Dynamics of Isolated Water Molecules in a Sea of Ions in a Room Temperature Ionic Liquid

Probing Isolated Water Molecules in Aqueous Acetonitrile Solutions Using Oxygen K-Edge X-ray Absorption Spectroscopy

Oxygen K-edge X-ray absorption spectroscopy (XAS) of an aqueous acetonitrile solution exhibited a sharp peak at approximately 537 eV, which was similar to that of water vapor and was not observed in liquid water. The inner-shell spectra of isolated water molecules and water clusters of different sizes surrounded by acetonitrile molecules were obtained by extracting these water structures from the liquid structures of aqueous acetonitrile solutions, as calculated using molecular dynamics simulations. The sharp peak profiles of the O K-edge XAS spectra were derived not from water clusters but from isolated water molecules surrounded by acetonitrile molecules. The present study proposes that isolated water molecules are easily formed in aqueous acetonitrile solutions and that the electronic structures of the isolated water molecules can be analyzed using O K-edge XAS spectra, which separates the contributions of small water clusters.

Ion/Water Network Structural Dynamics in Highly Concentrated Lithium Chloride and Lithium Bromide Solutions Probed with Ultrafast Infrared Spectroscopy

The structural dynamics of highly concentrated LiCl and LiBr aqueous solutions were observed from 1-4 to 1-16 water molecules per ion pair using ultrafast polarization-selective pump-probe (PSPP) experiments on the OD stretch of dilute HOD. At these high salt concentrations, an extended ion/water network exists with complex structural dynamics. Population decays from PSPP experiments highlight two distinct water components. From the frequency-dependent amplitudes of the decays, the spectra of hydroxyls bound to halides and to water oxygens are obtained, which are not observable in the FT-IR spectra. PSPP experiments also measure frequency-dependent water orientational relaxation. At short times, wobbling dynamics within a restricted angular cone occurs. At high concentrations, the cone angles are dependent on frequency (hydrogen bond strength), but at higher water concentrations (>10 waters per ion pair), there is no frequency dependence. The average cone angle increases as the ion concentration decreases. The slow time constant for complete HOD orientational relaxation is independent of concentration but slower in LiCl than in LiBr. Comparison to structural MD simulations of LiCl from the literature indicates that the loss of the cone angle wavelength dependence and the increase in the cone angles as the concentration decreases occur as the prevalence of large ion/water clusters gives way to contact ion pairs.

Direct Correlation between Short-Range Vibrational Spectral Diffusion and Localized Ion-Cage Dynamics of Water-in-Salt Electrolytes

The molecular dynamics simulations of a "water-in-salt" electrolyte, lithium bis(trifluoromethyl sulfonyl) imide (LiNTf₂), with a varying concentration range of 3 to 20 m were performed to establish a direct connection between a dynamic property like the ion-cage lifetime with the short-range vibrational stretching frequency shift of the used probe, HOD. The properties reported here are compared to that obtained from experiments performed at the same concentrations. The time-series wavelet transform was adopted as a preferable mathematical tool for calculating the instantaneous fluctuating frequencies of the probe O-D stretch mode and the concentration-dependent vibrational stretch spectral signature based on the variable functions associated with a particular chemical bond derived from classical molecular dynamics trajectories. The decay time constants of frequency fluctuations and the lifetime of the ion cage (τ_IC) were estimated as a function of salt concentration. Herein, we emphasize the correlation between the slowest time constant (τ₃) of the decay of O-D stretch frequency fluctuations and the timescales associated with the lifetime of ion cages (τ_IC). The results exhibit that the existing relationships were also concentration-dependent. Therefore, this study highlights the connection between the ionic motions that regulate the overall system dynamics with the short-range vibrational frequency shift of the used probe, which was used similar to experiments. It also provides an understanding of the interionic interactions and the dynamical and spectral properties of the electrolytic mixtures. We establish a direct correlation between short-range frequency profile and localized ion-cage lifetime, which can fill the gap of understanding between viscosity, vibrational frequency, and ion-cage dynamics of electrolytes.

Ionic Dynamics and Vibrational Spectral Diffusion of a Protic Alkylammonium Ionic Salt through Intrinsic Cationic N-H Vibrational Probe from FPMD Simulations

We employed density functional theory (DFT)-based molecular dynamics simulations to explore the structure, dynamics, and spectral properties of the protic ionic entity trimethylammonium chloride (TMACl). Structural investigations include calculating the site-site radial distribution functions (RDFs), the distribution of constituent cations and anions in three-dimensional space, and combined distribution functions of the hydrogen-bonded pair RDF versus angle, revealing the structural characteristics of the ionic solvation and the intermolecular interactions within ions. Further, we determined the instantaneous vibrational stretching frequencies of the intrinsic N-H stretch probe modes by applying the time-series wavelet method. The associated ionic dynamics within the protic ionic compound were investigated by calculating the time-evolution of the fluctuating frequencies and the frequency-time correlation functions (FFCFs). The time scale related to the local structural relaxation process and the average hydrogen bond lifetime, ion cage dynamics, and mean squared displacement were investigated. The faster decay component of the FFCFs, depicting the intermolecular motion of intact hydrogen bonds in TMACl, is 0.07 ps for the Perdew-Burke-Ernzerhof (PBE)-based simulation and 0.06 ps for the PBE-D2 representation. The slower time scale of the longer picosecond decay time component of PBE and PBE-D2 representations are 3.13 and 2.87 ps, respectively. These picosecond time scales represent more significant fluctuations of the hydrogen-bonding partners in the ionic entity and hydrogen-bond jump events accompanied by large angular jumps. The longest picosecond time scales represent structural relaxation, including large angular jumps and ion-pair dynamics. Also, ion cage lifetimes correlate with the slowest time scale of the associated dynamics of vibrational spectral diffusion despite the type of DFT functional. This study benchmarks DFT treatments of the exchange-correlation functional with and without the van der Waals (vdW) dispersion correction scheme. The inclusion of vdW interactions to the PBE functional represents a less structured state of the ionic entity and faster dynamics of the molecular motions relative to the one predicted by the PBE system. All the results illustrate the necessity of accurately describing the Coulomb interactions, vdW dispersive interactive forces, and localized hydrogen bonds required to sustain the energetic balance in this ionic salt.

Dynamics of Ionic Liquid through Intrinsic Vibrational Probes Using the Dispersion-Corrected DFT Functionals

First principles molecular dynamics simulations have been utilized to study the spectral properties of the protic ionic liquid, methylammonium formate (MAF). All simulations were performed using density functional theory (DFT) and various van der Waals-corrected exchange-correlation functionals. We calculated the vibrational stretch frequency distributions, determined the time-frequency correlations of the intrinsic vibrational probes, the N-H and C-O modes in MAF, and the frequency-structure correlations. We also estimated the average hydrogen-bond lifetimes and orientation dynamics to capture the ultrafast spectral response. The spectroscopic signature of the N-H stretching vibrations using the Becke-Lee-Yang-Parr (BLYP) and Perdew-Burke-Ernzerhof (PBE) functionals displays a spectral shift in the lower frequency side, suggesting stronger hydrogen-bonding interactions represented by the gradient approximation functionals than the van der Waals (vdW)-corrected simulations. The carboxylate frequency profiles with the dispersion-corrected representations are almost similar without a significant difference in the normalized distributions. Besides, the COO stretching frequencies at the peak maxima positions of the PBE functionals exhibit a lesser deviation from the experimental data. Spectral diffusion dynamics of the intrinsic vibrational probes on the cationic and anionic sites of the ionic liquid proceed through a short time relaxation of the intact hydrogen bonds followed by an intermediate time constant and a longer time decay indicating the switchover of hydrogen bonds. Dispersion-corrected atom-centered one-electron potential (DCACP) correction added to the BLYP system slows down the picosecond time scales of frequency correlation and the time constants of rotational motion, lengthening the overall system dynamics. The observed trends in the time-dependent decays of frequency fluctuations and the orientation autocorrelation functions correlate with the structural interactions in liquid MAF and hydrogen-bond dynamics. In this study, we examine the predictions made by different density functional treatments comparing the results of the uncorrected BLYP and PBE representations with the semiempirical vdW methods of Grimme and matching our calculated data with the experimental observations.

Ultrafast vibrational dynamics of a trigonal planar anionic probe in ionic liquids (ILs): A two-dimensional infrared (2DIR) spectroscopic investigation

A major impediment limiting the widespread application of ionic liquids (ILs) is their high shear viscosity. Incorporation of a tricyanomethanide (TCM^-) anion in ILs leads to low shear viscosity and improvement of several characteristics suitable for large scale applications. However, properties including interactions of TCM^- with the local environment and dynamics of TCM^- have not been thoroughly investigated. Herein, we have studied the ultrafast dynamics of TCM^- in several imidazolium ILs using linear IR and two-dimensional infrared spectroscopy techniques. The spectral diffusion dynamics of the CN stretching modes of TCM^- in all ILs exhibit a nonexponential behavior with a short time component of ∼2 ps and a long time component spanning ∼9 ps to 14 ps. The TCM^- vibrational probe reports a significantly faster relaxation of ILs compared to those observed previously using linear vibrational probes, such as thiocyanate and selenocyanate. Our results indicate a rapid relaxation of the local ion-cage structure embedding the vibrational probe in the ILs. The faster relaxation suggests that the lifetime of the local ion-cage structure decreases in the presence of TCM^- in the ILs. Linear IR spectroscopic results show that the hydrogen-bonding interaction between TCM^- and imidazolium cations in ILs is much weaker. Shorter ion-cage lifetimes together with weaker hydrogen-bonding interactions account for the low shear viscosity of TCM^- based ILs compared to commonly used ILs. In addition, this study demonstrates that TCM^- can be used as a potential vibrational reporter to study the structure and dynamics of ILs and other molecular systems.

Tuning Water Networks via Ionic Liquid/Water Mixtures

Water in nanoconfinement is ubiquitous in biological systems and membrane materials, with altered properties that significantly influence the surrounding system. In this work, we show how ionic liquid (IL)/water mixtures can be tuned to create water environments that resemble nanoconfined systems. We utilize molecular dynamics simulations employing ab initio force fields to extensively characterize the water structure within five different IL/water mixtures: [BMIM + ][BF 4 - ], [BMIM + ][PF 6 - ], [BMIM + ][OTf - ], [BMIM + ][NO 3 - ]and [BMIM + ][TFSI - ] ILs at varying water fraction. We characterize water clustering, hydrogen bonding, water orientation, pairwise correlation functions and percolation networks as a function of water content and IL type. The nature of the water nanostructure is significantly tuned by changing the hydrophobicity of the IL and sensitively depends on water content. In hydrophobic ILs such as [BMIM + ][PF 6 - ], significant water clustering leads to dynamic formation of water pockets that can appear similar to those formed within reverse micelles. Furthermore, rotational relaxation times of water molecules in supersaturated hydrophobic IL/water mixtures indicate the close-connection with nanoconfined systems, as they are quantitatively similar to water relaxation in previously characterized lyotropic liquid crystals. We expect that this physical insight will lead to better design principles for incorporation of ILs into membrane materials to tune water nanostructure.

Bulk-like and Interfacial Water Dynamics in Nafion Fuel Cell Membranes Investigated with Ultrafast Nonlinear IR Spectroscopy

The water confined in the hydrophilic domains of Nafion fuel cell membranes is central to its primary function of ion transport. Water dynamics are intimately linked to proton transfer and are sensitive to the structural features and length scales of confinement. Here, ultrafast polarization-selective pump-probe and two-dimensional infrared vibrational echo (2D IR) experiments were performed on fully hydrated Nafion membranes with sodium counterions to explicate the water dynamics. Like aerosol-OT reverse micelles (AOT RMs), the water dynamics in Nafion are attributed to bulk-like core water in the central region of the hydrophilic domains and much slower interfacial water. Population and orientational dynamics of water in Nafion are slowed by polymer confinement. Comparison of the observed dynamics to those of AOT RMs helps identify local interactions between water and sulfonate anions at the interface and among water molecules in the core. This comparison also demonstrates that the well-known spherical cluster morphology of Nafion is not appropriate. Spectral diffusion of the interfacial water, which arises from structural dynamics, was obtained from the 2D IR experiments taking the core water to have dynamics similar to bulk water. Like the orientational dynamics, spectral diffusion was found to be much slower at the interface compared to bulk water. Together, the dynamics indicate slow reorganization of weakly hydrogen-bonded water molecules at the interface of Nafion. These results provide insights into proton transport mechanisms in fuel cell membranes, and more generally, water dynamics near the interface of confining systems.

Dynamics and Vibrational Spectroscopy of Alcohols in Ionic Liquids: Methanol and Ethanol

The structure, dynamics, and vibrational spectroscopy of dilute HOD, methanol, and ethanol in the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][NTf₂], ionic liquid (IL) are investigated with molecular dynamics (MD) simulations. The structure of the ILs around the solutes is qualitatively similar, where the OD bond of the deuterated alcohols donates an interaction to an [NTf₂] anion and the [emim] cations interact with the oxygen atom of the OD group. The slowest time scale for the reorientational dynamics of the OD bond varied considerably for HOD, methanol, and ethanol (27, 71, and 87 ps, respectively). In contrast, the slowest time scales for spectral diffusion of the OD vibrational frequency were 11 ps for each of the three solutes, which indicates that the dynamics of the IL is relatively unchanged by the presence of the alcohols at dilute concentration. The theoretical results for the reorientational and spectral diffusion dynamics compare favorably with prior two-dimensional infrared (2D IR) spectroscopic measurements.

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: IV. Temperature Dependence

In previous papers in the series, the vibrational spectroscopy of CO₂ in ionic liquids (ILs) was investigated at ambient conditions. Here, we extend these studies to understand the temperature dependence of the structure, dynamics, and thermodynamics of CO₂ in the 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF₆], IL. Using spectroscopic mapping techniques, the infrared absorption spectrum of the CO₂ asymmetric stretch mode is simulated at a number of temperatures, and the results are found to be consistent with similar experimental studies. Structural correlation functions are used to reveal the thermodynamics of complete CO₂ solvent cage breakdown. The enthalpy and entropy of activation for solvent cage reorganization are found to be 6.9 and 7.6 (kcal/mol)/K, respectively, and these values are similar to the those for spectral, orientational, and translational diffusion. Caging times for CO₂ are calculated, and it is shown that the short time dynamics of CO₂ are unaffected by temperature, even though the long-time dynamics are highly sensitive to temperature.

Dynamically Disordered Lattice in a Layered Pb-I-SCN Perovskite Thin Film Probed by Two-Dimensional Infrared Spectroscopy

The dynamically flexible lattices in lead halide perovskites may play important roles in extending carrier recombination lifetime in 3D perovskite solar-cell absorbers and in exciton self-trapping in 2D perovskite white-light phosphors. Two-dimensional infrared (2D IR) spectroscopy was applied to study a recently reported Pb-I-SCN layered perovskite. The Pb-I-SCN perovskite was spin-coated on a SiO₂ surface as a thin film, with a thickness of ∼100 nm, where the S¹²CN^- anions were isotopically diluted with the ratio of S¹²CN:S¹³CN = 5:95 to avoid vibrational coupling and excitation transfer between adjacent SCN^- anions. The ¹²CN stretch mode of the minor S¹²CN^- component was the principal vibrational probe that reported on the structural evolution through 2D IR spectroscopy. Spectral diffusion was observed with a time constant of 4.1 ± 0.3 ps. Spectral diffusion arises from small structural changes that result in sampling of frequencies within the distribution of frequencies comprising the inhomogeneously broadened infrared absorption band. These transitions among discrete local structures are distinct from oscillatory phonon motions of the lattice. To accurately evaluate the structural dynamics through measurement of spectral diffusion, the vibrational coupling between adjacent SCN^- anions had to be carefully treated. Although the inorganic layers of typical 2D perovskites are structurally isolated from each other, the 2D IR data demonstrated that the layers of the Pb-I-SCN perovskite are vibrationally coupled. When both S¹²CN^- and S¹³CN^- were pumped simultaneously, cross-peaks between S¹²CN and S¹³CN vibrations and an oscillating 2D band shape of the S¹²CN^- vibration were observed. Both observables demonstrate vibrational coupling between the closest SCN^- anions, which reside in different inorganic layers. The thin films and the isotopic dilution produced exceedingly small vibrational echo signal fields; measurements were made possible using the near-Brewster's angle reflection pump-probe geometry.

Linear and non-linear infrared response of one-dimensional vibrational Holstein polarons in the anti-adiabatic limit: Optical and acoustical phonon models

The theory of linear and non-linear infrared response of vibrational Holstein polarons in one-dimensional lattices is presented in order to identify the spectral signatures of self-trapping phenomena. Using a canonical transformation, the optical response is computed from the small polaron point of view which is valid in the anti-adiabatic limit. Two types of phonon baths are considered: optical phonons and acoustical phonons, and simple expressions are derived for the infrared response. It is shown that for the case of optical phonons, the linear response can directly probe the polaron density of states. The model is used to interpret the experimental spectrum of crystalline acetanilide in the C=O range. For the case of acoustical phonons, it is shown that two bound states can be observed in the two-dimensional infrared spectrum at low temperature. At high temperature, analysis of the time-dependence of the two-dimensional infrared spectrum indicates that bath mediated correlations slow down spectral diffusion. The model is used to interpret the experimental linear-spectroscopy of model α-helix and β-sheet polypeptides. This work shows that the Davydov Hamiltonian cannot explain the observations in the NH stretching range.

Dynamics of the OH stretching mode in crystalline Ba(ClO4)2·3H2O

The vibrational dynamics of the OH stretching mode in Ba(ClO₄)₂ trihydrate are investigated by means of femtosecond infrared spectroscopy. The sample offers plane cyclic water trimers in the solid phase that feature virtually no hydrogen bond interaction between the water molecules. Selective excitation of the symmetric and asymmetric stretching leads to fast population redistribution, while simultaneous excitation yields quantum beats, which are monitored via a combination tone that dominates the overtone spectrum. The combination of steady-state and time-resolved spectroscopy with quantum chemical simulations and general theoretical considerations gives indication of various aspects of symmetry breakage. The system shows a joint population lifetime of 8 ps and a long-lived coherence between symmetric and asymmetric stretching, which decays with a time constant of 0.6 ps.

Vibrational Spectroscopy of Ionic Liquids

Vibrational spectroscopy has continued use as a powerful tool to characterize ionic liquids since the literature on room temperature molten salts experienced the rapid increase in number of publications in the 1990's. In the past years, infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and the resulting liquid structure in ionic liquids. A large body of information is now available concerning vibrational spectra of ionic liquids made of many different combinations of anions and cations, but reviews on this literature are scarce. This review is an attempt at filling this gap. Some basic care needed while recording IR or Raman spectra of ionic liquids is explained. We have reviewed the conceptual basis of theoretical frameworks which have been used to interpret vibrational spectra of ionic liquids, helping the reader to distinguish the scope of application of different methods of calculation. Vibrational frequencies observed in IR and Raman spectra of ionic liquids based on different anions and cations are discussed and eventual disagreements between different sources are critically reviewed. The aim is that the reader can use this information while assigning vibrational spectra of an ionic liquid containing another particular combination of anions and cations. Different applications of IR and Raman spectroscopies are given for both pure ionic liquids and solutions. Further issues addressed in this review are the intermolecular vibrations that are more directly probed by the low-frequency range of IR and Raman spectra and the applications of vibrational spectroscopy in studying phase transitions of ionic liquids.

A new interpretation of the meaning of the center of line slope from a two-dimensional infrared spectrum

This article presents a new approximation to understand the connection between the center of line slope from a single peak of a two-dimensional (2D) infrared spectrum and the frequency-frequency correlation function. This approximation which goes beyond the short-time approximation includes explicitly pure dephasing mechanisms by introducing a time parameter that separates the fast fluctuations and slow fluctuations. While in the short-time approximation, the center of line slope is given by the normalized frequency fluctuations auto-correlation function, I show using this new approximation that the center of line slope measures on long time scales a shifted and scaled correlation function. The results present a new interpretation of the meaning of the center of line slope that allows for a better understanding of what 2D experiments can measure. To illustrate these findings, I compare this approximation with the short-time approximation for several examples of frequency-frequency correlation functions. I also give an estimate of the value of the time separation parameter for a correlation function with a simple exponential decay.

Two-dimensional infrared spectroscopy of neat ice Ih

The assignment of the distinct peaks observed in the OH stretch lineshape of ice Ih is controversial. Recent two-dimensional infrared spectroscopic measurements provided new data. The spectra are, however, challenging to interpret and here we provide simulations that help overcome experimental issues as thermal signals and finite pulse duration. We find good agreement with experiment and the difference between H2O and D2O ices is well accounted for. The overall dynamics is demonstrated to be faster than observed for the corresponding liquid water. We find that excitonic cross peaks exist between the dominant exciton peaks. This leads us to conclude that the cross peaks arise due to the formation of delocalized exciton states, which have essentially no directional correlation between their transition dipoles as opposed to what is commonly seen, for example, in isolated water, where the transition dipoles of the eigenstates are perpendicular to each other.

Alcohols as molecular probes in ionic liquids: evidence for nanostructuration

A comprehensive study of the solution and solvation of linear alcohols (propan-1-ol, butan-1-ol and pentan-1-ol) in ionic liquids (ILs) is presented.

Ultrafast vibrational spectroscopy (2D-IR) of CO2 in ionic liquids: Carbon capture from carbon dioxide's point of view

The CO2ν3 asymmetric stretching mode is established as a vibrational chromophore for ultrafast two-dimensional infrared (2D-IR) spectroscopic studies of local structure and dynamics in ionic liquids, which are of interest for carbon capture applications. CO2 is dissolved in a series of 1-butyl-3-methylimidazolium-based ionic liquids ([C4C1im][X], where [X](-) is the anion from the series hexafluorophosphate (PF6 (-)), tetrafluoroborate (BF4 (-)), bis-(trifluoromethyl)sulfonylimide (Tf2N(-)), triflate (TfO(-)), trifluoroacetate (TFA(-)), dicyanamide (DCA(-)), and thiocyanate (SCN(-))). In the ionic liquids studied, the ν3 center frequency is sensitive to the local solvation environment and reports on the timescales for local structural relaxation. Density functional theory calculations predict charge transfer from the anion to the CO2 and from CO2 to the cation. The charge transfer drives geometrical distortion of CO2, which in turn changes the ν3 frequency. The observed structural relaxation timescales vary by up to an order of magnitude between ionic liquids. Shoulders in the 2D-IR spectra arise from anharmonic coupling of the ν2 and ν3 normal modes of CO2. Thermal fluctuations in the ν2 population stochastically modulate the ν3 frequency and generate dynamic cross-peaks. These timescales are attributed to the breakup of ion cages that create a well-defined local environment for CO2. The results suggest that the picosecond dynamics of CO2 are gated by local diffusion of anions and cations.

Line shape analysis of two-dimensional infrared spectra

Ultrafast two-dimensional infrared (2D IR) spectroscopy probes femtosecond to picosecond time scale dynamics ranging from solvation to protein motions. The frequency-frequency correlation function (FFCF) is the quantitative measure of the spectral diffusion that reports those dynamics and, within certain approximations, can be extracted directly from 2D IR line shapes. A variety of methods have been developed to extract the FFCF from 2D IR spectra, which, in principle, should give the same FFCF parameters, but the complexity of real experimental systems will affect the results of these analyses differently. Here, we compare five common analysis methods using both simulated and experimental 2D IR spectra to understand the effects of apodization, anharmonicity, phasing errors, and finite signal-to-noise ratios on the results of each of these analyses. Our results show that although all of the methods can, in principle, yield the FFCF under idealized circumstances, under more realistic experimental conditions they behave quite differently, and we find that the centerline slope analysis yields the best compromise between the effects we test and is most robust to the distortions that they cause.

Anharmonicities and coherent vibrational dynamics of phosphate ions in bulk H2O

2D IR spectroscopy reveals Fermi resonances and long lived quantum beats for phosphate ions in water.

Structural dynamics inside a functionalized metal-organic framework probed by ultrafast 2D IR spectroscopy

The structural elasticity of metal-organic frameworks (MOFs) is a key property for their functionality. Here, we show that 2D IR spectroscopy with pulse-shaping techniques can probe the ultrafast structural fluctuations of MOFs. 2D IR data, obtained from a vibrational probe attached to the linkers of UiO-66 MOF in low concentration, revealed that the structural fluctuations have time constants of 7 and 670 ps with no solvent. Filling the MOF pores with dimethylformamide (DMF) slows the structural fluctuations by reducing the ability of the MOF to undergo deformations, and the dynamics of the DMF molecules are also greatly restricted. Methodology advances were required to remove the severe light scattering caused by the macroscopic-sized MOF particles, eliminate interfering oscillatory components from the 2D IR data, and address Förster vibrational excitation transfer.

Molecular dynamics investigation of the vibrational spectroscopy of isolated water in an ionic liquid

Experimental studies examining the structure and dynamics of water in ionic liquids (ILs) have revealed local ion rearrangements that occur an order of magnitude faster than complete randomization of the liquid structure. Simulations of an isolated water molecule embedded in 1-butyl-3-methyl imidazolium hexafluorophosphate, [bmim][PF6], were performed to shed insight into the nature of these coupled water-ion dynamics. The theoretical calculations of the spectral diffusion dynamics and the infrared absorption spectra of the OD stretch of isolated HOD in [bmim][PF6] agree well with experiment. The infrared absorption line shape of the OD stretch is narrower and blue-shifted in the IL compared to those in aqueous solution. Decomposition of the OD frequency time correlation function revealed that translational motions of the anions dominate the spectral diffusion dynamics.

Quantum process tomography quantifies coherence transfer dynamics in vibrational exciton

Quantum coherence has been a subject of great interest in many scientific disciplines. However, detailed characterization of the quantum coherence in molecular systems, especially its transfer and relaxation mechanisms, still remains a major challenge. The difficulties arise in part because the spectroscopic signatures of the coherence transfer are typically overwhelmed by other excitation-relaxation processes. We use quantum process tomography (QPT) via two-dimensional infrared spectroscopy to quantify the rate of the elusive coherence transfer between two vibrational exciton states. QPT retrieves the dynamics of the dissipative quantum system directly from the experimental observables. It thus serves as an experimental alternative to theoretical models of the system-bath interaction and can be used to validate these theories. Our results for coupled carbonyl groups of a diketone molecule in chloroform, used as a benchmark system, reveal the nonsecular nature of the interaction between the exciton and the Markovian bath and open the door for the systematic studies of the dissipative quantum systems dynamics in detail.