Water Dynamics in 1-Alkyl-3-methylimidazolium Tetrafluoroborate Ionic Liquids.

The effects of water concentration and varying alkyl chain length on the dynamics of water in 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) were characterized using two-dimensional infrared (2D IR) vibrational echo spectroscopy and polarization-selective IR pump-probe experiments to study the water hydroxyl (OD) stretching mode of dilute HOD in H2O. Three imidazolium cation alkyl chain lengths, ethyl (Emim+), butyl (Bmim+), and decyl (Dmim+), were investigated. Both Bmim+ and Dmim+ cations have sufficiently long chains that the liquids exhibit polar-apolar segregation, whereas the Emim+ IL has no significant apolar aggregation. Although the OD absorption spectra are independent of the chain length, the measured reorientation and spectral diffusion dynamics are chain length dependent and tend to slow when the alkyl chain is long enough for polar-apolar segregation. As the water concentration is increased, a water-associated water population forms, absorbing in a new spectral region red-shifted from the isolated, anion-associated, water population. Furthermore, the anion-associated water dynamics are accelerated. At sufficiently high water concentrations, water in all of the RTILs experiences similar dynamics, the solvent structures having been fluidized by the addition of water. The water concentration at which the dilute water dynamics changes to fluidized dynamics depends on the alkyl chain length, which determines the extent and ordering of the apolar regions. Increases in both water concentration and alkyl chain length serve to modify the ordering of the RTIL, but with opposite and competing effects on the dissolved water dynamics.

Ionic Liquid versus Li(+) Aqueous Solutions: Water Dynamics near Bistriflimide Anions.

The ultrafast dynamics of concentrated aqueous solutions of the salt lithium bistriflimide and ionic liquid (IL) 1-ethyl-3-methylimidazolium bistriflimide was studied using two-dimensional infrared (2D IR) vibrational echo and polarization-selective IR pump-probe techniques to monitor water's hydroxyl stretch. Two distinct populations of hydroxyl groups, with differing vibrational lifetimes, are detected in solution: those engaged in hydrogen bonding with other water molecules and those engaged in hydrogen bonding with the bistriflimide anion. Water molecules with the same hydrogen bond partner exhibit similar vibrational lifetimes in the two solutions. The reorientation dynamics of the anion-associated waters is also similar in form in the two solutions, showing a restricted wobbling-in-a-cone motion followed by a slower diffusive orientational randomization. However, the wobbling motions are much more angularly restricted in the IL solution. Spectral diffusion dynamics, which tracks the structural fluctuations of water's hydrogen bonds, is very different in the two solutions. Water in the IL solution experiences much faster fluctuations overall and shows a greater extent of motional narrowing, resulting in a larger homogeneously broadened component in the spectral line, compared to those in the aqueous lithium salt. Thus, even when the hydroxyls of water associate with the same anion in solution, the cation identity and extent of ionic ordering (i.e., salt solution vs IL) can play an important role in determining the structural fluctuations experienced by a small hydrogen-bonded solute.

Structural and Rotational Dynamics of Carbon Dioxide in 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids: The Effect of Chain Length.

Ionic liquids (ILs) have been proposed as possible carbon dioxide (CO2) capture media; thus, it is useful to understand the dynamics of both the dissolved gas and its IL environment as well as how altering an IL affects these dynamics. With increasing alkyl chain length, it is well-established that ILs obtain a mesoscopic structural feature assigned to polar-apolar segregation, and the change in structure with chain length affects the dynamics. Here, the dynamics of CO2 in a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ILs, in which the alkyl group is ethyl, butyl, hexyl, or decyl, were investigated using ultrafast infrared spectroscopy by measuring the reorientation and spectral diffusion of carbon dioxide in the ILs. It was found that reorientation of the carbon dioxide occurs on three time scales, which correspond to two different time scales of restricted wobbling-in-a-cone motions and a long-time complete diffusive reorientation. Complete reorientation slows with increasing chain length but less than the increases in viscosity of the bulk liquids. Spectral diffusion, measured with two-dimensional IR spectroscopy, is caused by a combination of the liquids' structural fluctuations and reorientation of the CO2. The data were analyzed using a recent theory that takes into account both contributions to spectral diffusion and extracts the structural spectral diffusion. Different components of the structural fluctuations have distinct dependences on the alkyl chain length. All of the dynamics are fast compared to the complete orientational randomization of the bulk ILs, as measured with optical heterodyne-detected optical Kerr effect measurements. The results indicate a hierarchy of constraint releases in the liquids that give rise to increasingly slower dynamics.

Carbon dioxide in an ionic liquid: Structural and rotational dynamics.

Ionic liquids (ILs), which have widely tunable structural motifs and intermolecular interactions with solutes, have been proposed as possible carbon capture media. To inform the choice of an optimal ionic liquid system, it can be useful to understand the details of dynamics and interactions on fundamental time scales (femtoseconds to picoseconds) of dissolved gases, particularly carbon dioxide (CO2), within the complex solvation structures present in these uniquely organized materials. The rotational and local structural fluctuation dynamics of CO2 in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf2) were investigated by using ultrafast infrared spectroscopy to interrogate the CO2 asymmetric stretch. Polarization-selective pump probe measurements yielded the orientational correlation function of the CO2 vibrational transition dipole. It was found that reorientation of the carbon dioxide occurs on 3 time scales: 0.91 ± 0.03, 8.3 ± 0.1, 54 ± 1 ps. The initial two are attributed to restricted wobbling motions originating from a gating of CO2 motions by the IL cations and anions. The final (slowest) decay corresponds to complete orientational randomization. Two-dimensional infrared vibrational echo (2D IR) spectroscopy provided information on structural rearrangements, which cause spectral diffusion, through the time dependence of the 2D line shape. Analysis of the time-dependent 2D IR spectra yields the frequency-frequency correlation function (FFCF). Polarization-selective 2D IR experiments conducted on the CO2 asymmetric stretch in the parallel- and perpendicular-pumped geometries yield significantly different FFCFs due to a phenomenon known as reorientation-induced spectral diffusion (RISD), revealing strong vector interactions with the liquid structures that evolve slowly on the (independently measured) rotation time scales. To separate the RISD contribution to the FFCF from the structural spectral diffusion contribution, the previously developed first order Stark effect RISD model is reformulated to describe the second order (quadratic) Stark effect--the first order Stark effect vanishes because CO2 does not have a permanent dipole moment. Through this analysis, we characterize the structural fluctuations of CO2 in the ionic liquid solvation environment, which separate into magnitude-only and combined magnitude and directional correlations of the liquid's time dependent electric field. This new methodology will enable highly incisive comparisons between CO2 dynamics in a variety of ionic liquid systems.

Coupling of Carbon Dioxide Stretch and Bend Vibrations Reveals Thermal Population Dynamics in an Ionic Liquid.

The population relaxation of carbon dioxide dissolved in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf2) was investigated using polarization-selective ultrafast infrared pump-probe spectroscopy and two-dimensional infrared (2D IR) spectroscopy. Due to the coupling of the bend with the asymmetric stretch, excitation of the asymmetric stretch of a molecule with a thermally populated bend leads to an additional peak, a hot band, which is red-shifted from the main asymmetric absorption band by the combination band shift. This hot band peak exchanges population with the main peak through the gain and loss of bend excitation quanta. The isotropic pump-probe signal originating from the unexcited bend state displays a fast, relatively small amplitude, initial growth followed by a longer time scale exponential decay. The signal is analyzed over its full time range using a kinetic model to determine both the vibrational lifetime (the final decay) and rate constant for the loss of the bend energy. This bend relaxation manifests as the fast initial growth of the stretch/no bend signal because the hot band (stretch with bend) is "over pumped" relative to the ground state equilibrium. The nonequilibrium pumping occurs because the hot band has a larger transition dipole moment than the stretch/no bend peak. The system is then prepared, utilizing an acousto-optic mid-infrared pulse shaper to cut a hole in the excitation pulse spectrum, such that the hot band is not pumped. The isotropic pump-probe signal from the stretch/no bend state is altered because the initial excited state population ratio has changed. Instead of a growth due to relaxation of bend quanta, a fast initial decay is observed because of thermal excitation of the bend. Fitting this curve gives the rate constant for thermal excitation of the bend and the lifetime, which agree with those determined in the pump-probe experiments without frequency-selective pumping.

Dynamics of water, methanol, and ethanol in a room temperature ionic liquid.

The dynamics of a series of small molecule probes with increasing alkyl chain length: water, methanol, and ethanol, diluted to low concentration in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was investigated with 2D infrared vibrational echo (2D IR) spectroscopy and polarization resolved pump-probe (PP) experiments on the deuterated hydroxyl (O-D) stretching mode of each of the solutes. The long timescale spectral diffusion observed by 2D IR, capturing complete loss of vibrational frequency correlation through structural fluctuation of the medium, shows a clear but not dramatic slowing as the probe alkyl chain length is increased: 23 ps for water, 28 ps for methanol, and 34 ps for ethanol. Although in each case, only a single population of hydroxyl oscillators contributes to the infrared line shapes, the isotropic pump-probe decays (normally caused by population relaxation) are markedly nonexponential at short times. The early time features correspond to the timescales of the fast spectral diffusion measured with 2D IR. These fast isotropic pump-probe decays are produced by unequal pumping of the OD absorption band to a nonequilibrium frequency dependent population distribution caused by significant non-Condon effects. Orientational correlation functions for these three systems, obtained from pump-probe anisotropy decays, display several periods of restricted angular motion (wobbling-in-a-cone) followed by complete orientational randomization. The cone half-angles, which characterize the angular potential, become larger as the experimental frequency moves to the blue. These results indicate weakening of the angular potential with decreasing hydrogen bond strength. The slowest components of the orientational anisotropy decays are frequency-independent and correspond to the complete orientational randomization of the solute molecule. These components slow appreciably with increasing chain length: 25 ps for water, 42 ps for methanol, and 88 ps for ethanol. The shape and volume of the probe, therefore, impact reorientation far more severely than they do spectral diffusion at long times, though these two processes occur on similar timescales at earlier times.

Observation and theory of reorientation-induced spectral diffusion in polarization-selective 2D IR spectroscopy.

In nearly all applications of ultrafast multidimensional infrared spectroscopy, the spectral degrees of freedom (e.g., transition frequency) and the orientation of the transition dipole are assumed to be decoupled. We present experimental results which confirm that frequency fluctuations can be caused by rotational motion and observed under appropriate conditions. A theory of the frequency-frequency correlation function (FFCF) observable under various polarization conditions is introduced, and model calculations are found to reproduce the qualitative trends in FFCF rates. The FFCF determined with polarization-selective two-dimensional infrared (2D IR) spectroscopy is a direct reporter of the frequency-rotational coupling. For the solute methanol in a room temperature ionic liquid, the FFCF of the hydroxyl (O-D) stretch decays due to spectral diffusion with different rates depending on the polarization of the excitation pulses. The 2D IR vibrational echo pulse sequence consists of three excitation pulses that generate the vibrational echo, a fourth pulse. A faster FFCF decay is observed when the first two excitation pulses are polarized perpendicular to the third pulse and the echo, 〈XXY Y〉, than in the standard all parallel configuration, 〈XXXX〉, in which all four pulses have the same polarization. The 2D IR experiment with polarizations 〈XY XY〉 ("polarization grating" configuration) gives a FFCF that decays even more slowly than in the 〈XXXX〉 configuration. Polarization-selective 2D IR spectra of bulk water do not exhibit polarization-dependent FFCF decays; spectral diffusion is effectively decoupled from reorientation in the water system.

Dynamics of dihydrogen bonding in aqueous solutions of sodium borohydride.

Dihydrogen bonding occurs between protonic and hydridic hydrogens which are bound to the corresponding electron withdrawing or donating groups. This type of interaction can lead to novel reactivity and dynamic behavior. This paper examines the dynamics experienced by both borohydride and its dihydrogen-bound water solvent using 2D-IR vibrational echo and IR pump-probe spectroscopies, as well as FT-IR linear absorption experiments. Experiments are conducted on the triply degenerate B-H stretching mode and the O-D stretch of dilute HOD in the water solvent. While the B-H stretch absorption is well separated from the broad absorption band of the OD of HOD in the bulk of the water solution, the absorption of the ODs hydrogen bonded to BHs overlaps substantially with the absorption of ODs in the bulk H2O solution. A subtraction technique is used to separate out the anion-associated OD dynamics from that of the bulk solution. It is found that both the water and borohydride undergo similar spectral diffusion dynamics, and these are very similar to those of HOD in bulk water. Because the B-H stretch is triply degenerate, the IR pump-probe anisotropy decays very rapidly, but the decay is not caused by the physical reorientation of the BH4⁻ anions. Spectral diffusion occurs on a time scale longer than the anisotropy decay, demonstrating that spectral diffusion is not yet complete even when the transition dipole has completely randomized. To prevent chemical decomposition of the BH4⁻, 1 M NaOH was added to stabilize the system. 2D-IR experiments on the OD stretch of HOD in the NaOH/water liquid (no borohydride) show that the NaOH has a negligible effect on the bulk water dynamics.

Dynamics of isolated water molecules in a sea of ions in a room temperature ionic liquid.

The vibrational dynamics of the antisymmetric and symmetric stretching modes of very low concentration spatially isolated D(2)O molecules in the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate (BmImPF(6)) were examined using two-dimensional infrared (2D IR) vibrational echo spectroscopy and infrared pump-probe experiments. In BmImPF(6), D(2)O's antisymmetric and symmetric stretching modes are well resolved in the IR absorption spectrum in spite of the fact that the D(2)O is surrounded by a sea of ions, making it is possible to study inter- and intramolecular dynamics. Both population exchange between the modes and excited-state relaxation to the ground state contribute to the population dynamics. The kinetics for the incoherent population exchange (scattering) between the two modes was determined by the time dependence of the exchange peaks in the 2D IR spectrum. In addition, coherent quantum beats were observed at short time in both the amplitudes and 2D IR band shapes of the modes. The quantum beat decay is caused by dephasing due to both inhomogeneous and homogeneous broadening of the spectral lines. Analysis of the oscillations of the 2D line shapes demonstrates that there is some degree of anticorrelation in the inhomogeneous broadening of the two modes. It is proposed that a distribution in the coupling strength between the local modes that give rise to symmetric and antisymmetric eigenstates is responsible for the anticorrelation. Spectral diffusion, caused by structural evolution of the medium, occurs on multiple time scales and is identical for the two modes within experimental error. The spectral diffusion is fast compared to the time scale for complete orientational randomization of the RTIL. Spectral diffusion of the OD stretch of HOD in BmImPF(6) was also measured, and is essentially the same as that of the D(2)O modes. Orientational anisotropy measurements of HOD in BmImPF(6) determined the orientational relaxation dynamics of the isolated HOD molecules.

Water dynamics in divalent and monovalent concentrated salt solutions.

Water hydrogen bond dynamics in concentrated salt solutions are studied using polarization-selective IR pump-probe spectroscopy and 2D IR vibrational echo spectroscopy performed on the OD hydroxyl stretching mode of dilute HOD in H(2)O/salt solutions. The OD stretch is studied to eliminate vibrational excitation transfer, which interferes with the dynamical measurements. Though previous research suggested that only the anion affected dynamics in solution, here it is shown that the cation plays a role as well. From FT-IR spectra of the OD stretch, it is seen that replacing either ion of the salt pair causes a shift in absorption frequency relative to that of the OD stretch absorption in bulk pure water. This shift becomes pronounced with larger, more polarizable anions or smaller, high charge-density cations. The vibrational lifetime of the OD hydroxyl stretch in these solutions is a local property and is primarily dependent on the nature of the anion and whether the OD is hydrogen bonded to the anion or to the oxygen of another water molecule. However, the cation still has a small effect. Time dependent anisotropy measurements show that reorientation dynamics in these concentrated solutions is a highly concerted process. While the lifetime, a local probe, displays an ion-associated and a bulk-like component in concentrated solutions, the orientational relaxation does not have two subensemble dynamics, as demonstrated by the lack of a wavelength dependence. The orientational relaxation of the single ensemble is dependent on the identity of both the cation and anion. The 2D IR vibrational echo experiments measure spectral diffusion that is caused by structural evolution of the system. The vibrational echo measurements yield the frequency-frequency correlation function (FFCF). The results also show that the structural dynamics are dependent on the cation as well as the anion.

Water dynamics in water/DMSO binary mixtures.

The dynamics of dimethyl sulfoxide (DMSO)/water solutions with a wide range of water concentrations are studied using polarization selective infrared pump-probe experiments, two-dimensional infrared (2D IR) vibrational echo spectroscopy, optical heterodyne detected optical Kerr effect (OHD-OKE) experiments, and IR absorption spectroscopy. Vibrational population relaxation of the OD stretch of dilute HOD in H(2)O displays two vibrational lifetimes even at very low water concentrations that are associated with water-water and water-DMSO hydrogen bonds. The IR absorption spectra also show characteristics of both water-DMSO and water-water hydrogen bonding. Although two populations are observed, water anisotropy decays (orientational relaxation) exhibit single ensemble behavior, indicative of concerted reorientation involving water and DMSO molecules. OHD-OKE experiments, which measure the orientational relaxation of DMSO, reveal that the DMSO orientational relaxation times are the same as orientational relaxation times found for water over a wide range of water concentrations within experimental error. The fact that the reorientation times of water and DMSO are basically the same shows that the reorientation of water is coupled to the reorientation of DMSO itself. These observations are discussed in terms of a jump reorientation model. Frequency-frequency correlation functions determined from the 2D IR experiments on the OD stretch show both fast and slow spectral diffusion. In analogy to bulk water, the fast component is assigned to very local hydrogen bond fluctuations. The slow component, which is similar to the slow water reorientation time at each water concentration, is associated with global hydrogen bond structural randomization.

Dynamics of water at the interface in reverse micelles: measurements of spectral diffusion with two-dimensional infrared vibrational echoes.

Water dynamics inside of reverse micelles made from the surfactant Aerosol-OT (AOT) were investigated by observing spectral diffusion, orientational relaxation, and population relaxation using two-dimensional infrared (2D IR) vibrational echo spectroscopy and pump-probe experiments. The water pool sizes of the reverse micelles studied ranged in size from 5.8 to 1.7 nm in diameter. It is found that spectral diffusion, characterized by the frequency-frequency correlation function (FFCF), significantly changes as the water pool size decreases. For the larger reverse micelles (diameter 4.6 nm and larger), the 2D IR signal is composed of two spectral components: a signal from bulk-like core water, and a signal from water at the headgroup interface. Each of these signals (core water and interfacial water) is associated with a distinct FFCF. The FFCF of the interfacial water layer can be obtained using a modified center line slope (CLS) method that has been recently developed. The interfacial FFCFs for large reverse micelles have a single exponential decay (∼1.6 ps) to an offset plus a fast homogeneous component and are nearly identical for all large sizes. The observed ∼1.6 ps interfacial decay component is approximately the same as that found for bulk water and may reflect hydrogen bond rearrangement of bulk-like water molecules hydrogen bonded to the interfacial water molecules. The long time offset arises from dynamics that are too slow to be measured on the accessible experimental time scale. The influence of the chemical nature of the interface on spectral diffusion was explored by comparing data for water inside reverse micelles (5.8 nm water pool diameter) made from the surfactants AOT and Igepal CO-520. AOT has charged, sulfonate head groups, while Igepal CO-520 has neutral, hydroxyl head groups. It is found that spectral diffusion on the observable time scales is not overly sensitive to the chemical makeup of the interface. An intermediate-sized AOT reverse micelle (water pool diameter of 3.3 nm) is analyzed as a large reverse micelle because it has distinct core and interface regions, but its core region is more constrained than bulk water. The interfacial FFCF for this intermediate-sized reverse micelle is somewhat slower than those found for the larger reverse micelles. The water nanopools in the smaller reverse micelles cannot be separated into core and interface regions. In the small reverse micelles, the FFCFs are biexponential decays to an offset plus a fast homogeneous component. Each small reverse micelle exhibits an ∼1 ps decay time, which may arise from local hydrogen bond fluctuations and a slower, ∼6-10 ps decay, which is possibly due to slow hydrogen bond rearrangement of noninterfacial water molecules or topography fluctuations at the interface.