Structure of the complex of β-cyclodextrin with β-naphthyloxyacetic acid in the solid state and in aqueous solution Original Research Article

The structure of the complex of β-cyclodextrin (cyclomaltoheptaose) with β-naphthyloxyacetic acid was studied in solid state by X-ray diffraction and in aqueous solution by 1H NMR spectroscopy. The complex crystallizes in the channel mode, space group C2, with a stoichiometry of 2:1; two β-cyclodextrin molecules related by a twofold crystal axis form dimers, in the cavity of which one guest molecule is found on average. The above stoichiometry indicates one guest per β-CD dimer statistically oriented over two positions or two guest molecules in π–π interactions in half of the β-CD dimers and the rest of the β-CD dimers empty. In addition, occupancy of 0.5 for the guest per every β-CD dimer is in accord with the occupancy of the two disordered primary hydroxyls. These two hydroxyl groups, to which the carboxylic oxygen atoms of the guest are hydrogen bonded, point towards the interior of the β-CD cavity. In aqueous solution, the 1H NMR spectroscopic study indicated that there is a mixture of complexes with host–guest stoichiometries both 1:1 and 2:1.