Metastable Clusters Research Articles

Magic Number Pt13 and Misshapen Pt12 Clusters: Which One is the Better Catalyst?

A relationship between the size of metal particles and their catalytic activity has been established over a nanometer scale (2-10 nm). However, application on a subnanometer scale (0.5-2 nm) is difficult, a possible reason being that the activity no longer relies on the size but rather the geometric structure as a cluster (or superatomic) compound. We now report that the catalytic activity for the oxygen reduction reaction (ORR) significantly increased when only one atom was removed from a magic number cluster composed of 13-platinum atoms (Pt13). The synthesis with an atomic-level precision was successfully achieved by using a dendrimer ligand as the macromolecular template strictly defining the number of metal atoms. It was quite surprising that the Pt12 cluster exhibited more than 2-fold catalytic activity compared with that of the Pt13 cluster. ESI-TOF-mass and EXAFS analyses provided information about the structures. These analyses suggested that the Pt12 has a deformed coordination, while the Pt13 has a well-known icosahedral atomic coordination as part of the stable cluster series. Theoretical analyses based on density functional theory (DFT) also supported this idea. The present results suggest potential activity of the metastable clusters although they have been "missing" species in conventional statistical synthesis.

Self-interstitial clusters in radiation damage accumulation: coupled molecular dynamics and metadynamics simulations

Self-interstitial interactions causing volume expansion in bcc Fe are studied through an idealized microstructure evolution model in which only self-interstial atoms (SIAs) are inserted. Using a combination of non-equilibrium molecular dynamics simulations and a metadynamics algorithm, meta-stable SIA clusters are observed to nucleate and grow into dislocation loops or localized amorphous phases, both contributing to swelling behavior persisting well beyond the atomistic time scale. A non-monotonic local density variation with dose rate is found and attributed to competing evolutions of different defective structures.

More than one pathway

A predictive theoretical framework that incorporates both classical and non-classical crystal-nucleation pathways explains the observed rapid aggregation of metastable clusters in the nucleation process of minerals.

Synthesis, Characterization, and Dynamic Behaviour of Triosmium Clusters Containing the Tridentate Ligand {Ph2PCH2CH2}2S (PSP)

AbstractReaction of [Os3(CO)11(NCMe)] with bis‐diphenylphosphanylethylene sulfide, {Ph2PCH2CH2}2S (PSP), leads to the formation of [Os3(CO)11(PSP)] and [{Os3(CO)11}2(μ‐PSP)] in good yield. Similarly, treatment of [Os3(CO)10(NCMe)2] with PSP affords the cluster [Os3(CO)10(μ‐PSP)], in which the two phosphanes of the PSP ligand coordinate to different osmium atoms of the same triosmium unit. Reaction of [Os3(CO)11(PSP)] with [Os3(CO)10(NCMe)2] yields the compound 1,2‐[{Os3(CO)11}(μ3‐PSP){Os3(CO)10}] in which the thioether moiety and one of the phosphane groups of the PSP ligand are coordinated equatorially to the {Os3(CO)10} subunit. The cluster 1,2‐[{Os3(CO)11}(μ3‐PSP){Os3(CO)10}] is also formed when [Os3(CO)11(PSP)] is oxidatively decarbonylated by reaction with trimethylamine N‐oxide. The metastable cluster 1,2‐[{Os3(CO)11}(μ3‐PSP){Os3(CO)10}] undergoes slow isomerisation at room temperature to form 1,1‐[{Os3(CO)11}(μ3‐PSP){Os3(CO)10}] in which the thioether and phosphane moieties coordinate in a chelating mode to one of the {Os3(CO)10} subunits with the thioether moiety in an axial position. The dynamic behaviour of these clusters has been investigated by variable‐temperature 13C{1H} and 13P{1H} NMR spectroscopy. The solid‐state structures of [{Os3(CO)11}2(μ‐PSP)] and [Os3(CO)10(μ‐PSP)] are reported.

Evaluation of seismogenesis behavior in Himalayan belt using data mining tools for forecasting

AbstractIn the proposed study, non-linear behavioral patterns in the seismic regime for earthquakes in the Himalayan basin have been studied using a complete, verified EQ catalogue comprised of all major events and their aftershock sequences in the Himalayan basin for the past 110 years [1900–2010]. The dataset has been analyzed to give better decision making criteria for impending earthquakes. A series of statistical tests based on multi-dimensional rigorous statistical studies, inter-event distance analyses, and statistical time analyses have been used to obtain correlation dimensions. The time intervals of earthquakes within a seismic regime have been used to train the neural network to analyze the nature of earthquake patterns in the different clusters. The results obtained from descriptive statistics show high correlation with previously conducted gravity studies and radon anomaly variation. A study of the time of recurrence of the numerical properties of the regime for 60 years from 1950 to 2010 for the Himalayan belt for analysis of significant EQ failure events has been done to find the best fit for an empirical data probability distribution. The distribution of waiting time of swarm events occurring in the Himalayan basin follows a power-law model, while independent events do not fit the power-law distribution. This suggests that probability of the occurrence of swarm events [M ⩽ 6.0] with frequent shaking may be more frequent than that of the occurrence of independent events of magnitude [M >6.0] in the Himalayan belt. We propose a three-layer feed forward neural network model to identify factors, with the actual occurrence of the maximum earthquake level M as input and target vectors in Himalayan basin area. We infer through a series of statistical results and evaluations that probabilistic forecasting of earthquakes can be achieved by finding the meta-stable cluster zones of the Himalayan clusters for the spatio-temporal distribution of earthquakes in the area.

Fluctuations and Criticality of a Granular Solid-Liquid-Like Phase Transition

We present an experimental study of density and order fluctuations in the vicinity of the solid-liquid-like transition that occurs in a vibrated quasi-two-dimensional granular system. The two-dimensional projected static and dynamic correlation functions are studied. We show that density fluctuations, characterized through the structure factor, increase in size and intensity as the transition is approached, but they do not change significantly at the transition itself. The dense, metastable clusters, which present square symmetry, also increase their local order in the vicinity of the transition. This is characterized through the bond-orientational order parameter Q4, which in Fourier space shows an Ornstein-Zernike-like behavior. Depending on the filling density and vertical height, the transition can be of first- or second-order type. In the latter case, the associated correlation length ξ4, the relaxation time τ4, the zero k limit of Q4 fluctuations (static susceptibility), the pair correlation function of Q4, and the amplitude of the order parameter obey critical power laws, with saturations due to finite size effects. Their respective critical exponents are ν(perpendicular))=1, ν(parallel)=2, γ=1, η=0.67, and β=1/2, whereas the dynamical critical exponent z=ν(parallel)/ν(perpendicular)=2. These results are consistent with model C of dynamical critical phenomena, valid for a nonconserved critical order parameter (bond-orientation order) coupled to a conserved field (density).

Ostwald-Like Ripening of the Anomalous Mesoscopic Clusters in Protein Solutions

Metastable clusters of mesoscopic dimensions composed of protein-rich liquid exist in protein solutions, both in the homogeneous region of the solution phase diagram and in the region supersaturated with respect to an ordered solid phase, such as crystals; in the latter region they are crucial nucleation sites for ordered solids. We monitor, using three optical techniques, the long-term evolution of the clusters in lysozyme solutions at conditions where no condensed phases, liquid or solid, are stable or present as long-lived metastable domains. We show that cluster formation is a reversible process and that the clusters are in near equilibrium with the solution, up to a capillary correction. In contrast to classical phase transformations, the solution concentration at cluster-solution equilibrium is close to its initial value; this is akin to chemical reaction equilibria and demonstrates the complex chemical composition of the clusters. However, similar to classical phase transformations, en route to full equilibration, the average cluster size grows with time following a universal law t(0.26±0.03), independent of the cluster volume fraction; the cluster size distribution is scale-invariant at all stages of cluster evolution. Despite the correspondence of these behaviors to the Lifshitz-Slyozov-Wagner (LSW) theory predictions, the cluster sizes are about 10× smaller than the LSW prediction, likely due to the complex cluster composition. The observed cluster evolution helps us to understand nucleation mysteries, such as nucleation rates lower by orders of magnitude than classical theory predictions, nucleation rate variable under steady conditions, and others.

Template Synthesis of Subnanometer Gold Clusters in Interfacially Cross-Linked Reverse Micelles Mediated by Confined Counterions

A cationic surfactant with a triallylammonium headgroup was cross-linked photochemically in the presence of a hydrophilic dithiol in the reverse micelle (RM) configuration. The interfacially cross-linked reverse micelles (ICRMs) are unusual templates for nanomaterials synthesis. Our previous work indicated that the ICRMs could extract anionic metal salts such as tetracholoroaurate into the hydrophilic interior, and the entrapped aurate was reduced without externally added reducing agent to form subnanometer luminescent gold clusters [Zhang, S.; Zhao, Y. ACS Nano 2011, 5, 2637-2646]. In this work, the bromide counterions were established as the reducing agent in the template synthesis. The reduction of tetrachloroaurate was proposed to happen through ligand exchange on the aurate by the bromide ions, reductive elimination of halogen, and disproportionation of the Au(I) intermediate. The size of the gold clusters could be tuned rationally by the water-to-surfactant ratio (W(0)) and the reducing agent. Monodisperse Au(4) and Au(9-10) clusters as well as larger Au(18) and Au(23) clusters were obtained from the ICRM templates. The as-prepared, metastable gold clusters were subject to reconstruction triggered by ligand exchange on the surface but could be stabilized through proper surface protection using a chelating dithiol.

Kinetics of two-dimensional island nucleation on reconstructed surfaces

Nucleation of two-dimensional (2D) islands on reconstructed surfaces might involve formation of metastable nonepitaxial clusters as precursors for 2D islands. Being a thermally activated process the transformation of nonepitaxial clusters to epitaxial 2D islands might become a rate limiting step of the nucleation process at low deposition temperatures leading to substantial accumulation of the depositing atoms in nonepitaxial clusters. In the present paper, we study how slow cluster transformation influences the nucleation kinetics. For a simple model case of metastable nonepitaxial dimers and irreversible adatom incorporation into the epitaxial 2D islands, we analyze possible nucleation regimes and derive scaling relations for the density of stable 2D islands. We show that the 2D island density depends strongly on the cluster transformation rate as well as on the atomic mechanism of the dimer transformation. In particular, a steady-state nucleation regime different from the standard diffusion-mediated aggregation is possible if the transformation of a nonepitaxial dimer to the smallest epitaxial 2D island is triggered by attachment of an additional adatom. In this regime, the 2D island density obeys a nonstandard scaling $N\ensuremath{\sim}{(F/D)}^{1/4}$. Deposition conditions where this nonstandard scaling could be observed are determined.

Thermal stability of C4 + 4n H8 polycubanes

The temperature dependences of the lifetimes of polycubanes C4+4nH8 with n = 2 - 5 up to their decomposition have been directly calculated using the molecular dynamics method. It has been shown that the activation energy of decomposition of these metastable clusters, in which the C-C bonds form an angle of 90^0 that is not characteristic of carbon systems, rapidly decreases with an increase in n due to the lowering of the energy barrier that prevents the decomposition of the clusters. This has cast some doubt on the recently made suggestion that there exist nanotubes (n >> 1) with a square cross section. Nonetheless, the stability of bicubane (n = 2) and tricubane (n = 3) has proved to be sufficient for their existence at the liquid-nitrogen temperature.

Thermal stability of nitro derivatives of hydrocarbon cubane

An original nonorthogonal tight-binding model is used to determine the structural and energetic characteristics of the family of polynitrocubanes C8H8−q(NO2)q, where q = 1−8. The mechanisms of isomerization are studied in detail and the minimum heights of the energy barriers to the decay of the metastable clusters are calculated. It is shown that nitro groups destabilize the cubic carbon skeleton. For octanitrocubane C8(NO2)8, the temperature dependence of the characteristic decay time at temperatures 500–1000 K is obtained, and activation energy and frequency factor of the Arrhenius equation are calculated.

Nucleation of protein condensed phases

Proteins in solution form a number of condensed phases. Even omitting the amyloid structures formed after partial protein unfolding, these phases include crystals, polymers, and other solid aggregates, as well as dense liquids and gels. Some of these condensed phases underlie pathological conditions, while others play a crucial role in the biological function of the respective protein or are an essential part of its laboratory or industrial processing. In this review, we summarize the fundamentals and recent findings on the kinetics of nucleation of dense liquid droplets and crystals. We define the transition from nucleation to spinodal decomposition for these two phase transitions. We review the two-step mechanism of protein crystal nucleation, in which mesoscopic metastable protein clusters serve as precursors to the ordered crystal nuclei. The concepts and mechanisms reviewed here provide powerful tools for control of the nucleation process, by varying the solution thermodynamic parameters.

Fast and Efficient Numerical-Simulation Method for Supersonic-Gas Processing

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 131239, ’A Fast and Efficient Numerical-Simulation Method for Supersonic-Gas Processing,’ by Dengyu Jiang, Qitai Eri, Changliang Wang, Huoxing Liu, and Yao Yuan, Beijing University of Aeronautics and Astronautics, prepared for the 2010 CPS/SPE International Oil & Gas Conference and Exhibition in China, Beijing, 8-10 June. The paper has not been peer reviewed. Supersonic-swirling-separation technology separates heavy hydrocarbons and water vapor from natural gas. The de Laval nozzle, where condensation occurs, is used to generate supersonic flow and achieve a high degree of supersaturation in a natural-gas dehydration unit. To optimize the structure of the nozzle and achieve higher separation efficiency, numerical simulation was used to accelerate development cycles and reduce the cost of experiment. Rather than use multiphase models and a real-gas model, a quick and efficient method was validated and used to determine the location of the nucleation zone and the droplet-growth zone. Introduction Spontaneous condensation occurs when a supersaturated gas passes through a strong expansion process. As part of the process, latent heat is released to the environment and the latent heat makes the state of the parameters of the flow field (e.g., temperature, pressure, and density) change suddenly, the so-called condensation-jump phenomenon. If the amount of heat released exceeds some critical value, the flow becomes thermally choked and the structure of the flow field changes, even forming a condensation shock. Condensation can be divided into two physical processes: nucleation and droplet growth. Homogeneous nucleation occurs when the condensation nuclei are gas molecules that collect spontaneously rather than because of contact with dust particles. Then, the nuclei grow into droplets because of gas molecules colliding with and adhering to the surface of other gas molecules under chemical reaction. Nonequilibrium condensation occurs if the scale of the change of gas-state parameters is much less than the time scales of gas molecules adhering to and merging with other particles. At that moment, the gas is supersaturated, indicative of a metastable non-equilibrium state of the vapor phase. As these metastable clusters exceed some critical size, the process of droplet growth sets in, leading to the formation of a stable liquid phase. The full-length paper details the numerical-simulation method of condensation. Adapting This Method for a de Laval Nozzle The engineering objective of super-sonic-gas processing is superior dew-point depression and a high-pressure recovery; therefore, research on the interaction between the condensation and the flow condition provides some directive standards. To determine the interaction between condensation and the flow conditions, this method was adapted to model the nonequilibrium-condensation flow field in a de Laval nozzle. Fig. 1 shows the nozzle and structure mesh. By use of precision and efficiency, dramatic-change regions of density in the flow field and the near-wall area were refined, and wall functions were chosen for solving the boundary layer.

Nucleation.

Crystallization starts with nucleation and control of nucleation is crucial for the control of the number, size, perfection, polymorphism and other characteristics of crystalline materials. This is particularly true for crystallization in solution, which is an essential part of processes in the chemical and pharmaceutical industries and a major step in physiological and pathological phenomena. There have been significant recent advances in the understanding of the mechanism of nucleation of crystals in solution. The foremost of these are the two-step mechanism of nucleation and the notion of the solution-crystal spinodal. According to the two-step mechanism, the crystalline nucleus appears inside pre-existing metastable clusters of size several hundred nanometers, which consist of dense liquid and are suspended in the solution. While initially proposed for protein crystals, the applicability of this mechanism has been demonstrated for small molecule organic materials, colloids, polymers, and biominerals. This mechanism helps to explain several long-standing puzzles of crystal nucleation in solution: nucleation rates which are many orders of magnitude lower than theoretical predictions, the significance of the dense protein liquid, and others. At high supersaturations typical of most crystallizing systems, the generation of crystal embryos occurs in the spinodal regime, where the nucleation barrier is negligible. The solution-crystal spinodal helps to understand the role of heterogeneous substrates in nucleation and the selection of crystalline polymorphs. Importantly, these ideas provide powerful tools for control of the nucleation process by varying the solution thermodynamic parameters.

Individualization as Driving Force of Clustering Phenomena in Humans

One of the most intriguing dynamics in biological systems is the emergence of clustering, in the sense that individuals self-organize into separate agglomerations in physical or behavioral space. Several theories have been developed to explain clustering in, for instance, multi-cellular organisms, ant colonies, bee hives, flocks of birds, schools of fish, and animal herds. A persistent puzzle, however, is the clustering of opinions in human populations, particularly when opinions vary continuously, such as the degree to which citizens are in favor of or against a vaccination program. Existing continuous opinion formation models predict “monoculture” in the long run, unless subsets of the population are perfectly separated from each other. Yet, social diversity is a robust empirical phenomenon, although perfect separation is hardly possible in an increasingly connected world. Considering randomness has not overcome the theoretical shortcomings so far. Small perturbations of individual opinions trigger social influence cascades that inevitably lead to monoculture, while larger noise disrupts opinion clusters and results in rampant individualism without any social structure. Our solution to the puzzle builds on recent empirical research, combining the integrative tendencies of social influence with the disintegrative effects of individualization. A key element of the new computational model is an adaptive kind of noise. We conduct computer simulation experiments demonstrating that with this kind of noise a third phase besides individualism and monoculture becomes possible, characterized by the formation of metastable clusters with diversity between and consensus within clusters. When clusters are small, individualization tendencies are too weak to prohibit a fusion of clusters. When clusters grow too large, however, individualization increases in strength, which promotes their splitting. In summary, the new model can explain cultural clustering in human societies. Strikingly, model predictions are not only robust to “noise”—randomness is actually the central mechanism that sustains pluralism and clustering.

Free Heme and the Polymerization of Sickle Cell Hemoglobin

In search of novel control parameters for the polymerization of sickle cell hemoglobin (HbS), the primary pathogenic event of sickle cell anemia, we explore the role of free heme, which may be excessively released in sickle erythrocytes. We show that the concentration of free heme in HbS solutions typically used in the laboratory is 0.02–0.04 mole heme/mole HbS. We show that dialysis of small molecules out of HbS solutions arrests HbS polymerization. The addition of 100–260 μM of free heme to dialyzed HbS solutions leads to rates of nucleation and polymer fiber growth faster by two orders of magnitude than before dialysis. Toward an understanding of the mechanism of nucleation enhancement by heme, we show that free heme at a concentration of 66 μM increases by two orders of magnitude the volume of the metastable clusters of dense HbS liquid, the locations where HbS polymer nuclei form. These results suggest that spikes of the free heme concentration in the erythrocytes of sickle cell anemia patients may be a significant factor in the complexity of the clinical manifestations of sickle cell anemia. The prevention of free heme accumulation in the erythrocyte cytosol may be a novel avenue to sickle cell therapy.

Influence of casting temperature on the thermal stability of Zr-based metallic glasses

Influences of casting temperature on the thermal stability of Zr-based metallic glasses (MGs) were analyzed based on continuous heating transformation (CHT) diagrams built up by Kissinger analysis. The results indicate that increasing the casting temperature can enhance the thermal stability of MGs, which attributed to the dissipation of pre-existing metastable local-ordering clusters in the melt resulted from the elevating casting temperature.

Response of Observables for Cold Anionic Water Clusters to Cluster Thermal History

We have used mixed quantum classical molecular dynamics simulations to explore the role of structural relaxation when binding an excess electron to neutral water clusters. The structural and spectral properties of the water cluster anions were investigated as a function of the size (n = 45 and 104), nominal temperature (T(nom) = 50, 100, and 150 K), and preparation method of the parent neutral clusters. In particular, we consider two different protocols for preparing the initial neutral clusters, which differ markedly in their thermal history. In the first, warm equilibrium neutral clusters are gradually quenched to increasingly lower temperature. In the second, neutral clusters are formed spontaneously at approximately 0 K and then warmed to the same target temperatures, yielding inherently metastable, nonequilibrium structures. Electron attachment to these alternative sets of clusters shows that below a critical temperature (approximately 200 K), the metastable water clusters bind a surface state excess electron significantly more strongly than the quenched, equilibrium clusters. The structural analysis indicates that these cluster anions with larger vertical detachment energies (VDEs) more frequently stabilize the electron by double-acceptor-type water molecules and exhibit a weak temperature dependence of the VDE compared with the quenched clusters. These results suggest that the alternative classes of cluster anions seen experimentally may reflect differences in the thermal history of such clusters.

Evolutionary structure prediction and electronic properties of indium oxide nanoclusters

Indium sesquioxide is widely used as a transparent conducting oxide in modern optoelectronic devices; the rising cost of indium has generated interest in the nanoscale properties of In(2)O(3), and questions arise as to the nature of its physicochemical properties below the bulk regime. We report the stable and metastable stoichiometric clusters of (In(2)O(3))(n), where n = 1-10, as predicted from an evolutionary search within the classical interatomic potential and quantum density functional energy landscapes. In contrast to the paradigm set by ZnO, which favours high symmetry bubble-like structures, the In(2)O(3) nanoclusters are found to tend towards dense, low symmetry structures approaching the bulk system at remarkably small molecular masses. Electronic characterisation is performed at the hybrid density functional and many-body GW levels to obtain accurate predictions of the spectroscopic properties, with mean values of the ionisation potentials and electron affinities calculated as 7.7 and 1.7 eV, respectively.

Phase transitions of folded proteins

Proteins exhibit a rich phase behavior. Even omitting the amyloid structures formed after partial protein unfolding, proteins form crystals, polymers, and other solid aggregates, as well as dense liquids and gels. Some of these condensed phases underlie pathological conditions, others play a crucial role in the biological function of the respective protein or are an essential part of its laboratory or industrial processing. In this review, we first discuss the thermodynamic characteristics of the solution and the interactions between protein molecules in solution, which underlie the protein phase behavior. We highlight the role of water structured at the protein molecular surface as a main contributor to the free energy of the phase transition. We define the driving force for phase transformations. We then summarize the fundamentals and review recent findings on the kinetics of nucleation of dense liquid droplets and crystals. We define the transition from nucleation to spinodal decomposition for these two phase transitions. We review the two-step mechanism of protein crystal nucleation, in which mesoscopic metastable protein clusters serve as precursors to the ordered crystal nuclei. Lastly, we discuss the mechanisms of growth of crystals: the generation of new crystal layers, the pathways of the molecules from the solution into the growth sites, the density of the growth sites and the factors, which determine the activation barriers for association of the molecules to the growth sites.