Recent developments in polyolefin characterization

The synthesis and characterization of polyolefins continues to be one of the most important areas for academic and industrial research. One consequence of the development of new “tailor‐made” polyolefins is the need for new and improved analytical techniques for the analysis of polyolefins with respect to molar mass and chemical composition distribution. The present article briefly reviews different new and relevant techniques for polyolefin analysis. The analysis of copolymers by combining high‐temperature GPC and FTIR spectroscopy yields information on chemical composition as a function of molar mass. Crystallization analysis fractionation is a powerful new technique for the analysis of short‐chain branching in LLDPE and the analysis of polyolefin blends. Additives in polyolefins can be determined efficiently by pyrolysis‐gas chromatography‐mass spectrometry.     

Recent developments in chemical characterization with MeV ion beams

A multitude of ion-atom interactions are induced with projectiles of E0.1 MeV/nucleon. Analytical techniques derived from these include particle induced X-ray emission (PIXE), charged particle activation analysis (CPAA), prompt nuclear reactions (PNR), and Rutherford backscattering spectrometry (RBS). Among their features are broad elemental coverage (PIXE), subnanogram sensitivity (PIXE, CPAA), isotopic specificity (CPAA, PNR), and depth resolution (RBS, PNR). A limiting requirement with each technique is the need for high intensity ion beams. Novel approaches seek now to obtain analytical information with very small numbers of bombarding ions. Sample integrity is then maintained; moreover, they can be delivered in a microbeam (diameter 5 mm). A phenomenon which under these conditions provides useful analytical information is the particle induced desorption of molecular fragments. Thus, microscopic chemical analysis can be achieved with a small number (<10,000) of heavy fast projectiles and identification of the species desorbed from the sample surface via time-of-flight mass spectrometry. Experimental work with 84 MeV kr ions indicates the following: (a) high desorption yields can be obtained (>50%); (b) mass spectrometry on microspots (diameter of a few m) is feasible; (c) < 10⁶ atoms can be detected. Further capabilities of ion beams for minute, detailed, and comprehensive chemical characterization remain to be explored.     

Recent developments in electrochemical water oxidation

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Solid-state 17O NMR as a probe for structural studies of proteins in biomembranes

We report the first example of 17O NMR spectra from a selectively labeled transmembrane peptide, 17O-[Ala12]-WALP23, as a lyophilized powder and incorporated in hydrated phospholipid vesicles. It is shown that at high magnetic field it is feasible to apply 17O NMR to the study of membrane-incorporated peptides. Furthermore, we were able to estimate distances within the selectively labeled WALP peptide, which represents a consensus transmembrane protein sequence. This work opens up new applications of 17O solid-state NMR on biological systems.     

17O NMR spectra of alpha-diesters

17O NMR spectra of title compounds were measured at natural abundance in acetonitrile solutions. Intercarbonyl dihedral angles have been estimated by molecular mechanics, which show invariance except in one case. Because of this invariance, contrary to other alpha-dicarbonyl compounds, a correlation between chemical shifts and dihedral intercarbonyl angles could not be developed. Spectroscopic and computational results allowed us to evaluate other conformational features.     

17O solid-state NMR spectroscopic studies of the involvement of water vapor in molecular sieve formation by dry-gel conversion

Dry-gel conversion is a relatively new approach for molecular sieve synthesis. This method potentially has several advantages over the traditional hydrothermal synthesis and can be used to prepare molecular sieves with certain unique properties. The technique involves treating the predried reactive gel powder with water vapor at elevated temperature and pressure. The role of water vapor in this apparent solid transformation is, however, not clear. In this work, we directly monitored the involvement of 17O-enriched water vapor in crystallization of AlPO4-11 (an aluminophosphate-based molecular sieve) by 17O solid-state NMR spectroscopy. In addition to 17O magic-angle spinning technique, several dipolar-coupling based double-resonance methods including 17O[27Al], 17O[31P] rotational-echo double-resonance, 17O --> 31P and 1H --> 17O cross polarization techniques were used for spectral editing to select different 17O species. The results show that water from the vapor phase slowly exchanges with water molecules strongly bound to the AlPO intermediates first. Then 17O atoms are gradually incorporated in both P-O-H and P-O-Al units in the layered intermediate. There are three different P sites in AlPO4-11. Interestingly, during the transformation from the layered intermediate to AlPO4-11, the 17O atoms prefer to bond to the P2 and P3, but not to P1. The absence of 17O atoms in the first coordination sphere of P1 site suggests that some building units such as joint four- and six-membered rings involving hydrogen bonding with structure-directing agents are common in both layered intermediate and AlPO4-11 and they are not affected by the transformation from the layered phase to the AlPO4-11 framework.     

Recent developments in capillary zone electrophoresis of proteins.

This review article with 125 references describes recent developments in capillary zone electrophoresis of proteins. It encompasses approximately the last two years, from the previous review (V. Dolník, Electrophoresis 1997, 18, 2353-2361) through Spring 1999. Topics covered include modeling of the electrophoretic properties of proteins, sample preconcentration and derivatization, wall coatings, improving selectivity, special detection techniques, and applications.     

Recent developments in high-performance capillary electrophoresis of cereal proteins

Cereal proteins play important nutritional and functional roles in human foods and are also important components of animal feeds. As such, cereals are a major economic factor around the world. Because of their importance, cereal proteins have been widely studied. A new emerging technique for studying cereal proteins is high-performance capillary electrophoresis (HPCE). This review focuses mainly on new methods and applications of HPCE to cereal proteins that have been reported in the last three years.     

17O NMR data of phenoxyethyl derivatives

¹⁷O NMR data for 52 phenoxyethyl derivation have been measured and assigned. Copyright © 2003 John Wiley & Sons, Ltd.     

电解水金属单原子催化剂的研究进展

氢能因其能量密度高、清洁无污染等特点,作为替代化石燃料的能源载体得到了广泛的研究.如何清洁高效地制备氢气受到了大量研究者的关注.当前,以化石能源的热反应所得副产氢气是主要来源.然而,采用该类方法不仅不能摆脱化石能源的使用以及温室气体的排放,还会造成生产氢气的纯度不高,碳氧化物杂质浓度过高的问题,严重影响氢气的后续使用.采用可再生能源(太阳能、风能等)所产富余电,进行电解水制氢,产生的氢气不含碳氧化合物杂质,纯度很高,可以真正实现碳的零排放,被认为是未来氢气来源的重要方式.目前,电解水制氢在制氢市场的所占份额较小,而造成这样局面的主要因素是该过程中的高能耗问题.为了降低能耗,开发高效催化剂加速两个电极上的电解反应的动力学尤为重要.近年,金属单原子催化剂(SACs)因其独特的结构,在很多研究中被用作电解水催化剂,进而开发出大量高性能的金属单原子电解水催化剂.本文综述了近年SACs在电解水催化方面的应用.首先,针对电解水反应本身,总结了阴阳极两侧的电极反应机制以及影响电极催化性能的关键吸附中间物种;然后,根据载体的不同,即合金、碳以及其它化合物将SACs分为三类,总结了相关电解水催化研究现状,并且针对不同类型SACs目前的发展情况,提出了它们各自存在的问题.其次,进一步总结了影响SACs电解水催化活性的因素,提出了四种决定SACs催化性能的影响因子,分别为金属原子的固有元素性质、配位环境、几何结构和负载量;同时讨论了这四类影响因素对SACs催化活性的影响机制,总结了调控各类影响因素的方法,为SACs的设计提出了一些建议.最后,展望了SACs在电解水催化中的应用,探讨了SACs在催化剂设计及催化机制研究方面的问题,提出了SACs在电解水催化中的未来发展方向.     

17O NMR: 2J(17O1H) coupling constants by line shape analysis

Two bond spin-spin coupling constants ²J(¹⁷O¹H) are determined for the oxygen in ether, aldehyde, acid, ester and amide groups by line shape analysis of the corresponding ¹⁷O NMR spectra.     

17O NMR studies of substituted 1,3,4-oxadiazoles

Three series of substituted 1,3,4-oxadiazoles were studied by (17)O NMR spectroscopy. Chemical shifts values were correlated with empirical Hammett parameters as well as calculated bond lengths and chemical shielding values.     

17O NMR investigation of phosphite hydrolysis mechanisms

The use of solution 17O NMR spectroscopy in verifying the mechanism of trialkyl phosphite hydrolysis is presented. Trimethyl phosphite was reacted with 17O-labeled H2O at different temperatures and two reactant concentrations, with the reaction being monitored by 17O NMR. Kinetic details elucidated from the NMR spectra are also discussed.     

17O NMR Investigation of cyclic aromatic ethers

A linear relationship between the C-O-C angle and the molecular dihedral angle in a series of phenoxathi-ins and azaphenoxathiins is reported. ¹⁷O nmr spectroscopic data (natural abundance in acetonitrile at 75°C) were obtained on eight cyclic aromatic ethers 1-8 , including phenoxathiins, and two model compounds, acyclic aromatic ethers 9 and 10. The chemical shifts of the cyclic aromatic ethers were very sensitive to structural variations and were dependent upon electonic and conformational effects; however, no quantitative relationship between ¹⁷O chemical shift and geometric parameters was found.     

含氧催化剂的17O固体核磁共振谱学研究

含氧催化剂在工业催化等多个领域有重要应用.氧离子半径很大,而且往往出现在材料的关键位点,所以一般认为氧与吸附和催化过程密切相关.17O是氧的唯一有核磁共振响应的稳定同位素,其化学范围极宽(>1000 ppm),能灵敏反映结构信息;由于是四极核(I>1/2),其四极耦合作用也能用于结构研究.因此,17O固体核磁共振谱学应是一种能提供丰富催化剂结构信息的理想表征手段.然而,目前17O固体核磁共振研究催化剂并非常规手段,这主要是因为17O的天然丰度很低,同位素标记较为昂贵和困难,其较低的旋磁比和较大的四极耦合作用导致谱线加宽,难以获得高质量的谱图并加以解析.随着高磁场和高速魔角旋转等技术的发展,17O固体核磁共振谱学可以用于一系列简单氧化物和沸石等催化剂的结构研究.近年来,随着双旋转(DOR)、动态角旋转(DAS)、多量子魔角旋转(MQMAS)以及卫星跃迁魔角旋转(STMAS)等新技术的发展,能够消除二阶四极耦合作用带来的谱线展宽,显著提升谱图分辨率.而诸如交叉极化(CP)和旋转回波双共振(REDOR)技术,已经能用于探索氧与其它原子核空间相关方面的信息,成为研究催化剂相关作用的基础.本文综述了氧化物及相关催化剂17O固体核磁共振谱学研究的新进展.17O核磁共振谱学用于简单氧化物催化剂的结构研究,已经能够区分催化剂结构中不同晶相以及不同结晶学位点的氧物种,而1H→17O双共振实验也能用于选择表面羟基物种.对纳米氧化物结构的近期研究表明,17O核磁共振能将纳米氧化铈材料表面第1、2、3层、表面羟基、与氧空位靠近的氧物种与“体相”氧物种区分开来;此外借助17O-水和纳米氧化物作用,实现表面选择标记,为进一步探索催化剂结构和催化机理提供了新的可能.对于复合氧化物和负载催化剂,17O核磁共振谱学能够有效研究与催化性能最为相关的界面结构.在重要的氧化物催化材料沸石的研究中,17O核磁共振也发挥了巨大作用.借助高分辨率17O核磁共振方法,能够区分沸石中Si-O-Si和Si-O-Al物种,在一部分沸石中还能将不同结晶学位置的T-O-T’物种区分开来,并观测到天然沸石中违反Lowenstein规则,出现Al-O-Al物种的情况.借助双共振实验能够对与催化活性最为相关的B酸位Si-O(H)-Al结构和酸性进行研究,这一方法与探针分子相结合,已经能够对沸石和小分子的相互作用进行研究,提供吸附过程的重要信息.包括杂多酸和层状双氢氧化物在内的重要含氧催化材料也能够借助17O固体核磁共振进行局域结构和相互作用的研究.随着表面选择标记和动态核极化等选择表面研究的17O核磁共振技术的发展,我们能实现更为高效的表面结构的17O核磁共振观测,这一谱学方法将提供更多有关含氧催化剂和外来物种相互作用的信息,为研究氧化物催化剂及其催化应用提供新的策略.     

A 17O NMR study of the protonation of urea

¹⁷O NMR has been applied to the study of urea over a wide pH range, in fluorosulfonic acid and in magic acid (HFSO₃SbF₅). A 96 ppm diamagnetic chemical shift was observed on modifying the medium from an aqueous solution at pH 3 to a fluorosulfonic acid solvent. We attributed this shift to a protonation reaction at the urea oxygen having a pK_a of 0.1. A 36 ppm paramagnetic shift was observed in magic acid relative to the resonance position in fluorosulfonic acid, which is consistent with the formation of a diprotonated form of urea. Values of transverse and longitudinal relaxation rates were found to be equal as a function of pH and to double on lowering the pH from 2 to ?1.     

Recent developments in capillary electrophoresis-mass spectrometry of proteins and peptides.

Many researchers have invested considerable efforts toward improving capillary electrophoresis (CE)-mass spectrometry (MS) systems so they can be applied better to standard analyses. This review highlights the developments in CE-MS of proteins and peptides over the last five years. It includes the developments in interfaces, sample-enrichment techniques, microfabricated devices, and some applications, largely in capillary zone electrophoresis (CZE), capillary isoelectric focusing (CIEF) and capillary isotachophoresis formats.     

固体核磁共振研究分子筛的新进展

分子筛由于具有独特的孔道及可调控酸碱性等特征,被作为离子交换剂、吸附剂及催化剂而广泛应用于石油化工的各种催化过程中。固体核磁共振是研究分子筛结构、酸性及主客体相互作用的强有力谱学手段之一。简单概述了固体核磁共振研究分子筛的最近进展。