that is what chlorine does. If it meets an atom with a high-energy valence electron, such as sodium, the electron migrates to the chlorine atom and forms a chloride ion ... 

H.-A. Wagenknecht, Reductive electron transfer and excess electron migration in DNA. Angew. Chem., Int. Ed. 42, 2454-2460 (2003). 

Once the special pair has absorbed a photon of solar energy, the excited electron is rapidly removed from the vicinity of the reaction centre to prevent any back reactions. The path it takes is as follows within 3 ps (3 X 10 12 s) it has passed to the bacteriopheophytin (a chlorophyll molecule that has two protons instead of Mg2+ at its centre), without apparently becoming closely associated with the nearby accessory bacteriochlorophyll molecule. Some 200 ps later it is transferred to the quinone. Within the next 100 ps the special pair has been reduced (by electrons coming from an electron transport chain that terminates with the cytochrome situated just above it), eliminating the positive charge, while the excited electron migrates to a second quinone molecule. 

An excited atom (by thermal or electrical means) has its electrons migrate from inner orbitals (specifically valence electrons) to outer orbitals,... 

The first-formed carbocation is secondary. It is possible for this carbocation to become a more stable tertiary carbocation via rearrangement, in which a methyl group with its pair of electrons migrates from one carbon to the adjacent positive centre. Now the rearranged tertiary carbocation can yield SnI- and El-type products in much the same manner as the original secondary carbocation. A rearranged bromide is formed, together with two alkenes from an El... 

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

This solution describes a back-and-forth migration of the electron between the two protons. At r = 0, the electron is revolving about the left-hand-side proton with a frequency/= Eo lh. Then the electron starts migrating to the right-hand side. At r = tt/ IMI, the electron has migrated entirely to the right-hand side and at r = 2Tr/l A/, the electron comes back to the left-hand side, etc. In other words, the electron migrates back and forth between the two protons with a frequency v = M lh. Similarly, we have another solution ... 

ESR experiments at low temperatures with frozen aqueous solutions of DNA indicate electrons are trapped on cytosine (C, 50-85%) with the remainder on thymine (T) [6, 9,13]. After the initial electron attachment, electron migration is temporarily quenched at low temperatures. The reversible protonation of the cytosine anion, C at N3 to form the neutral species, C(N3)H is suggested to further stabilize this species (Scheme 1). Much experimental... 

For DNA at room temperature rapid protonation at carbon sites on both thymine and cytosine will permanently quench electron migration. Electron transfer therefore must take place in competition with these processes. At low temperatures, where irreversible protonations are prevented, the electron transfer process can be investigated and is discussed in Sect. 3. 

Of particular interest are reactions between molecular (solute) species in solution. This broad category may include reactions between small or moderately sized biological systems, but it explicitly excludes polymeric, colloidal and particulate species. Reactions involving exciton or electron migration in rigid crystalline or amorphous media are not considered here, nor are nucleation and growth discussed. There is, however, some considerable cross-fertilisation of ideas between these areas and that of diffusion-limited reaction rates in solution. 

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