Solvent effects activation energies

The steady-state polymerization in the presence of Cr (it-CsHb) was first order with respect to the monomer concentration (125) the effective activation energy was 4.7 0.5 kcal/mole. When the concentration of Crfir-CaHs was varied, first a linear rise of the polymerization rate occurred with an increase of tris-ir-allylchromium concentration to the upper limit then the rate does not depend on Crfx-CaHs concentration (126). The value of the upper limit of the polymerization rate increased with an increase in the water content of the solvent used. 

Solvation of, or the interaction of the solvents with, one of the reactants or reaction intermediates can have a significant influence on the rate or the selectivity of a reaction. When the solvation results in a lower potential energy, the activation energy increases, depressing the reaction rate. On the other hand, if the activated complex has a stabilizing interaction with the solvent, the activation energy decreases, which accelerates the reaction. If both the activated complex and the reactants are solvated, the effect of solvent on the rate may be negligible. The influence of the solvation of products does not have any effect on the rate except in the case of reversible reactions. Finally, in the case of ionic reactions, the dielectric constant of the solvents can have an important role. 

Figure 6.20. Energy diagram of the inner- and outer-sphere mechanisms, including solvent effects. Activation barriers are indicated by the small arcs a barrier as low as the reaction energy is designated by no act. The structure number refers to models shown in Pig.

The resonance stabilized intermediate, which can rotate about the erstwhile double bond, rapidly yields fumaric acid by loss of the elements of HCNS. In Seltzer s model of the transition state the carbon atom attacked by NCS is slightly more than halfway to tetrahedral. His calculation shows how well a secondary isotope effect can be accommodated with eq. (III-14). It also shows how many and how uncertain are the approximations that must go into it Use only of the six CH modes, with neglect of all others assumption of values of some of these frequencies in maleic acid by analogy with related molecules assumption of a functional dependence of force constants on interatomic distance and neglect of any possible contribution of solvent effects on energy or entropy of activation—for a reaction in aqueous solution. 

It should always be home in mind that solvent effects can modify the energy of both tile reactants and the transition slate. It is the difference in the two solvation effects that is the basis for changes in activation energies and reaction rates. Ihus, although it is conimon to express solvent effects solely in terms of reactant solvation or transition-slate solvation,... 

Another approach was used some years ago by Dewar and Storch (1989). They called attention to solvent effects in ion-molecule reactions which do not yield an activation energy in theoretical calculations related to gas-phase conditions, but which are known to proceed with measureable activation energy in solution. Dewar and Storch therefore make a distinction between intrinsic barriers due to chemical processes and desolvation barriers due to chemical processes. 

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