Complexes of cucurbituril with alkyl mono- and diammonium ions in aqueous formic acid studied by calorimetric titrations

The thermodynamic data of the complexation reaction between the ligand cucurbituril and different alkylmonoamine and alkyldiamine hydrochlorides have been studied in aqueous formic acid. The stability constants, the reaction enthalpies and the reaction entropies are obtained from calorimetric titrations. At both rims of the cavity of cucurbituril the carbonyl groups interact with ions by ion-dipole interactions. If ammonium ions with longer alkyl chains enter the hydrophobic cavity of cucurbituril hydrophobic interactions participate in the complex formation. The hydrophobic contribution to the reaction enthalpy is calculated for the alkyl monoammonium ions. In 50% (v/v) formic acid the enthalpies of the hydrophobic interactions contribute more to the experimental reaction enthalpy than the ion-dipole interactions. (C) 1997 Elsevier Science B.V.