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71.
A new method for six-membered ring annelation is described which involved reaction of hemiacetal vinylogs 1 with silyl enol ethers 2 in the presence of a Lewis acid. 相似文献
72.
Born-Oppenheimer (BO) potentials for the ground and first-excited electronic states of He2+ are determined using high level ab initio techniques for internuclear separations R of 1.2-100 bohrs and accurately fit to analytical functions. In the present formulation, the BO potentials are nuclear mass independent, and the corresponding BO approximation is obtained by ignoring four terms of the full rovibronic Hamiltonian. These four Born-Oppenheimer correction (BOC) terms are as follows: (1) mass polarization, (2) electronic orbital angular momentum, (3) first derivative with respect to R, and (4) second derivative with respect to R. In order to enable an exact rovibronic calculation, each of the four BOC terms are computed as a function of R, for the two electronic states and for their coupling, without any approximation or use of empirical parameters. Each of the BOC terms is found to make a contribution to the total energy over at least some portion of the range of R investigated. Interestingly, the most significant coupling contribution arises from the electronic orbital angular momentum term, which is evidently computed for the first time in this work. Although several BOC curves exhibit a nontrivial dependence on R, all are accurately fit to analytical functions. The resulting functions, together with the BO potentials, are used to compute exact rovibronic energy levels for 3He 3He+,3He 4He+) and 4He 4He+. Comparison to available high quality experimental data indicates that the present BOC potentials provide the most accurate representation currently available of both the low- and high-lying levels of the ground electronic state and the bound levels of the excited state. 相似文献
73.
Francis Carr Robert J. P. Corriu Agapit Kpoton Monique Poirier Grard Royo J. Colin YoungClaude Belin 《Journal of organometallic chemistry》1994,470(1-2):43-57
The application of dynamic NMR spectroscopy to the study of stereochemical non-rigidity in pentacoordinate chelated organosilicon compounds is described. It is shown that in the compounds Me2
iXYZ, non-dissociative ligand permutation at silicon can be distinguished unambiguously from processes associated with rupture of the chelate ring and nitrogen inversion. The crystal and molecular structure of 8-Me2NC10H6SiF3 has been determined. Pentacoordination of the silicon atom is confirmed, with the donor nitrogen atom and a fluorine atom occupying axial sites in an overall trigonal bipyramidal geometry. The N → Si separation is 2.3 Å (average of two distinct but closely related molecular conformations), which is less than the C1---C8 distance in the naphthalene nucleus, indicating a substantial bonding interaction. NMR studies of the dynamic behaviour of the Me2N group, and where possible (19F, 1H) of the monodentate ligands in 8-dimethylamino-1-silylnaphthalene compounds, together with the results for the chelated benzylaminosilicon compounds, confirm that inversion of the absolute configuration at the silicon atom is not achieved by this process. The free energies of activation for non-dissociative ligand permutation at a silicon range from less than 7 kcal mol−1 [SiH3, Si(OR)3], which is below the limit of direct measurement, to 13 kcal mol−1 for Me2NCH(Me)C6H4SiF3; difunctional silicon chelate compounds (Cl, F, OR) display values from 9–12 kcal mol−1. These are comparable with those determined for fluxional processes in acyclic pentacoordinate silicon compounds. 相似文献
74.
A new class of tungstate fluorophosphate glasses was identified in the NaPO3-BaF2-WO3 ternary system. The variation of several physical properties was determined with respect to chemical composition. Characteristic temperatures, density and refractive index increase as tungsten oxide content increases. The optical transmission range and specially the energy bandgap depend of the WO3 amount. No crystallization could be observed for the most WO3 concentrated vitreous samples (?20% molar). Color and optical properties of the glasses depend of the melting time because of the presence of reduced tungsten species like W5+ and W4+. In addition, photodarkening is observed in tungsten rich glass samples under UV laser illumination and this phenomenon can be reversible by heat treatment near the glass transition temperature. 相似文献
75.
A simple, customizable connectivity scheme is rigorously defined in which pairs of atoms are classified into three categories. The tools of graph theory are used to analyze the molecular graph and to efficiently find rings and ring assemblies through a combination of pruning and homeomorphic reduction. The definition of natural internal coordinates is extended in a nonredundant fashion for the various cases of weakly interacting components and for fused ring systems. The ring system coordinates were tested and found to be superior to Z-matrix coordinates. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 504–511, 1998 相似文献
76.
M A Poupart D R Cameron C Chabot E Ghiro N Goudreau S Goulet M Poirier Y S Tsantrizos 《The Journal of organic chemistry》2001,66(14):4743-4751
The NS3 serine protease enzyme of the hepatitis C virus (HCV) is essential for viral replication. Short peptides mimicking the N-terminal substrate cleavage products of the NS3 protease are known to act as weak inhibitors of the enzyme and have been used as templates for the design of peptidomimetic inhibitors. Automated solid-phase synthesis of a small library of compounds based on such a peptidomimetic scaffold has led to the identification of potent and highly selective inhibitors of the NS3 protease enzyme. 相似文献
77.
The sensitivity of mesh spacing on simulations of macrosegregation, particularly ‘freckles’, during vertical directional solidification of a superalloy in a rectangular mold was systematically analyzed to achieve accurate predictions in finite element calculations. It was observed that a coarser mesh spacing in the x‐direction horizontal tends to minimize the simulated macrosegregation, whereas a coarser mesh spacing in the y‐direction vertical artificially tends to make the system appear to have more macrosegregation. When solidification conditions either lead to a well‐established freckling case or to a well‐established non‐freckling case, the simulated results are not sensitive to the mesh spacing provided the elements are no larger than about 2d1 by 2 D/V and 3d1 by 4 D/V respectively, where d1 is the primary dendrite arm spacing, D is the diffusivity of the alloy solute with the smallest diffusivity in the liquid, and V is the growth rate. However, when solidification conditions are very close to the transition between freckling and no freckling, the simulated results are sensitive to the mesh spacing, especially in the y‐direction. Based on the mesh sensitivity analysis from the two‐dimensional simulations of rectangular castings of René N5, the mesh with element dimensions no larger than 2d1 in the x‐direction and 1.5 D/V in the y‐direction are recommended as the most stringent element size. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
78.
E.?Poirier R.?Chahine P.?BénardEmail author D.?Cossement L.?Lafi E.?Mélan?on T.K.?Bose S.?Désilets 《Applied Physics A: Materials Science & Processing》2004,78(7):961-967
The sorption of hydrogen on carbon structures and nanostructures offers a way to reduce the storage pressure of hydrogen with respect to compression storage while achieving interesting gravimetric storage densities. The most readily available carbon structures, activated carbons, can achieve reproducible, high gravimetric storage densities under cryogenic operating conditions: 5–6% at 35 bar and 77 K, in excess of the normal density that would be present in the pore volume under compression at the same temperature and pressure. We discuss and compare the adsorption of hydrogen on high specific surface activated carbons, nanofibres and nanotubes from experimental and theoretical considerations. In particular, we present gravimetric and volumetric hydrogen sorption measurements on single-walled carbon nanotubes (SWNTs) at (1 bar, 77 K) and (1 bar, 295 K) within the context of our ongoing work on the storage of hydrogen on activated carbon and carbon nanostructures. BET surface area and XRD characterization results on SWNTs are also presented. The experiments were performed on as received, chemically treated and metal-incorporated SWNT samples. Hydrogen sorption capacities measured on treated samples ranged from 0 to about 1 wt.% at 1 bar and 295 K and reached about 4 wt.% at 1 bar and 77 K. Our results show that under certain conditions, SWNTs have better hydrogen uptake performance than large surface area activated carbons. PACS 81.07.de; 81.05.Uw; 68.43.h 相似文献
79.
The solid-phase synthesis of 16-derivatives of 5-androstane-3,17-diol with one, two or three levels of molecular diversity was accomplished using the diethylsilyloxy linker. Libraries with one level of diversity (10 members) and two levels of diversity (40 members) were synthesized in a parallel fashion in good yields and acceptable HPLC purities for the majority of library members. Compounds with three levels of diversity (15 pools) were realized in a split and pool fashion to allow further deconvolution by the positional scanning method. The screening of the generated model libraries revealed interesting preliminary structure–activity relationships related to their antiproliferative activities on androgen-sensitive Shionogi cells. In the case of the two-level library, the presence of a hydrophobic amino acid at R1 (isoleucine (Ile) or phenylalanine (Phe)) and a six-membered ring (aromatic or not) at R2 seems an important requirement for activity. In the three-level library, the amino acid residues isoleucine and phenylalanine clearly provided a better antiproliferative activity than glycine (Gly) and proline (Pro). These model libraries will serve as basis for the generation of larger libraries of peptidosteroids toward the development of therapeutic agents. 相似文献
80.
G.T. Adylov F.K. Aliev D.Yu. Bardin W. Gajewski I. Ion B.A. Kulakov G.V. Micelmacher B. Niczyporuk T.S. Nigmanov E.N. Tsyganov M. Turala A.S. Vodopianov K. Wala E. Dally D. Drickey A. Liberman P. Shepard J. Tompkins J. Poirier 《Physics letters. [Part B]》1974,51(4):402-406
The pion radius has been measured by direct scattering of 50 GeV negative pions from stationary electrons in a hydrogen target. We find the square of the radius to be 〈rπ2〉 = (0.61 ± 0.15)fm2. 相似文献