Small Water Clusters Research Articles

Thermally Induced Hydrogen-Bond Rearrangements in Small Water Clusters and the Persistent Water Tetramer.

Small water clusters absorb heat and catalyze pivotal atmospheric reactions. Yet, experiments produced conflicting results on water cluster distribution under atmospheric conditions. Additionally, it is unclear which “phase transitions” such clusters exhibit, at what temperatures, and what are their underlying molecular mechanisms. We find that logarithmically small tails in the radial probability densities of (H2O)n clusters (n = 2 – 6) provide direct testimony for such transitions. Using the best available water potential (MB-pol), an advanced thermostating algorithm (g-BAOAB), and sufficiently long trajectories, we map the “bifurcation”, “melting”, and (hitherto unexplored) “vaporization” transitions, finding that both melting and vaporization proceed via a “monomer on a ring” conformer, exhibiting huge distance fluctuations at the vaporization temperatures (Tv). Tv may play a role in determining the atmospheric cluster size distribution such that the dimer and tetramer, with their exceptionally low/high Tv values, are under/over-represented in these distributions, as indeed observed in nondestructive mass spectrometric measurements.

DFT performance in the IQA energy partition of small water clusters

This paper addresses an assessment of the performance of a large set of exchange-correlation functionals in the description of hydrogen bonding within the interacting quantum atoms (IQA) energy partition. Specifically, we performed IQA analyses over a series of small water clusters $$(\hbox {H}_{2}\hbox {O})_{{n}}$$ with $$n \le 6$$. Apart from LDA-like approximations, all the considered families of exchange-correlation functionals (GGA, meta-GGA, and hybrid) reproduce the trends associated with hydrogen bond non-additive effects computed with reference Moller–Plesset and coupled cluster wave functions. In other words, the IQA energy partition together with most of the functionals addressed herein produce good results concerning the study of non-additivity in hydrogen bonds at a reduced cost as compared with correlated wave functions approximations. These conditions might be further exploited in the examination of larger hydrogen-bonded complexes.

Direct observation of the water dimer adsorbed on graphite

Water/solid interfaces are closely related to our daily lives and play an important role in nature and technology. In this paper, we investigate the water molecules adsorbed on highly oriented pyrolytic graphite (HOPG) surface. The frontier molecular orbitals of adsorbed water dimer are directly observed by means of the low-temperature scanning tunneling microscopy (LT-STM). Furthermore, we find there are many water dimers and few small water clusters on HOPG, since the average adsorption energy per molecule of water dimer are higher than that of others. These results shed important light on the water/graphite interactions.

Ionized water clusters , n = 2 to 6: A high‐accuracy study of structures and energetics

AbstractIonized water clusters, , have been of remarkable interest owing to their crucial roles in many chemical and biological processes. Small cationic water clusters , n = 2 to 6 serve as reasonable models for understanding the nature of the ionized water. In this study, employing high‐level ab initio quantum chemical methods, such as the density‐fitted orbital‐optimized linearized coupled‐cluster doubles (DF‐OLCCD), coupled‐cluster singles and doubles (CCSD), and coupled‐cluster singles and doubles with perturbative triples [CCSD(T)], a high‐accuracy study of structures and energetics for cationic water clusters [, n = 2‐6] is presented. In this study, 2 dimer, 8 trimer, 18 tetramer, 23 pentamer, and 25 hexamer clusters are reported. Most of the structures considered are reported for the first time. Relative, binding, and vertical attachment energies (VAEs), for the first time, are presented at the complete basis set (CBS) limit, extrapolating energies of the aug‐cc‐pVTZ and aug‐cc‐pVQZ basis sets, to provide the most accurate energetics to date. Our results demonstrate that as cluster size increases, the VAE value decreases, which indicates that large‐size clusters better compensate for the electron deficiency compared with small‐size clusters. The VAE values for pentamer and hexamer clusters are 118.5 to 165.5 and 121.9 to 153.7 kcal mol−1, respectively. Further, our binding energy results, at the CCSD(T)/CBS level, indicate strong bindings in cationic clusters due to hydrogen bond interactions. The average binding energy per water molecule varies from −16.6 to −21.8 kcal mol−1 for the clusters considered. Hence, we present the most extensive and accurate study on ionized water clusters to date. Further, our results indicate that the DF‐OLCCD method is very promising for ionic molecular clusters, and its accuracy approaches the CCSD(T) quality. The inexpensive analytic gradients of DF‐OLCCD, compared with CCSD(T), make it very helpful for high‐accuracy studies of molecular geometries.

Tunneling Splittings in Water Clusters from Path Integral Molecular Dynamics.

We present calculations of tunneling splittings in selected small water clusters, based on a recently developed path integral molecular dynamics (PIMD) method. The ground-rotational-state tunneling motions associated with the largest splittings in the water dimer, trimer, and hexamer are considered, and we show that the PIMD predictions are in very good agreement with benchmark quantum and experimental results. As the tunneling spectra are highly sensitive to both the details of the quantum dynamics and the potential energy surface, our calculations are a validation of the MB-Pol surface as well as the accuracy of PIMD. The favorable scaling of PIMD with system size paves the way for calculations of tunneling splittings in large, nonrigid molecular systems with motions that cannot be treated accurately by other methods, such as the semiclassical instanton.

Multistate Reactive Molecular Dynamics Simulations of Proton Diffusion in Water Clusters and in the Bulk.

The molecular mechanics with proton transfer (MMPT) force field is combined with multistate adiabatic reactive molecular dynamics (MS-ARMD) to describe proton transport in the condensed phase. Parametrization for small protonated water clusters based on electronic structure calculations at the MP2/6-311+G(2d,2p) level of theory and refinement by comparing with infrared spectra for a protonated water tetramer yields a force field which faithfully describes the minimum energy structures of small protonated water clusters. In protonated water clusters up to (H2O)100H+, the proton hopping rate is around 100 hops/ns. This rate converges for 21 ≤ n ≤ 31, and no further speedup in bulk water is found. This indicates that bulklike behavior requires the solvation of a Zundel motif by ∼25 water molecules, which corresponds to the second solvation sphere. For smaller cluster sizes, the number of available states (i.e., the number of proton acceptors) is too small and slows down proton-transfer rates. The cluster simulations confirm that the excess proton is typically located on the surface. The free-energy surface as a function of the weights of the two lowest states and a configurational parameter suggests that the "special pair" plays a central role in rapid proton transport. The barriers between this minimum-energy structure and the Zundel and Eigen minima are sufficiently low (∼1 kcal/mol, consistent with recent experiments and commensurate with a hopping rate of ∼100/ns or 1 every 10 ps), leading to a highly dynamic environment. These findings are also consistent with recent experiments which find that Zundel-type hydration geometries are prevalent in bulk water.

Probing the Partial Activation of Water by Open-Shell Interactions, Cl(H2O)1-4.

The partial chemical activation of water by reactive radicals was examined computationally for small clusters of chlorine and water, Cl•(H2O)n=1-4. Using an automated isomer-search procedure, dozens of unique, stable structures were computed. Among the resulting structural classes were intact, hydrated-chlorine isomers, as well as hydrogen-abstracted (HCl)(OH)(H2O)n-1 configurations. The latter showed increased stability as the degree of hydration increased, until n = 4, where a new class of structures was discovered with a chloride ion bound to an oxidized water network. The electronic structure of these three structural classes was investigated, and spectral signatures of this hydration-based evolution were connected to these electronic properties. An ancillary outcome of this detailed computational analysis, including coupled-cluster benchmarks, was the calibration of cost-effective quantum chemistry methods for future studies of these radical-water complexes.

Observation of the Chemical Structure of Water up to the Critical Point by Raman Spectroscopic Analysis of Fluid Inclusions.

Raman spectra were obtained simultaneously from the liquid and vapor phases of pure water trapped at the critical density (322 kg·m-3) within synthetic inclusions in quartz. As these inclusions are heated up to the critical temperature (373.946 °C), the liquid phase decreases in density and the maximum of the Raman OH-stretching band increases in wavenumber. Conversely, as the vapor phase increases in density, the maximum of the Raman OH-stretching band decreases in wavenumber. The Raman bands of the liquid and vapor phases converge to a single band at the critical point of water, where the fluid exists as a single phase. A comparison of the band centroids for the vapor and liquid phases of water indicates respective increases and decreases in the amount of hydrogen bonding in these phases as a function of increasing and decreasing density. These effects were further quantified by peak-fitting the Raman OH-stretching peak with five Gaussian components. All the Gaussian components of the liquid phase decrease in amplitude with increasing temperature with the exception of the double donor-single acceptor (H2O)4 cluster, which increases in amplitude and becomes the most intense component at temperatures above 300 °C. The Raman spectra of the vapor phase are dominated by the free OH component at temperatures below 300 °C, but, above this temperature, the double donor-single acceptor (H2O)4 cluster is again the most intense band. The results indicate that a significant quantity of water clusters is present in both liquid and vapor water at high temperatures and that supercritical water can be considered as a mixture of small water clusters [(H2O)n, n = 1-4] dominated by the double donor-single acceptor (H2O)4 cluster.

Influence of the solvent in the electronic excitation of aromatic alcohols: Excited state IR-UV of propofol(H2O)8.

It is well known that water plays an important role in the reactivity and dynamics in a solution of molecules in electronic excited states. For example, electronic excitation is usually accompanied by a solvent rearrangement that may also influence the redistribution of the excitation energy. However, there is a lack of experimental data on such processes. Here, we explore the structural changes that follow electronic excitation in aggregates of propofol (2,6-diisopropylphenol) with up to eight water molecules, using a combination of mass-resolved excitation spectroscopy and density functional theory calculations. The molecules of water form a polyhedron around the hydroxyl group of propofol, also interacting with the π cloud of the aromatic ring. Electronic excitation produces a strong structural change in the water superstructure, which moves to an interaction with one of the carbon atoms of the aromatic ring, producing its distortion into a prefulvenic structure. Such deformation is not observed in smaller water clusters or in propofol-phenol aggregates highlighting the decisive role played by the solvent.

Effect of argon environment on small water clusters in matrix isolation

The influence of cryogenic argon environment on small water clusters was investigated by quantum-chemical simulation of structure and vibrational spectra of water clusters consisting of different numbers of molecules. Comparison of calculation results for vacuum and argon environment shows a red shift of spectral bands in argon. Obtained IR frequencies and intensities for water clusters in argon are compared with experimentally registered FTIR spectra of water trapped in a low-temperature argon matrix.

Water in an electric field does not dance alone: The relation between equilibrium structure, time dependent viscosity and molecular motions

Dynamic structuring of water is a key player in a large class of processes underlying biochemical and technological developments today, the latter often involving electric fields. However, the anisotropic coupling between the water structure and the field has not been understood on a molecular level so far. Here we perform extensive molecular dynamics simulations to explore the influence of an externally imposed electric field on liquid water under ambient conditions. Using self-developed analysis tools and rigorous statistical analysis, we unambiguously show that water hydration shells break into subcompartments, which were hitherto not observed due to radial averaging. The shape of subcompartments is sensitive to the field magnitude and affects excitations of the hydrogen bond network including the femtosecond stretching and the sub-picosecond restructuring of hydrogen bonds. Furthermore, by analysing the reorientational dynamics of water molecules, we ascertain the existence of cooperative excitations of small water clusters. Enabled by the interplay between hydrogen bonding, and the coupling of water dipoles to the field, these coordinated motions, occurring on the picosecond time scale, are associated with fluctuations between torque-free states of water dipoles. We show that unlike the coupling between translation and reorientation of water molecules, which takes place on even longer time scales, these coordinated motions are the key for understanding the emergent anisotropy of diffusion and viscosity of water. Particular effort is invested to provide an analysis that allows for future experimental validation.

Making and breaking of small water clusters: A combined quantum chemical and molecular dynamics approach.

We present a combined quantum chemical and molecular dynamics study of cyclic and noncyclic water n-mers ([(H2 O]n , n = 2-6) at four different temperatures and showcase that the dynamics of small water clusters can reproduce the known properties of bulk water reasonably well. We investigate the making and breaking of the water clusters by computing the hydrogen bond strengths, average lifetimes, and relative stabilities, which are important to understand the complex solution dynamics. We compare the behavior of water clusters in the gas phase and in the solution phase as well as the variation in the properties as a function of cluster size and highlight the notably more interesting cluster dynamics of the water trimer when compared to the other water clusters. © 2019 Wiley Periodicals, Inc.

A combined experimental and theoretical study on the terahertz vibrations of water vapors.

Water vapor is of significance in global greenhouse effect. Its strong absorption at infrared region characterizes the fingerprint of water structure. We hereby performed THz time-domain spectroscopy (0.2-2 THz) study on water vapors at relative humidity from 10% to 50%, in which the intensities of vibrations were found not proportionally increased with relative humidity. Besides rotation of water monomer, the absorption at THz frequency was probably attributed to small water clusters (N < 9) based on quantum mechanics calculations, where the distinct absorption peaks were matched with the vibrations of clusters. Moreover, planar structures were found to prevail in clusters less than eight waters owing to increasingly strong hydrogen bonds while cube structure dominated in eight-water cluster. This study thus promotes our understanding of not only absorptions of water vapor at far-infrared region but also their structures.

Directing self-assembly in solution towards improved cooperativity in Fe(iii) complexes with amphiphilic tridentate ligands.

An amphiphilic iron(iii) complex with a tridentate Schiff-base ligand was prepared by condensation of a hexadecyloxy functionalised salycylaldehyde with a diamine followed by complexation with FeCl2 and anion methathesis with NaClO4. The complex shows spin crossover both in the solid state and solution. However in solution self-assembly and consequently aggregation of individual molecules form concentration dependent particles with sizes of 300 nm for higher concentrations, or 5 nm for lower concentrations. Aggregate formation was confirmed by NANO-flex 180° DLS Size, scan-rate dependent cyclic voltammetry and scanning electron microscopy. Molecular simulations were used to investigate the self-assembly of the complex in solution, including the role of residual water molecules. The simulations showed the self-assembly of reverse micelle-like structures when a small water cluster is inserted in solution, whereas no large aggregates formed in dehydrated environments. The perchlorate anions were found near the metal centres, stabilizing the aggregates around the water pool. Simulations of pre-assembled structures further showed the lack of stability of large aggregates in the absence of water. The larger aggregates promoted efficient communication between the iron(iii) centres and the compound displayed spin crossover in solution at around 220 K with a 10 K hysteresis window, as measured by NMR and SQUID magnetometry.

Ion reactions in atmospherically-relevant clusters: mechanisms, dynamics and spectroscopic signatures.

Reactions of nitrogen oxides with seawater are of major atmospheric importance, but microscopic understanding of these processes is still largely unavailable. In this paper we explore models of reactions of N2O4 with ions in water in order to provide molecular-level understanding of the processes. Presented here are studies of N2O4 interacting with two ions, SO42- and Cl-, in small water clusters. Reactions of the asymmetric conformer of N2O4 with SO42- ions in water clusters are studied via ab initio molecular dynamics (AIMD) simulations in order to unravel the microscopic mechanism of the processes and predict the timescales of different steps. Spectroscopic signatures of the reaction are proposed. The mechanisms of chloride substitution and hydrolysis of symmetric and asymmetric N2O4 are explored via intrinsic reaction coordinate (IRC) calculations. Spectroscopic calculations for relevant species suggest possible experimental signatures for the processes. The results of these model ion-N2O4 reactions in water throw light on the molecular-level mechanisms of the reactions of nitrogen oxides with seawater.

Thermodynamics and the potential energy landscape: case study of small water clusters.

We investigated the structure and the thermodynamic properties of small water clusters with the nested sampling computational technique, using two different water models, the coarse-grained mW (up to 25 molecules) and the flexible version of the TIP3P (up to 16 molecules). By mapping the entire potential energy landscape of the clusters, we calculated the heat capacity curves, located the structural transitions and identified those local minimum basins which contribute the most to the total partition function. We found that in the case of the mW model, trends in first-order-like and continuous-like transitions can be very well matched to the characteristics of the landscape: cluster sizes with fewer and narrower local minimum basins show a sharper 'melting' peak on the heat capacity curve. Trends in the case of the TIP3P model were not easily assigned to the changing occupation of basins, and the contribution of local minima was negligible, except for n = 7, 15 and 16.

The Relationship between Crystallization of Water Clusters Dispersed in Sephadex Resins in their Wet Form and the Polymeric Matrix Glass Transition Temperature

Abstract—Differential scanning calorimetry (DSC) analysis was used to study the dependences of the main parameters of crystallization and melting of small water clusters dispersed in Sephadex resins on the amount of water (30–55%) in the wet polymer. The temperatures and heats of the processes were shown to depend on the frozen water (FW) concentration in Sephadex, with the dependence being considered as a manifestation of the size effect. Hysteresis between FW melting and crystallization was detected in a certain Sephadex humidity range; i.e., Tm was higher than Tcr and Qm higher than Qcr. The two findings reflected the basic properties of low-dimensional systems. Sephadex and amorphous starch, which was examined previously, were compared as two polysaccharide systems with different structural organizations, and a significant difference was observed in FW crystallization in the systems. Nucleation and crystallite growth, which determine water crystallization, were found to occur separately in time in Sephadex with a low humidity under these experimental conditions. Nucleation occurred during cooling, while growth was observed mainly during heating. Both of the processes occurred during cooling and partly overlapped only at a high humidity, far from Tg of Sephadex. In contrast, the two processes always occurred during cooling in amorphous starch regardless of its humidity. The difference in FW crystallization was assumed to arise because water mobility differs between the systems as a result of their difference in molecular mobility of the biopolymer matrix proper near the glass transition region.

UV Absorption and Magnetic Circular Dichroism Spectra of Purine, Adenine, and Guanine: A Coupled Cluster Study in Vacuo and in Aqueous Solution.

The absorption and magnetic circular dichroism (MCD) spectra of purine and of the purine nucleobases adenine and guanine have been calculated in gas phase at the Coupled Cluster Singles and Doubles (CCSD) and Resolution-of-Identity Singles and Approximate Doubles (RI-CC2) levels of theory. Exploiting a new development in the TURBOMOLE program package for computing vertical excitation energies and Faraday [Formula: see text] terms in an implicit solvent approximated by the conductor-like screening model (COSMO) at the CC2 level, we have investigated the solvent effects on the relative positions of the ππ* and nπ* electronic transitions in these three molecules and compared them to the corresponding vacuum results. In the case of adenine, we also included specific solvent effects with a small water cluster. The spectra obtained with the implicit model COSMO are in qualitative agreement with those obtained with explicit water molecules both with and without the inclusion of the bulk solvent effects via the continuum solvent model. This suggests that the inclusion of the electrostatic contributions of the solvent can provide a sufficiently accurate description of the absorption spectra for adenine. The results for purine, adenine, and guanine show that, after the inclusion of bulk solvation, the ππ* states shift to lower energies while at the same time nπ* states show a reversed behavior. The computed MCD spectra show the characteristic bisignate profile found experimentally in all cases, despite, for adenine, remarkable differences in the origin of the individual peaks for different computational methods. Therefore, the ability (or inability) of MCD to determine the relative stability of the La and Lb states is critically reassessed. According to our best estimate for adenine in aqueous solution, the La state is more stable than Lb.

Competition between proton transfer and intermolecular Coulombic decay in water

Intermolecular Coulombic decay (ICD) is a ubiquitous relaxation channel of electronically excited states in weakly bound systems, ranging from dimers to liquids. As it is driven by electron correlation, it was assumed that it will dominate over more established energy loss mechanisms, for example fluorescence. Here, we use electron–electron coincidence spectroscopy to determine the efficiency of the ICD process after 2a1 ionization in water clusters. We show that this efficiency is surprisingly low for small water clusters and that it gradually increases to 40–50% for clusters with hundreds of water units. Ab initio molecular dynamics simulations reveal that proton transfer between neighboring water molecules proceeds on the same timescale as ICD and leads to a configuration in which the ICD channel is closed. This conclusion is further supported by experimental results from deuterated water. Combining experiment and theory, we infer an intrinsic ICD lifetime of 12–52 fs for small water clusters.

Modeling of Phase Transitions in Weakly Bound Molecular Clusters

The method of molecular dynamics was used for modeling the isomerization of a hydrogen bonding network in small water clusters (hexamer and octamer). The collective modes of the particles moving in the clusters were determined by applying principal component analysis. An entropy criterion for phase transitions in water clusters was suggested. This criterion can be used to study phase transitions in weakly bound atomic and molecular clusters.