Cold-Neutron Study of Hindered Rotations in Solid and Liquid Methylchloroform, Neopentane, and Ethane

Abstract

The low‐frequency modes of methylchloroform (1,1,1‐trichloroethane), neopentane, and ethane in their solid and liquid phases have been investigated by the scattering of cold neutrons. The energy‐gain spectra for these compounds in their low‐temperature solid phases exhibit broad bands peaked at about 52, 50, and 82 cm⁻¹ for CH₃CCl₃, C(CH₃)₄, and C₂H₆, respectively, which are attributed to whole‐molecule librations and translations in the lattice. In the high‐temperature phases of the ``globular'' compounds CH<sub>3</sub>CCl<sub>3</sub> and C(CH<sub>3</sub>)<sub>4</sub>, these spectral bands are no longer peaked, but are quite diffuse and blend into a considerably broadened elastic peak. These results show that the phase transitions below the melting points are associated with a change from strongly hindered to quasifree rotation, in agreement with the results of previous experiments. The barrier to molecular reorientation is estimated to be ≲1 kcal/mole in the high‐temperature phase of each compound. Bands are also observed in the CH<sub>3</sub>CCl<sub>3</sub> and C(CH<sub>3</sub>)<sub>4</sub> spectra peaked at about 300 and 286 cm<sup>−1</sup>, respectively, which are assigned primarily to the torsional oscillations of the methyl groups. These peak positions show no significant change in proceeding from the liquid to the solid phases, indicating little intermolecular contribution to the forces hindering methyl‐group reorientation. One of the liquid‐ethane spectra exhibits a shoulder around 260 cm<sup>−1</sup>, which is possibly due to torsional vibrations. Assuming threefold cosine potentials with no interaction between methyl groups, ``average'' barriers to rotation of 5.8 and 5.2 kcal/mole are calculated from the torsional peaks for CH₃CCl₃ and C(CH₃)₄. The CH₃CCl₃ barrier is considerably higher than the value derived from thermodynamic results for CH₃CCl₃ (and ethane) gas.