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

Temperature-dependent (2)H longitudinal spin relaxation times (T1) of dilute benzene-d6 in 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) and two deuterated variants of the 1-ethyl-3-methylimidazolium cation (Im21(+)-d1 and Im21(+)-d6) in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Im21][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 (2)H T1(T) data could be fit to within uncertainties over a broad region about the T1 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 system were therefore used to explore the nature of solute rotation in ionic liquids and how it might differ from rotation in conventional solvents. Whereas "spinning" about the C6 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°) jumps about in-plane axes lead to rotations being markedly nondiffusive, especially below room temperature.

Solvation dynamics in a prototypical ionic liquid + dipolar aprotic liquid mixture: 1-butyl-3-methylimidazolium tetrafluoroborate + acetonitrile.

Solvation energies, rotation times, and 100 fs to 20 ns solvation response functions of the solute coumarin 153 (C153) in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) + acetonitrile (CH3CN) at room temperature (20.5 °C) are reported. Available density, shear viscosity, and electrical conductivity data at 25 °C are also collected and parametrized, and new data on refractive indices and component diffusion coefficients presented. Solvation free energies and reorganization energies associated with the S0 ↔ S1 transition of C153 are slightly (≤15%) larger in neat [Im41][BF4] than in CH3CN. No clear evidence for preferential solvation of C153 in these mixtures is found. Composition-dependent diffusion coefficients (D) of Im41(+) and CH3CN as well as C153 rotation times (τ) are approximately related to solution viscosity (η) as D, τ ∝ η(p) with values of p = -0.88, -0.77, and +0.90, respectively. Spectral/solvation response functions (Sν(t)) are bimodal at all compositions, consisting of a subpicosecond fast component followed by a broadly distributed slower component extending over ps-ns times. Integral solvation times (⟨τ(solv)⟩ = ∫(0)(∞)Sν(t) dt) follow a power law on viscosity for mixturecompositions 0.2 ≤ x(IL) ≤ 1 with p = 0.79. With recent broad-band dielectric measurements [J. Phys. Chem. B 2012, 116, 7509] asinput, a simple dielectric continuum model provides predictions for solvation response functions that correctly capture thedistinctive bimodal character of the observed response. At x(IL) ∼ 1 predicted values of ⟨τ(solv)⟩ are smaller than those observed by a factor of 2-3, but the two become approximately equal at x(IL) = 0.2. Predictions of a recent semimolecular theory [J. Phys. Chem. B 2011, 115, 4011] are less accurate, being uniformly slower than the observed solvation dynamics.

Solute diffusion in ionic liquids, NMR measurements and comparisons to conventional solvents.

Diffusion coefficients of a variety of dilute solutes in the series of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imides ([Prn1][Tf2N], n = 3, 4, 6, 8, and 10), trihexyltetracedecylphosphonium bis(trifluoromethanesulfonyl)imide [P14,666][Tf2N], and assorted imidazolium ionic liquids are measured using pulsed field gradient (1)H NMR. These data, combined with available literature data, are used to try to uncover the solute and solvent characteristics most important in determining tracer diffusion rates. Discussion is framed in terms of departures from simple hydrodynamic predictions for translational friction using the ratio ζobs/ζSE, where ζobs is the observed friction, determined from the measured diffusion coefficient D via ζobs = kBT/D, and ζSE = 6πηR is the Stokes friction on a sphere of radius R (determined from the solute van der Waals volume) in a solvent with viscosity η. In the case of neutral solutes, the primary determinant of whether hydrodynamic predictions are accurate is the relative size of solute versus solvent molecules. A single correlation, albeit with considerable scatter, is found between ζobs/ζSE and the ratio of solute-to-solvent van der Waals volumes, ζobs/ζSE = {1 + a(VU/VV)(-p)}, with constants a = 1.93 and p = 1.88. In the case of small solutes, the observed friction is over 100-fold smaller than predictions of hydrodynamic models. The dipole moment of the solute has little effect on the friction, whereas solute charge has a marked effect. For monovalent solutes of size comparable to or smaller than the solvent ions, the observed friction is comparable to or even greater than what is predicted by hydrodynamics. These general trends are shown to be quite similar to what is observed for tracer diffusion in conventional solvents.

Bimolecular electron transfer in ionic liquids: are reaction rates anomalously high?

Steady-state and picosecond time-resolved emission spectroscopy are used to monitor the bimolecular electron transfer reaction between the electron acceptor 9,10-dicyanoanthracene in its S(1) state and the donor N,N-dimethylaniline in a variety of ionic liquids and several conventional solvents. Detailed study of this quenching reaction was undertaken in order to better understand why rates reported for similar diffusion-limited reactions in ionic liquids sometimes appear much higher than expected given the viscous nature of these liquids. Consistent with previous studies, Stern-Volmer analyses of steady-state and lifetime data provide effective quenching rate constants k(q), which are often 10-100-fold larger than simple predictions for diffusion-limited rate constants k(D) in ionic liquids. Similar departures from k(D) are also observed in conventional organic solvents having comparably high viscosities, indicating that this behavior is not unique to ionic liquids. A more complete analysis of the quenching data using a model combining approximate solution of the spherically symmetric diffusion equation with a Marcus-type description of electron transfer reveals the reasons for frequent observation of k(q) ≫ k(D). The primary cause is that the high viscosities typical of ionic liquids emphasize the transient component of diffusion-limited reactions, which renders the interpretation of rate constants derived from Stern-Volmer analyses ambiguous. Using a more appropriate description of the quenching process enables satisfactory fits of data in both ionic liquid and conventional solvents using a single set of physically reasonable electron transfer parameters. Doing so requires diffusion coefficients in ionic liquids to exceed hydrodynamic predictions by significant factors, typically in the range of 3-10. Direct, NMR measurements of solute diffusion confirm this enhanced diffusion in ionic liquids.