Effect of molecular topology on the transport properties of dendrimers in dilute solution at Theta temperature: a Brownian dynamics study

Structure and transport properties of dendrimers in dilute solution are studied with the aid of Brownian dynamics simulations. To investigate the effect of molecular topology on the properties, linear chain, star, and dendrimer molecules of comparable molecular weights are studied. A bead-spring chain model with finitely extensible springs and fluctuating hydrodynamic interactions is used to represent polymer molecules under Theta conditions. Structural properties as well as the diffusivity and zero-shear-rate intrinsic viscosity of polymers with varied degrees of branching are analyzed. Results for the free-draining case are compared to and found in very good agreement with the Rouse model predictions. Translational diffusivity is evaluated and the difference between the short-time and long-time behavior due to dynamic correlations is observed. Incorporation of hydrodynamic interactions is found to be sufficient to reproduce the maximum in the intrinsic viscosity versus molecular weight observed experimentally for dendrimers. Results of the nonequilibrium Brownian dynamics simulations of dendrimers and linear chain polymers subjected to a planar shear flow in a wide range of strain rates are also reported. The flow-induced molecular deformation of molecules is found to decrease hydrodynamic interactions and lead to the appearance of shear thickening. Further, branching is found to suppress flow-induced molecular alignment and deformation.     

On the Brownian motion of a massive sphere suspended in a hard-sphere fluid. II. Molecular dynamics estimates of the friction coefficient

The friction coefficient exerted by a hard-sphere fluid on an infinitely massive Brownian sphere is calculated for several size ratios , where and are the diameters of the Brownian and fluid spheres, respectively. The exact microscopic expression derived in part I of this work from kinetic theory is transformed and shown to be proportional to the time integral of the autocorrelation function of the momentum transferred from the fluid to the Brownian sphere during instantaneous collisions. Three different methods are described to extract the friction coefficient from molecular dynamics simulations carried out onfinite systems. The three independent methods lead to estimates of which agree within statisticalerrors (typically 5%). The results are compared to the predictions of Enskog theory and of the hydrodynamic Stokes law. The former breaks down as the size ratio and/or the packing fraction of the fluid increase. Somewhat surprisingly, Stokes' law is found to hold withstick boundary conditions, in the range 1/4.5 explored in the present simulations, with a hydrodynamic diameterd=. The analysis of the moleuclar dynamics data on the basis of Stokes' law withslip boundary conditions is less conclusive, although the right trend is found as / increases.     

Fuzzy Quantum Logics as a Basis for Quantum Probability Theory

Representation of an abstract quantum logic withan ordering set of states S in the form of a family L(S) of fuzzy subsets of S which fulfils conditionsanalogous to Kolmogorovian conditions imposed on -algebra of random events allows us toconstruct quantum probability calculus in a waycompletely parallel to the classical Kolmogorovianprobability calculus. It is shown that the quantumprobability calculus so constructed is a propergeneralization of the classical Kolmogorovian one. Someindications for building a phase-space representation ofquantum mechanics free of the problem of negativeprobabilities are given.     

Isomerization and Decomposition of a Criegee Intermediate in the Ozonolysis of Alkenes: Dynamics Using a Multireference Potential

The isomerization and decomposition dynamics of the simplest Criegee intermediate CH₂OO have been studied by classical trajectory simulations using the multireference ab initio MR‐PT2 potential on the fly. A new, accelerated algorithm for dynamics with MR‐PT2 was used. For an initial temperature of 300 K, starting from the transition state from CH₂OO→CH₂O₂ , the system reaches the dioxirane structure in around 50 fs, then isomerizes to formic acid (in ca. 2800 fs), and decomposes into CO+H₂O at around 2900 fs. The contributions of different configurations to the multiconfigurational total electronic wave function vary dramatically along the trajectory, with diradical contributions being important for transition states corresponding to H‐atom transfers, while being only moderately significant for CH₂OO. The implications for reactions of Criegee intermediates are discussed.     

Phenolic constituents of Knautia arvensis aerial parts

A new C-6 flavone glycoside (6), together with seven known compounds, cryptochlorogenic acid (1), chlorogenic acid (2), 2-O-trans-caffeoylhydrocitric acid (3), isovitexin 7-beta-D-glucopyranoside (4), 7,4'-dihydroxy-5-methoxyflavone-6-C-beta-D-glucopyranoside (5), 3,5-O-dicaffeoylquinic acid (7) and 4,5-O-dicaffeoylquinic acid (8), were isolated from the aerial parts of Knautia arvensis. Their structures were elucidated by extensive spectroscopic methods including 1D- (1H, 13C and TOCSY) and 2D-NMR (DQF-COSY, HSQC, HMBC) experiments, as well as ESIMS analysis. Compounds 1, 3-5 and 8 are reported for the first time in Knautia arvensis.     

Synthesis and characterization of monolayers and Langmuir-Blodgett films of an amphiphilic oligo(ethylene glycol)-C60-hexadecaaniline conjugate

A novel amphiphilic oligo(ethylene glycol)-C60-hexadecaaniline (A16) tricomponent conjugate, C60>(A16-EG43), possessing a well-defined number of repeating aniline donor units and a hydrophilic ethylene glycol oligomer chain was synthesized. The compound is composed of a covalently bound donor-acceptor chromophore structure. Molecular self-assembly of C60>(A16-EG43) at the air-water interface formed a densely packed Langmuir monolayer with all highly hydrophobic fullerene cages located above the liquid interface. The monolayer can then be transferred onto a glass substrate via Langmuir-Blodgett (LB) deposition. LB multilayered thin films formed by multiple deposition of the monolayer yielded broadened optical absorption peaks extending beyond 600 nm into the 950 nm region, suggesting strong intermolecular interactions among the C60 cages and the A16 moieties. An X-ray reflectometry study clearly reveals that the Langmuir film at the air-water interface consists of a C60 top layer and a bottom layer containing hexcadecaaniline and oligo(ethylene glycol) with gradually decreasing electron density over a distance of approximately 130 A above bulk water. The pressure isotherm shows that the packing density of the C60>(A16-EG43) monolayer, corresponding to a molecular area of approximately 95 A2/molecule, is similar to that of the surface area of the C60 monolayer. This result suggests that C60 packing plays a dominant role in guiding the formation of the monolayer structure. Further photoexcitation of hexadecaaniline moieties of aligned (C60>)-A16 layers by a flash light source induces cross linking between adjacent A16 segments forming an interlinked A16 array. Our results have demonstrated a unique fabrication method for preparing the aligned donor-acceptor array using strong intermolecular interactions between fullerenes as the molecular orientation guiding force in the Langmuir-Blodgett technique.     

Structure and dynamics of L-selenomethionine in the solid state

L-selenomethionine 1 crystallizes in P2(1) space group with two molecules in the asymmetric unit. Solid-state NMR spectroscopy is used for searching of structure and dynamics of 1 in the crystal lattice. The distinct molecular motion of side chains for A and B molecules of 1 is apparent from measurements of relaxation parameters (1H 1rho, 13C T1) and analysis of CSA data (2D-PASS experiment). The 13C delta(ii) and 77Se delta(ii) parameters are correlated with theoretical shielding parameters obtained by means DFT GIAO calculations. Attempt to explain the mechanism of phase transition of crystals of 1 at 313K is presented.     

Origins of thermodynamically stable superhydrophobicity of boron nitride nanotubes coatings

Superhydrophobic surfaces are attractive as self-cleaning protective coatings in harsh environments with extreme temperatures and pH levels. Hexagonal phase boron nitride (h-BN) films are promising protective coatings due to their extraordinary chemical and thermal stability. However, their high surface energy makes them hydrophilic and thus not applicable as water repelling coatings. Our recent discovery on the superhydrophobicity of boron nitride nanotubes (BNNTs) is thus contradicting with the fact that BN materials would not be hydrophobic. To resolve this contradiction, we have investigated BNNT coatings by time-dependent contact angle measurement, thermogravimetry, IR spectroscopy, and electron microscopy. We found that the wettability of BNNTs is determined by the packing density, orientation, length of nanotubes, and the environmental condition. The origins of superhydrophobicity of these BNNT coatings are identified as (1) surface morphology and (2) hydrocarbon adsorbates on BNNTs. Hydrocarbon molecules adsorb spontaneously on the curved surfaces of nanotubes more intensively than on flat surfaces of BN films. This means the surface energy of BNNTs was enhanced by their large curvatures and thus increased the affinity of BNNTs to adsorb airborne molecules, which in turn would reduce the surface energy of BNNTs and make them hydrophobic. Our study revealed that both high-temperature and UV-ozone treatments can remove these adsorbates and lead to restitution of hydrophilic BN surface. However, nanotubes have a unique capability in building a hydrophobic layer of adsorbates after a few hours of exposure to ambient air.     

Synthesis and Optical Properties of Various Thienyl Derivatives of Pyrene

A series of various thienyl derivatives of pyrene were synthesized by Stille cross-coupling procedure. Their structures were characterized by ¹H NMR, ¹³C NMR and elemental analysis. The spectroscopic characteristics were investigated by UV–vis absorption and fluorescence spectra. Based on quantum chemical calculations, the energy levels of investigated molecules with respect to the pyrene molecule were also discussed.     

The nature of variations of ammonia proton affinity in an argon environment

The presence of solvent molecules, even inert, may significantly influence processes taking place in the gas phase. The reason for the solvent activity may be found in studies of complexes originating from microsolvation. The structure, thermodynamics, and vibrational properties of NH(4)(+)Ar(n) (n = 0-5) and NH(3)Ar(n) (n = 0-4) complexes are presented in this work with the aim to elucidate the effect of microsolvation on protonation/deprotonation processes. The relation between the nature of interactions in cationic and neutral clusters and the proton affinity is studied as a function of the number of ligands in complexes.