Peaks in low field a.c. susceptibility of ferromagnetic Heusler alloys

The observation of a peak in the low field susceptibility need not always be taken to imply spin glass type of freezing. It may also be the consequence of a rapid build up of anisotropy field below the ordering temperature. The results of low field a.c. susceptibility measurements on Ru and Rh based ferromagnetic Heusler alloys presented here substantiate the above statement.     

The observation of resonance maxima in H ÷ CF4 and H + SF6 scattering

High resolution relative integral cross sections were measured for the interactions of low-velocity H atoms with CF₆ and SF₆. In each system two maxima were resolved, and the lower peak occurs at relative velocities of about 450–500 m/s. The shapes of the “spectra” are similar to those observed for Hrare gas scattering but differ in details.     

Alkaline bromine as volumetric reagent, analytical applications

Summary It has been shown that both isopropyl alcohol and acetone are quantitatively oxidised to acetic acid and carbon dioxide by alkaline hypobromite solutions and can be estimated volumetrically with the help of this reagent. It is already known that chromic acid oxidises isopropyl alcohol to acetone only under controlled conditions. A combination of the above two reagents can, therefore, be utilized for the volumetric estimation of isopropyl alcohol and acetone when present together in a mixture.The author's thanks are due to Dr. A. C. Chatterji, D. Sc., Head of the Department of Chemistry, Lucknow University, Lucknow, for the grant of laboratory facilities.     

UV-photometrisch bestimmte relative molare Massen von anionisch erzeugten Poly?thylenoxiden mit unterschiedlichen Endgruppen

Ohne Zusammenfassung     

Photonic logic NOR gate based on two symmetric microring resonators

We demonstrate an all-optical NOR logic gate based on symmetric GaAs-AlGaAs microring resonators whose resonances are closely matched. Two input pump data streams are tuned close to one resonance of the symmetric microrings to switch a probe beam tuned to another resonance by two-photon absorption. The switching energy of the gate is 20 pJ/pulse, and the switching window is 40 ps, limited by the carrier lifetime. The use of two rings provides for better cascading in photonic logic circuits because of the higher number of available ports.     

A phase cycle scheme that significantly suppresses offset-dependent artifacts in the R2-CPMG 15N relaxation experiment

R2-CPMG 15N relaxation experiments form the basis of NMR dynamics measurements, both for analysis of nano-pico second dynamics and milli-micro second dynamics (kinetics). It has been known for some time that in the practical limit of finite pulse widths, which becomes acute when using cryogenic probes, systematic errors in the apparent R2 relaxation behavior occur for spins far off-resonance from the RF carrier. Inaccurate measurement of R2 rates propagates into quantitative models such as model-free relaxation analysis, rotational diffusion tensor analysis, and relaxation dispersion. The root of the problem stems from evolution of the magnetization vectors out of the XY-plane, both during the pulses as well as between the pulses. These deviations vary as a function of pulse length, number of applied CPMG pulses, and CPMG inter-pulse delay. Herein, we analyze these effects in detail with experimentation, numerical simulations, and analytical equations. Our work suggests a surprisingly simple change in the phase progression of the CPMG pulses, which leads to a remarkable improvement in performance. First, the applicability range of the CPMG experiment is increased by a factor of two in spectral width; second, the dynamical/kinetic processes that can be assessed are significantly extended towards the slower time scale; finally, the robustness of the relaxation dispersion experiments is greatly improved.     

Synthesis of quantum superpositions by quantum computation

The quantum search algorithm can be looked at as a technique for synthesizing a particular kind of superposition-one whose amplitude is concentrated in a single basis state. This basis state is defined by a binary function f(&xmacr;) that is nonzero in this desired basis state and zero everywhere else. This paper extends the quantum search algorithm to an algorithm that can create an arbitrarily specified superposition on a space of size N in O(sqrt[N] ) steps. The superposition is specified by a complex valued function f(&xmacr;) that specifies the desired amplitude of the system in basis state &xmacr;.     

Non-directed Pd-catalysed electrooxidative olefination of arenes

The Fujiwara–Moritani reaction is a powerful tool for the olefination of arenes by Pd-catalysed C–H activation. However, the need for superstoichiometric amounts of toxic chemical oxidants makes the reaction unattractive from an environmental and atom-economical view. Herein, we report the first non-directed and regioselective olefination of simple arenes via an electrooxidative Fujiwara–Moritani reaction. The versatility of this operator-friendly approach was demonstrated by a broad substrate scope which includes arenes, heteroarenes and a variety of olefins. Electroanalytical studies suggest the involvement of a Pd(ii)/Pd(iv) catalytic cycle via a Pd(iii) intermediate.

The Fujiwara–Moritani reaction using electric current is a powerful tool for the olefination of arenes by Pd-catalysed C–H activation.

Transition metal-catalysed C–H functionalisation reactions have increasingly gained importance over the last few decades since they allow direct and rapid installation of functionality. Regardless of the undeniable synthetic value of such transformations, the need for superstoichiometric quantities of expensive and hazardous oxidants (e.g., silver and copper salts) remains a major drawback from a sustainable chemistry perspective.^1,2 Additionally, chemical oxidants often lead to the formation of by-products, hindering purification and decreasing atom economy. Nevertheless, very few reports were also reported in the literature wherein mild oxidant such as molecular oxygen can also serve as the oxidising agent.^2j To make chemical processes and transformations intrinsically sustainable, organic chemists re-discovered synthetic electrochemistry as an environmentally friendly approach.^3–6 In the domain of synthetic electrochemistry, the Lei group achieved a significant milestone and installed C–C bonds through a different cross-coupling strategy.^1k,2f–h Electroorganic synthesis utilizes electric current to realize redox processes and thereby avoids the use of dangerous, expensive, and polluting chemical oxidising or reducing agents. Precise control of electrochemical reaction parameters often leads to commendable reactivity and chemoselectivity and hence to an improved atom economy. In addition, electrochemical processes fulfil the expectations of sustainability since electricity can be generated from renewable energy sources, such as wind, sunlight or biomass. Recent efforts in the field of electrochemical C–H activation resulted in significant progress towards efficient C–C and C–heteroatom bond formations.^7–10 Hence, the utilization of electric current as an alternative oxidant in Pd-catalysed C–H functionalisations is emerging as an attractive alternative to stoichiometric reagents.^11–13The Fujiwara–Moritani reaction is one of the earliest known examples of Pd-catalysed oxidative C–H functionalisations for C–C bond formation.¹⁴ This extraordinary C(sp²)-H alkenylation reaction avoids the use of prefunctionalised starting materials; however, it suffers from the drawbacks of regioselectivity, reactivity and use of excess arenes.¹⁵ Since its development, a number of modified strategies have been reported by different research groups to address the issue of reactivity and selectivity.^16–21 In recent times, the ligand assisted oxidative C–H alkenylation of arenes without directing substituents has been established as one of the major strategies to overcome the reactivity issue and to elaborate the substrate scope.However, regioselectivity for most of the sterically and electronically unbiased arenes is still not up to the mark. The most recent studies on the non-directed oxidative C–H olefination of arenes were reported independently by Yu and van Gemmeren (Scheme 1). The Yu group employed electron-deficient 2-pyridone as an X-type ligand for the olefination of both electron-rich and electron-poor arenes including heteroarenes as the limiting reagent (Scheme 1a).¹⁸ The pyridone ligand improves the selectivity in a non-directed approach as compared to the directed C–H olefination reaction by enhancing the influence of steric effects. On the other hand, the van Gemmeren group utilizes two complementary ligands N-Ac–Gly–OH and a 6-methylpyridine derivative in a 1 : 1 ratio to accomplish the non-directed olefination reaction of arenes (Scheme 1b).²⁰ Despite the indisputable advances made by these research groups in the area of non-directed oxidative C–H olefination of arenes, the use of superstoichiometric amounts of toxic and waste-generating oxidants (Ag salts) deciphers into a strong call for an environmentally responsive and atom-economic protocol. To address these shortcomings, we recently introduced Pd-photoredox catalysed olefination of non-directed arenes with excellent site selectivity under oxidant free conditions.²¹Open in a separate windowScheme 1Recent approaches to sustainable C–H alkenylation reactions.In 2007, Jutand reported the directing group assisted Pd-electracatalysed ortho-olefination of acetyl protected aniline in a divided cell by utilizing catalytic amounts of benzoquinone as a redox mediator (Scheme 1c).^22a A Rh-catalysed ortho-C–H olefination of benzamide was developed through an electrooxidative pathway by the Ackermann group (Scheme 1d).^22b Simple arenes that bear no directing groups are cheap, easily available and very desirable starting materials. However, the use of such arenes is significantly more challenging for selective functionalisation as transformations often result in the formation of complex product mixtures. With no report of an electrooxidative Pd-catalysed C(sp²)-H alkenylation of simple arenes present, we wish to present such a variant of the Fujiwara–Moritani reaction (Scheme 1e). The developed method proceeds through a non-directed pathway and is controlled by stereoelectronic factors. This protocol does not require additional chemical oxidizing agents and is executed using an operator-friendly undivided cell setup.To start our study, naphthalene was chosen as a challenging substrate because of its ability to form α- and β-products. We examined various reaction conditions for the desired Pd-catalysed electrooxidative C–H alkenylation in a simple undivided cell setup (†) with n-butyl acrylate as the coupling partner. After rigorous optimisation, we found that naphthalene reacts with n-butyl acrylate in dichloroethane (DCE) in the presence of Pd(OAc)₂ (10 mol%), ligand L1 (20 mol%), and the electrolyte tetra-n-butylammonium hexafluorophosphate (TBAPF₆, 0.5 equiv.) while employing a graphite felt anode and a platinum cathode maintaining constant current electrolytic conditions (j = 2.5 mA cm⁻²,