Solvent reorganization, Marcus theory

Results from the dissociative reduction of aryl ethers led to a number of important generalizations. These are classical stepwise systems initially forming jr -type radical anions. In this case, the intrinsic barriers for the formation of the intermediate radical anions are dominated by solvent reorganization with only a small (sometimes negligible) contribution from internal reorganization. Thus, the ETs to these systems are properly described by Marcus theory, in which the harmonic approximation satisfactorily accounts for both outer and inner reorganization factors. On the... 

In this simple form of the Marcus theory, two terms, the reorganization of the inner coordination shell (AG m) and that of the solvent sheath (AG 0Ut), both describing the degenerate transition-state geometry, contribute to AG. While molecular mechanics might also be used to model the encounter complex, force field calculations have mainly been applied to the estimation of the inner coordination shell reorganization term (AGt,)19,143 1445. 

Picosecond absorption spectroscopy studies of the contact ion pairs formed in the photo-initiated, S N 1 reaction of three substituted benzhydryl acetates (18) provided the rate constants for the k and k2 steps of the reaction (Scheme 10), in acetonitrile and DMSO.83 The activation parameters for the k and k2 steps were obtained from the temperature dependence of these steps and the transition state energies were calculated from the rate constants. This allowed the energy surfaces for three substituted substrates to be calculated in each solvent. The effect of solvent reorganization on the reactions of the unsubstituted and methyl-substituted benzhydryl contact ion pairs (CIP) was significant, causing a breakdown of transition state theory for these reactions. The results indicated that it will be very difficult to develop a simple theory of nucleophilicity in, S N1 reactions and that Marcus theory cannot be applied to SnI processes. 

Solvent Reorganization as an Activation Barrier in the Marcus Theory 112... 

The Br0nsted plots (Fig. 3) give information on this point. The higher curvature of the plot for DMSO compared to methanol is indicative of a lower intrinsic barrier to proton transfer for the dipolar aprotic solvent. Since in the extended Marcus theory the solvent effect has already been taken into account, one would expect the intrinsic barrier for proton transfer to be identical in the two systems. This is not the case. Therefore it appears that separation of the mechanism into reagent positioning with concomitant solvent reorganization is not warranted. 

The use of the energy-gap reaction coordinate allows us to calculate solvent reorganization energies in a way analogous to that in the Marcus theory for electron transfer reactions.19 The major difference here is that the diabatic states for electron transfer reactions are well-defined, whereas for chemical reactions, the definition of the effective diabatic states is not straightforward. The Marcus theory predicts that... 

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