Temperature Effects on CD Spectra of β-Cyclodextrin Complexes with 2-Substituted Naphthalenes

CD spectra of β-cyclodextrin complexes with 2-naphthol, 2-naphthyloxyacetic acid, potassium 2-naphthoate, and potassium 2-naphthylacetate were measured at various concentrations of β-cyclodextrin and at temperatures ranging from 10 to 70°C. The temperature-dependent CD spectra of the 2-naphthyloxyacetic acid complex clearly show the 1 : 1 complex formation, but the other three complexes give CD intensity changes which are inexplicable in terms of the 1 : 1 stoichiometry. The molecular ellipticity and thermodynamic parameters were determined by the least-squares method. Enthalpy and entropy are in the ranges from −22.2 to −26.8 kJ·mol−1 and from −24 to −38 J·K−1·mol−1, respectively. They are strongly correlated to the volume of the substituent of the guest molecule, although the free energy is found in a quite narrow region (from −14.3 to −15.4 kJ·mol−1). The correlation is explained on the basis of the β-cyclodextrin-guest interaction and the solvation around the guest molecule.