Hydrogen Bonding and OH-Stretch Spectroscopy in Water: Hexamer (Cage), Liquid Surface, Liquid, and Ice

Probing the adsorption configuration of methanol at a charged air/aqueous interface using nonlinear spectroscopy

Investigating the effects of electrolyte ions on the adsorption configuration of methanol at a charged interface is important for studying the interface structure of electrolyte solutions and the oxidation mechanism of methanol in fuel cells. This study uses sum frequency generation (SFG) and heterodyne-detected second harmonic generation (HD-SHG) to investigate the adsorption configuration of methanol at the air/aqueous interface of 0.1 M NaClO4 solution, 0.1 M HClO4 solution and pure water. The results elucidate that the ion effect in the electrolyte solution affects the interface's charged state and the methanol's adsorption conformation at the interface. The negatively charged surface of the 0.1 M NaClO4 solution and the positively charged surface of the 0.1 M HClO4 solution arise from the corresponding specific ionic effects of the electrolyte solution. The orientation angle of methyl with respect to the surface normal is 43.4° ± 0.1° at the 0.1 M NaClO4 solution surface and 21.5° ± 0.2° at the 0.1 M HClO4 solution surface. Examining these adsorption configurations in detail, we find that at the negatively charged surface the inclined orientation angle (43.4°) of methanol favors the hydroxymethyl production by breaking the C-H bond, while at the positively charged surface the upright orientation angle (21.5°) of methanol promotes the methoxy formation by breaking the O-H bond. These findings not only illuminate the intricate ion effects on small organic molecules but also contribute to a molecular-level comprehension of the oxidation mechanism of methanol at electrode interfaces.

In Situ Probing the Structure Change and Interaction of Interfacial Water and Hydroxyl Intermediates on Ni(OH)2 Surface over Water Splitting

There is growing acknowledgment that the properties of the electrochemical interfaces play an increasingly pivotal role in improving the performance of the hydrogen evolution reaction (HER). Here, we present, for the first time, direct dynamic spectral evidence illustrating the impact of the interaction between interfacial water molecules and adsorbed hydroxyl species (OHad) on the HER properties of Ni(OH)2 using Au/core-Ni(OH)2/shell nanoparticle-enhanced Raman spectroscopy. Notably, our findings highlight that the interaction between OHad and interfacial water molecules promotes the formation of weakly hydrogen-bonded water, fostering an environment conducive to improving the HER performance. Furthermore, the participation of OHad in the reaction is substantiated by the observed deprotonation step of Au@2 nm Ni(OH)2 during the HER process. This phenomenon is corroborated by the phase transition of Ni(OH)2 to NiO, as verified through Raman and X-ray photoelectron spectroscopy. The significant redshift in the OH-stretching frequency of water molecules during the phase transition confirms that surface OHad disrupts the hydrogen-bond network of interfacial water molecules. Through manipulation of the shell thickness of Au@Ni(OH)2, we additionally validate the interaction between OHad and interfacial water molecules. In summary, our insights emphasize the potential of electrochemical interfacial engineering as a potent approach to enhance electrocatalytic performance.

Correlations between the Structures and Spectra of Protonated Water Clusters

Badger's rule-like correlations between OH stretching frequencies and intensities and the OH bond length are used to develop a spectral mapping procedure for studies of pure and protonated water clusters. This approach utilizes the vibrationally averaged OH bond lengths, which were obtained from diffusion Monte Carlo simulations that were performed using the general potential developed by Yu and Bowman. Good agreement is achieved between the spectra obtained using this approach and previously reported spectra for H+(H2O)n clusters, with n = 3, 4, and 5, as well as their perdeuterated analogues. The analysis of the spectra obtained by this spectral mapping approach supports previous work that assigned the spectrum of H+(H2O)6 to a mixture of Eigen and Zundel-like structures. Analysis of the calculated spectra also suggests a reassignment of the frequency of one of the transitions that involves the OH stretching vibration of the OH bonds in the hydronium core in the Eigen-like structure of H+(H2O)6 from 1917 cm-1 to roughly 2100 cm-1. For D+(D2O)6, comparison of the measured spectrum to those obtained by using the spectral mapping approach suggests that the carrier of the measured spectrum is one or more of the isomers of D+(D2O)6 that contain a four-membered ring and two flanking water molecules. While there are several candidate structures, the two flanking water molecules most likely form a chain that is bound to the hydronium core.

Selective extraction of anionic and cationic dyes using tailored hydrophobic deep eutectic solvents

In this work, eight kinds of hydrophobic deep eutectic solvents (DESs), including four types of ionic and four types of non-ionic DESs, were prepared and applied in vortex-assisted liquid-liquid microextraction (LLME) technology. To explore the extraction ability of the hydrophobic DESs-based vortex-assisted LLME, four types of dyes were selected as analytes, involving anionic tartrazine (TA), amaranth (AM) and cationic phenosafranine (PF), methylene blue (MB). It turned out that the ionic and non-ionic hydrophobic DESs showed selective extraction on anionic and cationic dyes, respectively. In particular, the extraction efficiency of TA could reach 99.3 % when trioctylmethylammonium chloride-thymol ([TMAC][Thy]) was utilized as extraction agent. The partitioning efficiency of PF was up to 99.9 % by using decanoic acid-thymol ([DecA][Thy]) as extraction agent. The limits of detection (LODs) of TA and PF were 0.06 and 0.14 μg mL-1, respectively. The limits of quantification (LOQs) obtained for TA and PF were 0.20 and 0.47 μg mL-1, respectively. Besides, FT-IR and 1H NMR were utilized to investigate the extraction mechanism. The results demonstrated that the hydrogen bonding and electrostatic force were the main driving forces in the extraction process. Furthermore, through separating various anionic and cationic dyes, the selective extraction ability of [TMAC][Thy] and [DecA][Thy] were successfully verified. Hence, the feasible operation, high extraction efficiency and excellent selectivity make the developed hydrophobic DESs-based vortex-assisted LLME attractive in dyes separation.

Appraisal of TIP4P-type models at water surface

In view of the current situation in which non-polarizable rigid water models have been scarcely examined against surface-specific properties, we appraise TIP4P-type models at the liquid water surface on the basis of heterodyne-detected sum frequency generation (HD-SFG) spectroscopy. We find in the HD-SFG spectrum of the water surface that the peak frequency of the hydrogen-bonded OH band, the half width at half maximum of the hydrogen-bonded OH band, and the full width at half maximum of the free OH band are best reproduced by TIP4P, TIP4P/Ew, and TIP4P/Ice, respectively, whereas it is already well known that TIP4P/2005 best reproduces the surface tension. These TIP4P-type models perform better at the water surface in terms of the present appraisal items than some polarizable models in the literature.

Exploring the Origins of the Intensity of the OH Stretch-HOH Bend Combination Band in Water

While the intensity of the OH stretching fundamental transition is strongly correlated to hydrogen-bond strength, the intensity of the corresponding transition to the state with one quantum of excitation in both the OH stretching and HOH bending vibrations in the same water molecule shows a much weaker sensitivity to the hydrogen-bonding environment. The origins of this difference are explored through analyses of the contributions of terms in the expansion of the dipole moment to the calculated intensity. It is found that the leading contribution to the stretch-bend intensity involves the second derivative of the dipole moment with respect to the OH bond length and HOH angle. While this is not surprising, the insensitivity of this derivative to the hydrogen-bonding environment is unexpected. Possible contributions of mode mixing are also explored. While mode mixing leads to splittings of the energies of nearly degenerate excited states, it does not result in significant changes in the sum of the intensities of these transitions. Analysis of changes in the partial charges on the hydrogen atoms upon displacement of the HOH angles shows that these charges generally increase with increasing HOH angle. This effect is partially canceled by a decrease in the charge of the hydrogen atom when a hydrogen bond is broken. The extent of this cancellation increases with the hydrogen bond strength, which is reflected in the observed insensitivity of the intensity of the stretch-bend transition to hydrogen-bond strength.

Enhanced Hydrogen Bonds of the (H2O)n (n = 4-8) Clusters Confined in Uranyl Peroxide Cluster Na20(UO2)20(O2)30

Water is a basic resource and an essential component of living organisms. It often exhibits some novel properties under confinement. The water clusters (H₂O)_n (n = 4-8) confined in the cavity of uranyl peroxide cluster Na₂₀(UO₂)₂₀(O₂)₃₀ (U₂₀) have been computationally investigated by using ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations in this study. The results show that the confined water clusters can form hydrogen bonds with the internal oxygen atoms (O_uranyl) of U₂₀, and their conformations changed significantly. The average lengths (2.553-2.645 Å) of hydrogen bonds in confined (H₂O)_n are shorter than those (2.731-2.841 Å) in the corresponding free water clusters. Moreover, these confined hydrogen bonds show better hydrogen bond patterns according to the quantified indices. The natural bond orbital (NBO) calculations determine that there is electron transferring from the U₂₀ to its interior (H₂O)_n. It is the main reason for enhancing hydrogen bond interactions among the confined water molecules because their oxygen atoms are more negatively charged and their hydrogen atoms are more positively charged. The quantum theory of atoms in molecules (QTAIM) and interacting quantum atoms (IQA) analyses indicate that the confined hydrogen bonds are more covalent, based on the significant electron density ρ(r) and local energy density H(r) at the bond critical points (BCPs), and the stronger energies of interatomic exchange interactions (V_xc). These findings may help to promote the communication of confined water clusters and enrich the understating of confined hydrogen bonds.

Ultrafast Water H-Bond Rearrangement in a Metal-Organic Framework Probed by Femtosecond Time-Resolved Infrared Spectroscopy

We investigated the water H-bond network and its dynamics in Ni₂Cl₂BTDD, a prototypical MOF for atmospheric water harvesting, using linear and ultrafast IR spectroscopy. Utilizing isotopic labeling and infrared spectroscopy, we found that water forms an extensive H-bonding network in Ni₂Cl₂BTDD. Further investigation with ultrafast spectroscopy revealed that water can reorient in a confined cone up to ∼50° within 1.3 ps. This large angle reorientation indicates H-bond rearrangement, similar to bulk water. Thus, although the water H-bond network is confined in Ni₂Cl₂BTDD, different from other confined systems, H-bond rearrangement is not hindered. The picosecond H-bond rearrangement in Ni₂Cl₂BTDD corroborates its reversibility with minimal hysteresis in water sorption.

Controlling Water Delivery to an Electrochemical Interface with Surfactants

Most electrochemical reactions require delivery of protons, often from water, to surface-adsorbed species. However, water also acts as a competitor to many such processes by directly reacting with the electrode, which necessitates using water in small amounts. Controlling the water content and structure near the surface is an important frontier in directing the reactivity and selectivity of electrochemical reactions. Surfactants accumulate near surfaces, and therefore, they can be used as agents to control interfacial water. Using mid-IR spectro-electrochemistry, we show that a modest concentration (1 mM) of the cationic surfactant CTAB in mixtures of 10 M water in an organic solvent (dDMSO) has a large effect on the interfacial water concentration, changing it by up to ∼35% in the presence of an applied potential. The major cause of water content change is displacement due to the accumulation or depletion of surfactants driven by potential. Two forces drive the surfactants to the electrode: the applied potential and the hydrophobic interactions with the water in the bulk. We have quantified their competition by varying the water content in the bulk. To our knowledge, for the first time, we have identified the electrochemical equivalent of the hydrophobic drive. For our system, a change in applied potential of 1 V has the same effect as adding a 0.55 mole fraction of water to the bulk. This work illustrates the significance of surfactants in the partitioning of water between the bulk and the surface and paves the way toward engineering interfacial water structures for controlling electrochemical reactions.

Infrared spectroscopic analysis of hydrogen-bonding interactions in cryopreservation solutions

BACKGROUND

In this study we investigated hydrogen bonding interactions in hydrated and frozen solutions of different cryoprotective agents (CPAs) including dimethyl sulfoxide, glycerol, ethylene glycol, propylene glycol, and trehalose. We also investigated the effect of CPAs on ice crystal growth during storage and correlated this with storage stability of liposomes.

METHODS

FTIR spectroscopy was used to study hydrogen bonding interactions in CPA solutions in H₂O and D₂O, and their thermal response was analyzed using van 't Hoff analysis. The effect of CPAs on ice crystal growth during storage was investigated by microscopy and correlated with storage stability of liposomes encapsulated with a fluorescent dye.

RESULTS

Principal component analyses demonstrated that different CPAs can be recognized based on the shape of the OD band region only. Chemically similar molecules such as glycerol and ethylene glycol closely group together in a principal component score plot, whereas trehalose and DMSO appear as condensed separated clusters. The OH/OD band of CPA solutions exhibits an overall shift to higher wavenumbers with increasing temperature and changed fractions of weak and strong hydrogen interactions. CPAs diminish ice crystal formation in frozen samples during storage and minimize liposome leakage during freezing but cannot prevent leakage during frozen storage.

CONCLUSIONS

CPAs can be distinguished from one another based on the hydrogen bonding network that is formed in solution. DMSO-water mixtures behave anomalous compared to other CPAs that have OH groups. CPAs modulate ice crystal formation during frozen storage but cannot prevent liposome leakage during frozen storage.

Hydrogen-Bond Configurations of Hydration Water around Glycerol Investigated by HOH Bending and OH Stretching Analysis

Toward a comprehensive understanding of the mechanism of glycerol as a moisturizer, studies on the hydrogen-bond (HB) structure of hydration water, which is known to be disordered by glycerol, are insufficient. To this aim, we evaluated the HB configurations based on the HOH bending and OH stretching spectra of the hydration water from those of glycerol/water mixtures by subtracting the contributions of bulk water and glycerol using dielectric relaxation spectroscopy. Analysis of the HOH bending band showed that hydration water-donating HBs lose the intermolecular bending coupling with increasing glycerol by replacing the water-water HBs with water-glycerol HBs. The OH stretching band provided more detailed insight into the HB configuration, indicating that the double-donor double-acceptor and double-donor single-acceptor configurations in bulk water change to a predominantly double-donor single-acceptor configuration in hydration water around glycerol. The formation of more donor HBs than acceptor HBs may be due to the steric constrains by glycerol and/or differences in the partial charge on the oxygen atom between water and glycerol.

Character of the OH Bend-Stretch Combination Band in the Vibrational Spectra of the "Magic" Number H3O+(H2O)20 and D3O+(D2O)20 Cluster Ions

The fundamental transitions that contribute to the diffuse OH stretching spectrum of water are known to increase in width and intensity with increasing red shift from the free OH frequency. In contrast, the profile of the higher-energy combination band involving the OH stretching and the intramolecular HOH bending modes displays a qualitatively different spectral shape with a much faster falloff on the lower-energy side. We elucidate the molecular origin of this difference by analyzing the shapes of the combination bands in the IR spectra of cryogenically cooled H₃O⁺(H₂O)₂₀ and D₃O⁺(D₂O)₂₀ clusters. The difference in the shapes of the bands is traced to differences in the dependence of their transition dipole matrix elements on the hydrogen-bonding environment. The fact that individual transitions across the combination band envelope have similar intensities makes it a useful way to determine the participation of various sites in extended H-bonding networks.

Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration

We study the bending mode of pure water and charged aqueous surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy. We observe a low (1626  cm^{-1}) and a high (1656  cm^{-1}) frequency component that can be unambiguously assigned to an interfacial dipole and a bulk quadrupolar response, respectively. We thus demonstrate that probing the bending mode provides structural and quantitative information on both the surface and the bulk.

The explicit role of electron exchange in the hydrogen bonded molecular complexes

We applied a set of advanced bonding descriptors to establish the hidden electron density features and binding energy characteristics of intermolecular DH∙∙∙A hydrogen bonds (OH∙∙∙O, NH∙∙∙O and SH∙∙∙O) in 150 isolated and solvated molecular complexes. The exchange-correlation and Pauli potentials as well as corresponding local one-electron forces allowed us to explicitly ascertain how electron exchange defines the bonding picture in the proximity of the H-bond critical point. The electron density features of DH∙∙∙A interaction are governed by alterations in the electron localization in the H-bond region displaying itself in the exchange hole. At that, they do not depend on the variations in the exchange hole mobility. The electrostatic interaction mainly defines the energy of H-bonds of different types, whereas the strengthening/weakening of H-bonds in complexes with varying substituents depends on the barrier height of the exchange potential near the bond critical point. Energy variations between H-bonds in isolated and solvated systems are also caused the electron exchange peculiarities as follows from the corresponding potential and the interacting quantum atom analyses complemented by electron delocalization index calculations. Our approach is based on the bonding descriptors associated with the characteristics of the observable electron density and can be recommended for in-depth studies of non-covalent bonding.

Demystifying the Diffuse Vibrational Spectrum of Aqueous Protons Through Cold Cluster Spectroscopy

The ease with which the pH is routinely determined for aqueous solutions masks the fact that the cationic product of Arrhenius acid dissolution, the hydrated proton, or H⁺(aq), is a remarkably complex species. Here, we review how results obtained over the past 30 years in the study of H⁺⋅(H₂O)_n cluster ions isolated in the gas phase shed light on the chemical nature of H⁺(aq). This effort has also revealed molecular-level aspects of the Grotthuss relay mechanism for positive-charge translocation in water. Recently developed methods involving cryogenic cooling in radiofrequency ion traps and the application of two-color, infrared-infrared (IR-IR) double-resonance spectroscopy have established a clear picture of how local hydrogen-bond topology drives the diverse spectral signatures of the excess proton. This information now enables a new generation of cluster studies designed to unravel the microscopic mechanics underlying the ultrafast relaxation dynamics displayed by H⁺(aq).

Isolating the Contributions of Specific Network Sites to the Diffuse Vibrational Spectrum of Interfacial Water with Isotopomer-Selective Spectroscopy of Cold Clusters

Decoding the structural information contained in the interfacial vibrational spectrum of water requires understanding how the spectral signatures of individual water molecules respond to their local hydrogen bonding environments. In this study, we isolated the contributions for the five classes of sites that differ according to the number of donor (D) and acceptor (A) hydrogen bonds that characterize each site. These patterns were measured by exploiting the unique properties of the water cluster cage structures formed in the gas phase upon hydration of a series of cations M⁺·(H₂O)_n (M = Li, Na, Cs, NH₄, CH₃NH₃, H₃O, and n = 5, 20-22). This selection of ions was chosen to systematically express the A, AD, AAD, ADD, and AADD hydrogen bonding motifs. The spectral signatures of each site were measured using two-color, IR-IR isotopomer-selective photofragmentation vibrational spectroscopy of the cryogenically cooled, mass selected cluster ions in which a single intact H₂O is introduced without isotopic scrambling, an important advantage afforded by the cluster regime. The resulting patterns provide an unprecedented picture of the intrinsic line shapes and spectral complexities associated with excitation of the individual OH groups, as well as the correlation between the frequencies of the two OH groups on the same water molecule, as a function of network site. The properties of the surrounding water network that govern this frequency map are evaluated by dissecting electronic structure calculations that explore how changes in the nearby network structures, both within and beyond the first hydration shell, affect the local frequency of an OH oscillator. The qualitative trends are recovered with a simple model that correlates the OH frequency with the network-modulated local electron density in the center of the OH bond.

Unusually strong hydrogen bond cooperativity in particular (H2O)20 clusters

Drawing upon an intuitive charge-transfer-based picture of hydrogen bonding, we demonstrate that cooperativity effects acting in concert can lead to unusually strong hydrogen bonds in neutral water clusters. The structure, vibrational, and NMR properties of a (H2O)20 pentagonal dodecahedron cluster containing such a strong hydrogen bond were studied using second-order perturbation theory and density functional theory. The hydrogen bond length was found to be shorter than 2.50 Å. A large redshift of over 2000 cm-1 with respect to the isolated water molecule was predicted for the OH stretching frequency of the donor water molecule. A large downfield shift to 13.5 ppm of the isotropic part of the 1H magnetic shielding tensor together with an unusually large shielding anisotropy of 49.9 ppm was obtained. The hydrogen bond energy was calculated using symmetry-adapted perturbation theory and was found to be more than three times stronger than a typical hydrogen bond in liquid water.

Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction

Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and interprotein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation. However, still, an all-encompassing theory that describes the vibrational solvatochromism, electrochromism, and dynamic fluctuation of vibrational frequencies has not been completely established mainly due to the intrinsic complexity of intermolecular interactions in condensed phases. In particular, the amount of data obtained from the linear and nonlinear vibrational spectroscopic experiments has been rapidly increasing, but the lack of a quantitative method to interpret these measurements has been one major obstacle in broadening the applications of these methods. Among various theoretical models, one of the most successful approaches is a semiempirical model generally referred to as the vibrational spectroscopic map that is based on a rigorous theory of intermolecular interactions. Recently, genetic algorithm, neural network, and machine learning approaches have been applied to the development of vibrational solvatochromism theory. In this review, we provide comprehensive descriptions of the theoretical foundation and various examples showing its extraordinary successes in the interpretations of experimental observations. In addition, a brief introduction to a newly created repository Web site (http://frequencymap.org) for vibrational spectroscopic maps is presented. We anticipate that a combination of the vibrational frequency map approach and state-of-the-art multidimensional vibrational spectroscopy will be one of the most fruitful ways to study the structure and dynamics of chemical, biological, and functional molecular systems in the future.

Capturing intrinsic site-dependent spectral signatures and lifetimes of isolated OH oscillators in extended water networks

The extremely broad infrared spectrum of water in the OH stretching region is a manifestation of how profoundly a water molecule is distorted when embedded in its extended hydrogen-bonding network. Many effects contribute to this breadth in solution at room temperature, which raises the question as to what the spectrum of a single OH oscillator would be in the absence of thermal fluctuations and coupling to nearby OH groups. We report the intrinsic spectral responses of isolated OH oscillators embedded in two cold (~20 K), hydrogen-bonded water cages adopted by the Cs⁺·(HDO)(D₂O)₁₉ and D₃O⁺·(HDO)(D₂O)₁₉ clusters. Most OH oscillators yield single, isolated features that occur with linewidths that increase approximately linearly with their redshifts. Oscillators near 3,400 cm^-1, however, occur with a second feature, which indicates that OH stretch excitation of these molecules drives low-frequency, phonon-type motions of the cage. The excited state lifetimes inferred from the broadening are considered in the context of fluctuations in the local electric fields that are available even at low temperature.

How cations affect the electric double layer and the rates and selectivity of electrocatalytic processes

Electrocatalysis is central to the production of renewable fuels and high-value commodity chemicals. The electrolyte and the electrode together determine the catalytic properties of the liquid/solid interface. In particular, the cations of the electrolyte can greatly change the rates and reaction selectivity of many electrocatalytic processes. For this reason, the careful choice of the cation is an essential step in the design of catalytic interfaces with high selectivity for desired high-value products. To make such a judicious choice, it is critical to understand where in the electric double layer the cations reside and the various distinct mechanistic impacts they can have on the electrocatalytic process of interest. In this perspective, we review recent advances in the understanding of the electric double layer with a particular focus on the interfacial distribution of cations and the cations' hydration states in the vicinity of the electrode under various experimental conditions. Furthermore, we summarize the different ways in which cations can alter the rates and selectivity of chemical processes at electrified interfaces and identify possible future areas of research in this field.

Hiding in the Crowd: Spectral Signatures of Overcoordinated Hydrogen-Bond Environments

Molecules with an excess number of hydrogen-bonding partners play a crucial role in fundamental chemical processes, ranging from anomalous diffusion in supercooled water to transport of aqueous proton defects and ordering of water around hydrophobic solutes. Here we show that overcoordinated hydrogen-bond environments can be identified in both the ambient and supercooled regimes of liquid water by combining experimental Raman multivariate curve resolution measurements and machine learning accelerated quantum simulations. In particular, we find that OH groups appearing in spectral regions usually associated with non-hydrogen-bonded species actually correspond to hydrogen bonds formed in overcoordinated environments. We further show that only these species exhibit a turnover in population as a function of temperature, which is robust and persists under both constant pressure and density conditions. This work thus provides a new tool to identify, interpret, and elucidate the spectral signatures of crowded hydrogen-bond networks.

Deconstructing water's diffuse OH stretching vibrational spectrum with cold clusters

The diffuse vibrational envelope displayed by water precludes direct observation of how different hydrogen-bond topologies dictate the spectral response of individual hydroxy group (OH) oscillators. Using cold, isotopically labeled cluster ions, we report the spectral signatures of a single, intact water (H₂O) molecule embedded at various sites in the clathrate-like cage structure adopted by the Cs⁺·(D₂O)₂₀ ion. These patterns reveal the site-dependent correlation between the frequencies of the two OH groups on the same water molecule and establish that the bound OH companion of the free OH group exclusively accounts for bands in the lower-energy region of the spectrum. The observed multiplet structures reveal the homogeneous linewidths of the fundamentals and quantify the anharmonic contributions arising from coupling to both the intramolecular bending and intermolecular soft modes.

Terahertz VRT Spectroscopy of the Water Hexamer-h12 Cage: Dramatic Libration-Induced Enhancement of Hydrogen Bond Tunneling Dynamics

We report the assignment and analysis of 176 transitions belonging to a librational band of the (H₂O)₆ cage isomer near 525 cm^-1(15 THz). From a fit of the transitions to an asymmetric top model, we observe both dramatic changes in the rotational constants relative to the ground state, indicating significant nonrigidity, and striking enhancement in the tunneling motions that break and reform the hydrogen bonds in the cluster. This is the fifth water cluster system to display such an enhancement in the 15 THz librational region, the details of which may help to elucidate the hydrogen bond dynamics occurring in bulk liquid water.

Decoding the spectroscopic features and time scales of aqueous proton defects

Acid solutions exhibit a variety of complex structural and dynamical features arising from the presence of multiple interacting reactive proton defects and counterions. However, disentangling the transient structural motifs of proton defects in the water hydrogen bond network and the mechanisms for their interconversion remains a formidable challenge. Here, we use simulations treating the quantum nature of both the electrons and nuclei to show how the experimentally observed spectroscopic features and relaxation time scales can be elucidated using a physically transparent coordinate that encodes the overall asymmetry of the solvation environment of the proton defect. We demonstrate that this coordinate can be used both to discriminate the extremities of the features observed in the linear vibrational spectrum and to explain the molecular motions that give rise to the interconversion time scales observed in recent nonlinear experiments. This analysis provides a unified condensed-phase picture of the proton structure and dynamics that, at its extrema, encompasses proton sharing and spectroscopic features resembling the limiting Eigen [H₃O(H₂O)₃]⁺ and Zundel [H(H₂O)₂]⁺ gas-phase structures, while also describing the rich variety of interconverting environments in the liquid phase.

Water at surfaces with tunable surface chemistries

Aqueous interfaces are ubiquitous in natural environments, spanning atmospheric, geological, oceanographic, and biological systems, as well as in technical applications, such as fuel cells and membrane filtration. Where liquid water terminates at a surface, an interfacial region is formed, which exhibits distinct properties from the bulk aqueous phase. The unique properties of water are governed by the hydrogen-bonded network. The chemical and physical properties of the surface dictate the boundary conditions of the bulk hydrogen-bonded network and thus the interfacial properties of the water and any molecules in that region. Understanding the properties of interfacial water requires systematically characterizing the structure and dynamics of interfacial water as a function of the surface chemistry. In this review, we focus on the use of experimental surface-specific spectroscopic methods to understand the properties of interfacial water as a function of surface chemistry. Investigations of the air-water interface, as well as efforts in tuning the properties of the air-water interface by adding solutes or surfactants, are briefly discussed. Buried aqueous interfaces can be accessed with careful selection of spectroscopic technique and sample configuration, further expanding the range of chemical environments that can be probed, including solid inorganic materials, polymers, and water immiscible liquids. Solid substrates can be finely tuned by functionalization with self-assembled monolayers, polymers, or biomolecules. These variables provide a platform for systematically tuning the chemical nature of the interface and examining the resulting water structure. Finally, time-resolved methods to probe the dynamics of interfacial water are briefly summarized before discussing the current status and future directions in studying the structure and dynamics of interfacial water.

The Interplay of Structure and Dynamics in the Raman Spectrum of Liquid Water over the Full Frequency and Temperature Range

While many vibrational Raman spectroscopy studies of liquid water have investigated the temperature dependence of the high-frequency O-H stretching region, few have analyzed the changes in the Raman spectrum as a function of temperature over the entire spectral range. Here, we obtain the Raman spectra of water from its melting to boiling point, both experimentally and from simulations using an ab initio-trained machine learning potential. We use these to assign the Raman bands and show that the entire spectrum can be well described as a combination of two temperature-independent spectra. We then assess which spectral regions exhibit strong dependence on the local tetrahedral order in the liquid. Further, this work demonstrates that changes in this structural parameter can be used to elucidate the temperature dependence of the Raman spectrum of liquid water and provides a guide to the Raman features that signal water ordering in more complex aqueous systems.

2D H-Bond Network as the Topmost Skin to the Air-Water Interface

We provide a detailed description of the structure of water at the interface with the air (liquid-vapor LV interface) from state-of-the-art DFT-based molecular dynamics simulations. For the first time, a two-dimensional (2D) H-bond extended network has been identified and fully characterized, demonstrating that interfacial water is organized into a 2D sheet with H-bonds oriented parallel to the instantaneous surface and following its spatial and temporal oscillations. By analyzing the nonlinear vSFG (vibrational sum frequency generation) spectrum of the LV interface in terms of layer-by-layer signal, we demonstrate that the 2D water sheet is solely responsible for the spectral signatures, hence providing the interfacial 3.5 Å thickness effectively probed in nonlinear interfacial spectroscopy. The 2D H-bond network unraveled here is the essential key to rationalize macroscopic properties of water-air interfaces, as demonstrated here for spectroscopy and the surface potential.

Dimension of discrete variable representation for mixed quantum/classical computation of three lowest vibrational states of OH stretching in liquid water

Three low-lying vibrational states of molecular systems are responsible for the signals of linear and third-order nonlinear vibrational spectroscopies. Theoretical studies based on mixed quantum/classical calculations provide a powerful way to analyze those experiments. A statistically meaningful result can be obtained from the calculations by solving the vibrational Schrödinger equation over many numbers of molecular configurations. The discrete variable representation (DVR) method is a useful technique to calculate vibrational eigenstates subject to an arbitrary anharmonic potential surface. Considering the large number of molecular configurations over which the DVR calculations are repeated, the calculations are desired to be optimized in balance between the cost and accuracy. We determine a dimension of the DVR method which appears to be optimum for the calculations of the three states of molecular vibrations with anharmonic strengths often found in realistic molecular systems. We apply the numerical technique to calculate the local OH stretching frequencies of liquid water, which are well known to be widely distributed due to the inhomogeneity in molecular configuration, and found that the frequencies of the 0-1 and 1-2 transitions are highly correlated. An empirical relation between the two frequencies is suggested and compared with the experimental data of nonlinear IR spectroscopies.

Water's dual nature and its continuously changing hydrogen bonds

A model is proposed for liquid water that is a continuum between the ordered state with predominantly tetrahedral coordination, linear hydrogen bonds and activated dynamics and a disordered state with a continuous distribution of multiple coordinations, multiple types of hydrogen bond, and diffusive dynamics, similar to that of normal liquids. Central to water's heterogeneous structure is the ability of hydrogen to donate to either one acceptor in a conventional linear hydrogen bond or to multiple acceptors as a furcated hydrogen. Linear hydrogen bonds are marked by slow, activated kinetics for hydrogen-bond switching to more crowded acceptors and sharp first peaks in the hydrogen-oxygen radial distribution function. Furcated hydrogens, equivalent to free, broken, dangling or distorted hydrogens, have barrierless, rapid kinetics and poorly defined first peaks in their hydrogen-oxygen radial distribution function. They involve the weakest donor in a local excess of donors, such that barrierless whole-molecule vibration rapidly swaps them between the linear and furcated forms. Despite the low number of furcated hydrogens and their transient existence, they are readily created in a single hydrogen-bond switch and free up the dynamics of numerous surrounding molecules, bringing about the disordered state. Hydrogens in the ordered state switch with activated dynamics to make the non-tetrahedral coordinations of the disordered state, which can also combine to make the ordered state. Consequently, the ordered and disordered states are both connected by diffusive dynamics and differentiated by activated dynamics, bringing about water's continuous heterogeneity.

Phosphate Ions Affect the Water Structure at Functionalized Membrane Surfaces

Antifouling surfaces improve function, efficiency, and safety in products such as water filtration membranes, marine vehicle coatings, and medical implants by resisting protein and biofilm adhesion. Understanding the role of water structure at these materials in preventing protein adhesion and biofilm formation is critical to designing more effective coatings. Such fouling experiments are typically performed under biological conditions using isotonic aqueous buffers. Previous studies have explored the structure of pure water at a few different antifouling surfaces, but the effect of electrolytes and ionic strength (I) on the water structure at antifouling surfaces is not well studied. Here sum frequency generation (SFG) spectroscopy is used to characterize the interfacial water structure at poly(ether sulfone) (PES) and two surface-modified PES films in contact with 0.01 M phosphate buffer with high and low salt (Ionic strength, I= 0.166 and 0.025 M, respectively). Unmodified PES, commonly used as a filtration membrane, and modified PES with a hydrophobic alkane (C18) and with a poly(ethylene glycol) (PEG) were used. In the low ionic strength phosphate buffer, water was strongly ordered near the surface of the PEG-modified PES film due to exclusion of phosphate ions and the creation of a surface potential resulting from charge separation between phosphate anions and sodium cations. However, in the high ionic strength phosphate buffer, the sodium and potassium chloride (138 and 3 mM, respectively) in the phosphate buffered saline screened this charge and substantially reduced water ordering. A much smaller water ordering and subsequent reduction upon salt addition was observed for the C18-modified PES, and little water structure change was seen for the unmodified PES. The large difference in water structuring with increasing ionic strength between widely used phosphate buffer and phosphate buffered saline at the PEG interface demonstrates the importance of studying antifouling coatings in the same aqueous environment for which they are designed. These results further suggest that strong long-range water structuring is limited in high ionic strength environments, such as within cells, facilitating chemical and biological reactions and processes.

Dielectric constant and low-frequency infrared spectra for liquid water and ice Ih within the E3B model

Two intrinsic difficulties in modeling condensed-phase water with conventional rigid non-polarizable water models are: reproducing the static dielectric constants for liquid water and ice Ih, and generating the peak at about 200 cm(-1) in the low-frequency infrared spectrum for liquid water. The primary physical reason for these failures is believed to be the missing polarization effect in these models, and consequently various sophisticated polarizable water models have been developed. However, in this work we pursue a different strategy and propose a simple empirical scheme to include the polarization effect only on the dipole surface (without modifying a model's intermolecular interaction potential). We implement this strategy for our explicit three-body (E3B) model. Our calculated static dielectric constants and low-frequency infrared spectra are in good agreement with experiment for both liquid water and ice Ih over wide temperature ranges, albeit with one fitting parameter for each phase. The success of our modeling also suggests that thermal fluctuations about local minima and the energy differences between different proton-disordered configurations play minor roles in the static dielectric constant of ice Ih. Our analysis shows that the polarization effect is important in resolving the two difficulties mentioned above and sheds some light on the origin of several features in the low-frequency infrared spectra for liquid water and ice Ih.