Librational motions in dimethylacetylene studied by inelastic neutron scattering

The librational motions of the methyl groups in solid dimethylacetylene at 4·8 K have been studied by inelastic incoherent neutron scattering. The observed libron density of states exhibits well-defined peaks, which have been compared with the theoretical results calculated with the time-dependent Hartree approximation using a simple model including only methyl librations about the C-C axes of the molecule. A close agreement between the experimental and calculated results was obtained by using a librational potential V 1 c/2 >cos 3α + V 2 c/2 >cos 6α with the parameters V 1 c = - 43·3 ± 1·7 meV and V 2 c = -3·5 ± 1·7 meV.