Topography of the 1 : 1 alpha-cyclodextrin nitromethane inclusion complex

Dissolution of alpha-cyclodextrin (alpha-CD) in 9:1 water-nitromethane smoo thly generates the title compound, which crystallizes as the pentahydrate i n the orthorhombic space group P2(1)2(1)2(1) with a = 9.452(4), b = 14.299( 3), c = 37.380(10) Angstrom, and Z = 4. Its crystal structure analysis reve aled the alpha-CD macrocycle in an unstrained conformation stabilized throu gh a ring of O-2 ... O-3' hydrogen bonds between five of the six adjacent g lucose residues. The nitromethane is located in the alpha-CD cavity in an o rientation parallel to the plane of the macrocycle, and assumes two sites o f equal population with the nitro group in excessive thermal motion; the gu est is held by van der Waals contacts and C-HO ... O-type hydrogen bonds to the pyranose H-3 and H-5 protons. The packing of the macrocycles in the cr ystal lattice is of cage herringbone-type with an extensive intra- and inte rmolecular hydrogen bonding network. The ready formation of a nitromethane inclusion complex in aqueous nitromethane, and the subtleties of its molecu lar structure amply demonstrate the ease with which water is expelled from the alpha-CD cavity by a more hydrophobic co-solvent. (C) 2000 Elsevier Sci ence Ltd. All fights reserved.