全文获取类型
收费全文 | 3802篇 |
免费 | 139篇 |
国内免费 | 45篇 |
专业分类
化学 | 2504篇 |
晶体学 | 28篇 |
力学 | 135篇 |
综合类 | 2篇 |
数学 | 563篇 |
物理学 | 754篇 |
出版年
2023年 | 24篇 |
2022年 | 38篇 |
2021年 | 49篇 |
2020年 | 48篇 |
2019年 | 43篇 |
2018年 | 43篇 |
2016年 | 74篇 |
2015年 | 69篇 |
2014年 | 96篇 |
2013年 | 194篇 |
2012年 | 249篇 |
2011年 | 266篇 |
2010年 | 137篇 |
2009年 | 141篇 |
2008年 | 231篇 |
2007年 | 219篇 |
2006年 | 220篇 |
2005年 | 192篇 |
2004年 | 173篇 |
2003年 | 149篇 |
2002年 | 112篇 |
2001年 | 94篇 |
2000年 | 97篇 |
1999年 | 58篇 |
1998年 | 56篇 |
1997年 | 34篇 |
1996年 | 74篇 |
1995年 | 52篇 |
1994年 | 50篇 |
1993年 | 46篇 |
1992年 | 43篇 |
1991年 | 37篇 |
1990年 | 28篇 |
1989年 | 22篇 |
1988年 | 27篇 |
1987年 | 24篇 |
1986年 | 20篇 |
1985年 | 47篇 |
1984年 | 29篇 |
1983年 | 29篇 |
1982年 | 24篇 |
1981年 | 38篇 |
1980年 | 29篇 |
1979年 | 25篇 |
1978年 | 30篇 |
1977年 | 27篇 |
1976年 | 20篇 |
1975年 | 25篇 |
1974年 | 37篇 |
1973年 | 19篇 |
排序方式: 共有3986条查询结果,搜索用时 11 毫秒
41.
The reaction of epibromohydrin with hydroxyquinazolin-2-(1H)one derivatives. Novel oxazine formation
Mary Lou Cotter Victor Bandurco Elizabeth Wong Zoltan G. Hajos 《Journal of heterocyclic chemistry》1979,16(3):623-624
Epibromohydrin was found to react with 7-hydroxy-2-methylpyrrolo[1,2-c]quinazolin-5(6H)-one ( 4 ) in the presence of sodium hydroxide to form the novel oxazine 5 . The structure of compound 5 was proven by cmr and pmr analysis. 相似文献
42.
[reaction: see text] An effective strategy has been developed for the rapid and efficient one-pot synthesis of 2-aryl-5-substituted-2,3-dihydrobenzofurans from readily available o-nitrotoluenes and aromatic aldehydes. This strategy allows access to a structurally diverse array of products for further manipulation. 相似文献
43.
J. B. Wong Shing W. E. Baker K. E. Russell 《Journal of polymer science. Part A, Polymer chemistry》1995,33(4):633-642
The kinetics of grafting a basic monomer, 2-(dimethylamino)-ethyl methacrylate (DMAEMA) to hydrocarbon substrates have been investigated. These systems were chosen as models for the grafting of a homopolymerizable monomer to polyolefins such as polyethylene. The reactions with squalane and n-eicosane were initiated by an organic peroxide, 2,5-dimethyl 2,5 dit-butylperoxy)-3-hexyne; grafting yields become significant at high reaction temperatures and low monomer concentrations. In squalane, the order of reaction with respect to monomer increased from about 1.1 for 0.22?0.44M DMAEMA to almost 2 at 0.69M DMAEMA; the order with respect to initiator was 0.56. The overall activation energy in the 130?160°C temperature range was, however, surprisingly low, 42±5 kJ mol?1. When analytical data were used to separate the overall rate into those for grafting and homopolymerization, different kinetic paths were observed for the competing reactions. These results are interpreted in terms of two different mechanisms; intramolecular chain transfer plays an important role in grafting, while depropagation becomes a major factor in homopolymerization at temperatures above 150°C. © 1995 John Wiley & Sons, Inc. 相似文献
44.
Using high-level ab initio MO methods, we have identified two reaction pathways with different thermodynamic and kinetic properties for the thermal decomposition of the three-membered heterocycle thiirane (C2H4S) and related derivatives. A homolytic ring opening, followed by attack of the generated diradical on another thiirane molecule, and subsequent elimination of ethene in a fast radical chain reaction results in the formation of disulfur molecules in their triplet ground state (3S2) and requires activation enthalpies of deltaH#(298) = 222 kJ mol(-1) and deltaG#(298) = 212 kJ mol(-1). This reaction mechanism would result in a first-order rate law in agreement with one reported gas-phase experiment but does neither match the experimental activation energy nor does it explain the observed retention of the stereochemical configuration in the thermal decomposition of certain substituted thiiranes. Alternatively, sulfur atoms can be transferred from one thiirane moleculeto another with the intermediate formation of thiirane 1-sulfide (C2H4S2). This molecule can either decompose unimolecularly to ethene and disulfur in its excited singlet state (1S2) or, by means of spin crossover, S2 in its triplet ground state may be formed. On the other hand, the thiirane 1-sulfide may react with itself and transfer one sulfur atom from one molecule to another with formation of thiirane 1,1-disulfide (C2H4S3), which is an analogue of thiirane sulfone; thiirane is formed as the second product. The 1,1-disulfide may then decompose to ethene and S3. In still another bimolecular reaction, the thiirane 1-sulfide may react with itself in a strongly exothermic reaction to give S4 and two equivalents of ethene. This series of reactions results in a second-order rate law and requires activation enthalpies of deltaH#(298) = 109 kJ mol(-1) and deltaG#(298) = 144 kJ mol(-1) for the formation of thiirane 1-sulfide, while the consecutive reactions require less activation enthalpy. Elemental sulfur (S8) is eventually formed by oligomerization of either S2, S3, or S4 in spin-allowed reactions. These findings are in agreement with most experimental data on the thermal desulfurization of thiirane and its substituted derivatives. Thiirane 1-persulfide (C2H4S3) with a linear arrangement of the three sulfur atoms as well as zwitterions and radicals derived from thiirane are not likely to be intermediates in the thermal decomposition of episulfides. 相似文献
45.
The dynamic response of the fluoride-selective electrode is shown to result from four distinct processes: ion diffusion, an undefined reaction, LaF3 dissolution and a calibration drift. Empirical equations are derived which describe the time—e.m.f. relationship over times of the order of days. Dissolution of LaF3 is shown to be a minor factor in determining the lower limit for measurement of fluoride concentration. The calibration drift process is the main obstacle. The time to thermodynamic equilibrium when the fluoride concentration is reduced becomes very long. A model is given for the calibration drift process. This model explains many of the anomalies reported in the literature for the behaviour of the fluoride-selective electrode. A much more detailed understanding of the calibration drift process is a prerequisite to general application of the fluoride electrode to the measurement of very low concentrations of fluoride. 相似文献
46.
47.
Prefractionation of protein samples prior to two-dimensional electrophoresis (2-DE) has the potential to increase the dynamic detection range for proteomic analysis. We evaluated a membrane-based electrophoretic separation technique (Gradiflow) for its ability to fractionate an exoproteome sample from the filamentous fungus Trichoderma reesei. The sample was separated on the basis of size and charge. Buffer optimization was found to be necessary for successful size fractionation. Fractionation by charge was used to resolve the sample into four fractions that were subjected to analysis by two-dimensional electrophoresis (2-DE). Enhanced detection of low-abundance proteins with selective removal of high-abundance species was achieved. Fractionated and unfractionated samples were examined for differences in the ability to identify proteins following 2-DE using trypsin in-gel digestion followed by peptide mass fingerprinting using matrix assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS). Fractionated samples showed marked improvement in protein identification ability and sequence coverage. This study demonstrates the utility of the Gradiflow for fractionation, resulting in an enhancement of resolution and characterization of a moderately complex proteome. 相似文献
48.
49.
The molecular structure of TTF dissolved in nematic liquid crystalline solvents has been determined from the proton magnetic resonance including couplings due to 13C in natural abundance. The molecule is puckered in a boat conformation with the SCHCHS planes making a dihedral angle of 13 ± 2° with the S2C CS2 plane. The other structural parameters obtained are rCH = 1.085 ± 0.014 Å and the angel CCH = 123.7 ± 1.5°. 相似文献
50.
A simple, rapid and sensitive high-performance liquid chromatographic method with fluorescence detection for the simultaneous determination of oxytetracycline, doxycycline, tetracycline and chlortetracycline was developed, and successfully applied to the analysis of commercial tetracycline antibiotics. The separation was performed on a reverse-phase C18 column with a gradient elution composed of methanol and sodium acetate buffer (containing disodium ethylenediaminetetraacetate and calcium chloride, pH 8.10) as the mobile phase, and fluorescence detection at 532 nm (excitation at 380 nm). The detection limits for oxytetracycline, doxycycline, tetracycline and chlortetracycline were 0.1, 0.5, 0.3 and 0.4 g L–1, respectively. Data with respect to precision and accuracy were reported and discussed. 相似文献