METHYL-GROUP TUNNELING STUDIES IN CALIXARENES

Inelastic neutron scattering has been used to study the tunnelling of methyl groups belonging to several guest molecules (toluene, p-xylene, gamma-picoline) incarcerated in a host calixarene matrix. In all the cases investigated, the low tunnel-split librational states of the gue st methyl groups. The main line occurs near 0.63 meV, very close to th e CH3 quantum free rotor limit. In the toluence complex, the quantum r egime persists at least up to 60 K. Effects of coupling between rotati onal and vibrational modes are discussed. Very subtle structural chang es not revealed by diffraction measurements are suggested by the neutr on spectroscopy results.