Structure and dynamics of acetonitrile: Molecular simulation and neutron scattering

We examine the interplay between organization and dynamics in bulk liquid acetonitrile. Using angularly resolved radial distribution functions, g(r,theta), derived from molecular simulations, we identify a complex microscopic structure in which most liquid molecules are associated with one or more neighboring molecules in an antiparallel, "octupole-paired" configuration and/or an offset, head-to-tail configuration. A detailed analysis of these structural motifs reveals that the offset head-to-tail dimers are the most prevalent. A time-dependent pairing analysis corroborates this picture of robust molecular associations favoring head-to-tail dimers, which last longer than antiparallel dimers. This organization, which is associated with pairing times (similar to ps) longer than the typical rotational time constant, cannot be explained on the basis of the Coulomb interactions in the dimer, and so must arise from collective effects that are neither strong nor specific. Finally, using both neutron-scattering techniques and molecular simulations, we study dynamics in liquid acetonitrile over time scales ranging from subpicosecond (the vibrational density of states) to picosecond (rotational/translational motions and the generalized density of states) to tens of picoseconds (self-diffusivity in the Fickian regime). (C) 2021 Elsevier B.V. All rights reserved.