Complexes of p-nitrophenol and p-nitrophenolate anion with heptakis-(2, 6-di-O-methyl)-β-cyclodextrin (DM-β-CD) in aqueous and organic solutions have been studied by spectroscopic analyses. Inclusion complex formation was observed on the basis of the changes of ultraviolet-visible absorption spectra in aqueous and organic solutions. The stability constants of the complexes were determined by the Benesi-Hildebrand equation. It was found that the stability constant of p-nitrophenolate anion-DM-β-CD complex was greater than that of p-nitrophenol-DM-β-CD complex in the aqueous solutions. Proton nuclear magnetic resonance data showed that the mode of complex formation of p-nitrophenol-DM-β-CD was the same as that of p-nitrophenolate anion-DM-β-CD in D2O. The apparent pKa value of p-nitrophenol decreased with increasing concentration of DM-β-CD.
