全文获取类型
收费全文 | 3340篇 |
免费 | 111篇 |
国内免费 | 8篇 |
专业分类
化学 | 2338篇 |
晶体学 | 24篇 |
力学 | 82篇 |
数学 | 505篇 |
物理学 | 510篇 |
出版年
2023年 | 33篇 |
2022年 | 50篇 |
2021年 | 83篇 |
2020年 | 81篇 |
2019年 | 75篇 |
2018年 | 49篇 |
2017年 | 38篇 |
2016年 | 113篇 |
2015年 | 95篇 |
2014年 | 101篇 |
2013年 | 211篇 |
2012年 | 227篇 |
2011年 | 276篇 |
2010年 | 135篇 |
2009年 | 124篇 |
2008年 | 168篇 |
2007年 | 178篇 |
2006年 | 184篇 |
2005年 | 147篇 |
2004年 | 99篇 |
2003年 | 106篇 |
2002年 | 94篇 |
2001年 | 36篇 |
2000年 | 25篇 |
1998年 | 22篇 |
1997年 | 31篇 |
1996年 | 33篇 |
1995年 | 33篇 |
1994年 | 27篇 |
1993年 | 21篇 |
1992年 | 21篇 |
1991年 | 22篇 |
1990年 | 27篇 |
1989年 | 23篇 |
1988年 | 17篇 |
1985年 | 15篇 |
1984年 | 21篇 |
1983年 | 20篇 |
1982年 | 30篇 |
1981年 | 31篇 |
1980年 | 31篇 |
1979年 | 24篇 |
1978年 | 20篇 |
1977年 | 20篇 |
1976年 | 18篇 |
1975年 | 18篇 |
1974年 | 16篇 |
1973年 | 20篇 |
1972年 | 17篇 |
1971年 | 13篇 |
排序方式: 共有3459条查询结果,搜索用时 515 毫秒
71.
The mechanism of the addition of carbonyl compounds to digermenes was investigated by examining the reaction between tetramesityldigermene and two mechanistic probes: trans-2-phenylcyclopropane carbaldehyde and trans,trans-2-methoxy-3-phenylcyclopropane carbaldehyde. In each reaction, two diastereomers of the formal [2+2] cycloadduct between the aldehyde and the digermene were obtained. These findings, together with the results of a recent theoretical study, indicate that the addition of carbonyl compounds to digermenes proceeds via a mechanism involving a zwitterionic intermediate. 相似文献
72.
We have designed beta-amino acid oligomers that are helical, cationic, and amphiphilic with the intention of mimicking the biological activity of amphiphilic, cationic alpha-helical antimicrobial peptides found in nature (e.g., magainins). We have previously identified a 17-residue beta-peptide (called beta-17) with antibiotic activity similar to that of a magainin derivative against four bacterial species, including two clinical isolates that are resistant to common antibiotics. This beta-peptide displays very low hemolytic activity against human red blood cells, which indicates selectivity for bacterial cells over mammalian cells. Here we examine some of the factors important for activity in this class of beta-peptides. An amphiphilic helix is necessary, because a nonamphiphilic isomer proved to be inactive. The ratio of cationic to hydrophobic residues is also important. Active beta-peptides induce the leakage of beta-galactosidase from treated Bacillus subtilis cells, as do alpha-helical antibiotic peptides, and this similarity suggests that the beta-peptide mode of action involves disruption of microbial membranes. This class of beta-peptides is not degraded by proteases, which bodes well for biological applications. 相似文献
73.
Bose D Martinavarro-Domínguez A Gil-Agustí M Carda-Broch S Durgbanshi A Capella-Peiró ME Esteve-Romero J 《Biomedical chromatography : BMC》2005,19(5):343-349
A micellar liquid chromatographic (MLC) procedure was developed for the clinical monitoring of imipramine and its active metabolite, desipramine. The determination of these highly hydrophobic substances was carried out after direct injection of the serum samples using a mobile phase composed of 0.15 m SDS--6% (v/v) pentanol buffered at pH 7, pumped at 1.5 mL/min into a C(18) column (250 x 4.6 mm), and electrochemical detection at 650 mV. Using this MLC method, calibration was linear (r > 0.995) and the limits of detection (ng/mL) were 0.34 and 0.24 for imipramine and desipramine, respectively. Repeatabilities and intermediate precision were tested at three different concentrations in the calibration range and a CV (%) below 2.2 was obtained. In this MLC procedure, the serum is determined without treatment, thus allowing repeated serial injections without changes in retention factors, and reducing the time and consumables required to carry out the pretreatment process. The assay method can be applied to the routine determination of serum imipramine and its metabolite in therapeutic drug monitoring. 相似文献
74.
Bismuth(III) salts are efficient catalysts for the selective allylic oxidation using tert-butyl hydroperoxide. BiCl3 is especially effective and can be easily recovered and reused as BiOCl. Using BiCl3/K-10 as catalyst, an increase in the reaction rate was observed. 相似文献
75.
Gaul C Njardarson JT Shan D Dorn DC Wu KD Tong WP Huang XY Moore MA Danishefsky SJ 《Journal of the American Chemical Society》2004,126(36):11326-11337
The first asymmetric total synthesis of (+)-migrastatin (1), a macrolide natural product with anti-metastatic properties, has been accomplished. Our concise and flexible approach utilized a Lewis acid-catalyzed diene aldehyde condensation (LACDAC) to install the three contiguous stereocenters and the trisubstituted (Z)-alkene of migrastatin (2 + 3 --> 21). Construction of the two remaining stereocenters and incorporation of the glutarimide-containing side chain was achieved by an anti-selective aldol addition of propionyl oxazolidinone 28 to angelic aldehyde 27, followed by a Horner-Wadsworth-Emmons (HWE) coupling of 32 with glutarimide aldehyde 5. Finally, the assembly of the macrocycle was realized by a highly (E)-selective ring-closing metathesis (35 --> 37). Utilizing the power of diverted total synthesis (DTS), a series of otherwise inaccessible analogues was prepared and evaluated for their potential as tumor cell migration inhibitors in several in vitro assays. These studies revealed a dramatic increase in activity when the natural motif was considerably simplified, presenting macrolactones 45 and 48, as well as macrolactam 55, macroketone 60, and CF(3)-alcohol 71 as promising anti-metastatic agents. 相似文献
76.
Johnson SA Liu FQ Suh MC Zürcher S Haufe M Mao SS Tilley TD 《Journal of the American Chemical Society》2003,125(14):4199-4211
The reaction of Cp(2)ZrCl(2) with 2 equiv of BuLi at -78 degrees C, followed by the addition of an unsymmetrical tetra- or pentafluorophenyl substituted alkyne R(1)C[triple bond]CAr(f) (R(1), Ar(f) = (CH(2))(4)Me, p-C(6)F(4)H; Me, p-C(6)F(4)H; Ph, C(6)F(5)), resulted in regioselective couplings of these alkynes to zirconacyclopentadienes in which the Ar(f) substituents preferentially adopt the 3,4-positions (beta beta) of the zirconacyclopentadiene ring. With Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) as the zirconocene reagent, the couplings could be carried out at room temperature; however, at higher temperatures significant quantities of the 2,4-fluoroaryl substituted (alpha beta) isomers were also formed. None of the conditions employed produced the 2,5-fluoroaryl substituted (alpha alpha) isomers. These fluoroaryl-substituted zirconacyclopentadienes were readily converted to butadienes via reactions with acids. The zirconacyclopentadiene Cp(2)ZrC(4)-2,5-Ph(2)-3,4-(C(6)F(5))(2), which resulted from the coupling of PhC[triple bond]C(C(6)F(5)), was converted to the corresponding thiophene by reaction with S(2)Cl(2), and to an arene by reaction with MeO(2)CC[triple bond]CCO(2)Me/CuCl. Mechanistic studies on zirconocene couplings of (p-CF(3)C(6)H(4))C[triple bond]C(p-MeC(6)H(4)) indicate that the observed regioselectivities are determined by an electronic factor that controls the orientation of at least one of the two alkynes as they are coupled. Additionally, these studies suggest an unsymmetrical transition state for the zirconocene coupling of alkynes, and this is supported by DFT calculations. The reaction of [(C(6)F(5))C[triple bond]CCH(2)](2)CH(2) with Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) resulted in a zirconacyclopentadiene in which the pentafluorophenyl substituents have been forced into the 2,5-positions (alpha alpha). Zirconocene coupling of the diyne (C(6)F(5))C[triple bond]C-1,4-C(6)H(4)-C[triple bond]C(C(6)F(5)) provided a route to conjugated polymers bearing electron-withdrawing pentafluorophenyl groups. 相似文献
77.
Samuel Melaku Ilse Gelaude Frank Vanhaecke Luc Moens Richard Dams 《Mikrochimica acta》2003,142(1-2):7-12
Microwave digestion reduction-aeration and pyrolysis combined with cold vapour atomic absorption and cold vapour atomic fluorescence
are compared for the determination of total mercury in several biological and environmental matrices. The biological samples
were digested in a mixture of HNO3/H2O2, the environmental samples in a mixture of HNO3/HClO4. After reduction with SnCl2, the mercury was collected by two-stage gold amalgamation. After microwave digestion reduction-aeration, detection limits
of 1.4 ng g−1 and 0.6 ng g−1 were obtained for cold vapour atomic absorption spectrometry (CVAAS) and cold vapour atomic fluorescence spectrometry (CVAFS),
respectively, for 250 mg of environmental samples. For biological samples (500 mg) the detection limits were 0.7 ng g−1 (CVAAS) and 0.4 ng g−1 (CVAFS). After pyrolysis, detection limits of 3.5 ng g−1 and 1.6 ng g−1 for CVAAS and CVAFS, respectively, were obtained for a 10 mg sample. Pyrolysis can only be applied when the organic content
of the sample is not too high. Accurate results were obtained for 8 certified reference materials of both environmental and
biological origin. In addition, a real sludge sample was analysed.
Author for correspondence. E-mail: richard.dams@rug.ac.be
Received September 18, 2002; accepted December 3, 2002
Published online May 5, 2003 相似文献
78.
[Re(CO)6][BF4] reacts with HMPA to form [Re(CO)3(HMPA)3][BF4] (4), whose structure was determined by X-ray crystallography and proves to be a key intermediate in the ligand exchange reaction between three CO and Cp; and may be related to other cations such as [Re(CO)3(H2O)3]+, [Re(CO)3(CH3CN)3]+, [Re(CO)3(DMSO)3]+, obtained by different ways, and important in the field of organometallic radiopharmaceuticals. 相似文献
79.
Samuel C. Perry Pui-ki Leung Ling Wang Carlos Ponce de León 《Current Opinion in Electrochemistry》2020
CO2 reduction processes continue to be developed for electrosynthesis, energy storage applications, and environmental remediation. A number of promising materials have shown high activity and selectivity to target reduction products. However, the progress has been mainly at a small laboratory scale, and the technical challenges of large scale CO2 reduction have not been considered adequately. This review covers recent advancements in catalyst materials and cell designs. The leading materials for CO2 reduction to a number of useful products are presented with their corresponding cell and reactor designs. The latest efforts to progress to industrially relevant scales are discussed, along with the challenges that must be met for carbon dioxide reduction to be a viable route for mass scale production. 相似文献
80.
The heavier alkali-metal monosulfides (KS, RbS, CsS, and FrS) have been studied by high-level ab initio calculations. The RCCSD(T) method has been employed, combined with large flexible valence basis sets. All-electron basis sets are used for potassium and sulfur, with effective core potentials being used for the other metals, describing the core electrons. Potential-energy curves are calculated for the lowest two neutral and cationic states: all neutral monosulfide species have a (2)Pi ground state, in contrast with the alkali-metal monoxide species, which undergo a change in the electronic ground state from (2)Pi to (2)Sigma(+) as the group is descended. In the cases of KS, RbS, and CsS, spin-orbit curves are also calculated. We also calculate potential-energy curves for the lowest (3)Sigma(-) and (3)Pi states of the cations. From the potential-energy curves, spectroscopic constants are derived, and for KS the spectroscopic results are compared to experimental spectroscopic values. Ionization energies, dissociation energies, and heats of formation are also calculated; for KS, we explore the effects of relativity and basis set extrapolation on these values. 相似文献