离子注入水稻干种子光声谱分析

<正> 中国科学院等离子体物理所与安徽省农科院水稻研究所合作,利用高技术——离子注入——于水稻种子诱变育种生物效应的研究,在物理学与生命科学的交叉上做出了有益的尝试。两年多的研究表明:离子注入水稻种子诱变,较其他方法诱变有突变率高、突变谱广的特点。作者认为:荷能粒子注入到水稻干种子中去,不仅有能量交换过程,引起生物体内很强的生理生化反应,而且发生了质量沉积过程,沉积的粒子同生物体分子基团直接键合,改变了生物体原来的分子基团结构。这一切,都会引起水稻种子遗传基因的突变。     

Formation of a carbon-carbon triple bond by coupling reactions in aqueous solution

Formation of a carbon-carbon triple bond by coupling reactions usually takes place at high temperatures, in anhydrous media and anaerobic conditions. We describe the formation of a carbon-carbon triple bond at room temperature in an aqueous solution exposed to the atmosphere. Two ethylidyne ligands of a trimolybdenum cluster coupled spontaneously to form 2-butyne. This unexpected result demonstrates the plausibility of alkylidyne chain lengthening and metathesis processes under ambient, environmentally friendly conditions.     

Catalysis by transition metals: metal-carbon double and triple bonds

Several well-characterized transition metal catalysts contain a metal-carbon double bond or a metal-carbon triple bond. In other homogeneous (or heterogeneous) catalyst systems in which the metal is likely to be in a relatively high oxidation state, such as molybdenum(VI) or tungsten(VI), metal-carbon multiple bonds may play an important role. Some recent results suggest that even supposedly well understood reactions such as ethylene polymerization may actually involve catalysts that behave as if they contained a metal-carbon double bond instead of a metal-carbon single bond. The chemistry of metal-carbon double and triple bonds should eventually complement and perhaps. overlap the known chemistry of complexes containing metal-oxygen double bonds or metal-nitrogen triple bonds, respectively; unique catalytic reactions involving carbon, nitrogen, and oxygen ligands multiply bonded to transition metals are therefore possible.     

A stable compound containing a silicon-silicon triple bond

The reaction of 2,2,3,3-tetrabromo-1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasilane with four equivalents of potassium graphite (KC8) in tetrahydrofuran produces 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyl-2-tetrasilyne, a stable compound with a silicon-silicon triple bond, which can be isolated as emerald green crystals stable up to 100 degrees C in the absence of air. The SiSi triple-bond length (and its estimated standard deviation) is 2.0622(9) angstroms, which shows half the magnitude of the bond shortening of alkynes compared with that of alkenes. Unlike alkynes, the substituents at the SiSi group are not arranged in a linear fashion, but are trans-bent with a bond angle of 137.44(4) degrees.     

Automatic determination of crystal structure

Several crystallographic computer programs have been organized into one large automatic program for solving crystal structures. The emphasis of this organization has been to produce a noninteractive system, that is, to have all decisions made by the computer. Input data are the raw intensity data, cell constants, space group, chemical formula, and other miscellaneous items. The output is a stereo picture of the contents in a unit cell. The program, operating in a noninteractive mode, has successfully solved compounds of unknown structure; in addition, for a test compound of completely unknown composition, this program deduced the correct structure with an average error in bond distance of 0.05 angstrom and an average error in bond angle of 7 degrees .     

A stable silicon(0) compound with a Si=Si double bond

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si-Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 +/- 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si-Si(Cl):L and L:Si=Si:L.     

Asbestiform chain silicates: new minerals and structural groups

The discovery and characterization of structurally ordered and disordered phases that are intermediate between amphiboles and micas have shown that the biopyriboles are a much more complex family of minerals than has previously been recognized. In addition to single-chain, double-chain, and sheet structures, there are also minerals with triple chains and with alternating double and triple chains. Many crystals exhibit disorder in the sequence of double and triple chains, and isolated chains that are wider than triple are common. This structural disorder helps to explain why asbestiform amphiboles are fibrous. The new phases have now been found in several localities, and it is possible that similar phenomena in other minerals could also have been overlooked. In particular, there is no reason to suppose that analogous substances and structures with both single and double chains do not occur between the pyroxenes and the amphiboles. Since the pyroxenes are used extensively by geologists to assess rock histories and formation temperatures and pressures, it is essential that the extent of this type of disorder be evaluated. It is possible that what appears to be only an interesting mineralogical problem may prove to be a petrological nightmare.     

Cleaving mercury-alkyl bonds: a functional model for mercury detoxification by MerB

The extreme toxicity of organomercury compounds that are found in the environment has focused attention on the mechanisms of action of bacterial remediating enzymes. We describe facile room-temperature protolytic cleavage by a thiol of the Hg-C bond in mercury-alkyl compounds that emulate the structure and function of the organomercurial lyase MerB. Specifically, the tris(2-mercapto-1-t-butylimidazolyl)hydroborato ligand [Tm(Bu(t))], which features three sulfur donors, has been used to synthesize [Tm(Bu(t))]HgR alkyl compounds (R = methyl or ethyl) that react with phenylthiol (PhSH) to yield [Tm(Bu(t))]HgSPh and RH. Although [Tm(Bu(t))]HgR compounds exist as linear two-coordinate complexes in the solid state, 1H nuclear magnetic resonance spectroscopy indicates that the complexes exist in rapid equilibrium with their higher-coordinate [kappa2-Tm(Bu(t))]HgR and [kappa3-Tm(Bu(t))]HgR isomers in solution. Facile access to a higher-coordinate species is proposed to account for the exceptional reactivity of [Tm(Bu(t))]HgR relative to that of other two-coordinate mercury-alkyl compounds.     

Dinitrogen Cleavage by a Three-Coordinate Molybdenum(III) Complex

Cleavage of the relatively inert dinitrogen (N(2)) molecule, with its extremely strong N identical withN triple bond, has represented a major challenge to the development of N(2) chemistry. This report describes the reductive cleavage of N(2) to two nitrido (N(3-)) ligands in its reaction with Mo(NRAr)(3), where R is C(CD(3))(2)CH(3) and Ar is 3,5-C(6)H(3)(CH(3))(2'), a synthetic three-coordinate molybdenum(III) complex of known structure. The formation of an intermediate complex was observed spectroscopically, and its conversion (with N identical withN bond cleavage) to the nitrido molybdenum(VI) product N identical withMo(NRAr)(3) followed first-order kinetics at 30 degrees C. It is proposed that the cleavage reaction proceeds by way of an intermediate complex in which N(2) bridges two molybdenum centers.     

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.     

N₂reduction and hydrogenation to ammonia by a molecular iron-potassium complex

The most common catalyst in the Haber-Bosch process for the hydrogenation of dinitrogen (N(2)) to ammonia (NH(3)) is an iron surface promoted with potassium cations (K(+)), but soluble iron complexes have neither reduced the N-N bond of N(2) to nitride (N(3-)) nor produced large amounts of NH(3) from N(2). We report a molecular iron complex that reacts with N(2) and a potassium reductant to give a complex with two nitrides, which are bound to iron and potassium cations. The product has a Fe(3)N(2) core, implying that three iron atoms cooperate to break the N-N triple bond through a six-electron reduction. The nitride complex reacts with acid and with H(2) to give substantial yields of N(2)-derived ammonia. These reactions, although not yet catalytic, give structural and spectroscopic insight into N(2) cleavage and N-H bond-forming reactions of iron.     

"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.     

稀土元素的开发应用

稀土元素具有独特的化学、光学、磁学性能。国内外对稀土元素及其化合物的研究不断深入,稀土元素的应用范围更为广泛。介绍了稀土元素的开发应用情况。     

An octahedral coordination complex of iron(VI)

The hexavalent state, considered to be the highest oxidation level accessible for iron, has previously been found only in the tetrahedral ferrate dianion, FeO4(2-). We report the photochemical synthesis of another Fe(VI) compound, an octahedrally coordinated dication bearing a terminal nitrido ligand. M?ssbauer and x-ray absorption spectra, supported by density functional theory, are consistent with the octahedral structure having an FeN triple bond of 1.57 angstroms and a singlet d2(xy) ground electronic configuration. The compound is stable at 77 kelvin and yields a high-spin Fe(III) species upon warming.     

Second structural motif for recognition of DNA by oligonucleotide-directed triple-helix formation

Relative orientations of the DNA strands within a purine.purine.pyrimidine triple helix have been determined by affinity cleaving. A purine-rich oligonucleotide bound in the major groove of double-helical DNA antiparallel to the Watson-Crick purine strand. Binding depended upon the concentration of multivalent cations such as spermine or Mg2+, and appeared to be relatively independent of pH. Two models with specific hydrogen-bonding patterns for base triplets (G.GC, A.AT, and T.AT) are proposed to explain the sequence specificity of binding. The two models differ in the conformation about the glycosyl bond (syn or anti) and the location of the phosphate-deoxyribose backbone in the major groove of DNA. This motif broadens the structural frameworks available as a basis for the design of sequence-specific DNA binding molecules.     

Solid-state chemistry: a a rediscovered chemical frontier

Chemical bonding in solids is not completely understood, mainly because of the wide variation in the chemical properties of the elements. Many difficult challenges remain in predicting the composition, structure, and the properties of new materials. Consequently, the synthesis of novel solids is as much an art as a science. Discoveries of new compounds and structure types highlight the versatility that nature has allowed with the relatively small number of elements. This article explores the long-term challenges in solid-state chemistry and then focuses on efforts at Cornell to prepare new solids.     

风化煤腐植酸增效尿素红外光谱分析

采用碱性混合活化剂处理风化煤,研制腐植酸添加剂,按不同比例添加到尿素熔融液中,采用模拟喷浆造粒的方法,制备3类增效尿素。通过风化煤与添加剂、尿素与增效尿素红外光谱特征对比发现:风化煤经过活化后,碳单键数量降低,碳链缩短,活性腐植酸HA类提取物出现活性官能团,FR类除单键数量减少和碳链缩短外,生成部分胺、酰胺结构;增效剂与尿素在反应过程中,出现了三键和累积双键破坏、稳定性复合物形成、碳链缩短、双键结构增加等变化,但不同类型增效剂对产品红外光谱特征影响存在差异。     

Synthesis of Linear Acetylenic Carbon: The "sp" Carbon Allotrope

A carbon allotrope based on "sp" hybridization containing alternating triple and single bonds (an acetylenic or linear carbon allotrope) has been prepared. Studies of small (8 to 28 carbon atoms) acetylenic carbon model compounds show that such species are quite stable (130 degrees to 140 degrees C) provided that nonreactive terminal groups or end caps (such as tert-butyl or trifluoromethyl) are present to stabilize these molecules against further reactions. In the presence of end capping groups, laser-based synthetic techniques similar to those normally used to generate fullerenes, produce thermally stable acetylenic carbon species capped with trifluoromethyl or nitrile groups with chain lengths in excess of 300 carbon atoms. Under these conditions, only a negligible quantity of fullerenes is produced. Acetylenic carbon compounds are not particularly moisture or oxygen sensitive but are moderately light sensitive.     

The nature of the metal-metal bond in bimetallic surfaces

The formation of a surface metal-metal bond can produce large perturbations in the electronic, chemical, and catalytic properties of a metal. Recent studies indicate that charge transfer is an important component in surface metal-metal bonds that involve dissimilar elements. The larger the charge transfer, the stronger the cohesive energy of the bimetallic bond. On a surface, the formation of a heteronuclear metal-metal bond induces a flow of electron density toward the element with the larger fraction of empty states in its valence band. This behavior is completely contrary to that observed in bulk alloys, indicating that the nature of a heteronuclear metal-metal bond depends strongly on the structural geometry of the bimetallic system.     

HD 181068: a red giant in a triply eclipsing compact hierarchical triple system

Hierarchical triple systems comprise a close binary and a more distant component. They are important for testing theories of star formation and of stellar evolution in the presence of nearby companions. We obtained 218 days of Kepler photometry of HD 181068 (magnitude of 7.1), supplemented by ground-based spectroscopy and interferometry, which show it to be a hierarchical triple with two types of mutual eclipses. The primary is a red giant that is in a 45-day orbit with a pair of red dwarfs in a close 0.9-day orbit. The red giant shows evidence for tidally induced oscillations that are driven by the orbital motion of the close pair. HD 181068 is an ideal target for studies of dynamical evolution and testing tidal friction theories in hierarchical triple systems.