Neutron scattering investigations of guest molecular dynamics in alpha , omega -dibromoalkane-urea inclusion compounds

Abstract

Incoherent quasi-elastic neutron scattering (IQNS) has been used to investigate the dynamic properties of alpha , omega -dibromoalkane (guest) molecules (Br(CH2)nBr) constrained within the urea channel structure (host) in the crystalline alpha , omega -dibromoalkane-urea inclusion compounds. These investigations have been carried out on samples of urea inclusion compounds containing urea-d4 to ensure that the incoherent scattering is dominated by the alpha , omega -dibromoalkane molecules, and the two experimental geometries of semi-oriented polycrystalline samples have been considered (specifically, with the channel axes of all crystals either parallel or perpendicular to the momentum transfer vector), allowing translational motions of the guest molecules along the channel to be investigated separately from reorientational motions of the guest molecules about the channel axis. The experiments have been performed as a function of firstly the experimental geometry, secondly the length of the alpha , omega -dibromoalkane molecule (n=8-10) and thirdly the temperature (with particular interest in changes in the dynamic properties above and below the well established phase transition temperatures for these inclusion compounds). It is shown that in the low-temperature phase there is an oscillatory motion along the channel axis which becomes overdamped above the phase transition temperature. Above the phase transition temperature, the dynamic properties of the guest molecules can be understood in terms of translational motions along the channel (modelled as continuous diffusion between rigid impermeable boundaries) and reorientational motions about the channel axis (modelled as uniaxial rotational diffusion in a onefold cosine potential). Quantitative dynamic information relating to these motional models is presented.