磁场作用下乙醇分子间氢键的变化初探

氢键的存在十分广泛,许多重要物质如水、醇、羧酸、碳水化合物.蛋白质、结晶水合物、氨基酸等都存在氢键。在研究这些物质的结构与性能关系中,氢键起重要的作用。在人类和动植物的生理生化过程中,氢键也起着十分重要的作用。我们通过研究在外磁场作用下,无水乙醇的′HNMR 谱,以及乙醇在磁场中体积的变化情况,发现磁场能影响乙醇分子间氢键的形成η率,改变分子间的缔合程度,从而影响到在磁场条件下有乙醇参加的化学反应。1 实验部分磁铁用复旦大学磁天平的电磁铁和永久磁铁,磁场强度用C7—3型交直流霍耳效应     

大学教材中氢键定义的探讨

氢键是分子内或分子间存在的一种相互吸引作用,对物质的性质有很大影响,仍然是生物、化学、材料等学科研究的热门领域之一。2011年,IUPAC对氢键已给出了最新的定义。文章在回顾了氢键定义研究历史的基础上,对比分析现代大学教材中氢键的定义,说明现代大学教材中氢键的定义已不能准确定义氢键,建议使用2011年IUPAC给出的最新氢键定义。     

有机颜料分子结构氢键特性及应用

有机颜料分子结构中存在氢键,包括分子内氢键与分子间氢键。氢键在很大程度上影响颜料的物理、化学性能,如:熔点、溶解度、耐热稳定性、耐光与耐气候牢度、耐溶剂性能等。本文较系统地讨论了不同颜料形成氢键的条件,氢键对有机颜料应用性能的影响。     

有机颜料分子结构氢键特性及应用(续)

有机颜料分子结构中存在氢键,包括分子内氢键与分子间氢键。氢键在很大程度上影响颜料的物理、化学性能,如:熔点、溶解度、耐热稳定性、耐光与耐气候牢度、耐溶剂性能等。本文较系统地讨论了不同颜料形成氢键的条件,氢键对有机颜料应用性能的影响。     

褐煤热转化过程氢键分布特征及调控手段研究进展

褐煤氧含量高且具备良好热反应性,是制备酚类、腐植酸等高附加值含氧化学品的重要原料。褐煤中的杂原子尤其是丰富的含氧官能团(酚羟基、羧基等)导致了大量氢键的存在,褐煤热转化过程(热解、干燥和直接液化等)中常见氢键包括煤中氢键和褐煤-溶剂间氢键等。煤中氢键和褐煤-溶剂间氢键在构型和强度上存在明显区别,目前可通过体积溶胀度、红外光谱等观测手段以及量子化学计算进行探究。煤中氢键和褐煤-溶剂间氢键广泛存在且明显作用于褐煤热解和直接液化等热转化过程。煤中氢键是维持褐煤大分子网络结构稳定的重要因素,能在热解过程中诱导酚羟基和羧基脱水,进而促进褐煤热转化过程中的低温交联反应,不利于焦油等轻质产物的生成。褐煤-溶剂间氢键是褐煤与溶剂相互作用的重要形式,其强度显著影响褐煤在热转化过程中的物理化学反应如萃取、溶胀以及脱氧等。充分认识褐煤相关氢键的存在形态和影响因素,并以此为基础进行氢键调控,对于褐煤清洁高效转化具有重要意义。现有的氢键调控主要目的在于破坏煤中氢键,从而在一定程度上抑制热转化过程中的交联反应。氢键调控方法主要包括低温预热以及使用吡啶、离子液体等强氢键受体进行溶剂预处理等。当前针对褐煤-溶剂间氢键的研究主要停留在定性层面,缺乏褐煤热转化过程中的原位观测以及定量分析。     

水在聚乙烯醇/水体系中的状态及氢键作用

水在聚乙烯醇(PVA)中的状态直接影响PVA的热塑加工.采用DSC和Raman光谱研究了水含量对其在PVA中的状态及氢键作用的影响.结果表明:水在PVA中以3种状态存在,随水含量增加,非冻结合水比例减小,可冻结合水和自由水比例增加.通过高斯分峰可将Raman光谱水的羟基伸缩振动峰分为5种羟基振动峰叠加,分别代表多氢键结合水分子的羟基对称与反对称伸缩振动,双氢键结合水分子羟基伸缩振动,单氢键结合水分子羟基伸缩振动,无氢键或弱氢键相互作用水分子的羟基伸缩振动.水含量增加,单氢键结合与多氢键结合水分子含量增加,而双氢键结合与无氢键结合水分子含量减少.     

胶束在中药制剂中的应用研究

两亲聚合物在选择性溶剂(对一片段为良溶剂同时对另一片段为劣溶剂)中,在分子间的氢键、静电相互作用和范德华力等推动下自发构筑形成聚合物胶束,溶解性差的片段形成胶束的核,溶解性好的片段形成溶剂化壳层。开始大量形成胶束时的聚合物浓度为临界胶束浓度(CMC)。聚合物胶束的形状以球形最为常见,此外还有棒状、囊状、片状、管状、星状等聚合物胶束的粒径通常在1~200 nm之间时,属于物质由宏观世界向微观世界的过渡区域。这种特殊结构可导致四种效应:小尺寸效应、表面与界面效应、量子尺寸效应和宏观量子隧道效应,并表现出传统固体物质所不具备的许多特殊性质。     

超临界水的特性及应用

介绍了水在高温高压下的热力学性质、氢键、离子积、扩散系数和粘度等在超临界区域的特殊性，以及超临界水溶液在介电常数、偏摩尔体积、溶解性和极性等方面的特殊性质，并阐述了超临界水在化学反应和废物处理中的特殊应用。     

苯-环己烷-环己烯分离效果的研究

针对苯-环己烷-环己烯分离效果中萃取剂与物质间的微观机制和实际生产中宏观条件上的调整进行了阐述,论述结果表明,萃取剂与物质间的相互作用在微观上主要由范德华力、氢键作用来影响,提出了在宏观上的调整方法。此研究对于其他企业提高苯-环己烷-环己烯的分离效果有一定的借鉴价值。     

关于氢及其化学键的教学思考

氢电子结构极为特殊,可以与其它元素形成离子键、共价键、氢键、氢桥键等诸多形式化学键,相关知识很分散。通过在教学过程中系统讲授,学生较好的掌握了相关知识,教学效果明显改善。     

Effect of fluorine chemistry in the remote plasma enhanced chemical vapor deposition of silicon films from Si2H6-SiF4-H2

SiF₄ was added into Si₂H₆-H₂ to deposit polycrystalline silicon films at low temperatures around 400°C in a remote plasma enhanced chemical vapor deposition reactor. It was found out that the fluorine chemistry obtained from SiF₄ addition had an influence on the chemical composition, crystallinity, and silicon dangling bond density of the film. The fluorine chemistry reduced the amount of hydrogen and oxygen incorporated into the film and also suppressed the formation of powders in the gas phase, which helped the crystallization at low temperatures. Effect of SiF₄ concentration as well as the deposition temperature was also significant.     

Dynamics of water interacting with interfaces, molecules, and ions

Water is a critical component of many chemical processes, in fields as diverse as biology and geology. Water in chemical, biological, and other systems frequently occurs in very crowded situations: the confined water must interact with a variety of interfaces and molecular groups, often on a characteristic length scale of nanometers. Water's behavior in diverse environments is an important contributor to the functioning of chemical systems. In biology, water is found in cells, where it hydrates membranes and large biomolecules. In geology, interfacial water molecules can control ion adsorption and mineral dissolution. Embedded water molecules can change the structure of zeolites. In chemistry, water is an important polar solvent that is often in contact with interfaces, for example, in ion-exchange resin systems. Water is a very small molecule; its unusual properties for its size are attributable to the formation of extended hydrogen bond networks. A water molecule is similar in mass and volume to methane, but methane is a gas at room temperature, with melting and boiling points of 91 and 112 K, respectively. This is in contrast to water, with melting and boiling points of 273 and 373 K, respectively. The difference is that water forms up to four hydrogen bonds with approximately tetrahedral geometry. Water's hydrogen bond network is not static. Hydrogen bonds are constantly forming and breaking. In bulk water, the time scale for hydrogen bond randomization through concerted formation and dissociation of hydrogen bonds is approximately 2 ps. Water's rapid hydrogen bond rearrangement makes possible many of the processes that occur in water, such as protein folding and ion solvation. However, many processes involving water do not take place in pure bulk water, and water's hydrogen bond structural dynamics can be substantially influenced by the presence of, for example, interfaces, ions, and large molecules. In this Account, spectroscopic studies that have been used to explore the details of these influences are discussed. Because rearrangements of water molecules occur so quickly, ultrafast infrared experiments that probe water's hydroxyl stretching mode are useful in providing direct information about water dynamics on the appropriate time scales. Infrared polarization-selective pump-probe experiments and two-dimensional infrared (2D IR) vibrational echo experiments have been used to study the hydrogen bond dynamics of water. Water orientational relaxation, which requires hydrogen bond rearrangements, has been studied at spherical interfaces of ionic reverse micelles and compared with planar interfaces of lamellar structures composed of the same surfactants. Water orientational relaxation slows considerably at interfaces. It is found that the geometry of the interface is less important than the presence of the interface. The influence of ions is shown to slow hydrogen bond rearrangements. However, comparing an ionic interface to a neutral interface demonstrates that the chemical nature of the interface is less important than the presence of the interface. Finally, it is found that the dynamics of water at an organic interface is very similar to water molecules interacting with a large polyether.     

双乙酸钠的晶体结构

在研究成功双乙酸钠零污染合成工艺的基础上 ,培养成功了晶型完美的单晶并应用四圆衍射仪对其晶体结构进行了测定 ,结果表明 :在双乙酸钠晶体中不存在相对独立的乙酸和乙酸钠分子 ;双乙酸钠分子内的羰基和羟基氢原子可形成氢键 (O—H…O) ,其H原子位于两个O原子连线的正中间 ,属于结构化学中极罕见的“对称氢键”。文中对双乙酸钠的结构和性能的关系以及“对称氢键”亦进行了探讨     

日本加快投资中国化工业扫描

简述了日本41家企业在我国化学工业领域的投资情况。     

Changes in hydrogen utilization with temperature during direct coal liquefaction

A method for quantitative measurement of the hydrogen utilized in different modes of reaction has been applied to hydroliquefaction reactions at temperatures ranging from 375 to 450 °C. The analytical approach is capable of differentiating hydrogen utilized in hydrogenation reactions from that used in bond scission chemistry (hydrogenolysis). The hydrogenolysis reactions result in breakdown of the coal matrix, formation of light hydrocarbon gas and elimination of organic heteroatoms. The results indicate that in this small continuous reactor, operated at 13.8 MPa (2000 psig) H₂, little net chemical activity of hydrogen occurs at 375 °C. However, at 400 °C, the slurry has been hydrogenated significantly with little net hydrogen incorporation. At 450 °C comparable amounts of hydrogen are consumed in gas generation, heteroatom removal, hydrogenation and matrix breakdown, with large net hydrogen incorporation. These results indicate that at temperatures below the thermolysis threshold of 400 °C, significant internal hydrogen redistribution occurs in the slurry. At higher temperatures, a more conventional hydroliquefaction chemistry involving significant bond cleavage and aromatization is indicated. This approach to analysis of hydrogen utilization requires integration of a variety of analytical data. The uncertainties in these data and their impact on the resultant utilization profile are discussed.     

Effect of surface chemistry on electrochemical storage of hydrogen in porous carbon materials

Porous carbon materials, with different porosities and surface chemistry have been prepared and characterized to obtain a better understanding of the mechanism of the electrochemical storage of hydrogen. The hydrogen storage capacity depends, not only on the porosity of the material, but also on the surface chemistry, which is a critical factor. The results show that the higher the amount of surface oxygen groups, the lower is the hydrogen uptake. Measurement of the number of active carbon sites shows the important role of the unsaturated carbon atoms in the process. In situ Raman spectroscopy has been used in order to further explore the structural changes in the carbon material during the charge-discharge processes. This technique has allowed us to observe the formation of the C(sp²)H bonds during the cathodic process and its reversibility during the oxidation step.     

氢键对有机取代反应和加成反应的影响

取代反应和加成反应是有机化学中最基础、最重要、也是研究最多的反应。影响取代反应和加成反应的因素很多,文章从氢键的形成、类型、键能大小出发,分析讨论了其对饱和碳原子上亲核取代反应、苯环上C-F键的亲核取代反应、亲电加成反应及羰基化合物亲核加成反应的影响,所得结论对有机化学理论教学和有机合成具有一定的指导意义。     

Influence of graphite surface properties on the first electrochemical lithium intercalation

The electrochemical insertion of lithium ions into graphite materials having different surface chemistry and defect concentration was studied during the first cycle in half-cell containing 1 M LiPF₆ in an electrolytic solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The graphite surface properties were varied by thermal treatments in either hydrogen, oxygen, or nitrogen oxide or chemical treatment in boiling nitric acid. The influence of the surface modifications on the course of the first electrolyte reduction was investigated. The surface group chemistry was analyzed by temperature-programmed desorption coupled with mass spectrometry. The surface defect concentration was determined in terms of the active surface area (ASA) measured by oxygen chemisorption and a subsequent temperature-programmed desorption. The experimental results showed that the ASA parameter governs the exfoliation tendency of the graphite negative electrode material with the existence of a critical value below which the graphite systematically exfoliates. The specific charge loss during the first electrochemical insertion of lithium and the exfoliation behavior of the graphite negative electrode material are not influenced by the type and amount of oxygen surface groups. But hydrogen present on the graphite surface increased the graphite exfoliation tendency even for graphite materials with an ASA above the critical value.     

Quantum Chemistry Derived Criteria for Impact Sensitivity

Energetic materials are a special and important kind of substance. Impact sensitivity, which refers to the vulnerability to explosion under external stimuli, measures the safety and reliability of an energetic material and is a critical property. Various efforts have been made to rationalize the impact sensitivity of different types of energetic materials. Since a chemical explosion is a chemical reaction dominated phenomenon, a comprehensive understanding of such explosive processes requires detailed information of chemical bonding and molecular interaction. Quantum chemistry provides a modern theory of chemical bonding and computational quantum chemistry is a powerful tool to investigate chemical phenomena. Even at the very beginning of computational quantum chemistry, researchers in the field of energetic materials have begun to apply quantum chemistry to explosive properties. In this paper we review the quantum chemistry studies on impact sensitivity and examine various quantum chemistry derived parameters used to rationalize the impact sensitivity ordering of various energetic materials.     

State of hydrogen electrochemically stored using nanoporous carbons as negative electrode materials in an aqueous medium

Nanoporous carbons were used as negative electrode material in aqueous KOH medium to store hydrogen by electrodecomposition of water at atmospheric pressure. The storage capacity by this process is approximately one order of magnitude higher than in the gas phase at ambient conditions. By considering the particularities of the electrochemical characteristics, this paper gives information on the mechanism and on the kind of bond between hydrogen and the carbon host. For most experiments, a self-standing porous carbon cloth electrode has been used in order to avoid any side effect which could be due to additives. After galvanostatic hydrogen charging, the carbon material was analyzed by galvanostatic discharge and temperature-programmed desorption in order to determine the nature of the carbon-hydrogen bond and the amount of hydrogen sorbed. The activation energy for hydrogen desorption was estimated to be 110 kJ/mol, that confirms a weak chemical character of the hydrogen-carbon bond. Although the bond is stronger than in the case of physisorption, the fraction of hydrogen irreversibly trapped is low compared to the reversible fraction. Finally, we show that the reversible capacity can be significantly enhanced by increasing the temperature to 60 °C during the electrochemical reduction of water. The well-defined plateau during the oxidation step demonstrates high potentialities of this technique for electrochemical energy storage in nanoporous carbons using an aqueous medium.