Dipole Moment Of Water Molecule Research Articles

Collision-Induced Dipole Moment and Millimeter and Submillimeter Continuum Absorption in Water Vapor

This work is devoted to estimation of the additional absorption of millimeter and submillimeter wavelengths in water vapor arising from collisional interaction of molecules due to the induced dipole moment. Absorption is modeled on the basis of ab initio data on the magnitude of the water molecule dipole moment at high densities, and common knowledge of the water vapor absorption spectrum. Using the model developed, we obtained a simple analytical expression for the absorption coefficient as a function of temperature, pressure, and frequency. Comparison of the results with known experimental data leads to the conclusion that in the range of pressures and temperatures typical of water vapor in the Earth’s atmosphere this type of absorption is negligible compared with the absorption arising due to association or dimerization of the water vapor molecules.

Effect of interfacial hydrogen bonds on the structure and dynamics of confined water

Reactive molecular dynamics (MD) is used to simulate the equilibrium process of water confined between two fully hydroxylated α-quartz (001) surfaces with separation distances from 7 to 20 Å. Effect of different patterns of interfacial hydrogen bonds on the structure and dynamics of confined water is investigated. Density profiles, radial distribution functions, number of interfacial hydrogen bonds, and mean square displacements are calculated. The α-quartz (001) surface is cut from an α-quartz crystal at a certain depth to construct a surface with geminal silanols after being fully hydroxylated. The silanol groups on the surface are treated in two different ways in the MD simulations. One of the silanol groups are treated as to be fixed, and the other one is treated as no constraint for the movement of surface silanols. Our results show that different patterns of hydrogen bonds are formed at the interface between SiO2 surface and water. For the fixed silanol surface there is one type of strong hydrogen bonds interacting between the oxygen atoms of water and the hydrogen atoms of surface silanols, leading to the dipole moment of water molecules pointing out from the surface. For the movable silanol surface there are two types of strong hydrogen bonds formed at the interface. One is between the oxygen atoms of water and the hydrogen atoms of surface silanols, and the other is between the oxygen atoms of surface silanols and the hydrogen atoms of water. The number of hydrogen bonds of the first type is much less than those of the second type, leading to the dipole moment of water molecules pointing to the surface. Moreover, the total number of interfacial hydrogen bonds formed on the fixed silanol surfaces is larger than that on the movable silanol surfaces. The density profiles of the confined water indicate the formation of a strong layering of water in the vicinity of the fixed silanol surface, and the water layer is also more ordered with an ice-like structure, as compared with a dense water layer with a liquid-like structure in the case of movable silanol surfaces. Thus the mean square displacements of confined water show that, as compared with interfacial hydrogen bonds formed on the fixed silanol surfaces, the weaker and the lesser interfacial hydrogen bonds formed on the movable silanol surfaces may be responsible for more intense movement of confined water between the movable silanol surfaces. Our simulation suggests that the different pattern of interfacial hydrogen bonds could signifiantly affect the structure and dynamic behaviors of the confined water between two fully hydroxylated silica surfaces.

Dipole Moment and Binding Energy of Water in Proteins from Crystallographic Analysis.

The energetics of water molecules in proteins is studied using the water placement software Dowser. We compared the water position predictions for 14 high-resolution crystal structures of oligopeptide-binding protein (OppA) containing a large number of resolved internal water molecules. From the analysis of the outputs of Dowser with variable parameters and comparison with experimental X-ray data, we derived an estimate of the average dipole moment of water molecules located in the internal cavities of the protein and their binding energies. The water parameters thus obtained from the experimental data are then analyzed within the framework of charge-scaling theory developed recently by this group; the parameters are shown to be in good agreement with the predictions that the theory makes for the dipole moment in a protein environment. The water dipole in the protein environment is found to be much different from that in the bulk and in such models as SPC or TIPnP. The role of charge scaling due to electronic polarizability of the protein is discussed.

Structure of hydrogen-bonded associates in supercritical water under low and high pressures

The character and structural features of hydrogen-bonded associates in sub- and supercritical water are studied by analyzing distributions of the dipole moments of water molecules at P = 40, 80, and 100 MPa and T = 373–773 K, calculated using Car-Parrinello molecular dynamics. The main types of hydrogen-bonded structures and their changes upon isobaric heating are determined. It is shown that clusters with tetrahedral configurations exist in supercritical water only under high pressure.

Properties of water in the region between a tubulin dimer and a single motor head of kinesin

A kinesin is a molecular motor that can perform movement on a microtubule track in a stepping-like manner. This motion is connected with processes of association and dissociation of kinesin and tubulin. Water is an important participant in these kinds of molecular interactions. This is why we have decided to investigate the dynamical and structural properties of water in the region between the kinesin catalytic domain and the tubulin dimer. Using the molecular dynamics method, we found that these properties are different from the ones of bulk water. The changes in structure and dynamics are visible for water beyond the first solvation layers, even for the longest analyzed distance between proteins equal to 2.0 nm. However, these changes are not always enhanced compared to the situation when only one protein surface is present. One factor that distinguishes the investigated situation from the one with a single protein is the presence of an additional electric field originating from the second protein. The tendency of vectors of dipole moments of water molecules between the proteins to follow the vectors of electric field generated by the proteins causes a distortion of the water-water hydrogen bond network. It has been shown that this distortion affects the properties of water in this region: it induces structural changes in solvation water, and leads to increased water density and increased stiffness of the water structure.

Changes of structure and dipole moment of water with temperature and pressure: A first principles study

The changes of structure and distribution of dipole moment of water with temperatures up to 2800 K and densities up to 2.2 g/cm(3) are investigated using ab initio molecular dynamics. Along the isochore of 1.0 g/cm(3), the structure of liquid water above 800 K is dramatically different from that at ambient conditions, where the hydrogen-bonds network collapses. Along the isotherm of 1800 K, the transition from the liquid state to an amorphous superionic phase occurs at 2.0 g/cm(3) (32.9 GPa), which is not observed along the isotherm of 2800 K. With increasing temperature, the average dipole moment of water molecules is decreased arising from the weakened polarization by the collapse of the hydrogen-bonds network, while it is contrarily increased with compression due to the strengthening effect upon the polarization of water molecules. Both higher temperature and pressure broaden the distribution of dipole moment of water molecules due to the enhanced intramolecular charge fluctuations.

The structure of H-bonded clusters in sub- and supercritical water

The structure of H-bonded complexes in sub- and supercritical water in regions close to and remote from the saturation curve was studied. The Car-Parrinello method was used to calculate water dipole moment distributions in 11 thermodynamic states. The dependence of the mean dipole moment of water molecules on the size of clusters and number of H-bonds was obtained.

Electronic properties of a methane–water solution

Electronic properties of a methane–water solution were investigated by a sequential quantum mechanical/molecular dynamics approach. Upon hydration methane acquires an induced dipole moment of ∼0.5 ± 0.2 D. This is related to polarisation effects and to weak methane–water hydrogen bond interactions. From gas phase to solution, the first vertical excitation and ionisation energies of methane are red-shifted by 0.45 ± 0.25 and 0.87 ± 0.40 eV, respectively. We also report results for the dynamic polarisability of methane in water. In comparison with water, no difference was found for the average monomeric dipole moment of water molecules in close interaction with methane.

Contributions of Ordered Solvent to Long-Range DNA-Dendrimer Interactions

Water is essential to nearly all biological reactions, and yet the role of solvent is often overlooked in studying such interactions. In particular, the ability of highly charged molecules to orient the dipole moments of water molecules has not been thoroughly explored. While short-range solvent ordering effects have been previously investigated, we report evidence of the existence of ordered waters between charged molecules at large distances and characterize the free energy contributions of this solvent ordering to the interaction. We present evidence from molecular dynamics simulations that the hydrogen bonding network in ordered waters between a strand of DNA and a highly charged nanoparticle, a generation 3 polyamidoamine (PAMAM) dendrimer, contribute significantly to the free energy and extend the interaction beyond the electrostatic range of the molecules. Such long-range water effects could potentially be of great importance to many biological systems, in which molecules appear to be able to recognize each other across significant distances, or for which the kinetic rates are too fast to be due to pure diffusion. Our results are in good agreement with experiments on the role of solvent in DNA condensation by multivalent cations.

Ab Initio Study of Water Polarization in the Hydration Shell of Aqueous Hydroxide: Comparison between Polarizable and Nonpolarizable Water Models.

Ab initio simulations of aqueous hydroxide are performed to study the structure and polarization of water molecules in the first solvation shell. Polarization is found to depend on the configuration of the hydrogen-bond (HB) donors. In the most common case of four HB donors, the dipole moment of water molecules is much larger than those in the first shell of monovalent ions. When there are only three HB donors, the water dipole moment exceeds even those in the first shell of a divalent cation. We also show that the dipole fluctuations in the first hydration shell of hydroxide are reduced compared to bulk water, which can provide a rationale for the propensity of hydroxide for interfaces with hydrophobes. Because of its unique properties, hydroxide provides a nontrivial test for benchmarking classical models. Comparison of the ab initio results with those obtained from the classical models indicates that the latter need to be further improved in order to yield reliable results.

Surface Potentials in Langmuir Monolayers of Unidirectionally Oriented α-Helical Diblock Copolypeptides

The surface potentials and effective dipole moments of alpha-helical amphiphilic diblock copolypeptides during monolayer compression at the air-water interface are reported. Amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) of various block lengths were studied during the double-brush formation process at the water surface. Upon monolayer spreading of PLGA-b-PMLGSLGs, surface potentials of hundreds of millivolts were recorded, attributed to the dipole moments of water molecules reorienting due to interactions with the monolayers. Upon compression, the effective dipole moments derived from the surface potentials of the PLGA-b-PMLGSLG monolayers decrease gradually, most likely as a result of the immersion of the hydrophilic block in water and cancellation of the interactions between the hydrophobic block and the underlying water molecules. The polypeptide macrodipole moment immersed in water was apparently effectively screened out. The remaining effective dipole moment of the monolayer contributes mainly to the hydrophobic block, and upon tilting away from the water surface toward the surface normal, it was found to increase with the hydrophobic block length, indicating the gradual formation of unidirectional aligned polypeptide molecules in the double-brush monolayer.

Born−Oppenheimer Molecular Dynamics of the Hydration of Na+ in a Water Cluster

The hydration of Na(+) in a water cluster is studied through all-electron Born-Oppenheimer molecular dynamics. The structure, dipole moment, and vibrational spectrum of the sodium cation hydration shells are examined. Emphasis is placed on the extent of the effect of the hydrated cation on the cluster properties. Our results show that hydration of Na(+) takes place in the interior of the cluster leading to significant changes in the hydrogen-bond (H-bond) network beyond the first hydration shell. In particular, we find that single acceptor-only H-bond arrangements increase significantly at the surface of the cluster relative to a neat water cluster. The vibrational spectrum of the first hydration shell of the cation, comprised mostly of H-bond double donor-single acceptor water molecules, is similar to that found for water molecules in the interior of a neat water cluster, although a small blue shift of the OH stretching band is observed. Further, a small reduction of the dipole moment of water molecules in the first hydration shell of the cation relative to a neat water cluster is also observed, and this persists to a minor extent when we move from the interior to the surface of the cluster. The present results indicate that the effect of the Na(+) on the cluster properties, although not pronounced, is not constrained to the first hydration shell. The reason appears to lie mostly in the specific orientation of the water molecules in the first coordination sphere, inducing modifications on the H-bond network topology of the cluster.

Polarization of Water in the First Hydration Shell of K+ and Ca2+ Ions

Accurate representation of the interactions of water molecules with charges is essential for correct description of biomolecules and their interactions and, hence, is a primary concern in the design of classical force fields. This task is made even more challenging by the fact that the charge distribution of water molecules in liquid is significantly altered by the local environment. To understand how such polarization effects would modify the force fields, we have performed density functional calculations for ion-water clusters using K+ and Ca2+ ions as probes. We find that the dipole moment of water molecules in the first hydration shell decreases with increasing number of waters, which is explained by the suppression of the ion's electric field by those of water dipoles. Adding further water beyond the first shell, the dipole moment of the first shell waters increases because water dipoles are strongly polarized in the presence of hydrogen bond acceptors. Thus the net polarization of water in the hydration shell of an ion is determined by two competing effects, of which only one directly depends on the ion. These observations explain why the dipole moment of waters in the first hydration shell of a K+ ion is smaller compared to those in bulk water while the opposite is true for Ca2+ ions and suggest new constraints to be used in the development of polarizable water models.

Interaction potential and selection rules in the rotational spectrum of water molecule during its collision with atoms

If water molecules are in an atmosphere of buffer monatomic gas, so that the collision frequency with atoms is much higher than the collision frequency between molecules, rotational levels of the molecule with the same angular momentum quantum number j are split into two sets with even and odd quantum numbers m′ (projection on the Oz′ axis of the molecular coordinate system). Such separation arises due to the interaction of the dipole moment of water molecule with the induced dipole on an atom during particle collision. The symmetry of the molecule-atom interaction potential allows transitions between levels of the same parity, but forbids transitions between sets. Experiments on saturation of rotational transitions in a rarefied gas mixture consisting of heavy water and argon vapors are interpreted. These experiments confirm the assumption that rotational levels of the water molecule are indeed separated into two sets due to the interaction of the molecule with the environment.

Car-Parrinello Molecular Dynamics Simulations of CaCl2 Aqueous Solutions.

Car-Parrinello molecular dynamics (CPMD) simulations are used to investigate the structural properties of 1 and 2 molal (m) CaCl2 aqueous solutions and, in particular, the radial distribution functions, coordination numbers, and dipole moments of water molecules in the first solvation shell. According to these simulations, the first solvation shell of the Ca(2+) ion consists of six water molecules, that are characterized by an increased averaged dipole moment compared to that of bulk water, and a first-shell Ca-O radial distribution function peak at 2.39 Å. The results are compared to those of CPMD simulations of Ca(2+) (no counterions), and no significant differences are found. This indicates that the homogeneous neutralizing background charge density implicitly included in simulations of non-neutral systems appropriately mimics the presence of the counterions (at least in terms of reproducing the solvation structure properties and for the box sizes considered). Classical molecular dynamics (MD) simulations of aqueous Ca(2+) using varying box sizes confirm this suggestion. The CPMD simulations at 2 m concentration also reveal additional possibilities for the structural arrangement of water molecules and chloride ions around Ca(2+). In particular, they support the stability of Ca(2+)-Cl(-) (contact) and Ca(2+)-H2O-Cl(-) (solvent-separated) ion pairs. In addition, the solvent-separated cation pair is found to occur in a deprotonated Ca(2+)-OH(-)-Ca(2+) form. The existence of such a species has, to our knowledge, never been invoked previously to account for experimental data on CaCl2 solutions.

Effect of Surface Polarity on Water Contact Angle and Interfacial Hydration Structure

We perform molecular dynamics simulations of water in the presence of hydrophobic/hydrophilic walls at T = 300 K and P = 0 GPa. For the hydrophilic walls, we use a hydroxylated silica model introduced in previous simulations [Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G.; Phys. Rev. E 2006, 73, 041604.]. By rescaling the physical partial atomic charges by a parameter 0 <or= k <or= 1, we can continuously transform the hydrophilic walls (hydroxylated silica, k = 1) into hydrophobic apolar surfaces (k = 0). From a physical point of view, k is the normalized magnitude of a surface dipole moment, and thus it quantifies the polarity of the surface. We calculate the contact angle of water for 0 <or= k <or= 1. We find that, at least for the present homogeneous, atomically flat, and defect-free surface model, the magnitude of the surface dipole correlates with the contact angle in a one-to-one correspondence. In particular, we find that polar surfaces with 0 < k <or= kc = 0.4 are macroscopically hydrophobic; that is, the contact angle is larger than 90 degrees . For the cutoff value k = kc, the magnitude of the dipole moment of the polar silica surface unit is 41% that of the water molecule dipole moment. We also study the water orientation distributions next to the walls (a microscopic property). We find that these distributions also correlate with the contact angle in a one-to-one correspondence. Thus, the structure of confined water, the surface polarity, and the contact angle are in a direct correspondence to each other, and therefore, each quantifies the hydrophobicity/hydrophilicity of the surface.

Effect of Electrification Conditions on the Freezing of Supercooled Water Droplets on a Hydrophobic Coating

Abstract The effects of electrification processes on ice nucleation on an octadecanethiol (ODT) coating were investigated under two different conditions. Electrification provides the driving force of the movement to the three-phase contact line. This change promotes ice nucleation on the supercooled water droplet. In contrast, electrification before cooling inhibits freezing, probably because of the limitation of orientational freedom by the interaction between the surface charge and the dipole moment of water molecules.

Dipole Correlation of the Electronic Structures of theConformations of Water Molecule Evolving Through theNormal Modes of Vibrations Between Angular (C2v) to Linear(D∝h) Shapes

In order to settle the issue of equivalence or non-equivalence of the two lone pairsof electrons on oxygen atom in water molecule, a quantum chemical study of the dipolecorrelation of the electronic structure of the molecule as a function of conformationsgenerated following the normal modes of vibrations between the two extremeconformations, C2v (∠HOH at 90o) and D∝h (∠HOH at 180o), including the equilibrium one,has been performed. The study invokes quantum mechanical partitioning of moleculardipoles into bond moment and lone pair moment and localization of delocalized canonicalmolecular orbitals, CMO’s into localized molecular orbitals, LMO’s. An earlier suggestion,on the basis of photoelectron spectroscopy, that one lone pair is in p-type and the other is ins-type orbital of O atom of water molecule at its equilibrium shape, and also the qualitative“Squirrel Ears” structure are brought under serious scrutiny. A large number ofconformations are generated and the charge density matrix, dipole moment of eachconformation is computed in terms of the generated canonical molecular orbitals, CMO’sand then Sinanoğlu’s localization method is invoked to localize the CMO’s of eachconformation and the quantum mechanical hybridizations of all the bonds and lone pairs onO center are evaluated in terms of the localized molecular orbitals. Computed datademonstrate that the electronic structures i.e. two bond pairs and two lone pairs and itshybridization status of all conformations of water molecule are straightforward in terms ofthe LMO’s. It is further revealed that the pattern of orbital hybridization changescontinuously as a function of evolution of molecular shape. The close analysis of thegenerated LMO’s reveals that one lone pair is accommodated in a pure p orbital and anotherlone pair is in a hybrid orbital in almost all conformations. One more important result of the present study is that, with the physical process of structural evolution from close angular shape to the linear transition state, the length of the σ (O–H) decreases and its strength increases as a monotone function of reaction coordinates. The bond length is shortest and the strength is largest at the transition state of structural inversion. Result of structural effect of the present study during the evolution of molecular conformations is quite consistent with the result of a very refined calculation that one physically significant feature of force field that the stretching force constants at the linear geometry are considerably larger than their equilibrium counter parts. The variation of bond strength and the hybridization of s and p orbitals on O atom center to form the σ (O–H) bond as a function of evolution of conformations is in accordance with Coulson’s prediction. The total dipole moment of all conformations is partitioned into the contribution from bonds and lone pairs and correlated in terms of the computed hybridization in lone pairs. The analysis of the variation of dipole moment as a function of angular to linear structural evolution reveals that the dipole moment of H2O molecule is not due to the bond moments only but a significant contribution comes from a lone pair. It is strongly established that the dipole moment of water molecule at and around the equilibrium geometry is not due to the bond moments only and the major part of the molecular dipole comes from the contribution of lone pair electrons. This necessitates the accommodation of a lone pair of electrons in a hybrid orbital on O atom. The computed LMO’s webbed with partitioned molecular dipole reveal that one lone pair is in a pure p- type orbital and the other lone pair is in a hybrid of s and p, and not in a pure s type orbital as suggested on the basis of photoelectron spectra. The possibility of qualitative “Squirrel Ears” structure is also ruled out. The problem of equivalence or non-equivalence of the two lone pairs of the O atom in water seems to have been finally resolved by the present quantum chemical calculation. An attempt of locating the origin of barrier to the physical process of inversion of water molecule is made in terms of energy partitioning method. It is found that the dipole can be used as a descriptor for the elucidation of electronic structure of molecules.

Dipole moments of water molecules confined in Na–LSX zeolites – Molecular dynamics simulations including polarization of water

Molecular dynamics simulations of hydrated Na–LSX zeolite at 300 K were performed with the explicit inclusion of the polarization of water. The Si/Al ratio of LSX is 1 and the number of water molecules per unit cell ranged from 0 to 224 to represent a range of hydration. The calculation results show that the dipole moments of water molecules increase with increasing hydration. By using the SPC–FQ water model instead of the SPC/E water model, the differential heat of adsorption showed similar trends in both models, whereas the differential potential energies between water–water and between water–zeolite are more sensitive to hydration.

Computer Simulation of the Absorption of CO2 Molecules by Water Cluster: 2. The Microstructure

The simulation of the absorption of CO2 molecules by the (H2O)10 cluster is performed by the molecular dynamics method using the modified TIP4P model of water. The detailed structure of (CO2)i(H2O)10 clusters (0≤i≤11) is analyzed by the statistic geometry method based on the construction of the Voronoi polyhedra. The obtained distributions of the geometric elements of polyhedra indicate the significant changes in the structure of a cluster after the absorption of one CO2 molecule. Only polyhedra characterizing the structure of unstable water clusters that absorbed six or seven CO2 molecules demonstrate a nonsphericity close to the ideal tetrahedron. Linear CO2 molecule tends to be oriented in a cluster so that the average angle formed by this molecule and permanent dipole moments of water molecules would be equal to about 30°.