Formation of phenylmagnesium halides in toluene

Formation reactions of phenylmagnesium chloride and bromide in toluene in the presence of one or two equivalents of diethyl ether or THF were investigated kinetically. Also, the reaction in diethyl ether and in chlorobenzene was addressed. Kinetic features of the reactions are similar to those found previously for the formation of alkylmagnesium halides in toluene and consist of rapid formation of a disolvated Grignard reagent followed by a slower formation of a monosolvated reagent. The latter is able of catalyzing the conversion of different halides into Grignard reagents. However, the contribution of Wurtz-type side reactions is considerable except when THF is used in toluene. Involving the kinetic data and the activation parameters some details of the reaction mechanism were discussed.     

A posteriori error estimates with the finite element method of lines for a Sobolev equation

A posteriori error estimates for semidiscrete finite element methods for a nonlinear Sobolev equation are considered. The error estimates are obtained by solving local nonlinear or linear pseudo‐parabolic equations for corrections to the solution on each element. The ratios of these estimates and the true errors are proved to converge to 1, implying that the estimates can be used as indicators in adaptive schemes for the problem. Numerical results underline our theoretical results. © 2004 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2005     

Chirped fiber Bragg gratings with arbitrary group delay responses using stepped-stretching fiber method

We present the design and development of piece-wise stepped-chirp fiber Bragg gratings (FBGs) with arbitrary group delay responses using a uniform phase mask in a pre-stretched fiber. The method involves writing a series of sub-gratings on a pre-stretched fiber whose length is varied during the UV exposure process. Two motorized stages are employed to control the length of the pre-stretched fiber in order to adjust the grating pitch at each writing step to achieve piece-wise stepped chirping of the FBG. To demonstrate the effectiveness of the method to produce chirped FBGs with arbitrary group delay responses, a linear-chirp FBG and a quadratic-chirp FBG were fabricated, and their measured performances agree well with analytical predictions. PACS 42.81.-i; 42.79.Dj; 42.79.Ci     

A Staged Continuous Tabu Search Algorithm for the Global Optimization and its Applications to the Design of Fiber Bragg Gratings

A novel staged continuous Tabu search (SCTS) algorithm is proposed for solving global optimization problems of multi-minima functions with multi-variables. The proposed method comprises three stages that are based on the continuous Tabu search (CTS) algorithm with different neighbor-search strategies, with each devoting to one task. The method searches for the global optimum thoroughly and efficiently over the space of solutions compared to a single process of CTS. The effectiveness of the proposed SCTS algorithm is evaluated using a set of benchmark multimodal functions whose global and local minima are known. The numerical test results obtained indicate that the proposed method is more efficient than an improved genetic algorithm published previously. The method is also applied to the optimization of fiber grating design for optical communication systems. Compared with two other well-known algorithms, namely, genetic algorithm (GA) and simulated annealing (SA), the proposed method performs better in the optimization of the fiber grating design.     

Une détermination du coefficient d'auto-diffusion de surface avec des pointes à émission de champ (tungstène)

To measure the surface self-diffusion coefficient, a method is described which is based on the increase of the curvature radius of a tip (Nichols and Mullins). This radius increase depending on temperature and time is measured under ultra high vacuum condition. The cleanliness of the surface is controlled by field electron microscopy. The radius variation is measured in situ by Fowler-Nordheim characteristics, and the initial and final radius by scanning electron microscopy. Measured are, for tungsten between 2100 and 2850°K, the activation energy ($\text{3.1}\text{eV}\text{atome}$) and the diffusivity ($\text{0.9}\text{cm}{}^2\text{s}\text{)}$.     

Grignard reaction with chlorosilanes in THF: a kinetic study

Kinetics of the reactions of phenylmagnesium chloride and bromide and diphenylmagnesium with chlorosilanes were investigated in tetrahydrofurane (THF) and in THF-hydrocarbon mixtures. The reaction in THF is much faster than that in diethyl ether. Assuming coordination of magnesium halides with three molecules of THF, concentrations of all the species involved in Schlenk equilibrium were calculated. In the Grignard reaction, species R(2)Mg and RMgX react competitively accompanied by additional reaction paths involving electrophilic catalysis by magnesium halide. This conclusion also proved to be valid for the Grignard reaction with a ketone and probably can be expanded to any Grignard reaction. When Schlenk equilibrium is shifted far to the RMgX species, the catalytic pathways are insignificant. Substituents at the silicon center control the rate of the reaction through their inductive and steric effects.     

Using indicator-displacement assays in test strips and to follow reaction kinetics

[structure: see text] An indicator-displacement assay was used to study the kinetics of reactions that produce tartrate. The same assay was also found to be useful in developing tartrate test strips, by coating filter paper with the same receptor and indicator.     

Monitoring the simultaneous ostwald ripening and solubilization of emulsions

The simultaneous Ostwald ripening of an emulsion and the solubilization of its oil droplets by added micellar surfactant solutions are monitored by measurements of time-averaged scattered intensities. A simple computer simulation model for the interpretation of the measurements is presented. Experimental data are analyzed with this model using one single parameter: an effective ratio of oil to surfactant molecules involved in the withdrawal of oil from the Ostwald ripening process by the added micelles. The fitted value of this parameter appears to be more than twice the one that can be predicted from the equilibrium solubilization of oil by the surfactant micelles, indicating that more oil is involved in the nonequilibrium exchange of oil and surfactant between micelles and droplets.     

Strong binding in the DNA minor groove by an aromatic diamidine with a shape that does not match the curvature of the groove

A combination of biophysical techniques has been used to characterize the interaction of an antitrypanosomal agent, CGP 40215A, with DNA. The results from a broad array of methods (DNase I footprinting, surface plasmon resonance, X-ray crystallography, and molecular dynamics) indicate that this compound binds to the minor groove of AT DNA sequences. Despite its unusual linear shape that is not complementary to that of the DNA groove, a high binding affinity was observed in comparison with other similar but more curved diamidine compounds. The amidine groups at both ends of the ligand and the -NH groups on the linker are involved in extensive and dynamic H-bonds to the DNA bases. Complementary and consistent results were obtained from both the X-ray and molecular dynamics studies; both of these methods reveal direct and water-mediated H-bonds between the ligand and the DNA.