Supramolecular Chemistry, Part 85[+] Flexibility, Association Constants, and Salt Effects in Organic Ion Pairs: How Single Bonds Affect Molecular Recognition

The free energies of association ΔGof ten organic ion pairs (such as the ones shown here) in water have a linear dependence on the number of rotatable bonds within the ions, but with a surprisingly small sensitivity of only 0.5 kJ mol−1 per single bond. The dependence of ΔG on the ionic strength I can be well described with the Debye–Huckel equation and yields an increment of 8 kJ mol−1 per salt bridge for I = 0.0.