Evaluation of NMR spectroscopy for the quantitative characterization of urea-formaldehyde resins

A comparative evaluation has been made of both proton and (13)C nuclear magnetic resonance techniques in the quantitative characterization of commercial urea-formaldehyde resins. There is good agreement between data derived from (13)C NMR spectra and from (1)H high-field continuous wave, or low-field (Fourier transform) NMR spectra. Low-field continuous wave proton spectra exhibit inferior resolution and provide inaccurate quantitative data. Combination of (13)C and proton NMR with nitrogen analysis gives a quantitative characterization technique for these resins.     

微乳液的热力学性质 I: 芳烃类的侧链链长对微乳液热力学函数影响

本文研究以非离子型表面活性剂-正辛醇-水-芳烃类所组成的微乳液, 探讨醇从油相转移到界面相时的自由能变化, 以及温度对自由能的影响。计算出熵和焓的变化, 发现在实验范围内, 上述热力学函数的对数值与芳烃侧链的碳原子数(n)呈线性关系。这些关系式对微乳液的构成和稳定性的讨论是重要的, 还对几种芳烃异构体所构成的微乳液的热力学函数进行了实验和讨论。     

抗坏血酸和尿酸在甘氨酸-壳聚糖修饰玻碳电极上的电化学行为及测定

制备了甘氨酸-壳聚糖复合膜修饰玻碳电极(Gly-CTS/GCE),研究了抗坏血酸(AA)和尿酸(UA)在该修饰电极上的电化学行为。结果表明在pH=5.59的磷酸盐缓冲溶液中,AA、UA在Gly-CTS/GCE上均产生灵敏的不可逆氧化峰,其峰电流与浓度在一定范围内呈良好的线性关系。对AA和UA混合溶液平行测定7次,相对标准偏差分别为4.6%、2.9%,表明该电极重现性和稳定性良好。AA、UA在Gly-CTS/GCE电极上的氧化峰峰电位相差340mV,据此可实现对二者的同时检测,并可应用于实际样品测定。     

Recent advances in the semi-pinacol rearrangement of alpha-hydroxy epoxides and related compounds

The semi-pinacol rearrangement is fast becoming an extremely reliable reaction in organic synthesis allowing the rapid construction of relatively complex stereodefined products in high yield. Recent advances in asymmetric synthesis have also enabled enantiopure precursors to partake in the rearrangement showing that extremely high levels of stereospecificity are observed. In this critical review recent advances in the semi-pinacol rearrangement over the past 15 years are examined which demonstrate the extremely high utility of this reaction towards the development of structurally diverse organic building blocks (74 references).     

C70X2(X=H,F, Cl)的稳定性和电子光谱

用INDO方法研究C70H2四种异构体的稳定性, 表明其最稳定异构体为1, 9-C70H2和7, 8-C70H2, 两者能量差为16.3KJ.mol^-^1, 与实验值及ab initio计算值接近; 光谱计算表明, 其特征吸收峰与实验值一致。在此基础上预测C70F2和C70Cl2的稳定性和电子光谱, 表明C70F2四种异构体的稳定性顺序与C70H2一致, 而C70Cl2则以21, 42-异构体最为稳定。二者的电子光谱与C70H2极其相似只是在500nm以上有细微差别。     

氢原子在镍台阶面上吸附、振动和表面扩散行为的理论研究

本文用对势方法研究了氢原子在Ni(510)台阶面上的吸附和振动, 计算结果与实验符合得很好。并考察了氢原子在Ni(997)台阶面上的吸附和扩散, 结果表明,台阶对下台面上扩散的氢原子开成捕获势阱, 对上台面扩散的氢原子形成反射势,这也很好地支持了实验结果。     

Application of NMR to fossil fuels: an assessment of recent advances

Summary Recent applications of high resolution solution- and solid-state NMR techniques to fossil fuels and their derivatives are assessed with emphasis on the quantification of aromatic, aliphatic and heteroatomic groups. ¹³C NMR spectral editing methods that generate C, CH, CH₂ and CH₃ sub-spectra for soluble materials are reasonably quantitative. Two-dimensional methods aid peak assignments in ¹H and ¹³C NMR analysis of distillates. There are few reports on the investigation of heteroatom environments by ¹⁷O, ¹⁴N, ¹⁵N and ³³S NMR, but derivatisation with groups containing a convenient magnetic label (¹H, ¹³C, ¹⁹F and ²⁹Si) has proved successful for estimating acidic OH, NH and SH concentrations in coal-derived liquids and petroleum residues. Solid-state ¹³C NMR analysis of coals and related materials is often only semi-quantitative because not all the carbon is observed using cross-polarisation.

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Anwendung der NMR auf fossile Brennstoffe: Bewertung neuerer Fortschritte

Zusammenfassung Neuere Anwendungen von hochauflösenden Lösungs- und Festkörper-NMR-Techniken auf fossile Brennstoffe und ihre Derivate werden vorgestellt mit der Betonung auf Quantifizierung der aromatischen, aliphatischen und heteroaromatischen Gruppen. Methoden der Editierung von ¹³C-NMR-Spektren, die separate Spektren für C, CH, CH₂ und CH₃-Gruppen liefern, sind bei löslichen Materialien ausreichend quantitativ. Zweidimensionale Methoden helfen bei der Signalzuordnung der ¹H- und ¹³C-NMR-Spektren von Destillaten. Zwar liegen einige Berichte über Untersuchungen von Heteroatomumgebungen mit ¹⁷O-, ¹⁴N-, ¹⁵N- und ³³S-NMR vor, doch hat sich die Derivatisierung mit Gruppen, die einen empfindlicheren Kern als Meßsonde besitzen (¹H, ¹³C, ¹⁰F, ²⁹Si), als erfolgreich zur Abschätzung der Konzentration acider OH-, NH- und SH-Gruppen in aus Kohle entstandenen Flüssigkeiten und in Erdölrückständen erwiesen. Die Analyse von Kohle und ähnlichen Materialien mit Hilfe der Festkörper-¹³C-NMR-Spektroskopie ist oft nur halbquantitativ, da nicht alle Kohlenstoffatome mit der Cross-Polarisations-Technik beobachtet werden.

     

Hydropyrolysis over a platinum catalyst as a preparative technique for the compound‐specific carbon isotope ratio measurement of C27 steroids

Compound‐specific stable carbon isotope analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is an important method for the determination of the ¹³C/¹²C ratios of biomolecules such as steroids, for a wide range of applications. However, steroids in their natural form exhibit poor chromatographic resolution, while derivatisation adds carbon thereby corrupting the stable isotopic composition. Hydropyrolysis with a sulphided molybdenum catalyst has been shown to defunctionalise the steroids, while leaving their carbon skeleton intact, allowing for the accurate measurement of carbon isotope ratios. The presence of double bonds in unsaturated steroids such as cholesterol resulted in significant rearrangement of the products, but replacing the original catalyst system with one of platinum results in higher conversions and far greater selectivity. The improved chromatographic performance of the products should allow GC/C/IRMS to be applied to more structurally complex steroid hormones and their metabolites. Copyright © 2010 John Wiley & Sons, Ltd.     

Helix persistence and breakdown in oligoureas of metaphenylenediamine: apparent diastereotopicity as a spectroscopic marker of helix length in solution

Oligomeric ureas derived from m-phenylenediamine with chain lengths of up to seven urea linkages were made by iterative synthetic pathways. Three families were synthesized: 4 and 20, bearing a terminal chiral sulfinyl group; 24, bearing a terminal rotationally restricted amide group, and 30 bearing a terminal achiral bromophenyl group. The distal end of the oligomers was capped with an N-benzyl group to act as a diastereotopic probe. With a terminal sulfinyl group, the 1H NMR signals arising from the CH2 group of the diastereotopic probe remained anisochronous even when separated from the stereogenic center by up to 24 bonds (in 20c). With a rotationally restricted amide, anisochronicity was no longer apparent beyond 17 bond lengths (in 24c). No anisochronicity was observable with a terminal bromophenyl group. We interpret these results as indicating that the oligoureas of short lengths adopt a defined helical secondary structure in solution, but that in longer oligomers the helicity breaks down and transmission of chirality in these systems is limited to about 24 bond lengths. We propose that "apparent diastereotopicity" (anisochronicity) provides a general empirical method for identifying secondary structure in solution.