Ion pairing and host-guest complexation in low dielectric constant solvents.

We report an equilibrium treatment for complexation of ionic species in low dielectric constant media that explicitly includes ion pairing of one of the components. Experimental validation was achieved through study of pseudorotaxane formation between dibenzylammonium salts and dibenzo-24-crown-8. In particular, we show that concentration-dependent fluctuations in the apparent K(a,exp) values as usually reported are attributable to ion pairing, with dissociation constant K(ipd), and that the constant K(ap) for complexation of the free cationic guest species, G(+), by the host crown ether is independent of counterion. More generally, using a simple extension of our model, we show the ability to diagnose the relative extent of ion pairing of the complex, which may be readily applied to other host-guest systems involving ionic species.