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The heat capacity C(T) of the interstitial solid solution (CH4)0.4C60 hasbeen investigated in the temperature interval 1.4-120 K. The contribution ofCH4 molecules to the heat capacity has been separated. The contributions of CH4and 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 ofthe rotational motion of CH4 and CD4 molecules changes from libration tohindered rotation. In the interval 14-35 K the heat capacities of CH4 and CD4molecules are satisfactorily described by contributions of the translationaland libration vibrations, as well as the tunnel rotation for the equilibriumdistribution of the nuclear spin species. The isotope effect in heat capacityof CH4 and CD4 molecules is due, mainly, the difference in the frequencies oflocal tranlational and libration vibrations of molecules CH4 and CD4. Thecontribution of the tunnel rotation of the CH4 and CD4 molecules to the heatcapacities of this gases is dominant below 8 K. The isotopic effect is causedby the difference between both the conversion rates and the rotational spectraof the nuclear spin species of CH4 and CD4 molecules. The conversion rate ofCH4 molecules is several times lower than that of CD4 ones. Weak featuresobserved 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 transitionsin the orientational glasses of these solutions.

The low temperature heat capacity of solutions of methane isotopes in fullerite C60. Isotope effects