Molecular recognition of organic anions by a water-soluble calix[4]arene: Evidence for enthalpy–entropy compensation

The inclusion of organic anions by the water-soluble tetracationic calix[4]arene TAC4 has been studied at neutral pH by 1H NMR and isothermal titration calorimetry. The aromatic anions are better included than their aliphatic analogues. The ΔH° and ΔS° values show that the inclusion processes are mostly enthalpically driven and entropically unfavoured; they also show that an enthalpy–entropy compensation balances the gain in one contribution against a corresponding loss in the other.