Molecular motion in hydroquinone hydrogen chloride clathrate compound

Heat capacities of [C₆H₄(OH)₂]₃·(HCl)_x with x=0.68 and 0.75 were measured from 1 to 15 K. Weak anomalies were found at 7 and 11 K, respectively. The rotational heat capacities of the hydrogen chloride molecules in the clathrate cavities were obtained by subtracting heat capacities due to the host lattice and the rattling motion of the guest molecules from the experimental values. The rotational heat capacity of the hydrogen chloride of the system with x = 0.68 agreed with the free rotational heat capacity of the gaseous hydrogen chloride with the modified moment of inertia I = 3.29 × 10^?47kgm² below 8 K. The dielectric constant of the system with x = 0.56 obeyed the Curie-Weiss law above 30 K. These results showed that the hydrogen chloride molecules in the clathrate cavities execute modified free rotation at low temperatures.