Combined electronic structure/molecular dynamics approach for ultrafast infrared spectroscopy of dilute HOD in liquid H2O and D2O

Drug binding disrupts chiral water structures in the DNA first hydration shell

Knowledge of how intermolecular interactions change hydration structures surrounding DNA will heighten understanding of DNA biology and advance drug development. However, probing changes in DNA hydration structures in response to molecular interactions and drug binding in situ under ambient conditions has remained challenging. Here, we apply a combined experimental and computational approach of chiral-selective vibrational sum frequency generation spectroscopy (chiral SFG) to probe changes of DNA hydration structures when a small-molecule drug, netropsin, binds the minor groove of DNA. Our results show that chiral SFG can detect water being displaced from the minor groove of DNA due to netropsin binding. Additionally, we observe that chiral SFG distinguishes between weakly and strongly hydrogen-bonded water hydrating DNA. Chiral SFG spectra show that netropsin binding, instead of displacing weakly hydrogen-bonded water, preferentially displaces water molecules strongly hydrogen-bonded to thymine carbonyl groups in the DNA minor groove, revealing the roles of water in modulating site-specificity of netropsin binding to duplex DNA rich in adenine-thymine sequences. The results convey the promise of chiral SFG to offer mechanistic insights into roles of water in drug development targeting DNA.

Effects of cations on the structure, dynamics and vibrational sum frequency generation spectroscopy of liquid/vapor interfaces of aqueous solutions of monovalent and divalent metal nitrates

We have employed molecular dynamics (MD) simulations and theoretical vibrational sum frequency generation spectroscopy (VSFG) to investigate the structure and interactions of water and ions at liquid/vapor interfaces of aqueous solutions of monovalent and divalent metal nitrates of NaNO3, Mg(NO3)2 and Ca(NO3)2. Structural properties, such as the number density profiles, average number of hydrogen bonds, slab radial distribution functions (SRDFs), and tetrahedral order parameter, are calculated to investigate the structure of water in bulk and at the interfaces. The ion pairing tendency is determined in terms of cation-anion SRDF and is found to be following the order Mg(NO3)2 < Ca(NO3)2 < NaNO3 both at the interfacial and bulk regions. An ionic double layer is found to be formed at the interface. A weak propensity of anions is found to be at the interface, while the cations are found to be present below the interfacial region. Three prominent features are observed in the VSFG spectrum of the liquid/vapor interfaces of metal nitrate solutions: a free (dangling) O-H peak at 3750 cm-1, a peak at 3589 cm-1 due to O-H groups hydrogen bonded to nitrates, and a broad peak at 3200-3500 cm-1 due to O-H modes hydrogen bonded to water. The charge density of cations affects the intensity of the 3200-3500 cm-1 peak. The presence of ions is found to have very little effect on the position and intensity of the dangling peak as compared to that for the neat water/vapor interface. The ionic double layers generate electric fields that reorganize and reorient the water molecules towards the vapor. This upward reorientation of water leads to a positive region in the VSFG spectrum of O-H modes which are hydrogen bonded to water in contrast to that of the neat water/vapor interface. The O-H groups hydrogen bonded to the nitrates are mostly oriented downwards and the strength of such hydrogen bonds is found to be weaker than those hydrogen bonded to water.

Theoretical study of the structure and vibrational sum frequency generation spectroscopy of liquid-vapor interface of aqueous acetic acid

We have investigated the liquid-vapor interface of aqueous acetic acid solution through calculations of the structure and vibrational sum frequency generation (VSFG) spectra of the interfaces for varying concentrations of the acetic acid (AA). Our findings reveal the surface propensity of the AA molecules. As the concentration of AA increases, more AA molecules are found to be present in the interfacial region. The AA molecules at the interface are found to be oriented in a manner where the hydrophobic part (methyl chain) is oriented toward the vapor phase and the hydrophilic part is pointed toward the liquid phase. The total VSFG spectrum reveals that the intensity of the peak of dangling OH groups of water around 3750 cm-1 decreases when the concentration of AA increases, while the intensity of the peak around 3550 cm-1 due to OH groups having hydrogen bonds between water and AA molecules increases. Furthermore, the latter peak is redshifted as the AA concentration increases, which is due to the partial cancellation between the positive response of water OH modes and the negative response of the OH modes of AA molecules. It is also noted that the AA molecules make the interfacial water more structured by making hydrogen bonds with its otherwise dangling OH modes at the surface.

Behavior of water at lipid/water interfaces upon phase transition of the lipid bilayer: Insights from 1D- and 2D-vibrational sum frequency generation spectral calculations from molecular dynamics simulations

We have investigated the structural and dynamical changes of the interfacial water near [1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine] (DMPC) lipid bilayer across various temperatures, ranging from 285 K (gel phase of lipid) to 320 K (liquid phase of lipid), through calculations of one-dimensional (1D) and two-dimensional (2D) vibrational sum frequency generation (VSFG) spectra from molecular dynamics simulations. The 1D-VSFG spectra show a broad positive peak in the hydrogen-bonded region, which means that water molecules are oriented upward toward the lipid bilayer. Although DMPC is a zwitterionic lipid, the negatively charged phosphate group primarily influences the orientation of the water molecules. The absence of a dangling peak in the 1D- and 2D-VSFG spectra shows that the water molecules form hydrogen bonds with the lipid headgroup atoms. The spectral diffusion timescales obtained from the 2D-VSFG metrics of the slope of the nodal line clearly reveal a dynamical crossover and exhibit Arrhenius behavior with different activation energies before and after the melting of the lipid bilayer. Apart from 2D-VSFG, the frequency fluctuation time correlation function also exhibits a dynamical crossover upon melting of the lipid bilayer.

A generalized van't Hoff relation for the temperature dependence of complex-valued nonlinear spectra

The temperature dependence of spectra can reveal important insights into the structural and dynamical behavior of the system being probed. In the case of linear spectra, this has been exploited to investigate the thermodynamic driving forces governing the spectral response. Indeed, the temperature derivative of a spectrum can be used to obtain effective energetic and entropic profiles as a function of the measured frequency. The former can further be used to predict the temperature-dependent spectrum via a van't Hoff relation. However, these approaches are not directly applicable to nonlinear, complex-valued spectra, such as vibrational sum-frequency generation (SFG) or two-dimensional infrared (2D-IR) photon echo spectra. Here, we show how the energetic and entropic driving forces governing such nonlinear spectra can be determined and used within a generalized van't Hoff relation to predict their temperature dependence. The central idea is to allow the underlying energetic profiles to themselves be complex-valued. We illustrate this approach for 2D-IR spectra of water and SFG spectra of the air-water interface and demonstrate the accuracy of the generalized van't Hoff relationship and its implications for the origin of temperature-dependent spectral changes.

Modeling of the Hydrogen Bond-Induced Frequency Shifts of the HOH and HOD Bending Modes of Water

The intramolecular bending mode of water is a possible useful probe of the hydrogen-bond situations in aqueous systems, but the behavior of its frequency and intensity should be further elucidated for better understanding on its nature and, hence, for its better utilization as a probe. Here, an analysis toward this goal is conducted by doing theoretical calculations on molecular clusters of normal isotopic and deuterated species of water and examining the correlations among the vibrational, structural, and electrostatic properties. It is shown that electrostatic interactions, particularly both of the in-plane components of the electric field along the OH bond and perpendicular to it, play a major role in controlling the hydrogen bond-induced shifts of the force constant, but additional factors, including the intermolecular structural and/or charge-transfer properties, are also important. Models of the hydrogen bond-induced shifts of the force constant are presented in a form that may be combined with classical molecular dynamics. With regard to the infrared intensity changes, it is shown on the basis of the electron density analysis that the intermolecular charge flux and polarization effect play an important role, depending on the angular characteristics of the hydrogen bond.

Effect of counterions on the structure and dynamics of water near a negatively charged surfactant: a theoretical vibrational sum frequency generation study

Charged aqueous interfaces are of paramount importance in electrochemical, biological and environmental sciences. The properties of aqueous interfaces with ionic surfactants can be influenced by the presence of counterions. Earlier experiments involving vibrational sum frequency generation (VSFG) spectroscopy of aqueous interfaces with negatively charged sodium dodecyl sulfate (Na+DS- or SDS) surfactants revealed that the hydrogen bonding strength of the interfacial water molecules follows a certain order when salts of monovalent and divalent cations are added. It is known that cations do not directly participate in hydrogen bonding with water molecules, rather they only influence the hydrogen bonded network through their electrostatic fields. In the current work, we have simulated the aqueous interfacial systems of sodium dodecyl sulfate in the presence of chloride salts of mono and divalent countercations. The electronic polarization effects on the ions are considered at a mean-field level within the electronic continuum correction model. Our calculations of the VSFG spectra show a blue shift in the presence of added countercations whose origin is traced to different relative contributions of water molecules from the solvation shells of the surfactant headgroups and the remaining water molecules in the presence of countercations. Furthermore, the cations shield the electric fields of the surfactant headgroups, which in turn influences the contributions of water molecules to the total VSFG spectrum. This shielding effect becomes more significant when divalent countercations are present. The dynamics of water molecules is found to be slower at the interface in comparison to the bulk. The interfacial depth dependence of various dynamical quantities shows that the interface is structurally and dynamically more heterogeneous at the microscopic level.

Development of a universal method for vibrational analysis of the terminal alkyne C≡C stretch

The terminal alkyne C≡C stretch has a large Raman scattering cross section in the "silent" region for biomolecules. This has led to many Raman tag and probe studies using this moiety to study biomolecular systems. A computational investigation of these systems is vital to aid in the interpretation of these results. In this work, we develop a method for computing terminal alkyne vibrational frequencies and isotropic transition polarizabilities that can easily and accurately be applied to any terminal alkyne molecule. We apply the discrete variable representation method to a localized version of the C≡C stretch normal mode. The errors of (1) vibrational localization to the terminal alkyne moiety, (2) anharmonic normal mode isolation, and (3) discretization of the Born-Oppenheimer potential energy surface are quantified and found to be generally small and cancel each other. This results in a method with low error compared to other anharmonic vibrational methods like second-order vibrational perturbation theory and to experiments. Several density functionals are tested using the method, and TPSS-D3, an inexpensive nonempirical density functional with dispersion corrections, is found to perform surprisingly well. Diffuse basis functions are found to be important for the accuracy of computed frequencies. Finally, the computation of vibrational properties like isotropic transition polarizabilities and the universality of the localized normal mode for terminal alkynes are demonstrated.

Theoretical basis for interpreting heterodyne chirality-selective sum frequency generation spectra of water

Chirality-selective vibrational sum frequency generation (chiral SFG) spectroscopy has emerged as a powerful technique for the study of biomolecular hydration water due to its sensitivity to the induced chirality of the first hydration shell. Thus far, water O-H vibrational bands in phase-resolved heterodyne chiral SFG spectra have been fit using one Lorentzian function per vibrational band, and the resulting fit has been used to infer the underlying frequency distribution. Here, we show that this approach may not correctly reveal the structure and dynamics of hydration water. Our analysis illustrates that the chiral SFG responses of symmetric and asymmetric O-H stretch modes of water have opposite phase and equal magnitude and are separated in energy by intramolecular vibrational coupling and a heterogeneous environment. The sum of the symmetric and asymmetric responses implies that an O-H stretch in a heterodyne chiral SFG spectrum should appear as two peaks with opposite phase and equal amplitude. Using pairs of Lorentzian functions to fit water O-H stretch vibrational bands, we improve spectral fitting of previously acquired experimental spectra of model β-sheet proteins and reduce the number of free parameters. The fitting allows us to estimate the vibrational frequency distribution and thus reveals the molecular interactions of water in hydration shells of biomolecules directly from chiral SFG spectra.

Theoretical Study of the Two-Dimensional Vibrational Sum Frequency Generation Spectroscopy of the Air-Water Interface at Varying Temperature and Its Connections to the Interfacial Structure and Dynamics

We performed a theoretical study of the temperature variation of two-dimensional vibrational sum frequency generation (2D-VSFG) spectra of the OH stretch modes at air-water interfaces in the mid-IR region. The calculations are performed at four different temperatures from 250 to 325 K by using a combination of techniques involving response function formalism of nonlinear spectroscopy, electronic structure calculations, and molecular dynamics simulations. Also, the calculations are performed for isotopically dilute solutions so that the intra- and intermolecular coupling between the vibrational modes of interest can be ignored. We have established the connections of temperature variation of various frequency- and time-dependent features of the calculated spectra to the changes in the underlying structure and dynamics of the interfaces. The results reveal that interfacial water is dynamically more heterogeneous than bulk water, with three dominant dynamical processes exhibiting their corresponding time-dependent features in the 2D-VSFG spectrum. These are the spectral diffusion of hydrogen-bonded OH groups at the interface, conversion of an initially hydrogen-bonded OH group to a dangling OH which is a stable state for surface water, unlike the bulk water, and the third one, which involves the conversion of an initially free or dangling OH group to its hydrogen-bonded state at the interface. The temporal appearance of the cross peaks corresponding to interconversion of the hydrogen-bonded state to the dangling state or vice versa of an interfacial OH group is found to take place at a slower rate than the dynamics of spectral diffusion of hydrogen-bonded molecules at the interface, which, in turn, is slower than the corresponding spectral diffusion of bulk water molecules. The temperature variation of these dynamic processes can be linked to the decay of appropriate hydrogen-bond and non-hydrogen-bond time correlation functions of interfacial water molecules for the different air-water systems studied in this work.

Experimental Access to Mode-Specific Coupling between Quantum Molecular Vibrations and Classical Bath Modes

The interaction of quantum-mechanical systems with a fluctuating thermal environment (bath) is fundamental to molecular mechanics and energy transport/dissipation. Its complete picture requires mode-specific measurements of this interaction and an understanding of its nature. Here, we present a combined experimental and theoretical study providing detailed insights into the coupling between a high-frequency vibrational two-level system and thermally excited terahertz modes. Experimentally, two-dimensional terahertz-infrared-visible spectroscopy reports directly on the coupling between quantum oscillators represented by CH3 streching vibrations in liquid dimethyl sulfoxide and distinct low-frequency modes. Theoretically, we present a mixed quantum-classical formalism of the sample response to enable the simultaneous quantum description of high-frequency oscillators and a classical description of the bath. We derive the strength and nature of interaction and find different coupling between CH3 stretch and low-frequency modes. This general approach enables quantitative and mode-specific analysis of coupled quantum and classical dynamics in complex chemical systems.

Structural evolution of water-in-propylene carbonate mixtures revealed by polarized Raman spectroscopy and molecular dynamics

The liquid structure of systems wherein water is limited in concentration or through geometry is of great interest in various fields such as biology, materials science, and electrochemistry. Here, we present a combined polarized Raman and molecular dynamics investigation of the structural changes that occur as water is added incrementally to propylene carbonate (PC), a polar, aprotic solvent that is important in lithium-ion batteries. Polarized Raman spectra of PC solutions were collected for water mole fractions 0.003 ≤ χwater ≤ 0.296, which encompasses the solubility range of water in PC. The novel approach taken herein provides additional hydrogen bond and solvation characterization of this system that has not been achievable in previous studies. Analysis of the polarized carbonyl Raman band in conjunction with simulations demonstrated that the bulk structure of the solvent remained unperturbed upon the addition of water. Experimental spectra in the O-H stretching region were decomposed through Gaussian fitting into sub-bands and comparison to studies of dilute HOD in D2O. With the aid of simulations, we identified these different bands as water arrangements having different degrees of hydrogen bonding. The observed water structure within PC indicates that water tends to self-aggregate, forming a hydrogen bond network that is distinctly different from the bulk and dependent on concentration. For example, at moderate concentrations, the most likely aggregate structures are chains of water molecules, each with two hydrogen bonds.

Counteracting Effects of Trimethylamine N-Oxide against Urea in Aqueous Solutions: Insights from Theoretical Two-Dimensional Infrared Spectroscopy

The study of small osmolytes in their aqueous solutions has gained significant attention because of their relevance to structure and thermodynamics of proteins in aqueous media. Special attention has been given to the binary and ternary aqueous solutions of urea and trimethylamine N-oxide (TMAO). Urea is a well-known protein denaturant, while TMAO protects proteins in their native states. Interestingly, TMAO counteracts urea's ability to denature proteins when present in solutions with approximately half of the concentration of urea. Vibrational spectroscopy can improve our understanding of the molecular origin of this counteracting effect because of its sensitivity to local structure and dynamics. We present results of theoretical linear vibrational and two-dimensional infrared (2DIR) spectroscopy of water in the binary and ternary aqueous solutions of TMAO and urea. The 2DIR spectra are calculated using the electronic structure/molecular dynamics approach. The non-Condon effects in spectral transitions are incorporated in the theoretical calculations of 2DIR spectra. It is found that TMAO disrupts the local structure of water, while urea leaves it essentially unaffected. The 2DIR results show that both TMAO and urea slow down the dynamics of spectral diffusion of water. The extent of slowing down is found to be particularly significant for both hydration and bulk water in the presence of TMAO which can be attributed to strong hydrogen bonds between the water and TMAO molecules. The water molecules present in the hydration layer of the solutes in the ternary solutions are found to relax at even slower rates compared to that in their binary solutions in water. The hydrogen bonds between TMAO and urea are found to be not stable. Thus, the counteracting effect of TMAO against urea is seen to take place mainly through water-mediated interactions. Such TMAO-induced effects giving rise to more structured and slower hydrogen-bonded network are successfully captured through 2DIR spectroscopic calculations.

Detecting Interplay of Chirality, Water, and Interfaces for Elucidating Biological Functions

Chemists have long been fascinated by chirality, water, and interfaces, making tremendous progress in each research area. However, the chemistry emerging from the interplay of chirality, water, and interfaces has been difficult to study due to technical challenges, creating a barrier to elucidating biological functions at interfaces. Most biopolymers (proteins, DNA, and RNA) fold into macroscopic chiral structures to perform biological functions. Their folding requires water, but water behaves differently at interfaces where the bulk water hydrogen-bonding network terminates. A question arises as to how water molecules rearrange to minimize free energy at interfaces while stabilizing the macroscopic folding of biopolymers to support biological function. This question is central to solving many research challenges, including the molecular origin of biological homochirality, folding and insertion of proteins into cell membranes, and the design of heterogeneous biocatalysts. Researchers can resolve these challenges if they have the theoretical tools to accurately predict molecular behaviors of water and biopolymers at various interfaces. However, developing such tools requires validation by the experimental data. These experimental data are scarce because few physical methods can simultaneously distinguish chiral folding of the biopolymers, separate signals of interfaces from the overwhelming background of bulk solvent, and differentiate water in hydration shells of the polymers from water elsewhere.We recently illustrated these very capacities of chirality-sensitive vibrational sum frequency generation spectroscopy (chiral SFG). While chiral SFG theory dictates that the method is surface-specific under the condition of electronic nonresonance, we show the method can distinguish chiral folding of proteins and DNA and probe water structures in the first hydration shell of proteins at interfaces. Using amide I signals, we observe protein folding into β-sheets without background signals from α-helices and disordered structures at interfaces, thereby demonstrating the effect of 2D crowding on protein folding. Also, chiral SFG signals of C-H stretches are silent from single-stranded DNA, but prominent for canonical antiparallel duplexes as well as noncanonical parallel duplexes at interfaces, allowing for sensing DNA secondary structures and hybridization. In establishing chiral SFG for detecting protein hydration structures, we observe an H218O isotopic shift that reveals water contribution to the chiral SFG spectra. Additionally, the phase of the O-H stretching bands flips when the protein chirality is switched from L to D. These experimental results agree with our simulated chiral SFG spectra of water hydrating the β-sheet protein at the vacuum-water interface. The simulations further reveal that over 90% of the total chiral SFG signal comes from water in the first hydration shell. We conclude that the chiral SFG signals originate from achiral water molecules that assemble around the protein into a chiral supramolecular structure with chirality transferred from the protein. As water O-H stretches can reveal hydrogen-bonding interactions, chiral SFG shows promise in probing the structures and dynamics of water-biopolymer interactions at interfaces. Altogether, our work has created an experimental and computational framework for chiral SFG to elucidate biological functions at interfaces, setting the stage for probing the intricate chemical interplay of chirality, water, and interfaces.

Calculating Vibrational Excited State Absorptions with Excited State Constrained Minimized Energy Surfaces

The modeling and interpretation of vibrational spectra are crucial for studying reaction dynamics using vibrational spectroscopy. Most prior theoretical developments focused on describing fundamental vibrational transitions while fewer developments focused on vibrational excited state absorptions. In this study, we present a new method that uses excited state constrained minimized energy surfaces (CMESs) to describe vibrational excited state absorptions. The excited state CMESs are obtained similarly to the previous ground state CMES development in our group but with additional wave function orthogonality constraints. Using a series of model systems, including the harmonic oscillator, Morse potential, double-well potential, quartic potential, and two-dimensional anharmonic potential, we demonstrate that this new procedure provides good estimations of the transition frequencies for vibrational excited state absorptions. These results are significantly better than those obtained from harmonic approximations using conventional potential energy surfaces, demonstrating the promise of excited state CMES-based methods for calculating vibrational excited state absorptions in real systems.

Characterizing Interfaces by Voronoi Tessellation

The chemistry of interfaces differs markedly from that of the bulk. Calculation of interfacial properties depends strongly on the definition of the interface, which can lead to ambiguous results that vary between studies. There is a need for a method that can explicitly define the interfaces and boundaries in molecular systems. Voronoi tessellation offers an attractive solution to this problem through its ability to determine neighbors among specified groups of atoms. Here we discuss three cases where Voronoi tessellation combined with modeling of vibrational sum frequency generation (SFG) spectroscopy yields relevant insights: the breakdown of the air-water interface into clear and intuitive molecular layers, the study of the hydration shell in biological systems, and the acceleration of difficult spectral calculations where intermolecular vibrational couplings dominate. The utility of Voronoi tessellation has broad applications that extend beyond any single type of spectroscopy or system.

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.

Theoretical Two Dimensional Infrared Spectroscopy of Aqueous Solutions of tert-Butyl Alcohol: Variation of the Dynamics of Spectral Diffusion along the Percolation Transition

Binary mixtures of water and tert-butyl alcohol (TBA) are known to exhibit the so-called percolation transition where small clusters of TBA molecules span into large aggregates beyond a threshold concentration of the alcohol. In the present study, we have investigated the linear and two-dimensional infrared spectral features of aqueous solutions of TBA for varying concentration of the alcohol along the percolation transition. The percolation transition is characterized through calculations of intermolecular radial distribution functions and average size of the largest cluster of TBA molecules. It is found that, with variation of alcohol concentration, the radial distribution functions of the central carbon atoms of TBA molecules show a nonmonotonic change in the height of the first peak and also the size of the largest cluster of TBA molecules show a jump in the increase of its size for TBA mole fraction between 0.04 and 0.06 corresponding to a transition from smaller clusters to larger spanning aggregates. However, it is found that the linear infrared spectrum of water does not exhibit any noticeable changes on variation of TBA concentration along the percolation transition. Subsequently, two-dimensional infrared (2DIR) spectra and vibrational frequency time correlation function of water are calculated for all the TBA-water solutions considered in this study. The spectral diffusion of water calculated from 2DIR is found to slow down with increase of the TBA concentration. The time scales of spectral diffusion of water, as characterized by the relaxation of frequency time correlation function, 2DIR metric of central line slope, and also the hydrogen bond time correlation functions, are found to exhibit a noticeable jump along the percolation transition. The hydrophilic group of TBA is found to retard the water dynamics more effectively than the hydrophobic groups. Also, the jump in the dynamical slowdown along the percolation transition is found to be more significant for water molecules at the hydrophilic sites.

Design of an Electrostatic Frequency Map for the NH Stretch of the Protein Backbone and Application to Chiral Sum Frequency Generation Spectroscopy

We develop an electrostatic map for the vibrational NH stretch (amide A) of the protein backbone with a focus on vibrational chiral sum frequency generation spectroscopy (chiral SFG). Chiral SFG has been used to characterize protein secondary structure at interfaces using the NH stretch and to investigate chiral water superstructures around proteins using the OH stretch. Interpretation of spectra has been complicated because the NH stretch and OH stretch overlap spectrally. Although an electrostatic map for water OH developed by Skinner and co-workers was used previously to calculate the chiral SFG response of water structures around proteins, a map for protein NH that is directly responsive to biological complexity has yet to be developed. Here, we develop such a map, linking the local electric field to vibrational frequencies and transition dipoles. We apply the map to two protein systems and achieve much better agreement with experiment than was possible in our previous studies. We show that couplings between NH and OH vibrations are crucial to the line shape, which informs the interpretation of chiral SFG spectra, and that the chiral NH stretch response is sensitive to small differences in structure. This work increases the utility of the NH stretch in biomolecular spectroscopy.

Temperature Dependence of Non-Condon Effects in Two-Dimensional Vibrational Spectroscopy of Water

Non-Condon effects in vibrational spectroscopy refers to the dependence of a molecule's vibrational transition dipole and polarizability on the coordinates of the surrounding environment. Earlier studies have shown that such effects can be pronounced for hydrogen-bonded systems like liquid water. Here, we present a theoretical study of two-dimensional vibrational spectroscopy under the non-Condon and Condon approximations at varying temperatures. We have performed calculations of both two-dimensional infrared and two-dimensional vibrational Raman spectra to gain insights into the temperature dependence of non-Condon effects in nonlinear vibrational spectroscopy. The two-dimensional spectra are calculated for the OH vibration of interest in the isotopic dilution limit where the coupling between the oscillators is ignored. Generally, both the infrared and Raman line shapes undergo red shifts with decrease in temperature due to strengthening of hydrogen bonds and decrease in the fraction of OH modes with weaker or no hydrogen bonds. The infrared line shape is further red-shifted under the non-Condon effects at a given temperature, while the Raman line shape does not show any such red shift due to non-Condon effects. The spectral dynamics becomes slower on decrease of temperature due to slower hydrogen bond relaxation and, for a given temperature, the spectral diffusion occurs at a faster rate upon inclusion of non-Condon effects. The time scales of spectral diffusion extracted from different metrics agree well with each other and also with experiments. The changes in the spectrum due to non-Condon effects are found to be more significant at lower temperatures.

Direct calculation of the temperature dependence of 2D-IR spectra: Urea in water

A method for directly calculating the temperature derivative of two-dimensional infrared (2D-IR) spectra from simulations at a single temperature is presented. The approach is demonstrated by application to the OD stretching spectrum of isotopically dilute aqueous (HOD in H₂O) solutions of urea as a function of concentration. Urea is an important osmolyte because of its ability to denature proteins, which has motivated significant interest in its effect on the structure and dynamics of water. The present results show that the temperature dependence of both the linear IR and 2D-IR spectra, which report on the underlying energetic driving forces, is more sensitive to urea concentration than the spectra themselves. Additional physical insight is provided by calculation of the contributions to the temperature derivative from different interactions, e.g., water-water, water-urea, and urea-urea, present in the system. Finally, it is demonstrated how 2D-IR spectra at other temperatures can be obtained from only room temperature simulations.

Quantum-chemical calculation of two-dimensional infrared spectra using localized-mode VSCF/VCI

Computational protocols for the simulation of two-dimensional infrared (2D IR) spectroscopy usually rely on vibrational exciton models which require an empirical parameterization. Here, we present an efficient quantum-chemical protocol for predicting static 2D IR spectra that does not require any empirical parameters. For the calculation of anharmonic vibrational energy levels and transition dipole moments, we employ the localized-mode vibrational self-consistent field (L-VSCF)/vibrational configuration interaction (L-VCI) approach previously established for (linear) anharmonic theoretical vibrational spectroscopy [P. T. Panek and C. R. Jacob, ChemPhysChem 15, 3365-3377 (2014)]. We demonstrate that with an efficient expansion of the potential energy surface using anharmonic one-mode potentials and harmonic two-mode potentials, 2D IR spectra of metal carbonyl complexes and dipeptides can be predicted reliably. We further show how the close connection between L-VCI and vibrational exciton models can be exploited to extract the parameters of such models from those calculations. This provides a novel route to the fully quantum-chemical parameterization of vibrational exciton models for predicting 2D IR spectra.

Detecting the First Hydration Shell Structure around Biomolecules at Interfaces

Understanding the role of water in biological processes remains a central challenge in the life sciences. Water structures in hydration shells of biomolecules are difficult to study in situ due to overwhelming background from aqueous environments. Biological interfaces introduce additional complexity because biomolecular hydration differs at interfaces compared to bulk solution. Here, we perform experimental and computational studies of chiral sum frequency generation (chiral SFG) spectroscopy to probe chirality transfer from a protein to the surrounding water molecules. This work reveals that chiral SFG probes the first hydration shell around the protein almost exclusively. We explain the selectivity to the first hydration shell in terms of the asymmetry induced by the protein structure and specific protein-water hydrogen-bonding interactions. This work establishes chiral SFG as a powerful technique for studying hydration shell structures around biomolecules at interfaces, presenting new possibilities to address grand research challenges in biology, including the molecular origins of life.

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.

Shining (Infrared) Light on the Hofmeister Series: Driving Forces for Changes in the Water Vibrational Spectra in Alkali-Halide Salt Solutions

The Hofmeister series is frequently used to rank ions based on their behavior from chaotropes ("structure breakers"), which weaken the surrounding hydrogen-bond network, to kosmotropes ("structure makers"), which enhance it. Here, we use fluctuation theory to investigate the energetic and entropic driving forces underlying the Hofmeister series for aqueous alkali-halide solutions. Specifically, we exploit the OH stretch infrared (IR) spectrum in isotopically dilute HOD/D₂O solutions as a probe of the effect of the salt on the water properties for different concentrations and choice of halide anion. Fluctuation theory is used to calculate the temperature derivative of these IR spectra, including decomposition of the derivative into different energetic contributions. These contributions are used to determine the thermodynamic driving forces in terms of effective internal energy and entropic contributions. This analysis implicates entropic contributions as the key factor in the Hofmeister series behavior of the OH stretch IR spectra, while the effective internal energy is nearly ion-independent.

Probing electrolyte–silica interactions through simulations of the infrared spectroscopy of nanoscale pores

The structural and dynamical properties of nanoconfined solutions can differ dramatically from those of the corresponding bulk systems. Understanding the changes induced by confinement is central to controlling the behavior of synthetic nanostructured materials and predicting the characteristics of biological and geochemical systems. A key outstanding issue is how the molecular-level behavior of nanoconfined electrolyte solutions is reflected in different experimental, particularly spectroscopic, measurements. This is addressed here through molecular dynamics simulations of the OH stretching infrared (IR) spectroscopy of NaCl, NaBr, and NaI solutions in isotopically dilute HOD/D2O confined in hydroxylated amorphous silica slit pores of width 1–6 nm and pH [Formula: see text]. In addition, the water reorientation dynamics and spectral diffusion, accessible by pump–probe anisotropy and two-dimensional IR measurements, are investigated. The aim is to elucidate the effect of salt identity, confinement, and salt concentration on the vibrational spectra. It is found that the IR spectra of the electrolyte solutions are only modestly blue-shifted upon confinement in amorphous silica slit pores, with both the size of the shift and linewidth increasing with the halide size, but these effects are suppressed as the salt concentration is increased. This indicates the limitations of linear IR spectroscopy as a probe of confined water. However, the OH reorientational and spectral diffusion dynamics are significantly slowed by confinement even at the lowest concentrations. The retardation of the dynamics eases with increasing salt concentration and pore width, but it exhibits a more complex behavior as a function of halide.

Field-Induced Electron Generation in Water: Solvation Dynamics and Many-Body Interactions

The solvated electron represents an elementary quantum system in a liquid environment. Electrons solvated in water have raised strong interest because of their prototypical properties, their role in radiation chemistry, and their relevance for charge separation and transport. Nonequilibrium dynamics of photogenerated electrons in water occur on ultrafast time scales and include charge transfer, localization, and energy dissipation processes. We present new insight into the role of fluctuating electric fields of the liquid for generating electrons in the presence of an external terahertz field and address polaronic many-body properties of solvated electrons. This Perspective combines a review of recent results from experiment and theory with a discussion of basic electric interactions of electrons in water.

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.

Effects of Stearyl Alcohol Monolayer on the Structure, Dynamics and Vibrational Sum Frequency Generation Spectroscopy of Interfacial Water

The structure, dynamics and vibrational spectroscopy of water surface covered by a monolayer of stearyl alcohol (STA) are investigated by means of molecular dynamics simulations and vibrational sum frequency generation...

The mutual interactions of RNA, counterions and water - quantifying the electrostatics at the phosphate-water interface

The structure and dynamics of polyanionic biomolecules, like RNA, are decisively determined by their electric interactions with the water molecules and the counterions in the environment. The solvation dynamics of the biomolecules involves a subtle balance of non-covalent and many-body interactions with structural fluctuations due to thermal motion occurring in a femto- to subnanosecond time range. This complex fluctuating many particle scenario is crucial in defining the properties of biological interfaces with far reaching significance for the folding of RNA structures and for facilitating RNA-protein interactions. Given the inherent complexity, suited model systems, carefully calibrated and benchmarked by experiments, are required to quantify the relevant interactions of RNA with the aqueous environment. In this feature article we summarize our recent progress in the understanding of the electrostatics at the biological interface of double stranded RNA (dsRNA) and transfer RNA (tRNA). Dimethyl phosphate (DMP) is introduced as a viable and rigorously accessible model system allowing the interaction strength with water molecules and counterions, their relevant fluctuation timescales and the spatial reach of interactions to be established. We find strong (up to ≈90 MV cm-1) interfacial electric fields with fluctuations extending up to ≈20 THz and demonstrate how the asymmetric stretching vibration νAS(PO2)- of the polarizable phosphate group can serve as the most sensitive probe for interfacial interactions, establishing a rigorous link between simulations and experiment. The approach allows for the direct interfacial observation of interactions of biologically relevant Mg2+ counterions with phosphate groups in contact pair geometries via the rise of a new absorption band imposed by exchange repulsion interactions at short interatomic distances. The systematic extension to RNA provides microscopic insights into the changes of the hydration structure that accompany the temperature induced melting of the dsRNA double helix and quantify the ionic interactions in the folded tRNA. The results show that pairs of negatively charged phosphate groups and Mg2+ ions represent a key structural feature of RNA embedded in water. They highlight the importance of binding motifs made of contact pairs in the electrostatic stabilization of RNA structures that have a strong impact on the surface potential and enable the fine tuning of the local electrostatic properties which are expected to be relevant for mediating the interactions between biomolecules.

Computational spectroscopy of complex systems

Numerous linear and non-linear spectroscopic techniques have been developed to elucidate structural and functional information of complex systems ranging from natural systems, such as proteins and light-harvesting systems, to synthetic systems, such as solar cell materials and light-emitting diodes. The obtained experimental data can be challenging to interpret due to the complexity and potential overlapping spectral signatures. Therefore, computational spectroscopy plays a crucial role in the interpretation and understanding of spectral observables of complex systems. Computational modeling of various spectroscopic techniques has seen significant developments in the past decade, when it comes to the systems that can be addressed, the size and complexity of the sample types, the accuracy of the methods, and the spectroscopic techniques that can be addressed. In this Perspective, I will review the computational spectroscopy methods that have been developed and applied for infrared and visible spectroscopies in the condensed phase. I will discuss some of the questions that this has allowed answering. Finally, I will discuss current and future challenges and how these may be addressed.

Electric fields and potentials in condensed phases

The electric fields and potentials inside and at the interface of matter are relevant to many branches of physics, chemistry, and biology. Accurate quantification of these fields and/or potentials is essential to control and exploit chemical and physical transformations. Before we understand the response of matter to external fields, it is first important to understand the intrinsic interior and interfacial fields and potentials, both classically and quantum mechanically, as well as how they are probed experimentally. Here we compare and contrast, beginning with the hydrogen atom in vacuum and ending with concentrated aqueous NaCl electrolyte, both classical and quantum mechanical electric potentials and fields. We make contact with experimental vibrational Stark, electrochemical, X-ray, and electron spectroscopic probes of these potentials and fields, outline relevant conceptual difficulties, and underscore the advantage of electron holography as a basis to better understand electrostatics in matter.

Coupled Local-Mode Approach for the Calculation of Vibrational Spectra: Application to Protonated Water Clusters

Spectroscopic studies of protonated water clusters (PWCs) have yielded enormous insights into the fundamental nature of the hydrated proton. Here, we introduce a new coupled local-mode (CLM) approach to calculate PWC OH stretch vibrational spectra. The CLM method combines a sampling of representative configurations from density functional theory (DFT)-based ab initio molecular dynamics (AIMD) simulations with DFT calculations of local-mode vibrational frequencies and couplings. Calculations of inhomogeneous OH stretch vibrational spectra for H⁺(H₂O)₄ and H⁺(H₂O)₂₁ agree well with experiment and higher-level calculations, and decompositions of the calculated spectra in terms of the coupled modes aids in the interpretation of the spectra. This observation is consistent with the idea that capturing anharmonicity and coupling is as important to accuracy as the underlying level of electronic structure theory. The CLM calculations can easily discern the configuration that dominates the experimental measurement for H⁺(H₂O)₅, which can adopt several low-energy conformations.

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.

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.

Computing the frequency fluctuation dynamics of highly coupled vibrational transitions using neural networks

The description of frequency fluctuations for highly coupled vibrational transitions has been a challenging problem in physical chemistry. In particular, the complexity of their vibrational Hamiltonian does not allow us to directly derive the time evolution of vibrational frequencies for these systems. In this paper, we present a new approach to this problem by exploiting the artificial neural network to describe the vibrational frequencies without relying on the deconstruction of the vibrational Hamiltonian. To this end, we first explored the use of the methodology to predict the frequency fluctuations of the amide I mode of N-methylacetamide in water. The results show good performance compared with the previous experimental and theoretical results. In the second part, the neural network approach is used to investigate the frequency fluctuations of the highly coupled carbonyl stretch modes for the organic carbonates in the solvation shell of the lithium ion. In this case, the frequency fluctuation predicted by the neural networks shows a good agreement with the experimental results, which suggests that this model can be used to describe the dynamics of the frequency in highly coupled transitions.

An electrostatic energy-based charge model for molecular dynamics simulation

The interactions of the polar chemical bonds such as C=O and N-H with an external electric field were investigated, and a linear relationship between the QM/MM interaction energies and the electric field along the chemical bond is established in the range of weak to intermediate electrical fields. The linear relationship indicates that the electrostatic interactions of a polar group with its surroundings can be described by a simple model of a dipole with constant moment under the action of an electric field. This relationship is employed to develop a general approach to generating an electrostatic energy-based charge (EEC) model for molecules containing single or multiple polar chemical bonds. Benchmark test studies of this model were carried out for (CH₃)₂-CO and N-methyl acetamide in explicit water, and the result shows that the EEC model gives more accurate electrostatic energies than those given by the widely used charge model based on fitting to the electrostatic potential (ESP) in direct comparison to the energies computed by the QM/MM method. The MD simulations of the electric field at the active site of ketosteroid isomerase based on EEC demonstrated that EEC gave a better representation of the electrostatic interaction in the hydrogen-bonding environment than the Amber14SB force field by comparison with experiment. The current study suggests that EEC should be better suited for molecular dynamics study of molecular systems with polar chemical bonds such as biomolecules than the widely used ESP or RESP (restrained ESP) charge models.

Simulations of the IR and Raman spectra of water confined in amorphous silica slit pores

Water in nano-scale confining environments is a key element in many biological, material, and geological systems. The structure and dynamics of the liquid can be dramatically modified under these conditions. Probing these changes can be challenging, but vibrational spectroscopy has emerged as a powerful tool for investigating their behavior. A critical, evolving component of this approach is a detailed understanding of the connection between spectroscopic features and molecular-level details. In this paper, this issue is addressed by using molecular dynamics simulations to simulate the linear infrared (IR) and Raman spectra for isotopically dilute HOD in D₂O confined in hydroxylated amorphous silica slit pores. The effect of slit-pore width and hydroxyl density on the silica surface on the vibrational spectra is also investigated. The primary effect of confinement is a blueshift in the frequency of OH groups donating a hydrogen bond to the silica surface. This appears as a slight shift in the total (measurable) spectra but is clearly seen in the distance-based IR and Raman spectra. Analysis indicates that these changes upon confinement are associated with the weaker hydrogen-bond accepting properties of silica oxygens compared to water molecules.

On the role of hydrogen-bond exchanges in the spectral diffusion of water

The dynamics of a vibrational frequency in a condensed phase environment, i.e., the spectral diffusion, has attracted considerable interest over the last two decades. A significant impetus has been the development of two-dimensional infrared (2D-IR) photon-echo spectroscopy that represents a direct experimental probe of spectral diffusion, as measured by the frequency-frequency time correlation function (FFCF). In isotopically dilute water, which is perhaps the most thoroughly studied system, the standard interpretation of the longest timescale observed in the FFCF is that it is associated with hydrogen-bond exchange dynamics. Here, we investigate this connection by detailed analysis of both the spectral diffusion timescales and their associated activation energies. The latter are obtained from the recently developed fluctuation theory for the dynamics approach. The results show that the longest timescale of spectral diffusion obtained by the typical analysis used cannot be directly associated with hydrogen-bond exchanges. The hydrogen-bond exchange time does appear in the decay of the water FFCF, but only as an additional, small-amplitude (<3%) timescale. The dominant contribution to the long-time spectral diffusion dynamics is considerably shorter than the hydrogen-bond exchange time and exhibits a significantly smaller activation energy. It thus arises from hydrogen-bond rearrangements, which occur in between successful hydrogen-bond partner exchanges, and particularly from hydrogen bonds that transiently break before returning to the same acceptor.

Role of Intermolecular Charge Fluxes in the Hydrogen-Bond-Induced Frequency Shifts of the OH Stretching Mode of Water

The relation between the vibrational properties and the electrostatic situations of the vibrating functional group is useful to predict vibrational spectroscopic features based on, for example, classical molecular dynamics of liquids or biomolecular systems, but to pursue its generality or the extent of applicability, it is required to understand the mechanisms giving rise to it. Here such an analysis is carried out for the OH stretching mode of water. By examining the correlations among various (structural, vibrational, and electrostatic) properties and by analyzing the spatial characteristics of the behavior of electrons occurring upon the vibration, it is shown that the dependence of the vibrational frequency and the dipole derivative of the OH stretching mode on the electric field is not of purely electrostatic origin, and the delocalized electronic motions occurring with this mode, called intermolecular charge fluxes, related to both the dipole first and second derivatives play important roles.

Evaluating Simple Ab Initio Models of the Hydrated Electron: The Role of Dynamical Fluctuations

Despite its importance in electron transfer reactions and radiation chemistry, there has been disagreement over the fundamental nature of the hydrated electron, such as whether or not it resides in a cavity. Mixed quantum/classical simulations of the hydrated electron give different structures depending on the pseudopotential employed, and ab initio models of computational necessity use small numbers of water molecules and/or provide insufficient statistics to compare to experimental observables. A few years ago, Kumar et al. (J. Phys. Chem. A 2015, 119, 9148) proposed a minimalist ab initio model of the hydrated electron with only a small number of explicitly treated water molecules plus a polarizable continuum model (PCM). They found that the optimized geometry had four waters arranged tetrahedrally around a central cavity, and that the calculated vertical detachment energy and radius of gyration agreed well with experiment, results that were largely independent of the level of theory employed. The model, however, is based on a fixed structure at 0 K and does not explicitly incorporate entropic contributions or the thermal fluctuations that should be associated with the room-temperature hydrated electron. Thus, in this paper, we extend the model of Kumar et al. by running Born-Oppenheimer molecular dynamics (BOMD) of a small number of water molecules with an excess electron plus PCM at room temperature. We find that when thermal fluctuations are introduced, the level of theory chosen becomes critical enough when only four waters are used that one of the waters dissociates from the cluster with certain density functionals. Moreover, even with an optimally tuned range-separated hybrid functional, at room temperature the tetrahedral orientation of the 0 K first-shell waters is entirely lost and the central cavity collapses, a process driven by the fact that the explicit water molecules prefer to make H-bonds with each other more than with the excess electron. The resulting average structure is quite similar to that produced by a noncavity mixed quantum/classical model, so that the minimalist 4-water BOMD models suffer from problems similar to those of noncavity models, such as predicting the wrong sign of the hydrated electron's molar solvation volume. We also performed BOMD with 16 explicit water molecules plus an extra electron and PCM. We find that the inclusion of an entire second solvation shell of explicit water leads to little change in the outcome from when only four waters were used. In fact, the 16-water simulations behave much like those of water cluster anions, in which the electron localizes at the cluster surface, showing that PCM is not acceptable for use in minimalist models to describe the behavior of the bulk hydrated electron. For both the 4- and 16-water models, we investigate how the introduction of thermal motions alters the predicted absorption spectrum, vertical detachment energy, and resonance Raman spectrum of the simulated hydrated electron. We also present a set of structural criteria that can be used to numerically determine how cavity-like (or not) a particular hydrated electron model is. All of the results emphasize that the hydrated electron is a statistical object whose properties are inadequately captured using only a small number of explicit waters, and that a proper treatment of thermal fluctuations is critical to understanding the hydrated electron's chemical and physical behavior.

Temperature Dependence of the Water Infrared Spectrum: Driving Forces, Isosbestic Points, and Predictions

The temperature derivative of the infrared (IR) spectrum of HOD/D₂O is directly calculated from simulations at a single temperature using a fluctuation theory approach. It is demonstrated, on the basis of an energetic decomposition of the derivative, that the blue shift with increasing temperature is associated with the competition between electrostatic and Lennard-Jones interactions. The same competition gives rise, where their contributions cancel, to a near isosbestic point. The derivative is further used to define an effective internal energy (and entropy) associated with the IR spectrum, and it is shown how a van't Hoff relation can be used to accurately predict the spectrum over a wide range of temperatures. These predictions also explain why a precise isosbestic point is not observed.

Two-Dimensional Infrared Spectroscopy of Aqueous Solutions of Metal Nitrates: Slowdown of Spectral Diffusion in the Presence of Divalent Cations

The hydrogen-bonded network of water can be affected both structurally and dynamically by the presence of ions. In the present study, we have considered three aqueous solutions of metal nitrates to investigate the effects of divalent cations (Mg²⁺ and Ca²⁺), compared to that of monovalent Na⁺ ions, on hydrogen-bond fluctuations and vibrational spectral diffusion through calculations of linear and two-dimensional infrared spectra of these solutions at room temperature. We have employed the methods of molecular dynamics simulations using effective polarizable models of ions combined with quantum mechanical calculations of transition variables and statistical mechanical calculations of spectral response functions of vibrational spectroscopy. Divalent cations are found to have much stronger and longer-ranged effects on the structure and dynamics of the hydrogen-bonded network than that induced by the monovalent sodium ions. The blue shifts in the calculated linear spectra are found to follow the Hofmeister trend for the cations. The 2D-IR spectral lineshape and intensity corresponding to three-pulse echo peak shift (3PEPS) experiments are calculated. The timescales of these nonlinear spectral responses and also frequency-time correlations show significant slowing down of spectral diffusion for solutions containing divalent Mg²⁺ and Ca²⁺ ions compared to the corresponding dynamics of the solution containing Na⁺ ions. Unlike NaNO₃ solution, the relaxation of frequency and dipole orientational fluctuations of anion-bound water in Mg(NO₃)₂ and Ca(NO₃)₂ solutions are found to be somewhat slower than bulk water, which can be attributed to the presence of divalent cations whose effects go beyond their first solvation shells. This is also seen in the dynamics of bulk water in these solutions which is found to be notably slower for the solutions containing divalent cations than that in the NaNO₃ solution. Unlike Mg²⁺ and Ca²⁺ ions, no specific cationic effect is observed for the Na⁺ ions.