全文获取类型
收费全文 | 4670篇 |
免费 | 207篇 |
国内免费 | 32篇 |
专业分类
化学 | 3518篇 |
晶体学 | 25篇 |
力学 | 119篇 |
数学 | 574篇 |
物理学 | 673篇 |
出版年
2023年 | 38篇 |
2022年 | 57篇 |
2021年 | 80篇 |
2020年 | 102篇 |
2019年 | 108篇 |
2018年 | 72篇 |
2017年 | 61篇 |
2016年 | 140篇 |
2015年 | 128篇 |
2014年 | 155篇 |
2013年 | 242篇 |
2012年 | 311篇 |
2011年 | 372篇 |
2010年 | 169篇 |
2009年 | 137篇 |
2008年 | 337篇 |
2007年 | 326篇 |
2006年 | 325篇 |
2005年 | 268篇 |
2004年 | 231篇 |
2003年 | 197篇 |
2002年 | 167篇 |
2001年 | 51篇 |
2000年 | 46篇 |
1999年 | 48篇 |
1998年 | 44篇 |
1997年 | 44篇 |
1996年 | 65篇 |
1995年 | 40篇 |
1994年 | 41篇 |
1993年 | 45篇 |
1992年 | 29篇 |
1991年 | 42篇 |
1990年 | 30篇 |
1989年 | 15篇 |
1988年 | 20篇 |
1987年 | 20篇 |
1986年 | 12篇 |
1985年 | 26篇 |
1984年 | 35篇 |
1983年 | 18篇 |
1982年 | 24篇 |
1981年 | 23篇 |
1980年 | 25篇 |
1979年 | 19篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 14篇 |
1975年 | 10篇 |
1974年 | 20篇 |
排序方式: 共有4909条查询结果,搜索用时 15 毫秒
161.
Reaction of the cationic complex [WI(CO)(NCMe){Ph2P(CH2)PPh2}(η2-MeC2ME)][BF4] with an equimolar amount of MX (MX = NaCl, NaBr, NaI, KNO2, KNO3, NaNCS or KOH) in acetone at room temperature gave the neutral complex [WIX(CO){Ph2P(CH2)PPh2}(η2-MeC2Me)] (1–7) in good yield. Complexes 1–7 have been characterized by elemental analysis (C, H and N), IR and 1H NMR spectroscopy. 相似文献
162.
Scheuermann JE Sibbons KF Benoit DM Motevalli M Watkinson M 《Organic & biomolecular chemistry》2004,2(18):2664-2670
A number of chiral unsymmetrically N-substituted 1,4,7-triazacyclononane ligands have been prepared by modular methods. The key step in the synthesis centres on the macrocyclisation of three tertiary amide precursors under standard Richman-Atkins conditions which allows for subsequent N-functionalisation. 相似文献
163.
Good K Kuksenok O Buxton GA Ginzburg VV Balazs AC 《The Journal of chemical physics》2004,121(12):6052-6063
We investigate the structural evolution of an A/B/C ternary mixture in which the A and B components can undergo a reversible chemical reaction to form C. We developed a lattice Boltzmann model for this ternary mixture that allows us to capture both the reaction kinetics and the hydrodynamic interactions within the system. We use this model to study a specific reactive mixture in which C acts as a surfactant, i.e., the formation of C at the A/B interface decreases the interfacial tension between the A and B domains. We found that the dynamics of the system is different for fluids in the diffusive and viscous regimes. In the diffusive regime, the formation of a layer of C at the interface leads to a freezing of the structural evolution in the fluid; the values of the reaction rate constants determine the characteristic domain size in the system. In the viscous regime, where hydrodynamic interactions are important, interfacial reactions cause a slowing down of the domain growth, but do not arrest the evolution of the mixture. The results provide guidelines for controlling the morphology of this complex ternary fluid. 相似文献
164.
[reaction: see text] By using a sequence of regiocontrolled halogenation and palladium-catalyzed coupling reactions, the synthesis of variously substituted oxazoles from ethyl 2-chlorooxazole-4-carboxylate (2) was accomplished. The methodology was applied to the synthesis of a series of 2,4-disubstituted, 2,5-disubstituted, and 2,4,5-trisubstituted oxazoles. 相似文献
165.
Kevin M. Downard Klaus Biemann 《Journal of the American Society for Mass Spectrometry》1993,4(11):874-831
Ammo acid sequence prerequisites are described for the formation of c, ions observed in high-energy collision-induced decomposition spectra of peptides. It is shown that the formation of cn ions is promoted by the nature of the amino acid C-terminal to the cleavage site. A propensity for cn cleavage preceding threonine, and to a lesser extent tryptophan, lysine, and serine, is demonstrated where fragmentation is directed N-terminally at these residues. In addition, the nature of the residue N-terminal to the cleavage site is shown to have little effect on cn ion formation. A mechanism for cn ion formation is proposed and its applicability to the results observed is discussed. 相似文献
166.
Elettra L. Piacentino Edwin Rodriguez Kevin Parker Thomas M. Gilbert Richard A. J. O'Hair Victor Ryzhov 《Journal of mass spectrometry : JMS》2019,54(6):520-526
Gas‐phase C―C coupling reactions mediated by Ni (II) complexes were studied using a linear quadrupole ion trap mass spectrometer. Ternary nickel cationic carboxylate complexes, [(phen)Ni (OOCR1)]+ (where phen = 1,10‐phenanthroline), were formed by electrospray ionization. Upon collision‐induced dissociation (CID), they extrude CO2 forming the organometallic cation [(phen)Ni(R1)]+, which undergoes gas‐phase ion‐molecule reactions (IMR) with acetate esters CH3COOR2 to yield the acetate complex [(phen)Ni (OOCCH3)]+ and a C―C coupling product R1‐R2. These Ni(II)/phenanthroline‐mediated coupling reactions can be performed with a variety of carbon substituents R1 and R2 (sp3, sp2, or aromatic), some of them functionalized. Reaction rates do not seem to be strongly dependent on the nature of the substituents, as sp3‐sp3 or sp2‐sp2 coupling reactions proceed rapidly. Experimental results are supported by density functional theory calculations, which provide insights into the energetics associated with the C―C bond coupling step. 相似文献
167.
Wyche KP Blake RS Willis KA Monks PS Ellis AM 《Rapid communications in mass spectrometry : RCM》2005,19(22):3356-3362
The technique of proton transfer reaction mass spectrometry (PTR-MS) couples a proton transfer reagent, usually H3O+, with a drift tube and mass spectrometer to determine concentrations of volatile organic compounds. Here we describe a first attempt to use chemical ionization (CI) reagents other than proton transfer species inside a PTR-MS instrument. The ability to switch to other types of CI reagents provides an extra dimension to the technique. This capability is demonstrated by focusing on the ability to distinguish several isobaric aldehydes and ketones, including the atmospherically important molecules methacrolein and methyl vinyl ketone. Two CI reagents were selected, H3O+ and NO+, both being cleanly generated in a low intensity radioactive source prior to injection into the drift tube. By recording spectra with both of these reagents, the contributions from different isobaric molecules can be separated by virtue of their unique spectrometric 'fingerprints'. The work demonstrates that this form of instrumentation is not restricted to proton transfer reagents and is the basis of a more general technique, chemical ionization reaction mass spectrometry (CIRMS). 相似文献
168.
The fluorescence of the water-soluble anionic conjugated polymer, poly[lithium 5-methoxy-2-(4-sulfobutoxy)-1,4-phenylenevinylene] (MBL-PPV), is quenched in dilute aqueous solution by cytochrome c, a small, naturally occurring electron-transfer protein. The large value obtained for the Stern-Volmer constant (K(sv) = 3.2 x 10(8) at pH 7.4, and approximately 10(9) in acidic solutions) is attributed to a combination of two factors: (1) facile ET between the luminescent semiconducting polymer and the protein and (2) the Columb attraction between the oppositely charged polyelectrolytes. This system shows significant potential for biosensor applications. 相似文献
169.
Bruce S. Dien Nick Nagle Kevin B. Hicks Vijay Singh Robert A. Moreau Melvin P. Tucker Nancy N. Nichols David B. Johnston Michael A. Cotta Quang Nguyen Rodney J. Bothast 《Applied biochemistry and biotechnology》2004,115(1-3):937-949
Approximately 9% of the 9.7 billion bushels of corn harvested in the United States was used for fuel ethanol production in 2002, half of which was prepared for fermentation by dry grinding. The University of Illinois has developed a modified dry grind process that allows recovery of the fiber fractions prior to fermentation. We report here on conversion of this fiber (Quick Fiber [QF]) to ethanol. QF was analyzed and found to contain 32%wt glucans and 65%wt total carbohydrates. QF was pretreated with dilute acid and converted into ethanol using either ethanologenic Escherichia coli strain FBR5 or Saccharomyces cerevisiae. For the bacterial fermentation the liquid fraction was fermented, and for the yeast fermentation both liquid and solids were fermented. For the bacterial fermentation, the final ethanol concentration was 30 g/L, a yield of 0.44 g ethanol/g of sugar(s) initially present in the hydrolysate, which is 85% of the theoretical yield. The ethanol yield with yeast was 0.096 gal/bu of processed corn assuming a QF yield of 3.04 lb/bu. The residuals from the fermentations were also evaluated as a source of corn fiber oil, which has value as a nutraceutical. Corn fiber oil yields were 8.28%wt for solids recovered following prtetreatment. 相似文献
170.
Lynch CL Hale JJ Budhu RJ Gentry AL Finke PE Caldwell CG Mills SG MacCoss M Shen DM Chapman KT Malkowitz L Springer MS Gould SL DeMartino JA Siciliano SJ Cascieri MA Carella A Carver G Holmes K Schleif WA Danzeisen R Hazuda D Kessler J Lineberger J Miller M Emini E 《Organic letters》2003,5(14):2473-2475
[reaction: see text] A novel approach to alpha,alpha-disubstituted-beta-amino acids (beta(2,2)-amino acids) was employed in the synthesis of a series of 3-(pyrrolidin-1-yl)propionic acids possessing high affinity for the CCR5 receptor and potent anti-HIV activity. The rat pharmacokinetics for these new analogues featured higher bioavailabilities and lower rates of clearance as compared to cyclopentane 1. 相似文献