Inelastic incoherent neutron scattering study of the methyl rotation in various methyl halides

Publisher Website
Google Scholar
Abstract
The inelastic incoherent neutron scattering technique has been applied in an energy range from 0.4 μeV to 60 meV to measure the rotational excitations of the methyl groups in polycrystals of CH3I, CH2DI, CD3I, CH3Br, CD3Br, and CH3Cl in a temperature range 5<T[K]3I and CH3Br, respectively. Excitations to the higher excited librational states are identified via the temperature dependence and the isotope effects of the modes in the meV regime. The rotor–rotor coupling between neighboring methyl groups is found to be small. Thus single particle rotational potentials were derived from the observed eigenenergies. The potential strength increases going from the iodide via the bromide to the chloride. In CH3Br the rotational potential does not change on deuteration. In CH3I a very unusual increase in the rotational potential with deuteration is found. Besides the methyl librations, large amplitude motions around axes perpendicular to the molecular symmetry axis are observed at energies comparable to the methyl libration. The coupling of the methyl rotor to lattice modes modulating the strength of the orientational potential in CH3I gives rise to a peculiar temperature dependence of the tunnel splitting.

This publication has 29 references indexed in Scilit: