Solvent exchange transition state

That is, terms of the form (1 — jr -I- jr< )/) appear in the denominator for all reactant sites having exchangeable protons and similarly in the numerator for all transition state sites. If there is no change in the fractionation factor for a site, its contribution cancels. If the solvent is a reactant, its term disappears because the solvent fractionation factor is unity by definition. 

Fig. 3. Representations of the transition states for the spectrum of solvent or symmetrical ligand exchange processes (7). 

The observation of a primary solvent deuterium isotope effect (kH/fa>) = 2-4 on the specific acid-catalyzed hydrolysis of vinyl ethers provides evidence for reaction by rate-determining protonation of the alkene.69 Values of kHikD 1 are expected if alkene hydration proceeds by rate-determining addition of solvent to an oxocarbenium ion intermediate, since there is no motion of a solvent hydron at the transition state for this step. However, in the latter case, determination of the solvent isotope effect on the reaction of the fully protonated substrate is complicated by the competing exchange of deuterium from solvent into substrate (see above). 

Presumably, the function of M+ is to "cushion the repulsion of the two negative ions. The larger, softer Cs+ can do this more effectively than the smaller, harder ions such as Li+ or Na+. Also, to form these bridged transition states, solvent molecules must be displaced from the solvation sphere of the cations. That process, because of their smaller sizes, would require more energy for the more strongly solvated Li+ and Na+. For the Cs+ ion, which forms effective bridges, the rate of electron exchange has been found to be linearly related to Cs+ concentration. 

All employed methods to include solvent effects corroborate the nature of the transition state for water exchange via an interchange mechanism and show very satisfactorily the mode... 

The HCN exchange itself proceeds through a trigonal bipyr-amidal intermediate [Li(NCH)5]+ reached via a late transition state. The entering HCN molecule approaches the lithium cation directly, and pushes three coordinated solvent molecules away toward the equatorial positions. In line with the experimental observation of a very fast exchange process, the computed... 

Fig. 2. Volume profiles, connected to the transition states, for the spectrum of solvent exchange processes.

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