全文获取类型
收费全文 | 3970篇 |
免费 | 233篇 |
国内免费 | 67篇 |
专业分类
化学 | 1612篇 |
晶体学 | 27篇 |
力学 | 153篇 |
数学 | 333篇 |
物理学 | 788篇 |
无线电 | 1357篇 |
出版年
2023年 | 27篇 |
2022年 | 49篇 |
2021年 | 82篇 |
2020年 | 67篇 |
2019年 | 67篇 |
2018年 | 44篇 |
2017年 | 53篇 |
2016年 | 98篇 |
2015年 | 108篇 |
2014年 | 150篇 |
2013年 | 213篇 |
2012年 | 233篇 |
2011年 | 250篇 |
2010年 | 165篇 |
2009年 | 167篇 |
2008年 | 276篇 |
2007年 | 204篇 |
2006年 | 211篇 |
2005年 | 206篇 |
2004年 | 161篇 |
2003年 | 139篇 |
2002年 | 143篇 |
2001年 | 110篇 |
2000年 | 113篇 |
1999年 | 80篇 |
1998年 | 81篇 |
1997年 | 54篇 |
1996年 | 79篇 |
1995年 | 64篇 |
1994年 | 55篇 |
1993年 | 71篇 |
1992年 | 42篇 |
1991年 | 56篇 |
1990年 | 32篇 |
1989年 | 34篇 |
1988年 | 36篇 |
1987年 | 20篇 |
1986年 | 16篇 |
1985年 | 23篇 |
1984年 | 25篇 |
1983年 | 30篇 |
1982年 | 22篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1977年 | 14篇 |
1976年 | 9篇 |
1972年 | 4篇 |
1970年 | 6篇 |
排序方式: 共有4270条查询结果,搜索用时 21 毫秒
111.
[structure: see text] We have synthesized novel fluorophores by using Sonogashira reactions of 1,4-bis(dibromovinyl)benzene and 2,5-bis(dibromovinyl)thiophene with various aromatic bromides. The emission maxima of these fluorophores vary from the indigo blue to the reddish-orange region, depending on the structures of aromatic nuclei and peripheral moieties. 相似文献
112.
A standard curve for the quantification of L-ascorbic acid (L-AA) by capillary zone electrophoresis (CZE) was established, and the quantification of ascorbic acid and total ascorbic acid in fruits (lemon, Sunkist, and pineapple) and spinach were performed using D-isoascorbic acid (D-IAA) as an internal standard. The minimum detection limits (MDLs) for L-AA and D-IAA were determined to be 1 and 2 microg/mL, respectively, at 265 nm. Dehydroascorbic acid (DHAA) in fruits and spinach was quantified in the presence of DL-homocysteine. The recoveries for L-AA in these juices were between 95 and 105%. 相似文献
113.
The vanadium trapping effect of Mg and La containing additives in cracking catalyst contaminated with 2300 ppm Ni and 4700 ppm V has been analyzed by microactivity test (MAT) and imaging secondary ion mass spectrometry (SIMS). The results of SIMS imaging are consistent with cracking activity data and show that the La/spinel is a superior vanadium trap for the fluid catalytic cracking of hydrocarbons (FCC) operation. La/spinel serves as a dual function additive for both vanadium trap and SOx removal. The optimum amount of La/spinel added to the cracking catalyst is about 15% by weight. This results in an increased catalytic activity, an increase in gasoline yield, and a decrease in coke and gas factors. The MgAl2O4 phase of Mg/Al2O3 additive is found capable of trapping vanadium while its MgO of Mg/Al2O3 phase can migrate to zeolite particles (the active component of the FCC catalyst) that, in turn, causes a decline in the activity of that catalyst. 相似文献
114.
An efficient eight-step synthesis (53% overall) and the evaluation of 1,2,9,9a-tetrahydrocyclopropa[c]benz[e]-3-azaindol-4-one (CBA) and its derivatives containing an aza variant of the CC-1065/duocarmycin alkylation subunit are detailed. This unique deep-seated aza modification provided an unprecedented 2-aza-4,4-spirocyclopropacyclohexadienone that was characterized chemically and structurally (X-ray). CBA proved structurally identical with CBI, the carbon analogue, including the stereoelectronic alignment of the key cyclopropane, its bond lengths, and the bond length of the diagnostic C3a-N2 bond, reflecting the extent of vinylogous amide (amidine) conjugation. Despite these structural similarities, CBA and its derivatives were found to be much more reactive toward solvolysis and hydrolysis, much less effective DNA alkylating agents (1000-fold), and biologically much less potent (100- to 1000-fold) than the corresponding CBI derivatives. 相似文献
115.
Huang HS Chiou JF Chiu HF Hwang JM Lin PY Tao CW Yeh PF Jeng WR 《Chemical & pharmaceutical bulletin》2002,50(11):1491-1494
The synthesis of a series of anthraquinone moieties bearing symmetrical sulfur-linked substituents in the 1 and 5 positions is described. These compounds were evaluated for their ability to inhibit the growth of suspended rat glioma C6 cells and human hepatoma G2 cells, respectively. In addition, the redox property of the compounds was determined based on the inhibition of lipid peroxidation in model membranes. Compounds 2a and 2h in this series compared favorably and exhibited the most potent cytotoxicity (0.02, 0.05 microM) against C6 cells in the XTT colorimetric assay. As far as redox properties are concerned, all bis-thio-anthraquinones show potential lipid peroxidation in model membranes very close to that of mitoxantrone (MX), and 2a, 2d, 2e, 2i, 2j, and 2k have more potential than that of MX. The lack of cytotoxicity of compound 2i cannot be related to lipid peroxidation, but the steric and electronic properties of the side-chain substituent maybe impair effective recognition of the cleavable complex. In contrast to MX, 2a and 2h are cytotoxic in rat glioma C6 cells and do not enhance lipid peroxidation in model membranes. 相似文献
116.
Direct reaction of iron pentacarbonyl with 1-N,N-disubstituted-cyclohexa-2,5-dicnecarboxamide and 1-N,N-disubstituted-cyclohexa-1,3-dienecarboxamide mixture (in which the disubstkuted group is diethyl or diphenyl) gave the isomeric tricarbonyl iron complexes of 2-N,N-disubstituted-1,4-η-cyclohexa-1,3-dienccarboxamide (1), 1-N,N-disubslituted-1,4-η-cyclohexa-1,3-dienecarboxamide (2), and 5-N,N-disubstituted-1,4-η-cyclohexa-1,3-dienecarboxamide (3) and tricarbonyliron complexes of l-N,N-disubstituted-cyclohexenecarboxamide (4). These complexes were separated and characterized by IR, UV-VIS, 1H NMR, elemental analysis, and mass spectra. Only 1 isomerized to give 2 under acidic conditions; both 1 and 2 undergo hydride abstraction with triphenylmethyl hexafluorophosphatc. Complexes 3, undergo neither isomcrization nor hydride abstraction. According to the spectral data, the possible interaction between carboxamide and iron carbonyl moiety was investigated. The irreversible electrochemical behavior of these complexes were studied. 相似文献
117.
Ronald A. Buono Nathalie Kucharczyk Magrit Neuenschwander Johan Kemmink Lih-Yueh Hwang Jean-Luc Fauchère Carol A. Venanzi 《Journal of computer-aided molecular design》1996,10(3):213-232
Summary The design of enzyme mimics with therapeutic and industrial applications has interested both experimental and computational chemists for several decades. Recent advances in the computational methodology of restrained molecular dynamics, used in conjunction with data obtained from two-dimensional 1H NMR spectroscopy, make it a promising method to study peptide and protein structure and function. Several issues, however, need to be addressed in order to assess the validity of this method for its explanatory and predictive value. Among the issues addressed in this study are: the accuracy and generizability of the GROMOS peptide molecular mechanics force field; the effect of inclusion of solvent on the simulations; and the effect of different types of restraining algorithms on the computational results. The decapeptide Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly, which corresponds to the sequence of ACTH1–10, has been synthesized, cyclized, and studied by two-dimensional 1H NMR spectroscopy. Restrained molecular dynamics (RMD) and time-averaged restrained molecular dynamics (TARMD) simulations were carried out on four different distance-geometry starting structures in order to determine and contrast the behavior of cyclic ACTH1–10 in vacuum and in solution. For the RMD simulations, the structures did not fit the NOE data well, even at high values of the restraining potential. The TARMD simulation method, however, was able to give structures that fit the NOE data at high values of the restraining potential. In both cases, inclusion of explicit solvent molecules in the simulation had little effect on the quality of the fit, although it was found to dampen the motion of the cyclic peptide. For both simulation techniques, the number and size of the NOE violations increased as the restraining potential approached zero. This is due, presumably, to inadequacies in the force field. Additional TARMD vacuum-phase simulations, run with a larger memory length or with a larger sampling size (16 additional distance-geometry structures), yielded no significantly different results. The computed data were then analyzed to help explain the sparse NOE data and poor chymotryptic activity of the cyclic peptide. Cyclic ACTH1–10, which contains the functional moieties of the catalytic triad of chymotrypsin, was evaluated as a potential mimic of chymotrypsin by measurement of the rate of hydrolysis of esters of L-and d-phenylalanine. The poor rate of hydrolysis is attributed to the flexibility of the decapeptide, the motion of the side chains, which result in the absence of long-range NOEs, the small size of the macrocycle relative to that of the substrate, and the inappropriate orientation of the Gly, His, and Ser residues. The results demonstrate the utility of this method in computer-aided molecular design of cyclic peptides and suggest structural modifications for future work based on a larger and more rigid peptide framework. 相似文献
118.
The formation of Schiff bases from the reaction of primary amines and several aromatic aldehydes has been studied. In many cases the Schiff bases were too unstable or feebly-fluorescent to be of analytical value. 1-Pyrenealdehyde and 2-fluorenealdehyde, however, were found to be suitable fluorigenic reagents for primary aliphatic amines, forming Schiff bases that were very stable and intensely fluorescent in acidic ethanol. The derivatives of 1-pyrenealdehyde could be detected at concentrations less than 1 ng ml-1 in pure solution. Derivatives of 1-pyrenealdehyde could be readily produced by reactions at the surface of a t.l.c. plate. Combination of this approach with a simple deproteinizing procedure permitted analysis for nanograna quantities of primary amines in blood serum. 相似文献
119.
Chen CH Hwang BJ Wang GR Sarma LS Tang MT Liu DG Lee JF 《The journal of physical chemistry. B》2005,109(46):21566-21575
We report in situ X-ray absorption spectroscopy (XAS) investigations on the formation of palladium-platinum (Pd/Pt) bimetallic clusters at the early stage within the water-in-oil microemulsion system of water/AOT/n-heptane. The reduction of palladium and platinum ions and the formation of corresponding clusters are monitored as a function of dosage of reducing agent, hydrazine (N(2)H(5)OH). Upon successive addition of the reducing agent, hydrazine (N(2)H(5)OH), five distinguishable steps are observed in the formation process of Pd/Pt clusters at the early stage. Both in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis for both the Pd K-edge and Pt L(III)-edge revealed the formation of Pd/Pt bimetallic clusters. A corresponding structural model is proposed for each step to provide a detailed insight into the nucleation and growth mechanism of Pd/Pt bimetallic clusters. We also discussed the atomic distribution of Pd and Pt atoms in Pd/Pt bimetallic clusters based on the calculated XAS structural parameters. 相似文献
120.
Hwang S Lee KH Ryu GH Jang YH Lee SB Lee WY Hong JI Chung DS 《The Journal of organic chemistry》2000,65(2):536-542
The binding affinity and selectivity of a new ionophore, [1(8)]starand (1), toward alkali metal cations in methanol were examined through NMR titration experiments and free energy perturbation (FEP) and molecular dynamics simulations. The preference was determined to be K(+) > Rb(+) > Cs(+) > Na(+) > Li(+) in both FEP simulations and NMR experiments. The FEP simulation results were able to predict the relative binding free energies with errors less than 0.13 kcal/mol, except for the case between Li(+) and Na(+). The cation selectivity was rationalized by analyzing the radial distribution functions of the M-O and M-C distances of free metal cations in methanol and those of metal-ionophore complexes in methanol. 相似文献