The origin and temperature dependence of the single particle, methyl-group rotational potential in acetic acid Original Research Article

Inelastic neutron scattering (INS) is used to obtain a direct measurement of the tunnel frequency of the methyl groups at 2 K in acetic acid (ν0 = 1.52 μeV) and its temperature dependence up to 38 K. Corresponding measurements of the libration frequency up to 90 K are also reported. In conjunction with a new 2 K crystal-structure determination, a recently published method for calculating rotational potentials of methyl groups from crystal structures is re-examined. Crystal structures at four different temperatures, and the improved calculation, are used to predict the temperature dependence of the rotational potential correctly.