The synthesis and reactivity of polysilylacetylenes: Diels—Alder reactions to give bis (silyl) benzenes

Six bis(silyl)acetylenes (XMe²Si? C?C? SiMe₂X) with the following varied silicon substituents X were prepared: 1 (Me, Me); 2 (H, H); 3 (C1, H); 4 (CI, CI); 5 (MeO, H); 6 (MeO, MeO). While 1 and 2 may be prepared by the reaction of dilithio- or bis(bromomagnesium)-acetylide with the appropriate chlorosilane, similar reactions designed to give 3–6 yielded oligomers, XMe₂Si? (? C?C? SiMe₂)_n? X, 7, X=CI, MeO, as the major products, indicating that the acetylenic functionality on silicon activates the chlorosilane towards nucleophilic substitution. Compounds 3 and 4 were prepared by free radical chlorination of 2. Methanolysis of 3 and 4 gave quantitative yields of 5 and 6 respectively. Compounds 1–6 undergo a Diels–Alder reaction with α-pyrone to produce, after loss of carbon dioxide, bis(silyl)-substituted benzene derivatives. The order of reactivity has been determined to be: 4=6>3=5>1>2, indicating that chloro or alkoxy substituents favor the cycloaddition with 2- pyrone. The adducts formed by compounds 3–6 undergo an unusually facile hydrolysis or elimination to give 1,1,3,3-tetramethyl-1,3-disila-2-oxaindane.     

Evolving access networks: a European perspective

The diversity of telecommunications markets and access network infrastructure among the countries of Europe is a complicating factor in the development of effective standards for Europe's future access networks. The need for evolving access networks to make efficient use of a wide variety of existing infrastructure may at times conflict with the long-term requirement for a stable platform of self-consistent standards which allow future-proof evolution. In light of these constraints, the authors summarize progress made in the European Telecommunications Standards Institute (ETSI) toward standards for future access networks     

Spectroscopic analysis and kinetics of intermolecular hydrogen bond formation in poly‐pyridobisimidazole (M5) fiber

Poly‐pyridobisimiazole (M5) single filaments subjected to varying degrees of heat treatment have been analyzed using Fourier Transform Infrared (FTIR) microspectroscopy in transmission mode to detect changes in the state of intermolecular hydrogen bonding as a function of fiber annealing conditions. The FTIR absorbance bands associated with hydrogen bonding in M5 fiber have been identified, and the integrated molar absorption coefficients for the bands of interest have been determined experimentally, which allows to quantify the concentration of N? H vibration groups hydrogen‐bonded (H‐bonded) to water molecules, and the concentration of N? H vibration groups H‐bonded to adjacent polymer chains in the fiber. A dual mechanism kinetic rate expression is used to describe intermolecular H‐bond formation in M5 fiber as a function of annealing conditions, from which an activation energy for H‐bond formation of 14.8 kJ/mol is obtained. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1809–1824, 2009     

Diagonally neighbour transitive codes and frequency permutation arrays

Constant composition codes have been proposed as suitable coding schemes to solve the narrow band and impulse noise problems associated with powerline communication, while at the same time maintaining a constant power output. In particular, a certain class of constant composition codes called frequency permutation arrays have been suggested as ideal, in some sense, for these purposes. In this paper we characterise a family of neighbour transitive codes in Hamming graphs in which frequency permutation arrays play a central rode. We also classify all the permutation codes generated by groups in this family.     

Toward making the mean spherical approximation of primitive model electrolytes analytic: an analytic approximation of the MSA screening parameter

The mean spherical approximation (MSA) for the primitive model of electrolytes provides reasonable estimates of thermodynamic quantities such as the excess chemical potential and screening length. It is especially widely used because of its explicit formulas so that numerically solving equations is minimized. As originally formulated, the MSA screening parameter Γ (akin to the reciprocal of the Debye screening length) does not have an explicit analytic formula; an equation for Γ must be solved numerically. Here, an analytic approximation for Γ is presented whose relative error is generally ?10(-5). If more accuracy is desired, one step of an iterative procedure (which also produces an explicit formula for Γ) is shown to give relative errors within machine precision in many cases. Even when ion diameter ratios are ～10 and ion valences are ～10, the relative error for the analytic approximation is still ?10(-3) and for the single iterative substitution it is ?10(-9).     

Automated estimation of rare event probabilities in biochemical systems

In biochemical systems, the occurrence of a rare event can be accompanied by catastrophic consequences. Precise characterization of these events using Monte Carlo simulation methods is often intractable, as the number of realizations needed to witness even a single rare event can be very large. The weighted stochastic simulation algorithm (wSSA) [J. Chem. Phys. 129, 165101 (2008)] and its subsequent extension [J. Chem. Phys. 130, 174103 (2009)] alleviate this difficulty with importance sampling, which effectively biases the system toward the desired rare event. However, extensive computation coupled with substantial insight into a given system is required, as there is currently no automatic approach for choosing wSSA parameters. We present a novel modification of the wSSA--the doubly weighted SSA (dwSSA)--that makes possible a fully automated parameter selection method. Our approach uses the information-theoretic concept of cross entropy to identify parameter values yielding minimum variance rare event probability estimates. We apply the method to four examples: a pure birth process, a birth-death process, an enzymatic futile cycle, and a yeast polarization model. Our results demonstrate that the proposed method (1) enables probability estimation for a class of rare events that cannot be interrogated with the wSSA, and (2) for all examples tested, reduces the number of runs needed to achieve comparable accuracy by multiple orders of magnitude. For a particular rare event in the yeast polarization model, our method transforms a projected simulation time of 600 years to three hours. Furthermore, by incorporating information-theoretic principles, our approach provides a framework for the development of more sophisticated influencing schemes that should further improve estimation accuracy.     

N.M.R. studies of liquid boron trifluoride

Previous ¹⁹F N.M.R. studies of liquid BF₃ have been extended by obtaining the ¹¹B spectrum of BF₃ and the ¹⁹F spectrum of ¹⁰BF₃ at various temperatures. Values for the B-F coupling constant and the boron relaxation times have been obtained by computer matching and visual matching of observed and calculated spectra. Relaxation times have the Arrhenius temperature dependence found previously. The temperature dependence of the B-F coupling constant is discussed. Some of the possible advantages of ‘high spin spectra’ are discussed.     

Asymptotic approximations in radome analysis

A detailed analysis is presented of assumptions and limitations of physical optics and stationary phase techniques for radome analysis. A two-dimensional ogival structure with an electric or magnetic line source inside is investigated. A method-of-moments analysis using J.H. Richmond's (1965) polarization current technique is performed to obtain reference results. Physical optics, uniform geometrical theory of diffraction, and stationary phase techniques of order 1 and 2 are analyzed