Dynamics of methane trapped in C{sub 60} interstices
- Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
- Argonne National Laboratory, Argonne, Illinois 60636 (United States)
- Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States)
- Department of Chemistry, State University of New York, Buffalo, New York 14260 (United States)
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{yields}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_equal}2.6 h) triplet{yields}quintet nuclear spin conversion that necessarily accompanies the J=1{yields}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 {approx}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. (c) 2000 American Institute of Physics.
- OSTI ID:
- 20216848
- Journal Information:
- Journal of Chemical Physics, Vol. 113, Issue 1; Other Information: PBD: 1 Jul 2000; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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