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971.
Brian Harbourne 《Transactions of the American Mathematical Society》1997,349(3):1191-1208
A determination of the fixed components, base points and irregularity is made for arbitrary numerically effective divisors on any smooth projective rational surface having an effective anticanonical divisor. All of the results are proven over an algebraically closed field of arbitrary characteristic. Applications, to be treated in separate papers, include questions involving: points in good position, birational models of rational surfaces in projective space, and resolutions for 0-dimensional subschemes of defined by complete ideals.
972.
Forman Mark A. Zanoni Brian C. Chopko Emily C. Carroll Patrick J. 《Structural chemistry》1998,9(1):27-32
The synthesis and X-ray crystal structure of the p-bromoanilide derivative of bicyclo[2.2.0]hexane-l-carboxylic acid (1), N-(4-bromophenyl)-bicyclo[2.2.0]hexane-l-carboxamide (2), is reported. N-(4-Bromophenyl)-bicyclo[2.2.0]hexane-l-carboxamide is synthesized in good yield via the DCC coupling of bicyclo[2.2.0]hexane-l-carboxylic acid (1) with p-bromoaniline. Low-temperature X-ray analysis of 2 reveals that the
bonds of the bicyclo[2.2.0]hexane skeleton vicinal to the amide group show a slight lengthening due to conjugative interaction with the -accepting amide group. Ab initio calculations at the 6-31G* level of theory on bicyclo[2.2.0]hexane-l-carboxamide, a model for 2, and for other 1-substituted bicyclo[2.2.0]hexanes are also reported. 相似文献
973.
This paper is concerned with the numerical simulation of a random walk in a random environment in dimension d = 2. Consider a nearest neighbor random walk on the 2-dimensional integer lattice. The transition probabilities at each site are assumed to be themselves random variables, but fixed for all time. This is the random environment. Consider a parallel strip of radius R centered on an axis through the origin. Let X
R be the probability that the walk that started at the origin exits the strip through one of the boundary lines. Then X
R is a random variable, depending on the environment. In dimension d = 1, the variable X
R converges in distribution to the Bernoulli variable, X
= 0, 1 with equal probability, as R . Here the 2-dimensional problem is studied using Gauss-Seidel and multigrid algorithms. 相似文献
974.
William Clegg Dale R. Harbron Brian P. Straughan 《Journal of chemical crystallography》1990,20(1):17-22
The crystal structure of [Zn(O2CC(CH3)CHCH3)0.54(O2CC6H5)1.46]x has been determined by single crystal X-ray diffraction. The compound crystallizes in the monoclinic system, space group P21/c witha=10.311(1),b=13.170(1),c=19.000(2) Å,=91.895(7)°,V=2578.7 Å3,Z=4,D
x=1.525g cm–3, (Mo K)=0.71073 Å,=1–95 mm–1, F(000)=1206,T=295K,R=0.046 for 4548 unique reflections. The two different carboxylates are randomly scrambled along a one dimensional polymer, made up of binuclear units connected by a syn-anti carboxylate bridge. Each binuclear unit contains three syn-syn carboxylates which bridge two zinc atoms. Vibrational data indicate that [Zn(O2CC(CH3)CHCH3)2]x has the same polymeric structure; orientation of tiglate is the same as for the mixed ligand compound. Recrystallization of mixed carboxylate species produces either an alteration in carboxylate ligand ratio, or a total separation of the two carboxylates, giving two different compounds. 相似文献
975.
Ge2.5PSx glasses were studied with a combination of Raman spectroscopy, nuclear magnetic resonance, and neutron diffraction. From these experiments the distribution of bonding configurations was determined, and used to explain the compositional dependence of the index of refraction and the glass transition temperature. On reducing the sulfur content of these glasses below the stoichiometric amount, the sulfur deficit is accommodated by the progressive loss of the non-bridging sulfur of SPS3/2 groups, followed by the conversion of the resultant PS3/2 groups into species such as P4S3 characterized by P-P bonding. The presence of metal-metal bonds involving germanium, found in samples with the lowest sulfur content, was found to be the most important structural feature in determining the optical response. 相似文献
976.
Cullen T. Vogelson Christopher L. Edwards Andrew N. Kobylivker Star B. Chacko Cristin E. Moran Kathryn Dalton Shawn M. Stewart Brian C. Werner Simon G. Bott Andrew R. Barron 《Journal of chemical crystallography》1998,28(11):815-824
The molecular structures of Al(tfac)3 (1), Co(tfac)3 (2) (H-tfac = 1,1,1-trifluoroacetylacetone) and Cu(H2O)(fod)2 (3) (H-fod = 1,1,1,2,2,3,3-hepta-fluoro-7,7-dimethyloctane-4,6-dione) have been determined. The metal coordination spheres in compounds 1 and 2 are essentially the same as the respective M(acac)3 derivatives. Despite the isomorphous nature of the structures of compounds 1 and 2, the identity of the nearest intermolecular van der Waals contacts are altered by minor changes in the metal coordination sphere. The geometry about copper in compound 3 is close to that of an ideal square bipyramid with the -diketonate ligands occupying the basal plane. The water ligand in each molecule of compound 3 is hydrogen bonded to an oxygen of a -diketonate ligand on an adjacent molecule resulting in the formation of dimers, which form rods along the y-axis due to weak C–F···Cu interactions. Crystal data: (1) orthorhombic, Pca21, a = 14.949(3), b = 19.806(4), c = 13.624(3) Å, V = 4033(1) Å3, and Z = 8, and (2) orthorhombic, Pca21, a = 14.930(3), b = 19.620(4), c = 13.540(3) Å, V = 3966(1) Å3, and Z = 8,; (3) monoclinic, P21/c, a = 12.447(3), b = 10.486(2) c = 21.980(4) Å, = 102.65(3)°, V = 2799(1) Å3, and Z = 4. 相似文献
977.
A highly diastereoselective intramolecular hydroamination is the key step in a modular synthesis of 2,6-disubstituted piperazines. The requisite hydroamination substrates were prepared in excellent yields by nucleophilic displacement of cyclic sulfamidates derived from amino acids. A variety of alkyl and aryl substituents at the 2-position were tolerated. The stereochemistry of the piperazines was determined to be trans by X-ray crystallography, which also showed the preferred conformation of the 2,6-disubstituted piperazine to be a twist-boat due to A(1,3) strain. 相似文献
978.
Sadar MD Williams DE Mawji NR Patrick BO Wikanta T Chasanah E Irianto HE Soest RV Andersen RJ 《Organic letters》2008,10(21):4947-4950
The new chlorinated peptides sintokamides A to E (1-5) have been isolated from specimens of the marine sponge Dysidea sp. collected in Indonesia. Their structures were elucidated by a combination of spectroscopic and single-crystal X-ray diffraction analyses. Sintokamide A (1) is an inhibitor of N-terminus transactivation of the androgen receptor in prostate cancer cells. 相似文献
979.
Anderson BJ Guino-o MA Glueck DS Golen JA DiPasquale AG Liable-Sands LM Rheingold AL 《Organic letters》2008,10(20):4425-4428
Enantioselective tandem alkylation/arylation of primary phosphines with 1-bromo-8-chloromethylnaphthalene catalyzed by Pt(DuPhos) complexes gave P-stereogenic 1-phosphaacenaphthenes (AcePhos) in up to 74% ee. Diastereoselective formation of four P-C bonds in one pot with bis(primary) phosphines gave C2-symmetric diphosphines, including the o-phenylene derivative DuAcePhos, for which the rac isomer was formed with high enantioselectivity. These reactions, which appear to proceed via an unusual metal-mediated nucleophilic aromatic substitution pathway, yield a new class of heterocycles with potential applications in asymmetric catalysis. 相似文献
980.
Markt P McGoohan C Walker B Kirchmair J Feldmann C De Martino G Spitzer G Distinto S Schuster D Wolber G Laggner C Langer T 《Journal of chemical information and modeling》2008,48(8):1693-1705
The cysteine protease cathepsin S (CatS) is involved in the pathogenesis of autoimmune disorders, atherosclerosis, and obesity. Therefore, it represents a promising pharmacological target for drug development. We generated ligand-based and structure-based pharmacophore models for noncovalent and covalent CatS inhibitors to perform virtual high-throughput screening of chemical databases in order to discover novel scaffolds for CatS inhibitors. An in vitro evaluation of the resulting 15 structures revealed seven CatS inhibitors with kinetic constants in the low micromolar range. These compounds can be subjected to further chemical modifications to obtain drugs for the treatment of autoimmune disorders and atherosclerosis. 相似文献