Dynamics of orientationally disordered hydrogen bonds and of water molecules in a molecular cage

Abstract

The crystalline complex between the ring-shaped β-cyclodextrin (β-CD) and 11 water molecules (β-CD · 11H2O) shows extensive orientational disorder of β-CD hydroxyl groups and water molecules and in addition positional disorder of six water molecules which are enclosed in the β-CD molecular cavity. A quasielastic incoherent neutron scattering (QINS) study was performed employing three different energy resolutions. The quasielastic lineshape can be approximated by a sum of two Lorentzian curves of constant width and intensity ratio and intensities decreasing with the scattering angle φ and extrapolating to zero for φ = 0 consistently for all three energy resolutions. This is indicative of local diffusion of hydrogen atoms in the sample. A previous high resolution (0.6Å) neutron crystallographic study provides the knowledge of the number and jump distances of the disordered hydrogen atoms. This gives a sound basis for model calculations which aim to describe the experimental spectra in terms of theoretical scattering functions. A very simple two-site jump model with two different jump distances already describes the data well. It can be interpreted as the observation of reorientational jumps of hydroxyl groups and water molecules over H ··· H distances around 1.5Å and of diffusive motions of water molecules enclosed in the β-CD molecular cavity over H ··· H (and O ··· O) distances around 3 Å. At room temperature both types of motion occur with a similar distribution of jump rates extending at least from about 2 × 1010 to 2 × 1011 s-1. The temperature dependence of the elastic peak intensity shows that the diffusive motions can be followed down to at least T = 240K.