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1.
Alkanes at the air-sea interface were analyzed in 118 surface samples collected at five different intervals over a 12-month period from Timbalier Bay (Louisiana), ogfshore Louiisiana, and offshore Florida. The alkanes were characterized by gas chromatography and mass spectrometry. Unexpectedly, methyl branched alkanes ranging in chain length froin C(15) to C(35) and cycloalkanes were frequently the predominant components. This suggests that the alkanes are produced by natural biological sources as well as human activities. 相似文献
2.
Motivated by increased losses of Antarctic stratospheric ozone and by improved understanding of the mechanism, a concept is suggested for action to arrest this ozone loss: injecting the alkanes ethane or propane (E or P) into the Antarctic stratosphere. A numerical model of chemical processes was used to explore the concept. The model results suggest that annual injections of about 50,000 tons of E or P could suppress ozone loss, but there are some scenarios where smaller E or P injections could increase ozone depletion. Further, key uncertainties must be resolved, induding initial concentrations of nitrogen-oxide species in austral spring, and several poorly defined physical and chemical processes must be quantifed. There would also be major difficulties in delivering and distributing the needed alkanes. 相似文献
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4.
Ochiai M Miyamoto K Kaneaki T Hayashi S Nakanishi W 《Science (New York, N.Y.)》2011,332(6028):448-451
Amination of alkanes has generally required metal catalysts and/or high temperatures. Here we report that simple exposure of a broad range of alkanes to N-triflylimino-λ(3)-bromane 1 at ambient temperature results in C-H insertion of the nitrogen functionality to afford triflyl-substituted amines in moderate to high yields. Marked selectivity for tertiary over secondary C-H bonds was observed; primary (methyl) C-H bonds were inert. Addition of hexafluoroisopropanol to inhibit decomposition of 1 dramatically improved the C-H amination efficiencies. Second-order kinetics, activation parameters (negative activation entropy), deuterium isotope effects, and theoretical calculations suggest a concerted asynchronous bimolecular transition state for the metal-free C-H amination event. 相似文献
5.
低温敏显性核不育水稻“8987”的育性转换与遗传研究 总被引：11，自引：0，他引：11
１９８９年在３３０４／明恢６３的Ｆ５代发现一个水稻雄性不育材料８９８７。它具有育性转换特性，温度是引起育性转换的主导因素，较低温时完全雄性不育，较高温时正常可育，结实率可达８０％。８９８７与其它品种正、反交Ｆ１代具有与它相同的育性转换，但组合间表现程度不完全一致。Ｆ２代在较低温度下发生育性分离时，有些组合Ｆ２不育株数：可育株数符合３：１，但有些组合可育株较多，分离比偏离３：１。这表明８９８７具有低温敏不育特性，它的不育性主要受细胞核内一对显性基因控制，遗传背景（微效修饰基因）对育性也有一些影响，是一种雄性不育新类型。 相似文献
6.
Bergman RG 《Science (New York, N.Y.)》1984,223(4639):902-908
Alkanes, although plentiful enough to be considered for use as feedstocks in large-scale chemical processes, are so unreactive that relatively few chemical reagents have been developed to convert them to molecules having useful functional groups. However, a recently synthesized iridium (lr) complex successfully converts alkanes into hydridoalkylmetal complexes (M + R-H --> R-M-H). This is a dihydride having the formula Cp(*)(L)lrH(2), where Cp(*) and L are abbreviations for the ligands (CH(3))(5)C(5) and (CH(3))(3)P, respectively. Irradiation with ultraviolet light causes the dihydride to lose H(2), generating the reactive intermediate Cp(*)lrL. This intermediate reacts rapidly with C-H bonds in every molecule so far tested (including alkanes) and leads to hydridoalkyliridium complexes Cp(*)(L)lr(R)(H). Evidence has been obtained that this C-H insertion, or oxidative addition, reaction proceeds through a simple three-center transition state and does not involve organic free radicals as intermediates. Thus the intermediate Cp(*)lrL reacts most rapidly with C-H bonds having relatively high bond energies, such as those at primary carbon centers, in small organic rings, and in aromatic rings. This contrasts directly with the type of hydrogen-abstraction selectivity that is characteristic of organic radicals. The hydridoalkyliridium products of the insertion reactions can be converted into functionalized organic molecules-alkyl halides-by treatment with mercuric chloride followed by halogens. Expulsion (reductive elimination) of the hydrocarbon from the hydridoalkyliridium complexes can be induced by Lewis acids or heat, regenerating the reactive intermediate Cp(*)lrL. Oxidative addition of the corresponding rhodium complexes Cp(*)RhL to alkane C-H bonds has also been observed, although the products formed in this case are much less stable and undergo reductive elimination at -20 degrees C. These and other recent observations provide an incentive for reexamining the factors that have been assumed to control the rate of reaction of transition metal complexes with C-H bonds-notably the need for electron-rich metals and the proximity of reacting centers. 相似文献
7.
Sphere-forming block copolymers are known to self-assemble into body-centered cubic crystals near the order-disorder transition temperature. Small-angle x-ray scattering and transmission electron microscopy experiments on diblock and tetrablock copolymer melts have revealed an equilibrium phase characterized by a large tetragonal unit cell containing 30 microphase-separated spheres. This structure, referred to as the sigma (σ) phase by Frank and Kasper more than 50 years ago, nucleates and grows from the body-centered cubic phase similar to its occurrence in metal alloys and is a crystal approximant to dodecagonal quasicrystals. Formation of the σ phase in undiluted linear block copolymers (and certain branched dendrimers) appears to be mediated by macromolecular packing frustration, an entropic contribution to the interparticle interactions that control the sphere-packing geometry. 相似文献
8.
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. 相似文献
9.
The formation of a single product from terminal functionalization of linear alkanes from a transition metal-catalyzed reaction is reported. The rhodium complex Cp*Rh(eta(4)-C(6)Me(6)) (Cp*, C(5)Me(5); Me, methyl) catalyzes the high-yield formation of linear alkylboranes from commercially available borane reagents under thermal conditions. These reactions now allow catalytic, regiospecific functionalization of alkanes under thermal conditions. The organoborane products are among the most versatile synthetic intermediates in chemistry and serve as convenient precursors to alcohols, amines, and other common classes of functionalized molecules. 相似文献
10.
The vibrational Raman spectrum of solid hydrogen has been measured from 77 to 295 K in the vicinity of the recently observed insulator-metal transition and low-temperature phase transition at 150 gigapascals (1.5 megabars). The measurements provide evidence for a critical point in the pressure-temperature phase boundary of the low-temperature transition. The result suggests that below the critical temperature the insulator-metal transition changes from continuous to discontinuous, consistent with the general criteria originally proposed by Mott for metallization by band-gap closure. The effect of temperature on hydrogen metallization closely resembles that of the lower pressure insulator-metal transitions in doped V(2)O(3) alloys. 相似文献
11.
含支链氨基酸饮料对高温环境下小鼠体能恢复的影响 总被引：5，自引：0，他引：5
合支链氨基酸饮料是一种体能型特需饮料，能迅速恢复高温条件下机体体能的损失。本研究以BALB／C近交系小鼠作为实验动物模型，人工模拟30℃的高温条件，将小鼠间断地暴露于高温中，在高温处理期间，给小鼠分别喂饮含支链氨基酸饮料和雪碧对照饮料，然后进行体能各项指标的测定。结果表明：饮用含支链氨基酸饮料组小鼠体质量增加显著，运动能力增强，运动后血糖含量高，血乳酸含量低，同时其耐热、抗疲劳、耐缺氧能力显著增强。 相似文献
12.
A novel concept for performing stoichiometric and catalytic chemical transformations has been developed that is based on the limited miscibility of partially or fully fluorinated compounds with nonfluorinated compounds. A fluorous biphase system (FBS) consists of a fluorous phase containing a dissolved reagent or catalyst and another phase, which could be any common organic or nonorganic solvent with limited or no solubility in the fluorous phase. The fluorous phase is defined as the fluorocarbon (mostly perfluorinated alkanes, ethers, and tertiary amines)-rich phase of a biphase system. An FBS compatible reagent or catalyst contains enough fluorous moieties that it will be soluble only or preferentially in the fluorous phase. The most effective fluorous moieties are linear or branched perfluoroalkyl chains with high carbon number; they may also contain heteroatoms. The chemical transformation may occur either in the fluorous phase or at the interface of the two phases. The application of FBS has been demonstrated for the extraction of rhodium from toluene and for the hydroformylation of olefins. The ability to separate a catalyst or a reagent from the products completely at mild conditions could lead to industrial application of homogeneous catalysts or reagents and to the development of more environmentally benign processes. 相似文献
13.
Tarascon JM Greene LH McKinnon WR Hull GW Geballe TH 《Science (New York, N.Y.)》1987,235(4794):1373-1376
Structural, magnetic, and electronic properties of compounds in the series La2-xSrx CuO4-y for 0.05 相似文献
14.
A Fluorite Isotype of SnO2 and a New Modification of TiO2: Implications for the Earth's Lower Mantle
Liu LG 《Science (New York, N.Y.)》1978,199(4327):422-425
The existence of a cubic fluorite-type SnO(2) and a hexagonal TiO(2) (which may be related to the fluorite structure) have been demonstrated by an in situ x-ray diffraction study in which a diamond-anvil pressure cell was used after the samples had been heated by a continuous yttrium-aluminum-garnet laser. At room temperature, the lattice parameter for SnO(2) (fluorite) is a = 4.925 +/- 0.005 angstroms and those for TiO(2) (fluorite-related) are a = 9.22 +/- 0.01 angstroms and c = 5.685 +/- 0.006 angstroms at about 250 kilobars. The volume change associated with the transition from rutile to fluorite (or related structure) is about -8 percent for SnO(2) and -10.5 percent for TiO(2) at transition. Upon release of pressure, both the fluorite-type SnO(2) and the TiO(2) reverted to the alpha-PbO(2) structure at room temperature. The hypothesis that the earth's lower mantle is composed of oxide phases might be feasible if it were possible for SiO(2) to possess the fluorite structure or its related forms at high pressure, as shown for SnO(2) and TiO(2) in this study. The oxide hypothesis proposed here differs from that postulated by Birch in that the primary coordination of silicon is 6 for Birch's hypothesis and 8 for the hypothesis presented here. 相似文献
15.
Dimeric products are formed by riboflavin-sensitized photooxidation of 2,4-dichlorophenol. The products of this reaction were examined to determine whether chlorinated dibenzo-p-dioxins could be formed from chlorophenols in water by the action of light of wavelengths greater than 280 nanometers. Dimers are formed by union of phenoxy radicals through carbon-carbon or carbon-oxygen bonds. 4,6-Dichloro-2-(2,4-dichlorophenoxy) phenol was obtained in greater quantity than other dimers. Products were characterized by combined gas chromatography and mass spectrometry. Chlorinated dibenzo-p-dioxins which could result from ring closure of a 2-phenoxyphenol derivative were not detected in the products of photolysis. The failure to detect chlorinated dibenzo-p-dioxins may result from the rapid photolytic breakdown of the lower chlorinated dibenzop-dioxins. Under environmental conditions, dioxins are unlikely products of the lower chlorinated phenols or phenoxyalkanoic acids. 相似文献
16.
Measurements of electrical conductivity at high pressure and temperature were taken on the lower mantle phase magnesiowustite with varying Fe3+ content. Although previous measurements at atmospheric pressure suggest Fe2+-Fe3+ hopping (small polaron) as the dominant conductivity mechanism, the present experiments show a change in charge transport mechanism with temperature. The lower temperature measurements are consistent with small polaron conduction, but at higher temperatures, which are more applicable to the lower mantle, a large polaron mechanism is suggested. Because these mechanisms have different temperature and compositional dependencies, this transition has important implications for extrapolation to mantle conditions. 相似文献
17.
以碳原子数Ｎ和Ｗｉｅｎｅｒ指数Ｗ,Ｐ为结构参数,建立了烷烃同分异构体的沸点与其分子结构间的定量关系式,用此式计算了７６个支链烷烃的沸点。结果表明,该式适用范围广泛,计算结果较准确,可用于预测烷烃同分异构体的沸 相似文献
18.
Antonangeli D Siebert J Aracne CM Farber DL Bosak A Hoesch M Krisch M Ryerson FJ Fiquet G Badro J 《Science (New York, N.Y.)》2011,331(6013):64-67
Seismic discontinuities in Earth typically arise from structural, chemical, or temperature variations with increasing depth. The pressure-induced iron spin state transition in the lower mantle may influence seismic wave velocities by changing the elasticity of iron-bearing minerals, but no seismological evidence of an anomaly exists. Inelastic x-ray scattering measurements on (Mg(0.83)Fe(0.17))O-ferropericlase at pressures across the spin transition show effects limited to the only shear moduli of the elastic tensor. This explains the absence of deviation in the aggregate seismic velocities and, thus, the lack of a one-dimensional seismic signature of the spin crossover. The spin state transition does, however, influence shear anisotropy of ferropericlase and should contribute to the seismic shear wave anisotropy of the lower mantle. 相似文献
19.
The nature of chain folding in polymers and the determination of the chain length at which folding occurs have been central questions in polymer science. The study of the formation of lamellar polymer crystals through chain folding has received a new impetus as a result of the recent synthesis of normal alkanes of strictly uniform chain lengths up to C(390) H(782). Chain folding is found in all such paraffins starting with C(150)H(302). As with polyethylenes obtained by conventional polymerization, the fold length in the normal alkanes varies with crystallization temperature, but it is always an integral reciprocal of the full chain length. This behavior indicates that the methyl end groups are located at the lamellar surface and that the fold itself must be sharp and adjacently reentrant. 相似文献
20.
风化煤腐植酸增效尿素红外光谱分析 总被引：2，自引：1，他引：1
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。 相似文献