  The heat capacity C(T) of the interstitial solid solution (CH4)0.4C60 has
been investigated in the temperature interval 1.4-120 K. The contribution of
CH4 molecules to the heat capacity has been separated. The contributions of CH4
and CD4 molecules to the heat capacity of the solutions (CH4)0.40C60 and
(CD4)0.40C60 have been compared. It is found that above 80K the character of
the rotational motion of CH4 and CD4 molecules changes from libration to
hindered rotation. In the interval 14-35 K the heat capacities of CH4 and CD4
molecules are satisfactorily described by contributions of the translational
and libration vibrations, as well as the tunnel rotation for the equilibrium
distribution of the nuclear spin species. The isotope effect in heat capacity
of CH4 and CD4 molecules is due, mainly, the difference in the frequencies of
local tranlational and libration vibrations of molecules CH4 and CD4. The
contribution of the tunnel rotation of the CH4 and CD4 molecules to the heat
capacities of this gases is dominant below 8 K. The isotopic effect is caused
by the difference between both the conversion rates and the rotational spectra
of the nuclear spin species of CH4 and CD4 molecules. The conversion rate of
CH4 molecules is several times lower than that of CD4 ones. Weak features
observed in the curves of heat capacity of CH4 and CD4 near 6 K and 8 K,
respectively, are most likely a manifestation of first-order phase transitions
in the orientational glasses of these solutions.
