NEUTRON-SCATTERING INVESTIGATIONS OF GUEST MOLECULAR-DYNAMICS IN ALPHA, OMEGA-DIBROMOALKANE UREA INCLUSION-COMPOUNDS

Incoherent quasi-elastic neutron scattering (IQNS) has been used to in vestigate 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 co mpounds. 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 s amples have been considered (specifically, with the channel axes of al l crystals either parallel or perpendicular to the momentum transfer v ector), allowing translational motions of the guest molecules along th e channel to be investigated separately from reorientational motions o f the guest molecules about the channel axis. The experiments have bee n performed as a function of firstly the experimental geometry, second ly the length of the alpha, omega-dibromoalkane molecule (n = 8-10) an d thirdly the temperature (with particular interest in changes in the dynamic properties above and below the well established phase transiti on temperatures for these inclusion compounds). It is shown that in th e low-temperature phase there is an oscillatory motion along the chann el axis which becomes overdamped above the phase transition temperatur e. Above the phase transition temperature, the dynamic properties of t he guest molecules can be understood in terms of translational motions along the channel (modelled as continuous diffusion between rigid imp ermeable boundaries) and reorientational motions about the channel axi s (modelled as uniaxial rotational diffusion in a onefold cosine poten tial). Quantitative dynamic information relating to these motional mod els is presented.