Modeling of Platinum Clusters in H-Mordenite

The size, location and structure of Pt clusters in H-mordenite have been investigated by molecular mechanics energy minimization and molecular dynamics simulation techniques using the Catalysis software of Molecular Simulations (MSI). Lattice energy minimizations are performed to study the effects of the specific framework aluminum positions on the location and stability of monoatomic Pt sites in H-mordenite. The lattice energies relative to the siliceous platinum-aluminosilicate structure reveal that the stability of a single Pt atom in H-mordenite is remarkably influenced by the specific location of the Al atoms in the lattice. At the studied Si/Al ratio of two Al ions per unit cell, a stabilization of the H-mordenite lattice upon Pt deposition is obtained. Moreover, lattice energy calculations on Pt/aluminosilicate mordenites of different metal contents per unit cell have been performed. An optimum size for the aggregate confined to the 12-ring main channel that is almost independent of the Pt content per mordenite unit cell has been found. The structural features of the resulting clusters at the end of molecular dynamics simulations on Pt/alumina-mordenites reflect a strong metal-zeolite interaction. The present results are consistent with a previous molecular dynamics simulation study on the structure of platinum deposited on SiO₂ surfaces.     

Effect of increased food abundance near forest edges on flocking patterns of Coal Tit Parus ater winter groups in mountain coniferous forests

CapsuleIndividuals concentrated near forest edges in bigger social groups than in forest interiors and foraged more on pine cones which were more abundant there.

Aims To evaluate differences in food distribution between forest edges and forest interiors and their effects on the non-breeding flocking patterns of Coal Tit populations inhabiting mountain coniferous forests.

Methods We collected cone production data at forests edges and interiors in mountain pine forests located in the Pyrenees (northeast Iberian peninsula). At the same sites, we also quantified Coal Tit abundance, flocking patterns and foraging behaviour by means of paired bird surveys during autumn and early winter.

Results We recorded a larger abundance of pine cones available on trees along forest edges compared with forest interiors. Coal Tit groups were of bigger size along forest edges, although the number of social groups detected did not differ from forest interiors. Our observations on foraging behaviour supported the hypothesis that differences in flock sizes and overall abundances associated with distance to the edge are due to differences in the availability of pine cones and to the heavier use of these foraging substrates by birds along forest edges.

Conclusions Our results suggest that by changing food distribution, edge effects on pine cone production may be significantly involved in local changes in the social structure of the Coal Tit. An increase in resource heterogeneity and local population density may have important implications at a population level, such as favouring mobility of individuals searching for food resources and thus a transient life, and increasing the costs of territory defence to resident individuals.     

Implanting Isolated Ru Atoms into Edge‐Rich Carbon Matrix for Efficient Electrocatalytic Hydrogen Evolution

Although the maximized dispersion of metal atoms has been realized in the single‐atom catalysts, further improving the intrinsic activity of the catalysts is of vital importance. Here, the decoration of isolated Ru atoms into an edge‐rich carbon matrix is reported for the electrocatalytic hydrogen evolution reaction. The developed catalyst displays high catalytic performance with low overpotentials of 63 and 102 mV for achieving the current densities of 10 and 50 mA cm^?2, respectively. Its mass activity is about 9.6 times higher than that of the commercial Pt/C‐20% catalyst at an overpotential of 100 mV. Experimental results and density functional theory calculations suggest that the edges in the carbon matrix enhance the local electric field at the Ru site and accelerate the reaction kinetics for the hydrogen evolution. The present work may provide insights into electrocatalytic behavior and guide the design of advanced electrocatalysts.     

Reaction of [Pt(Gly-Gly-N,N',O)I]- with the N-acetylated dipeptide L-methionyl-L-histidine: selective platination of the histidine side chain by intramolecular migration of the platinum(II) complex

The reaction of the monofunctional [Pt(Gly-Gly-N,N′,O)I]⁻ complex, in which Gly-Gly is the dipeptide glycyl-glycine coordinated through two nitrogen and oxygen atoms, with the N-acetylated dipeptide l-methionyl-l-histidine (MeCOMet-His) studied by ¹H NMR spectroscopy. All reactions were carried out in 50 mM phosphate buffer at pD 7.4 and at 25 °C. In the initial stage of the reaction, the platinum(II) complex forms the kinetically favored [Pt(Gly-Gly-N,N′,O)(MeCOMet-His-S)]⁻ complex, with unidentate coordination of the MeCOMet-His dipeptide through the sulfur atom of the methionine residue. In the second stage of the reaction, complete intramolecular migration of the [Pt(Gly-Gly-N,N′,O)] unit from the sulfur to the N3 nitrogen atom of imidazole was observed and a new platinum(II)-peptide complex, [Pt(Gly-Gly-N,N′,O)(MeCOMet-His-N3)]⁻ was formed. In comparison with previous results obtained for the reaction of [Pt(dien)Cl]⁺ with different methionine- and histidine-containing peptides, this migration reaction was sufficiently fast and strongly selective to the N3 atom of the imidazole ring of the histidine side chain. This study is an important step in the development of new platinum(II) complexes for selective covalent modification of peptides and proteins.     

Condensed State Molecular Dynamics in Sorbitol and Maltitol: Mobility Gradients and Conformation Transitions

Abstract

Molecular mobility in sorbitol and maltitol is studied in order to understand their differences near the junction between the α and β relaxations. The molecular dynamics simulations performed on the polyols in their bulk state give support to the ¹³C NMR results and imply that the mobility of a carbon atom located at the extremity of the chain is higher than that of any other carbon. Moreover, the difference in carbon atoms mobility is greater within the sorbitol moiety of maltitol than in sorbitol and seems intimately related to the junction temperature of the α and β relaxation processes. The reorientation of the C–H vectors as probed by NMR is shown to be mainly the effect of conformation transitions in the case of a carbon atom located at the end of the chain.     

Single‐Atom to Single‐Atom Grafting of Pt1 onto FeN4 Center: Pt1@FeNC Multifunctional Electrocatalyst with Significantly Enhanced Properties

Nonprecious metal catalysts (NPMCs) Fe?N?C are promising alternatives to noble metal Pt as the oxygen reduction reaction (ORR) catalysts for proton‐exchange‐membrane fuel cells. Herein, a new modulation strategy is reported to the active moiety Fe?N₄ via a precise “single‐atom to single‐atom” grafting of a Pt atom onto the Fe center through a bridging oxygen molecule, creating a new active moiety of Pt₁?O₂?Fe₁?N₄. The modulated Fe?N?C exhibits remarkably improved ORR stabilities in acidic media. Moreover, it shows unexpectedly high catalytic activities toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), with overpotentials of 310 mV for OER in alkaline solution and 60 mV for HER in acidic media at a current density of 10 mA cm^?2, outperforming the benchmark RuO₂ and comparable with Pt/C(20%), respectively. The enhanced multifunctional electrocatalytic properties are associated with the newly constructed active moiety Pt₁?O₂?Fe₁?N₄, which protects Fe sites from harmful species. Density functional theory calculations reveal the synergy in the new active moiety, which promotes the proton adsorption and reduction kinetics. In addition, the grafted Pt₁?O₂? dangling bonds may boost the OER activity. This study paves a new way to improve and extend NPMCs electrocatalytic properties through a precisely single‐atom to single‐atom grafting strategy.     

PtL3 X-ray absorption studies of Pt model compounds and [(NH3)2Pt(OH)2Pt(NH3)2]2+ bound to DNA

The Pt L₃ X-ray absorption spectra of a series of Pt compounds have been recorded and their extended fine structure (EXAFS) analysed to investigate the sensitivity of EXAFS to non-first-shell PtPt distances. The Pt L₃ EXAFS spectra of complexes formed between [(NH₃)₂Pt(OH)₂Pt(NH₃)₂]²⁺ and calf thymus DNA were also recorded. PtPt vectors could not be detected in these spectra. When combined with the model compound studies, this result rules out Pt dimer structures for the PtDNA complex which involve rigidly bridged, adjacent Pt atoms. Such structures, based on dimeric bonding of a hydroxo dimer intermediate to DNA, have been proposed as models for cisplatin antitumor activity. These types of models now seem unlikely.     

Effect of temperature,Si/Al and metal loading on the positioning of Pt atoms in MOR-type zeolites

Metropolis Monte Carlo (MMC) and molecular dynamics (MD) simulations were performed to study the feasibility of synthesising Pt nanowires inside the pores of zeolites with the MOR framework, such as mordenite. Results show that the temperature, the silicon to aluminium ratio (Si/Al) of the zeolite and the Pt metal loading have an important combined effect on the positioning of Pt atoms inside the framework. MMC simulations indicate that low Si/Al and high metal loadings promote the positioning of Pt atoms inside the main pore channels, which are the places where metal nanowires could be formed. On the other hand, high Si/Al and low metal loadings favour the positioning of Pt atoms in the side pockets of the MOR structure. For Pt loadings of 5% (mol/mol), the guest metal atoms were positioned mainly in the main pore channels for all conditions studied. MD simulation results are in agreement with MMC simulations, showing that when Si/Al = ∞, the trend of single Pt atoms is to move towards the side pockets of the MOR framework, while for Si/Al = 5, the Pt atoms remain in the main pore channel of the structure. Temperature had the effect of increasing the mobility of Pt atoms through the MOR framework.     

Coordination of 9-methyladenine to [cis-Pt(NH3)2]2+ and [Pt(dien)]2+ as studied by proton NMR

Reaction products of 9-methyladenine (mAde) with [Pt(dien)Cl]Cl and cis-Pt(NH₃)₂Cl₂ have been separated using CM-Sephadex C25 cation exchange chromatography. NMR and UV characteristics are presented; the platinum binding sites were established by studying the pH dependence of the ¹H-NMR chemical shifts and of UV difference absorption. It is shown that the N 1 atom of the ligand can be protonated in Pt(mAde-N7) adducts, while the N7 atom can be protonated in Pt(mAde-N1).     

Platinum(II) and palladium(II) complexes with 2-Acetyl pyridine 4N-ethyl thiosemicarbazone able to overcome the cis-Platin resistance. Structure, antibacterial activity and DNA strand breakage

The reactions of Pd(II) and Pt(II) with 2-Acetyl Pyridine N(4)-Ethyl-Thiosemicarbazones, HAc4Et and 2-Acetyl Pyridine N(4)-1-(2-pyridyl)-piperazinyl Thiosemicarbazone, HAc4PiPiz and 2-Formyl Pyridine N(4)-1-(2-pyridyl)-piperazinyl Thiosemicarbazone, HFo4PiPiz afforded the complexes, [Pd(Ac4Et)], 1, [Pd(HAc4Et)₂]Cl₂, 2 and [Pd(Ac4Et)₂], 3[Pt(Ac4Et)], 4, [Pt(HAc4Et)₂]Cl₂, 5, [Pt(Ac4Et)₂], 6 and [Pd(Fo4PipePiz)Cl], 7, [Pd(Fo4PipePiz)₂], 8, [Pd(Ac4PipePiz)Cl], 9 and [Pd(Ac4PipePiz)₂], 10. The crystal structure of the complex [Pt(Ac4Et)₂], 6 has been solved. The platinum(II) atom is in a square planar environment surrounded by two cis nitrogen atoms and two cis sulfur atoms. The ligands are not equivalent, one being tridentate with (N,N,S) donation, the other being monodentate using only the sulfur atom to coordinate to the metal. The tridentate ligand shows a Z, E, Z configuration while the monodentate ligand shows an E, E, Z. Inter-molecular hydrogen bonds stabilize the structure, while the crystal packing is determined by –, and Pt – C interactions. The antibacterial effect of Pd(II) and Pt(II) complexes were studied in vitro. The complexes were found to have effect on Gram(+) bacteria, while the same complexes showed no bactericidal effect on Gram(–) bacteria. The effect of the Pd(II) and Pt(II) complexes on the in vitro DNA strand breakage was studied by agarose gel electrophoresis. The complexes 1-6 were found to exhibit a cytotoxic potency in a very low micromolar range and to be able to overcome the cisplatin resistance of A2780/Cp8 cells (Kovala-Demertzi et al. 2000).     

NMR study on Pt(II) interaction with Amadori compounds

Here we report the first evidence of Pt(II) interaction with Amadori compound [N-(1-deoxy-d-Fructos-1-yl)glycine (Fru-Hgly)]. The ¹H and ¹⁹⁵Pt NMR results show that complexation of Pt(II) by Fru-Hgly is strongly dependent on pH and reaction molar ratio. In 1/1 Pt/Fru-Hgly molar ratio, at acidic pH, the first coordination site is the carboxylic oxygen, while at physiological pH the anchoring group is the aminic one, in both cases the system slowly evolves towards an N,O chelating mode. In 1/2 Pt/Fru-Hgly molar ratio the only coordination site is nitrogen atom while the carboxylic oxygen is not involved in metal coordination.     

A comparative theoretical study for the methanol dehydrogenation to CO over Pt3 and PtAu2 clusters

The density functional theory (DFT) calculations are carried out to study the mechanism details and the ensemble effect of methanol dehydrogenation over Pt(3) and PtAu(2) clusters, which present the smallest models of pure Pt clusters and bimetallic PtAu clusters. The energy diagrams are drawn out along both the initial O-H and C-H bond scission pathways via the four sequential dehydrogenation processes, respectively, i.e., CH(3)OH → CH(2)OH → CH(2)O → CHO → CO and CH(3)OH → CH(3)O → CH(2)O → CHO → CO, respectively. It is revealed that the reaction kinetics over PtAu(2) is significantly different from that over Pt(3). For the Pt(3)-mediated reaction, the C-H bond scission pathway, where an ensemble composed of two Pt atoms is required to complete methanol dehydrogenation, is energetically more favorable than the O-H bond scission pathway, and the maximum barrier along this pathway is calculated to be 12.99 kcal mol(-1). In contrast, PtAu(2) cluster facilitates the reaction starting from the O-H bond scission, where the Pt atom acts as the active center throughout each elementary step of methanol dehydrogenation, and the initial O-H bond scission with a barrier of 21.42 kcal mol(-1) is the bottom-neck step of methanol decomposition. Importantly, it is shown that the complete dehydrogenation product of methanol, CO, can more easily dissociate from PtAu(2) cluster than from Pt(3) cluster. The calculated results over the model clusters provide assistance to some extent for understanding the improved catalytic activity of bimetal PtAu catalysts toward methanol oxidation in comparison with pure Pt catalysts.     

Pt(II) compounds with sulfoxide ligands and crystal structures of complexes of the types I(R2SO)Pt(μ-I)2Pt(R2SO)I and trans-Pt(R2SO)(L)X2 (L = amine,pyridine and pyrimidine)

The crystal structures of several Pt(II) complexes containing sulfoxide ligands are described. The two iodo bridged dimers of the type I(R₂SO)Pt(μ-I)₂Pt(R₂SO)I (where R is ethyl or n-butyl) are twinned structures. The dinuclear species are the trans isomers. Two compounds of the type trans-Pt(DMSO)(amine)X₂ were studied by X-ray diffraction methods. The diiodo MeNH₂ compound forms H-bonded chains, formed by maximizing the H-bonds between the amine group with the O atom of DMSO and one iodo ligand. The H-bonding pattern is quite different in the dichloro t-BuNH₂ complex. In the latter crystal, there are two independent molecules which are H-bonded in pairs. The methyl groups of DMSO and the t-butyl group of the amine are oriented towards the outside of the pairs of molecules, while the H-bonds link the two independent molecules. Again, the amino group forms the maximum H-bonds with the O atom of DMSO and one chloro ligand. The crystal structures of trans-Pt(DMSO)(pyridine)I₂ and of trans-Pt(MeBzSO)(pyrimidine)I₂ (Bz = benzyl) were also studied. In the pyridine complex, the O atom of DMSO is in the Pt(II) plane by symmetry, while in the pyrimidine compound, the C atom of the –CH₃ group is in the Pt(II) plane. The pyridine and the pyrimidine ligands are perpendicular to the Pt(II) square plane. The trans influence of the different ligands is discussed.     

Low-dimensional compounds containing cyano groups. XIX. Crystal structure, spectroscopic, thermal and magnetic properties of {[Cu(tn)2]3[Pt(CN)4]2}[Pt(CN)4] (tn = 1,3-diaminopropane) complex

Violet prismatic crystals of {[Cu(tn)₂]₃[Pt(CN)₄]₂}[Pt(CN)₄] (tn = 1,3-diaminopropane) were crystallized from the water-methanol solution containing CuCl₂·2H₂O, tn and K₂[Pt(CN)₄]·3H₂O. Prepared complex was characterized using elemental analysis, infrared and UV-Vis spectroscopy, magnetic measurement and thermal analysis. X-ray analysis revealed an ionic character of the complex containing mononuclear square planar [Pt(CN)₄]²⁻ complex anions and penta-nuclear [Cu(tn)₂-Pt(CN)₄-Cu(tn)₂-Pt(CN)₄-Cu(tn)₂]²⁺ complex cations. The inner Cu(II) atom of the complex cation is hexa-coordinated, whereas two crystallographically equivalent peripheral Cu(II) atoms are penta-coordinated in the shape of a deformed square pyramid. Four v(CN) absorption bands observed in the IR spectrum are in agreement with the higher number of crystallographically different cyano groups and a broad highly asymmetric band observed in the reflectance UV-Vis spectrum is consistent with the presence of both hexa- and penta-coordinated Cu(II) atoms in the structure. The temperature dependence of the inverse susceptibility suggests the presence of a weak antiferromagnetic exchange coupling between Cu(II) ions. The complex is stable up to 210 °C when its two-stage thermal decomposition starts.     

MMO: P450 in wolf's clothing?

 Methane monooxygenase (MMO) catalyzes the oxidation of stable hydrocarbons that are not attacked by cytochrome P450 monooxygenase. A key transient intermediate in the catalytic cycle of the soluble form of MMO termed compound Q (Q) has been trapped and characterized through spectroscopic comparisons with novel high valent model complexes. Q appears to contain a non-heme dinuclear Fe(IV) cluster bridged by at least two single oxygen atoms to form a so-called diamond core. Q has the ability to react directly with unactivated hydrocarbons to yield oxidized products. Several types of experiments indicate that this reaction involves formation of an intermediate, probably with radical character. This is consistent with a hydrogen atom abstraction mechanism analogous to that ascribed to cytochrome P450. However, these same experiments show that a pure hydrogen atom abstraction mechanism is unlikely for many substrates without an additional interaction between the intermediate that is formed and the high valent cluster. The results may be of general relevance to monooxygenase catalysis. Received: 15 January 1998 / Accepted: 9 March 1998     

Molecular dynamics simulation of the size-dependent morphological stability of cubic shape silver nanoparticles

The morphological stability of sharp-edged silver nanoparticles is examined by the classical molecular dynamics (MD) simulations. The crystalline structure and the perfect fcc atom packing of a series of silver nanocubes (AgNC) of different sizes varying from 63 up to 1099 atoms are compared against quasi-spherical nanoparticles of the same sizes at temperature 303 K. Our MD simulations demonstrate that starting from the preformed perfect crystalline structures the cubic shape is preserved for AgNCs composed of 365–1099 atoms. Surprisingly, the rapid loss of the cubic shape morphology and transformation into the non-fcc-structure are found for smaller AgNCs composed of less than ~256 atoms. No such loss of the preformed crystalline structure is seen for quasi-spherical nanoparticles composed of 38–1007 atoms. The analysis of the temperature dependence and the binding energy of outermost Ag surface atoms suggests that the loss of the perfect cubic shape, rounding and smoothing of sharp edges and corners are driven by the tendency towards the increase in their coordination number. In addition, we revealed that AgNC₁₀₉₉ partially loses its sharp edges and corners in the aqueous environment; however, the polymer coating with poly(vinyl alcohol) (PVA) was able to preserve the well-defined cubic morphology. Finally, these results help improve the understanding of the role of surface capping agents in solution phase synthesis of Ag nanocubes.     

Reactions of [PtCl(dien)]Cl with glutathione,oxidized glutathione and S-methyl glutathione. Formation of an S-bridged dinuclear unit

The reaction of the monofunctional platinum compound [PtCl(dien)]Cl with the tripeptide glutathione (GSH), oxidized glutathione (GSSG) and S-methyl glutathione (GS-Me) has been investigated by ¹H, ¹³C and ¹⁹⁵Pt magnetic resonance spectroscopy and by potentiometric titrations. It appears that platinum binds with a high degree of specificity to the GSH sulfhydryl group. The reaction of platinum with GSH proceeds in two steps. In the first step only one platinum binds to the sulfur atom and, in the second step, another [Pt(dien)]²⁺ unit binds to [Pt(dien)GS]⁺ forming an S-bridged dinuclear unit [{Pt(dien)}₂GS]³⁺. The rate of the first binding step is pH-dependent, whereas the rate of the second step is not. At pH < 7 the rate of the first binding step is slow compared to the rate of the second binding step. At pH > 10, on the other hand, the rate of the first binding step is faster than the rate of the second binding step. Consequently, at pH < 7 one can only isolate the [{Pt(dien)}₂GS]³⁺ complex. In the presence of free GSH, at pH > 7, one [Pt(dien)]²⁺ unit of [{Pt(dien)}₂GS]³⁺ dissociates forming [Pt(dien)GS]⁺. The mechanism of the pH-dependent rate of the first platinum binding step and the ligand-exchange reaction are discussed. GSSG reacts with [Pt(dien)]²⁺, also forming the S-bridged dinuclear unit [{Pt(dien)}₂GS]³⁺, probably through a redox disproportionation reaction with a catalytic function of [PtCl(dien)]Cl. GS-Me reacts with [Pt(dien)]²⁺ forming the S-coordinated [Pt(dien)GS-Me]²⁺. [Pt(dien)GS-Me]²⁺ exists as a pair of diastereomers due to different configurations about sulfur. The rate of the inversion of configuration at the coordinated sulfur atom is slow on the NMR time-scale.     

Adsorption and spin state properties of Cr,Ni, Mo,and Pt deposited on Li+ and Na+ monovalent cation impurities of MgO (001) surface: DFT calculations

We have analyzed, by means of density functional theory calculations and the embedded cluster model, the adsorption and spin-state properties of Cr, Ni, Mo, and Pt deposited on a MgO crystal. We considered deposition at the Mg²⁺ site of a defect-free surface and at Li⁺ and Na⁺ sites of impurity-containing surfaces. To avoid artificial polarization effects, clusters of moderate sizes with no border anions were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The interaction between a transition metal atom and a surface results from a competition between Hund's rule for the adsorbed atom and the formation of a chemical bond at the interface. We found that the adsorption energies of the metal atoms are significantly enhanced by the cation impurities, and the adsorption energies of the low-spin states of spin-quenched complexes are always more favorable than those of the high-spin states. Spin polarization effects tend to preserve the spin states of the adsorbed atoms relative to those of the isolated atoms. The metal–support interactions stabilize the low-spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. Spin quenching occurs for Cr and Mo complexes at the Mg²⁺ site of the pure surface and at Li⁺ and Na⁺ sites of the impurity-containing surfaces. Variations of the spin-state properties of free metals and of the adsorption and spin-state properties of metal complexes are correlated with the energies of the frontier orbitals. The electrostatic potential energy curves provide further understanding of the nature of the examined properties.     

Molecular Dynamics Simulation Studies of Zeolite-A. IV. Structure and Dynamics of NH4 + ions in a Rigid Dehydrated Zeolite-A

Abstract

In recent papers [1–3] we reported molecular dynamics simulation studies of ions and water molecules adsorbed in a rigid zeolite-A framework using a simple Lennard-Jones potential plus Coulomb potential with Ewald summation to investigate the structure and dynamics of the adsorbates. In the present paper the same technique is applied to study the local structure and dynamics of NH₄ ⁺ ions in a rigid dehydrated zeolite-A. During the preliminary equilibration, the unstable NH₄(4) type ion (the 12th ion) is pushed down to near a more stable 6-ring position in the α-cage that is already associated with an NH₄(1) type ion (the 1st) in the β-cage, which moves to another 6-ring position in the β-cage that is already associated with an NH₄(2) type ion (the 7th) in the α-cage. Calculated x, y, and z coordinates of some NH₄ ⁺ ions are in good agreement with those obtained from an X-ray diffraction experiment except that no NH₄(4) type ion is found and there are six NH₄(2) type ions instead of 0.5 and 5.5 occupancy. The analyses of calculated interatomic distances and time correlation functions of these ions indicate that the NH₄(1 – 1) and NH₄(3) type ions are associated loosely with only one O (3) atom of the 6-ring and with only one O (1) atom of the 8-ring windows, respectively, while the NH₄(1–2) and NH₄(2) type ions are associated strongly with two or three O (3) atoms of the 6-ring windows in the α- and β-cages, respectively. The analysis of hydrogen bond time correlation functions of these ions indicate that about one, two or three, three, and one hydrogen bond of each NH₄(1–1), NH₄(1–2), NH₄(2) and NH₄(3) type ion is kept for 1.4, 21, 75, and 1.4 ps, respectively, before breakup of the hydrogen bond occurs and significant exchange of O atom hydrogen-bonded to the ion.     

Mechanism of the Atomic Intermixing Process in Crystalline Microclusters

Abstract

The Mechanism of atomic intermixing process in crystalline microclusters is studied by molecular dynamics simulation for a two-dimensional system with the Lennard-Jones potential. Temperature is chosen so that a cluster consists of solid-like core region and the region of surface melting. It is found that atomic intermixing in the solid-like core region is caused by the motion of a dislocation through the cluster as well as the random walk of a vacancy in the cluster. Generation of a dislocation or a vacancy occurs at the interfacial region between the liquid-like surface and the solid-like core regions due to large scale fluctuation of the configuration of atoms in the region of surface melting and the opportune collective motion of atoms in the solid-like core region. The rate per atom of atomic intermixing, the basic quantity of our interest (for the definition see the text), in the solid-like core of the microcluster is three to four orders of magnitude larger than that in the bulk crystal.