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排序方式: 共有195条查询结果,搜索用时 515 毫秒
91.
92.
Abdel-Sattar S. Hamad Dieter Schinzer 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):187-199
Abstract (R) and (S) of 2-Methy1-3-oxa-5-(tert-butyldiphenylsilyloxyl)methy1pentanoate (4a. 6a) and (R) and (S) of 2-methy1-3-oxa-5-(tert-butyldimethylsilyloxyl)methylpentanoate (4b, 6b) were synthesized by the reaction of the preparative compounds (2a, 2b) with (R) and (S) methy1lactate (3, 4) in the presence of a silver (1) oxide catalyst (Method B). The title compounds are useful for the synthesis of different natural products such as epothilones. 相似文献
93.
S. Munavalli D. K. Rohrbaugh F. J. Berg L. R. McMahon F. R. Longo H. D. Durst 《Phosphorus, sulfur, and silicon and the related elements》2013,188(5):1117-1125
The reaction of 1,5-cyclooctadiene with F 3 CSCl at m 80;C has been examined and found to furnish both di- and tetrasubstituted adducts. Their mass spectra show the presence of the intact cyclooctyl ring. However, photolyis of a solution of 1,5-cyclooctadiene and F 3 CSCl in dry pentane yields addition, isomerization and dimerization as well as free radical products. The rationalization of the formation of the above products along with their mass spectral characterization is described in this communication. 相似文献
94.
Isoplagiochin D is a ring‐strained macrocyclic bisbibenzylis, which showed stable axial chirality in one biaryl structure, and semistable axial chirality in the other biaryl moiety. We reported here an unprecedented example for the catalytically asymmetric synthesis of ring‐strained atropisomers via Pd‐catalyzed macrocyclization between benzyl halides and carbenes. This newly developed Pd‐catalyzed asymmetric macrocyclization protocol enabled us a quick synthesis of isoplagiochin D in a highly enantioselective manner. 相似文献
95.
A well-established generalization of graph coloring is the concept of list coloring. In this setting, each vertex v of a graph G is assigned a list L(v) of k colors and the goal is to find a proper coloring c of G with c(v)∈L(v). The smallest integer k for which such a coloring c exists for every choice of lists is called the list chromatic number of G and denoted by χl(G).We study list colorings of Cartesian products of graphs. We show that unlike in the case of ordinary colorings, the list chromatic number of the product of two graphs G and H is not bounded by the maximum of χl(G) and χl(H). On the other hand, we prove that χl(G×H)?min{χl(G)+col(H),col(G)+χl(H)}-1 and construct examples of graphs G and H for which our bound is tight. 相似文献
96.
研究了用火焰原子吸收光谱法在同一溶液体系中直接快速测定金川铜镍矿浮选产品中铜、镍、镁含量新的新方法。考察了产品中高含量的SiO2对测定镁的干扰情况,并提出了排除这项干扰简便方法,根据各元素的标准溶液系列的测定数据,推算出了铜、镍、镁3元素的线性回归方程及线性相关系数,特征浓度铜为0.40mg.L^-1/1%,镍为0.48mg.L^-1/1%,Mg为0.018mg.L^-1/1%;线性浓度范围铜,镍均为0-10mg.L^-1,镁为0.2-1.2mg.L^-1。相对标准偏差铜为2.5%(n=6),镍为2.3%(n=6),镁为2.8%(n=6),加标回收率铜为96.5%-102.5%,镍为98.8%-103.5%,镁为98.2%-102.5%,这种方法操作简便,快速,实用,具有较好的精密度和准确度,实际产品分析结果与外检单位及金川公司测定的结果吻合。 相似文献
97.
白金制品的X射线荧光检验 总被引:1,自引:0,他引:1
应用含量归一原理,解决样品形状和大小不确定的问题,可用X射线荧光法直接测定白金制品成色。 相似文献
98.
Chaohui ZHANG 《数学年刊B辑(英文版)》2011,32(6):885-894
Let S be a Riemann surface that contains one puncture x. Let ℐ be the collection of simple closed geodesics on S, and let ℱ denote the set of mapping classes on S isotopic to the identity on S ∪ {x}. Denote by t
c
the positive Dehn twist about a curve c ∈ ℐ. In this paper, the author studies the products of forms (t
b
−m
∘ t
a
n
) ∘ f
k
, where a, b ∈ ℐ and f ∈ ℱ. It is easy to see that if a = b or a, b are boundary components of an x-punctured cylinder on S, then one may find an element f ∈ ℱ such that the sequence (t
b
−m
∘ t
n
a
) ∘ f
k
contains infinitely many powers of Dehn twists. The author shows that the converse statement remains true, that is, if the
sequence (t
b
−m
∘ t
a
n
) ∘ f
k
contains infinitely many powers of Dehn twists, then a, b must be the boundary components of an x-punctured cylinder on S and f is a power of the spin map t
b
−1 ∘ t
a
. 相似文献
99.
吴群英 《数学年刊A辑(中文版)》2014,35(2):225-234
考虑独立但不同分布的正随机变量序列,建立了关于部分和乘积的弱不变原理的一个普遍性结果,推广和改进了由Matula和Stepien在2009年以及Rempala和Wesolowski在2002年获得的弱不变原理. 相似文献
100.
In this contribution the influence of various stresses and their combined impact on the electrostatic charging tendency of oil is studied. Various physicochemical properties were measured according to ASTM Standards to detect changes in oil quality. A free radical reagent, 2.2 diphenyl-1-picrylhydrazyl (DPPH), was added to oil before and after the application of stresses to determine free radical concentration. The results obtained show that the application of stresses contributes to an increase in the electrification current. These results also demonstrate that electrification current is affected by the quantity of the free radicals. 相似文献