Charge separated states solvent effect

Finally, an effective influence of the environment (solvent polarity) on the wavefunctions and energies of the low-lying excited states of the complex dative bond will be qualitatively considered. An influence of a polar solvent stabilizing the charge-separated state might play a key role for the process considered. An example of an extreme strong... 

An unusual feature is that the luminescence intensity and lifetime of 12b increases substantially in nonpolar solvents relative to the lifetime in polar solvents. Moreover, as the emission lifetime increases, the decay kinetics become distinctly nonexponential. The explanation for the unusual solvent-dependent luminescence properties of 12b is that in a low dielectric solvent the MLCT and charge separated states (14 and 15, respectively, in Scheme 8) are similar in energy and equilibrium is established between the two states. In polar solvents 15 is stabilized with respect to 14, and photoinduced ET is irreversible. Similar, but attenuated, solvent dependent luminescence is observed for 12c and 12d. The effect is attenuated in these complexes because the amine donors are easier to oxidize and thus the charge separated state is stabilized with respect to the MLCT state. 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

In the last part of this chapter, intramolecular charge transfer (ICT) in anthryl derivatives with linked donor-acceptor parts was discussed. Ultrafast spectroscopy has been applied both for structural characterization and for real-time probing of the ICT in this case. Microscopic solvation effects on the torsional motions and the ICT in the molecules have been examined by the use of their clusters with polar solvents. One of the most important problems which awaits for further studies is an ambiguous description of the electronic character of the charge-separated states in the systems. So far, high-level quantum-mechanical calculations have not been able to deal with such large molecular systems, but reliable evaluation of electronically... 

The presence of solution can dramatically affect dissociative chemisorption. In the vapor phase, most metal-catalyzed reactions are homolyticlike, whereby the intermediates that form are stabilized by interactions with the surface. Protic solvents, on the other hand, can more effectively stabilize charge-separated states and therefore aid in heterolytic activation routes. Heterolytic paths can lead to the formation of surface anions and cations that migrate into solution. This is directly relevant to methanol oxidation over PtRu in the methanol fuel cell. The metal-catalyzed route in the vapor phase would involve the dissociation of methanol into methoxy or hydroxy methyl and hydrogen surface intermediates. Subsequent dehydrogenation eventually leads to formation of CO and hydrogen. In the presence of an aqueous media, however, methanol will more likely decompose heterolytically into hydroxy methyl (—1) and intermediates. 

The expression (68) was applied [44] to the interpretation of the effect of electrolyte on the stability of a photoinduced charge-separated states of such probe molecules as p-aminonitrobiphenyl and p-aminonitroterphenyl in solution of different tetrabutulammonium salts in nonpolar solvents such as benzene (e - 2.28) and toluene (e = 2.38). According to the experimen-... 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

The neutral reactants possess permanent dipoles, the product is ionic, and the transition state must be intermediate in its charge separation, so an increase in solvent polarity should increase the rate. Except for selective solvation effects of the type cited in the preceding section, this qualitative prediction is correct. 

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