Energy wasting back reactions

This charge separation is of paramount importance. The key problem is maintaining the charge separation, which involves minimization of the energy-wasting back reaction. Reaction centers contain an ordered array of secondary electron acceptors (Ai, A2, A3 ) that optimize the AG° that occurs at each step ... 

Similarly, the reaction of photoexcited 9,10-dicyanoanthracene (DCA) with a benzylstannane yields the contact ion pair in which the cation radical undergoes rapid mesolytic cleavage of the C—Sn bond to afford benzyl radical and tributyltin cation (which then adds to DCA- )77 (Scheme 14). When such unimolecular processes are faster than the energy-wasting back electron transfer (/cbet) within the contact ion pair, the D/A reactions occur rapidly despite unfavorable driving forces for electron transfer. 

The thermal and photochemical activations of EDA complexes by electron transfer are both enhanced when the radical ions D+- or A--(either paired or free) undergo a facile first-order (unimolecular) transformation such as fragmentation, rearrangement, bond-formation, etc., which pulls the redox equilibrium and thus renders the competition from the energy-wasting back electron transfer less effective (compare Scheme 5). Critical to the quantitative evaluation of the reaction dynamics is the understanding that the typical [D+% A--] intermediates, as described in... 

Even if the primary electron-transfer process occurs efficiently, spontaneous energy-wasting back-electron transfer can always occur unless elaborate methods are employed to minimize this ubiquitous reaction. 

Charge transfer between amphipathic ruthenium(II) complexes and N-butylphenothiazine in micelles, synthetic bilayers and liposomes has been studied by flash photolysis (Takayanagi et al., 1980). It was shown that the energy wasting back electron-transfer reaction is less efficient in the vicinity of the charged surface and that it is disfavoured by an increase in temperature. 

Thus, the tendency is to advocate the use of dry lime scrubbing systems thereby producing a waste stream that can be handled by conventional fly ash removal procedures. There are also dry processes, such as the metal oxide processes, in which sulfur dioxide can be removed from gas streams by reaction with a metal oxide. These processes, which are able to operate at high temperatures (approximately dOO C [TSO F]), are suitable for hot gas desulfurization without an energy-wasteful cooUng step. The metal oxides can usually be regenerated by aerial oxidation to convert any metal sulfide(s) back to the oxide(s) or by the use of a mixture of hydrogen and steam. 

The primary charge separation in photosynthetic bacterial reaction centres (RC) is charactrized by three unique features (i) Ultrafast rate in the psec domain which precludes energy waste due to back transfer to the antenna, (ii) non-Arrhenius temperature dependence of the rate, which manifests optimal coupling to the medium nuclear motion, and (iii) unidirectionality across the A branch of the quasisymmetric RC. 

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