Dynamics of methane trapped in C[sub 60] interstices.

We have carried out inelastic neutron scattering studies of the methane/C[sub 60] system in order to understand the hindered rotational and vibrational dynamics of methane trapped in C[sub 60] 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 CH[sub 4]. 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 (t[sub 1/2]approx. 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 CH[sub 4] rattling in its cage at ∼2.6 THz. Other peaks involving higher-energy vibrational excitations in CD[sub 4]/C[sub 60] correspond in energy to assigned peaks in the inelastic neutron scattering spectra of C[sub 60], albeit sometimes with different intensities. Taken together, these findings suggest the rotation of CH[sub 4] is hindered by the C[sub 60] cage and that the vibrations of CH[sub 4] and C[sub 60] are loosely coupled. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]