Rotational dynamics of simple asymmetric molecules

Molecular dynamic simulations were carried out on rigid diatomic molecules, which exhibit both alpha (structural) and beta (secondary) dynamics. The relaxation scenarios range from onset behavior, in which a distinct alpha process emerges on cooling, to merging behavior, associated with two relaxation peaks that converge at higher temperature. These properties, as well as the manifestation of the beta peak as an excess wing, depend not only on thermodynamic conditions, but also on both the symmetry of the molecule and the correlation function (odd or even) used to analyze its dynamics. These observations help to reconcile divergent results obtained from different experiments. For example, the beta process is more intense and the alpha-relaxation peak is narrower in dielectric relaxation spectra than in dynamic light scattering or NMR measurements. In the simulations herein, this follows from the weaker contribution of the secondary relaxation to even-order correlation functions, related to the magnitude of the relevant angular jumps.