Viscoelastic properties of dendrimers in the melt from nonequlibrium molecular dynamics

The viscoelastic properties of dendrimers of generation 1-4 are studied using nonequilibrium molecular dynamics. Flow properties of dendrimer melts under shear are compared to systems composed of linear chain polymers of the same molecular weight, and the influence of molecular architecture is discussed. Rheological material properties, such as the shear viscosity and normal stress coefficients, are calculated and compared for both systems. We also calculate and compare the microscopic properties of both linear chain and dendrimer molecules, such as their molecular alignment, order parameters and rotational velocities. We find that the highly symmetric shape of dendrimers and their highly constrained geometry allows for substantial differences in their material properties compared to traditional linear polymers of equivalent molecular weight.     

Hydrogen bond directed synthesis of pyridazine and naphthyridine containing macrocycles

This work describes a high-yielding, one-step synthesis of pyrizadine and naphthyridine containing macrocycles directed by intramolecular H-bonding.     

Valence bond studies of AH2 molecules

Minimal basis set (STO) molecular orbital and valence-bond calculations are reported for the³ B ₁ and¹ A ₁ states of CH₂. The open-shell molecular orbital calculations used the Roothaan formulation. The valence-bond calculations used the Prosser-Hagstrom biorthogonalisation technique to evaluate the cofactors required in using Löwdin's formulae. Optimisation of geometry and orbital exponents in the molecular orbital calculation on the³ B ₁ state gave a geometry of R_C-H=2.11 a.u. and H-C-H=123.2 °. The energy obtained was ?38.8355 a.u. The molecular orbital and valencebond calculations are compared. In the valence-bond calculations the variation with bond-length and bond-angle of the configuration energies was studied. Valence bond “build-up” studies are also reported. Valence-bond calculations using hybrid orbitals instead of natural atomic orbitals showed that the perfect-pairing approximation is not as good for CH₂ as BeH₂. The nature of the lone-pair and bonding orbitals is found to be significantly different between the³ B ₁ and¹ A ₁ states. In the³ B ₁ state the 2s and 2p orbitals are fairly equally mixed between both types of orbital. However in the¹ A ₁ state the bonding orbitals have mainly 2p character and the lone pair orbitals have mainly 2s character. As was found for H₂O, the bonding hybrid orbitals do not follow the hydrogen nuclei as the bond angle varies but continue to point approximately in their equilibrium directions.     

Kinetics and mechanism of the oxidation of iron(II) ion by chlorine dioxide in aqueous solution

The mechanism by which an excess of iron(II) ion reacts with aqueous chlorine dioxide to produce iron(III) ion and chloride ion has been determined. The reaction proceeds via the formation of chlorite ion, which in turn reacts with additional iron(II) to produce the observed products. The first step of the process, the reduction of chlorine dioxide to chlorite ion, is fast compared to the subsequent reduction of chlorite by iron(II). The overall stoichiometry is The rate is independent of pH over the range from 3.5 to 7.5, but the reaction is assisted by the presence of acetate ion. Thus the rate law is given by At an ionic strength of 2.0 M and at 25°C, k_u = (3.9 ± 0.1) × 10³ L mol^?1 s^?1 and k_c = (6 ± 1) × 10⁴ L mol^?1 s^?1. The formation constant for the acetatoiron(II) complex, K_f, at an ionic strength of 2.0 M and 25°C was found to be (4.8 ± 0.8) × 10^?2 L mol^?1. The activation parameters for the reaction were determined and compared to those for iron(II) ion reacting directly with chlorite ion. At 0.1 M ionic strength, the activation parameters for the two reactions were found to be identical within experimental error. The values of ΔH? and ΔS? are 64 ± 3 kJ mol^?1 and + 40 ± 10 J K^?1 mol^?1 respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 554–565, 2004     

C ϰ(X) and a property of (db)-spaces

Summary A new class of locally convex linear topological spaces, the (db)-spaees, recently defined by Robertson, Tweddle and Yeomans, interpolates the classes of Baire-like and unordered Baire-like spaces. Saxon and Narayanaswami have given convex metrizable spaces that distinguish among these classes. This paper gives a new characterization of (db)-spaces from which is extracted the class of b-spaces. This class interpolates the classes of -spaces and -spaces of Lehner. Let C(X) be the space of all real-valued continuous functions on the completely regular Sausdorf space X, supplied with the topology of uniform convergence on compact sets of X. It is shown that C(X) is a b-space if and only if it is an -space. A characterization of X for which C(X) is a (db)-space is unknown. Other open questions are stated in the paper.     

Superselection rules in quantum theory: Part I. A new proposal for state restriction violation

It is argued that preparation of a quantum state characterized by density operator not commuting with a superselection operatorQ does not by itself constitute an instance of superselection rule violation. It would, however, be an instance of state restriction violation. It is held that superselection rule violation is only possible with simultaneous observable and state restriction violations. It is shown that it is a priori conceivable to subdivide an ensemble whose satisfies[, Q] = 0 into subensembles whose density operators violate the state restrictions. The dynamics of the subdivision process is not considered.     

Backbone-controlled LUMO energy induces intramolecular C–H activation in ortho-bis-9-borafluorene-substituted phenyl and o-carboranyl compounds leading to novel 9,10-diboraanthracene derivatives

The choice of backbone linker for two ortho-bis-(9-borafluorene)s has a great influence on the LUMO located at the boron centers and, therefore, the reactivity of the respective compounds. Herein, we report the room temperature rearrangement of 1,2-bis-(9-borafluorenyl)-ortho-carborane, C₂B₁₀H₁₀-1,2-[B(C₁₂H₈)]₂ ([2a]) featuring o-carborane as the inorganic three-dimensional backbone and the synthesis of 1,2-bis-(9-borafluorenyl)benzene, C₆H₄-1,2-[B(C₁₂H₈)]₂ (2b), its phenylene analog. DFT calculations on the transition state for the rearrangement support an intramolecular C–H bond activation process via an S_EAr-like mechanism in [2a], and predicted that the same rearrangement would take place in 2b, but at elevated temperatures, which indeed proved to be the case. The rearrangement gives access to 3a and 3b as dibora-benzo[a]fluoroanthene isomers, a form of diboron polycyclic aromatic hydrocarbon (PAH) that had yet to be explored. The isolated compounds 2b, 3a, and 3b were fully characterized by NMR, HRMS, cyclic voltammetry (CV), single-crystal X-ray diffraction analysis, and photophysical measurements, supported by DFT and TD-DFT calculations.

The backbone linker for two ortho-bis-(9-borafluorene)s influences the LUMO at the boron centers and the reactivity of the respective compounds.     

Ergodicity of a Single Particle Confined in a Nanopore

We analyze the dynamics of a gas particle moving through a nanopore of adjustable width with particular emphasis on ergodicity. We give a measure of the portion of phase space that is characterized by quasiperiodic trajectories which break ergodicity. The interactions between particle and wall atoms are mediated by a Lennard-Jones potential, so that an analytical treatment of the dynamics is not feasible, but making the system more physically realistic. In view of recent studies, which proved non-ergodicity for systems with scatterers interacting via smooth potentials, we find that the non-ergodic component of the phase space for energy levels typical of experiments, is surprisingly small, i.e. we conclude that the ergodic hypothesis is a reasonable approximation even for a single particle trapped in a nanopore. Due to the numerical scope of this work, our focus will be the onset of ergodic behavior which is evident on time scales accessible to simulations and experimental observations rather than ergodicity in the infinite time limit.     

Synthesis of water‐soluble zwitterionic polysiloxanes

A series of water‐soluble siloxane polymers with pendent phosphorylcholine (PC) and sulfobetaine (SB) zwitterions was prepared using thiol‐ene “click” chemistry. Specifically, well‐defined vinyl‐substituted siloxane homopolymers and block copolymers were functionalized with small molecule zwitterionic thiols at room temperature. Rapid and quantitative substitution of the pendent vinyl groups was achieved, and zwitterionic polysiloxanes of narrow molecular weight distribution were obtained. The PC‐ and SB‐substituted polymers were found to be readily soluble in pure, salt‐free water. Critical micelle concentrations (CMCs) of these polymers in water were measured using a pyrene fluorescence probe, with CMC values estimated to be <0.01 g/L. Polymer aggregates were studied by dynamic light scattering, and the micelles generated from the PC block copolymers were visualized, after drying, by transmission electron microscopy. Aqueous solutions of these zwitterionic polysiloxanes significantly reduced the oil‐water interfacial surface tension, functioning as polymer amphiphiles that lend stability to oil‐in‐water emulsions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 127–134