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   402篇
  免费   7篇
  国内免费   3篇
化学   301篇
晶体学   6篇
力学   27篇
数学   20篇
物理学   58篇
  2023年   9篇
  2022年   47篇
  2021年   28篇
  2020年   14篇
  2019年   17篇
  2018年   12篇
  2017年   9篇
  2016年   12篇
  2015年   13篇
  2014年   15篇
  2013年   28篇
  2012年   25篇
  2011年   22篇
  2010年   5篇
  2009年   9篇
  2008年   11篇
  2007年   16篇
  2006年   10篇
  2005年   10篇
  2004年   12篇
  2003年   7篇
  2002年   5篇
  2001年   3篇
  2000年   9篇
  1999年   4篇
  1998年   3篇
  1997年   2篇
  1996年   2篇
  1995年   1篇
  1994年   3篇
  1993年   3篇
  1992年   3篇
  1991年   3篇
  1990年   3篇
  1989年   5篇
  1988年   4篇
  1987年   3篇
  1986年   4篇
  1985年   4篇
  1984年   3篇
  1983年   2篇
  1982年   3篇
  1981年   2篇
  1979年   2篇
  1978年   1篇
  1976年   1篇
  1971年   1篇
  1969年   1篇
  1964年   1篇
排序方式: 共有412条查询结果,搜索用时 33 毫秒
61.
Chemical investigation of the sponge Ircinia formosana resulted in the isolation of seven new linear C22‐sesterterpenoids, irciformonins E–K ( 1 – 7 ) in addition to irciformonin A ( 8 ), a previously isolated furanosesterterpenoid (=a furan‐moiety‐containing sesterterpenoid) from the same species. The structures were determined by interpretation of HR‐ESI‐MS and 2D‐NMR spectra. The structure of irciformonin A ( 8 ) was revised. Compound 5 exhibited significant inhibition of peripheral blood mononuclear cell proliferation induced by phytohemaglutinin.  相似文献   
62.
Little is known about the effect of conformation on passive membrane diffusion rates in small molecules. Evidence suggests that intramolecular hydrogen bonding may play a role by reducing the energetic cost of desolvating hydrogen bond donors, especially amide N-H groups. We set out to test this hypothesis by investigating the passive membrane diffusion characteristics of a series of cyclic peptide diastereomers based on the sequence cyclo[Leu-Leu-Leu-Leu-Pro-Tyr]. We identified two cyclic hexapeptide diastereomers based on this sequence, whose membrane diffusion rates differed by nearly two log units. Results of solution NMR studies and hydrogen/deuterium (H/D) exchange experiments showed that membrane diffusion rates correlated with the degree of intramolecular hydrogen bonding and H/D exchange rates. The most permeable diastereomer, cyclo[d-Leu-d-Leu-Leu-d-Leu-Pro-Tyr] (1), exhibited a passive membrane diffusion rate comparable to that of the orally available drug cyclosporine A.  相似文献   
63.
64.
Molecular electronic structure calculations, employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methodologies, have been carried out to improve the performance of the synthesised dye YD2-o-C8 which is characterised by 11.9%–12.7% efficiencies. We aimed to narrow the band gap of YD2-o-C8 to extend the light-harvesting region to near-infrared (NIR). This was done by incorporating Cd instead of Zn onto the porphyrin ring and elongating the length of π-conjugation by adding ethynylene link and anthracene unit, so that the performances of the suggested cells could be expected to exceed the 11.9%–12.7% efficiencies with TiO2, ZnO2, and WO3 oxide electrodes. The effects of modifying the central metal and elongating the length of π-conjugation on cell performance are confirmed in terms of frontier molecular orbital (FMO) energy gaps, density of states (DOS), molecular electrostatic potentials (MEPs), non-linear optical (NLO) properties, ultraviolet–visible (UV–vis) electronic absorption, and 1H nuclear magnetic resonance chemical shifts. Increasing the length of π-conjugation of the D–π–A dyes leads to increasing the DOS near Fermi levels, more active NLO performance, strong response to the external electric field, delocalisation of the negative charges near the anchoring groups, deep electron injection, suppressing macrocycle aggregation, active dye regeneration, and inhibited dye recombination. The calculated band gap/eV of the present DMP-Zn is correlated with the experimental (E1/2(oxidation)–E1/2(reduction)/V) potentials of the identical YD2-o-C8. A co-sensitiser is suggested for NIR sensitisation (550–950 nm) to increase the power-to-conversion efficiency beyond 14%.  相似文献   
65.
Optical, microstructural, and thermal properties of the investigated silver nitrate samples were characterized by various techniques, such as X-ray analysis, scanning electron microscopy, UV–Vis–NIR absorption and differential scanning calorimetry (DSC). The presence of structural phase transition [orthorhombic structure (phase II) to rhombohedral structure (phase I)] was checked by DSC and X-ray analysis measurements. The thermal energy required for such transformation is found to be 11.6 J/g. The optical band gaps of AgNO3 are 1.4 and 2.02 eV for phase II and phase I, respectively, at the low-energy region. But at high-energy region, the optical band gaps are 3.41 and 3.43 eV for phase II and phase I, respectively. Characteristic peaks for AgNO3 corresponding to (2 1 1), (0 0 4) and (3 5 1) for phase II and (0 0 4), (3 1 1) and (0 2 4) for phase I have been observed. The average crystalline size for AgNO3 samples and the values of dislocation density δ and the strain ε for the planes of two phases II and I are calculated and also the texture coefficient is determined. Such information can considerably aid in understanding the process of phase transformations in AgNO3.  相似文献   
66.
Solvatochromic mixed ligand complexes of copper(II) with malonate and diamine derivatives, Cu(n)(RMal)(diam)(n)X(m) (where n=1 or 2, m=1-4, RMal, malonic acid (H(2)Mal), diethylmalonate (HDEtMal) or diethylethoxyethylenemalonate (DEtEMal), and diam, ethylenediamine (en), 1,3-propylenediamine (1,3-pn), N,N,N'-trimethylethylenediamine (Me(3)en), N,N,N'-triethylethylenediamine (Et(3)en), N,N,N',N'-tetramethylethylenediamine (Me(4)en), N,N,N',N'-tetramethylpropylenediamine (Me(4)pn), or N-methyl-1,4-diazacycloheptane (medach); and X=ClO(4)(-) or Cl(-)), has been synthesized and characterized by spectroscopic, magnetic, molar conductance and electrochemical measurements. The mass spectra along with the analytical data of the complexes show peaks with m/e corresponding to a bridged binuclear structure for the chloride complexes, while perchlorate complexes showed either mononuclear structure for DEtMal and DEtEMal or bridged binuclear structure for Mal complexes. These results correspond to IR spectral data, which indicated that the modes of ester and carboxylato coordination sites are mono- and/or bidentate. The d-d absorption bands in weak donor solvents suggest square-planar and distorted square pyramidal-trigonal bipyramid geometries for the perchlorate and chloride complexes; respectively. On the other hand, an octahedral structure is identified for complexes in strong donor solvents. Perchlorate complexes show a drastic color change from violet to green as the donation ability of solvent increases, whereas chloride complexes are highly affected by the acceptor properties of the solvent. Cyclic voltammetric measurements on the complexes, proposed a quasi-reversible or irreversible and mainly diffusion controlled reduction process. Such behavior has been explained according to the ECE mechanism. A linear correlation has been found between the Cu(II) reduction potential and the spectral data. Molecular orbital calculations were performed for the ligands on the bases of PM3 level and the results corresponded to the experimental data. The data are discussed in terms of chromotropic concept and its applications as a Lewis acid-base color indicator.  相似文献   
67.
Treatment of indole with substituted aldehyde in the presence of equimolar amount of sodium bromate and sodium hydrogen sulfite mixture in water yielded corresponding substituted bis(indolyl)methanes in good yields. This provides a facile and environmentally friendly method towards the synthesis of an important class of organic compounds.  相似文献   
68.
Two rare metal coordination complexes of yttrium(III) including 1,10‐phenanthroline, Y(phen)2(NO3)3 and (phenH)2[Y2(pydc)3(NO3)2·6H2O] (phen=1,10‐phenanthroline, pydc=2,6‐pyridinedicarboxylate), and a proton transfer compound (phenH+)2(pydc2?) were synthesized and characterized by elemental analysis, molar conductance, infrared spectra (IR), nuclear magnetic resonance (NMR) and thermal analysis. The proposed structures of yttrium complexes were exhibited. The in vitro biological activities of the newly synthesized complexes have also been investigated against Bacillus coli, Staphylococcus aureus and Candida albicans. The results showed that yttrium(III) complexes including 1,10‐phenanthroline exhibited better antibacterial/antifungal activity than their ligands and corresponding compounds.  相似文献   
69.
Amino cellulose sulfate (ACS); namely 6-deoxy-6-(ω-aminoethyl) amino cellulose-2,3(6)-O-sulfate (AECS) and 6-deoxy-6-(2-(bis-N′,N′-(2-aminoethyl)aminoethyl)) amino cellulose-2,3(6)-O-sulfate (BAECS) were prepared by a three step synthesis starting with the functionalization of microcrystalline cellulose with p-toluenesulfonyl (tosyl) groups (degree of substitution, DSTos between 0.55 and 1.37). Subsequently the introduction of the sulfate moieties was carried out (DSSulf between 1.09 and 1.27) and the tosyl groups at position 6 were replaced by a nucleophilic substitution reaction. As nucleophilic agents 1,2-diaminoethane and tris-(2-aminoethyl)amine were applied, yielding AECS (DSAEA values between 0.41 and 0.86) and BAECS (DSBAEA values between 0.32 and 0.74), respectively. The ACS samples were characterized by means of elemental analysis, 13C-NMR-, FT-IR-, and UV–Vis spectroscopy. Moreover, the solubility of the samples in water at different pH values and the molecular weights of the samples in aqueous solution were studied.  相似文献   
70.
A simple, facile, efficient and three-components procedure for the synthesis of pyrimido[1,2-a]benzimidazoles and pyrazolo[3,4-b]pyridines utilizing phenylsulfone synthon, under ultrasonic irradiation was developed.  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