Crystal and molecular structure of β-cyclodextrin inclusion complex with succinic acid

Abstract

The crystal complex of β-cyclodextrin with succinic acid, intermediate product of hydrolysis reaction of succinic anhydride in the presence of β-cyclodextrin, was isolated and studied by X-ray analysis (monoclinic, space group P21, a = 15.1977(7) A, b = 10.1763(5) A, c = 20.6943(6) A, β = 109.239(4)°, V = 3021.8(2) A3, Z = 2, R 1 = 0.0359, wR 2 = 0.0947). It was proved that β-cyclodextrin and succinic acid form an inclusion complex, which exists in crystal state as a heptahydrate. The molecule of succinic acid is fully included in the β-cyclodextrin cavity with its carboxyl groups accessible for water molecules. Water molecules located at borders of cavity rims and in interstices between molecules of β-cyclodextrin participate in formation of intermolecular hydrogen bonds. The overall structure does not contain disordered fragments. The crystal conformation of succinic acid corresponds to one of possible conformers of the molecule in vacuo and is almost not disturbed by intermolecular interactions in crystal. Based on the analysis of structural features of the crystal conformation of succinic acid and character of its location in the β-cyclodextrin cavity, it was suggested that hydrolysis of succinic anhydride via ring opening and formation of succinic acid is mediated by cyclodextrin microenvironment and it likely occurs near the narrow rim of the macrocycle cavity.