Charge-separation reaction

Since the values of i/ depend on several factors noted above, in the absence of additional data such as the temperature dependence of the electron transfer rate constants for i-2 it is difficult to analyze the apparent difference between i/ for the charge separation reaction and that of the radical ion pair recombination reaction. However, the difference between these two values of u is not unreasonable given that the charge separation involves oxidation of an excited state of the donor, while radical ion pair recombination involves two ground state radicals. Small changes in the nuclear coordinates of the donor and acceptor for these two reactions should be sufficient to produce the observed difference in i/. The electronic coupling factor between ZnTPP and AQ should be different than that between ZnTPP " and AQ". 

The light energy stored in the radical ion pair is utilized to split water or to produce other high-energy chemicals. While impressive progress has been made in increasing the efficiency of this charge separation reaction (Fox Chanon, 1988 Norris Meisel, 1989), inhibition of the exothermic back-reaction to neutral species,... 

The second article deals with probably the most fascinating predictions of modem electron transfer theories i.e. the Marcus Inverted Region (M.I.R.) . It was shown only one decade ago, nearly 20 years after the first formulation of the Marcus theory, that the M.I.R. does indeed exist First for thermal charge shifts and later for charge recombination. Even a charge separation reaction was recently found to behave according to the Marcus theory. Nevertheless, many reactions do not follow the Marcus model and therefore the second contribution of this issue is mainly concerned with this question. 

Fig. 1.35. Schematic diagram describing energy levels of singlet (So, Si), triplet (Ti, Tn), and charge-separated (CS) states of OPVn-C6o dyads. The energy transfer (A et) and indirect (fc s) and direct (k s) charge separation reactions are indicated with curved dotted arrows. The solid arrow describes the initial excitation of the OPVn moiety. Other symbols are kr and k[ for the radiative rate constants, knr and k m for the non-radiative decay constants, fcjcs and k ics for the intersystem crossing rate constants, and fcx and fcxX for the rate constants for triplet-energy transfer, in each case for OPVn and MP-Ceo, respectively...

One of the ongoing mysteries of the OEC is how PSII could have evolved the ability to use water as an electron donor. It would seem that a number of changes, such as formation of a very high-potential oxidant in the charge-separation reaction and incorporation of a four-electron water-oxidation catalyst, would have had to occur simultaneously in order to convert an anaerobic photosynthetic reaction centre into PSII. 

This protocol has been extended to produce the S3 state by using the redox-active herbicides 1 and 2 (Figure 3) (16). In this case, two sequential charge-separation reactions occur as shown in equation 6 ... 

Table 3 Mapping of the Q Model on Simulation Data for Charge Separation Reactions (Energies are in kcal/mole) ...

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 free energy profile for the electron transfer reaction in a polar solvent is examined based on the extended reference interaction site method (ex-RISM) applying it to a simple model of a charge separation reaction which was previously studied by Carter and Hynes with molecular dynamics simulations. Due to the non-linear nature of the hypemetted chain (HNC) closure to solve the RISM equation, our method can shed light on the non-linearity of the free energy profiles, and we discuss these problems based on the obtained free energy profile. 

These values are similar, but slightly smaller than those observed for the charge separation reaction (Eq. 10). Clearly, the charge recombination process is also taking place by a TB-mediated mechanism in these dyads. 

A detailed study[81] of the solvent non-equilibrium response to electron transfer reactions at the interface between a model diatomic non-polar solvent and a diatomic polar solvent has shown that solvent relaxation at the liquid/liquid interface can be significantly slower than in the bulk of each liquid. In this model, the solvent response to the charge separation reaction A + D —> A + D+ is slow because large structural rearrangements of surface dipoles are needed to bring the products to their new equilibrium state. 

A.25.5 This charge-separation reaction demonstrates an important difference between the kinetics of a reaction and the thermodynamics of a reaction. Thermodynamics shows us where a reaction would like to go given an infinite of time, but most reactions happen much faster than that. In this case, tlie large -AG° reduces that rate of tlie back reaction so much that the forward reaction is kinetically favored, so the reaction proceeds forward. 

As shown in the scheme at the beginning of this section, the reduction ofQa to QbH2 in a photochemical cycle involves two charge-separation reactions which result in the transfer of two electrons and two protons, as well as the oxidation of two cytochrome molecules to restore the (twice) oxidized primary donor P870 to its original reduced state. In Fig. 6 we present the details ofthe quinone-reduction cycle, as currently perceived, to aid our discussion ofthe individual reactions that are to be monitored. 

Hammarstrbm and coworkers have conducted a thorough investigation of dissociative PCET from tyrosine (Y) using ruthenium-tyrosine model complexes. The work is motivated by the crucial role that tyrosine residues, play in charge-separation reactions in biology, particularly in Photosystem II (PS II). The model systems do not contain a specific PT coordinate as in PSII [148-156], yet the simplicity of the system permits the balance of stepwise versus concerted PCET to be studied in a controlled fashion. 

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