Direct Identification of the Carbon Skeleton of Organic Compounds using Double Quantum Coherence 13C-NMR Spectroscopy. The INADEQUATE Pulse Sequence

NMR spectroscopy plays an important part in the determination of the structures of organic compounds. The parameters of importance here are the chemical shifts of the ¹H and ¹³C nuclei and the spin-spin interactions both between ¹H nuclei and between ¹H and ¹³C nuclei. Couplings between ¹³C nuclei were almost completely neglected until a few years ago, since they were extremely difficult to observe because of the low natural abundance of ¹³C. However, it is these couplings which afford information directly on the carbon-carbon connectivities in the molecule. It is now possible to use a special NMR pulse sequence to make these couplings more readily visible: the result of using this sequence is a ¹³C-NMR spectrum from which the carbon skeleton concerned can be directly read off. Two-dimensional spectra in particular are very easy to evaluate. The pulse sequence involved, which bears the somewhat puzzling name INADEQUATE, produces double-quantum coherences from which the NMR signals of the coupled carbon nuclei can be obtained. In this article the principle of double-quantum coherence is described and a number of examples for the application of the INADEQUATE pulse sequences to problems in synthetic organic chemistry, biosynthesis and natural products chemistry are presented; in addition, the possibility of applying the INADEQUATE method to other nuclei is considered.     

A quantitative method for dual‐pass electrostatic force microscopy phase measurements

Electrostatic force microscopy (EFM) has become a powerful tool for investigating charges on surfaces. The use of phase measurement in EFM is a direct and fast way to detect electrostatic force gradients, but only qualitatively. With the dual‐pass scheme, the phase signal at lifted height is often assumed to exclude any influences from the topography, but it does not. We report the collection of both topography and phase data by EFM on charged, micron‐sized metal wires. In order to quantify the electrostatic force, a cone model and finite element analysis are provided to integrate the force gradient from the phase signal. Copyright © 2007 John Wiley & Sons, Ltd.     

对在建标技术报告中正确贯彻《测量不确定度表示指南》的理解

针对计量人员在贯彻《测量不确定表示指南》,尤其是在撰写“建标技术报告”时遇到的一些体问题,进行了探讨。     

Electroreduction of the antifouling agent TCMTB and its electroanalytical determination in tannery wastewaters

The electrochemical reduction of antifouling agent 2-thiocyanomethylthiobenzothiazole (TCMTB) was investigated by cyclic and pulse differential voltammetry. The irreversible electrode reduction of TCMTB proceeded by ECEC reaction mechanism by two electrons transfer with one irreversible wave. Upon the basis of electrochemical evidence, the electrodic reaction mechanism was suggested to formation of mercaptobenzothiazole (MTB) in solution.Subsequently, a pulse differential method is described for the formation of TCMTB based on this electroreduction. Having been obtained a detection limit of 1.0 × 10⁻⁷ mol L⁻¹ and recovery to 98% to concentration of 2.0 × 10⁻⁶ mol L⁻¹. Therefore, the proposed method in this study is practical, sensitive and accurate for the analysis of TCMTB in tannery wastewater samples.     

The assessment of compliance with legal limits. Part 2. How to consider both type I and type II errors when working in the concentration domain

Considering the uncertainty of measurement when assessing compliance with reference values given in compositional specifications and statutory limits is still a controversial matter. In theory, assessing compliance requires considering both type I (false positive) and type II (false negative) errors. The more the concentration of the analyte in the sample under investigation is close to the allowed concentration limit, the more critical it is to consider both types of errors. This paper describes how this could be done. The matter is discussed in the light of the most recent literature information.     

Ion association in calcium phosphate solutions at 37°C

Ion association in the system Ca(OH)₂–H₃PO₄–KCl–H₂O at 37°C has been studied potentiometrically over a range of pH from 3 to 9. The experimental conditions were optimized for the accurate determination of the association constants for the formation of the ion pairs CaH₂PO ₄ ⁺ , CaHPO₄ and CaPO ₄ ^– which were found to be 27.9±0.1, 591±2, and (1.35±0.02)×10⁶ L-mol^–1, respectively.     

Solid-state,optical-rotation measurements on macromolecules using powder-suspensions

Summary We report techniques for measuring optical rotation of suspended solids, including dissymmetric molecular crystals (e.g. sodium chlorate and bromate), synthetic polymers, polypeptides and biopolymers. Examples include poly(triphenylmethyl methacrylate), poly-S-lactide, poly-L-hydroxyproline, hemocyanine, cellulose acetate andRNA. The results indicate that optical rotation measurements are sensitive to differences in secondary structure and can serve as a probe for structural differences between solid-state and solution.

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Messung der optischen Rotation von Makromolekülen im Festzustand unter Verwendung von Pulversuspensionen

Zusammenfassung Es wird über eine Methode zur Messung der optischen Rotation von Festkörpern in Suspensionen berichtet, wobei dissymetrische Kristalle (z. B. Natriumchlorat und -bromat), synthetische Polymere, Polypeptide und Biopolymere behandelt werden. Die Beispiele umfassen Poly(triphenylmethylmethacrylat), Poly-S-lactid, Poly-L-hydroxyprolin, Hemocyanin, Celluloseacetat undRNA. Die Ergebnisse zeigen, daß die optischen Rotationen bezüglich Unterschiede in der Sekundärstruktur empfindlich sind und daß diese Messungen als ein Maß für Strukturunterschiede zwischen Festzustand und Lösung herangezogen werden können.

     

Novel rhenium(III) complexes with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-trazine: X-ray structures and DFT calculations for [ReCl3(OPPh3)(dppt)] and [ReCl3(PPh3)(dppt)] complexes

The reaction of [ReOCl₃(PPh₃)₂] with 5,6-diphenyl-3-(2-pyridyl)-1,2,4-trazine (dppt) has been examined and [ReCl₃(OPPh₃)(dppt)] has been obtained. The triphenylphosphine oxide can be easily replaced by PPh₃ in the reaction of [ReCl₃(OPPh₃)(dppt)] with an excess of triphenylphosphine. The [ReCl₃(OPPh₃)(dppt)] and [ReCl₃(PPh₃)(dppt)] complexes have been structurally and spectroscopically characterized. Their molecular orbital diagrams have been calculated with the density functional theory (DFT) method, and their electronic spectra have been discussed on the basis of time-dependent DFT calculations. The compound [ReCl₃(OPPh₃)(dppt)] has been studied additionally by magnetic measurement. The magnetic behavior is characteristic of mononuclear complexes with d⁴ low-spin octahedral Re(III) complexes (³T_1g ground state) and arise because of the large spin–orbit coupling (ζ = 2500 cm⁻¹), which gives diamagnetic ground state.     

A novel approach for introducing particulate matter into thermal plasmas: The triple-cathode arc

A coalesced high-intensity dc discharge is maintained between three cathodes and a single anode, stabilized by using resistors on each cathode leg. Jets of plasma gas are produced from either the cathode area or the anode area of the device. Cathode jets are generated by the self-induced pumping at the cathode tips and augmented by central gas injection. Arc voltage-current characteristics show classical convection-stabilized arc behavior. Anode heat transfer rates may be substantially increased by central gas injection toward the anode. Temperature fields in the coalesced, axially symmetric portion of the arc are determined spectrometrically and compared to those of a classical single-cathode free-burning arc.