NMR Determination of Association Constants for Aqueous Calixarene Complexes and Guest Template Effects on the Conformational Freedom

Abstract

Abstract The association constants (K) for aqueous complexes with water-soluble p-sulfonatocalix[n]arene (1n: n=4, 6, and 8) were determined by the NMR method. It was found that 14 and 16 form 1 : 1 complexes with guest molecules (trimethylanilinium chloride and 1-adamantyltrimethylammonium chloride), whereas 18 forms 1 : 2 18/guest complexes. The finding supports the view that calixarenes 1n’s are capable of molecular recognition on the basis of the ring size. Examination of the thermodynamic parameters for the association process established that the complexation with 14 is mainly due to the electrostatic force, whereas that with 16 and 18 is mainly due to the hydrophobic force. The coalescence temperatures (Tc’s) for calixarene ring inversion were subject to the template effect by these molecules. The activation parameters for the rate of ring inversion were estimated by a computer-assisted spectrum simulation: it was found that the Tc increase, observed in the presence of these guest molecules acting as templates, is caused by the increase in ΔH\eweq. These findings provide new insights into the association characteristics of water-soluble calixarenes.