Universality of Viscosity Dependence of Translational Diffusion Coefficients of Carbon Monoxide, Diphenylacetylene, and Diphenylcyclopropenone in Ionic Liquids under Various Conditions

Do Electrostatics Control the Diffusive Dynamics of Solitary Water? NMR and MD Studies of Water Translation and Rotation in Dipolar and Ionic Solvents

NMR-based measurements of the diffusion coefficients and rotation times of solitary water and benzene at 300 K are reported in a diverse collection of 13 conventional organic solvents and 10 imidazolium ionic liquids. Proton chemical shifts of water are found to be correlated to water OH-stretching frequencies, confirming the importance of electrostatic interactions in these shifts. However, the influence of magnetic interactions in aromatic solvents renders chemical shifts a less reliable indicator of electrostatics. Diffusion coefficients (DB) and rotational correlation times (τB) of benzene in the solvents examined are accurately described as functions of viscosity (η) by DB ∝ η-0.81 and τB ∝ η0.64. Literature values of DB and τB in alkane and normal alcohols, which were not included among the solvents studied here, are systematically faster than predicted by these correlations, indicating that factors beyond solvent viscosity play a role in determining the friction on benzene. In contrast to benzene, water diffusion and rotation are poorly described in terms of viscosity alone, even in the dipolar and ionic solvents measured here. The present data and the substantial literature data already available on dilute water diffusion show a systematic dependence of DW on solvent polarity among isoviscous solvents. The aspect of solvent polarity most relevant to water dynamics is the ability of a solvent to accept hydrogen bonds from water, as conveniently quantified by the frequency of water's OH stretching band, ΔνOH. The friction on translation, ζtr = kBT/DW, and rotation, ζrot = kBTτW, are both well correlated by functions of the form ζ(η, ΔνOH) = a1ηa2 exp (a3ΔνOH), where the ai are adjustable parameters. Molecular dynamics simulations reveal a strong coupling between electrostatic and nonelectrostatic water-solvent interactions, which makes it impossible to dissect the friction on water into additive dielectric and hydrodynamic components. Simulations also provide a tentative explanation for the unusual form of the correlating function ζ(η, ΔνOH), at least in the case of ζrot.

Solvent Role of Ionic Liquids in Fundamental Chemical Reaction Dynamics Analyzed by Time-Resolved Spectroscopy

Ionic liquids (ILs), which are used as solvents for chemical reactions, are different from conventional organic solvents owing to their designability. Physicochemical parameters of the ILs, such as polarity and viscosity, that affect chemical equilibria and reaction kinetics can be tuned by changing the combination of anions and cations or by varying the lengths of the alkyl chains present in the cations. We were interested in knowing how these physicochemical parameters affect fundamental chemical reactions in ILs. Therefore, in this personal account, we investigate our recent work on two different photochemical reactions in ILs, namely excited-state intramolecular proton transfer of hydroxyflavone and photodissociation of aminodisulfide, using time-resolved spectroscopic techniques. Interestingly, the roles of the ILs in these chemical reactions are quite different. The effect of the cationic species of the ILs (i. e., the head groups and number of alkyl carbons) on the solvation environment upon photoexcitation and reaction rate are discussed.

Anomalous Dependence of Translational Diffusion on the Water Mole Fraction for Solute Molecules Dissolved in a 1-Butyl-3-methylimidazolium Tetrafluoroborate/Water Mixture

Translational diffusion coefficients of carbon monoxide (CO), diphenylacetylene (DPA), and diphenylcyclopropenone (DPCP) were determined in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim]BF₄) and water using transient grating spectroscopy at different mole fractions of water (x_w). While DPA exhibited a larger diffusion coefficient than DPCP at low water mole fractions (x_w < 0.7), as observed for conventional liquids and ionic liquids (ILs), it was smaller at high mole fractions (x_w > 0.9). The apparent molecular radius of DPA determined using the Stokes-Einstein equation at x_w > 0.9 is close to the radius of an IL cluster in a water pool as determined from small-angle neutron scattering experiments (J. Bowers et al., Langmuir, 2004, 20, 2192-2198), suggesting that the DPA molecules are trapped in IL clusters in the water pool and move together. The solvation state of DPCP in the mixture was studied using Raman spectroscopy. Dramatically strong water/DPCP hydrogen bonding was observed at higher water mole fractions, suggesting that DPCP is located near the cluster interfaces. The large diffusion coefficient of DPCP suggests that hopping of DPCP between IL clusters occurs through hydrogen bonding with water.

Electrochemical Properties and Local Structure of the TEMPO/TEMPO+ Redox Pair in Ionic Liquids

Redox-active organic species play an important role in catalysis, energy storage, and biotechnology. One of the representatives is the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical, used as a mediator in organic synthesis and considered a safe alternative to heavy metals. In order to develop a TEMPO-based system with well-controlled electrochemical and catalytic properties, a reaction medium should be carefully chosen. Being highly conductive, stable, and low flammability fluids, ionic liquids (ILs) seem to be promising solvents with easily adjustable physical and solvation properties. In this work, we give an insight into the local structure of ILs around TEMPO and its oxidized form, TEMPO⁺, underlining striking differences in the solvation of these two species. The analysis is coupled with a study of thermodynamics and kinetics of oxidation in the frame of Marcus theory. Our systematic investigation includes imidazolium, pyrrolydinium, and phosphonium families combined with anions of different size, polarity, and flexibility, opting to provide a clear and comprehensive picture of the impact of the nature of IL ions on the behavior of radical/cation redox pairs. The obtained results will help to explain experimentally observed effects and to rationalize the design of TEMPO/IL systems.

Temperature-Dependent Constrained Diffusion of Micro-Confined Alkylimidazolium Chloride Ionic Liquids

Alkylimidazolium chloride ionic liquids (ILs) have many uses in a variety of separation systems, including micro-confined separation systems. To understand the separation mechanism in these systems, the diffusion properties of analytes in ILs under relevant operating conditions, including micro-confinement dimension and temperature, should be known. For example, separation efficiencies for various IL-based microextraction techniques are dependent on the sample volume and temperature. Temperature-dependent (20-100 °C) fluorescence recovery after photobleaching (FRAP) was utilized to determine the diffusion properties of a zwitterionic, hydrophilic dye, ATTO 647, in alkylimidazolium chloride ILs in micro-confined geometries. These micro-confined geometries were generated by sandwiching the IL between glass substrates that were separated by ∼1 to 100 μm. From the measured temperature-dependent FRAP data, we note alkyl chain length-, thickness-, and temperature-dependent diffusion coefficients, with values ranging from 0.021 to 46 μm2/s. Deviations from Brownian diffusion are observed at lower temperatures and increasingly less so at elevated temperatures; the differences are attributed to alterations in intermolecular interactions that reduce temperature-dependent nanoscale structural heterogeneities. The temperature- and thickness-dependent data provide a useful foundation for efficient design of micro-confined IL separation systems.

Coupling between Translational Diffusion of a Solute and Dynamics of the Heterogeneous Structure: Higher Alcohols and Ionic Liquids

Translational diffusion of nonpolar monoatomic solutes in a room-temperature ionic liquid and 1-octanol was studied by molecular dynamics simulation. The diffusion coefficient was evaluated in two different ways: (1) from the mean-square displacement of a freely diffusing solute and (2) from the time correlation function of force acting on a fixed solute. The diffusion of the free solute is much greater than the prediction of the Stokes-Einstein (SE) relation when the size of the solute is small, as has been reported by many experimental works. In contrast, the friction on fixed small solutes follows the SE relation. The mechanism of the solute diffusion in both solvents was then analyzed based on the coupling between the translational motion of the solute and the collective dynamics of the heterogeneous intermediate-range structure characteristic to these solvents. Analysis revealed that the coupling is present in all systems, but the relaxation is fast in the cases of free and small solutes. This suggests that the coupling can relax through the motion of the solute when the solute is free and small, while the relaxation of the heterogeneous structure itself is required for large or fixed solutes. The difference in the relaxation dynamics of the friction on the solute and the shear viscosity is explained as the coupling with different dynamic modes of the solvent. Therefore, the validity of the SE relation may not be a good criterion to judge whether the mechanisms of the diffusion and the viscosity are the same or not.

Decoupling between Solvent Viscosity and Diffusion of a Small Solute Induced by Self-Motion

The self-diffusion of a monatomic solute in liquid 1-octanol and n-tetradecane was investigated by means of a molecular dynamics simulation. The diffusion coefficient of a solute as small as argon is much greater than that obtained from the hydrodynamic-based Stokes-Einstein (SE) relation, as was reported experimentally. A relaxation of the memory function of a freely diffusing solute is much faster than that of the autocorrelation function of a shear stress. However, the SE behavior is recovered when the solute is spatially fixed, and the diffusion coefficient is calculated from the force-force autocorrelation function. A relaxation of the autocorrelation function of the force also follows that of shear stress. The fast diffusion of a small solute is thus ascribed to the decoupling between the structural relaxation of solvent and the solute diffusion induced by the self-motion of the solute.

Relationship between the Relaxation of Ionic Liquid Structural Motifs and That of the Shear Viscosity

In a set of recent articles, we have highlighted that friction is highly inhomogeneous in a typical ionic liquid (IL) with charge networks that are stiff and charge-depleted regions that are soft. This has consequences not only for the dynamics of ILs but also for the transport properties of solutes dissolved in them. In this article, we explore whether the family of alkylimidazolium ILs coupled with bis(trifluoromethylsulfonyl)imide (with similar Coulombic interactions but different alkyl tails), when dynamically “equalized” by having a similar shear viscosity, display q-dependent structural relaxation time scales that are the same across the family. Our results show that this is not the case, and in fact, the relaxation of in-network charge alternation appears to be significantly affected by the presence of separate polar and apolar domains. However, we also find that if one was to assign weight factors to the relaxation of the structural motifs, charge alternation always contributes about the same amount (between 62.1 and 66.3%) across systems to the running integral of the stress tensor correlation function from which the shear viscosity is derived. Adjacency correlations between positive and negative moieties also contribute an identical amount if a prepeak is not present (about 38%) and a slightly smaller amount (about 28%) when intermediate range order exists. The prepeak only contributes about 6% to viscoelastic relaxation, highlighting that the dynamics of the smaller scale motifs is the most important.

Excited-State Intramolecular Proton Transfer Reaction and Ground-State Hole Dynamics of 4'-N,N-Dialkylamino-3-hydroxyflavone in Ionic Liquids Studied by Transient Absorption Spectroscopy

The excited-state intramolecular proton transfer (ESIPT) of 4'-N,N-dialkylamino-3-hydroxyflavone (C_nHF) having different alkyl chain lengths (ethyl, butyl, and octyl chains) was investigated in ionic liquids (ILs) by steady-state fluorescence and transient absorption spectroscopy. Upon photoexcitation, C_nHF underwent ESIPT from the normal form to the tautomer form, and dual emissions from both states were detected. For C₄HF and C₈HF, the tautomerization yields determined from the fluorescence intensity ratios increased with the increasing number of alkyl chain carbon atoms in the cation and on reducing the excitation wavelength as reported for C₂HF [K. Suda et al., J. Phys. Chem. B. 117, 12567 (2013)]. The transient absorption spectra of C_nHF were measured at excitation wavelengths of 360, 400, and 450 nm. The ESIPT rate determined from the induced emission of the tautomer was correlated with the tautomerization yield for C₂HF and C₄HF. In addition, the recovery of the ground-state bleach was found to be strongly dependent on the excitation wavelength. This result indicates that the solvated state of the molecule before photoexcitation is dependent on the excitation wavelengths. The time constant for the ground-state relaxation was slower than that for the excited state.

Experimental and theoretical study on p-aminophenylthyil radical geminate recombination in ionic liquids; analysis using the Smoluchowski-Collins-Kimball equation

Recombination dynamics of geminate p-aminophenylthiyl (PAPT) radicals produced from the photodissociation of bis(p-aminophenyl) disulfide in ionic liquids (ILs) were investigated by transient absorption spectroscopy. ILs with various cationic species were used to examine the effect of viscosity and polarity on recombination dynamics. Experimentally obtained recombination yields and dynamics were found to be virtually independent of the cation species, despite the viscosity range of the solvent ILs being extensive, spanning from a few tens of mPa s to several hundred mPa s. We applied a theoretical analysis model based on the diffusion equation to the time profiles of the experimentally determined recombination yields of geminate PAPT radicals. The square well potential was incorporated into the diffusion equation to consider the concerted dynamics of solvent cage formation and recombination. A long-time asymptotic expression for the survival probability of the photodissociated products was derived and used to simulate the experimentally obtained time profile of the recombination yield. The time profiles in the range of 20-1000 ps and the final yield were successfully simulated by the asymptotic expression of the square well potential model. The optimized parameters used for the fit, including the mutual diffusion coefficient of the radical pairs, cage radius of the potential well, and well depth, were discussed in terms of the diffusion coefficient conventional theory and the potential mean force estimated from the molecular dynamics simulation for the photodissociation reaction in ILs.

Heterogeneous Structures of Ionic Liquids as Probed by CO Rotation with Nuclear Magnetic Resonance Relaxation Analysis and Molecular Dynamics Simulations

The rotational dynamics of carbon monoxide (CO) in ionic liquids (ILs) was investigated by nuclear magnetic resonance (NMR) relaxation measurements and molecular dynamics (MD) simulations. NMR spin-lattice relaxation time measurements were performed for ¹⁷O-enriched CO in 10 ILs (four imidazolium-cation-based, four phosphonium-cation-based, and two ammonium-cation-based ILs, all paired with the bis(trifluorosulfonylmethane)imide anion). In combination with previously reported data for five ILs and viscosity data, our results indicated that the obtained rotational relaxation times (τ_2R) were much smaller than those predicted using the Stokes-Einstein-Debye (SED) theory. For the same viscosity/temperature values, the τ_2R^-1 value increased linearly with increasing carbon number of the alkyl group in the cation. The deviation from the SED equation was due to the insensitivity of τ_2R to the carbon number, even though a higher carbon number generally leads to higher viscosity values for ILs. To investigate the unique rotational properties of CO in the ILs, MD simulations were performed on five representative ILs (two imidazolium, two phosphonium, and one ammonium) containing CO solutes. From rotational correlation function analyses, the CO rotation mainly occurred in a free rotation-like manner within 1 ps, which explained the relative insensitivity of CO rotation to viscosity. In the subsequent time scale (>1 ps), the minor component of the CO rotation was discriminated among different ILs. It was strongly suggested that, because CO preferably locates in the outer part of the alkyl groups in the cation, the slow CO rotation is correlated with the outer alkyl dynamics, which are decoupled from the whole cation rotation.

Solvation heterogeneity in ionic liquids as demonstrated by photo-chemical reactions

It has been recognised that ionic liquids (ILs) with long alkyl-chains have a segregated structure due to the inhomogeneous distribution of polar parts and non-polar parts. This inhomogeneity of ILs brings about unique solvation phenomena of solute molecules dissolved in ILs. We have investigated various solvation-state selective phenomena by using laser spectroscopic techniques such as solvation state selective vibrational spectroscopy, translational and rotational dynamics of small molecules in ILs, and solvation state selective fundamental chemical reactions. In this paper, we have reviewed an intramolecular electron transfer (ET) reaction in the Marcus inverted region of N,N-dimethyl-p-nitroaniline and an intramolecular proton transfer (IPT) reaction in 4′-N,N-diethylamino-3-hydroxyflavone as examples of chemical reactions affected by unique solvation in ILs.

Transient binding accounts for apparent violation of the generalized Stokes-Einstein relation in crowded protein solutions

The effect of high concentration, also referred to as crowding conditions, on Brownian motion is of central relevance for the understanding of the physical, chemical and biological properties of proteins in their native environment. Specifically, the simple inverse relationship between the translational diffusion coefficient and the macroscopic solution viscosity as predicted by the generalized Stokes-Einstein (GSE) relation has been the subject of many studies, yet a consensus on its applicability has not been reached. Here, we use isotope-filtered pulsed-field gradient NMR to separately assess the μm-scale diffusivity of two proteins, BSA and an SH3 domain, in mixtures as well as single-protein solutions, and demonstrate that transient binding can account for an apparent violation of the GSE relation. Whereas GSE behavior applies for the single-protein solutions, it does not hold for the protein mixtures. Transient binding behavior in the concentrated mixtures is evidenced by calorimetric experiments and by a significantly increased apparent activation energy of diffusion. In contrast, the temperature dependence of the viscosity, as well as of the diffusivity in single-component solutions, is always dominated by the flow activation energy of pure water. As a practically relevant second result, we further show that, for high protein concentrations, the diffusion of small molecules such as dioxane or water is not generally a suitable probe for the viscosity experienced by the diffusing proteins.