Till now, there has been ambiguity about the structural heterogeneity inside a solute solvation shell and the dynamical response of the surrounding solvent molecules. To address the dynamics and spectral response of solvent molecules, we performed first-principles molecular dynamics simulations for the comprehensive study of water’s hydroxyl stretch frequency evolution due to environmental variations (also called “spectral diffusion”) in the vicinity of a hydrophobe, tetramethylammonium (TMA) cation. The N–Ow radial distribution function (RDF), spatial distribution function (SDF), and combined distribution function (CDF) were calculated to provide information about the arrangement of water molecules around TMA. In the probability distribution plot of the cosine of the angle (θ) between Ow–Hw and NTMA–Ow bond vectors, the hydrogen atoms are observed being oriented toward TMA and the oxygen atoms aligned away. The decaying dynamics of the orientation autocorrelation function (OACF) reveals the reorientation time is more inside the solvation shell (4.1 ps) as compared to bulk (2.8 ps), matching with the trend obtained from water’s orientational dynamics in tetraalkylammonium salts. Wavelet transform of the obtained trajectory was used to calculate the time-dependent vibrational stretching frequencies of the OH modes of water molecules. The normalized frequency distribution in the aqueous solvation shell of TMA, tagging a particular water molecule within the N–Ow cutoff distance 5.5 Å, displays an intense peak at 3661 cm–1 representing non-hydrogen bonded or dangling or free OH modes. Simulations around aromatic solutes and Raman-MCR studies in the hydration shell of hydrophobic TBA reported a distinctive dangling OH band at 3660 cm–1 (range: 3661 ± 2 cm–1). Besides dangling water molecules in the first hydration layer of ammonium nitrogen, few OH modes are strongly hydrogen-bonded having an average frequency at 3300 cm–1. The predominance of dangling hydroxyl modes around apolar hydrophobic TMA was further explored by comparing the dangling lifetime (∼0.68 ps) with the lifespan of hydrogen bonded OH modes (∼0.48 ps). At our simulation temperature 330 K, a significant fraction of the water molecules in the vicinity of TMA ion are free or dangling, and a few of them form an ordered structure with enhanced hydrogen bonding. Structural analysis, orientation correlation, frequency fluctuations, dangling, and hole dynamics calculations provide the evidence of the existence of dangling OH modes dominating over highly ordered strong hydrogen bonded structure in the cationic TMA solvation shell at an elevated temperature.

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, (LiNTf 2 ), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, ([EMIm] [NTf 2 ]), 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][NTf 2 ] shows a heterogeneous bimodal line shape associated with both water and ion hydrogen-bonded OD populations. However, H 2 O/LiNTf 2 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][NTf 2 ] solution. At a time-lapse of 0.1 ps, the 2D IR correlation spectrum in aqueous LiNTf 2 appears elongated along with the frequency diagonal. The spectral signature in the water− [EMIm][NTf 2 ] 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 LiNTf 2 solution might be engaged in the structural arrangement of the restricted IL environment. In contrast, water in aqueous [EMIm][NTf 2 ] 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.

We have investigated the liquid phase of an ionic liquid (IL), methylammonium formate (MAF), through the first principles molecular dynamics simulations using van der Waals (vdW) corrected exchange and correlation functionals of the density functional theory. The simulations were carried out to obtain a comparative study of various properties of the MAF using two different generalized gradient approximation functionals (Becke− Lee−Yang−Parr (BLYP) and Perdew−Burke−Ernzerhof (PBE)) along with three types of dispersion corrections (D2, D3, and dispersion-corrected atom-centered one-electron potentials), and two values of the plane-wave cutoff (300 and 600 Ry). We have evaluated the effects of various electronic parameters in describing the hydrogen-bonded structure and dynamical properties of MAF by performing 10 sets of molecular dynamics simulations. Thermodynamic properties are found to be sensitive to the details of electronic structure calculations. Our results of PBE functionals with the semiempirical vdW method provide the best agreement with experimental density. The overall density predictions match the cohesive energy trends, and the calculations incorporating dispersion forces exhibit enhanced intermolecular interactions within the hydrogen-bonded IL framework. All of the vdW-corrected BLYP functionals, mainly the dispersion-corrected atom-centered one-electron potential (DCACP) method, illustrate a well-defined structure of liquid MAF. To look into the dynamical perspective of the hydrogen-bond descriptions, we elucidate two possible mechanistic pathways of the hydrogen-bond jump events between the counterions. The hydrogen-bond breaking and forming mechanism along with the collision dynamics can be best described by incorporating dispersion interactions alongside the exchange and correlation functionals within the Kohn−Sham scheme. The rattling dynamics of ions are observed for dispersion-corrected functionals. Hence, an accurate representation of the delicately balanced interactive forces within ionic liquids is a necessary step toward a better description of its thermophysical and structural properties along with the associated ionic dynamics.

Water is believed to be a heterogeneous liquid comprising multiple density regions that arise because of the presence of interstitial molecules and can be differentiated by their structure as well as the existence of hydrogen-bonded pairs with varying strengths. First-principles molecular dynamics studies were performed at six different temperatures to investigate the effect of temperature on the thermophysical, structure, dynamics, and vibrational spectral properties of the water molecules using dispersion-corrected density functional theory. The variation of properties like density, cohesive energy, and compressibility with a change in temperature produces a trend that matches with the experiments and resembles the experimentally observed anomalous behavior. We explore the possibility of explaining the trends in calculated properties by analyzing the structure and dynamics of the water molecules in terms of instantaneous low- and instantaneous high-density regions that are found during the simulation time. The dynamics of these two types of water molecules were studied by calculating the lifetime from the proposed autocorrelation functions. The lifetime of formation of instantaneous low-density water is found to decrease with an increase in temperature, whereas the lifetime of instantaneous high-density water is found to be maximum at 298 K among all the considered temperatures. The presence of more interstitial water molecules is observed at this temperature. The signature of these water molecules is found in the radial distribution function, spatial distribution function, void distribution, configurational space, orientational dynamics, and spectral diffusion calculations. It is also found that around 298 K, these water molecules are present distinctively that mix up with the first and second solvation shells with the rise of the temperature. The outlook of the reported results can be extended to other thermodynamic conditions to explain some of the anomalous properties, which can be related to the presence of the interstitial molecules in water.

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.