Dynamics Of Water Clusters Research Articles

The structure of water

Images of water clusters have been reported recently. In this paper, the dynamics of water clusters when dissolving crystals of sodium chloride is studied under a microscope. Images of water clusters have two different colours, a dark outer layer and a shallow inner layer as a water cluster is composed of two phases. Since the experiments were conducted at room temperature there should be no solid phase (ice) in water clusters. Therefore, it is proposed that individual water clusters are composite of two phases, an outer liquid phase and an inner gas phase. Obviously, there will be water clusters everywhere in bulk water and they maintain a certain distance among them; there are spaces among water clusters which are filed with gases and named free volume. At a molecular level, hydrogen atom in a water cluster are pointing outward while oxygen atoms are pointing inward. In this configuration, the cluster is more stable. A formula for calculating the forces inside a water cluster is developed. In this study the structure of water is presented in a new light. The study has opened a door for further understanding of water and its mysterious properties.

Molecular Dynamics of Water Clusters and Interaction Potentials

The molecular dynamics method was used to analyze the dynamic characteristics of water clusters in the solid and liquid phase. A criterion is proposed for choosing the interaction potential, which is based on the distributions of the potential energy of cluster molecules in different phases. The connection of the obtained distributions with the dynamics and structure of the hydrogen bonds’ net of the cluster is shown.

Dynamics of water clusters in solution with LiCl

In this work we study by means of Nuclear Magnetic Resonance spectroscopy the dynamics of the different water clusters that form within a solution with LiCl at eutectic concentration in the temperature range 320–205 K. This solution is considered a model system allowing the investigation of water properties in the deep supercooled regime in its bulk phase. Our data reveal two important dynamical changes occurring at two relevant temperatures for water: the highest temperature coincides with that of the water density maximum (277 K) and the lowest with that of the so-called dynamical crossover (≃225K). We interpret our data in terms of the different influence that the ions exert on water by lowering the temperature and of the tendency that water displays to develop its characteristic hydrogen bond network.

Dynamics of water clusters upon UV‐excitation leading to ionization: Nonempirical study

AbstractNonempirical direct molecular dynamics of neutral and cationic water clusters involving four to six molecules is studied to shed light on the character of the ionization of water clusters. Potential energy and forces are calculated in the second‐order of the Møller‐Plesset perturbation theory with a 6–31++G(1d, 1p) basis set. Upon vertical ionization of a water cluster, H3O+ and OH fragments appear. The process needs no less than 40 fs, and in some cases, up to 100 or 150 fs. The resulting configuration of the cluster (neglecting its generally larger volume) is close to that obtained in the geometry optimization run. Vibrational excitation of a neutral cluster with a total energy about half its adiabatic ionization potential may promote the appearance of structures similar to that of the stable cation at certain moments in time. In particular, a configuration in which only one water molecule separates H3O+ and OH− fragments forms much faster than upon vertical electron detachment from a stable neutral cluster, in nearly 18 fs. Such structure reorganization is noticeably impeded by the rotational–vibrational excitation of the whole cluster, especially when hydrogen bonds are substantially distorted. The effect of distorting motions becomes very pronounced after 20 fs. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Molecular Dynamics Simulation Study of Water Adsorption on Hydroxylated Graphite Surfaces

In this paper, we present results from molecular dynamic simulations devoted to the characterization of the interaction between water molecules and hydroxylated graphite surfaces considered as models for surfaces of soot emitted by aircraft. The hydroxylated graphite surfaces are modeled by anchoring several OH groups on an infinite graphite plane. The molecular dynamics simulations are based on a classical potential issued from quantum chemical calculations. They are performed at three temperatures (100, 200, and 250 K) to provide a view of the structure and dynamics of water clusters on the model soot surface. These simulations show that the water-OH sites interaction is quite weak compared to the water-water interaction. This leads to the clustering of the water molecules above the surface, and the corresponding water aggregate can only be trapped by the OH sites when the temperature is sufficiently low, or when the density of OH sites is sufficiently high.

Structure and dynamics of water in aqueous methanol

We have performed classical molecular dynamics simulations to investigate thestructure and dynamics of water clusters in aqueous methanol across a range ofsolution compositions. Investigation of the effect of different model potentials andcluster membership criteria indicates that the qualitative trends in cluster sizedistributions are preserved. Characterization of the water molecules on the interiorof such clusters using a tetrahedral order parameter previously used to studypure water suggests that they are over-structured compared to the pure liquidfor a broad range of cluster sizes and at different compositions. Calculation ofthe hydrogen-bond dynamics indicates that the water–water hydrogen-bondsin the solution are longer lived than those in the pure liquid for all solutionsstudied and increase almost linearly as a function of methanol concentration.

Formation and dynamics of water clusters on Ru(001)

The adsorption kinetics of water on Ru(001) was simulated using molecular dynamics (MD) and equilibrium-model approach. The results nicely reproduce observations from STM imaging, work function change, and IR measurements. The agreement with experimental results is based on the formation of stable clusters already at very low surface coverage and temperature. Tetramers are predicted to be relatively stable compared to smaller and larger clusters. The dipole moment per water molecule continuously decreases from 2.2D for the monomer down to 1.1D for pentamer and larger clusters. Dimers are found to diffuse faster than monomers or larger clusters, with activation energy for diffusion of 2.9 kcal/mol, in agreement with recent STM measurements. A unique mechanism for dimers diffusion is proposed. Temperature programmed desorption (TPD) spectra from a metal surface were calculated by employing the MD scheme. These spectra were found identical to the standard Redhead line-shape analysis of the experimental TPD spectra of water from Pt(111) and Ru(001), an observation that was used to verify the consistency of the MD procedure. Finally, a kinetic model, fed by the MD calculated decreasing dipole moments per water molecule at larger clusters, explains well the highly nonlinear initial stages (up to 0.35BL) of the work function change data determined experimentally.

Proton Transfer Processes in Water Clusters

AbstractElucidation of some of the basic mechanisms and principles of proton transfer in clusters is achieved by systematic evaluation of experimental and theoretical results that were obtained throughout the last decade from the authors' studies on ionic water clusters and, more recently, on protonated rare gas clusters. Among the processes studied are: Blackbody radiation‐induced, stepwise desolvation of clusters when stored in an ion trap; ionic dissociation and recombination of HCl in water clusters; metal oxidation in aqueous clusters; formation of nascent hydrogen in reactions of solvated metal cations with acids; and the behavior of ionic water clusters as acidic or basic micro‐solutions. Infrared spectroscopic studies of size‐selected water clusters are expected to advance our knowledge of structure and dynamics of water clusters and of excess protons therein.

Classical dynamics of hydrogen bonded systems: Water clusters

The minimum energy structures and the classical dynamics of (H2O)n clusters, with n=3–6,8, have been investigated with the potential function of Cieplak, Kollman, and Lybrand. The potential is the sum of a pairwise additive part plus a polarization term calculated iteratively from the permanent and induced dipole moments. It is found that at the minimum energy structures the polarization energy makes a contribution of about 13% in the total energy. Caloric curves and rms fluctuations of the oxygen–oxygen bond lengths show that the tetramer, with a tetragonal minimum geometry, and the octamer, with a cubic minimum geometry, are more stable clusters than the pentamer and hexamer. The geometries of absolute minima for the trimer, tetramer, and octamer agree with those predicted by other potentials. For the pentamer and hexamer the present potential predicts floppy type structures instead of cyclic ones, which are supported by other functions and ab initio calculations. The dynamics of water clusters is chaotic with a characteristic time for forgetting the initial conditions equal to the period of the stretching mode of the hydrogen bond. Phenomena of coexistence appear for the hexamer and larger clusters for which classes of minima of different geometrical type exist. For all studied clusters the power spectra of the oxygen velocity autocorrelation function are characterized by two frequency bands; one around 30 cm−1, torsional, and one at 200 cm−1, stretching. Generally, no significant influence on the dynamics of the clusters is found by the inclusion of polarization interaction, although for the hexamer and octamer polarization energy results in interchanging the geometries of absolute minima.