Ligand binding dielectric constant

The generally accepted process for metal ion-catalyzed reactions of the sort we consider here involves pre-equilibrium binding with the substrate, followed by a reaction of the complex as schematized in Equation (1). Whether the metal ion is free or complexed by ligands, or bears an associated lyate, or whether the substrate is neutral or anionic, these appear to be just the sort of processes one might expect to experience large rate accelerations in passing from water to a medium of reduced dielectric constant such as alcohols or other lower polarity solvents. 

Ae Dielectric constant Ellipticity in circular dichroism n.i Statistical factor for ligand-i binding at n sites on a macromolecule... 

Effects due to solvent dielectric constant in terms of the contribution AGb (equation I) have already been considered. The destabilization effect due to a decrease in dielectric constant is relatively small as long as s S 10. On the other hand, during the complexation process the ligand binding sites have to be set free by breaking intermolecular solvent-ligand bonds. This is more difficult in polar solvents of high than for solvents of low e (5, 16). 

Comparing ligands 22 and 38 which contain the same number of binding sites, we note that the special complexation features of 38 result from the cryptate nature of its complexes. Indeed, whereas in complexes of 22 polar solvent molecules may approach the cation from top and bottom, it is much more shielded in complexes of 38. This difference in behaviour is reflected in the corresponding change in ligand thickness (Table 12). The results in Table 11 also display the expected decrease in M2+/M+ stability ratio as the dielectric constant decreases from water to methanol. 

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