Direct Observation of Methyl Librations in Neopentane

The "small-κ" method of neutron molecular spectroscopy has been used to observe for the first time the two librational modes, A₂ and F₂, of the methyl groups in neopentane. Two inelastic-scattering peaks corresponding to librational modes were measured at 26 and 33 meV, the separation resulting primarily from a difference in the six-fold energy barriers governing the two rotations. Both librational motions were restricted by a dominant threefold energy barrier of +870±{}30 MeV.