Fig. 3. Measurement of the bromide ion binding constants of TK⁶⁺ complexes by titration. (a) Computed B3LYP/6-31G(d) electrostatic potential of TK⁶⁺ on the molecular surface defined by the 0.001 electrons bohr^–3 contour of the electron density (b) schematic representation of bromide ion binding. (c) Stacked plots of ¹H NMR (600 MHz, 298 K) spectra of 1.87 mM solutions of TK⁶⁺ in D₂O titrated with, bottom to top, increasing amounts of tetrabutylammonium bromide. (d) Binding isotherm obtained by plotting H_j signal shift versus bromide ion concentration. In most cases, addition of more than two equivalents of anion precipitated the knot and prevented further measurements.