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91.
Ahmed M. Assaf 《组合设计杂志》1993,1(6):453-458
A (v, k. λ) covering design of order v, block size k, and index λ is a collection of k-element subsets, called blocks, of a set V such that every 2-subset of V occurs in at least λ blocks. The covering problem is to determine the minimum number of blocks, α(v, k, λ), in a covering design. It is well known that $ \alpha \left({\nu,\kappa,\lambda } \right) \ge \left\lceil {\frac{\nu}{\kappa}\left\lceil {\frac{{\nu - 1}}{{\kappa - 1}}\lambda} \right\rceil} \right\rceil = \phi \left({\nu,\kappa,\lambda} \right) $, where [χ] is the smallest integer satisfying χ ≤ χ. It is shown here that α (v, 5, λ) = ?(v, 5, λ) + ? where λ ≡ 0 (mod 4) and e= 1 if λ (v?1)≡ 0(mod 4) and λv (v?1)/4 ≡ ?1 (mod 5) and e= 0 otherwise With the possible exception of (v,λ) = (28, 4). © 1993 John Wiley & Sons, Inc. 相似文献
92.
Ahmed Shalabi 《中国化学会会志》1990,37(3):247-253
Ab initio and MNDO calculations of VSEPR model were carried out on CH2, CH2+, CH2?, SiH2, SiH2+ and SiH2?. Comparisons between the second row carbon and its third row silicon analogue as unshared electron pair contributors are considered. The repulsion effects as well as the volume requirement of the unshared electron pair on several structural and energetic properties arc investigated. 相似文献
93.
Summary A sensitive and specific method for quantitation of the steroids betamethasone, prednisolone and cortisone acetate commonly used as adulterants in locally produced herb extracts and in certain homeopathic drugs is described. Reverse-phase liquid chromatography with UV detection has been used. 相似文献
94.
[reaction: see text] Diastereoselective hydroborations of enantiopure 3,6-dihydro-2H-1,2-oxazines led to dihydroxy-substituted 1,2-oxazines. Samarium diiodide-induced N-O bond cleavage generated 1,4-amino alcohols which were recyclized to polyhydroxylated pyrrolidines which are potential glycosidase inhibitors. 相似文献
95.
Gamil A. Al-Hazmi Mohammed S. El-Shahawi Ahmed A. El-Asmy 《Transition Metal Chemistry》2005,30(4):464-470
The redox properties of the title mono- and binuclear copper(II) chelates have been investigated by cyclic voltammetry in DMF at a working platinum electrode. The cathodic reduction and anodic oxidation of the investigated chelates produced the corresponding electrochemical CuI and CuIIIspecies stable only in the voltammetric time scale, The effects of substituents on E1/2, redox properties and stability towards oxidation of the complexes were related to the electron-withdrawing or releasing ability of the substituents on the C=N1[H, CH3 or C6H5] and/or N4H [H, C2H5, C6H5 or pClC6H4] groups, The electron attracting substituents stabilize the Cu(II) complexes while electron-donating groups favor oxidation to Cu(III). Changes in the E1/2 for the complexes due to remote substituent effects could be related to changes in basicity of N4H.Thus, variation in N41-J has more influence on E1/2 than changes in C=N1. The correlation between E1/2 of the complexes and pKa of the ligands has been attributed to the spherical potential generated by the electron density of the donor atoms at the antibonding d orbitals. 相似文献
96.
[reaction: see text] N-Acylsulfonamides usually react with nucleophiles by acyl transfer and C-N bond fission. However, the hydrolysis of N-acyl beta-sultams is a sulfonyl transfer reaction that occurs with S-N fission and opening of the four-membered ring. Similar to other beta-sultams, the N-acyl derivatives are at least 10(6)-fold more reactive than N-acyl sulfonamides. 3-Oxo-beta-sultams are both beta-lactams and beta-sultams but also hydrolyze with preferential S-N bond fission. 相似文献
97.
Assefa Z Omary MA McBurnett BG Mohamed AA Patterson HH Staples RJ Fackler JP 《Inorganic chemistry》2002,41(24):6274-6280
The structures and temperature-dependent photoluminescence properties of the one-dimensional compounds [(TPA)(2)Au][Au(CN)(2)], 1, and (TPA)AuCl, 2, are reported. An extended linear chain with weak Au.Au interactions along the c-axis is evident in the structure of 1, and a helical chain with a pitch of 3.271 A is seen for 2. The intrachain Au...Au separation is 3.457(1) and 3.396(2) A in 1 and 2, respectively. As a result of this weak Au...Au interaction, the physical properties of these compounds are anisotropic. Scanning electron microscopy (SEM) studies indicate that single crystals of both compounds are noninsulating. Single crystals of 1 do not luminesce visibly, but grinding the crystals finely initiates a strong green emission under UV irradiation at room temperature. Further interesting optical properties include the dependence of the emission profile of the powder on the exciting wavelength and luminescence thermochromism. When excited at wavelengths < 360 nm, the powder exhibits a blue emission at 425 nm while excitation with longer wavelengths leads to a green emission near 500 nm. While the green emission dominates at ambient temperature, cooling to cryogenic temperatures leads to the dominance of the blue emission. Fibers of 2 are luminescent at 78 K with an emission band centered at 580 nm. Compound 1 crystallizes in the orthorhombic space group Cccm (No. 66), with Z = 2, a = 6.011(1) A, b = 23.877(6) A, c = 6.914(1) A, V = 992.3(3) A(3), and R = 0.0337. Compound 2 crystallizes in the trigonal space group R3 (No. 148), with Z = 18, a = 22.587(2) A, b = 22.587(2) A, c = 9.814(2) A, V = 4336 A(3), and R = 0.0283. 相似文献
98.
Ahmed A. El-Asmy Yehia M. Shaibi Abdallah S. Babaqi Mohamed Mounir Salah A. Ashour 《Transition Metal Chemistry》1988,13(5):332-335
Summary The interaction of 1-benzoin-4-phenylthiosemicarbazone (H2 BPS) with some transition metal ions has been investigated. The ligand can function as a tridentate chelating agent, giving M(HBPS)2 and M(BPS). Potentiometric studies proved that the mechanism of chelation is based on hydrogen ion liberation. Spectral studies in solution show that the ligand could be used for the microdetermination of CuIIions. On the basis of magnetic and spectral data, an octahedral structure is proposed for the CoII and NiII complexes and a square-planar structure for the CuII complex. The corrosion inhibition of aluminium in Cl3CCO2H using H2BPS is studied. The electrical conductivity of H2BPS and of its complexes have been measured. The ligand shows an activation energy in the range of semiconducting materials. The antimicrobial activity of all compounds has also been demonstrated. 相似文献
99.
Zafar Ahmed 《Tetrahedron》2005,61(8):2055-2063
A number of pulvinic acid natural products were prepared based on Suzuki cross coupling reactions of α-hydroxy-γ-alkylidenebutenolides which are readily available by cyclization of 1,3-bis-silyl enol ethers with oxalyl chloride. The formal total synthesis of pulvinic acid, atromentic acid, gomphidic acid and of 4-hydroxypulvinic acid, 4′-hydroxypulvinic acid and iso-gomphidic acid are reported. In addition, total syntheses of pinastric acid, xerocomic acid and variegatic acid were accomplished. 相似文献
100.
Summary 3,4-Dicyano-5-aminopyrazole, H3,4(CN)25NH2pz (L) reacts either with anhydrous MCl2 or with [M(PPh3)2Cl2] to yield ML4Cl2 complexes (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd or Hg), whose monomeric and covalent natures have been confirmed by their solubility in most non-polar solvents and their low electrical conductivities. The bonding mode of substituted pyrazole is inferred from the position of the (C-N) band in the i.r. spectra. The electronic spectra and the magnetic moments of these compounds were recorded. 相似文献