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

Abstract

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 ([Im][BF]) and simulations in the generic ionic liquid model ILM2 as well as a more detailed representation of [Im][BF]. H longitudinal spin relaxation times (T) 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 ∝ (η/T) 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. When rotational diffusion coefficients (D) rather than correlation times were considered, much larger departures from hydrodynamic predictions are found in many cases (p ∼ 0.5 and D ≫ slip predictions). Rotational van Hove functions and trajectory analyses reveal the importance of large-angle jumps about some axes, even in the larger solutes.

The effect of structural heterogeneity upon the microviscosity of ionic liquids

“… 34 The same effects have also been observed on the rotational dynamics of charged and uncharged solutes. 35 …”

“…The study of the diffusion and rotational dynamics of charged solutes in various ionic liquids shows that charged solutes will partition into the ‘stiff’ polar domain generated by the charge carrying moieties in the cation and anion. 30 – 35 The partitioning into the polar domain is very strong for solutes that are significantly smaller than the ionic liquid ions, while solutes that are larger in comparison to the ionic liquid will partition less strongly. 34 …”

“…In their study of the size, shape, and charge of a solute rotating in [C 4 C 1 im][BF 4 ] Rumble et al demonstrated that such diffusive jumps were much more common and of greater amplitude for the smaller probes, where they significantly contributed to rotational diffusion coefficients, but rarer in the case of larger probes. 35 Hence, one would expect Cy3 to experience a large friction to its diffusive motion. However, within the same study, Rumble et al also showed other faster, spatially restricted motions of all of their solutes, regardless of size.…”

“…These consisted of oscillations within a slowly relaxing environment which, along with other similar motions, are often described as ‘rattling’ inside the cage. 35 , 53 – 55 …”

Rotational and translational dynamics and their relation to hydrogen bond lifetimes in an ionic liquid by means of NMR relaxation time experiments and molecular dynamics simulation

“…In principle, single particle correlation times can be determined by means of NMR relaxation time experiments. This has been done for molecular solutes such as benzene, or for molecular vectors of cations which are not involved in hydrogen bonding [13][14][15][16][17][18]. However, hydrogen bonding is often subject to substantial proton transfer, which is fast on the NMR time-scale.…”