Short Linear Chains Research Articles

Fe nanostructures stabilized by long-range interactions on Cu(111): kinetic Monte Carlo simulations

The magnetic Fe nanostructures formed on a Cu(111) surface were simulated by the kinetic Monte Carlo (kMC) method, in which the substrate-mediated long-range interactions between Fe adatoms are involved. The dependence of coverage and temperature on the formation of local nanostructures was investigated to reveal the microprocess of Fe nanostructures formed on a Cu(111) substrate. The simulation results show that the long-range interactions between Fe adatoms lead first to the formation of short linear chains and then to locally ordered nanostructures when they become steady at low coverages between 10 and 18 K. Based on the analysis of the formation mechanism of the Fe superlattice, it is found that the size of the repulsive ring, the distance to the first minimum potential and the diffusion barrier are the important parameters for Fe adatoms to form a superlattice on the Cu(111) surface.

Short linear atomic chains in copper nanowires

We have performed realistic molecular dynamics simulations of copper nanowires (NWs)under stress along some crystallographic directions until their rupture to help understandthe properties of these NWs at an atomistic level. We compare the structural arrangementduring the elongation with the dynamical evolution of copper NWs observed in highresolution transmission electron microscopy (HRTEM) experiments, and they are in goodagreement. Finally, we report the formation of short linear atomic chains (LACs) beforebreaking that occurs in all cases indicating the possibility to use copper as metallicnanocontacts.

Direct laser manufacturing with coaxial powder injection: Modelling of structure of deposited layers

Direct Laser Manufacturing (DLM) with coaxial powder injection is applied for fabrication of near net shape objects from metallic powder. The proposed model includes analysis of the two-phase flow formed by the coaxial nozzle, particles heating by the laser beam, and deposit formation. The experimentally observed deposit looks like a porous array of separate intersecting particles. The density of the deposit is determined by competition between kinetic energy of the liquid droplets at impact and surface tension forces preserving their spherical shape. It is shown that the density, the mean coordination number and the maximum radius of the necks formed between particles increase with the impact velocity. The particles are bound preferentially in the vertical direction. The bounding anisotropy decreases with the impact velocity. The neck size distribution changes from a very narrow one at low velocities to a wide distribution at high velocities. The radial distribution function indicates that short linear chains of particles are formed at low impact velocities, these chains disappearing at higher velocities.

Rice Amylopectin Fine Structure Variability Affects Starch Digestion Properties

The possibility to identify or develop new rice cultivars with low glycemic response was investigated. Twelve rice cultivars with a narrow range of amylose contents were selected based on their wide variation in rapid viscoanalyzer (RVA) pasting breakdown to study the relationship between starch digestibility and amylopectin fine structure and pasting properties. Rice flour samples were cooked for in vitro digestibility analysis using the standard Englyst assay. RVA was performed for pasting properties of starches. Results showed that rapidly digestible starch (RDS) was highly and negatively correlated (r = -0.86, p < 0.01; r = -0.81, p < 0.01) with FrI long and FrII intermediate/short debranched amylopectin linear chains, respectively, and positively correlated (r = 0.79; p < 0.01) with FrIII very short linear chains. Slowly digestible (SDS) starch was positively correlated (r = 0.80, p < 0.01; 0.76, p < 0.01) with FrI and FrII, respectively, and negatively correlated (r = -0.76, p < 0.01) with FrIII. RVA breakdown viscosity was positively correlated (r = 0.88, p < 0.01) with RDS and negatively correlated (r = -0.89, p < 0.01) with SDS. Thus, the RVA method potentially could be used as a screening tool for starch digestion properties. This study reveals a molecular basis in amylopectin fine structure variability for starch digestion properties in rice cultivars and could have value in identifying slowly digesting cultivars as well as developing a breeding strategy to produce low glycemic rice cultivars. Rice; starch; RVA; amylopectin; digestibility.

Linear Melt Rheology of Pom-Pom Polystyrenes with Unentangled Branches

We measured the linear viscoelastic properties of a series of polystyrene melts with pom-pom architecture consisting of backbones ranging from marginally to well-entangled (of molecular weight Mb), end-grafted with q unentangled branches (Mbr) per backbone end. In this case, the branches relax very fast and act primarily as solvents for the backbone. Using a time-marching tube-based model, we showed that, in such a case only, the pom-pom polymer is equivalent to a blend of long (ML ≈ Mb + 2Mbr) and short (MS ≈ Mbr) linear chains with respective proportion (Mb + 2Mbr)/2(q − 1)Mbr.

Dilute Cu nanostructure stabilized by substrate-mediated interactions onCu(111): Kinetic Monte Carlo simulations

The homoepitaxial growth of Cu on $\mathrm{Cu}(111)$ is studied by kinetic Monte Carlo simulations at temperature $15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and at coverage ranging from 0.005 to 0.04 monolayer (ML). It is shown that substrate-mediated interactions between adatoms lead to the formation of dilute nanostructures with an average nearest-neighbor distance a few times longer than an equilibrium adatom-adatom distance. Morphology of the nanostructure depends on coverage and annealing time. At coverage lower than $0.02\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ short linear chains develop. During annealing the chains evolve into small dilute clusters. At coverage in the range of $0.02--0.04\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ the Cu adatoms need $10--20\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ to self-organize into dilute islands with local hexagonal structure. The simulation reveals the existence of energetically favorable adsorption sites at the step edge on the $\mathrm{Cu}(111)$ surface that stimulate aggregation of adatoms near the step.

Violation of the des Cloizeaux relation for self-avoiding walks on Sierpinski square lattices

The statistics of self-avoiding walks (SAWs) on deterministic fractal structures with infinite ramification, modeled by Sierpinski square lattices, is revisited in two and three dimensions using the reptation algorithm. The probability distribution function of the end-to-end distance of SAWs, consisting of up to 400 steps, is obtained and its scaling behavior at small distances is studied. The resulting scaling exponents are confronted with previous calculations for much shorter linear chains (20 to 30 steps) based on the exact enumeration (EE) technique. The present results coincide with the EE values in two dimensions, but differ slightly in three dimensions. A possible explanation for this discrepancy is discussed. Despite this, the violation of the so-called des Cloizeaux relation, a renormalization result that holds on regular lattices and on deterministic fractal structures with finite ramification, is confirmed numerically.

Effect of the long-range adsorbate interactions on the atomic self-assembly on metal surfaces

Recent experimental studies have demonstrated that short linear chains are often formed in the early stage of heteroepitaxy on the (1 1 1) noble metal surfaces at low temperatures. Here, we show that the surface-state mediated long-range interaction between adsorbates is the driving force for the self-organization of adsorbates at very low temperatures. Our kinetic Monte Carlo simulations for Co adatoms on Cu(1 1 1) and for Ce adatoms on Ag(1 1 1) reveal that these interactions can lead to the formation of linear chains.

Inside Front Cover: One‐Dimensional Plasmon Coupling by Facile Self‐Assembly of Gold Nanoparticles into Branched Chain Networks (Adv. Mater. 21/2005)

AbstractShort, single‐particle‐wide chains and complex networks of interconnected chains are easily self‐assembled from 13 nm Au nanoparticles by inducing a surface electrostatic dipolar moment in a controlled manner. Mann and co‐workers further demonstrate both experimentally and theoretically on p. 2553 that efficient surface plasmon coupling takes place in these extensive networks, thus opening a new bottom–up approach to subwavelength optical‐waveguiding devices. The left panel in the image shows isolated 13 nm Au nanoparticles; the back panel, short linear chains; the bottom panel, complex branched network of chains; and the right panel, a graphical rendering of optical spectroscopic properties during the self‐assembly process.

Age progression along the Society hotspot chain (French Polynesia) based on new unspiked K-Ar ages

Abstract New K-Ar dates of volcanic rocks from five of the nine islands of the Society Archipelago (Moorea, Huahine, Raiatea, Bora Bora and Maupiti), confirm a Pacific plate velocity of around 11 cm/a during the last 4.3 m.y. These new data allow us to analyse the age-distance relationship along the chain and to evaluate possible temporal variations in the activity of the Society hotspot. A clear increase of ages is observed along the linear chain away from the present Society hotspot location. The time-space relationship between Taiarapu, Tahiti-Nui and Moorea can be explained by a simple hotspot model. Nevertheless, the simple fixed hotspot model assuming constant Pacific plate velocity may need adjustments to fully explain the age progression along the Archipelago. The slight departures from a linear age distribution can be explained by changes in Pacific plate motion which occurred at 5 and 3 Ma. In addition, the contemporaneous magmatic activities in the pairs Bora-Bora/Tahaa, Raiatea/Huahine, Maiao/Moorea require additional lithospheric control on magma transport. Combined with the hotspot activity, lithospheric loading may have produced extension and triggered volcanism along already existing fractures linking paired islands. The most likely model for the Society chain, proposed by McNutt [1998], involves a plume originating from a wide deep thermally anomalous zone (the Pacific Superswell) as a rising diapir (hotspot of secondary type according to the classification of Courtillot et al. [2003]). It melted during ascent and ponded beneath the Pacific plate to form short linear island chains showing rather good age vs. distance correlations.

Size and Shape Selectivity of Host Networks Built Based on Tunable Secondary Building Units

By modulating the secondary building units derived from the primary building units, N-acylsalicylhydrazides (H3 xshz), we have been able to construct isostructural but tunable host networks, [Mn6(xshz)6(dmf)2(bpea)2], with different cavity sizes and shapes where the secondary building units, [Mn6(xshz)6], were linked through exo-bidentate bridging ligand, 1,2-bis(pyridyl)ethane (bpea) to form 3-D networks. With a short length linear N-acyl side chain at the primary building unit, the host networks have a 3-D network with three-dimensional cavities. With an appropriate length linear N-acyl side chain at the primary building unit, the host network keeps the isostructural 3-D network but with two types of one-dimensional channels of reduced cavity volume. The tuned host networks showed not only size selectivity for the guest molecules but also shape selectivity. While the three-dimensional channeled host showed selectivity depending on the length of the podal guests, the one-dimensional channeled host showed selectivity depending on both the length and/or the podality of the guest molecules.

Hylins: Bombinins H Structurally Related Peptides from the Skin Secretion of the Brazilian Tree-Frog Hyla biobeba

Two hemolytic peptides were isolated from the skin secretion of the Brazilian Hylidae frog Hyla biobeba, using reversed-phase chromatographic procedures. Hylins b1 and b2 exhibit short linear polypeptide chains, a large number of hydrophobic residues, amidated C termini and hemolytic properties. These two novel peptides are the first examples of bombinins H-like peptides isolated from anurans species not related to Bombina species.

The liquid–vapour interface of chain molecules investigated using a density functionalapproach

A microscopic density functional theory is used to investigate the liquid–vapour interface offluids composed of short linear chains. We analyse the structure of the interface andevaluate the dependence of the surface tension and of the interfacial width on thetemperature. The difference in chain length leads to differences in the thermodynamicproperties of the fluids. The liquid-phase parts of the interfacial profiles of shorter chainsexhibit oscillations at low temperatures. These oscillations vanish for longer chains. Thesurface tension and the interfacial width at a given temperature are found to increase withthe chain length. Both the surface tension and the interfacial width scale as powerlaws upon approaching the critical point with critical exponents characteristicof mean-field-type theories and with prefactors depending on the chain lengthonly.

ENTANGLEMENT DEGRADATION IN A STRING OF ATOMS COUPLED BY THE ELECTROMAGNETIC FIELD

We investigate the dynamics of short linear chains consisting of two-level systems (atoms) coupled by the electromagnetic field. The environment of photon modes acts as a source of noise and leads to the disappearance of the initially present multipartite entanglement. The rate of this process (entanglement degradation) depends on the separation of the atoms, and also on the initial state. With the aid of the appropriate entanglement witnesses we show that this rate is exceptionally low for the so-called subradiant states. Below one resonant wavelength of atomic separation the effect of the environmental noise is weaker than the dipole-dipole interaction and multipartite entanglement can be formed in the initial stage of the time evolution.

Split crystallization during debranching of maltodextrins at high concentration by isoamylase.

Debranching and crystallization occurring during the enzymatic treatment of 25% (w/v) aqueous solutions of maltodextrins by isoamylase at 52 degrees C were studied. The morphology as well as the crystal and molecular structures of the precipitates formed at different stages of the reaction were characterized. Two types of resulting products, differing in terms of structure and morphology, were evidenced. A loose B-type network, containing linear and branched chains of highest molecular weight, was mainly formed during the first 12 h of reaction, whereas aggregates of A-type lamellar crystals, made of short linear chains, were predominantly obtained between 12 and 48 h. The aggregation behavior as a function of temperature and molecular weight distribution of such substrates was discussed and compared to that of related starch products.

Decoherence and entanglement of a linear chain of dipole coupled atoms

The time evolution of short linear chains consisting of dipole coupled two-level systems (atoms) is investigated under the influence of oscillator modes, which represent the environment. The dynamics of the system is calculated using the super-radiant master equation. The focus is on phenomena related to the environment induced decoherence and entanglement degradation. For large atomic distances the initially present pair entanglement is destroyed independently from the spatial position of the two atoms, while when the distance between the atoms becomes comparable to the resonant wavelength the rate of entanglement degradation corresponds to the strength of the pair interaction. Below one resonant wavelength of atomic separation, formation of pair entanglement is also possible in the initial stage of the time evolution. In this case it is found that pair entanglement is formed and destroyed periodically, and the strengths of the consecutive 'entanglement revivals' are damped depending on the initial state. The subradiant states, for which decoherence is slow, also preserve pair entanglements for an exceptionally long time.

Structural analysis of a pectic polysaccharide from the leaves of Diospyros kaki

A pectic polysaccharide DL-2A with a molar mass of 8.5 × 10 5, was obtained from the boiling water extract of Diospyros kaki leaves. It had [ α] 20 D −21.8° ( c 0.22, H 2O) and consisted of rhamnose, arabinose, galactose, xylose and galacturonic acid units in the molar ratio of 0.4:3.4:2.4:1.0:0.8, along with traces of glucuronic acid. About 16.7% of galacturonic acid existed as the methyl ester. A combination of linkage analyses, periodate oxidation, partial acid hydrolysis, selective lithium-degraded reaction, ESIMS, 1H- and 13C- NMR spectral analyses revealed its structural features. It was found that DL-2A possessed an α-(1→4)-galacturonan backbone with some insertions of α-1,2-Rha p residues. The side-chains of arabino-3,6-galactan were attached to the backbone via O-4 of Rha p residues and O-3 of GalA p residues, while 4-linked xylose residues (forming short linear chains) were directly linked to O-4 of rhamnose residues, not as part of the xylogalacturonan. These novel structural features enlarge the knowledge on the fine structure of pectic substances in the plant kingdom.

On the calculation of vibrational energy relaxation rate constants from centroid molecular dynamics simulations

We explore the use of centroid molecular dynamics (CMD) for calculating vibrational energy relaxation (VER) rate constants of high-frequency molecular vibrations in the condensed phase. We employ our recently proposed linear-response-theory-based approach to VER [Q. Shi and E. Geva, J. Chem. Phys. 118, 7562 (2003)], to obtain a new expression for the VER rate constant in terms of a correlation function that can be directly obtained from CMD simulations. We show that the new expression reduces to a centroid Landau-Teller-type formula in the golden-rule regime. Unlike previously proposed CMD-based approaches to VER, the new formula does not involve additional assumptions beyond the inherent CMD approximation. The new formula has the same form as the classical Landau–Teller formula, and quantum effects enter it in two ways: (1) The initial sampling and subsequent dynamics are governed by the centroid potential, rather than the classical potential; (2) The classical force is replaced by the corresponding centroid symbol. The application of the new method is reported for three model systems: (1) A vibrational mode coupled to a harmonic bath, with the coupling exponential in the bath coordinates; (2) A diatomic molecule coupled to a short linear chain of Helium atoms; (3) A “breathing sphere” diatomic molecule in a two-dimensional monoatomic Lennard-Jones liquid. It is confirmed that CMD is able to capture the main features of the force–force correlation function rather well, in both time and frequency domains. However, we also find that CMD is unable to accurately predict the high-frequency tail of the quantum-mechanical power spectrum of this correlation function, which limits its usefulness for calculating VER rate constants of high-frequency molecular vibrations. The predictions of CMD are compared with those obtained via the linearized-semiclassical initial-value-representation (LSC-IVR) method, which does yield accurate predictions of high-frequency VER rate constants. The reasons underlying these observations are discussed in terms of the similarities and differences between these two approaches.

Copoly(γ, dl-glutamate)s containing short and long linear alkyl side chains

Copoly(α-alkyl γ, dl-glutamate)s containing octadecyl and ethyl side groups were prepared from racemic poly(γ, dl-glutamic acid) of bacterial origin, and their structure in the solid state examined by DSC, polarized IR spectroscopy and X-ray diffraction methods. Copolymers with a blocky microstructure were prepared by transesterification of poly(α-ethyl γ, dl-glutamate) with octadecanol for a wide range of compositions. Random copolymers were prepared by sequential alkylation of the polyacid with ethyl bromide and octadecyl bromide in solution. All the multiblock copolymers were found to adopt the layered structure characteristic of comb-like polypeptides with the octadecyl side chains crystallized in extended conformation in a separate phase, and aligned normal to the main chains, which are arranged in helical conformation. Melting of paraffinic crystallites reversibly happened in the 50–55 °C range without significant alteration of the layered arrangement. Upon heating at 150 °C, the structure contracted but no indication on the existence of a second transition, as it is known to occur in comb-like poly(α-alkyl γ-glutamate) homopolymers, was observed by DSC. Random copolymers did show nor thermal transition neither evidence of any organized supramolecular structure.

Semiclassical Theory of Vibrational Energy Relaxation in the Condensed Phase

This paper presents the first application of semiclassical methodology to the calculation of vibrational energy relaxation (VER) rate constants in condensed phase systems. The VER rate constant is treated within the framework of the Landau-Teller formula and is given in terms of the Fourier transform, at the vibrational frequency, of the force-force correlation function (FFCF). Due to the high frequency of most molecular vibrations, predictions based on the classical FFCF are often found to deviate by orders of magnitude from the experimentally observed values. In this paper, we employ a semiclassical approximation for the quantum-mechanical FFCF, that puts it in terms of a classical-like expression, where Wigner transforms replace the corresponding classical quantities. The multidimensional Wigner transform is performed via a novel implementation of the local harmonic approximation (LHA). The resulting expression for the FFCF is exact at t = 0, and converges to the correct classical limit when h → 0. Quantum effects are introduced via a nonclassical initial sampling of both positions and momenta, as well as by accounting for delocalization in the calculation of the force at t = 0. The application of the semiclassical method is reported for three model systems: (1) a vibrational mode coupled to a harmonic bath, with the coupling exponential in the bath coordinates; (2) a diatomic coupled to a short linear chain of helium atoms; (3) a breathing sphere diatomic in a two-dimensional monatomic Lennard-Jones liquid. Good agreement is found in all cases between the semiclassical predictions and the exact results, or their estimates. It is also found that the VER of highfrequency molecular vibrations is dominated by a purely quantum-mechanical term, which vanishes in the classical limit.