Diffusion of a Highly Charged Supramolecular Assembly: Direct Observation of Ion Association in Water¹

The diffusion coefficient of the self-assembled supramolecular cluster [Ga₄L₆]¹²⁻ depends on the cationic counterions in solution. Diffusion coefficients were determined using the pulsed-gradient spin–echo ¹H NMR method and fit using nonlinear least-squares refinement. Saturation studies revealed a small number of ion-association sites on the exterior of the assembly and the direct observation of ion association in water. The addition of excess alkali-metal cations displaces the ion-associated hydrophobic tetraalkylammonium cations. Comparisons between tetraethyl- and tetrapropylammonium show a preference for ion association with the more hydrophobic cation.