Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses)


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Large ions do not exclude hydrophobes because large ions do not bind waters so tightly. Therefore, adding large ions to solution does not drive increased hydrophobe concentration in the remaining solution volume. Hydrophobes are excluded around small ions. Hydrophobic solutes gold insert more readily into the inner solvation shells of large ions than small ions.

It shows how the solvation-shell waters are structured at different ion separations. When the two ions are separated by about one layer of water, the bridging waters between them will be structured by multiple interactions. Each bridging water interacts with other bridging waters through hydrogen bonding, and each bridging water interacts with each ion through its water dipole.

When the two mobile ions come into contact, the ion—ion electrostatics can also contribute substantially to the free energy. Extensive computer simulations using different water models show that the shape of the PMF depends on all these factors. The resulting free energy from the sum of the factors can be quite different for ions of different sizes and shapes. The water structure around an ion pair depends on the cation—anion distance. At large separations, each ion has its own solvation shell.

At intermediate separations, the ion pair is stabilized by bridging waters. Ion—ion contacts of opposite charge are stabilized by electrostatic attractions, in addition to the water forces. There is an interesting puzzle of ion pairing. Some salts are more soluble in water than others. When a salt is composed of a small anion and a small cation, say LiF, it is relatively insoluble in water. When a salt is composed of a big anion and big cation, say CsI, it is also relatively insoluble in water. But, when a salt is composed of a small ion and a large ion, say CsF, then it is relatively soluble.

Collins explained this through his law of matching water affinities. Two small ions stick together their contact state is most stable because their ion—ion charge attractions dominate the energetics. Two large ions stick together because they act like hydrophobes since their charge interactions are weak, because the two ions cannot come sterically close together. The reason this state is stable is because the small ion attracts a water cage around it for electrostatic reasons, and the large ion is compatible with a water cage for hydrophobic reasons. The water structure around an ion pair depends on the size of each ion.

Water is more electrostricted around small ions. Therefore, the properties of aqueous solutions of even the simplest ions result from a subtle balance of geometry, hydrogen bonding, and charge interactions. When additional forces are also involved, such as when ions are near curved surfaces or protein binding sites, it can further tip that balance. In these cases, charge and geometry are less complicating factors. Water is commonly in contact with surfaces.

Examples include water permeating through granular or porous or supramolecular structures or gels, or inside crowded biological cells, or bound to proteins or DNA, or at interfaces with air or oils. For example, since water cannot form hydrogen bonds to hydrophobic surfaces, water tends to move away from them toward locations most compatible with forming water—water hydrogen bonding instead.

Water molecules orient at interfaces to favor hydrogen bonding. Air above liquid water acts like a hydrophobic surface.

In order to minimize the loss of H-bonding interactions, interfacial water will tend to orient such that either a single proton or electron lone pair points toward the air. Hydrophobes in water tend to concentrate at air—water interfaces. This is because hydrophobes tend to localize wherever they are able to break the fewest water—water hydrogen bonds.

Surfaces have a lower density of water—water hydrogen bonds than bulk water has. For example, consider acetic acid in a solution with water at a bulk pH equal to the p K a i. In the bulk, there will be equal concentrations of the protonated and deprotonated forms. According to semiempirical simulations by Hummer et al. Its high mobility in nanotubes is because water does not break or make hydrogen bonds to the nanotube walls as it flows.

However, other studies show the opposite; namely that water flow is retarded in certain kind of nanotubes. The hydrogen bonding between two waters or between water and the surface depends on the size and shape of the confinement. Depending on the diameter, as well as their interior surface characteristics, nanotubes may exhibit ion selectivity, similar to that of the ion channels, which is attributed mostly to the formation of ion hydration shells.

What happens if water is squeezed in only a single direction, such as between two smooth plates? Sometimes ice itself is the constraining vessel. Other molecules can be captured within ices, in the form of clathrate hydrates. Clathrate hydrates are ice cages that encapsulate small molecules, typically hydrocarbons. The shape and size of the caged solutes, as well as the pressure and temperature, will direct the type of clathrate structure adopted by the surrounding water.

Clathrate ice structures can act as cages for gas molecules. Among the most notable clathrate hydrates is methane clathrate. Clathrate hydrates can be problematic commercially because they limit gas and oil extraction by clogging up the transport of hydrocarbons through pipelines. Despite this troublesome aspect, their large natural abundance and high methane density mean methane clathrates represent an enormous potential energy source. Under the proper conditions, clathrate hydrates can be grown, often by doping ice using a hydrogen-bonding solute that can catalyze hydrate formation through the uptake of surrounding gas solutes.

Growing such clathrate hydrates also provides an avenue for finding new structures of pure-water ices. Solutes can be extracted from clathrate hydrates by vacuum evacution to empty the ice cages, leading to new potential phases of pure ice. Not unexpectedly, it is more delicate than the unevacuated Ne clathrate, collapsing at temperatures above K. Cooling a liquid slowly causes it to freeze to a stable crystalline state. But cooling a liquid rapidly can lead to kinetically trapped states that are glassy , i.

In water, the characteristic relaxation times become of the order of s and the rate of change of the volume and entropy decreases abruptly but continuously to that of a crystalline solid. But, such simulations have been challenging because the time scales are very slow—15 orders of magnitude slower than in normal liquid water. But, even as glassy materials go, water is unusual. He defines fragile glass formers as having curved lines, where the viscosity is insensitive to temperature in the hot liquid and strongly dependent on temperature in the cold liquid.

Strong glass formers are regarded as having memory of their molecular arrangements above their glass transition. Fragile liquids forget their glass structure much faster upon crossing the glass transition into the liquid phase. Water is more complicated than either of these behaviors, because of the complexity of structure in the supercooled liquid state.

The high-density and low-density components of water each have different viscosity characteristics. In simple liquids and in hot water , increasing the pressure increases the viscosity and decreases the diffusivity because applying pressure leads to crowding of the molecules, making their motion more sluggish. However, cold water behaves differently. At temperatures below K the viscosity of water decreases with increasing pressure. In the case of cold water, applying pressure shifts water structures, crunching cage-like waters into van der Waals clusters.

This breakage of hydrogen bonding frees up waters to move faster. Small molecules and ions diffuse through liquids. Their diffusion rates typically depend on the radius of the diffusant and the viscosity of the solvent. However, when the liquid is water, there is a remarkable exception. Ionic diffusion in water plays crucial roles in processes in biology e. Water ionization is a key component of aqueous acid—base chemistry.

Hydronium makes strong hydrogen bonds, which can influence more than surrounding waters, and can form hydronium chains. In liquid water, protons are hydrated. Recent computer simulations give additional insights. First, it is found that the Eigen and Zundel cations are only limiting ideal structures, and that there are delocalized defects, giving a broader distribution of structures. Fluctuations driving the autoionization of water involve the concerted motion of many atoms. The Grotthuss mechanism A of proton propagation involves three consecutive steps 1—3.

In contrast, a cooperative motion of the water wire B that results in a concerted motion of three protons is shown. Water molecules can diffuse rapidly through nanotubes. It has been suggested that an excess of proton charge defects near the entrance of dry hydrophobic carbon nanotube can aid the loading of water. A wetting mechanism of this type can be important in biological systems, for example in understanding the hydration of hydrophobic protein pores, where the charge defect is, for instance, created by peripheral amino acid residue deprotonation.

To create new technologies for producing clean water, reclaiming polluted water, predicting climate and weather, inventing green chemistry, separating chemicals and biomolecules, and designing new drugs to cure diseases requires an ever deeper understanding of water structure—property relationships. It requires ever better models of various types and at different levels. Modeling is needed that can handle water that contains salts and oils and biomolecules, often in high concentrations, in the presence of complex and structured media and surfaces, and often under different conditions of temperature and pressure.

Semiempirical models of water have generated insights and quantitative predictions over a broad spectrum of inquiries. But, there are opportunities for the future. We need improved water models for solvating ions of high charge density. We need continued work in polarizable models. We need to go beyond current fixed-charge models, if we are to study pH or acid—base behavior, because protons cannot dissociate in present models. There is also great value in improved analytical and semianalytical and coarse-grained models.

They can give insights into principles; they give ways to explore dependences on variables such as temperatures, pressures, and concentrations; and they should be able to give computationally efficient ways to address engineering questions in complex systems. Moreover, combining methods can also be valuable: quantum plus semiempirical modeling for bond-breaking reactions, or coarse-grained plus atomistic semiempirical models for noncovalent changes in large biomolecular complexes, for example.

Anomalous properties of water arise from the cage-like features of its molecular organization, arising from the tetrahedral hydrogen bonding among neighboring molecules. The challenge in modeling is due to the coupling between rotational and translational freedom of neighboring molecules. We thank Thomas M. Truskett, Nico F. We thank Sarina Bromberg for help with some figures. Emiliano Brini received his M. He is currently a postdoc with Ken A. His research focuses on solvation and protein physics.

Christopher J. Fennell received his Ph. He is currently an assistant professor of chemistry at Oklahoma State University with a research focus in computational and theoretical chemistry. Marivi Fernandez-Serra obtained her B. She received her Ph. Currently she is an associate professor of physics and has been at Stony Brook University since Her research is in the field of computational condensed matter physics.

She develops and applies methods to study the atomic and electronic dynamics of complex materials. One of her main research areas is the study of fundamental properties of liquid water using quantum mechanical simulations. In her group they apply their methods to study the interface between water and functional elements such as electrodes, photocatalytic semiconductors, and oxide materials. In — she did her postdoctoral research with Ken A. Dill at the University of California, San Francisco.

Her research is in the fields of statistical mechanical theories of water and hydration, partly quenched systems, solutions of poly electrolytes, and intermolecular interactions and their role in the stability of protein solutions. In he obtained his Ph. His research interests are in statistical mechanical theory of water and hydration, partly quenched systems, solutions of poly electrolytes, and lately in electrochromic cell devices.

Ken A. Dill received S. Flory at Stanford University. His research has been on water statistical mechanics, the protein-folding problem, aspects of nonequilibrium statistical physics, and biophysical mechanisms and evolution of cells. For studying water that contacts large or complex surfaces, efficient computational methods are needed. Several fast approximate models, described below, have been useful.

For modeling how water molecules interact with solutes, a simple and computationally inexpensive strategy is to treat water as a miniature unstructured continuous medium. For solutes having multiple chemical moieties, the total solvation free energy is assumed to be the sum of free energies of the component moieties.

A different continuum approximation is useful for treating the solvation of ions or polar molecules. When two charged objects interact with each other in a polarizable medium, such as liquid water, their interactions are weakened by the screening effect that results from the effect of the ions polarizing the medium.

Implicit-solvent models treat water as a continuum, rather than as individual molecules. Implicit-solvent models treat water as having a dielectric constant, which reduces the ion—ion interaction energy. For solute molecules that have both nonpolar and charged moieties, another type of additivity relationship is used.

Hydrogen Bonding

One type of implicit-solvent model is the Poisson or Poisson—Boltzmann approach. In the presence of mobile charges , such as dissolved salt ions—which are free to distribute around the fixed charges—the Poisson—Boltzmann equation is solved for the electrostatic potential due to both fixed and mobile charges. As implicit-solvent models go, solving the Poisson equation can be computationally expensive. A less computationally expensive way to approximate the screening effect of water around charges is the generalized Born model. In the case where a solute molecule can be modeled as several charges embedded in spheres, and the separation between them is sufficiently large compared to the radii, the solvation free energy of a molecule can be given by a sum of individual Born terms, and pairwise Coulomb terms.

GB simulations tend to be faster than those of PB. To achieve similar accuracies, GB models are often parametrized on a corresponding PB model by choosing proper effective atomic radii. The advantage of continuum models is their computational speed. Therefore, they are often applied in modeling large or complex solutes, such as biomolecules, or multicomponent systems, such as in chemical separations.

A compromise between explicit-solvent and continuum models are the integral-equation theories of molecular liquids. They aim to combine a relatively detailed molecular description with relatively low computational cost. The fundamental relationship for the integral equation theories is the Ornstein—Zernike OZ integral equation which, for a m -component system, is Here, c r is the direct correlation function.

To solve the OZ equation, another relation is needed between the functions c r and h r , called a closure relation. Different closures are used for different problems. One such approach to solvation in aqueous solutions is the reference interaction site model RISM , proposed by Chandler and Andersen. The RISM theory has been used in combination with the hypernetted-chain HNC closure to study the solvation of monatomic solutes alkali halides and argon-like particles in aqueous solutions, , small peptides, and polar organic molecules.

Further developments led to the 3D-RISM model , in which a set of three-dimensional 3D integral equations one equation for each solvent site was derived by integrating over the orientational degrees of freedom. Improved methods to predict hydration free energies and entropies of small drug-like molecules are now also available. SEA semi-explicit assembly is a model that aims to compute solvation free energies using the physics of semiempirical models, such as TIP3P, but at much faster computational speeds. SEA is as fast as implicit-solvent models, because of its free-energy additivities in the assembly step.

While SEA is only as accurate as its underlying explicit-solvent model, nevertheless its accuracy stems, in part, from its intrinsic capture of water—water multibody effects in the presimulations. The water-accessible surface depends on the charge on the solute gray. In the spirit of the SEA approach, i-PMF is a method for fast and accurate calculations of the potential of mean force PMF between pairs of charged or uncharged solutes in water. The i-PMF method is divided into presimulation and runtime steps. In the presimulation step, extensive MD simulations are performed with a classical force field to compute the PMFs of various pairs of solutes in the solvent.

These computed PMFs are compiled into tables. Next, interpolations are performed to fill in additional solute radii and separations r. At runtime, a PMF can be computed rapidly for particles of arbitrary charges, interaction energies, and radii. How can we evaluate and improve computational models of solvation? Different methods have different errors and different computational speeds.

A community-wide mechanism has recently been developed for blind testing of solvation models. In the SAMPL event, a set of small-molecule often drug-like structures is provided to the predictor community, who then uses their various methods to predict the solvation free energies. Experimental measurements are then performed subsequently, and the prior blind predictions of the different groups are then evaluated comparatively. The total amount of water on Earth is around 1. Of this, only around 0. Table 3 lists selected physicochemical properties of liquid water.

The crystal structures and densities of various ice forms are listed in Table 4. The parameters for some water molecular models are listed in Table 5. Different water models used in Table 5. The water—water pair interaction potential u ww for non-polarizable pointcharge water models is calculated using the parameters from Table 5 through eq 4 :. The calculated physicochemical properties of some water models are listed in Table 6.

National Center for Biotechnology Information , U. Chemical Reviews. Chem Rev. Published online Sep Author information Article notes Copyright and License information Disclaimer. Received May 8. This is an open access article published under an ACS AuthorChoice License , which permits copying and redistribution of the article or any adaptations for non-commercial purposes. This article has been cited by other articles in PMC. Open in a separate window. Water Is Essential for Life All forms of life depend on water.

Water Is a Basic Human Need. Figure 1. Water Is Crucial for Industrial Processes Almost every manufactured product uses water in at least one part of its production process. Figure 2. Figure Figure 3. Figure 4. Water Is Often Modeled through Semiempirical Classical Simulations Using Atomistic Potentials Beginning around , a popular approach has been to model water using semiempirical classical i.

Figure 5. Water Is More Cohesive than Simpler Liquids, due to Its Hydrogen Bonding Liquid water tends to be a more cohesive than other simple liquids, because water—water attractions arise from hydrogen bonding in addition to van der Waals interactions that are typical in simpler liquids. Figure 6. Figure 7. Figure 8. Figure 9. Water Forms Solvation Structures around Ions 8.

Cold and Supercooled Water Diffusion and Viscosity Depend on the Relative Population of High and Low Density Water In simple liquids and in hot water , increasing the pressure increases the viscosity and decreases the diffusivity because applying pressure leads to crowding of the molecules, making their motion more sluggish. Challenges for Improving Water Models To create new technologies for producing clean water, reclaiming polluted water, predicting climate and weather, inventing green chemistry, separating chemicals and biomolecules, and designing new drugs to cure diseases requires an ever deeper understanding of water structure—property relationships.

Appendix A. Modeling Large Complex Solutes Requires Approximations and Efficient Computational Methods For studying water that contacts large or complex surfaces, efficient computational methods are needed. Modeling Water as a Continuum: Surface Tension, Born and Poisson Models For modeling how water molecules interact with solutes, a simple and computationally inexpensive strategy is to treat water as a miniature unstructured continuous medium. Born and Generalized Born Implicit-Solvent Models of Water A less computationally expensive way to approximate the screening effect of water around charges is the generalized Born model.

Table 2 Global Distribution of Water on Earth a. Data collected from refs 2 and 3. Selected Physicochemical Properties of Liquid Water Table 3 lists selected physicochemical properties of liquid water. See also Table 1. Data collected from ref Parameters for Some Water Models The parameters for some water molecular models are listed in Table 5.

Calculated Physicochemical Properties of Some Water Models The calculated physicochemical properties of some water models are listed in Table 6. Notes The authors declare no competing financial interest. Author Status C. References Coonfield T.. Geological Survey. Shiklomanov I. NMR , 36 , 1— Water Resources Group. United Nations. Water; Environmental Outlook to Water Res. Water and Conflict. Security , 18 , 79— Water Conflict Chronology List; Masahiro M.. Global Water Budget; Seinfeld J.

Springer Theses

Water Pollution; Petrie B. Applications of Nanotechnology in Water and Wastewater Treatment. KGaA: Innovations and Green Chemistry. Kuchment A. Drilling for Earthquakes. Data , 31 , — Statistical Thermodynamics for Chemists and Biochemists ; Springer: ; pp — Water: A Comprehensive Treatise.

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IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters

Volume 1. Molecular Theory of Water and Aqueous Solutions. Water Structure and Science; Sun C. The Nature of the Chemical Bond , 2nd ed. Molecular Association in Liquids. A Theory of the Structure of Water. London, Ser. A , , — B , , — Anomalous Nuclear Quantum Effects in Ice. Faraday Soc. Molecular Dynamics Study of Liquid Water. Aspects of the Statistical-Mechanical Theory of Water.

Theory and Molecular Models for Water. Inherent Structure in Water. Quantum and Statistical Mechanical Studies of Liquids. Application to Liquid Water. In Intermolecular Forces ; Pullman B. On the Time Required to Freeze Water. A simple model for the intermolecular potential of water. Dynamical properties of the soft sticky dipole model of water: Molecular dynamics simulations. A temperature of maximum density in soft sticky dipole water. On the structural and transport properties of the soft sticky dipole and related single point water models.

Theory Comput. Dielectric Constant of Water Confined in a Nanocavity. Multipole moments of water molecules and the aqueous solvation of monovalent ions.

IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters - PDF Free Download

E: Soft Matter Biol. Faraday Discuss. The Multiscale Coarse-Graining Method. Soft Matter , 9 , — Statistical Mechanics of Dissipative Particle Dynamics. PLoS Comput. Solids , , — Molecular Model of Hydrophobic Solvation. How Ions Affect the Structure of Water. E , 85 , Simple Model of Hydrophobic Hydration. A Tetrahedral Entropy for Water. Matter , 24 , B: Condens. Matter Mater. Effect of quantum nuclear motion on hydrogen bonding. Competing quantum effects in the dynamics of a flexible water model.

Quantum nature of the hydrogen bond. Unraveling quantum mechanical effects in water using isotopic fractionation. Growth , 12 , 21— Snow and Ice Crystals. Today , 60 12 70— The glass-liquid transition of hyperquenched water. Nature , , — Calorimetric study of the vitrified liquid water to cubic ice phase transition. The heat capacity and glass transition of hyperquenched glassy water.

B , 60 , — Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature. Isochoric Temperature Differential of the x-ray structure factor and structural rearrangements in low-temperature heavy water. A: At. Isochoric Differential Scattering Functions in Liquid water: the fifth neighbor as a network defect.

Phase Behaviour of Metastable Water. Origin of Temperature and Pressure Effects on the Radial distribution function of water. Supercooled and Glassy Water. Matter , 15 , R A coherent picture of water at extreme negative pressure. Waters second glass transition. Anomalies in bulk supercooled water at negative pressure. Density Fluctuations in Liquid Water. Metastable liquid-liquid transition in a molecular model of water. Nature , , Evidence for a liquid-liquid critical point in supercooled water within the E3B3 model and a possible interpretation of the kink in the homogeneous nucleation line.

Tuning the liquid-liquid transition by modulating the hydrogen-bond angular flexibility in a model for water. Additivity Principles in Biochemistry. Physical Modeling of Aqueous Solvation. Thermodynamics of Aqueous Solutions of Noble Gases. Water-Mediated Hydrophobic Interactions. A Thermodynamic Analysis of Solvation. Energetics of Interactions of Aromatic Hydrocarbons With. Theory of the Hydrophobic Effect.

The Hydrophobic Effect. Colloid Interface Sci. A View of the Hydrophobic Effect.


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Interfaces and the Driving Force of Hydrophobic Assembly. Generalized Solvation Heat Capacities. Hydrophobic Effects on a Molecular Scale. New Perspectives on Hydrophobic Effects. Hydrophobicity at Small and Large Length Scales. Hydrophobic Collapse in Multidomain Protein Folding. Science , , — Necessity of capillary modes in a minimal model of nanoscale hydrophobic solvation. Drying and Hydrophobic Collapse of Paraffin Plates. Molecular Shape and the Hydrophobic Effect. Water Determines the Structure and Dynamics of Proteins. Langmuir , 32 , — Hydrophobic Ambivalence: Teetering on the Edge of Randomness.

Structure and Dynamics of Hydrated Ions. Single-Ion Solvation. Free Energy of Ionic Hydration. Acta , , — Thermodynamic Origin of Hofmeister Ion Effects. Surfaces Affect Ion Pairing. Water at Interfaces. Protons and Hydroxide Ions in Aqueous Systems. Water Surface Is Acidic. Thermodynamics of Hydronium and Hydroxide Surface Solvation. Nano Lett. Viscous Water Meniscus Under Nanoconfinment. Confined Water as Model of Supercooled Water. Phase Diagram of Water in Carbon Nanotubes. Transformations in methane hydrates. Current perspectives on gas hydrate resources.

WR1 and RW2 have the same transverse frequency and RW1 has a smaller infrared intensity, which may help explain why the librational band is well fit by a single optical mode. Similar results hold for TTM3F. To verify that the modes we observe are actually propagating and to further quantify the range of propagation we study the spatial extent of polarization dipole correlations as a function of frequency.

We investigated several different methodologies that can be used to decompose a spectra into distance-dependent components Supplementary Note 1. We choose to start with the polarization correlation function:. Here p i k , t represents either the longitudinal or transverse molecular polarization vector of molecule i.

We now limit the molecules in the second sum to those in a sphere of radius R around each molecule i :. R can be increased to the largest R in the system , where the full response function for the simulation box is recovered. As R increases, the contributions of the distinct term add constructively and destructively to the self term, illustrating the contributions from molecules at different distances. Similar results are obtained for TTM3F, but with additional contributions in the stretching band.

Figure 8 shows the distance decomposed longitudinal susceptibility for TTM3F in a 1. The self part is the same in both the longitudinal and transverse cases, reflecting an underlying isotropy, which is only broken when dipole—dipole correlations are introduced. Further insight into the self-distinct cancellation comes from the results of Bopp, et al.

Transverse a and longitudinal b susceptibilities, calculated with a 4 nm box at K, using the smallest k vector in the system. Gaussian smoothing was applied. In both the transverse and longitudinal cases as R increases a new peak emerges, corresponding to the propagating mode.

Incidentally, the shift in the peak between the self and distinct parts rules out the possibility that the propagating mode is the proposed dipolar plasmon resonance, since the dipolar plasmon must be a resonance of both the of single molecule and collective motion 45 , 46 , Interestingly, there are very long-range contributions to this peak.

As noted, recent studies of ice XI suggest that the propagating modes consist of coupled wagging and rocking librations 42 , The results for the transverse mode seem to confirm this hypothesis for liquid water, since the propagating mode peak lies between the single-molecule rocking and wagging peaks. In the longitudinal case the propagating mode overlaps more with the wagging peak, suggesting a greater role for these type of librations in the longitudinal phonon.

To provide further evidence the aforementioned optical modes propagate through the hydrogen-bond network of water we decided to repeat our analysis for other polar liquids, both H-bonding and non H-bonding. As an H-bonding liquid we choose methanol, which is known to contain winding H-bonded chains.

Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters

According to results from MD simulation, most of these chains have around 5—6 molecules 48 , 49 , with a small percentage of chains containing 10—20 molecules Chain lifetimes have been estimated to be about 0. Therefore, we expect methanol can also support a librational phonon mode that propagates along hydrogen bonds, but perhaps with a shorter lifetime and range than water. As a non H-bonding polar liquid we choose acetonitrile, because it has a structure similar to methanol, but with the hydroxyl group replaced by a carbon atom.

As with water, the transverse spectra also exhibits dispersion, but to a much lesser extent. In this work, we have presented several lines of evidence for short-lived optical phonons that propagate along the H-bond network of water. The longitudinal and transverse nonlocal susceptibility exhibit dispersive peaks with dispersion relations resembling optical phonons. As the temperature is lowered, the resonance frequencies and LO—TO splittings of these modes converge towards the values for phonons in ice Ih. By comparing our results with a recent study of ice XI we believe both modes likely consist of coupled wagging and rocking librations 42 , This work fundamentally changes our understanding of the librational band in the Raman spectra of water by assigning the lower and higher frequency peaks to transverse and longitudinal optical modes.

We are also led to a new interpretation the librational region of the real part of the dielectric function. The presence of dampening smooths the divergence leading to a peak followed by a sharp dip. Acoustic modes, which are observable through the dynamic structure factor, have been explored as means of understanding the hydrogen-bond structure and low-temperature anomalies of water 5. In this work, we have argued that optical modes can also provide insight into water's structure and dynamics.

The fast sound mode lies at much lower frequencies than the librational and OH stretch modes that we studied. The H-bond bending and stretching modes also primarily lie at at frequencies below the librational region. However, normal mode analysis of liquid water and clusters shows that the H-bond stretching modes have a wide distribution of frequencies, which overlaps with the librational modes, so some coupling between these modes is possible 54 , Recently, it was shown that there is coupling between the acoustic and optic modes in water—that is, between fluctuations in mass density and fluctuations in charge density The large spatial range and coherent propagation of these modes is surprising and implies the existence of an extended hydrogen-bond network, in contrast to earlier ideas about the structure of water which emphasize dynamics as being confined within small clusters Simulations with larger simulation boxes are needed to fully quantify the extent of the longitudinal modes.

The ability of water to transmit phonon modes may be relevant to biophysics, where such modes could lead to dynamical coupling between biomolecules, a phenomena that is currently only being considered at much lower frequencies 58 , 59 , The methodology used in this paper to analyse LO—TO splitting opens up a new avenue to understanding the structure and dynamics of water. The fact that the librational LO—TO splitting increases with temperature instead of the expected decrease is likely due to significant changes in the structure of the liquid.

This could be caused by the local quasi-structure determined by H-bonding changing from a more ice-like structure four molecules per unit cell to a more cubic structure 1 molecule per unit cell. More research is needed to understand the microscopic origin of the LO—TO splitting in water, both in the librational and stretching modes. If the external field is sufficiently small, then the relation between the polarization response of a medium and the electric displacement field D for a spatially homogeneous system is given by:.

For isotropic systems, the tensor can be decomposed into longitudinal and transverse components:. This expression relates the susceptibility to the time correlation function of the polarization in equilibrium. To calculate the transverse part of the polarization we use the method of Raineri and Friedman to find the polarization vector of each molecule Supplementary Note 2 We can rewrite equation 13 in terms of the normalized polarization correlation function equation 7 , and taking into account the isotropy of water:.

To simulate methanol and acetonitrile we used the General AMBER Forcefield GAFF 66 , a forcefield with full intramolecular flexibility, which has been shown to satisfactory reproduce key properties of both liquids Other simulations were 1—2 ns long. All simulations were equilibrated for at least 50 ps before outputting the data. Because of periodic boundary conditions, the possible k vectors are limited to the form , where n x , n y and n z are integers.

We calculated correlation functions separately for each k and then average over the results for k vectors with the same magnitude, a process we found reduced random noise. Previously it was shown that the widely used harmonic correction does not change the spectrum For the OH-stretching peak, however, quantum effects are known to be very important.

To obtain resonance frequencies and lifetimes for the librational peak in the imaginary part of the response we used a damped oscillator model. A Debye peak overlaps significantly with the librational band in both the longitudinal and transverse cases and must be included in the peak fitting. Equation 14 can be used to relate the form of the time correlation function to the absorption peak lineshape.

For Debye response one has the following expressions:. Because of this overlap and due to the broad nature of the transverse band, the fitting in the transverse case is only approximate. We found this was especially true for TTM3F and the experimental data, so we do not report lifetimes for such cases. How to cite this article: Elton, D. The hydrogen-bond network of water supports propagating optical phonon-like modes.

DE-SC M. Author contributions D. Performed the simulations and helped prepare the manuscript. S supervised the project and helped prepare the manuscript. Europe PMC requires Javascript to function effectively. Recent Activity. In the long wavelength limit these optical modes exhibit longitudinal-transverse splitting, indicating the presence of coherent long-range dipole-dipole interactions, as in ice.

The snippet could not be located in the article text. This may be because the snippet appears in a figure legend, contains special characters or spans different sections of the article. Nat Commun. Published online Jan 4. PMID: Daniel C. Received Aug 10; Accepted Nov All Rights Reserved. This work is licensed under a Creative Commons Attribution 4. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material.

This article has been cited by other articles in PMC. Abstract The local structure of liquid water as a function of temperature is a source of intense research. Open in a separate window. Figure 1. Dielectric susceptibilities of ice and water. Polarization correlation functions The normalized longitudinal and transverse polarization correlation functions are defined as:. Figure 2. Polarization correlation functions. Dispersion of the librational peak Figure 3 shows the imaginary part of the longitudinal and transverse susceptibility for TTM3F.

Figure 3. Imaginary part of longitudinal and transverse susceptibility. Figure 4. Dispersion relations for the propagating librational modes. Table 1 Resonance frequencies and lifetimes. Figure 5. Imaginary part of the longitudinal susceptibility. Figure 6. Imaginary parts of dielectric susceptibility. LO—TO splitting versus temperature The frequencies of the librational and stretching modes are shown in Table 1. Relation to phonons in ice Naturally we would like to find corresponding optical phonon modes in ice. Figure 7.

Figure 8. Long-range contributions are observed in the OH-stretching band. Methanol and acentonitrile To provide further evidence the aforementioned optical modes propagate through the hydrogen-bond network of water we decided to repeat our analysis for other polar liquids, both H-bonding and non H-bonding. Discussion In this work, we have presented several lines of evidence for short-lived optical phonons that propagate along the H-bond network of water.

Methods Theory of the nonlocal susceptibility If the external field is sufficiently small, then the relation between the polarization response of a medium and the electric displacement field D for a spatially homogeneous system is given by:. Fitting the librational band To obtain resonance frequencies and lifetimes for the librational peak in the imaginary part of the response we used a damped oscillator model.

Additional information How to cite this article: Elton, D. Footnotes Author contributions D. References Santra B. Local structure analysis in ab initio liquid water. Errington J. Relationship between structural order and the anomalies of liquid water. Nature , — Density fluctuations in liquid water. The inhomogeneous structure of water at ambient conditions. Natl Acad. USA , — Possible relation of water structural relaxation to water anomalies. Microscopic structure of water at elevated pressures and temperatures. Raman spectral studies of water structure.

Carey D. Measurement of the Raman spectrum of liquid water. Walrafen G. Raman spectral studies of the effects of temperature on water structure. Temperature dependence of the low and high frequency Raman scattering from liquid water. Castner E. The intermolecular dynamics of liquid water. Zelsmann H. Fukasawa T. Relation between dielectric and low-frequency Raman spectra of hydrogen-bond liquids. The translational lattice-vibration Raman spectrum of single crystal ice 1 h.

Klug D. The longitudinaloptictranverseoptic mode splitting in ice Ih. Whalley E. A detailed assignment of the oh stretching bands of ice i. Galeener F. Longitudinal optical vibrations in glasses: GeO 2 and SiO 2. Resat H. Studies of the optical like high frequency dispersion mode in liquid water. Bopp P. Frequency and wave-vector dependent dielectric function of water: Collective modes and relaxation spectra. Ricci M. Theoretical and computer-simulation study of the density fluctuations in liquid water. A 40 , — On the polar vibrations of alkali halides.

Barker A. Long-wavelength soft modes, central peaks, and the Lyddane-Sachs-Teller relation. B 12 , — Sievers A.

Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses) Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses)
Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses) Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses)
Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses) Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses)
Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses) Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses)
Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses) Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses)
Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses) Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses)
Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses) Spectroscopic Investigations of Hydrogen Bond Network Structures in Water Clusters (Springer Theses)

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