Solute Rotation in Ionic Liquids: Size, Shape, and Electrostatic Effects

Herein are reported temperature-dependent measurements and molecular dynamics simulations designed to investigate the effects of molecular size, shape, and electrostatics on rotational dynamics in ionic liquids. Experiments were performed in the representative ionic liquid 1-butyl-3-methylimadazolium tetrafluoroborate ([Im₄₁]­[BF₄]) and simulations in the generic ionic liquid model ILM2 as well as a more detailed representation of [Im₄₁]­[BF₄]. ²H longitudinal spin relaxation times (T₁) were measured for deuterated versions of 1,4-dimethylbenzene, 1-cyano-4-methylbenzene, and 1,4-dimethylpyridinium between 296 and 337 K. Fluorescence anisotropy measurements were made on the larger solutes 9,10-dimethylanthracene, 9-cyano-10-methylanthracence, and 9,10-dimethylacridnium between 240 and 292 K. Both experiment and simulation showed the nonpolar solutes rotate ∼2-fold faster than their dipolar and charged counterparts. The rotational correlation functions measured in fluorescence experiments are significantly nonexponential and can be fit to stretched exponential functions having stretching exponents 0.4 ≤ β ≤ 0.8, with β decreasing with decreasing temperature. Rotational correlation times in both the NMR and fluorescence experiments conform approximately to the hydrodynamic expectation τ_rot ∝ (η/T)^p with p ≅ 1, and observed times are reasonably close to slip hydrodynamic predictions. Simulations, even with the idealized ILM2 solvent model, are in semiquantitative agreement with experiment when compared on the basis of equal values of ηT^–1. When rotational diffusion coefficients (D_i) rather than correlation times were considered, much larger departures from hydrodynamic predictions are found in many cases (p ∼ 0.5 and D_i ≫ slip predictions). Rotational van Hove functions and trajectory analyses reveal the importance of large-angle jumps about some axes, even in the larger solutes.