TOPOGRAPHY OF CYCLODEXTRIN INCLUSION COMPLEXES .25. COOPERATIVE O H=O HYDROGEN-BONDS IN BETA-CYCLODEXTRIN ETHANOL OCTAHYDRATE AT 15-K - A NEUTRON-DIFFRACTION STUDY

A single crystal neutron crystallography study of β-cyclodextrin-ethanol-octahydrate was carried out at T = 15 K. The composition is (C6H7O5D3)7-C2D5OD-8D2O with space group P21, and cell constants a = 20.460 (1) Å,b= 10.003 (1) Å, c-15.227 (1) Å, β = 109.023 (4)°, λ = 1.3167 Å, 4264 unique neutron data, and nominal resolution 0.93 Å (2λ = 90°). All H and D atoms are located and the structure is refined to R = 4.1%. All hydroxyl groups, the ethanol, and six water molecules are well ordered, two water molecules are disordered-in contrast to the room temperature structure, where orientational disorder of hydroxyl groups and extensive disorder of the ethanol and several water molecules is observed and gives rise to flip-flop hydrogen bonds. This ordering upon cooling indicates that the disorder at room temperature is of dynamic and not of static nature. At 15 K all hydrogen bonds in the structure are interconnected to form an infinite ordered network extending through the crystal. Water molecules (except W6 and W8 which are located in the β-cyclodextrin cavity) accept two hydrogen bonds and the hydroxyl groups accept one except for O(2)H of glucoses 3, 5, and 6 which accept two hydrogen bonds and O(3)4 which acts only as donor. Homodromic arrangements of hydrogen bonds dominate and show the strong influence of the cooperative effect. The general observations concerning hydrogen bond patterns are similar to those in (3-cyclodextrin undecahydrate, indicating that these characteristics are not unique but are an intrinsic feature of β-cyclodextrin complexes. There is systematic three-center hydrogen bonding with the minor components intramolecular, O(6)H…O(5) in five glucoses and O(2)H…O(4′) or O(3)H…O(4) in all seven glucoses, with the major component intramolecular interresidue O(2)…O(3) or O(3)H…O(2), with a distribution of 4 to 3. The included ethanol and three water molecules form hydrogen bonds between themselves and the adjacent β-cyclodextrin molecules but are not in hydrogen-bonding contact with the host cavity. This contrasts β-cyclodextrin. 11H2O where the included cluster of eight water molecules forms two hydrogen bonds to O(6) hydroxyls. © 1990, American Chemical Society. All rights reserved.