Methyl quantum rotation in solid CH3F: An inelastic neutron scattering study

Inelastic incoherent neutron spectra of solid CH3F were measured in the μeV and in the meV range. The methyl groups display the effect of rotational tunneling with a splitting of the librational ground state ℏωt =23.1 μeV. All CH3 groups are equivalent. The tunnel splitting and the transition energy to the first excited librational state E01=10.1 meV are reproduced by a rotational potential V(φ)=11.8 meV⋅cos 3φ. A crystal structure different to those of the other halides has to be postulated to understand this potential.