Rotational tunnelling and molecular disorder of 4-iodo-toluene

Abstract High resolution neutron spectroscopy is used to study head-to-tail disorder in 4-iodo-toluene via methyl tunnelling rotation. The observed three tunnelling bands represent three different nearest neighbourhoods. Their relative intensities are explained on the basis of ∼10% misoriented molecules and a repulsive methyl–methyl interaction which is the basis of the observed disorder. The shape of the broad tunnelling bands can be described by Gaussian potential distributions characteristic of disorder at larger distances. This model yields an average potential strength and its width for each tunnelling band. The shape of the average potential is refined to second order by including librational modes extracted from the density of states. Dissolution of the molecules in the isomorphous host 1,4-diiodo-benzene modifies the strength of the rotational potential only weakly. Spectral intensities at intermediate concentration are consistent with a miscibility gap.