What do far-infrared spectra of solitary water in "water-in-solvent" systems reveal about water's solvation and dynamics?

Classical molecular dynamics simulations of water in ionic and dipolar solvents were used to interpret the far-infrared (FIR) rotation/libration spectra of "solitary water" in terms of water's rotational dynamics and interactions with solvents. Seven solvents represented by nonpolarizable all-atom force fields and a series of idealized variable-charge solvents were used to span the range of solvent polarities (hydrogen bonding) studied experimentally. Simulated spectra capture the solvent dependence observed, as well as the relationship between the frequencies of water libration (νL) and OH stretching bands (νOH). In more strongly interacting solvents, simulated νL are ∼20% higher than those of experiment. In all solvents, the simulated spectra are composites of rotational motions about the two axes perpendicular to water's dipole moment, and the different frequencies of these two motions are responsible for the breadth of the libration band and the bimodal shape observed in halide ionic liquids. Simulations overestimate the separation of these two components in most solvents. The character of water rotational motions changes markedly with solvent polarity, from quasi-free rotation in nonpolar and weakly polar solvents to highly constrained libration in strongly hydrogen bonding environments. The changeover to librational motions dominating the spectrum occurs between solvents such as benzene (νL ∼ 250 cm-1) and acetonitrile (νL ∼ 400 cm-1). For solvents in the latter category, the mean frequency of the experimental FIR band provides a direct measure of mean-squared torques and, therefore, force constants associated with interactions constraining water's librational motion.

Rapid water dynamics structures the OH-stretching spectra of solitary water in ionic liquids and dipolar solvents.

In a recent study [J. Phys. Chem. B 126, 4584-4598 (2022)], we have used infrared spectroscopy to investigate the solvation and dynamics of solitary water in ionic liquids and dipolar solvents. Complex shapes observed for water OH-stretching bands, common to all high-polarity solvents, were assigned to water in several solvation states. In the present study, classical molecular dynamics simulations of a single water molecule in four ionic liquids and three dipolar solvents were used to test and refine this interpretation. Consistent with past assignments, simulations show solitary water usually donates two hydrogen bonds to distinct solvent molecules. Such symmetrically solvated water produces the primary pair of peaks identified in the OH spectra of water in nearly all solvents. We had further proposed that additional features flanking this main peak are due to asymmetric solvation states, states in which only one OH group makes a hydrogen bond to solvent. Such states were found in significant concentrations in all of the systems simulated. Simulations of the OH stretching spectra using a semiclassical description and the vibrational map developed by Auer and Skinner [J. Chem. Phys. 128, 224511-224512 (2008)] provided semi-quantitative agreement with experiment. Analysis of species-specific spectra confirmed assignment of the additional features in the experimental spectra to asymmetrically solvated water. The simulations also showed that rapid water motions cause a marked motional narrowing compared with the inhomogeneous limit. This narrowing is largely responsible for making the additional features due to minority solvation states manifest in the spectra.

Solvent controlled intramolecular electron transfer in mixtures of 1-butyl-3-methylimidizolium tetrafluoroborate and acetonitrile.

Time-resolved emission techniques were used to study the excited-state intramolecular electron transfer of 9-(4-biphenyl)-10-methylacridinium (BPAc+) in mixtures of 1-butyl-3-methylimidizolium tetrafluoroborate ([Im41][BF4])+ acetonitrile (ACN), a mixture previously shown to be of nearly constant polarity and nearly ideal mixing behavior. Reaction times (τ rxn) track solvation times (τ solv) as a function of mixture composition over a range of more than 3 orders of magnitude in τ solv. This same correlation extends to a variety of neat dipolar solvents and ionic liquids. Reaction times are ∼2-fold larger than τ solv over most of the range studied but appear to reach a limiting value of ∼3 ps in the fastest solvents.

Photoinduced Bimolecular Electron Transfer in Ionic Liquids.

The present work seeks to better understand the role of solute diffusion and solvation dynamics on bimolecular electron transfer in ionic liquids (ILs). Steady-state and time-resolved measurements of the reductive fluorescence quenching of five fluorophores ("F") by six quenchers ("Q"; electron donors) are reported in acetonitrile and two ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide. Data were collected on 66 different F-Q-solvent systems, which span a 2.0 eV range in driving force and viscosities that vary 1000-fold, enabling stringent tests of bimolecular electron transfer models. A Stern-Volmer analysis yielded much larger diffusion-limited rates than simple kinetic theory predictions in the ILs and the absence of a Marcus turnover. Use of an approximate solution to the spherical diffusion-reaction equation enabled testing of several models for the reaction rate distance dependence. The Smoluchowski and Collins-Kimball models, which assume reaction at a single distance, are able to fit the data collected in acetonitrile solutions reasonably well, but not the data in the IL solvents. An extended sink model, incorporating a finite reaction zone, was able to fit all data satisfactorily with only three adjustable parameters. Diffusion coefficients extracted from these fits were much larger for the neutral versus anionic quenchers and close to predicted values. Molecular dynamics simulations and density-functional methods were then used to explore solvation structures and electronic couplings. The electronic coupling between contact F-Q pairs was found to vary strongly with the relative location and orientation of the reactants. Information from these simulations was used to constrain a model based on classical Marcus theory, which provided physically reasonable fits with only two adjustable parameters, but required systematic reduction of the driving forces in order to suppress a rate turnover at large driving force. The latter requirement indicates that reaction rates in ionic liquids are limited by some factor not properly accounted for in bimolecular electron transfer models based on a spherical diffusion-reaction approach. Small-amplitude motions within contact F-Q pairs, which gate the electronic coupling, are suggested to be the limiting dynamics.

Observations of probe dependence of the solvation dynamics in ionic liquids.

Solvation and rotational dynamics of 4-aminophthalimide (4AP) in four ionic liquids (ILs) are measured using a combination of fluorescence upconversion spectroscopy and time-correlated single photon counting. These data are compared with previously reported data for coumarin 153 (C153) to investigate the probe dependence of solvation dynamics. No fast component (<15 ps) in the fluorescence anisotropy is observed with 4AP. The differences between the solvation response functions of 4AP and C153 are significant in all four ILs, but these differences can be reduced by applying a correction for solute rotation using measured emission anisotropies. Response functions of other probes available in the literature are used to further examine the validity of this correction. The corrected data are also compared to predictions of dielectric continuum models of solvation. By replacing the measured static conductivity of the ILs with an estimated value, such predictions show good agreement with the observed spectral response functions, especially when the anion size is small.

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.

Infrared Spectroscopy of Li+ Solvation in Diglyme: Ab Initio Molecular Dynamics and Experiment.

Infrared (IR) spectra of solutions of the lithium salt LiBF4 in diglyme, CH3O(CH2CH2O)2CH3, are studied via IR spectroscopy and ab initio molecular dynamics (AIMD) simulations. Experiments show that the major effects of LiBF4, compared to neat diglyme, are the appearance of a new broad band in the 250-500 cm-1 frequency region and a broadening and intensity enhancement of the diglyme band in the 900-1150 cm-1 region accompanied by a red-shift. Computational analysis indicates that hindered translational motions of Li+ in its solvation cage are mainly responsible for the new far-IR band, while the changes in the mid-IR are due to Li+-coordination-dependent B-F stretching vibrations of BF4- anions coupled with diglyme vibrations. Molecular motions in these and lower frequency regions are generally correlated, revealing the collective nature of the vibrational dynamics, which involve multiple ions/molecules. Herein, a detailed analysis of these features via AIMD simulations of the spectrum and its components, combined with analysis of the generalized normal modes of the solution components, is presented. Other minor spectral changes as well as diglyme conformational changes induced by the lithium salt are also discussed.

Ionic liquids: structure and photochemical reactions.

Ionic liquids are subjects of intense current interest within the physical chemistry community. A great deal of progress has been made in just the past five years toward identifying the factors that cause these salts to have low melting points and other useful properties. Supramolecular structure and organization have emerged as important and complicated topics that may be key to understanding how chemical reactions and other processes are affected by ionic liquids. New questions are posed, and an active debate is ongoing regarding the nature of nanoscale ordering in ionic liquids. The topic of reactivity in ionic liquids is still relatively unexplored; however, the results that have been obtained indicate that distributed kinetics and dynamical heterogeneity may sometimes, but not always, be influencing factors.

CCVJ is not a simple rotor probe.

The photochemistry of the rotor probe 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ) was studied to elucidate a curious effect of fluid flow previously reported. The apparent sensitivity to fluid motion observed in CCVJ but not in the closely related molecule 9-(dicyanovinyl)julolidine (DCVJ) is found to be an indirect effect of a photoisomerization reaction. The results presented here demonstrate that it is this isomerization, rather than the commonly assumed TICT process, that confers viscosity-sensing ability on these fluorophores. In micromolar solutions in hydroxylic solvents CCVJ exists primarily in the carboxylate form. Only the E isomer of this anion is initially present in solutions prepared from the solid, but in room light such solutions rapidly achieve a photostationary state in which the E isomer and an essentially nonfluorescent Z isomer exist in comparable concentrations. The Z isomer is metastable in S(0) such that in the absence of light the solution reverts slowly to pure E. Unlike DCVJ where only a single isomer is possible, the production of long-lived photoproducts in CCVJ and other asymmetrically substituted styryenyl probes complicates their fluorescence response. Considerable care is needed when such fluorphores are used as steady-state sensors of environmental fluidity are used.

2-Cyano-3-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij]quinolizin-9-yl)prop-2-enoic acid dimethyl sulfoxide monosolvate.

In dimethyl sulfoxide solvated 9-(2-carb-oxy-2-cyano-vin-yl)julolidine, C(16)H(16)N(2)O(2)·C(2)H(6)OS, the essentially planar -CH=(CN)-CO(2)H substituent (r.m.s. deviation = 0.014 Å) is almost coplanar with respect to the benzene ring, the dihedral angle between the two planes being 0.48 (2)°. The conformations of the fused, non-aromatic rings were found to be half-chair. In the crystal, the acid molecule forms a hydrogen bond to the O atom of the solvent mol-ecule. The acid mol-ecule is disordered over two positions with respect to the methyl-ene C atoms in a 1:1 ratio. The crystal studied was found to be a racemic twin.

OH Stretching and Libration Bands of Solitary Water in Ionic Liquids and Dipolar Solvents Share a Single Dependence on Solvent Polarity.

Ionic liquids are an emerging class of materials which are finding application in a variety of technologically important areas. Because of their hydrophilic character, at least a small concentration of water is often present when ionic liquids are used in practical applications. This study employs infrared spectroscopy in the OH stretching and libration regions together with DFT calculations to better characterize the state of dilute water in ionic liquids. Water mole fractions (xw ∼ 0.1) are chosen such that nearly all water occurs in monomeric form and spectra probe the solvation structure and dynamics of solitary water molecules. New data are reported for a series of 1-ethyl-3-methylimidazolium liquids [Im21][X] with X- = (C2F5)3F3P-, (CF3SO2)2N-, BF4-, B(CN)4-, CF3SO3-, C2H5SO4-, NO3-, SCN-, and CH3CO2-, as well as for the two 1-hexyl-3-methylimidazolium liquids [Im61][Cl] and [Im61][I]. For comparison, spectra are also recorded in a variety of dipolar solvents, and much of the available literature data are summarized, providing a comprehensive perspective on monomeric water in homogeneous solution. Most prior studies of dilute water in ionic liquids interpreted OH stretching spectra only in terms of water being specifically bonded to two anions in A-···H-O-H···A- type solvates. The more detailed analysis presented here indicates the additional presence of asymmetrically solvated water, which in some cases includes both singly solvated (A-···H-O-H) and more subtle forms of asymmetric solvation. The same pattern of solvation also pertains to dipolar solvents capable of accepting hydrogen bonds from water. No clear distinction is found between OH spectra in high-polarity conventional solvents and ionic liquids. In all solvents, OH frequencies are strongly correlated to measures of solvent basicity or hydrogen bond accepting ability. Far-infrared spectra of the water libration band also show common trends in ionic and dipolar solvents. Despite the different character of the libration and OH modes, the frequencies of these vibrations show virtually the same solvent dependence (apart from sign) except in weakly polar or nonpolar solvents.

Infrared Spectroscopy of Li+ Solvation in EmimBF4 and in Propylene Carbonate: Ab Initio Molecular Dynamics and Experiment.

Infrared (IR) spectra of solutions of the lithium salt LiBF4 in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) and in the organic solvent propylene carbonate (PC) are studied via infrared spectroscopy and ab initio molecular dynamics (AIMD) simulations. The measurements show that the major effects of LiBF4 in both solutions, compared to their neat counterparts, are the appearance of a new broad band in the 300-450 cm-1 frequency region and a broadening of the IR structure in the 900-1200 cm-1 region with the development of a new peak at 980 cm-1. Computational analysis indicates that hindered translational motions of Li+ in its solvation cage are mainly responsible for the former, while the latter is due to Li+-induced structural changes and accompanying vibrational frequency shifts of constituent ions and molecules of the solutions. In addition, molecular motions in these and lower-frequency regions are generally correlated, disclosing the collective nature of the vibrational dynamics, which involve multiple ions/molecules. Herein, a detailed analysis of these features via AIMD simulations of the spectrum and its components arising from auto- and cross-correlations of motions of constituent molecular species, combined with generalized normal modes of the solutions and normal modes of small Li+-containing clusters, is presented. Other minor spectral changes caused by the lithium salt as well as the interaction-induced effect on IR spectra are also discussed.

Solvent-controlled electron transfer in crystal violet lactone.

Steady-state and picosecond time-resolved emission experiments are used to examine the excited-state charge transfer reaction of crystal violet lactone (CVL) in aprotic solvents. Solvatochromic analysis using a dielectric continuum model suggests dipole moments of 9-12 D for the initially excited (LE) state and ∼24 D for the charge-transfer (CT) state. Intensities of steady-state emission as well as kinetic data provide free energies for the LE → CT reaction that range from +12 kJ/mol in nonpolar solvents to -10 kJ/mol in highly polar solvents at 25 °C. Reaction rates constants, which lie in the range of 10-100 ns(-1) in most solvents, depend on both solvent polarity and solvent friction. In highly polar solvents, rates are correlated to solvation times in a manner that indicates that the reaction is a solvent-controlled electron transfer on an adiabatic potential surface having a modest barrier.

Electron Transfer Kinetics between an Electron-Accepting Ionic Liquid and Coumarin Dyes.

Study of electron transfer in ionic liquids is of interest for what it may reveal about the effects of solvent dynamics on electron transfer as well as for helping to inform current efforts to employ ionic liquids as electrolytes in energy-related applications. The present report describes time-resolved fluorescence quenching measurements of electron transfer between electronically excited 7-aminocoumarin dyes and a redox-active pyridinium ionic liquid, 1-butylpyridinium bis(trifluoromethylsulfonyl)imide ([Py4][Tf2N]). Comparable measurements of fluorescence quenching in conventional dipolar solvents were made over 20 years ago, primarily in aromatic amine liquids. Like these prior experiments, use of commercially available coumarin dyes allowed the driving force for electron transfer (-ΔGET) to be varied over a 0.7 V range, leading to electron transfer rates that increase with driving force over the range 1010-1012 s-1. These rates are similar to rates previously measured in aromatic amine solvents, despite the much greater polarity of the ionic liquid, which increases the driving force by more than 0.5 eV. Fluorescence decays of most of the fluorophores in [Py4][Tf2N] were found to be highly non-exponential functions of time, including both subpicosecond components and components in the 102-103 ps range. Such broadly distributed emission dynamics were not observed in prior studies. Emission decays in [Py4][Tf2N] resemble the broadly distributed solvation response characteristic of ionic liquids, suggesting that solvent motions may control the rate of electron transfer, at least in the more slowly reacting dyes. This similarity could be interpreted either in terms of solvent motions being responsible for varying the energy gap or the electronic coupling between the reactant and product states.

2-[(2,3,6,7-Tetra-hydro-1H,5H-benzo[ij]quinolizin-9-yl)methyl-ene]propane-dinitrile.

The π system of the title compound, known as julolidinemalononitrile, C(16)H(15)N(3), is nearly planar, with a 3.5 (1)° twist between the aromatic and dicyano-vinyl groups. The bond lengths indicate significant zwitterionic character in the ground state.

Characterization of a New Electron Donor-Acceptor Dyad in Conventional Solvents and Ionic Liquids.

Ionic liquids are being tested as potential replacements for current electrolytes in energy-related applications. Electron transfer (ET) plays a central role in these applications, making it essential to understand how ET in ionic liquids differs from ET in conventional organic solvents and how these differences affect reaction kinetics. A new intramolecular electron donor-acceptor probe was synthesized by covalently linking the popular photoacceptor coumarin 152 with the donor dimethylaniline to create the dyad "C152-DMA" for potential use in probing dynamical solvent effects in ionic liquids. Molecular dynamics simulations of this dyad show the considerable conformational flexibility of the linker group but over a range of geometries in which the ET rate parameters vary little and should have minimal effect on reaction times >100 ps. Steady-state and time-resolved fluorescence methods show the spectra of C152-DMA to be highly responsive to solvent polarity, with ET rates varying over the range of 108 to 1012 s-1 between nonpolar and high-polarity conventional solvents. The sensitivity to hydrolysis in the presence of acidic impurities limits the dyad's use to ionic liquids of high purity. The results in the few ionic liquids examined here suggest that in addition to solvent polarity, electron transfer in C152-DMA also depends on solvent fluidity or solvation times.

Solvation and solvatochromism in CO2-expanded liquids. 3. The dynamics of nonspecific preferential solvation.

Subpicosecond time-resolved fluorescence of trans-4-dimethylamino-4'-cyanostilbene (DCS) is used to measure solvation dynamics in the gas-expanded liquid (GXL) system CH(3)CN + CO(2) at 25 degrees C along the liquid-vapor coexistence curve. These measurements are supplemented by measurements of the steady-state solvatochromism of DCS and of its rotation and isomerization times. Molecular dynamics computer simulations and a semiempirical spectral model that reproduces the observed solvatochromism in this system are used to interpret the experimental results. Simulations indicate that at compositions of x(CO2) > 0.5, the cybotactic region surrounding DCS is enriched in CH(3)CN molecules, and the extent of this enrichment is greater in S(1) than that in S(0). Solvation dynamics are dominated by the CH(3)CN component. These dynamics are biphasic, consisting of a subpicosecond inertial component, followed by a slower picosecond component, related to the redistribution of CH(3)CN molecules between the cybotactic region and the bulk solvent.

Physical properties of ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various anions and the bis(trifluoromethylsulfonyl)imide anion with various cations.

Physical properties of 4 room-temperature ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various perfluorinated anions and the bis(trifluoromethylsulfonyl)imide (Tf2N-) anion with 12 pyrrolidinium-, ammonium-, and hydroxyl-containing cations are reported. Electronic structure methods are used to calculate properties related to the size, shape, and dipole moment of individual ions. Experimental measurements of phase-transition temperatures, densities, refractive indices, surface tensions, solvatochromic polarities based on absorption of Nile Red, 19F chemical shifts of the Tf2N- anion, temperature-dependent viscosities, conductivities, and cation diffusion coefficients are reported. Correlations among the measured quantities as well as the use of surface tension and molar volume for estimating Hildebrand solubility parameters of ionic liquids are also discussed.

Simulations of 1-Butyl-3-methylimidazolium Tetrafluoroborate + Acetonitrile Mixtures: Force-Field Validation and Frictional Characteristics.

To temper their prohibitively high viscosities, ionic liquids are commonly mixed with polar cosolvents to retain favorable physical properties and make them suitable for industrial applications. Molecular dynamics simulations of 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) mixed with acetonitrile (CH3CN) are conducted primarily to test the accuracy of a composite force field (FF) and also to provide some insights into the solvation and frictional characteristics of this mixture. The FF combines the united-atom model for imidazolium ionic liquids of Zhong et al. [ J. Phys. Chem. B 2011, 115, 10027] with the acetonitrile model of Nikitin and Lyubartsev [ J. Comput. Chem. 2007, 28, 2020]. Comparison of simulated properties such as mixture densities, viscosities, electrical conductivities, and component diffusion coefficients to experimental data at 298 K shows that this combined FF provides reasonable accuracy for both static and dynamic properties. Component rotational dynamics, as well as those of a dilute benzene solute, probed via new 2H NMR T1 measurements, are also reasonably reproduced. Simulated coordination numbers reveal virtually random mixing between the [Im41][BF4] and CH3CN components in this system. Comparison of translational diffusion coefficients and rotation times to simple hydrodynamic predictions indicates that the friction on most motions becomes increasingly decoupled from solution viscosity as the [Im41][BF4] concentration increases.

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump-Dump-Probe Spectroscopy.

4'- N, N-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump-dump-probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump-probe and pump-dump-probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization.