Solute-solvent interactions vertical transitions

This process is illustrated schematically in Fig. 2 [40b]. Figure 2(a) traces the alterations in the free energy of the reacting species the states O and R refer to the chemical free energies, G°B, of the oxidized and reduced forms of the redox couple, respectively, that are associated with solute-solvent interactions. Step 1 entails moving down the curve OSR the bowed shape arises from the quadratic dependence of G°s on the ionic charge that is anticipated from the Born model. State S is that formed upon completion of step 1, so that the vertical line ST refers to step 2 yielding the transition state... 

Given tlie disparate nature of the physical interactions between die different electronic states and the solvent, and the non-equilibrium nature of the solvation of at least one state in die vertical process, theoretical models require a fairly high degree of sophistication in their construction to be applicable to predicting spectroscopic properties in solution. This requirement, coupled with the rather poor utility of available experimental data (most solution spectra show very broad absorption peaks, making it difficult to assign vertical transitions accurately in the absence of a very complex dynamical analysis), has kept most theory in this area at the developers level. A full discussion is beyond the scope of an introductory text, but we will briefly touch on a few of die key issues. 

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