Entrance channel-electron transfer

All chemical reactions begin as the reagents approach and end as the products separate. The system must traverse both the entrance channel and the exit channel. Our hope is that, by specifying the reagent orientation in the entrance channel and monitoring the appearance of ions in the exit channel, we may learn how orientation affects the transfer of an electron from one species to another. Strictly speaking, we cannot separate the electron transfer, presumed to occur in the entrance channel, from the process of the ions separating, which is in the exit channel. Under certain circumstances, one or the other of these interactions may dominate. We have thus interpreted the data under certain assumptions, but what we really learn is how the... 

One of the first attempts to compare the effect of different forms of energy on gas-phase reactions was done in a very general way by John Polanyi [70, 73]. When the reaction barrier is in the entrance or the exit channel of the reaction, translational or vibrational energy, respectively, is more efficient at promoting the reaction. These rules, known as Polanyi s rules, are not linked specifically to electron-transfer reactions. On the contrary, they were derived without reference to a specific reaction. As an illustration of these rules in an electron-transfer reaction, vibrational excitation of HCl gives easier access than translation to a late barrier in the K-hHCl reaction [123, 124]. 

Competition with this channel can occur in three ways. Firstly, energy transfer or ionization may occur from the entrance channel before the electron-jump can take place this is clearly favoured if Rc is small, but Rm is large, as for polyatomic molecules with small electron affinities. This is likely to be operative if the total quenching cross-section for A -f BC is larger than the reaction cross-section for the analogous alkali -I- BC system. 

Fig. 14.27. Electron transfer in the reaction DA -> D+A , as well as the relation of the Marcus parabolas to the concepts of the conical intersection, diabatic and adiabatic states, entrance and exit channels and the reaction barrier. Panel (a) shows two diabatic surfaces as functions of the and 2 variables that describe the deviation from the comical intersection point (within the...

Fig. 14.25. Electron transfer in the reaction DA- -D+A " as well as the relation of the Marcus parabolas to the concepts of the conical intersection, diabatic and adiabatic states, entrance and exit channels and the reaction barrier. Fig. (a) shows two diabatic (and adiabatic) surfaces of the electronic energy as functions of the f and 2 variables that describe the deviation from the conical intersection point (cf. p. 262). Both diabatic surfaces are shown schematically in the form of the two paraboloids one for the reactants (DA), the second for products (D+A ). The region of the conical intersection is also indicated. Fig. (b) also shows the conical intersection, but the surfaces are presented more realistically. The upper and lower parts of Fig. (b) touch at the conical intersection point. On the lower part of the surface we can see two reaction channels each with its reaction barrier (see the text), on the upper part (b) an energy valley is shown that symbolizes a bound state that is separated from the conical intersection by a reaction barrier.

The effects of molecular structure on the rates of energy transfer are manifested in many ways. Additional pathways are available in collisions with molecules, such as the influence of ionic potential surfaces [35], the availability of near resonant electronic-to-vibrational and rotational energy transfer pathways [36], and the introduction of nonadiabatic transitions due to the breaking of the molecular orbital symmetry [37]. For the studies considered here, we might also add the competition between reaction and the desired energy transfer process, the possibility of energy transfer processes in the entrance or exit channels, selective changes in... 

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