Isolated Water Molecules Research Articles

Quadrupole coupling constants of deuterons in molecular clusters Ca2+(D2O) n (n = 6, 8, 10, 18)

Characteristic features of the structure of Ca2+ hydration shells were considered. The results of quantum chemical calculations were compared with experimental data obtained from the study of nuclear magnetic relaxation of deuterons in aqueous solutions of calcium salts. The influence of the basis set and computational procedure on the calculated 2D quadrupole couling constants (QCC) in isolated water molecule was investigated. The 2D QCC in molecular clusters (D2O)5 and Ca2+(D2O) n (n =6, 8, 10, 18) were calculated using the B3LYP/6-31++G** density functional method.

Composition, structure, and stability of the rutileTiO2(110)surface: Oxygen depletion, hydroxylation, hydrogen migration, and water adsorption

A comprehensive phase diagram of lowest-energy structures and compositions of the rutile ${\text{TiO}}_{2}(110)$ surface in equilibrium with a surrounding gas phase at finite temperatures and pressures has been determined using density-functional theory in combination with a thermodynamic formalism. The exchange of oxygen, hydrogen, and water molecules with the gas phase is considered. Particular attention is given to the convergence of all calculations with respect to lateral system size and slab thickness. In addition, the reliability of semilocal density functionals in describing the energetics of the reduced surfaces is critically evaluated. For ambient conditions the surface is found to be fully covered by molecularly adsorbed water. At low coverages, in the limit of single isolated water molecules, molecular and dissociative adsorption modes become energetically degenerate. Oxygen vacancies form in strongly reducing, oxygen-poor environments. However, already at slightly more moderate conditions it is shown that removing full ${\text{TiO}}_{2}$ units from the surface is thermodynamically preferred. In agreement with recent experimental observations it is furthermore confirmed that even under extremely hydrogen-rich environments the surface cannot be fully hydroxylated, but only a maximum coverage with hydrogen of about 0.6--0.7 monolayer can be reached. Finally, calculations of migration paths strongly suggest that hydrogen prefers diffusing into the bulk over desorbing from the surface into the gas phase.

Structures of Thermal, Mass-Selected Water Clusters Probed with Hydrophobic Ion Tags and Infrared Photodissociation Spectroscopy

The structures of tetramethylammonium (TMA(+)) and phenyltrimethylammonium (PTMA(+)) with 1-8 water molecules attached were investigated using infrared photodissociation spectroscopy (IRPD), blackbody infrared radiative dissociation, and theory to elucidate the effects of ion-water interactions and water-water hydrogen bonding in clusters containing these hydrophobic ions. Several pieces of evidence suggest that these ions have only a small effect on water structure. The binding energy of a water molecule to TMA(+) is 44 kJ/mol, a value that is significantly lower than that reported for other cations and close to the condensation energy of bulk water. The OH asymmetric stretch of the water molecule in TMA(+).(H(2)O)(1) at 3718 cm(-1) is less red-shifted from that of an isolated water molecule (3756 cm(-1)) than for those of other cations, and water-water hydrogen bonding is extensive for all clusters with two or more water molecules. These results indicate that charge transfer from water to these hydrophobic ions is much less than that for other cations. There are two predominant structures for these ions with three water molecules: one in which water molecules form a chain and one with a cyclic structure analogous to that of neutral water trimer. With four water molecules attached, water forms a cyclic structure similar to that of neutral water tetramer. The spectra of the larger clusters follow trends reported for neutral water molecule clusters, which indicates that these hydrophobic ions have only a minimal effect on the structure of water in clusters with four or more water molecules. These results suggest that these hydrophobic ions can be used as "tags" that could make possible acquisition of IRPD spectra of even larger clusters, such as clathrates, for which precise mass selection of neutral clusters is not possible by other methods.

Investigation of water signatures at gully-exposed sites on Mars by hyperspectral image analysis

High-resolution images from the Mars Obiter Camera (MOC) onboard the Mars Global Surveyor (MGS) show a variety of gully features on sloped surfaces of Mars. The mechanism of gully formation is still under debate, although a majority of studies tend to favor a mechanism related to liquid water flow based on geomorphology and fluid mechanics considerations. In this study, we examined four known gully sites using Visible and Infrared Mineralogical Mapping Spectrometer (OMEGA) imagery. In particular, we analyzed the absorption depths of the water-associated absorption bands and concluded that there are stronger water signatures at the gully-exposed sites than in the surrounding areas. This implies that the water signatures, most likely representing water ice, isolated water molecules, and/or hydroxyl molecules incorporated into minerals, are still present in the shallow unconsolidated soils. This study provides additional evidence that water was likely involved in the formation of the gully features and is still locally active on the Martian surface in the present time.

A polyethylene-glycol-functionalized ring-like isopolymolybdate cluster

A new organic–inorganic hybrid compound, Na 6[HO(CH 2CH 2O) 4H] 3{Mo 36O 108(H 2O) 14(OH) 6[HO(CH 2CH 2O) 3H] 2} · ∼75H 2O ( 1) has been obtained in polyethylene glycol (PEG)/H 2O system with a good yield, and characterized by element analysis, IR, UV, TG, 13C NMR spectroscopy, electrochemistry and X-ray single crystal diffraction analysis. Compound 1 consists of the {Mo 36} clusters as the structural motif covalently decorated by PEG fragments. Such hybrid polyoxoanions are linked by Na + counter-cations, leading to one-dimensional chains. The adjacent chains are closely packed together into a three-dimensional supramolecular framework via extensive H-bonding interactions among polyoxoanions and the isolated water molecules.

Adsorption and Protonation of CO2 on Partially Hydroxylated γ-Al2O3 Surfaces: A Density Functional Theory Study

Adsorption and protonation of CO2 on the (110) and (100) surfaces of gamma-Al2O3 have been studied using density functional theory slab calculations. On the dry (110) and (100) surfaces, the O-Al bridge sites were found to be energetically favorable for CO2 adsorption. The adsorbed CO2 was bound in a bidentate configuration across the O-Al bridge sites, forming a carbonate species. The strongest binding with an adsorption energy of 0.80 eV occurs at the O3c-Al5c bridge site of the (100) surface. Dissociation of water across the O-Al bridge sites resulted in partially hydroxylated surfaces, and the dissociation is energetically favorable on both surfaces. Water dissociation on the (110) surface has a barrier of 0.42 eV, but the same process on the (100) surface has no barrier with respect to the isolated water molecule. On the partially hydroxylated gamma-Al2O3 surfaces, a bicarbonate species was formed by protonating the carbonate species with the protons from neighboring hydroxyl groups. The energy difference between the bicarbonate species and the coadsorbed bidentate carbonate species and hydroxyls is only 0.04 eV on the (110) surface, but the difference reaches 0.97 eV on the (100) surface. The activation barrier for forming the bicarbonate species on the (100) surface, 0.42 eV, is also lower than that on the (110) surface (0.53 eV).

On the transition between directed bonding and helium-like angular correlation in a modified Hooke-Calogero model

Angular correlation in three-body systems varies between the limiting cases of slightly perturbed equi-distribution, as in the electronic ground state of helium and directed bond-type bent structure, as in the isolated water molecule. In an exactly solvable modification of the Hooke-Calogero model, it is shown that there is a sharp transition between the two cases if the particles’ masses are suitably varied. In the Hooke-Calogero model attraction between different particles is harmonic and the repulsion between equal particles is given by a 1/r 2 potential. The bent structure appears in the angular distribution function if the masses of the two equal particles are below a critical value, which depends on the mass of the third particle. Above the critical value, the angular correlation is of helium type and exhibits a minimum at 0° corresponding to the Coulomb hole and a maximum at 180°. The model thus demonstrates the modulating role of mass in the transition between semi-rigid structure and more diffuse nuclear states.

Uranyl interaction with the hydrated (1 1 1) nickel face: A periodic density functional theory investigation

Periodic DFT calculations using plane waves basis sets with the GGA formalism were performed in order to study the behavior of the UO 2 2 + uranyl ion at the water–Ni(1 1 1) interface. First, the adsorption of water molecules interacting with an optimized surface model has been studied. At low coverage, isolated water molecules adsorb preferentially on top of the surface nickel atoms, the plane defined by the H 2O molecule being almost parallel to the surface. When the water coverage increases from 1/9 ML to 2/3 ML (ML = monolayer coverage), hydrogen bonds are created leading to the formation of water hexamers, as suggested experimentally on Ni(1 1 1) and other metallic surfaces [A. Michaelides, A. Alavi, D.A. King, Physical Review B 69 (2004) 113404. M. Nakamura, M. Ito, Chemical Physics Letters 325 (2000) 293–298]. Higher water coverage induces the formation of an additional water layer physisorbed over the cyclic hexamers. In a second step, the behavior of UO 2 2 + with respect to the hydrated Ni(1 1 1) face has been investigated. Evidence that two adsorption modes can take place was put forward: a first one with an outer sphere adsorption mechanism, where two water molecules of the uranyl ion first hydration shell are shared with four water hexamers, and a second one through a strong Ni–O −yle bond formation. Even though the second surface complex is energetically the most stable, the necessary activation energy to reach it makes it improbable.

Long-Range Proton Transport for the Water Reaction on Si(001): Study of Hydrogen-Bonded Systems with a Model Liquid−solid Interface

The reaction pathways for water dissociation at a model liquid−solid interface have been investigated by a combination of experimental and theoretical approaches. By scanning tunneling microscopy (STM) and high-resolution electron energy-loss spectroscopy (HREELS), we revealed that the fragments of condensed water molecules, i.e., OH and H, efficiently terminate the isolated dangling bonds on a precovered Si(001) surface, in comparison with those of the isolated water molecules on the same surface. The most favorable reaction mechanism was predicted by first-principles calculations. At the first stage, the condensed water molecules create a new surface OH group at one of the isolated dangling bond sites. Simultaneously, counter fragment H and surrounding water molecules form a flexible hydronium complex along hydrogen bonds, because the fragment H takes a certain positive charge. Then, another dangling bond is terminated by a H fragment under the proton relay mechanism via the hydronium complex, in which a very low activation energy is expected because the hydronium complex near the surface is not sufficiently stabilized as in the case of aqueous liquid but is hindered in shallow potential energy surfaces. Since the spatial hindrance near solid surfaces is a common property, the characteristic proton pathway should appear at various aqueous liquid−solid interfaces and enhance the surface reactions involving proton transfer.

Vibrational study of water dimers on Pt(1 1 1) using a scanning tunneling microscope

Adsorption and diffusion of isolated water molecules such as monomers and dimers on Pt(1 1 1) surface were directly observed by the scanning tunneling microscopy (STM) at very low coverage and at low temperature. A water dimer appears as a 6-fold symmetric “flower-like” protrusion, which can be explained based on the model that a hydrogen bond acceptor molecule in the dimer is rotating around a donor molecule. We obtained vibrational spectrum of individual water dimers by means of single-molecule vibrational spectroscopic method from the vibrationally-induced lateral hopping motions. Analyzing the spectra suggests a detailed model for adsorption structure of a water dimer where an acceptor molecule of the dimer points toward the surface to form O–H–Pt hydrogen bonding.

Dynamic Ion Pairs in the Adsorption of Isolated Water Molecules on Alkaline-Earth Oxide (001) Surfaces

Low coverage water adsorption on alkaline-earth oxides is studied by first principles methods and ab initio molecular dynamic simulations. On CaO and BaO(001), water dissociation is thermodynamically favored in contrast with the molecular adsorption found for MgO(001). On CaO(001), the barrier for water splitting is very small leading to a tight ion pair H-OH that exhibits a dynamic character with the hydroxyl group able to visit four equivalent adsorption sites while remaining attached to the proton. In contrast, ion pair separation is endothermic by 0.8 eV. These results are common to other basic surfaces such as BaO(001) and have important implications in the chemistry of partially hydroxylated oxide surfaces.

Hopping Motion and Reaction of a Single Water Molecule on Cu(110)

The authors studied the isolated water molecules and hydroxyl species on Cu(110) using a low-temperature scanning tunneling microscope (STM). A water molecule adsorbs on top of Cu atom and thermally hops along the atomic row at 6 K. The migration is also induced by STM via the excitation of molecule-substrate vibration. An isolated hydroxyl species, produced by dissociating a water molecule with the STM, adsorbs at the short-bridge site with its axis inclined along [001]. The orientation was found to be dynamically flipped, wherein substantial isotope effect was observed in the rate.

The Water Forcefield: Importance of Dipolar and Quadrupolar Interactions

It is widely recognized that dipolar interactions play a fundamental role in water. Less emphasis is put on the effects dues to the quadrupole moment. The recent calculation of the phase diagram for several rigid, nonpolarizable water models has shown that this is a rather severe test for water potentials. In this work, we analyze the results yielded by popular three-point-charge water models (TIP3P, SPC, SPC/E, TIP4P, TIP4PEw, TIP4P/2005, and TIP4P/Ice) and attempt to correlate them with the molecular dipoles and quadrupoles. It is shown that the melting temperatures of the proton disordered ices depend almost linearly on the quadrupole moments. However, the relative stability of ice II with respect to ices III, V, and VI seems to be rather dependent on the ratio of dipolar/quadrupolar forces. Small departures from a given ratio increase the stability of ice II and result in a serious deterioration of the phase diagram. Simple expressions are derived for the dipole and quadrupole moments, which allow us to analyze the effect of these multipole moments in the phase diagram when the rest of the molecular parameters are fixed. Satisfactory results for both the melting temperatures and for the relative stability of the different ices are obtained only when the molecular quadrupole approaches that of the isolated water molecule. Or, expressed in terms of the model parameters, acceptable results require that the negative charge be shifted from the oxygen toward the hydrogen positions by 0.14 A or more. I. Introduction Water is probably the most studied substance in nature. This is not only a consequence of its importance in our everyday life, but it is also due to its peculiar behavior which makes it the target of a great number of theoretical investigations. Both sources of interest (theoretical and practical) are closely related since many industrial and biochemical processes ultimately rely on its unusual physicochemical properties. Hence, there is interest in an adequate description of the intermolecular interactions in water. The complexity of the water properties together with the different possible levels of description have led to the proposal of hundreds of models (see the excellent review by Guillot 1 for a critical analysis of the results yielded by these models). At first, it would seem feasible that an analytical fit of ab initio calculations of the potential energy surface (PES) of water dimers could provide accurate potential functions. 2 Unfortunately, the results did not confirm the expectations. This is because the procedure has a number of limitations. The first is due to the reduced number of points used to represent the potential energy surface. Besides, since the potential energy represents only a small fraction of the total energy of the system, the precision of the calculations may compromise the results. Moreover, calculations based on the water dimer or more complex water clusters do not necessarily give an accurate representation of water in condensed phases. For these reasons, most of the popular water models are still empirical. However, promising results have been recently obtained with more refined electronic calculations. This is the case, for instance, of the MCDHO model 3 obtained from a fit of a refined ab initio single molecule deformation PES. 4 These calculations provide quali

Coordination geometry induced control of channel morphology in divalent metal pyridinedicarboxylate coordination polymers containing a kinked tethering organodiimine

Hydrothermal synthesis has afforded two divalent metal coordination polymers incorporating tridentate 2,6-pyridinedicarboxylate (PDC) ligands and the kinked dipodal organodiimine 4,4′-dipyridylamine (dpa), {[Ni(PDC)(dpa)(H 2O)] · 2H 2O} ( 1) and {[Zn(PDC)(dpa)] · 3H 2O} ( 2), which were characterized by single crystal X-ray diffraction and spectral and thermogravimetric analyses. Although both 1 and 2 display one-dimensional (1-D) polymeric chain motifs, the different coordination environments (octahedral in 1, distorted square pyramidal in 2) provoke divergence in the structures and aggregations of the chain subunits. Compound 1 manifests both polycatenation and interdigitation of its 1-D polymeric chains, while 2 exhibits only interdigitation, resulting in widely disparate morphologies for water molecule-bearing channels within the extended structures. Compound 1 possesses three distinct channel types occupied by isolated water molecules. Compound 2 presents only one type of channel, larger than those in 1, filled with D(5) discrete-chain water molecule aggregations. In both cases the co-crystallized water molecules are anchored to the coordination polymer matrix by hydrogen bonding involving PDC carboxylate oxygen atoms and the central amine unit of the dpa ligands. These supramolecular interactions are crucial for stability, as 1 and 2 both undergo irreversible loss of crystallinity upon dehydration.

Energetics of ice nanotubes and their encapsulation in carbon nanotubes from density-functional theory

We have performed total-energy electronic-structure calculations based on the density-functional theory that clarify atomic and electronic structures of ice nanotubes encapsulated in carbon nanotubes. We have found that three tubular structures of ice are stable in carbon nanotubes. The structural characteristics of these ice nanotubes is a straight stacking or helical arrangement of polygonal units consisting of water molecules. In these stable ice nanotubes, each water molecule is fourfold coordinated with neighboring water molecules through hydrogen bonds. The energy gain upon encapsulation of ice nanotubes in carbon nanotubes is found to be an order of $10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ per water molecule, whereas the energy gain to form ice nanotubes from isolated water molecules is about a half eV per molecule. Carbon nanotubes act as a template to form the new polymorph of ice. Detailed structural analysis has revealed a role of hydrogen-bond angles in determining energetics of the ice nanotubes. We have also clarified that energy gaps of ice nanotubes are comparable with the gap of the most stable ice polymorph Ih and that the electronic structure of ice nanotubes encapsulated in carbon nanotubes is a superposition of the energy bands of the constituents.

Water as a molecular hinge in amidelike structures

The authors have studied the reorientational dynamics of isolated water molecules in a solution of N,N-dimethylacetamide (DMA). From linear spectra, the authors find that the water in this solution forms double hydrogen bond connections to the DMA molecules, resulting in the formation of DMA-water-DMA complexes. The authors use polarization-resolved mid-infrared pump-probe spectroscopy on the water in these complexes to measure the depolarization of three distinct transition dipole moments, each with a different directionality relative to the molecular frame (OH stretch in HDO, symmetric and asymmetric stretch normal modes in H(2)O). By combining these measurements, the authors find that the system exhibits bimodal rotational dynamics with two distinct time scales: a slow (7+/-1 ps) reorientation of the entire DMA-water complex and a fast (0.5+/-0.2 ps) "hinging" motion of the water molecule around the axis parallel to the connecting hydrogen bonds. Additionally, the authors observe an exchange of energy between the two normal modes of H(2)O at a time scale of 0.8+/-0.1 ps and find that the vibrational excitation decays through the symmetric stretch normal mode with a time constant of 0.8+/-0.2 ps.

Transient diffusion and cluster formation of water molecules on Rh(111) at 20K

The authors investigated the initial stage of water adsorption on Rh(111) at 20 K, using infrared reflection absorption spectroscopy. In this low coverage region, isolated water molecules and small water clusters are observed. Since thermal diffusion is suppressed at 20 K, the formation of water clusters at low coverage is controlled by both coverage and transient diffusion on the surface. Within a simple random walk model of the transient diffusion and clustering process, the authors estimate the mean lateral displacement from the first impact point to the final adsorption site to be 7.6 A; an incoming water molecule on Rh(111) is trapped with eight postcollision hops on the average.

Surface Dynamics of Water Monomers and Dimers on Pt(111) Surface

Isolated water molecules such as monomers and dimers on Pt(111) surfaces with low degree of coverage were directly observed by scanning tunneling microscopy (STM) at low temperatures. A water monomer is shown as a single protrusion in the STM image. While, a dimer appears as a 6-fold symmetric “flower-like” protrusion, which can be explained by the model that one of the water molecules in the dimer is rotating around the other. The adsorption sites of a dimer were also determined as both molecules on atop sites. Indeed, we have succeeded in forming and breaking dimers by exciting molecular vibrations with inelastically tunneled electrons from an STM tip. It turns out that a dimer is more stable than a monomer when the sample bias voltage is shifted. We also report on vibrationally induced lateral hopping motion of both monomer and dimer molecules by injecting tunneling electron.

An 2H(D) isotope shift in the 1H NMR spectra of water in gaseous environment of fluoromethanes

Measurements of 1H chemical shifts in H 2 16O and HD 16O molecules in gaseous mixtures with excess of fluoromethanes CH n F 4 − n with n ⩽ 4 have been obtained. Over the range of accessible pressures of the gaseous buffers used at room temperature the proton chemical shift vs. density of the medium relation are linear for either isotopomer and can be subjected to linear regression analysis. This analysis allowed to find the proton shielding constants in isolated water molecules. The isotope effect associated with this shieldings 2Δ 1H ( 2/1H) was evaluated to be 0.0386 ppm and the value was compared with the available literature data. The new result for the isotope effect shows a better fit to the results obtained by the ab initio calculations, than the experimental data thus far known for liquid phase. Obviously, for the strongly polar water molecules the interactions in liquids are very strong and significantly affect the 1H shielding parameters. Thus measurements in gaseous solutions proved expedient, with extrapolation of the results to a zero pressure. The effect of molecular interactions in the CH n F 4 − n /H 2O/HDO systems on the shielding of protons of water in the gaseous complexes formed were discussed. The spin–spin coupling 2 J( 1H, 2H) was measured experimentally (−1.06 Hz) in the partly deuterated water in gas phase for the first time. The coupling constant 2 J( 1H, 1H) found on this basis is −6.89 Hz.

Water Adsorption and Diffusion on NaCl(100)

At low coverage and temperature the water-surface interaction determines the adsorption geometry of the water molecule on the NaCl(100) surface. However, at room temperature the molecules are also able to move on the surface and form islands where the water molecules are held together by hydrogen bonds. As a step toward the description of such complex phenomenology, in this work we have used density functional theory calculations to study the most favorable adsorption geometry of an isolated water molecule and the energy barriers associated with different hopping mechanisms between equivalent adsorption configurations on this surface. We propose different hopping processes that can be classified as translations, if the molecule moves from one adsorption site to the adjacent one, or reorientations, if the molecule only changes its orientation on the surface and remains in the same adsorption site. The straightforward parallel translation of the water molecule along the surface exhibits the highest barrier. All other processes, either translations or reorientations, involve the rotation of the water molecule around certain axes and present much smaller barriers (at least 50% smaller). To obtain a net movement of the molecule along the surface it is always necessary to combine one of these translational and reorientational processes. Such combinations provide favorable and plausible pathways for the diffusion of the water molecule on the NaCl(100) substrate.