Electron association reactions Subject

The theoretical modeling of electron transfer reactions at the solution/metal interface is challenging because, in addition to the difficulties associated with the quantitative treatment of the water/metal surface and of the electric double layer discussed earlier, one now needs to consider the interactions of the electron with the metal surface and the solvated ions. Most theoretical treatments have focused on electron-metal coupling, while representing the solvent using the continuum dielectric media. In keeping with the scope of this review, we limit our discussion to subjects that have been adi essed in recent years using molecular dynamics computer simulations. 

The relevance of adiabatic electron transfer to the primary charge separation reaction has been the subject of considerable discussion, mainly due to the observation of undamped low-frequency nuclear motions associated with the P state (see Section 5.5). More recently, sub-picosecond time-scale electron transfer has been observed at cryogenic temperatures, driven either by the P state in certain mutant reaction centres (see Section 5.6) or by the monomeric BChls in both wild-type and mutant reaction centres (see Section 5.7). These observations have led to the proposal that such ultra-fast electron transfer reactions require strong electronic coupling between the co-factors and occur on a time-scale in which vibrational relaxation is not complete, which would place these reactions in the adiabatic regime. Finally, as discussed in Section 2.2, evidence has been obtained that electron transfer from QpJ to Qg is limited by nuclear rearrangement, rather than by the driving force for the reaction. 

The metal atoms of the neutral metal layers are subject to charge transfer ionization by the principal molecular ions of the ionospheric E-region, NO+ and 02" . The highly stable atomic metal ions are either transported to higher altitudes, where they can undergo electron-ion recombination, or they can be removed by three-body association reactions with atmospheric molecules at lower altitudes, such as N2 ... 

Fig. 3. Vibrational population distributions of N2 formed in associative desorption of N-atoms from ruthenium, (a) Predictions of a classical trajectory based theory adhering to the Born-Oppenheimer approximation, (b) Predictions of a molecular dynamics with electron friction theory taking into account interactions of the reacting molecule with the electron bath, (c) Born—Oppenheimer potential energy surface, (d) Experimentally-observed distribution. The qualitative failure of the electronically adiabatic approach provides some of the best available evidence that chemical reactions at metal surfaces are subject to strong electronically nonadiabatic influences. (See Refs. 44 and 45.)...

One of the major difficulties associated with catalytic photochemical water decomposition reactions is the requirement that four electrons be provided for each molecule of oxygen that is formed and there are very few compounds which allow this reaction to take place without the intermediacy of high energy species such as hydroxyl radicals. We therefore treat this subject in some detail. 

At present the most entertained model which incorporates the above principles is the Q cycle proposed by Mitchell [246], shown in Fig. 3.11. Many discussions of the Q cycle or variants thereof are available [8,14,67,87,178,179,199,211,221,229,238,246]. In essence, ubiquinol is oxidised at centre o , which is closely associated with the FeS centre and the haem of b-566. This results usually in transient generation of SQ bound to this centre, as identified by EPR spectroscopy [239]. Then there is rapid electron transfer from SQ to b-566 with formation of Q. The electron in FeS is transferred rapidly to cytochrome c via cytochrome c,. The electron in b-566 moves transmembranously to haem 6-562 (Figs. 3.9,11). The latter may then reduce either SQ or Q at centre i , of which SQ has again been identified by EPR [239-244]. This reaction is a weak point of the Q cycle, and has been subject to modifications by several workers (see Refs. 270, 271 and Refs, above). 

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