| In this work, we have investigated both translational and rotational diffusion in neat ionic liquids (ILs) and in IL solutions using nuclear magnetic resonance (NMR) methods. Translational diffusion studies focus on ionic liquid solvents N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imides [Prn1][Tf2N], with n = 3, 4, 5, 6, 8, 10, and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide [P14,6,6,6][Tf2N]. Solutes include fused or bridged aromatics, fluorinated and nitrile-substituted benzenes, tetraphenylphosphonium benzoate, and other ionic liquids. Translational diffusion coefficients were measured using the longitudinal-eddy-current delay (LED) stimulated echo NMR pulse sequence with bipolar gradient pulse pairs. Applied field strengths were 400 and 850 MHz for 1H frequency. Additional data were collected, from various sources in the literature, for both IL solvent and conventional solvent systems. These data were used both as a point of comparison for our own measurements and as a broader sampling of solutes and solvents, allowing for an assessment of the effect of solute-solvent properties on the friction coefficient.;Although the diffusion of solutes has been widely studied in conventional solvents and, to a lesser degree, in ionic liquids, many deviations from hydrodynamic predictions continue to be reported, often accompanied by their own competing models and hypotheses. One common deviation is that of sub-slip diffusion of small solutes in dilute solution. Study of such cases has been difficult because many of the more commonly-used analysis techniques are unable to measure small solutes or are prone to error. By contrast, NMR spectroscopy is ideal for such studies in that it is applicable for nearly all solutes, and provides more reproducible data than do several competing techniques. Despite this fact, NMR has been little used in studying dilute small molecule diffusion in ionic liquids. As a result, our work in this area provides a significant amount of new data and insight.;We find that deviations of translational diffusion coefficients from the Stokes-Einstein (SE) equation in ILs are analogous but more pronounced than those in conventional solvents, in part due to the typically larger size of IL solvents. The ratio of solute-to-solvent size in IL solutions has a significant effect on the friction coefficient for translational diffusion, as it does for conventional solutions. The friction coefficient is also affected, in both conventional solvents and ILs, by the difference in the intermolecular forces of the solute and of the solvent. We find that the effect of solute shape on translational friction coefficient is minimal with respect to other sources of deviation from SE behavior. We also consider several SE corrections which were proposed by other researchers for conventional solutions, and assess their accuracy for IL solutions.;In comparing rotational diffusion measurements made over a range of temperatures with three different magnetic field strengths to molecular dynamics simulations, we are able to fit various possible models for rotational correlation functions. Specifically, we generate time correlation functions via variable-temperature MD simulations, fit them to a parameterized functional form, in order to represent the simulated time and temperature dependence. Varying the parameters of such representations enables us to extract more meaningful rotational correlation functions and their temperature dependence from the measured NMR T1 data. For both benzene and 1-ethyl-3-methyl-imidazolium, we find very fast dynamics that fall within the extreme narrowing regime at laboratory-accessible conditions, as well as slower dynamics. The faster dynamics likely correspond to in-plane rotations, while the slower correspond to tumbling. The slower component may be described with a single exponential or stretched exponential decay, but the faster component requires a bi-exponential. Solutions of [Im 21][Tf2N] and THF display a single-exponential decay with increasing viscosity. (Abstract shortened by UMI.). |
