Direct Observation of Methyl Librations in Neopentane

Abstract

The "small-$\kappa$" method of neutron molecular spectroscopy has been used to observe for the first time the two librational modes, $A_2$ and $F_2$, 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.