Deuterium quadrupole-echo studies of molecular motion in a biphenyl–β-cyclodextrin clathrate

Abstract

Deuterium quadrupole spin-echo spectra have been obtained at 12 temperatures between 103 and 303 K for deuterated derivatives of biphenyl in polycrystalline β-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^–1. 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 kJ mol^–1. 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.