Free energy barrier polarization effect

In PET, the rate can be markedly affected by the solvent polarity. With the formation of each new charge-transfer intermediate, solvent dipoles undergo reorientation in response to the new charge distribution on the reactants [49]. The solvent response influences the free-energy barrier of the reaction by altering the potential energy surface of the electron transfer. We consider this facet of solvent motion in this section. In a later section, we examine dynamical solvent effects. 

The thermodynamic effects of finite size and the kinetic barriers, AG, for the formation of vapor phase have been fully developed [41-46]. Macroscopic thermodynamics predicts that when we have two non-polar surfaces immersed in a liquid and bring them closer together, at a critical distance, Dc, liquid will be replaced by vapor (2). Due to a considerable free energy barrier for confinement-induced evaporation, however, the liquid phase is often metastable below Dc [36, 41, 43, 45, 47]. Coarse-grained simulations confirmed [45] macroscopic scaling predictions [48, 49] ... 

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

Arnett and coworkers later examined the reaction of lithium pinacolone enoiate with substituted benzaldehydes in THE at 25 °C. The determination of the heat of reaction indicated that the Hammett p value for the process is 331. Although the aldol reaction was instantaneous in THF at 25 °C, the reaction with o- or p-methylbenzaldehyde could be followed using a rapid injection NMR method in methylcyclohexane solvent at —80 °C. Application of Eberson s criterion based on the Marcus equation, which relates the free energy of ET determined electrochemically and the free energy of activation determined by kinetics, revealed that the barriers for the ET mechanism should be unacceptably high. They concluded that the reaction proceeds via the polar mechanism . Consistent with the polar mechanism, cyclizable probe experiments were negative . The mechanistic discrepancy between the reactions of benzaldehyde and benzophenone was later solved by carbon kinetic isotope effect study vide infraf. ... 

Solvent tends to attenuate the a-branching effect in Sj,f2 reactions (see Table 6.6). The gas-phase barrier increases twice as rapidly with the addition of each methyl group compared to that in acetone. Jensen" performed B3LYP/6-31-I-G optimizations of the reactants and TSs in the gas phase and then obtained free energies in water using the polarized continuum mediod (PCM). These results are also listed in Table 6.6. The gas-phase computational values match up well with the experimental values. However, the relative activation barriers computed with PCM are quite similar to the gas-phase values and, therefore, overestimate the solution-phase barriers. 

One application of PI-QTST to PT has been to study a model A-H-A PT solute in a polar solvent [77]. This computational study provided a detailed examination of the specific features of PT, including the competition between proton tunneling and solvent activation, the influence from intramolecular vibrational modulation of the PT barrier, and the role of electronic polarizability of both the solute and the solvent. Changes in the total quantum activation free energy, and hence the reaction probability, due to these different effects were calculated (cf. Fig. 18). By virtue of these studies, it was found that to fully understand the rate of a given PT reaction, one must deal with a number of complex, nonlinear interactions. Examples of such interactions include the nonlinear dependence of the solute dipole on the position of the proton, the coupling of the solute dipole to both the proton coordinate and to other vibrational modes, and the intrinsically nonlinear interactions arising from both solute and solvent polarizability effects. Perhaps the most important conclusion... 

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