Terahertz spectroscopy study of the confining potential for methane in the endofullerene CH4@C60

We used terahertz spectroscopy to study the non-covalent interaction between CH4 and the confining fullerene cage in endofullerene CH4@C60. The temperature dependence of the THz absorption spectra of powdered CH4@C60 was measured between 5 and 300 K. At temperatures below 50 K, the THz spectrum of CH4 shows a single line centered at 214 cm-1, which broadens and shifts to a higher energy as the temperature increases. These effects are explained by the anharmonicity of the CH4-C60 interaction potential function. The model involves the center-of-mass motion of CH4 in a spherically symmetric potential well. Line intensities are modeled by invoking an electric dipole moment induced by the translational displacement of CH4 from the center of the cage C60. The potential function and the dipole moment parameters were derived from the temperature dependence of the THz absorption spectra and compared with the parameters of previously studied endofullerenes. The quantum chemistry calculations reproduce the CH4 translation motion frequency and the potential function remarkably well.