Electrostatic activation energy

Near the surface, AG is dominated by the Coulomb energy profile and, therefore, it is approximately equal to the difference of the electrostatic potentials at the proton positions before and after the transfer. This difference depends strongly on the distance of the proton from the surface. Values of AG were found in the range of 0.5 eV. This value decreases, however, to the activation energy of proton transport in bulk water when the proton-surface separation exceeds 3 A (the thickness of one monolayer of water). Moreover, the electrostatic activation energy is a function of the separation between surface charges, which lies in the range of 7 to 15 A. 

What other factors might be responsible for difference in activation energies Compare atomic charges anc electrostatic potential maps for the Sn2 transition states Does the increase in steric crowding lead to enhanced o diminished charge delocalization Explain. How, if at all would this be expected to affect the energy barrier Why ... 

Calculate activation energies for Sn2 reactions of ammonia and trimethylamine with methyl iodide via transition states ammonia+methyl iodide and trimethyl-amine+methyl iodide, respectively. Is attack by ammonia or trimethylamine more facile Rationalize your observation by comparing electrostatic potential maps for the two transition states. Which transition state requires more charge separation Is this also the higher-energy transition state ... 

The general principle that activation of para substitution is greater than of ortho substitution holds true also for an azinium moiety in the one instance studied. Thus, the activation energy for the 4-chloropyridine quaternary salt 280 (Table II, line 9) is 1 kcal lower than that for the 2-isomer (line 5). The rate relation (2- > 4-isomer) is controlled by the entropies of activation in this reaction due to electrostatic attraction in the transition state (281). The reverse rate relation (4- > 2-position) is predicted for aminations of such quaternary compounds due to electrostatic repulsion (282) plus the difference in E. A kinetic study of the 2- and 4-pyridine quaternary salts... 

For comparison purposes activation energies for reaction 12, in both gas and liquid phase, are collected in Table 5. The value for the a-toluenesulfonyl radical is in line with the activation energies of the other RS02 desulfonylations, provided allowance is made for differences in the stabilization energies of the organic radicals that are produced. However, electrostatic repulsions within the radical molecule and phase effect may considerably influence activation energies. 

As discussed and demonstrated in the previous chapters, the catalytic effect of several classes of enzymes can be attributed to electrostatic stabilization of the transition state by the surrounding active site. Apparently, enzymes can create microenvironments which complement by their electrostatic potential the changes in charges during the corresponding reactions. This provides a simple and effective way of reducing the activation energies in enzymatic reactions. 

The concept of electrostatic complimentarity is somewhat meaningless without the ability to estimate its contribution to AAg. Thus, it is quite significant that the electrostatic contribution to AAthat should be evaluated by rigorous FEP methods can be estimated with a given enzyme-substrate structure by rather simple electrostatic models (e.g., the PDLD model). It is also significant that calculated electrostatic contributions to A A g seem to account for its observed value (at least for the enzymes studied in this book). This indicates that simple calculations of electrostatic free energy can provide the correlation between structure and catalytic activity (Ref. 10). 

In a recent paper. Mo and Gao [5] used a sophisticated computational method [block-localized wave function energy decomposition (BLW-ED)] to decompose the total interaction energy between two prototypical ionic systems, acetate and meth-ylammonium ions, and water into permanent electrostatic (including Pauli exclusion), electronic polarization and charge-transfer contributions. Furthermore, the use of quantum mechanics also enabled them to account for the charge flow between the species involved in the interaction. Their calculations (Table 12.2) demonstrated that the permanent electrostatic interaction energy dominates solute-solvent interactions, as expected in the presence of ion species (76.1 and 84.6% for acetate and methylammonium ions, respectively) and showed the active involvement of solvent molecules in the interaction, even with a small but evident flow of electrons (Eig. 12.3). Evidently, by changing the solvent, different results could be obtained. 

Kds are the constants of rates of chemical reactions of oxygen adsorption and desorbtion from ZnO film and Aq are electron work function from ZnO before oxygen gets adsorbed and its variation caused by dipole moment of adsorbed complexes being formed U is the adsorption activation energy of non-electrostatic nature [ M] is the concentration of solvent molecules. Apparently we can write down the following expression for the stationary system ... 

If stabilization of the transition state (TS) is more effective than that of the ground state (GS), this results in enhancement of reactivity as a result of a decrease in the activation energy (Fig. 3.5), because of electrostatic (dipole-dipole type) interactions of polar molecules with the electric field. 

Another approach to ET reactions originated in the work of Weiss and Libby, who suggested that activation energy for ET reactions in solution does not arise from the collisional-vibrational or electrostatic interaction of the solvent in the first and second solvation sphere, but rather from continuum solvent polarization fluctuation far out in solution. This approach, called continuum theory, was further developed by Kubo and Toyozawa, " Marcus, Platzmann and Erank, and by Levich and Dogo-nadze. ... 

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