首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   99篇
  免费   3篇
化学   90篇
力学   1篇
数学   1篇
物理学   10篇
  2020年   1篇
  2016年   1篇
  2014年   3篇
  2013年   3篇
  2012年   3篇
  2011年   14篇
  2010年   5篇
  2009年   3篇
  2008年   5篇
  2007年   5篇
  2006年   6篇
  2005年   5篇
  2004年   9篇
  2003年   10篇
  2002年   7篇
  2001年   4篇
  2000年   2篇
  1999年   4篇
  1998年   1篇
  1997年   2篇
  1993年   2篇
  1991年   2篇
  1988年   1篇
  1987年   1篇
  1982年   1篇
  1972年   1篇
  1937年   1篇
排序方式: 共有102条查询结果,搜索用时 15 毫秒
41.
The reaction of a 2-[(dimethylamino)methyleneamino]pteridine, two 2-[(dimethylamino)methyleneamino]pyrimidines, and a 2-[(dimethylamino)methyleneamino]pyrido[2,3-d]pyrimidine with trimethylsilylethynylithium in the presence of benzyl chloroformate leads to the corresponding 2-[bis(trimethylsilylethynyl)methylamino]-substituted heterocycles. A series of such substrates was prepared and some of the factors which permit this transformation were delineated. An X-ray crystal structure was determined of one of the products - 2-[bis(trimethylsilylethynyl)methylamino]-5,6,7,8-tetrahydro-3-(2,2-dimethylpropanoyloxymethyl)quinazolin-4-one.  相似文献   
42.
A new continuous-flow automated enzymatic method suitable for the direct determination of creatinine in physiological samples is described. The proposed system utilizes an on-line gas predialysis unit in conjuction with a flow-through enzyme reactor coil and a potentiometric ammonia detector. The enzyme reactor contains immobilized creatinine iminohydrolase (EC 3.5.4.21) which converts creatinine to ammonia and N-methylhydantoin. Ammonia liberated from this reaction is detected downstream with the membrane electrode-based detector. The novel gas predialysis unit effectively removes >99.8% of endogenous ammonia (up to 1 mM) present in the sample. Thus, final peak potentials recorded by the electrode detector are directly proportional to the logarithm of creatinine concentrations present. The method is shown to be precise (<3%), selective, and capable of accurately determining creatinine in serum and urine samples containing abnormally high endogenous ammonia levels. Determinations of creatinine in serum samples (n = 30) using this new method correlate well with an existing Technicon AutoAnalyzer colorimetric method (r = 0.996).  相似文献   
43.
The synthesis, structure, magnetic and electronic properties of soluble transition metal phosphonate cages utilizing tritylphosphonic acid (TPA) as ligand are reported.  相似文献   
44.
The dodecametallic Cr(III) cluster has an S = 6 ground state with an axial zero-field splitting (ZFS) of DS=6 = +0.088 cm-1. Analysis of high-resolution optical data (MCD) allows us to determine the single-ion ZFS of the constituent Cr(III) ions directly (D = -1.035 cm-1). A vector coupling analysis demonstrates that the cluster ZFS is almost entirely single-ion in origin. Thus, the relative orientations of the local and cluster magnetic axes can lead to cluster ZFS of opposite sign to the single-ion even when this is the only significant contribution.  相似文献   
45.
We report the syntheses and crystal structures of (NH4)11[Ln(III)(PMo11O39)2.xH2O (where Ln = every trivalent lanthanide cation except promethium) in which two lacunary [PMo11O39]7- anions sandwich an 8-coordinate Ln(III) cation to yield the complex anion, [LnIII(PMo11O39)2]11-. The 14 salts crystallise in two different space groups, C2/c or P1, but the LnIII containing anions are isostructural across the whole series, a very rare example of such a complete study. Solid state and solution 31P NMR, Raman and IR spectroscopies have been used to prove the stability of [Ln(PMo11O39)2]11- in aqueous solution. As expected, the LnIII cation contracts across the series and the Ln-O bond distances decrease uniformly. Interestingly, the splitting in the nu(P-O) mode within the [PMo11O39]7- unit increases uniformly across the series, which we attribute to the stronger interaction with the smaller, higher charge density LnIII cation as the series is traversed. For the 31P NMR measurements a direct comparison of Lanthanide Induced (paramagnetic) Shift could be made with the analogous [P(W11O39)2]11- complexes.  相似文献   
46.
An interlaboratory study was conducted to evaluate a method for determining total soy isoflavones in dietary supplements, dietary supplement ingredients, and soy foods. Isoflavones were extracted using aqueous acetonitrile containing a small amount of dimethylsulfoxide (DMSO) and all 12 of the naturally occuring isoflavones in soy were determined by high-performance liquid chromatography (HPLC) with UV detection using apigenin as an internal standard. Fifteen samples (6 pairs of blind duplicates plus 3 additional samples) of soy isoflavone ingredients, soy isoflavone dietary supplements, soy flour, and soy protein products were successfully analyzed by 13 collaborating laboratories in 6 countries. For repeatability, the relative standard deviations (RSDr) ranged from 1.07 for samples containing over 400 mglg total isoflavones to 3.31 for samples containing 0.87 mg/g total isoflavones, and for reproducibility the RSDR values ranged from 2.29 for samples containing over 400 mg/g total isoflavones to 9.36 for samples containing 0.87 mg/g total isoflavones. HorRat values ranged from 1.00 to 1.62 for all samples containing at least 0.8 mg/g total isoflavones. One sample, containing very low total isoflavones (< 0.05 mg/g), gave RSDR values of 175 and a HorRat value of 17.6. This sample was deemed to be below the usable range of the method. The method provides accurate and precise results for analysis of soy isoflavones in dietary supplements and soy foods.  相似文献   
47.
The synthesis and magnetic properties of enneametallic and octametallic Fe(III) cage complexes from tetraazamacrocycle ligands (1,7-H2DO2A) and (H3DO3A) respectively, are reported.  相似文献   
48.
The paramagnetic aryl-alkynyl complexes [Mo(C≡CAr)(dppe)(η-C(7)H(7))](+) (dppe = Ph(2)PCH(2)CH(2)PPh(2); Ar = C(6)H(5), [1](+); C(6)D(5), [2](+); C(6)H(4)-4-F, [3](+); C(6)H(4)-4-Me, [5](+)) and [Mo(C≡CBu(t))(dppe)(η-C(7)H(7))](+) [4](+), have been investigated in a combined EPR and ENDOR study. Direct experimental evidence for the delocalisation of unpaired spin density over the framework of an aryl-alkynyl ligand has been obtained. The X-band solution EPR spectrum of the 4-fluoro derivative, [3](+), exhibits resolved hyperfine coupling to the remote para position of the aryl group [a(iso)((19)F) = 4.5 MHz, (1.6 G)] in addition to couplings attributable to (95/97)Mo, (31)P and (1)H of the C(7)H(7) ring. A full analysis of the (1)H ENDOR spectra is restricted by the low g anisotropy of the system which prevents the use of orientation selection. However, inter-comparison of the (1)H cw-ENDOR frozen solution spectra of [1](+), [2](+), [4](+) and [5](+), combined with spectral simulation informed by calculated values derived from DFT investigations, has facilitated estimation of the experimental a(iso)((1)H) hyperfine couplings of [1](+) including the ortho, ±3.7 MHz (±1.3 G) and para, ±3.9 MHz (±1.4 G) positions of the C(6)H(5) substituent of the aryl-alkynyl ligand.  相似文献   
49.
The synthesis, crystal structure and preliminary magnetic studies are reported for a very highly reduced, VIV8VIII2, polyoxo(alkoxo)vanadate with a unique square prismatic topology.  相似文献   
50.
We report the synthesis and structural characterization of [UO(2)(ReO(4))(DPPMO(2))(2)][ReO(4)] and [UO(2)(Cl)(DPPMO(2))(2)][Cl] (where DPPMO(2) = bis(diphenylphosphino)methane dioxide). In both complexes, the linear uranyl dication is coordinated to two bidentate DPPMO(2) ligands in the equatorial plane with one coordinated and one non-coordinated anion (either perrhenate or chloride). We have also prepared the pertechnetate analogue, and, through (31)P and (99)Tc NMR, we have shown that the cation, [UO(2)(TcO(4))(DPPMO(2))(2)](+), is stable in solution.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号