Charge-separated activated complexes

However, modifications are required for reactions involving ions and charged or charge-separated activated complexes (Problem 4.5(ii) and Tables 7.1 to 7.3). 

If reaction involves molecular reactants and a charge-separated activated complex, the activated complex is likely to be more solvated than the reactants. The activation energy is reduced by an amount similar to the A H of solvation, leading to an increase in rate relative to the gas phase reaction (Figure 7.10). 

Reaction between two heavily solvated oppositely charged ions could produce a neutral, possibly charge-separated activated complex that is only lightly solvated,... 

Figure 7.10 Potential energy profile showing the effect of the solvent for reaction between uncharged reactants— charge-separated activated complex...

For an ion-molecule reaction the effect will be small, while for reaction of neutral molecules there will be an overall decrease in volume if a charge-separated activated complex is formed. The greater the charge separation, the more negative will be the overall volume change, and the bigger the positive pressure effect. 

Molecule-molecule with a charge-separated activated complex expect a decrease. A S is slightly — ve. 

When proteolytic removal of Dl from inactive PSII cannot proceed at a sufficient rate (in vivo under high stress, or in vitro), Dl degradation becomes much slower and a severe inactivation of PSII charge separation and disassembly of the complex occurs. This process is most probably induced by ROS and radicals produced by aberrant PSII photochemistry, since loss of charge separation activity and PSII disassembly has not been observed under anaerobic conditions. So, besides its function in restoring PSII activity, selective Dl replacement may also represent a means of preventing the formation of ROS and their attack on PSII components. ... 

The DPA moiety is less active in forming the CT complex with viologens than the pyrene moiety e.g., for PMAvDPA the KCT values with MV2+ and SPV are 1.3 x 103 M 1 and almost zero, respectively, at pH 8-9 [60, 77], whereas for PMAvPY they are 7.8 xlO4 and 6.3 x 102 M, respectively, at pH 11 [77]. Therefore, the polymer-bound pyrene system undergoes much more static quenching than the polymer-bound DPA system. As will be discussed in Chapter 6, it is very important for charge separation whether the fluorescence quenching is static or dynamic. 

Figure 16(a) (O) shows the EMF responses of a 1,2-dichloroethane membrane containing anionic sites (KT/ C1PB). A Nernstian response was obtained. An SHG response to KCl was observed at activities of the latter above 10 M [Fig. 16(b), O]-These results can be interpreted in the same way as for ionophore-incorporated PVC liquid membranes, for which we have shown that the concentration of oriented cation complexes at the liquid-liquid interface can explain both the observed SHG signal and EMF response. The present SHG responses thus suggest primary ion concentration dependent charge separation at the interface of the 1,2-dichloroethane membranes incorporated with ionic sites. 

Isopolar activated complexes differ very little or not at all in charge separation or charge distribution from the corresponding initial reactants. These complexes are formed in pericyclic reactions such as Diels-Alder cydoadditions and the Cope rearrangement. 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

In the one-step symmetry-allowed mechanism, with little charge separation in the activated complex, the Hammett p-values for p-XCgH4- substituted substrates are very small. As anticipated, the reaction has a negative volume of activation. 

On the basis of kinetic data, it was suggested that appreciable charge separation in the activated complex (equation 13) could be avoided by means of such proton transfers, where HA is a general acid (H2O, ROH, RO—OH). Upon change from a polar protic solvent to the nonpolar solvent dioxane, the reaction was observed to be second-order in hydrogen peroxide and the second molecule of H2O2 obviously played the role of HA in the 1,4-proton shift. The rate of oxidation was shown to increase linearly with the pfsTa of solvent HA. In general, it was concluded that solvent interactions provide a... 

The observed positive value for the second term (see slope, Fig. 1) indicated that the activated complex (created between monomer and catalyst, or propagating anion) has a larger separation of charges than the reactants in the initial stage. 

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