Thermodynamic and 13C NMR studies of the inclusion complexes of α- and β-cyclodextrins with cyano- and nitrophenols in aqueous solution

Equilibrium constants for the formation of 1 : 1 inclusion complexes of alpha-cyclodextrin (alpha-CD) with neutral and anionic phenol derivatives (3- and 4-cyanophenols and 3- and 4-nitrophenols) have been evaluated at 5, 12, 25, and 35 degrees C by means of spectrophotometry. Similarly, the equilibrium constants have been determined for the inclusion complexes of beta-cyclodextrin (beta-CD) with the phenols. Enthalpy and entropy changes for the formation of the inclusion complexes have been estimated from the temperature dependences of the equilibrium constants. With alpha-CD, the enthalpy and entropy changes for the anionic species have been found to be more negative than those for the neutral ones, except for 4-cyanophenol, suggesting that the inclusion complexes of the anionic species are more rigid than those of the neutral species. From analyses of chemical shift differences in C-13 NMR spectra of 3- and 4- cyanophenols and 3- and 4-nitrophenols in aqueous solutions with and without CDs, a nitro or a cyano group has been found to be first bound to the alpha- and beta-CD cavities.