Dynamics of Some He and Ar Clathrate Hydrates. Computer Simulation Study

Molecular dynamics simulation of solid solutions of He and Ar inice II at T 200 K has shown that amplitudes of water moleculeoscillations diminish when noble gas atoms fill the cavities ofthe hydrogen-bonded framework. The effect of Ar atoms ismore pronounced. Slow diffusion of He along trigonal axis isobserved when not all the cavities are filled. He and Ar atomsexert little effect on frequencies of translational and librationalvibrations of the water molecules. Type II empty gas hydrateframework (CSII) is quite stable at T 200 K. Amplitudes ofoscillations of water molecules which occupy differentcrystallographic positions are different. Filling of the cavitiesin the CSII framework with Ar atoms causes diminutionof the amplitudes of water molecule vibrations, and differencebetween amplitudes of vibration of molecules occupyingdifferent positions becomes less pronounced. Large cavities inthe CSII framework can accommodate two Ar atoms withoutdistortion. No diffusion of guest Ar atoms was observed at 200 Kin CSII framework.     

Noble gas clusters in model zeolite cavities

Noble gas atoms trapped in the intracrystalline cavities of zeolites may form clusters. A classical-mechanical isoenergetic molecular dynamics simulation is performed to simulate the dynamical behavior of noble gas clusters in zeolite cavities. To implement the simulation, a model is adopted of a homogeneous spherical cavity with Morse interaction between the noble gas atoms and cavity walls. The results for Ar₆ clusters indicate that the noble gas clusters in the cavity undergo the same solid/liquid phase changes as in free space, and, at high enough energies, a rapid exchange between atoms adsorbed on the inner surface and thosein the interior of the cavity. Mathematical quenching is used to investigated the multidimensional potential surface of Ar clusters in the cavity.     

Crystal structure and physical properties of a new CuTi2S4 modification in comparison to the thiospinel

A new modification of CuTi(2)S(4) was prepared from the elements at 425 degrees C. It crystallizes in the rhombohedral space group Rm, with lattice parameters of a = 7.0242(4) A, c = 34.834(4) A, and V = 1488.4(2) A(3) (Z = 12). Two topologically different interlayer regions exist between the close-packed S layers that alternate along the c axis: one comprises both Cu (in tetrahedral voids) and Ti atoms (in octahedral voids), and the second exclusively Ti atoms (again in octahedral voids). In contrast to the known modification, the spinel, Cu-Ti interactions of 2.88 A occur that have bonding character according to the electronic structure calculations. Both CuTi(2)S(4) modifications are metallic Pauli paramagnets due to Ti d contributions. The Pauli susceptibility of the Rm form is larger than that of the thiospinel in quantitative agreement with the LMTO-ASA band structure calculations. The irreversible transformation to the spinel takes place at temperatures above 450 degrees C.     

Bis-catecholate complexes of the IV group elements in the radical polymerization of styrene: ESR studies

The main features of the radical polymerization of styrene in the presence of germanium(IV) and tin(IV) bis-catecholate complexes were studied in the temperature range 70?C110 °C. Depending on the initial composition and structure, as well as on the reaction conditions, the complexes used can play the role of inhibitors or regulators directing the polymerization process. The ESR data in the styrene medium showed formation of germanium and tin o-semiquinolate derivatives, that indicates a direct involvement of the organoelement additives into the propagation step. The polymers synthesized in the presence of the bis-catecholate complexes of the IV group elements can be used as macroinitiators of styrene polymerization.     

A new HG/LT-GC/ICP-MS multi-element speciation technique for real samples in different matrices

An improved speciation technique is presented for metal(loid)organic compounds, enabling identification and quantification of species from twelve elements: germanium, arsenic, selenium, molybdenum, tin, antimony, tellurium, iodine, tungsten, mercury, lead and bismuth. At this time it is possible to identify 29 species with boiling points between -88.5 degrees C and 250 C in gaseous, liquid and solid samples in a few minutes. This study shows as an example results from measurements of soil samples from municipal waste deposits. The HG/LT-GC/ICP-MS-(hydride generation/low temperature-gas chromatography/inductively coupled plasma-mass spectrometry) apparatus contains a home-built gas chromatograph that enables satisfactory separation of various species with a boiling point difference of > or = 14 degrees C. The absolute detection limits for the elements mentioned above were below 0.7 pg.     

A new HG/LT-GC/ICP-MS multi-element speciation technique for real samples in different matrices

An improved speciation technique is presented for metal(loid)organic compounds, enabling identification and quantification of species from twelve elements: germanium, arsenic, selenium, molybdenum, tin, antimony, tellurium, iodine, tungsten, mercury, lead and bismuth. At this time it is possible to identify 29 species with boiling points between –88.5?°C and 250?°C in gaseous, liquid and solid samples in a few minutes. This study shows as an example results from measurements of soil samples from municipal waste deposits. The HG/LT-GC/ICP-MS-(hydride generation/low temperature-gas chromatography/inductively coupled plasma-mass spectrometry) apparatus contains a home-built gas chromatograph that enables satisfactory separation of various species with a boiling point difference of ≥ 14?°C. The absolute detection limits for the elements mentioned above were below 0.7 pg.     

Field ionization of high velocity neutral species. Rydberg states in noble gas atoms; the measurement of translational energy loss in neutralization-reionization mass spectra

The modification of a double-focusing mass spectrometer of BE geometry (VG-Analytical ZAB-2F) to permit the field ionization of fast atoms in high Rydberg states is described. Field ionization was achieved by means of a pair of closely spaced, very fine metal meshes with a (kV) potential difference between them. High Rydberg noble gas atoms were generated from their ions by electron transfer from noble gas targets. Also described is a method, involving a field ionization observation, for measuring the net kinetic energy loss associated with the collision-induced neutralization-reionization of polyatomic ions.     

Oxygen K-edge X-ray absorption fine structure studies of vacuum-deposited ice films

Multilayer ice films deposited on polycrystalline Au(111) substrates at 90 and 153 K under ultra-high-vacuum conditions are studied using O K-edge X-ray absorption fine structure spectroscopy in the surface-sensitive partial electron yield mode. Both near-edge and extended oscillatory fine structures are analyzed in combination with the theoretical real-space full-multiple-scattering simulations based on the FEFF8 code. The experimental data consistently indicate that the local structure of the near-surface regions in ice films at both substrate temperatures resembles that of high-density crystalline modifications of ice (e.g., ice II, ice III, or high-density amorphous ice). In addition, the ice films deposited at 153 K most probably contain a minor fraction of low-density cubic ice (I(c) phase).     

Coordination of a chiral tin(ii) cation bearing a bis(oxazoline) ligand with tetrahydrofuran derivatives

The reaction of SnCl(2) with the lithio derivative of a bis(oxazoline) ligand precursor afforded the enantiomeric chlorostannylene whose chloride ion can be substituted by several neutral or anionic Lewis donors. Abstraction of the chloride ion from the chlorostannylene with silver salts gave the corresponding tetrahydrofuran (THF) complexes of a chiral tin(ii) cation in 1,2-dimethoxyethane (DME) containing THF. That is, the reaction with silver hexafluoroantimonate (AgSbF(6)) afforded the THF complex without interaction with the counteranion. In contrast, reaction with silver triflate (AgOTf) gave the THF complex whose tin center had a pseudo-trigonal bipyramidal structure with two nitrogen atoms of a bidentate ligand and a lone pair at the equatorial positions and one of the oxygen atoms of triflate and an oxygen atom of THF at the apical positions in the solid state. Use of 3-methyltetrahydrofuran (3-MeTHF) instead of THF afforded the 3-MeTHF complexes, where the R-enantiomer of 3-MeTHF predominantly coordinates to the tin center. The previously reported germanium(ii) analogue of the tin(ii) cation indicated a similar enantioselectivity for the coordination of 3-MeTHF on the germanium center.     

石墨炉原子吸收光谱法测定锡和锗时基体改进剂硝酸钙的作用

石墨炉原子吸收光谱法测定低温易挥发元素锡和锗时灵敏度较低。本文以硝酸钙为基体改进剂,使锡和锗的灵敏度分别比不加基体改进剂时提高9倍和50倍,同时也显著提高灰化温度,并降低原子化温度。本文也探讨了基体改进剂硝酸钙对锡和锗的增感机理。锡的增感是由于固相和气相中钙的作用,而锗的增感仅是气相钙的作用。     

Thin Film Growth of Germanium Selenides from PECVD of GeCl<Subscript>4</Subscript> and Dimethyl Selenide

Plasma enhanced chemical vapor deposition (PECVD) of germanium selenide thin films from germanium tetrachloride and dimethyl selenide was studied to determine the viability of these reagents for thin film deposition. Germanium tetrachloride and alkylselenides were selected as candidates for these reactions due to their lower toxicities and higher availabilities compared to the more typical substitutes: germane and hydrogen selenide in the formation of germanium selenides. Dimethyl selenide was used successfully for the deposition of germanium selenides. Variation in film stoichiometry was observed by the modification of reactant gas flow ratios. Relative mass flow rates were varied in order to determine their effect on germanium chalcogenide deposition, and the effect of these flow rate modifications on the film thickness, structural properties, and composition are reported.     

Emissionsspektrographische bestimmung von germanium und zinn in galliumarsenid

An emission spectrographic determination of germanium and tin in gallium arsenide is described. For a sample of 5 mg the limit of detection is 3·1 ppm germanium and 7·7 ppm tin. The relative standard deviations are 11% for germanium and 16% for tin. The evaporation of the elements during the excitation is discussed.     

HG/LT-GC/ICP-MS coupling for identification of metal(loid) species in human urine after fish consumption

Human urine samples after fish consumption have been investigated by low-temperature gas chromatography with inductively coupled plasma mass spectrometric detection after sample derivatization by hydride generation (HG/LT-GC/ICP-MS). This analytical technique enabled the identification of organometal(loid) compounds in human urine; species of the six elements germanium, arsenic, selenium, tin, antimony, and mercury were determined. Three different organic selenium species, two germanium species, seven arsenic species, four tin species, five antimony species, and one species of mercury were found; 18 of the 22 species detected could be identified. The relative detection limits ranged between 2 and 12 pg x L(-1) (x=element). These organometal(loid) compounds probably build up in the human body under the influence of micro-organisms, in the presence of hydrogen sulfide and methane, in the human intestine.     

Composition of gas hydrates of CHClF2, CCl2F2 and SF6

Gas hydrates are forms of ice stabilized by the presence of molecules of gas occupying cavities in the solid water lattice. There are two common forms: structure I and structure II. The mean free diameters of the two types of cavities in structure I are about 5.0 and 5.8 angstroms. Very small gas molecules such as Xe or H₂S can occupy both. In the past it has been considered that gas molecules of larger diameter than 5.0 angstroms could not occupy the smaller cavities. It has now been shown through measurement of hydration numbers of CHClF₂ (diameter about 5.4 angstroms) under various pressures at 0°, that some of the small cavities are filled. This state of affairs also exists for CH₃Br. In structure II, the mean diameters of the two types of cavities are about 5.0 and 6.6 angstroms. Hydration numbers of SF₆ (diameter about 5.8 angstroms) and CCl₂F₂ (about 6.2 angstromsm) show that nearly all of the large cavities but essentially none of the small cavities are occupied.     

Carbosilanes

Carbosilanes are compounds in which the elements carbon and silicon occupy alternate positions in the molecular framework. Formal replacement of every second carbon atom in the diamond lattice by a silicon atom gives silicon carbide, which exists, however, in several modifications characterized by different stacking orders. The SiC lattice is the basis of most carbosilanes.^[1] These are divided on the basis of structural differences into carborundanes, silascaphanes, and molecules that no longer contain elements of the silicon carbide structure. In the carborundanes the Si–C six-membered rings are arranged in the chair form, in the silascaphanes in the boat form. The molecular framework of the third group is derived from barrelane and asterane. The reactivity of the carbosilanes is determined mainly by the substituents on the skeletal atoms. The presence of SiH or Si–halogen groups leads to a high reactivity, while derivatives with organic groups on the silicon atoms and CH₂ or CH groups in the skeleton are much less reactive. However, even such CH₂ and CH groups permit certain characteristic reactions. Because of their structural features and the range of substituents that can be introduced, the carbosilanes are presently of considerable interest with respect to technological developments as precursors for ceramics.     

Structural Investigations of Argon Hydrates at Pressures up to 10 kbar

Neutron diffraction patterns of three argon hydrates which exist at the pressures up to 10 kbar has been studied; Rietveld refinement of their structures has been done. The phase which is stable from 1 bar to 4.6 kbar appears to be typical cubic structure II gas hydrate with variable degree of filling of the large cavities. Stoichiometry of this compound under high-pressure conditions has been determined for the first time and appears to be ArW4.5H₂O and Ar·4H₂O at 3.4 and 4.3 kbar, respectively. Gas hydrate existing in the pressure range of 4.6–7.7 kbar has a hexagonal structure (hexagonal structure III, so-called structure H). Refinement of the structure has shown that the best agreement between calculated and experimental pattern can be reached in the case of accommodation of five (!) argon atoms in the large cavity. Indexing of the neutron diffraction pattern of the hydrate stable in the 7.7–9.5 kbar range leads to the primitive tetragonal unit cell with parameters a = 6.342 Å, c = 10.610 Å at 9.2 kbar, which does not correspond to any known type of gas hydrates. The water framework of this structure was found by idealizing the structure of pinacol semiclathrate hydrate. This hydrate belongs to a new, earlier unknown, tetragonal structural type of gas hydrates. It contains only one type of polyhedral cavities with 14 faces. This type of polyhedrons are space-filling; two argon atoms occupy each cavity. This structure gives the first example of the gas hydrate water framework which contains only one type of polyhedral cavities.     

轻元素调控的贵金属催化剂在能源相关领域的应用

贵金属广泛用于多相催化研究,对于诸多具有重要科学意义和工业应用价值的化学反应展现出优异的催化活性和选择性.引入轻合金元素(如C,H,B和N),可以调控贵金属的晶体结构和电子性质,是进一步提高贵金属催化性能的重要策略.与传统的金属合金催化剂相比,这种轻元素合金化的催化剂具有一些独特性:(1)轻元素由于原子尺寸很小,容易溶于金属晶格的间隙位点;(2)一些轻元素(如C,N和S)的电负性与金属的差别很大,能够在相邻原子间引起较大的电荷转移;(3)轻元素-金属合金中的电子相互作用主要由金属的d轨道和轻元素的sp轨道杂化主导,这与金属合金中的d-d轨道杂化显著不同.这些独特性为贵金属原子结构和电子结构的调控以及催化性能的优化带来了更多的可能性.轻合金元素研究的主要瓶颈在于其原子尺寸小、分布不均匀、难以直接观察和精准控制,从而限制了对活性提升机制和构效关系的研究.近几十年来,纳米合成技术和材料表征技术的长足发展使得轻合金元素改性的催化剂研究渐入佳境.此外,计算化学在结构分析和催化应用中的日趋成熟为揭示轻合金元素对贵金属晶体结构、电子结构和催化性质的调控作用提供了有力工具.本文综述了引入轻合金元素改性的贵金属催化剂在不同催化应用中的主要研究进展,总结了贵金属催化性能的主要影响因素(包括轻合金元素的种类、位置、浓度和有序度等),阐述了轻合金原子如何影响催化反应性能,介绍了轻元素的实验引入策略以及揭示轻元素合金效应的实验表征和理论研究方法.重点讨论了不同轻合金原子改性的贵金属基催化剂在催化反应中的广泛应用,并试图建立其结构特征与催化性能之间的密切联系.总的来说,轻合金原子的活性调控作用主要表现在以下几个方面:(1)晶相转变:轻元素的引入能够改变金属原子的堆积模式,产生有利于催化反应的晶相结构;(2)电荷转移:轻元素和母体金属的电负性差异能够导致电荷重新分布,影响金属的电子结构;(3)应力效应:轻元素的引入会导致金属晶格膨胀,产生拉伸应力,引起电子结构变化;(4)配体效应:轻元素的sp轨道和金属的d轨道杂化,引起d带中心下移,降低表面吸附性质;(5)集团效应:轻元素的引入能够孤立金属原子,产生特定的表面金属位点,有利于促进催化反应;(6)次表面化学:在氢相关的催化反应中,次表面的间隙轻元素能够阻止氢的渗入,抑制活性衰减或不利的副反应发生.最后,本文对于当前该领域存在的挑战和未来的发展前景进行了分析,以期促进该合金体系的合成、理解和催化应用,内容包括:(1)开发更精确可控的轻元素掺入策略;(2)合理阐明轻合金元素与宏观催化性能之间的关系;(3)发展新型的轻元素改性催化剂;(4)扩展轻元素改性催化剂的催化应用范围.     

多体展开势能函数在碳族元素原子簇研究中的应用III. 锗原子族的结构和相对稳定性

基于从晶体锗确立的多体展开势能函数, 本文通过坐标完全优化, 发现小的锗原子簇分子(Ge~2~Ge~14)倾向于形成密堆积结构, 表面原子分布以蝶形四元环(D~2d)为主; 常见立方晶体“微观晶体碎片”的分层优化结果表明, 在Ge~15~Ge~100范围内, 多数壳层的原子到分子中心的距离均受到压缩, 且以畸变的简单立方、面心立方及体心立方较为稳定; 在这些畸变密堆积结构中, 表面原子向内压缩最为严重, 使整个分子趋于球形化。较为开放的金刚石类层状原子族只有当所含原子数达数百以上时才可能相对更为稳定。     

A Novel High-Pressure Tin Oxynitride Sn2N2O

We report the first oxynitride of tin, Sn₂N₂O (SNO), exhibiting a Rh₂S₃-type crystal structure with space group Pbcn. All Sn atoms are in six-fold coordination, in contrast to Si in silicon oxynitride (Si₂N₂O) and Ge in the isostructural germanium oxynitride (Ge₂N₂O), which appear in four-fold coordination. SNO was synthesized at 20 GPa and 1200–1500 °C in a large volume press. The recovered samples were characterized by synchrotron powder X-ray diffraction and single-crystal electron diffraction in the TEM using the automated diffraction tomography (ADT) technique. The isothermal bulk modulus was determined as B_o=193(5) GPa by using in-situ synchrotron X-ray diffraction in a diamond anvil cell. The structure model is supported by DFT calculations. The enthalpy of formation, the bulk modulus, and the band structure have been calculated.     

Ab‐Initio study of Helium‐small carbon cage systems

The traditional method for opening the electron shells of noble gas atoms involves a strong electronegative atom or group. However, this approach is limited to only heavy noble‐gas atoms, such as Kr and Xe. In this paper, we performed accurate calculations of He@C₈H₈ and He@C10H16 and showed the possibility of opening the electron shell of a light noble‐gas atom. © 2012 Wiley Periodicals, Inc.