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1.
Laser vaporization of graphite generates C(60)(+) cluster ions that are fullerenes and a mixture of roughly planar polycyclic polyyne ring isomers. Experimental studies of the annealing of the non-fullerene C(60)(+) ions indicate that they can be converted (in the gas phase) into the fullerene and an isomer that appears to be a large monocyclic ring. Some fragmentation is associated with conversion to the fullerene geometry, but the majority of the non-fullerene C(60)(+) isomers are cleanly converted into an intact fullerene. The emergence of the monocyclic ring (as the clusters are annealed) suggests that this is a relatively stable non-spheroidal form of these all carbon molecules. The estimated activation energies for the observed structural interconversions are relatively low, suggesting that these processes may play an important role in the synthesis of spheroidal fullerenes. 相似文献
2.
The distinct properties of fluorous phases are practically useful for separation, purification, and reaction control in organic synthesis. Here, we report the formation of a liquid-like fluorous droplet, composed of 24 perfluoroalkyl chains confined in the interior of a 5-nanometer-sized, roughly spherical shell that spontaneously assembled in solution from 12 palladium ions and 24 bridging ligands. Crystallographic analysis confirmed the rigid shell framework and amorphous interior. Perfluoroalkanes can dissolve in this well-defined fluorous phase, whereas they can hardly dissolve in a surrounding polar organic solution, and their solubility (up to approximately eight perfluoroalkane molecules per spherical complex) can be finely controlled by tuning the length of perfluoroalkyl chains tethered to the shell. 相似文献
3.
Difficulties in preparing cluster ions for spectroscopic studies have limited our understanding of intermolecular forces in charged complexes that are typical of many reactive intermediates. Here, the infrared spectrum of the charge transfer complex [Ar-N2]+, recorded in a supersonic planar plasma with a tunable diode laser spectrometer, is presented. More than 70 adjacent rovibrational transitions were measured near 2272 wave numbers and assigned to the molecular nitrogen stretching fundamental in the 2Sigma+ ground state of [Ar-N2]+. The accurate structural parameters that were determined confirm a linear structure and show that the major part of the charge is located at the argon atom. The latter result is surprising and implies a charge switch of the cationic center upon complexation. 相似文献
4.
以常州市为研究区域,采用现场调查和实验室分析相结合的方法,对常州市浅层地下水HCO3-、Cl-、SO42-、F-、Na+、K+等10种指标进行检测,并分析浅层地下水的水质状况、地球化学特征和成因机理。在常州市浅层地下水样中,第四系松散岩类孔隙潜水有2个水样总硬度超标,微承压水和第Ⅰ承压水各有1个水样总硬度超标。常州市浅层地下水化学类型主要为HCO3-Ca.Na(Na.Ca)和HCO3-Ca.Mg,在常州的东南和西部少部分地区出现几种类型的地下水。这与常州全区地势平坦、地下水径流微弱具有一致性。常州市孔隙潜水、微承压水和第Ⅰ承压水中,各种离子存在一定的分布规律,且在各含水层的分布规律不同。常州市浅层地下水化学特征主要与地下水离子间的交换吸附作用、人类生产生活活动和浅层地下水的酸碱性有关。 相似文献
5.
Chao EC 《Science (New York, N.Y.)》1967,156(3772):192-202
Shock effects in quartz, plagioclase, biotite, amphibole, and some accessory minerals have been observed in rocks subjected to various degrees of meta morphism by meteoritic impact. The shock features described are unique; they are never observed in rocks from normal geologic environments. Such features are described: 1) Multiple sets of closely spaced planar microstructures occur in quartz, plagioclase, and other rock-forming minerals. Those characteristic of shock consist of alternating platelets, with a range of reduced mean index of re fraction and birefringence; they con sist of platelets that have been partially or completely transformed to an amor phous phase. 2) Quartz and plagioclase are selec tively and completely transformed to silica glass and plagioclase glass in the solid state, whereas the associated mafic minerals remained crystalline. There is no reaction between adjacent minerals. 3) High-pressure polymorphs occur, such as coesite or stishovite. Coesite oc Curs exclusively within silica glass; it has not been observed as a reaction or breakdown product. 4) Nickel-iron spherules occur in the fused glass or impactites. 5) The occurrence of droplets of ilmenite, rutile, pseudobrookite, and baddaleyite in impactites indicates a temperature of formation exceeding 150 degrees C. 6) Dense glass occurs, similar in composition to bulk rock, in which iron oxide, such as fine particles of mag netite, is completely dissolved. All these features are characteristic of a process involving the rapid rise and fall of extremiiely high pressures and temperatures. Minerals and mineral as semblages experiencing such high strain rates and sudden changes of pressures and temperatures react and change in dependently to the bulk chemical com position, under nonequilibrium condi tions. Many aspects of shock features re quire careful study. Kink bands in biotite and deformation lamellae in quartz occur in tectonically deformed rocks. These features should be studied with great care in order to determine whether reduction in mean index of refraction and total birefringence along the planar structures have resulted from vitrification or phase transition; their presence is additional evidence in favor of a shock mechanism. Vitreous phases or glasses formed by shock also have many unique prop erties; they have not been studied by such methods as thermoluminescence, electron spin resonance, low-angle x ray diffraction, or infrared spectroscopy. Shock-fused glass of high density needs to be studied in detail in carefully con trolled laboratory conditions. Experimental shock-wave studies of the equation-of-state of single minerals and mineral assemblages, under care fully controlled conditions, must pre cede estimates of peak pressures and peak and residual temperatures of shocked natural mineral assemblages. Detailed petrographic and mineralogic studies, however, have provided useful and definitive criteria for characteriza tion of impact events. Such data should be of paramount importance in the study of samples brought back from Moon. 相似文献
6.
Ions typically pass with difficulty from water into organic phases because of water's superior solvation power. This inhibits such processes as ion transport in batteries or in lipid bilayers of cells. Ion penetration across such an interface was studied with unusual structural control. Hydronium ions were soft-landed at 1 electron volt on cold films of 3-methylpentane ("oil") on a metal substrate. The field produced by these ions drove them through the films when warmed. Coadsorption of water (0.14 to 35 bilayers) inhibited the ion penetration by creating a solvation energy trap. A Born solvation model successfully predicted the trapping energies (0 to 38 kilojoules per mole). 相似文献
7.
Evidence has been found that the earth is immersed in a large volume of glowing helium ions. This ionic glow, at 304 angstroms, is similar to the geocoronal hydrogen glow in that it extends to very high altitudes, but it is unique in that it is largely confined to the closed field line portion of the magnetosphere. Because of its magnetic containment, this ionic radiation is called the "magnetoglow." Observations of the magnetoglow from inside and outside the magnetospheric cavity promise to provide a valuable means of studying the structural dynamics of the magnetosphere. 相似文献
8.
Kroto H 《Science (New York, N.Y.)》1988,242(4882):1139-1145
Although carbon has been subjected to far more study than all other elements put together, the buckminsterfullerene hollow-cage structure, recently proposed to account for the exceptional stability of the C(60) cluster, has shed a totally new and revealing light on several important aspects of carbon's chemical and physical properties that were quite unsuspected and others that were not previously well understood. Most significant is the discovery that C(60) appears to form spontaneously, and this has particularly important implications for particle formation in combustion and in space as well as for the chemistry of polyaromatic compounds. The intriguing revelation that 12 pentagonal "defects" convert a planar hexagonal array of any size into a quasi-icosahedral cage explains why some intrinsically planar materials form quasi-crystalline particles, as appears to occur in the case of soot. Although the novel structural proposal has still to be unequivocally confirmed, this article pays particular attention to the way in which it provides convincing explanations of puzzling observations in several fields, so lending credence to the structure proposed for C(60). 相似文献
9.
Gas-phase ion chromatography can separate ions that have the same mass but differ in isomeric structure or electronic configuration. The main features of this technique are briefly outlined, and applications to a series of problems in transition metal chemistry and carbon cluster chemistry are described. Examples in transition metal chemistry include state-selective reactivity, excited state deactivation, and state-selective ligand binding energies. For clusters, ion chromatography was used to determine the structure of pure carbon cluster ions as a function of size from C(4) to C(84). The results indicate that carbon grows first in linear chains, transforms to monocyclic planar rings at about C(10), and forms new families of planar bi-, tri-, and tetracyclic rings at C(20), C(30), and C(40), respectively. Fullerenes, which mysteriously appear at C(30) and dominate by C(50), are generated by heating the planar ring systems above an isomerization barrier rather than by growth of graphite precursors. 相似文献
10.
Bowman KP 《Science (New York, N.Y.)》1988,239(4835):48-50
Satellite ozone data from the Total Ozone Mapping Spectrometer from 1979 through 1986 show that recent decreases of total ozone have not been confined to the Antarctic spring season (the Antarctic ozone hole), but are global in extent. The losses are about twice the estimated uncertainty in the satellite data. The decreases are largest in middle and high latitudes and occur in all seasons of the year. The decreases for this 8-year period are comparable in magnitude to the increases observed during the 1960s. Southern Hemisphere values from 1986 are generally greater than those from 1985. 相似文献
11.
Degenerate exciton systems have been produced in quasi-two-dimensional confined areas in semiconductor coupled quantum well structures. We observed contractions of clouds containing tens of thousands of excitons within areas as small as (10 micron)2 near 10 kelvin. The spatial and energy distributions of optically active excitons were determined by measuring photoluminescence as a function of temperature and laser excitation and were used as thermodynamic quantities to construct the phase diagram of the exciton system, which demonstrates the existence of distinct phases. Understanding the formation mechanisms of these degenerate exciton systems can open new opportunities for the realization of Bose-Einstein condensation in the solid state. 相似文献
12.
非饱和黄土的结构变化特性 总被引：16，自引：0，他引：16
以陕西杨凌原状黄土和人工结构性黄土为研究对象,通过非饱和黄土的压缩试验获取的基本资料,求取非饱和黄土在不同结构状态下的综合结构势参数,研究了黄土在压缩试验状态下的结构变化特性。结果表明,对于原状结构黄土,含水率对结构性参数影响明显,含水率愈低同一压力下对应的结构性参数愈大,尤其在低含水率区域,含水率变化强烈影响黄土的结构性;对于人工结构性黄土,黄土的结构性受密度、含水率和水泥含量的耦合因素影响,总体表现为干密度愈低、含水率愈小、水泥含量愈高,黄土的结构性愈强。 相似文献
13.
Idziak SH Safinya CR Hill RS Kraiser KE Ruths M Warriner HE Steinberg S Liang KS Israelachvili JN 《Science (New York, N.Y.)》1994,264(5167):1915-1918
An x-ray surface forces apparatus for simultaneously measuring forces and structures of confined complex fluids under static and flow conditions is described. This apparatus, combined with an intense synchrotron x-ray source, allows investigation of molecular orientations within a thin liquid crystal film confined between two shearing mica surfaces 3900 angstroms apart. The layer-forming smectic liquid crystal 8CB (4-cyano-4'-octylbiphenyl) adopted a series of distinct planar layer orientations, including the bulk flow-forbidden b orientation. 相似文献
14.
The confined electronic structure of nanoscale materials has increasingly been shown to induce behavior quite distinct from that of bulk analogs. Direct atomic-scale visualization of nanowires of zinc oxide was achieved through their unique pancake-type diffraction by using four-dimensional (4D) ultrafast electron crystallography. After electronic excitation of this wide-gap photonic material, the wires were found to exhibit colossal expansions, two orders of magnitude higher than that expected at thermal equilibrium; the expansion is highly anisotropic, a quasi-one-dimensional behavior, and is facilitated by the induced antibonding character. By reducing the density of nanowires, the expansions reach even larger values and occur at shorter times, suggesting a decrease of the structural constraint in transient atomic motions. This unanticipated ultrafast carrier-driven expansion highlights the optoelectronic consequences of nanoscale morphologies. 相似文献
15.
C(60) powders were shock-compressed quasi-isentropically and quenched from pressures in the range 10 to 110 GPa (0.1 to 1.1 Mbar). Recovered specimens were analyzed by Raman spectroscopy and optical microscopy. C(60) fullerenes are stable into the 13- to 17-GPa pressure range. The onset of a fast ( approximately 0.5 micros) reconstructive transformation to graphite occurs near 17 GPa. The graphite recovered from 27 GPa and about 600 degrees C is relatively well ordered with crystal planar domain size of about 100 A. Above 50 GPa a continuous transformation to an amorphous state is observed in recovered specimens. The fast transformation to graphite is proposed to occur by pi-electron rehybridization which initiates breakup of the ball structure and formation of the graphite structure at high density. 相似文献
16.
Bridge HS Bagenal F Belcher JW Lazarus AJ McNutt RL Sullivan JD Gazis PR Hartle RE Ogilvie KW Scudder JD Sittler EC Eviatar A Siscoe GL Goertz CK Vasyliunas VM 《Science (New York, N.Y.)》1982,215(4532):563-570
Results of measurements of plasma electrons and poitive ions made during the Voyager 2 encounter with Saturn have been combined with measurements from Voyager 1 and Pioneer 11 to define more clearly the configuration of plasma in the Saturnian magnetosphere. The general morphology is well represented by four regions: (i) the shocked solar wind plasma in the magnetosheath, observed between about 30 and 22 Saturn radii (RS) near the noon meridian; (ii) a variable density region between approximately 17 RS and the magnetopause; (iii) an extended thick plasma sheet between approximately 17 and approximately 7 RS symmetrical with respect to Saturn's equatorial plane and rotation axis; and (iv) an inner plasma torus that probably originates from local sources and extends inward from L approximately 7 to less than L approximately 2.7 (L is the magnetic shell parameter). In general, the heavy ions, probably O(+), are more closely confined to the equatorial plane than H(+), so that the ratio of heavy to light ions varies along the trajectory according to the distance of the spacecraft from the equatorial plane. The general configuration of the plasma sheet at Saturn found by Voyager 1 is confirmed, with some notable differences and additions. The "extended plasma sheet," observed between L approximately 7 and L approximately 15 by Voyager 1 is considerably thicker as observed by Voyager 2. Inward of L approximately 4, the plasma sheet collapses to a thin region about the equatorial plane. At the ring plane crossing, L approximately 2.7, the observations are consistent with a density of O(+) of approximately 100 per cubic centimeter, with a temperature of approximately 10 electron volts. The location of the bow shock and magnetopause crossings were consistent with those previously observed. The entire magnetosphere was larger during the outbound passage of Voyager 2 than had been previously observed; however, a magnetosphere of this size or larger is expected approximately 3 percent of the time. 相似文献
17.
Distribution of enkephalin immunoreactivity in germinative cells of developing rat cerebellum 总被引：4，自引：0，他引：4
The cellular distribution of enkephalin, an endogenous opioid, in the developing rat cerebellum was determined by immunocytochemistry. Methionine and leucine enkephalin were concentrated in the external germinal layer, a matrix of proliferative cells; staining was confined to the cortical cytoplasm. Enkephalin was not detected by immunocytochemistry in differentiated neural cells. These results indicate that endogenous opioids are involved specifically in early phases of nervous system development, particularly cell proliferation and differentiation. 相似文献
18.
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. 相似文献
19.
Newkome GR Wang P Moorefield CN Cho TJ Mohapatra PP Li S Hwang SH Lukoyanova O Echegoyen L Palagallo JA Iancu V Hla SW 《Science (New York, N.Y.)》2006,312(5781):1782-1785
Mathematics and art converge in the fractal forms that also abound in nature. We used molecular self-assembly to create a synthetic, nanometer-scale, Sierpinski hexagonal gasket. This nondendritic, perfectly self-similar fractal macromolecule is composed of bis-terpyridine building blocks that are bound together by coordination to 36 Ru and 6 Fe ions to form a nearly planar array of increasingly larger hexagons around a hollow center. 相似文献
20.
Knowing the behavior of a fluid in small volumes is essential for the understanding of a vast array of common problems in science, such as biological interactions, fracture propagation, and molecular tribology and adhesion, as well as pressure solvation and other geophysical processes. When a fluid is confined, its phase behavior is altered and excluded-volume effects become apparent. Pioneering measurements performed with the surface forces apparatus have revealed so-called structural or oscillatory solvation forces as well as the occurrence of a finite shear stress, which was interpreted as a solidification transition. Here, we report measurements obtained with an extended surface forces apparatus, which makes use of fast spectral correlation to gain insight into the behavior of a thin film of cyclohexane confined within attoliter volumes, with simultaneous measurement of film thickness and refractive index. With decreasing pore width, cyclohexane is found to undergo a drastic transition from a three-dimensional bulk fluid to a two-dimensional adsorbate with strikingly different properties. Long-range density fluctuations of unexpected magnitude are observed. 相似文献