Angular momentum conservation in dipolar energy transfer

Conservation of angular momentum is a familiar tenet in science but has seldom been invoked to understand (or predict) chemical processes. We have developed a general formalism based on Wigner's original ideas concerning angular momentum conservation to interpret the photo-induced reactivity of two molecular donor-acceptor assemblies with physical properties synthetically tailored to facilitate intramolecular energy transfer. Steady-state and time-resolved spectroscopic data establishing excited-state energy transfer from a rhenium(I)-based charge-transfer state to a chromium(III) acceptor can be fully accounted for by F?rster theory, whereas the corresponding cobalt(III) adduct does not undergo an analogous reaction despite having a larger cross-section for dipolar coupling. Because this pronounced difference in reactivity is easily explained within the context of the angular momentum conservation model, this relatively simple construct may provide a means for systematizing a broad range of chemical reactions.     

Turning down the heat: design and mechanism in solid-state synthesis

Solid-state compounds have historically been prepared through high-temperature solid-solid reactions. New mechanistic understanding of these reactions suggests possible routes to metastable compositions and structures as well as to thermodynamically stable, low-temperature phases that decompose at higher temperatures. Intermediate-temperature synthetic techniques, including flux and hydrothermal methods, as well as low-temperature intercalation and coordination reactions, have recently been developed and have been used to prepare unprecedented materials with interesting electronic, optical, and catalytic properties. The trend in modern solid-state synthesis resembles increasingly the approach used in small-molecule chemistry, in the sense that attention to reaction mechanism and the use of molecular building blocks result in an ability to prepare new materials of designed structure.     

Preparation of organometallic compounds from highly reactive metal powders

The formation of novel organometallic compounds by the reaction of organic substrates with finely divided metal powders represents a powerful tool for the synthetic chemist. The direct reaction with a zerovalent metal is the only viable method of synthesis for many of these compounds. Accordingly, chemists have been actively developing new methods for increasing the reactivity of metal powders toward organic substrates. Three principal approaches have been developed in recent years: the metal vaporization method, sonochemistry, and the preparation of finely divided metal powders by the reduction of metal salts. A number of new methods of synthesis have resulted from these studies.     

Mass spectrometry: analytical capabilities and potentials

The mass range of mass spectrometers has been extended by almost an order of magnitude in the past decade, ionization procedures have been introduced which allow ionic, nonvolatile compounds to be examined, and new capabilities have been achieved through the successful integration of separation and analysis techniques. In combination with other techniques, mass spectrometry has been used in biological and environmental research to characterize constituents of mixtures, including those present in trace amounts; in metabolic profiling, where high throughput and large dynamic range are important; and in protein structure determinations. Measurements of stable isotope abundances by mass spectrometry have been used in enzymology, studies of photosynthesis, and carbon dating. Outside the area of chemical analysis, mass spectrometry has been used to study gas-phase acidities and basicities and to study organic reaction mechanisms in the gas phase. Trends in mass spectrometry include multidimensional experiments, use of ionization methods, direct analysis without extensive sample preparation, and the development of advanced instrumentation including an ion trap and an inductively coupled plasma mass spectrometer. It is likely that mass spectrometry will come to be much more widely used and that data will increasingly be other than conventional mass spectra.     

Ambient-temperature isolation of a compound with a boron-boron triple bond

Homoatomic triple bonds between main-group elements have been restricted to alkynes, dinitrogen, and a handful of reactive compounds featuring trans-bent heavier elements of groups 13 and 14. Previous attempts to prepare a compound with a boron-boron triple bond that is stable at ambient temperature have been unsuccessful, despite numerous computational studies predicting their viability. We found that reduction of a bis(N-heterocyclic carbene)-stabilized tetrabromodiborane with either two or four equivalents of sodium naphthalenide, a one-electron reducing agent, yields isolable diborene and diboryne compounds. Crystallographic and spectroscopic characterization confirm that the latter is a halide-free linear system containing a boron-boron triple bond.     

The hydrogen atom and its reactions in solution

Hydrogen atoms have been generated in solution by photolysis of thiols in solutions of organic compounds, and the relative rate constants, k(H), have been measured for the reaction H* + QH --> H(2) + Q*, where QH is any organic compound which contains hydrogen. This represents the first kinetic study of the hydrogen atom in which it is generated in solution by a technique not involving ionizing radiation. The relative values of k(H) are in agreement with the values from radiolysis for most of the substances studied; however, for some compounds significantly different results have been obtained.     

Low-temperature synthesis of zintl compounds with a single-source molecular precursor

Thermolysis of the heterobimetallic phosphinidene complex [Sb(PCy)3]2- Li6.6HNMe2 (Cy = C6H11) at 303 to 313 kelvin gives Zintl compounds containing (Sb7)3- anions. The complex thus constitutes a stable molecular single-source precursor to Zintl compounds and provides a potential low-temperature route to photoactive alkali metal antimonates. The new chemical reaction involved, which is driven thermodynamically by the formation of P-P bonds, has implications in the low-temperature synthesis of other technologically important materials (such as gallium arsenide).     

邻-[(5(6)-取代苯并咪唑)-2-基]-苯氧乙酸乙酯的合成与表征

通过2－（2‘-羟苯基）－5（6）－取代苯并咪唑和溴乙酸乙酯反应，合成了7个标题化合物，通过元素分析，IR，^1HNMR及MS，并确证了它们的结构。     

SSR分子标记方法鉴定杂交向日葵品种纯度研究

通过对向日葵总DNA提取方法的比较、PCR扩增反应体系和程序的筛选、电泳检测方法的优化,以15个杂交组合为材料,从165对引物中筛选出扩增带型稳定、多态性丰富的8对核心引物。最终建立了一套适用于杂交向日葵品种纯度鉴定的技术体系。     

"Frozen" transition States: pentavalent carbon et Al

Organic ligands have been designed for the stabilization of specific geometries of compounds of nonmetallic elements. These ligands have made possible the isolation, or direct observation, of large numbers of trigonal bipyramidal organo-nonmetallic species. Many of these species are analogs of transition states for nucleophilic displacement reactions and have been stabilized by the ligands to such a degree that they have become ground-state energy minima. Ideas derived from research on these species have been applied to carbon species to generate a molecule that is an analog of the transition state for the associative nucleophilic displacement reaction. The molecule is a pentavalent carbon species that has been observed by nuclear magnetic resonance spectroscopy.     

A whiff of oxygen before the great oxidation event?

High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 +/- 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O2 in the environment more than 50 million years before the start of the Great Oxidation Event.     

Iron catalysts for selective anti-Markovnikov alkene hydrosilylation using tertiary silanes

Alkene hydrosilylation, the addition of a silicon hydride (Si-H) across a carbon-carbon double bond, is one of the largest-scale industrial applications of homogeneous catalysis and is used in the commercial production of numerous consumer goods. For decades, precious metals, principally compounds of platinum and rhodium, have been used as catalysts for this reaction class. Despite their widespread application, limitations such as high and volatile catalyst costs and competing side reactions have persisted. Here, we report that well-characterized molecular iron coordination compounds promote the selective anti-Markovnikov addition of sterically hindered, tertiary silanes to alkenes under mild conditions. These Earth-abundant base-metal catalysts, coordinated by optimized bis(imino)pyridine ligands, show promise for industrial application.     

花生香味物质的研究进展

花生是我国重要的油料作物,花生仁和其加工品可以产生独特的香气,其香味品质是决定市场经济价值的最重要指标之一。花生仁通过不同的加工方式可以产生不同的香味物质,而花生香味由多种挥发性物质共同组成,包括吡嗪类、吡咯类、呋喃类、吡啶类、醛类、酮类、酸类、烃类、芳香类等化合物,且不同挥发性物质具有不同的香味。花生的香味物质主要来源途径有美拉德反应、脂肪氧化和热降解反应、氨基酸和糖类降解反应 3 种途径,目前花生香味物质的生物合成途径研究还较少。随着科技发展,越来越多的挥发性物质被鉴定,目前主要通过顶空固相微萃取对花生香味进行提取,并结合气相色谱 - 质谱联用仪对香味进行检测。花生香味物质的形成主要受基因型、加工方式和时间、籽粒含水量等因素影响,已有大量文献表明耕作栽培模式影响水稻香味物质的积累,但不同耕作栽培模式影响花生香味物质积累的文章未见报道。同时,利用花生香味物质还可以抑制黄曲霉的污染,减少花生的经济损失,以及利用挥发性物质鉴别不同种类的植物油。该文以花生仁为对象,通过对花生香味物质成分分析、花生香味物质分类、花生香味物质的提取和鉴定方法,影响花生香味物质形成的因素以及花生香味物质抑制黄曲霉的污染等方面进行综述,为研究花生香味品质形成提供理论依据。     

Permethylpolysilanes: Silicon Analogs of Hydrocarbons: Cyclic polysilanes resemble aromatic hydrocarbons in electronic and chemical properties

Studies of the spectra of cyclopolysilanes have led to several significant conclusions. These molecules possess both easily ionized Si-Si bonding electrons and low energy delocalized antibonding orbitals, and can therefore serve either as electron donors or acceptors. This model explains the similarity in properties between cyclopolysilanes and aromatic hydrocarbons. Further investigations of cyclopolysilanes seem likely to provide the key to understanding of controversial questions of bonding in metalloid compounds even as studies of their carbon analogs, the cyclic and cage hydrocarbons, have been crucial to present knowledge of organic stereochemistry and reaction mechanisms. The reactions of cyclopolysilanes are not only interesting in themselves, but have opened the way to the synthesis of complex polysilanes and thus to whole new areas of study. Improved methods of synthesis and isolation are needed, but the number and kinds of compounds that can be prepared seem almost limitless. Perhaps a polymetal chemistry comparable in breadth and variety to carbon chemistry is now developing.     

城市污泥热解重金属Cd的形态变化机理

利用纯化合物CdS、CdCO3模拟污泥中重金属Cd的主要存在形式,在污泥的热解气氛下,对CdS、CdCO3进行加热处理,并对热解产物进行X-射线衍射分析,确定产物的组成,从而探究出污泥的热解时重金属Cd的形态变化机理。结果表明,Cd的硫化物在低于800℃热解时以稳定的形态存在于污泥中,而在高于900℃会分解形成Cd单质,其反应可以用以下式子表示CdS→Cd（g）＋S;镉的碳酸盐热解时发生了以下变化：CdCO3→CdO＋CO2,CdO＋硫化物（g）→CdS,其中硫化物是污泥热解过程中产生的,高温（〉800℃）热解时生成的CdS发生再分解。     

A major meteorite impact on the Earth 65 million years ago: evidence from the cretaceous-tertiary boundary clay

Evidence for a major meteorite impact on the earth 65 million years ago is shown by the presence of meteoritic debris in the "fish clay" from Denmark representing the Cretaceous-Tertiary boundary. Noble metals (iridium, osmium, gold, platinum, rhenium, ruthenium, palladium, nickel, and cobalt), which are sensitive indicators of meteorites and are normally depleted on the terrestrial surface by factors of 10(4) to 10(2) relative to cosmic abundances, are enriched in this boundary clay by factors of 5 to 100 over the expected abundances. With the exception of rhenium, all the enriched noble metals in the clay are present in cosmic proportions, indicating that the impacting celestial body had not undergone gross chemical differentiation. The major extinction of life on the earth at the end of the Cretaceous Period may be related to the meteorite impact.     

含双肟醚结构的2-甲氧亚氨基-2-(2-(取代苯基)苯基)乙酰甲胺系列衍生物的合成和抑菌活性研究

2-甲基苯甲酰甲酸甲酯通过与甲胺、甲氧基胺的盐酸盐、N-溴代丁二酰亚胺(NBS)和取代苯乙酮肟多步反应合成了11个含双肟醚结构的2-甲氧亚氨基-2-(2-(取代苯基)苯基)乙酰甲胺的衍生物(G1～G11)。目标化合物的结构经IR、1H NMR和GC-MS确认。用10μg.mL-1目标化合物进行初步离体抑菌活性试验,结果表明:G4对油菜菌核病、蔬菜灰霉病、小麦赤霉病、小麦纹枯病有抑菌活性;G11对蔬菜灰霉病、小麦赤霉病、小麦纹枯病有抑菌活性。     

Coordination polymers of osmium: the nature of osmium black

The design of cytochemical reagents that yield osmiophilic products from which an osmium black may be derived on exposure to osmium tetroxide has resulted in new methods described previously for the ultrastructural demonstration of enzyme activity and functional groups of macromolecules with the electron microscope. Attempts to determine the nature of the osmium black end products have been frustrated by their insolubility. The preparation of watersoluble analogs and their characterization as polymers suggest that the insoluble osmium blacks are coordination polymers. This is consonant with the unusually favorable properties of osmium black in electron microscopy. Although polymers of osmium have frequently been postulated as the end products of reaction of osmium tetroxide with tissue conistituents or with other organic compounds, this is the first example of their characterization.     

Discovery of an α-amino C-H arylation reaction using the strategy of accelerated serendipity

Serendipity has long been a welcome yet elusive phenomenon in the advancement of chemistry. We sought to exploit serendipity as a means of rapidly identifying unanticipated chemical transformations. By using a high-throughput, automated workflow and evaluating a large number of random reactions, we have discovered a photoredox-catalyzed C-H arylation reaction for the construction of benzylic amines, an important structural motif within pharmaceutical compounds that is not readily accessed via simple substrates. The mechanism directly couples tertiary amines with cyanoaromatics by using mild and operationally trivial conditions.     

Revealing atom-radical reactivity at low temperature through the N + OH reaction

More than 100 reactions between stable molecules and free radicals have been shown to remain rapid at low temperatures. In contrast, reactions between two unstable radicals have received much less attention due to the added complexity of producing and measuring excess radical concentrations. We performed kinetic experiments on the barrierless N((4)S) + OH((2)Π) → H((2)S) + NO((2)Π) reaction in a supersonic flow (Laval nozzle) reactor. We used a microwave-discharge method to generate atomic nitrogen and a relative-rate method to follow the reaction kinetics. The measured rates agreed well with the results of exact and approximate quantum mechanical calculations. These results also provide insight into the gas-phase formation mechanisms of molecular nitrogen in interstellar clouds.