Solid-liquid phase transition in small water clusters: a molecular dynamics simulation study

Water clusters, (H₂O) _n , of varying sizes (n = 8, 12, 16, 20, 24, 28, 32, 36, and 40) have been studied at different temperatures from 0 to 200 K using molecular dynamics simulations. Transitions between solid and liquid phases were investigated to estimate the melting temperature of the clusters. Although the melting temperatures showed non-monotonic behaviour as a function of cluster size, their general tendency follows the classical relationship T _m ∝ n ^?1/3 to the cluster size n. Moreover, it was observed that the liquid-solid surface tension decreased with the cluster size in a similar way to the liquid-vapour surface tension in bulk water. Upon cooling, ice-like crystals were formed from the smaller clusters with n up to 20, while the larger clusters were transformed to glassy structures. The decrease in the glass transition temperature with the cluster size was observed to be much less than the corresponding melting temperature. The mutual order of the melting and glass-transition temperatures were found to be reversed compared with that observed for bulk water.     

Solvation properties of Li+ and CI− in water: molecular dynamics simulation with a non-rigid model

Molecular dynamics (MD) simulations were carried out to investigate the solvation properties of Li⁺ and C1^? ions in water with a relatively accurate but rarely used non-rigid model, RWK2, in this study. A new set of ion-water interaction parameters was evaluated from the experimental data and first principles calculation results of stable clusters, Li⁺(H₂O) _n (n = 1- 6) and CI^?(H₂O) _n (n = 1–4). With the ion-water potential parameters evaluated from the data of the clusters and the water-water potential predetermined from the non-rigid RWK2 model, the structural (radial distribution functions, angular distribution functions, spatial distribution functions, coordination number), dynamical (residence time) and energetic properties of the ionic salvations in bulk water were studied through a comprehensive analysis of our MD simulation outputs. These results not only agree well with experimental data and first principles calculations, but also reveal some new insights into the microscopic ionic salvation processes.     

Investigation of structural evolution of hydrogen-bond networks of water clusters at 1 and 300 K

Computer simulation of water clusters (H₂O) _n , Na⁺(H₂O) _n and K⁺(H₂O) _n (n ?? 8) at temperatures of 1 and 300 K was carried out using the Monte Carlo method. All the types of the hydrogen-bond network structures at equilibrium configurations of the water clusters and, among them, the most probable ones at temperatures of 1 and 300 K, were detected. The analysis of the most probable types of hydrogen-bond networks at equilibrium configurations of water clusters revealed that clusters with n water molecules inherit the configuration of clusters with n ? 1 water molecules with an accuracy of one hydrogen bond.     

Drainage and water clusters in Gillette foam

Raman measurements on Gillette foam have been carried out to analyze different phases of water in the system. We have shown that in addition to free water molecules, which drain out with aging of foam, water clusters of only a few water molecules are also present in foam. We have analyzed the rate of drainage with the existing theory available in the literature. The nature of water clusters in Gillette foam has also been obtained from ab initio self‐consistent field calculations for [H₂O]_n clusters. Copyright © 2008 John Wiley & Sons, Ltd.     

Cluster Structure of Liquid Alcohols,Water, and <Emphasis Type="Italic">n</Emphasis>-Hexane

The processes of cluster formation in liquid alcohols, water, methanol, n-hexanol, and n-hexane have been investigated by the method of flicker-noise spectroscopy. Two types of clusters — clusters with a close-packed structure and clusters with a loose structure — have been detected. The energy of formation of different clusters in methanol and n-hexane ranges, respectively, from −250 to +250 J/mole and from −50 to +50 J/mole. The smallest clusters of methanol, n-hexanol, water, and n-hexane consist, respectively, of six, two, eleven, and two molecules, and their largest clusters represent oscillators consisting, respectively, of 50,400, 17,200, 93,500, and 33,150 molecules at 274 K. In methanol at 271 K, more than 44 types of clusters consisting of 6, 97, 152, 219, 297, 492, 1029, 1368, 1560, etc. molecules were detected. In n-hexanol at 273 K, 57 types of clusters were detected. Models of small clusters are proposed. In water, the content of close-packed clusters is maximum at 277 K. The energy of formation/decomposition of small clusters in water ranges from −0.4 to +0.4 kJ/mole and increases with increase in the water temperature. The hysteresis of transformation of the (H₂O)₂₈₀ cluster in the process of heating and cooling of water in the temperature range 273–280 K was detected. Series of energy spectra of clusters in liquids at different temperatures are presented and discussed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 305–312, May–June, 2005.     

From collisions to clusters: first steps of sulphuric acid nanocluster formation dynamics

The clustering of sulphuric acid with base molecules is one of the main pathways of new-particle formation in the Earth's atmosphere. First step in the clustering process is likely the formation of a (sulphuric acid)₁(base)₁(water)_n cluster. Here, we present results from direct first-principles molecular dynamics collision simulations of (sulphuric acid)₁(water)_{0, 1} + (dimethylamine) → (sulphuric acid)₁(dimethylamine)₁(water)_{0, 1} cluster formation processes. The simulations indicate that the sticking factor in the collisions is unity: the interaction between the molecules is strong enough to overcome the possible initial non-optimal collision orientations. No post-collisional cluster break up is observed. The reasons for the efficient clustering are (i) the proton transfer reaction which takes place in each of the collision simulations and (ii) the subsequent competition over the proton control. As a consequence, the clusters show very dynamic ion pair structure, which differs from both the static structure optimisation calculations and the equilibrium first-principles molecular dynamics simulations. In some of the simulation runs, water mediates the proton transfer by acting as a proton bridge. In general, water is able to notably stabilise the formed clusters by allocating a fraction of the released clustering energy.     

Spectral characteristics of water clusters in the interaction with ozone molecules and bromide ions

The molecular dynamics method is used to study the interaction of the (Br⁻) _i (H₂O)_{50 − i} clusters in a medium of water vapor with ozone molecules. The clusters absorb O₃ molecules and retain them, along with Br⁻ ions, for a 25-ps-long calculation procedure. The presence of bromide ions results in significant increases in the values of the real and imaginary parts of the relative permittivity. The addition of bromide ions causes a significant increase in the integrated IR radiation absorption intensities and in the radiant power emitted by the clusters. The addition of Br⁻ ions only slightly affects the intensity of the Raman spectra until the number of Br⁻ reaches six, when a dramatic decrease of the integrated intensity of this spectrum occurs. Bromide ions absorbed by water clusters produce a much more lasting impact on the ozone molecules trapped by the cluster than chlorine ions do, all other things being equal.     

Ab Initio Investigation of the Microspecies and Energy in Hydrated Strontium Ion Clusters

Quantum chemistry calculations were used to study the structure and energy of strontium (Sr) ion hydrated clusters [Sr(H₂O)_1?25]²⁺. The saturated hydration number of the first hydration layer of Sr²⁺ was 8, and the hydration distance was 2.58 Å. The second hydration layer had 1–9 hydration numbers, and the hydration distance was in the range of 4.4–4.6 Å. This work also developed the relationship between the thermodynamic data (average water binding energy E_n and successive water binding energy ΔE_n,n?1, etc.) of the aforementioned low-energy structure and the hydration structures. The first hydration layer was formed by the strong electrostatic interaction between Sr²⁺ and water molecules, and the decrease in ΔE_n,n?1 was relatively large. Hydrogen bonds were formed between water molecules of the second hydration layer and water molecules of the inner layer, and the decrease in ΔE_n,n?1 was relatively small. When one water molecule was added beyond the second hydration layer, ΔE_n,n?1 was close to the hydrogen bond energy 8.88 kcal/mol (37.1 kJ/mol) of dimer water molecule, indicating that there was very weak interaction between Sr²⁺ and the water molecules beyond the second hydration layer.     

A computer study of the absorption spectra of the water-carbon monoxide disperse system

The IR absorption, emission, and reflectance spectra of (CO) _i (H₂O)₂₀, 1 ≤ i ≤ 10, clusters were calculated using the molecular dynamics model. After the adsorption of CO molecules by clusters, IR radiation absorption and reflection by the system composed of them weakened, whereas thermal radiation power increased. We tracked changes in spectral characteristics as the number of molecules in clusters increased. Growing water clusters decreased absorption and gradually increased IR emission power. The growth of a water cluster with the addition of CO molecules to it as a rule caused the opposite effect. On the whole, clusterization and the entrapment of CO molecules by clusters had an antigreenhouse effect.     

The Structure of Water Clusters Interacting with Gaseous Acetylene

The interaction of water clusters with acetylene molecules at T = 230 K was studied by the molecular dynamics method. The structure of clusters was analyzed by constructing Voronoi polyhedra. Water clusters interacting with C₂H₂ molecules are characterized by a diversity of H-bond orientations, a more uniform distribution of H-bonds over the cluster volume, a larger number of bonds per atom, and smaller bond lengths. The spectrum of bond lengths broadens as the number of acetylene molecules interacting with the water cluster increases. C₂H₂ molecules have a pressing action on water clusters.     

Solvation and chemical reaction of sodium in water clusters

Na_m(H₂O)_n Clusters ( n = 1...200, m = 1...50) are formed in a recently build pick-up arrangement. Preformed water clusters traverse a sodium oven, where sodium atoms are picked up. At low sodium vapour pressure ( < 1×10^-4 mbar) pure Na(H₂O)_n clusters are observed in the mass spectra. At high sodium vapour pressure ( > 1×10^-3 mbar) the water cluster pick up more than 50 Na atoms and reaction products Na(NaOH)_n ( n = 2, 4...50) dominate the mass spectra. The even number of NaOH units in the products indicate that also in a finite cluster the reaction occurs in pairs as in the macroscopic reaction. Received 4 December 2000     

Ionic solvation and association in LiCl aqueous solution: a density functional theory,polarised continuum model and molecular dynamics investigation

In this work, the ionic solvation and association behaviours in the LiCl aqueous solution were investigated using density functional theory (DFT), a polarised continuum model and classical molecular dynamics simulations. DFT calculations of LiCl(H₂O)_1–6,8 clusters show that contact ion pair (CIP) and solvent-shared ion pair (SSIP) conformers of LiCl(H₂O)_n (n ≥ 4) clusters are generally energetic both in the gas phase and in the aqueous solution. Some SSIP conformers may be slightly more stable than their CIP isomers when at least eight water molecules are incorporated in the inner hydration shells of LiCl hydrates. The transformation between CIP and SSIP conformers is easy by overcoming a small energy barrier, which mainly results from the hydration shell reorganisation of Li⁺. Molecular dynamics simulations show that ion pairs or ion clusters can be found in the LiCl aqueous solution, and the probability of CIP conformers or ion clusters presented in the LiCl solution generally increases with rise in temperature. However, the presentation of ion pairs or ion clusters in the LiCl aqueous solution does not inevitably lead to the nucleation of LiCl crystallisation.     

Dynamical and structural properties of adsorbed water molecules at the TiO2 rutile-(110) surface: interfacial hydrogen bonding probed by ab-initio molecular dynamics

ABSTRACT

Ab-initio molecular dynamics (AIMD) simulations have been carried out to study a range of different and energetically-accessible adsorbed-water configurations and motifs for their vibrational and structural characteristics, in contact with rutile-(110) interfaces at 100?K. The radial pair distribution function between the titanium atoms at the interface and the hydrogen and oxygen atoms in the water monolayer show an orientation of the water molecules parallel to the surface of titania, and with hydrogen atoms pointed in the opposite direction to the surface. In some cases, a distinctive vibrational frequency region between 2500 and 3000?cm^?1 has also been observed, due to a strong dispersion interaction between water molecules. This behaviour is also seen in experimental studies of thin-film water coverage on TiO₂ surfaces.     

Dipole moment of a small water cluster. The effect of size,temperature, and electric field

A molecular dynamics simulation of neutral clusters (H₂O) _{n ≤ 21} has been performed in the framework of the flexible polarized model. The formation and evolution of the dipole moment of the cluster have been investigated with a change in the size and temperature of the cluster and an external electric field. It has been shown that at low electric fields corresponding to the experiments on the deflection of clusters in the transverse inhomogeneous electrostatic field (Moro et al., 2006), the induced polarization of the cluster is determined by the orientational polarizability of the “rigid” cluster, rather than by the intracluster reorientation of the molecules. The calculated dependence of the effective polarizability of the cluster in the low field on n qualitatively reproduces the experimental results, but the calculated polarizability is numerically much higher than the experimental value by, e.g., a factor of 4 for n ≈ 20.     

Comprehensive facilitating of water oxidation reaction by ultrasonic attenuation of hydrogen-bonded structure of water

The balance between water-metal interactions and water-water hydrogen bonding (HBs) controls the process of water adsorption on metallic surfaces. In other hand, the yield of oxygen evolution reaction (OER) is dependent on the binding energy of H₂O at electrode surface. Therefore, on a specific metal substrate, attenuation of HBs may be a promising route for improving OER. In this study, the computational and experimental evidences indicate that the performance of ultrasonically irradiated deionized water (USI-DW), participated in water oxidation reaction (WOR), is different from its in the intact bulk water. To date, establishing of new electrocatalysts with lower overpotentials (η) and higher current densities (J) in OER have been mostly considered based on metals and oxide materials. Here, we ultrasonically agitated the water clusters formed by strong HBs, and as a sustainable improvement route explored its particular effects on the efficiency of OER. The molecular modeling (MM) of the (H₂O)_n clusters (n = 1–100 molecules), the corresponding IR spectra, the molecular orbitals energy levels and the adsorption of free and cluster confined H₂O molecules on the Pt surface were studied by the appropriate quantum mechanical (QM) methods. The result of deconvolution of FTIR spectra recorded for USI-DW in the –OH stretching region (∼2600–3900 cm⁻¹) properly confirmed the expected increase of the single water molecules. The reduction in overpotentials was 82 ± 8 mV and 158 ± 12 mV, to reach the J of 1 mA cm⁻¹ at the typical pHs 12.2 and 13.1, respectively.     

单母体遗传算法优化(H2O)n(n≤14)的结构

在传统遗传算法的基础上提出了单母体遗传算法(single-parent genetic algorithm, SPGA)，通过对母体团簇实施两种不同的变异操作对结构进行优化，给出了分子团簇结构优化的算法实现. 结合TIP3P模型势函数，研究了水分子团簇(H₂O)_n(n≤14)的稳定结构. 优化结构和已有理论及实验结果一致. 计算结果表明当n<8时，平均结合能随n增加较快；当n≥8时有小的起伏. n=4,8,10,12的团簇结构具有较高对称性，比较稳定. 关键词： 单母体遗传算法 水分子团簇 结构优化     

Isotope effect and cluster size dependence for water and hydrated hydrogen halide clusters: multi-component molecular orbital approach

In order to explore the proton/deuteron (H/D) isotope effect on the structures, wavefunctions, and size dependence of water clusters, both electronic and nuclear wavefunctions are determined simultaneously. The optimized centres and the exponents for the nuclear orbitals indicate the Ubbelohde effect, i.e. the deuteron has weaker hydrogen bonding than the proton. Calculations are made also of hydrogen halide water clusters, Such as HF(H₂O)_n, HB_r(H₂O)_n, (n = 0–4), and their deuterated species. Only the hydrogen transferred ring structure is optimized for the protonic HBr (H₂O)₃ cluster, while both the hydrogen transferred and the non-transferred structures are obtained for the deuterated DBr (H₂O)₃ cluster under the one-particle multi-component treatment. The proton in the HF molecule is localized more than those in the HCl and HBr molecules, and no hydrogen transferred structures are obtained for HF water clusters.     

Hyperthermal surface-collisions of water cluster cations

Size-selected, protonated water cluster cations (H₂O)_nH⁺, 4 n 32, are scattered at normal incidence from the surface of a diamond-coated silicon wafer at collision energies 0 E _coll 500 eV. The size distribution of collision-induced fragment-ions and the ion yield of scattered particles are analyzed, using a secondary time-of-flight mass spectrometer, as a function of the cluster size, n, and the collision energy, E _coll. Even at low impact energies only very small fragment-ions can be detected, with a maximum fragment size of 35% of the colliding parent cluster ions. For clusters consisting of more than 10 molecules, the protonated water dimer (H₂O)₂H⁺ becomes the predominant fragment-ion. The total charge survival yield obeys a nonlinear increase with cluster size; for the largest clusters investigated, more than 35% of the impacting ions survive the surface collision in the cationic charge state.     

Wn (n= 1–6)团簇吸附CO的密度泛函研究

采用密度泛函理论(density functional theory, DFT) 在B3LYP/LANL2DZ基组水平上对钨团簇吸附CO分子进行了系统研究. 结果表明, W_nCO团簇的基态结构是在W_n团簇中性或阴离子基态结构的基础上吸附CO生长而成; CO的吸附以端位吸附为主,桥位吸附为辅; CO分子在W_n团簇表面发生的是非解离性吸附. 与优化的CO键长(0.116 nm)相比,吸附后C-O键长变长(0.120–0.123 nm), 表明吸附后C-O键被削弱, CO分子被活化了.稳定性分析表明,在所研究的团簇中, W₃CO和W₅CO团簇的稳定性较强;自然键轨道(NBO)分析表明, W原子与CO分子相互作用的本质是CO分子内的杂化轨道与W原子6s, 5d, 6p和6d轨道相互作用的结果. 关键词： nCO (n= 1–6)团簇')" href="#">W_nCO (n= 1–6)团簇 基态构型 稳定性 电子性质     

Magnetic properties of stable structures of small binary clusters

We have studied the optimum geometries and the magnetic behavior of small binary Fe_nGe_m (n+m≤4) clusters usingab initio spin-polarized density functional calculations. Our results reveal that the optimized clusters present high values in the HOMO–LUMO gap and generally prefer structures with high geometries, the local magnetic moments at Fe atoms present an enhancement with respect to Fe bulk magnetization, whereas the Ge atoms present local magnetic moments whose modulus take significative values. The magnetic coupling between Fe and Ge atoms is mainly antiferromagnetic-like. All the clusters studied here present a charge transference from Fe atoms to Ge atoms.