全文获取类型
收费全文 | 2810篇 |
免费 | 98篇 |
国内免费 | 16篇 |
专业分类
化学 | 1775篇 |
晶体学 | 13篇 |
力学 | 84篇 |
数学 | 317篇 |
物理学 | 735篇 |
出版年
2023年 | 10篇 |
2022年 | 42篇 |
2021年 | 39篇 |
2020年 | 40篇 |
2019年 | 53篇 |
2018年 | 44篇 |
2017年 | 47篇 |
2016年 | 116篇 |
2015年 | 56篇 |
2014年 | 98篇 |
2013年 | 180篇 |
2012年 | 165篇 |
2011年 | 156篇 |
2010年 | 102篇 |
2009年 | 98篇 |
2008年 | 160篇 |
2007年 | 132篇 |
2006年 | 109篇 |
2005年 | 114篇 |
2004年 | 106篇 |
2003年 | 99篇 |
2002年 | 93篇 |
2001年 | 78篇 |
2000年 | 80篇 |
1999年 | 51篇 |
1998年 | 28篇 |
1997年 | 32篇 |
1996年 | 50篇 |
1995年 | 58篇 |
1994年 | 56篇 |
1993年 | 48篇 |
1992年 | 55篇 |
1991年 | 29篇 |
1990年 | 34篇 |
1989年 | 23篇 |
1988年 | 18篇 |
1987年 | 21篇 |
1986年 | 26篇 |
1985年 | 25篇 |
1984年 | 29篇 |
1983年 | 12篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 9篇 |
1979年 | 13篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1975年 | 7篇 |
1974年 | 7篇 |
1973年 | 7篇 |
排序方式: 共有2924条查询结果,搜索用时 15 毫秒
81.
de Oliveira Neto Marçal Pires José M. Giambiagi Mario de Giambiagi Myriam Segre Alvarez Fernando A. 《Structural chemistry》1998,9(5):339-348
Electronic properties of lamotrigine (LTG) and two analogues (A1 and A2) are compared through MOPAC-AM1 calculations. Two stable conformers of LTG are calculated to exist in agreement with X-ray crystallography. In the three compounds and the two conformers for each of them, the more favorable protonation sites are N2 and N4; these should then be the sites appropriate for interaction with a receptor, and group valence reinforces the supposition. The molecular electrostatic potentials show that a region between the two chlorine atoms in LTG could be the site for an electrostatic interaction with a corresponding site in the receptor. The fluorine atom in A1 would play an equivalent role. A simple model for LTG-receptor interaction is proposed. 相似文献
82.
Kumar K Michalik D Garcia Castro I Tillack A Zapf A Arlt M Heinrich T Böttcher H Beller M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(3):746-757
A practical route for the synthesis of new biologically active 5-HT(2 A) receptor antagonists has been developed. In only three catalytic steps, this class of central nervous system (CNS) active compounds can be synthesized efficiently with high diversity. As the initial step, an anti-Markovnikov addition of amines to styrenes provides an easy route to N-(arylalkyl)piperazines, which constitute the core structure of the active molecules. Here, base-catalyzed hydroamination reactions of styrenes with benzylated piperazine proceeded in high yield even at room temperature. After catalytic debenzylation, the free amines were successfully carbonylated with different aromatic and heteroaromatic halides and carbon monoxide to yield the desired compounds in good to excellent yields. The two key reactions, base-catalyzed hydroamination of styrenes and palladium-catalyzed aminocarbonylation of haloarenes/heterocycles, showed tolerance towards various functional groups, thereby demonstrating the potential to synthesize a wide variety of new derivatives of this promising class of pharmaceuticals. 相似文献
83.
The use of an amperometric biosensor for the salicylate determination in blood serum is described. The biosensor is based on salicylate hydroxylase (EC 1.14.13.1) electropolymerized onto a glassy carbon-working electrode with polypyrrole and glutaraldehyde, to improve the biosensor lifetime. The hexacyanoferrate (II) was also incorporated to work as a redox mediator to minimize possible interferences. The salicylate is enzymatically converted to catechol, which is monitored amperometrically by its electrooxidation at+0.170 V versus SCE (saturated calomel electrode). Salicylate determination was carried out maintaining the ratio between beta-NADH and salicylate at 4:1 (30 degrees C). The amperometric response of the biosensor was linearly proportional to the salicylate concentration between 2.3x10(-6) and 1.4x10(-5) mol l(-1), in 0.1 mol l(-1) phosphate buffer (pH 7.8), containing 0.1 mol l(-1) KCl and 5.0x10(-4) mol l(-1) Na(2)H(2)EDTA, as supporting electrolyte. The recovery studies, in the presence of several interfering compounds, showed recoveries between 96.4 and 104.8%. The useful lifetime of the biosensor in the concentration range evaluated was at least 40 days, in continuous use. Blood serum samples analyzed by this biosensor showed a good correlation compared to the spectrophotometric method (Trinder) used as reference, presenting relative deviations lower than 7.0%. 相似文献
84.
S. R. de Miguel G. Torres A. A. Castro O. A. Scelza 《Reaction Kinetics and Catalysis Letters》1993,51(2):443-447
The hydrogenation activity of Pt supported on activated carbon would depend on both the Pt dispersity (which can be modified by a partial carbon burn-off in air) and the impurities concentration. 相似文献
85.
Laura M. Bruno José L. de Lima Filho Eduardo H. de M. Melo Heizir F. de Castro 《Applied biochemistry and biotechnology》2004,113(1-3):189-199
Mucor miehei lipase was immobilized on magnetic polysiloxane-polyvinyl alcohol particles by covalent binding with high activity recovered.
The performance of the resulting immobilized biocatalyst was evaluated in the synthesis of flavor esters using heptane as
solvent. The impact on reaction rate was determined for enzyme concentration, molar ratio of the reactants, carbon chain length
of the reactants, and alcohol structure. Ester synthesis was maximized for substrates containing excess acyl donor and lipase
loading of 25 mg/mL. The biocatalyst selectivity for the carbon chain length was found to be different concerning the organic
acids and alcohols. High reaction rates were achieved for organic acids with 8 or 10 carbons, whereas increasing the alcohol
carbon chain length from 4 to 8 carbons gave much lower esterification yields. Optimal reaction rate was determined for the
synthesis of butyl caprylate (12 carbons). Esterification performance was also dependent on the alcohol structure, with maximum
activity occurring for primary alcohol. Secondary and tertiary alcohols decreased the reaction rates by more than 40%. 相似文献
86.
Andrés Izquierdo Pilar Linares M. D. Luque de Castro Miguel Valcarcel 《Fresenius' Journal of Analytical Chemistry》1990,336(6):490-493
Summary Two fluorimetric methods are proposed for the simultaneous determination of ammonia and urea based on the reaction of the former with o-phthalaldehyde/2-mercaptoethanol and on the use of a dual injection system in which the valves are coupled in series or in parallel. An enzymatic reactor converts urea into ammonia which is determined by the same reaction. These configurations provide very similar results with a linear determination range between 0.5 and 10.0 g ml–1 and a sampling frequency of 60 h–1. The proposed methods are very tolerant to foreign species commonly found in water, as shown by applying them to samples of different origins. 相似文献
87.
Correlation between the temperature at which a base is desorbed and the acid strength (Ho) of the sites set free by it, was looked for. Ammonia was employed as base. A linear correlation of the type Ho=+/T (K) is valid with and constants, dependent on the adsorbate and the adsorbent.
(Ho) , . Ho=+/T (K), .相似文献
88.
89.
Lisardo Núñez F. Fraga A. Castro L. Fraga 《Journal of Thermal Analysis and Calorimetry》1998,52(3):1013-1022
The influence of the resin/diamine ratio on the properties of the system diglycidyl ether of bisphenol A (BADGE n=0/m-xylylenediamine)
(m-XDA) was studied. Variation of this ratio resulted in significant effects on the cure kinetics and final dynamic mechanical
properties of the product material.
The study was made in terms of storage modulus (E′), vss modulus (E″) and molecular mass between cross-links (Mc) at different ratios. Two geometries (cylindrical and rectangular) were considered. The influence of temperature was studied
through the activation energy (Ea>), which depends on the epoxy/amine ratio and the geometry of the samples. Glass transition temperatures (Tg>) and glass transition temperatures for thermosets with null degree of conversion (Tgo>) were determined by DSC. Tg> decreases when amounts of curing agent greatly in excess of the stoichiometric composition were used.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
90.
A continuous method, easy to automate, for the determination of anthocyanins in wine based on the coupling of continuous liquid–solid extraction, evaporation, HPLC individual separation and photometric detection is proposed. The target analytes are removed from the wine in a continuous fashion using a C18 minicolumn and eluted with an aqueous solution (pH 2) with 16% acetonitrile. The eluted fraction is concentrated by solvent evaporation assisted by heat and dragging off the vapour using a flow of N2. For in-line preconcentration, a continuous evaporation module was designed and located in the manifold between the solid-phase minicolumn and the injection valve of the chromatograph. In this way, injection of the sample into the dynamic system leads the plug through it for liquid–solid extraction of the anthocyanins, partial evaporation of the eluent (with a preconcentration factor as required) and transport to the high-pressure injection valve of the chromatograph, where individual separation and subsequent photometric detection take place. The method thus developed for the determination of malvidin-3-glucoside, cyanidin-3-glucoside and peonidin-3-glucoside anthocyanins in Spanish red wines is more sensitive than the batch manual method based on the same steps, has better linearity of the calibrations curves with lower detection limits and much wider determination range for the most abundant anthocyanins in wine. In addition, the method can be fully automated with low acquisition and maintenance costs. 相似文献