OD Stretch Research Articles

Vibrational relaxation of HOD by collisions with Ar atoms.

Vibrational relaxation of HOD(v12, v3) molecules by collisions with Ar was studied at 298K (v12 denotes coupled bending, v2, and OD stretching, v1, vibrational modes and v3 denotes OH stretching mode). The vibrationally excited HOD molecules were generated by exothermic abstraction reactions of OD radicals with 13 different RH reactants and observed by infrared emission from a fast-flow reactor as a function of Ar pressure and reaction time. State-specific relaxation rate constants were obtained by comparison of the time evolution of the experimental vibrational distributions with numerical kinetic calculations for vibrational populations. The relaxation mechanism was based on the relaxation scheme of H2O studied earlier with the addition of specific channels for HOD(v12, v3). Unlike H2O, energy in stretching and bending vibrations of HOD cannot be separated due to close ν1 and 2ν2 energies, which leads to fast collisional equilibration between these Fermi-resonant levels. For relaxation of the only pure bending state (10), a rate constant of (1.5 ± 0.3) × 10-13cm3molecule-1s-1 was obtained. The relaxation rate of higher v12 states linearly increases with quantum number and very likely includes transfer of population from OD stretch levels, v1, to a lower energy bend level. The average rate constants for the loss of population from (01), (02), and (03) stretching states are (1.1 ± 0.3) × 10-14, (3.2 ± 1.0) × 10-14, and (5.6 ± 1.2) × 10-14cm3molecule-1s-1, respectively.

Dynamics of Formamide-Water Mixtures Investigated by Linear and Nonlinear Infrared Spectroscopy.

Formamide (FA) exhibits complete miscibility with water, offering a simplified model for exploring the solvation dynamics of peptide linkages in biophysical processes. Its liquid state demonstrates a three-dimensional hydrogen bonding network akin to water, reflecting solvent-like behavior. Analyzing the microscopic structure and dynamics of FA-water mixtures is expected to provide crucial insights into hydrogen bonding dynamics─a key aspect of various biophysical phenomena. This study is focused on the dynamics of FA-water mixtures using linear and femtosecond infrared spectroscopies. By using the intrinsic OD stretch and extrinsic probe SCN-, the local vibrational behaviors across various FA-water compositions were systematically investigated. The vibrational relaxation of OD stretch revealed a negligible impact of FA addition on the vibrational lifetime of water molecules, underscoring the mixture's water-like behavior. However, the reorientational dynamics of OD stretch slowed with increasing FA mole fraction (XFA), plateauing beyond XFA > 0.5. This suggests a correlation between OD's reorientational time and the strength of the hydrogen bond network, likely tied to the solution's changing dielectric constant. Conversely, the vibrational relaxation dynamics of SCN- was strongly correlated with XFA, highlighting a competition between water and FA molecules in solvating SCN-. Moreover, a linear relationship between rising viscosity and the prolonged correlation time of SCN-'s slow dynamics indicates that the solution's macroscopic viscosity is dictated by the extended structures formed between FA and water molecules. The relation between the reorientation dynamics of the SCN- and the macroscopic viscosity in aqueous FA-water mixture solutions was analyzed by using the Stokes-Einstein-Debye equations. The direct viscosity-diffusion coupling is observed, which can be attributed to the homogeneous dynamics feature in FA-water mixture solutions. The inclusion of these intrinsic and extrinsic probes not only enhances the comprehensiveness of our analysis but also provides valuable insights into various aspects of the dynamics within the FA-water system. This investigation sheds light on the fundamental dynamics of FA-water mixtures, emphasizing their molecular-level homogeneity in this binary mixture solution.

Investigating Intramolecular H Atom Transfer Dynamics in β-Diketones with Ultrafast Infrared Spectroscopies and Theoretical Modeling.

The vibrational signatures and ultrafast dynamics of the intramolecular H-bond in a series of β-diketones are investigated with 2D IR spectroscopy and computational modeling. The chosen β-diketones exhibit a range of H atom donor-acceptor distances and asymmetry along the H atom transfer coordinate that tunes the intramolecular H-bond strength. The species with the strongest H-bonds are calculated to have very soft H atom potentials, resulting in highly red-shifted OH stretch fundamental frequencies and dislocation of the H atom upon vibrational excitation. These soft potentials lead to significant coupling to the other normal mode coordinates and give rise to the very broad vibrational signatures observed experimentally. The 2D IR spectra in both the OH and OD stretch regions of the light and deuterated isotopologues reveal broadened and long-lived ground-state bleach signatures of the vibrationally hot molecules. Polarization-sensitive transient absorption measurements in the OH and OD stretch regions reveal notable isotopic differences in orientational dynamics. Orientational relaxation was measured to occur on ∼600 fs and ∼2 ps time scales for the light and deuterated isotopologues, respectively. The orientational dynamics are interpreted in terms of activated H/D atom transfer events driven by collective intramolecular structural rearrangements.

Systematic investigation of coupling between symmetric and antisymmetric stretches of D2O in CHCl3 by 2D IR.

The coupling between the symmetric (νs) and antisymmetric (νa) OD stretch modes of monomeric D2O in CHCl3 is investigated using polarization-dependent two-dimensional infrared (2D IR) spectroscopy supported by numerical 2D IR simulations based on the exciton-band theory. The relationship between the local modes' and the exciton states' parameters is systematically studied, including center frequencies, diagonal anharmonicities, coupling, and off-diagonal anharmonicity. The mean coupling between νs and νa is accurately evaluated to be -49.96 ± 0.14cm-1. The degree of relaxation in the harmonic approximation is quantified, and the angle between the exciton-state dipoles is accurately evaluated to be 101.4° ± 3.6°. In addition, the effect of the local-mode frequency correlation on the resulting exciton-state frequency correlation and the spectral shape of the linear and 2D IR spectra are also investigated.

Combinational Vibration Modes in H2O/HDO/D2O Mixtures Detected Thanks to the Superior Sensitivity of Femtosecond Stimulated Raman Scattering.

Overtones and combinational modes frequently play essential roles in ultrafast vibrational energy relaxation in liquid water. However, these modes are very weak and often overlap with fundamental modes, particularly in isotopologues mixtures. We measured VV and HV Raman spectra of H2O and D2O mixtures with femtosecond stimulated Raman scattering (FSRS) and compared the results with calculated spectra. Specifically, we observed the mode at around 1850 cm-1 and assigned it to H-O-D bend + rocking libration. Second, we found that the H-O-D bend overtone band and the OD stretch + rocking libration combination band contribute to the band located between 2850 and 3050 cm-1. Furthermore, we assigned the broad band located between 4000 and 4200 cm-1 to be composed of combinational modes of high-frequency OH stretching modes with predominantly twisting and rocking librations. These results should help in a proper interpretation of Raman spectra of aqueous systems as well as in the identification of vibrational relaxation pathways in isotopically diluted water.

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.

Origins of the diffuse shared proton vibrational signatures in proton-coupled electron transfer model dyad complexes.

Phenol-benzimidazole and phenol-pyridine dyad complexes have served as popular model systems for the study of proton-coupled electron transfer (PCET) kinetics in solution-phase experiments. Interpretation of measured PCET rates in terms of key structural parameters, such as the H-bond donor-acceptor distance, however, remains challenging. Herein, we report vibrational spectra in the electronic ground state for a series of phenol-benzimidazole and phenol-pyridine complexes isolated and cryogenically cooled in an ion trap. The four models studied each display highly red-shifted and broadened OH stretching transitions that arise from strong H-bonding interactions between the phenol OH group and the basic N site on benzimidazole/pyridine rings. The OH stretch transition in each model displays relatively strong absorption onsets near 2500cm-1 with broad shoulders that extend asymmetrically to higher frequencies over hundreds of wavenumbers. In contrast, the deuterated isotopologues yield much weaker OD stretch transitions that appear symmetrically broadened. The spectral breadth and shape of the OD stretch transitions are ascribed to variations in OD stretch frequencies that arise from zero-point distributions in the proton donor-acceptor low-frequency soft mode vibration. The asymmetric structure of the OH stretch transitions is attributed to a set of combination bands between the OH stretch and a series of low-frequency H-bond soft modes. The spectra and modeling highlight the importance of OH stretch-soft mode couplings, which are thought to play important roles in PCET and proton transfer dynamics.

Vibrational dynamics of the OD stretch in an atomistic simulation of HDO in H2O

An all-atom molecular dynamics (MD) simulation of dilute HDO in H2O was used to obtain vibrational frequency spectra from the classical MD trajectories and a wavelet-based method was developed to obtain the frequency-frequency correlation functions (FFCFs) of the OD stretch vibrations. Time constants were obtained on the order of 30 fs, 150–260 fs and 3.2–3.6 ps. An OD covalent bond length correlation function was calculated and a long-timescale decay constant of 1.73 ps was obtained, similar to experimental values that range from 1.45 ps to 1.8 ps. An O-O hydrogen bond length correlation function was calculated and found to exhibit an oscillation at 150 fs and a long-timescale decay time of 3.47 ps, both in good agreement with the FFCFs. Such results suggest that the spectral diffusion observed in FFCFs can be primarily attributed to fluctuations in O-O distances. An OD stretch energy correlation function was calculated, which consists of exponential decay with a time constant of 1.13 ps. The energy correlation function also exhibits two oscillations that are indicative of resonant energy transfer processes involving the OD stretch.

Water Dynamics in Aqueous Poly-N-Isopropylacrylamide Below and Through the Lower Critical Solution Temperature.

Poly-N-isopropylacrylamide (PNIPAM) is a thermo-responsive polymer that exhibits a reversible structural change from extended chains to aggregates in aqueous solution above its lower critical solution temperature (LCST). Using polarization-selective IR pump-probe spectroscopy, the water orientational dynamics in PNIPAM from below to above the LCST were examined and compared to those of its monomer solution, N-isopropylacrylamide (NIPAM), polyacrylamide, and an acrylamide monomer solution, which are not thermo-responsive. The OD stretch of dilute HOD in H2O is used as a vibrational probe of water orientational dynamics. Below the LCST of the polymer, NIPAM and PNIPAM solutions exhibited identical water dynamics that were significantly different from those of bulk water, containing both faster and slower components due to solute-water interactions. Therefore, there is no difference in the nature of water interactions with a single NIPAM moiety and a long polymer chain. For all systems, including PNIPAM below and above the LCST, the orientational dynamics were modeled with a bulk water component and a polymer/monomer-associated component based on previous experimental and computational findings. Above the LCST, PNIPAM showed fast water orientational relaxation but much slower long-time dynamics compared to those of NIPAM. The slow component in PNIPAM, which was too slow to be accurately measured due to the limited OD vibrational lifetime, is ascribed to water confined in small voids (<2 nm in diameter) of PNIPAM globules. These results highlight important details about thermo-responsive polymers and the dynamics of their solvation water as they undergo a significant structural change.

Ultrafast vibrational dynamics of the free OD at the air/water interface: Negligible isotopic dilution effect but large isotope substitution effect.

Vibrational relaxation dynamics of the OH stretch of water at the air/water interface has been a subject of intensive research, facilitated by recent developments in ultrafast interface-selective nonlinear spectroscopy. However, a reliable determination of the vibrational relaxation dynamics in the OD stretch region at the air/D2O interface has not been yet achieved. Here, we report a study of the vibrational relaxation of the free OD carried out by time-resolved heterodyne-detected vibrational sum frequency generation spectroscopy. The results obtained with the aid of singular value decomposition analysis indicate that the vibrational relaxation (T1) time of the free OD at the air/D2O interface and air/isotopically diluted water (HOD-H2O) interfaces show no detectable isotopic dilution effect within the experimental error, as in the case of the free OH in the OH stretch region. Thus, it is concluded that the relaxation of the excited free OH/OD predominantly proceeds with their reorientation, negating a major contribution of the intramolecular energy transfer. It is also shown that the T1 time of the free OD is substantially longer than that of the free OH, further supporting the reorientation relaxation mechanism. The large difference in the T1 time between the free OD and the free OH (factor of ∼2) may indicate the nuclear quantum effect on the diffusive reorientation of the free OD/OH because this difference is significantly larger than the value expected for a classical rotational motion.

Vibrational Dynamics of the Intramolecular H-Bond in Acetylacetone Investigated with Transient and 2D IR Spectroscopy.

Acetylacetone (AcAc) has proven to be a fruitful but highly challenging model system for the experimental and computational interrogation of strong intramolecular hydrogen bonds. Key questions remain, however, regarding the identity of the minimum-energy structure of AcAc and the dynamics of intramolecular proton transfer. Here, we investigate the OH/OD stretch and bend regions of the enol tautomer of AcAc and its deuterated isotopologue with transient absorption and 2D IR spectroscopy. The OH bend region reveals a single dominant diagonal transition near 1625 cm-1 with intense cross peaks to lower-frequency modes, demonstrating highly mixed fingerprint transitions that contain OH bend character. The anharmonic coupling of the OH bend results in a highly elongated OH bend excited-state absorption transition that indicates a large manifold of OH bend overtone/combination bands in the OH stretch region that leads to strong bend-stretch Fermi resonance interactions. The OH and OD stretch regions consist of broad ground-state bleach signals, but there is no clear evidence of ω21 excited-state absorptions due to rapid population relaxation arising from strong intramolecular coupling to bending, fingerprint, and low-frequency H-bond modes. Orientational relaxation dynamics persist for timescales longer than the vibrational lifetimes, with polarization anisotropy components decaying within approximately 2 and 10 periods of the O-O oscillation for the OH and OD stretch, respectively. The significant isotopic dependence of the orientational dynamics is discussed in the context of intramolecular mode coupling, diffusional processes, and contributions from proton/deuteron transfer dynamics.

Water Dynamics and Structure of Highly Concentrated LiCl Solutions Investigated Using Ultrafast Infrared Spectroscopy.

In highly concentrated salt solutions, the water hydrogen bond (H-bond) network is completely disrupted by the presence of ions. Water is forced to restructure as dictated by the water-ion and ion-ion interactions. Using ultrafast polarization-selective pump-probe (PSPP) spectroscopy measurements of the OD stretch of dilute HOD, we demonstrate that the limited water-water H-bonding present in concentrated lithium chloride solutions (up to four waters per ion pair) is, on average, stronger than that occurring in bulk water. Furthermore, information on the orientational dynamics and the angular restriction of water H-bonded to both water oxygens and chloride anions was obtained through analysis of the frequency-dependent anisotropy decays. It was found that, when the salt concentration increased, the water showed increasing restriction and slowing at frequencies correlated with strong H-bonding. The angular restriction of the water molecules and strengthening of water-water H-bonds are due to the formation of a water-ion network not present in bulk water and dilute salt solutions. The structural evolution of the ionic medium was also observed through spectral diffusion of the OD stretch using 2D IR spectroscopy. Compared to bulk water, there is significant slowing of the biexponential spectral diffusion dynamics. The slowest component of the spectral diffusion (13 ps) is virtually identical to the time for complete reorientation of HOD measured with the PSPP experiments. This result suggests that the slowest component of the spectral diffusion reflects rearrangement of water molecules in the water-ion network.

Ab Initio Molecular Dynamics Study of Aqueous Solutions of Magnesium and Calcium Nitrates: Hydration Shell Structure, Dynamics and Vibrational Echo Spectroscopy.

Ab initio molecular dynamics simulations are performed to study the hydration shell structure, dynamics, and vibrational echo spectroscopy of aqueous Mg(NO3)2 and Ca(NO3)2 solutions. The hydration shell structure is probed through calculations of various ion-ion and ion-water radial and spatial distribution functions. On the dynamical side, calculations have been made for the hydrogen bond dynamics of hydration shells and also residence dynamics and lifetimes of water in different solvation environments. Subsequently, we looked at the dynamics of frequency fluctuations of OD modes of heavy water in different hydration environments. Specifically, the temporal decay of spectral observables of two-dimensional infrared (2DIR) spectroscopy, three pulse echo peak shift (3PEPS) measurements and also of time correlations of frequency fluctuations are calculated to investigate the dynamics of vibrational spectral diffusion of water in different hydration environments in these solutions. The OD stretch frequencies of water molecules in the vicinity of both divalent cations are found to be red-shifted and also fluctuating at a slower rate than other water molecules present in the solutions. The Mg2+ ions are found to be strongly hydrated which can be linked to their lower tendency to form contact ion-pairs and essentially no water exchange between the cationic hydration shells and bulk during the time scale of the current simulations. The stronger hydration of Mg2+ ions make their hydration shells structurally and dynamically more rigid and make the dynamics of hydrogen bonds and vibrational spectral diffusion, as revealed through spectral observables of 2DIR and 3PEPS slower than that for the Ca2+ ions. The structural and spectral dynamics of water molecules outside the cationic solvation shells in the Mg(NO3)2 solution are also found to be relatively slower than that of the Ca(NO3)2 solution and pure water which show the effects of stronger electric fields of Mg2+ ions extending beyond their first hydration shells. Also, water molecules in the hydration shells of the NO3- ions are found to relax at a slower rate in the Mg(NO3)2 solution which manifests the effect countercations have on anionic hydration shells for divalent metal nitrate solutions.

Hydrogen Delocalization in an Asymmetric Biomolecule: The Curious Case of Alpha-Fenchol.

Rotational microwave jet spectroscopy studies of the monoterpenol -fenchol have so far failed to identify its second most stable torsional conformer, despite computational predictions that it is only very slightly higher in energy than the global minimum. Vibrational FTIR and Raman jet spectroscopy investigations reveal unusually complex OH and OD stretching spectra compared to other alcohols. Via modeling of the torsional states, observed spectral splittings are explained by delocalization of the hydroxy hydrogen atom through quantum tunneling between the two non-equivalent but accidentally near-degenerate conformers separated by a low and narrow barrier. The energy differences between the torsional states are determined to be only 16(1) and 7(1) cm for the protiated and deuterated alcohol, respectively, which further shrink to 9(1) and 3(1) cm upon OH or OD stretch excitation. Comparisons are made with the more strongly asymmetric monoterpenols borneol and isopinocampheol as well as with the symmetric, rapidly tunneling propargyl alcohol. In addition, the third—in contrast localized—torsional conformer and the most stable dimer are assigned for -fenchol, as well as the two most stable dimers for propargyl alcohol.

Adsorbed Water Structure on Acrylate-Based Biocompatible Polymer Surface.

The role of water in the excellent biocompatibility of the acrylate-based polymers widely used for antibiofouling coating material has been realized previously. Here, we report femtosecond mid-infrared pump-probe spectroscopy of the OD stretch band of HOD molecule adsorbed on highly biocompatible poly(2-methoxyethyl) acrylate [PMEA] and poorly biocompatible poly(2-phenoxyethyl) acrylate [PPEA], both of which reveal that there are two water species with significantly different vibrational lifetime. PMEA interacts more strongly with water than PPEA through the H-bonding interaction between carbonyl (C═O) and water. The vibrational lifetime of the OD stretch in PPEA is notably longer by factors of 3 and 7 than those in PMEA and bulk water, respectively. The IR-pump visible-probe photothermal imaging further unravels substantial spatial overlap between polymer CO group and water for hydrated PMEA and a significant difference in surface morphology than those in PPEA, which exhibits the underlying relationships among polymer-water interaction, surface morphology, and biocompatibility.

Identical Water Dynamics in Acrylamide Hydrogels, Polymers, and Monomers in Solution: Ultrafast IR Spectroscopy and Molecular Dynamics Simulations.

The dynamics and structure of water in polyacrylamide hydrogels (PAAm-HG), polyacrylamide, and acrylamide solutions are investigated using ultrafast infrared experiments on the OD stretch of dilute HOD/H2O and molecular dynamics simulations. The amide moiety of the monomer/polymers interacts strongly with water through hydrogen bonding (H-bonding). The FT-IR spectra of the three systems indicate that the range of H-bond strengths is relatively unchanged from bulk water. Vibrational population relaxation measurements show that the amide/water H-bonds are somewhat weaker but fall within the range of water/water H-bond strengths. A previous study of water dynamics in PAAm-HG suggested that the slowing observed was due to increasing confinement with concentration. Here, for the same concentrations of the amide moiety, the experimental results demonstrate that the reorientational dynamics (infrared pump-probe experiments) and structural dynamics (two-dimensional infrared spectroscopy) are identical in the three acrylamide systems studied. Molecular dynamics simulations of the water orientational relaxation in aqueous solutions of the acrylamide monomer, trimer, and pentamer are in good agreement with the experimental results and are essentially chain length independent. The simulations show that there is a slower, low-amplitude (<7%) decay component not accessible by the experiments. The simulations examine the dynamics and structure of water H-bonded to acrylamide, in the first solvent shell, and beyond for acrylamide monomers and short chains. The experiments and simulations show that the slowing of water dynamics in PAAm-HG is not caused by confinement in the polymer network but rather by interactions with individual acrylamide moieties.

Asymmetric response of interfacial water to applied electric fields.

Our understanding of the dielectric response of interfacial water, which underlies the solvation properties and reaction rates at aqueous interfaces, relies on the linear response approximation: an external electric field induces a linearly proportional polarization. This implies antisymmetry with respect to the sign of the field. Atomistic simulations have suggested, however, that the polarization of interfacial water may deviate considerably from the linear response. Here we present an experimental study addressing this issue. We measured vibrational sum-frequency generation spectra of heavy water (D2O) near a monolayer graphene electrode, to study its response to an external electric field under controlled electrochemical conditions. The spectra of the OD stretch show a pronounced asymmetry for positive versus negative electrode charge. At negative charge below 5×1012 electrons per square centimetre, a peak of the non-hydrogen-bonded OD groups pointing towards the graphene surface is observed at a frequency of 2,700per centimetre. At neutral or positive electrode potentials, this 'free-OD' peak disappears abruptly, and the spectra display broad peaks of hydrogen-bonded OD species (at 2,300-2,650per centimetre). Miller's rule1 connects the vibrational sum-frequency generation response to the dielectric constant. The observed deviation from the linear response for electric fields of about ±3×108volts per metre calls into question the validity of treating interfacial water as a simple dielectric medium.

Ultrafast Aqueous Dynamics in Concentrated Electrolytic Solutions of Lithium Salt and Ionic Liquid.

In the last few decades, aqueous electrolytes based on ionic liquids (ILs) have been attractive because of their various uses in chemical sciences and potential applications in Li-ion batteries. The presence of water molecules influences the molecular structure and complex transformations occurring at ultrafast timescales because of the interaction between the water and the ionic entities of ILs. In this study, we investigate water-IL interactions that correlate the vibrational dynamics of the associated infrared probe using a fast and accurate computational approach. The obtained results from our approach are directly compared with those of the ultrafast two-dimensional infrared spectroscopy experiments. We investigated the dynamics of ion-water interactions of aqueous lithium bis(trifluoromethylsulfonyl)imide, (LiNTf2), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, ([EMIm] [NTf2]), with the help of vibrational dynamics of the deuteroxyl probe of water molecules using wavelet analysis of trajectories of classical molecular dynamics simulations. The normalized OD stretch frequency distribution of aqueous [EMIm][NTf2] shows a heterogeneous bimodal line shape associated with both water and ion hydrogen-bonded OD populations. However, H2O/LiNTf2 solution depicts a broad spectral band without a distinct spectral feature. We observe faster timescales of frequency fluctuations, time-dependent hole propagation, and reorientation dynamics in aqueous [EMIm][NTf2] solution. At a time-lapse of 0.1 ps, the 2D IR correlation spectrum in aqueous LiNTf2 appears elongated along with the frequency diagonal. The spectral signature in the water-[EMIm][NTf2] mixture is relatively symmetric in shape, indicating faster dynamics. Our results show the characteristic behavior of water molecules present in the aqueous ionic mixtures that distinguish two mixtures dynamically. While examining water interactions in aqueous ionic solutions, we predict that the additional water in the aqueous LiNTf2 solution might be engaged in the structural arrangement of the restricted IL environment. In contrast, water in aqueous [EMIm][NTf2] enhances the dynamics. Hence, we observe that a change in the cationic constituent of the IL solution results in a substantial change in the overall system dynamics.

Two-dimensional infrared spectroscopy of aqueous solutions from first principles simulations

Vibrational spectral diffusion of OH/OD modes in liquid water can be influenced by the presence of ions, dipolar solutes or by the change of temperature. In the present work, we have calculated the two-dimensional infrared (2D-IR) spectrum of aqueous NaBr and urea solutions at room temperature and also liquid water at two different temperatures to study the effects of these solutes and varying temperature on the dynamics of vibrational spectral diffusion of OD stretch modes in aqueous media. Time-dependent evolution of the ellipticity of 2D-IR spectra is calculated to obtain timescales of the dynamics of vibrational spectral diffusion.

Vibrational echo spectroscopy of aqueous sodium bromide solutions from first principles simulations.

A theoretical study of the time-dependent vibrational echo spectroscopy of sodium bromide solutions in deuterated water at two different concentrations of 0.5 and 5.0 M and at temperatures of 300 and 350 K is presented using the method of ab initio molecular dynamics simulations. The instantaneous fluctuations in frequencies of local OD stretch modes are calculated using time-series analysis of the simulated trajectories. The third-order polarization and intensities of three pulse photon-echo are calculated from ab initio simulations. The timescales of vibrational spectral diffusion are determined from the frequency time correlation functions (FTCF) and short-time slope of three pulse photon echo (S3PE) calculated within the second-order cumulant and Condon approximations. It is found that under ambient conditions, the rate of vibrational spectral diffusion becomes slower with increase in ionic concentration. Decay of S3PE calculated for different systems give timescales, which are in close agreement with those of FTCF and also with the results of experimental time-dependent vibrational spectroscopic experiments. © 2019 Wiley Periodicals, Inc.