Low-temperature thermal properties of the Werner calthrate inclusion compounds. Isotopic p-xylenes in tetrakis (4-methylpyridine) nickel(II) thiocyanate

The heat capacities of the clathrate compounds formed by inclusion of p-C6H 4(CH3)(2) and p-C6D4(CD3)(2) in tetrakis(4-methylpyridine) nickel(II) thioc yanate, and the empty host compound have been measured at temperatures betw een (12 and 300) K. Two temperature regions of anomalous heat capacity were found for leach of the clathrate compounds. The molar heat capacity divide d by the temperature (C-p,C-m/T) plotted against the temperature gave clear representation of the anomalies, making it possible to estimate the temper ature, enthalpy, and entropy associated with the heat capacity peaks. Stand ard thermodynamic functions were calculated and tabulated for rounded tempe ratures. These data were combined with those of p-xylene to give the entrop y of clathrate formation. Experimental evidence is presented that helium is captured into the empty host cavity to form a clathrate compound at low te mperatures. (C) 1998 Academic Press.