Molecular Simulation and Spectroscopy of a Strong Multipolar Fluid

Abstract

Acetonitrile (CH3CN) is a small, aprotic molecule. The apparent simplicity of this species belies great complexity in its organization and dynamics in the liquid state. Additionally, at silica surfaces, liquid acetonitrile takes on a lipid-bilayer-like structure that has great practical importance in batteries, chromatography, and heterogeneous catalysis. In this thesis, I use molecular simulations and neutron scattering to explore the structure and dynamics of liquid acetonitrile in the bulk and at silica interfaces. First, using angularly resolved radial distribution functions, I identify a microscopic structure in bulk acetonitrile in which most liquid molecules form antiparallel and/or head-to-tail dimers. In contrast to the traditional view of acetonitrile, antiparallel dimers are shown to be octupole-paired, as opposed to dipole-paired. Further analysis reveals that head-to-tail dimers are the dominant motif and live longer than antiparallel dimers. I also use angularly resolved radial distribution functions to show that this propensity to form head-to-dimers has signatures in the observed crystalline polymorphs, and I connect my results using molecular simulation to neutron-scattering data that I also collected. Second, I present findings on the structure and transport properties of acetonitrile at the liquid/silica interface. I use molecular simulations to show that the bilayer is exceptionally robust with large changes in temperature. The effects of Poiseuille hydrodynamic flow on the surface bilayer suggest where the flow boundary may lie, because there is a departure in the vicinity of the walls from the standard parabolic fit describing Poiseuille flow. These results will help guide ion-separation and ion-current experiments using acetonitrile as a solvent. In a separate set of chapters, I present a data analysis protocol for the characterization of absorptive linearities and nonlinearities measured using 2-beam action spectroscopy. This nonlinear-optical technique, along with the protocol described in this thesis, enables the simultaneous determination of the effective order(s) of absorption. This method is important in the study of many nonlinear-optical phenomena, including multiphoton absorption polymerization.