Understanding chemical reactivity in solution

Let us bring together several key ideas that we have discussed. We seek a imified approach where both the role of the solvent and the rearrangement of the reactants to form products are taken into consideration. We further want the approach to center attention on the correlation of reactivity with slmcture, a theme that we started in Chapter In essence, we generalize the one-coordinate discussion of solvation in Section 11.1 to a two-dimensional world that consists of a solvation coordinate and a reaction coordinate. Starting from the gas phase, what we do is generalize the one-coordinate Evans-Polanyi model to include the role of the solvent. 

The systematics of the energy profile along the reaction coordinate are obtained from the Evans-Polanyi model. Recall, Sections 5.1.4 and 7.0.2, that this model regards the adiabatic potential energy profile leading from reactants to prodncts 

The approximation of the energies of different reactions in the series as congruent parabolas allows us to determine the free energy height of the barrier, in analogy to the derivation of the Marcus Eq. (11.13), as 

But you also must consider the free-eneigy changes that accompany the separated solvated reactants coming together into the cage. We will introduce this correction. 

Not every reaction series has a physically clear example of such a symmetric reaction. So this reference case can be hypothetical, but it need not be. Well-studied reaction series for which there is an obvious exanple of a symmetric case are proton transfers between two bases, AH -I- B  

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