Electrothermal atomic absorption spectrometric determination of cadmium in environmental samples with niobium wire preconcentration method

A preconcentration method by adsorption of cadmium on a niobium wire was developed for the environmental waters, followed by electrothermal atomic absorption spectrometry with a tungsten tube atomizer. After the preconcentration, the niobium wire was directly inserted into the tungsten tube atomizer. In the preconcentration (adsorption) process of cadmium, the optimal immersing time was 60?s. The effects of large amounts of concomitants on the preconcentration of cadmium were evaluated. When 10³–10⁴ fold excess of matrix elements existed in aqueous solution at pH 4 and 9, the cadmium response was profoundly affected by the matrix elements. However, the cadmium absorption signal was not significantly influenced at pH 7. Therefore, pH 7 was selected for the application into the real environmental samples. Under the optimal conditions, the detection limit (3S/N) for cadmium by the niobium wire preconcentration method was 7.0?pg?mL^?1 and the relative standard deviation was 6.8%. The method with preconcentration on a niobium wire was applied to the determination of cadmium in water and proved to be sensitive, simple and convenient. Because this preconcentration method can be utilized in the in situ treatment of trace cadmium in environmental water samples, it was unnecessary to carry the water samples to the analytical work place. The technique was shown to be useful for the determination of cadmium in environmental water samples at 0.1–1?µg?L^?1 levels.     

Production of a positron microprobe using a transmission remoderator.

A production method for a positron microprobe using a beta+-decay radioisotope (22Na) source has been investigated. When a magnetically guided positron beam was extracted from the magnetic field, the combination of an extraction coil and a magnetic lens enabled us to focus the positron beam by a factor of 10 and to achieve a high transport efficiency (71%). A 150-nm-thick Ni(100) thin film was mounted at the focal point of the magnetic lens and was used as a remoderator for brightness enhancement in a transmission geometry. The remoderated positrons were accelerated by an electrostatic lens and focused on the target by an objective magnetic lens. As a result, a 4-mm-diameter positron beam could be transformed into a microprobe of 60 microm or less with 4.2% total efficiency. The S parameter profile obtained by a single-line scan of a test specimen coincided well with the defect distribution. This technique for a positron microprobe is available to an accelerator-based high-intensity positron source and allows 3-dimensional vacancy-type defect analysis and a positron source for a transmission positron microscope.     

Stability and vibrational spectra of toroidal isomers of C 240

To study the thermodynamic and mechanical stability of toroidal isomers of C ₂₄₀, we use a semi-empirical tight-binding theory and calculate their electronic structure, cohesive energy and vibrational spectra within the harmonic approximation. From these, we deduce their free energy at temperatures up to 1500K. The results are also compared to the isomer with icosahedral symmetry. Finally, we discuss within this approach, their stability and abundance.     

Multiple Reaction Pathways Operating in the Mechanism of Vinylogous Mannich‐Type Reaction Activated by a Water Molecule

A systematic search for reaction pathways for the vinylogous Mannich‐type reaction was performed by the artificial force induced reaction method. This reaction affords δ‐amino‐γ‐butenolide in one pot by mixing 2‐trimethylsiloxyfuran, imine, and water under solvent‐free conditions. Surprisingly, the search identified as many as five working pathways. Among them, two concertedly produce anti and syn isomers of the product. Another two give an intermediate, which is a regioisomer of the main product. This intermediate can undergo a retro‐Mannich reaction to give a pair of intermediates: an imine and 2‐furanol. The remaining pathway directly generates this intermediate pair. The imine and 2‐furanol easily react with each other to afford the product. Thus, all of these stepwise pathways finally converge to give the main product. The rate‐determining step of all five (two concerted and three stepwise) pathways have a common mechanism: concerted Si? O bond formation through the nucleophilic attack of a water molecule on the silicon atom followed by proton transfer from the water molecule to the imine. Therefore, these five pathways have comparable barriers and compete with each other.     

Phase-transfer reactions catalyzed by polymer-supported crown ethers

Polymer-supported crown ethers were prepared from chloromethylated or ω-bromoalkylated polystyrene resins and hydroxymethylbenzo-18-crown-6 and 15- or 18-membered monoazacrown ethers. Effects of the cavity size of crown ethers, the degree of crosslinking, the degree of ring substitution, particle size, spacer chains, and solvents on the activity of the polymer-supported crown ethers in the reaction of 1-chloro- or 1-bromo-octane with aqueous NaI or KI were investigated and mechanisms of the reaction were discussed in terms of mass transfer, intraparticle diffusion, and intrinsic reactivity.     

Phosphonation of arenes with dialkyl phosphites catalyzed by Mn(II)/Co(II)/O2 redox couple

[reaction: see text]. Arylphosphonates were first synthesized through a catalytic phosphonation of various arenes with dialkyl phosphites under the influence of an Mn(OAc)2/Co(OAc)2/O2 redox couple. For instance, the reaction of benzene with diethyl phosphite in the presence of Mn(OAc)2 (5 mol %) and Co(OAc)2 (1 mol %) under a mixed gas of O2 (0.5 atm) and N2 (0.5 atm) at 45 degrees C led to diethyl phenylphosphonate in 81% selectivity at 62% conversion. This is the first successful phosphonation of benzene with dialkyl phosphites through a catalytic radical process.     

Dynamics of order formation by colloidal adsorption onto a substrate studied with Brownian dynamics

Colloidal adsorption and spontaneous ordering of adsorbed particles on a substrate was simulated using a three-dimensional simulation model for colloidal dispersion system with an adsorptive surface under a specified bulk concentration, where the particle-particle and particle-substrate interactions were modeled on the DLVO theory. The key process for order formation is considered to be the adsorption of a particle that induces the transition from incomplete order to perfect order, and is found to involve a stochastic nature due to an energy barrier which must be overcome for the system to reach ordered state. Also, a model was developed to predict the energy barrier for order formation based on direct observation of the key process. Further, a model to describe the stochastic nature of the process was developed and its quantitative validity was demonstrated. Through the examination of the key process, it is concluded that the mechanism of the order formation is composed of two successive processes and the rate-determining step varies depending on the ionic strength.     

Interaction between peroxide ion and phenol group which are coordinated to the same iron(III) ion

We have prepared several new iron(III) complexes with ligands which contain a phenol group; these are tetradentate [(X-phpy)H, X and H(phpy) represent the substituents on the phenol ring and N,N-bis(2-pyridylmethyl)-N-(2-hydroxybenzyl)amine, respectively] and pentadentate ligands [(R-enph-X)H; R=ethyl(Et) or methyl(Me) derivative and H(Me-enph) denotes N,N-bis(2-pyridylmethyl)-N″-methyl-N″-(2″-hydroxyl-benzylamine)ethylenediamine] and have determined the crystal structures of Fe(phpy)Cl₂, Fe(5-NO₂-phpy)Cl₂, and Fe(Me-enph)ClPF₆, which are of a mononuclear six-coordinate iron(III) complex with coordination of one or two chloride ion(s). These compounds are highly colored (dark violet) due to the coordination of phenol group to an iron(III) atom. When hydrogen peroxide was added to the solution of the iron(III) complex, a color change occurs with bleaching of the violet color, indicating that oxidative degradation of the phenol moiety occurred in the ligand system. The bleaching of the violet color was also observed by the addition of t-butylhydroperoxide. The rate of the disappearance of the violet color is highly dependent on the substituent on the phenol ring; introduction of an electron-withdrawing group in the phenol ring decreases the rate of bleaching, suggesting that disappearance of the violet band should be due to a chemical reaction between the phenol group and a peroxide adduct of the iron(III) species with an η¹-coordination mode and that in this reaction the peroxide adduct acts as an electrophile towards phenol ring. The intramolecular interaction between the phenol moiety and an iron(III)-peroxide adduct may induce activation of the peroxide ion, and this was supported by several facts that the solution containing an iron(III) complex and hydrogen peroxide exhibits high activities for degradation of nucleosides and albumin.     

Hydrogen‐Bonding Catalysis of Tetraalkylammonium Salts in an Aza‐Diels–Alder Reaction

A piperidine‐derived tetraalkylammonium salt with a non‐coordinating counteranion worked as an effective hydrogen‐bonding catalyst in an aza‐Diels–Alder reaction of imines and a Danishefsky diene. The hydrogen‐bonding interaction between the ammonium salt and an imine was observed as part of a ¹H NMR titration study.