Zirconium catalyzed amide formation without water scavenging

A scalable homogeneous metal‐catalyzed protocol for direct amidation of carboxylic acids is presented. The use of 2–10 mol% of the commercially available Zr(Cp)₂(OTf)₂·THF results in high yields of amides at moderate temperature, using an operationally convenient reaction protocol that circumvents the use of water scavenging techniques.     

Molecular Basis of the Chemiluminescence Mechanism of Luminol

Light emission from luminol is probably one of the most popular chemiluminescence reactions due to its use in forensic science, and has recently displayed promising applications for the treatment of cancer in deep tissues. The mechanism is, however, very complex and distinct possibilities have been proposed. By efficiently combining DFT and CASPT2 methodologies, the chemiluminescence mechanism has been studied in three steps: 1) luminol oxygenation to generate the chemiluminophore, 2) a chemiexcitation step, and 3) generation of the light emitter. The findings demonstrate that the luminol double-deprotonated dianion activates molecular oxygen, diazaquinone is not formed, and the chemiluminophore is formed through the concerted addition of oxygen and concerted elimination of nitrogen. The peroxide bond, in comparison to other isoelectronic chemical functionalities (−NH−NH−, −N⁻−N⁻−, and −S−S−), is found to have the best chemiexcitation efficiency, which allows the oxygenation requirement to be rationalized and establishes general design principles for the chemiluminescence efficiency. Electron transfer from the aniline ring to the OO bond promotes the excitation process to create an excited state that is not the chemiluminescent species. To produce the light emitter, proton transfer between the amino and carbonyl groups must occur; this requires highly localized vibrational energy during chemiexcitation.     

Properties of magnetocaloric La(Fe,Co,Si)13 produced by powder metallurgy

We present a comprehensive study of the magnetocaloric materials series La(Fe_1−xCo_x)_11.9Si_1.1 with 0.055<x<0.122. The ferromagnetic samples were manufactured using a novel powder metallurgy process by which industrial scale production is feasible. This new production method makes the materials more attractive as magnetic refrigerants for room temperature magnetic refrigeration. The Curie temperature of the compounds can be easily tuned by altering the Co content and all samples have little magnetic anisotropy and present a second-order magnetic transition so that thermal and magnetic hysteresis is absent. For all seven samples, we have calculated the magnetic entropy change, ΔS_M, from initial curve measurements and measured the adiabatic temperature change, ΔT_ad, directly. In addition, for two of the samples, we determined the heat capacity as a function of applied magnetic field and the thermal conductivity. Where relevant, the results are compared with those of Gd, the benchmark material for room temperature magnetic refrigeration.     

The emulsion copolymerization of styrene and sodium styrene sulfonate

The emulsion copolymerization of styrene and sodium styrene sulfonate has been shown to be a feasible preparative route to ionomeric sulfonated polystyrene. The properties of these copolymers are reported elsewhere. The copolymerization rate was found to be dramatically enhanced when compared to that for the emulsion copolymerization of styrene under identical conditions. This copolymerization was studied in detail and two mechanisms were proposed to account for these rate differences. An increase in the number of polymerizing particles in the copolymerization with consequent rate enhancement was substantiated by electron microscopy. However, the data indicate that the rate differences cannot be fully accounted for by this effect. In addition, a gel effect is proposed as a second contributor to the enhanced rate. This gel effect is believed to result from the intermolecular association of the incorporated metal sulfonate units in the growing polymer particles. When a third monomer that plasticizes the ionic interactions is used the polymerization rate decreases. This supports the gel effect hypothesis.     

Natural radiative lifetimes in the2 S 1/2 and2 D 3/2, 5/2 sequences of neutral copper

Natural radiative lifetimes have been measured of the 3d ¹⁰ ns ² S _1/2 (n=6?10) and of the 3d ¹⁰ nd ² D _{3/2, 5/2} (n=5?9) sequences by using two step excitation of copper atoms in an atomic beam. The states investigated were populated by the light from two pulsed dye lasers pumped by the same Nd:YAG laser. The lifetimes of the² D sequence increase regularly with increasing principal quantum number. This is not the case for the² S series, because of configuration interaction in the² P series. In addition the lifetime of the 3d ¹⁰5p ² P _3/2 state has been measured together with its branching ratio.     

Electrothermal atomic absorption spectroscopy - preseht understanding and future needs

A comparison is made between Massmann-type furnaces (with and without the L'vov platform) and constant temperature atomizers. It is shown that there is no major difference between these types of furnaces with regard to peak height sensitivities. On the other hand, the Massmann-type furnaces shoved to a greater extent susceptibilities towards matrix interference effects. The effect of the sample residence time on gas phase interference effects has been investigated at various constant temperatures for lead in large excesses of iron chloride and sodium sulphate, respectively. These experimental results are discussed and they are correlated to data obtained by high temperature equilibrium calculations. As a conclusion we found that there is a need for a better control of the gas phase inside graphite tubes. Advantages of separating the volatilization and atomization processes are discussed. The potentialities of constant temperature atomizers for atomic emission spectroscopy are lined out.Since its inception, conventional GFAAS has been developed considerably with regard to methodology and instrumentation. The technique has been essentially improved by the introduction of e.g., automatic sample devices, the L'vov platform technique, matrix modifications, pyrolytically coated graphite, automatic background correctors, adequate signal evaluation and rapid controlled heating of the atomizers. In spite of this progress there still remain problems in connection with the vaporization/atomization of samples. In conventional Massmann-type furnaces, the temperature at which an element is vaporized depends on its volatility and usually effective atomization temperatures are often too low for complete atomization. An additional disadvantage comprises difficulties in relating absorbance signals, which may originate from different atomization intervals, to true amounts of an element. Many of these problems inherent in Massmann-type furnaces can be eliminated by vaporizing samples into atomizers which are kept at a constant temperature. This concept was employed in the first graphite furnace ever built for analytical AAS [l], but due to the technical complexity of the isothermal approach, it has only been realized on a minor scale and therefore little is known about its limitations.By vaporizing samples from a platform [2,3] inserted into Massmann-type furnaces, the problems arising from non-isothermal atomization can often be minimized in a relatively simple way. In particular for volatile elements it is possible to approach conditions of constant temperature atomizers by the combined use of the platform technique with an element stabilizing modifier solution [4,5].The aim of this paper is to characterize isothermal as well as Massmann-type atomizers (equipped with and without platforms) with respect to sensitivity and susceptibility to interference effects as well as identifying future needs in order to develop the graphite furnace technique further.     

Perceived sound quality of reproductions with different frequency responses and sound levels

Three programs (female voice, jazz music, and pink noise) were reproduced using four different frequency responses and two different sound levels. Fourteen normal hearing subjects listened to the reproduction via earphones and judged the sound quality on seven perceptual scales (loudness, clarity, fullness, spaciousness, brightness, softness/gentleness, and nearness) and a fidelity scale. Significant differences among the reproductions appeared in all scales and could be attributed to the differences in frequency response or sound level or both. Interactions between the reproductions and the programs could be explained by the relations between the spectrum of the programs and the frequency responses used. The results for the noise program were similar to those for the jazz music program.     

Impact-induced vibrations of a simple viscoelastic column model

A two-degree-of-freedom model for an almost-axially impacted viscoelastic cantilever column is analyzed. The impact load is produced by a mass striking the free end of the column. Under the assumption of small displacements two second-order non-linear ordinary differential equations for the coupled longitudinal and transverse vibrations of the column are derived. In the absence of damping these equations of motion are reduced to Mathieu's equation through the use of a perturbation method. The excitation parameters are (i) the natural frequency of small amplitude transverse vibrations of the undamped column and (ii) the initial velocity of the end of the column. The boundaries of two unstable regions are obtained. In the stable regions the solution of Mathieu's equation for the transverse displacement is close to that of the original non-linear equations of motion. In the first unstable region there is agreement only for early time. With increasing damping the peak of the maximum transverse displacement in the first unstable region decreases or even vanishes.