Neutron Inelastic Scattering Studies of Globular Compounds

Abstract

The use of neutron inelastic scattering measurements to determine the nature of the solid‐state transitions in globular compounds is proposed and exemplified. Patterns were taken of the high‐ and low‐temperature phases of solid cyclohexane, 2–2‐dimethylbutane, and 1–4‐diazo‐bicyclo‐(2.2.2)‐octane. The first two have broad inelastic scattering peaks in the low‐temperature phase. In the high‐temperature phase these peaks disappear and the elastic peak broadens markedly. These changes are attributed to ``quasi‐free‐rotation'' of the molecules. Inelastic peaks do not appear in 1–4‐diazo‐bicyclo‐(2.2.2)‐octane, and the elastic peak does not broaden appreciably in the high‐temperature phase. Self‐diffusion coefficients D<sub>s</sub>=4.5×10<sup>−5</sup> cm<sup>2</sup>/sec for cyclohexane and D<sub>s</sub>=8.6×10<sup>−5</sup> cm<sup>2</sup>/sec for 2–2‐dimethylbutane were determined from the patterns of the liquids at 300°K using the ``small motions'' approximation. These values are larger than those obtained from NMR measurements, presumably as a result of irreversible rotation of the molecules.