Dynamics of methane trapped in C60 interstices

We have carried out inelastic neutron scattering studies of the methane/C60 system in order to understand the hindered rotational and vibrational dynamics of methane trapped in C60 interstices and to determine the structure around the interstitial site. At temperatures of 20 K and below, we observe inelastic peaks from rotational transitions of the CH4. These transitions allow unambiguous assignment of the hindered rotational energy levels and a determination of the interaction potential. The appearance of two peaks for one of the J=0→3 transitions implies the existence of two distinct kinds of interstitial sites and the measured transition energies suggest rotational barriers of about 26 and 16 meV for these sites. Time-dependent changes in peak heights indicate slow (t1/2≈2.6 h) triplet→quintet nuclear spin conversion that necessarily accompanies the J=1→0 rotational relaxation. We also have observed an inelastic peak at 10.9 meV in a region where there is a gap in the calculated, and otherwise dense, rotational spectrum. We believe that this must correspond to a local vibrational mode of CH4 rattling in its cage at ∼2.6 THz. Other peaks involving higher-energy vibrational excitations in CD4/C60 correspond in energy to assigned peaks in the inelastic neutron scattering spectra of C60, albeit sometimes with different intensities. Taken together, these findings suggest the rotation of CH4 is hindered by the C60 cage and that the vibrations of CH4 and C60 are loosely coupled.