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.     

The (Me5C5)Si+ cation: a stable derivative of HSi+

The reaction of decamethylsilicocene, (Me5C5)2Si, with the proton-transfer reagent Me5C5H2+B(C6F5)4- produces the salt (Me5C5)Si+ B(C6F5)4(2), which can be isolated as a colorless solid that is stable in the absence of air and moisture. The crystal structure reveals the presence of a cationic pi complex with an eta5-pentamethylcyclopentadienyl ligand bound to a bare silicon center. The 29Si nuclear magnetic resonance at very high field (delta = - 400.2 parts per million) is typical of a pi complex of divalent silicon. The (eta5-Me5C5)Si+ cation in 2 can be regarded as the "resting state" of a silyliumylidene-type (eta1-Me5C5)Si+ cation. The availability of 2 opens new synthetic avenues in organosilicon chemistry. For example, 2 reacted with lithium bis(trimethylsilyl)amide to give the disilene E-[(eta1-Me5C5)[N(SiMe3)2]Si]2(3).     

Fabrication of atomic-scale structures on si(001) surfaces

The scanning tunneling microscope has been used to define regular crystalline structures at room temperature by removing atoms from the silicon (001) surface. A single atomic layer can be removed to define features one atom deep and create trenches with ordered floors. Segments of individual dimer rows can be removed to create structures with atomically straight edges and with lateral features as small as one dimer wide. Conditions under which such removal is possible are defined, and a mechanism is proposed.     

A hexagonal (wurtzite) form of silicon

The existence of a hexagonal (wurtzite) form of silicon, similar to that form of diamond (carbon) observed in meteorites and in the laboratory, has been identified by x-ray diffraction in reaction-bonded silicon nitride containing unreacted silicon. The presence of this phase is due to stresses created in the silicon by the nitridation reaction.     

Atomic resolution of the silicon (111)-(7x7) surface by atomic force microscopy

Achieving high resolution under ultrahigh-vacuum conditions with the force microscope can be difficult for reactive surfaces, where the interaction forces between the tip and the samples can be relatively large. A force detection scheme that makes use of a modified cantilever beam and senses the force gradient through frequency modulation is described. The reconstructed silicon (111)-(7x7) surface was imaged in a noncontact mode by force microscopy with atomic resolution (6 angstroms lateral, 0.1 angstrom vertical).     

Adatom pairing structures for Ge on si(100): the initial stage of island formation

With the use of scanning tunneling microscopy, it is shown that germanium atoms adsorbed on the (100) surface of silicon near room temperature form chainlike structures that are tilted from the substrate dimer bond direction and that consist of two-atom units arranged in adjoining substrate troughs. These units are distinctly different from surface dimers. They may provide the link missing in our understanding of the elementary processes in epitaxial film growth: the step between monomer adsorption and the initial formation of two-dimensional growth islands.     

Shape transition of germanium nanocrystals on a silicon (001) surface from pyramids to domes

Chemical vapor deposition of germanium onto the silicon (001) surface at atmospheric pressure and 600 degrees Celsius has previously been shown to produce distinct families of smaller (up to 6 nanometers high) and larger (all approximately 15 nanometers high) nanocrystals. Under ultrahigh-vacuum conditions, physical vapor deposition at approximately the same substrate temperature and growth rate produced a similar bimodal size distribution. In situ scanning tunneling microscopy revealed that the smaller square-based pyramids transform abruptly during growth to significantly larger multifaceted domes, and that few structures with intermediate size and shape remain. Both nanocrystal shapes have size-dependent energy minima that result from the interplay between strain relaxation at the facets and stress concentration at the edges. A thermodynamic model similar to a phase transition accounts for this abrupt morphology change.     

Identification of the products from the reaction of chlorine with the silicon(111)-(7x7) surface

The various products from the reaction of chlorine (Cl) with the adatom layer of the Si(111)-(7x7) surface have been identified with scanning tunneling microscopy (STM). Initially, a single Cl atom reacts with the adatom dangling bond. At higher surface coverage, additional Cl atoms insert themselves into the Si-Si backbonds between the adatom and rest-atom layers, producing adatoms that have reacted with two or three Cl atoms. These products are characterized by different registries with respect to the underlying rest layer and appear in STM images as adatoms of different sizes, consistent with the breaking of Si-Si backbonds and the formation ofnew Si-Cl bonds.     

Reversible rotation of antimony dimers on the silicon (001) surface with a scanning tunneling microscope

The scanning tunneling microscope (STM) was used to control the configuration of antimony clusters on the (001) surface of silicon. In particular, the STM tip induced a reversible rotation between two orthogonal orientations of individual antimony dimers on the surface. This simple rotation can be explained by an atomic-scale torque exerted on the antimony dimers by the STM tip. The reversibility of this process could provide a basis for making atomic-scale memory cells.     

Energy dependence of abstractive versus dissociative chemisorption of fluorine molecules on the silicon (111)-(7x7) surface

Scanning tunneling microscopy and monoenergetic molecular beams have been used to obtain real-space atomic images of the competition between abstractive and dissociative chemisorption. The size distribution of Si-F adsorbates on the Si(111)-(7x7) surface was examined as a function of the incident translational energy of the F(2) molecules. For F(2) molecules with 0.03 electron volt of incident energy, the dominant adsorbate sites were isolated Si-F species. As an F(2) molecule with low translational energy collides with the surface, abstraction occurs and only one of the F atoms chemisorbs; the other is ejected into the gas phase. For F(2) molecules with 0.27 electron volt of incident energy, many adjacent Si-F adsorbates (dimer sites) were observed because F(2) molecules with high translational energy collide with the surface and chemisorb dissociatively so that both F atoms react to form adjacent Si-F adsorbates. For halogens with very high incident energy (0.5-electron volt Br(2)), dissociative chemisorption is the dominant adsorption mechanism and dimer sites account for nearly all adsorbates.     

真核细胞翻译起始因子5A(eIF5A)研究进展

真核细胞翻译起始因子5A(eIF5A)是真菌、动植物体内普遍存在的蛋白质翻译起始因子。研究发现其不仅仅在部分蛋白质翻译起始中发挥作用,还在人体癌症发生、促进动植物细胞增殖、细胞衰老和死亡以及一些植物环境胁迫应答等方面都有一定的调控作用。进一步研究真核细胞翻译起始因子5A(eIF5A)的功能,对其生产实践中应用具有非常重要的意义。     

Dangling bond dynamics on the silicon (100)-2x1 surface: dissociation, diffusion, and recombination

The dynamics of dangling bond (DB) diffusion was studied after deuterium desorption from the silicon (100)-2x1 surface. At elevated temperatures, paired DB sites produced after desorption unpaired as deuterium atoms hopped from adjacent dimers. Below 620 kelvin, the unpaired configuration most commonly observed corresponded to two DBs on adjacent silicon dimers. At higher temperatures, unpaired DBs executed one-dimensional walks along the dimer rows, and recombination was observed with the same partner after walks lasting many minutes. The frequency and extent of these excursions increased with temperature. Above 660 kelvin, complete dissociation was observed and was sometimes followed by recombination by means of partner exchange. The implications for low-temperature materials growth are discussed.     

A potent greenhouse gas identified in the atmosphere: SF(5)CF(3)

We detected a compound previously unreported in the atmosphere, trifluoromethyl sulfur pentafluoride (SF(5)CF(3)). Measurements of its infrared absorption cross section show SF(5)CF(3) to have a radiative forcing of 0.57 watt per square meter per parts per billion. This is the largest radiative forcing, on a per molecule basis, of any gas found in the atmosphere to date. Antarctic firn measurements show it to have grown from near zero in the late 1960s to about 0.12 part per trillion in 1999. It is presently growing by about 0.008 part per trillion per year, or 6% per year. Stratospheric profiles of SF(5)CF(3) suggest that it is long-lived in the atmosphere (on the order of 1000 years).     

A 5-micron-bright spot on Titan: evidence for surface diversity

Observations from the Cassini Visual and Infrared Mapping Spectrometer show an anomalously bright spot on Titan located at 80 degrees W and 20 degrees S. This area is bright in reflected light at all observed wavelengths, but is most noticeable at 5 microns. The spot is associated with a surface albedo feature identified in images taken by the Cassini Imaging Science Subsystem. We discuss various hypotheses about the source of the spot, reaching the conclusion that the spot is probably due to variation in surface composition, perhaps associated with recent geophysical phenomena.     

5-coordinate si compounds as intermediates in the synthesis of silicates in nonaqueous media

Sodium silicoglycolate, an organosilicon compound in which silicon is in 5-coordination with respect to oxygen, is shown to be a reaction intermediate in the synthesis of a purely siliceous sodalite with ethylene glycol as solvent. The presence of silicon-29 nuclear magnetic resonances at -105.5 +/- 0.3 parts per million in solution and -102.7 parts per million in the solid state as well as x-ray powder diffraction patterns demonstrate that 5-coordinate silicon is intimately involved in the synthesis. Silicon compounds of the 5-coordinate type are highly reactive and are promising starting materials for the synthesis of novel silicone polymers, molecular sieves, glasses, semiconductors, and ceramics. We have used sodium silicoglycolate to prepare the molecular sieve silicalite as well as sodalites of different compositions.     

Structure of a dinucleotide: thymidylyl-(5'-3')-thymidylate-5' (pTpT)

The crystal and molecular structure of the naturally occurring deoxyribose dinucleotide sodium thymidylyl-(5'-->3')-thymidylate-5' has been determined by x-ray diffraction. There are four molecules of dinucleotide and 52 water molecules in an orthorhombic unit cell of dimensions (in angstroms) a = 16.06, b = 15.13, c = 15.65, space group P2(1)2(1)2. There is a very high degree of conformational consistency between the two halves of the molecule when the dinucleotide is viewed as the combination of two 5'-mononucleotides. The planes of the two thymines are not parallel, but are tilted 38 degrees with respect to each other. An extensive system of hydrogen bonding exists involving the bases, waters, phosphates, and sodiums; no base-base hydrogen bonding occurs. The dinucleotide structural parameters should be of assistance in interpreting DNA fiber diagrams in terms of possible structures.     

转cry1Ab/cry2Aj基因玉米双抗12-5对意大利蜜蜂成虫的影响

转cry1Ab/cry2Aj基因玉米双抗12-5是我国自主研发的抗虫耐除草剂玉米品系,目前已获批准进入生产性试验阶段。蜜蜂是具有重要生态和经济意义的昆虫,是转基因植物环境安全评价中主要的非靶标节肢动物。在转基因抗虫植物种植过程中,蜜蜂可能通过采集花粉暴露于转基因抗虫植物表达的杀虫蛋白。本实验采用室内生测的方法评价了转基因玉米双抗12-5花粉和Cry1Ab杀虫蛋白对意大利蜜蜂(Apis mellifera)的影响。结果显示,喂食转基因玉米花粉处理组的意大利蜜蜂体内能检测到Cry1Ab杀虫蛋白;与对照相比,饲喂转基因玉米花粉和8 μg·mL^-1纯化的Cry1Ab杀虫蛋白对意大利蜜蜂的存活率没有显著影响,吡虫啉处理组意大利蜜蜂存活率显著低于花粉和蔗糖处理组。以上结果表明,转基因玉米双抗12-5或Cry1Ab杀虫蛋白对意大利蜜蜂的风险较小。     

甘蓝型油菜透明种皮12（TT12）基因家族反义抑制植物表达载体的构建

【研究目的】构建一个同时反义共抑制甘蓝型油菜(Brassica napus)透明种皮12基因家族(BnTT12)转录的植物表达载体。【方法】通过PCR扩增得到BnTT12家族一段503bp的特异保守片段TT12A,通过亚克隆整合到中间载体pCambia2301G,构建TT12反义植物表达载体pCambi-a2301G-TT12A。采用液氮冻融法将其转化到根癌农杆菌。【结果】经过PCR鉴定、酶切鉴定和DNA测序证实载体构建成功,并转化到根癌农杆菌LBA4404菌株中形成TT12反义表达载体的工程菌株。【结论】pCambia2301G-TT12A的成功构建为利用此反义载体通过遗传转化获得转基因新型黄籽油菜和进一步探明TT12基因在甘蓝型油菜种皮色素合成途径中的分子生物学机理奠定了基础。     

A homomolecular porous network at a Cu(111) surface

Anthraquinone molecules self-assemble on a Cu(111) surface into a large two-dimensional honeycomb network (square root of 304 x square root of 304)R23 degrees with pore diameters of approximately 50 A. The spontaneous formation of a pattern containing pores roughly five times larger than the size of the constituent molecules is unprecedented. The network originates from a delicate balance between substrate-mediated repulsion and intermolecular attraction involving an unusual chemical motif: hydrogen bonding between a carbonyl oxygen and an aromatic hydrogen atom. Substrate-mediated long-range adsorbate-adsorbate repulsion has been observed on anisotropic surfaces and in the context of the absence of pattern formation. Its applicability for the design of tailored molecular films is explored here.