Deuterium Quadrupole-echo Studies of Molecular Motion in a Biphenyl-P-Cyclodextrin Clathrate

Deuterium quadrupole spin-echo spectra have been obtained at 12 temperatures between 103 and 303 K for deuterated derivatives of biphenyl in polycrystalline /3-cyclodextrin clathrates. Reorientation of biphenyl in the clathrate channel may be modelled by constructing a grid of positions for the p,p'-axis and using a multisite simulation program to calculate spectra for different patterns of site-to-site hopping. It is found that above 240 K the biphenyl p,p'-axis librates in a cone of half angle ca. 27", with an Arrhenius activation energy 58 +4 kJ mol-'. Between 163 K and the lowest measured temperature the p,p'-axis librates over a more restricted range of angles, with a much lower activation energy, 4 kJmol-I. The simple models developed here do not suffice to account quantitatively for the transition from this motion to the thermally activated motion observed at higher temperatures.