Local solvation state

Several picoseconds after photon adsorption, the electrons end up in a localized solvated state at distances of l-5nm from the electrode surface. The electron s hydration energy in aqueous solutions is about 1.5-1.6eV. 

The generation of molecular imprints at surfaces is an obvious solution to the problems of mass transfer and accessibility, and potentially affords control over more subtle parameters such as binding site orientation and local solvation state. While a standard general protocol does not as yet exist for surface imprinting, a number of strategies that create the required recognition cavities at a surface or interface have now been developed and selected examples of these are considered in this chapter. 

Thus from pulse radiolysis, mobility measurements, and electron reaction studies, we have information on the absorption spectra, the cavity volume, and the energy of the trapped or solvated state. The nature of this state seems to be an electron that is localized in a cavity in the liquid. 

All the above-considered photoelectrochemical phenomena are based on the transition of light-excited electrons into a localized state in the solution, namely at the energy levels associated with individual ions or molecules. However, the phototransition is also possible when the electrons pass into a qualitatively different delocalized state in the solution it is this type of phototransition that represents photoemission (Barker et al, 1966). The emitted delocalized electron in the solution is then thermalized and localized to form a solvated (hydrated in aqueous solution) electron. The energy level, which corresponds to the solvated electron, lies below the bottom of the band of permitted delocalized states in the solution. Finally, the electron may pass from the solvated state to an even lower local energy level associated with an electron acceptor in the solution (see Fig. 30). 

The formal potential, E0/, contains useful information about the ease of oxidation of the redox centers within the supramolecular assembly. For example, a shift in E0/ towards more positive potentials upon surface confinement indicates that oxidation is thermodynamically more difficult, thus suggesting a lower electron density on the redox center. Typically, for redox centers located close to the film/solution interface, e.g. on the external surface of a monolayer, the E0 is within 100 mV of that found for the same molecule in solution. This observation is consistent with the local solvation and dielectric constant being similar to that found for the reactant freely diffusing in solution. The formal potential can shift markedly as the redox center is incorporated within a thicker layer. For example, E0/ shifts in a positive potential direction when buried within the hydrocarbon domain of a alkane thiol self-assembled monolayer (SAM). The direction of the shift is consistent with destabilization of the more highly charged oxidation state. 

Direct information about the local solvation structure for the solvated electron in condensed media is scarce, although having an accurate picture of, at least, the ground state of the solvated electron, is important to interpret its properties. Steady-state electron paramagnetic resonance (EPR) and electron spin-echo modulation (ESEM) experiments as well... 

Even proteins that contain only a single Trp residue generally exhibit multiexponential decays. Several hypotheses have been proposed to explain why. Pirst, multiple conformational states may exist for the single Trp such as different rotameric configurations (orientations about the Trp xi or X2 C-C bond) (18). Even in the absence of mnltiple rotamers, the electron-transfer qnenching rate is extremely sensitive to the local environment, so a distribntion of local microconformational states may cause a nonexponential flnorescence decay. Other possible sonrces of nonexponential flnorescence decay inclnde the response of the protein and surronnding solvent to the change in dipole moment of Trp on excitation ( solvation ) (19). 

Other reactions of aromatic hydrocarbon anion radicals and amine cation radicals lead to exciplex emission, particularly in nonpolar solvents [15], Luminescence from exciplexes is most definitively observed in systems for which the redox reaction is energetically unable to yield a localized excited state. The free energy of exciplex formation, jE exc, is associated with solvation and geometry optimization in the encounter complex of and A+. 

Let us now consider one of the phases, thus a region in space occupied by a continuous multicomponent mixture. The actual nature (molecular configuration) of the mixture may be little known imagine, e.g., a liquid ionic solution with various degrees of dissociation, solvatation, etc. One then usually assumes that some equilibria (such as ionic equilibria) are installed very rapidly so that the (instantaneous local) thermodynamic state at a point of the mixture can be defined by the temperature, pressure, and mass (or mole) fractions of certain K components C, —, . The Q are some formal chemical species the... 

In a few exceptional cases, it was possible to separate the NMR signals from the two solvated states of an ion and infer from the areas under the separate signals the amount of preferential solvation that occurred. Such a case is the preferential solvation of Mg in methanol+water mixtures cooled down to -75°C as reported by Swinehart and Taube [43]. At x =0.076, the local water mole fraction in the first solvation shell of Mg + is 0.128, that is, the ion is preferentially solvated by the aqueous component of the mixture. Other examples of separated signals from the two solvated forms of ions, applicable to room temperature, are confined to ions of a high charge density—small multivalent ions. This was demonstrated by Strehlow et al. [44] for AP in mixtures of A=ethylene carbonate and B=acetonitrile, in which at Xg=0.6, the H NMR spectrum shows the cation to be much more solvated by A than by B. 

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