Rotational Dynamics in Ionic Liquids from NMR Relaxation Experiments and Simulations: Benzene and 1-Ethyl-3-Methylimidazolium

Temperature-dependent H-2 longitudinal spin relaxation times (T-1) Of dilute benzene-d(6) in 1-butyl-3-methylimidazolium tetrafluoroborate ([Im(41)][BE4) and two deuterated variants of the 1-ethyl-3-methylimidazolium cation (Im(21)(+)-d(1) and Im(21)(+)-d(6)) in 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([Im(21)][Tf2N]), measured at multiple Larmor frequencies, were used to probe rotational dynamics in ionic liquids. Rotational correlation times significantly faster than predicted by slip hydrodynamic calculations were observed for both solutes. Molecular dynamics simulations of these systems enabled extraction of more information about the rotational dynamics from the NMR data than rotation times alone. The multifrequency H-2 T-1(T) data could be fit to within uncertainties over a broad region about the T-1 minimum using models of the relevant rotational time correlation functions and their viscosity/temperature dependence derived from simulation. Such simulation-guided fitting provided confidence in the semiquantitative accuracy :of the simulation models and enabled interpretation of NMR measurements to higher viscosities than previously possible: Simulations of the benzene systeni were therefore used to explore the nature of solute rotation in ionic liquids and how it might differ froth rotation in conventional solvents. Whereas spinning about the C-6 axis of benzene senses similarly weak solvent :friction in both types of solvents, tumbling (rotations about in-plane axes) differs significantly in conventional solvents and ionic liquids. In the sluggish environment provided by ionic liquids, orientational caging and the presence of rare but influential large-amplitude (180 degrees) jumps about in-plane axes lead to rotations being markedly nondiffusive, especially below room temperature.