全文获取类型
收费全文 | 4085篇 |
免费 | 115篇 |
国内免费 | 22篇 |
专业分类
化学 | 2537篇 |
晶体学 | 29篇 |
力学 | 122篇 |
数学 | 602篇 |
物理学 | 932篇 |
出版年
2023年 | 22篇 |
2022年 | 45篇 |
2021年 | 82篇 |
2020年 | 66篇 |
2019年 | 54篇 |
2018年 | 52篇 |
2017年 | 49篇 |
2016年 | 129篇 |
2015年 | 113篇 |
2014年 | 102篇 |
2013年 | 221篇 |
2012年 | 225篇 |
2011年 | 260篇 |
2010年 | 161篇 |
2009年 | 165篇 |
2008年 | 239篇 |
2007年 | 229篇 |
2006年 | 239篇 |
2005年 | 221篇 |
2004年 | 200篇 |
2003年 | 159篇 |
2002年 | 146篇 |
2001年 | 80篇 |
2000年 | 67篇 |
1999年 | 52篇 |
1998年 | 46篇 |
1997年 | 41篇 |
1996年 | 54篇 |
1995年 | 33篇 |
1994年 | 34篇 |
1993年 | 33篇 |
1992年 | 23篇 |
1991年 | 17篇 |
1990年 | 31篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 13篇 |
1986年 | 23篇 |
1985年 | 52篇 |
1984年 | 47篇 |
1983年 | 26篇 |
1982年 | 38篇 |
1981年 | 29篇 |
1980年 | 37篇 |
1979年 | 40篇 |
1978年 | 15篇 |
1977年 | 22篇 |
1976年 | 20篇 |
1974年 | 14篇 |
1973年 | 16篇 |
排序方式: 共有4222条查询结果,搜索用时 31 毫秒
61.
An expeditious synthesis of t-butyldimethylsilyl triflate is described from triflic acid and t-butyldimethylsilane. Reaction between trichloromethyltrimethylsilane and triflic acid is also reported. 相似文献
62.
Highly reliable and accurate analytical methods are needed for the determination of magnetic resonance imaging (MRI) contrast agents in complex matrices of clinical interest. We demonstrate the reliability of capillary zone electrophoresis (CZE) coupled with electrospray ionization-mass spectrometry (ESI-MS) for the analysis of MultiHance (gadobenate dimeglumine), a gadolinium-based MRI agent. A sheath liquid interface connected the CE system with an electrospray mass spectrometer equipped with an ion-trap analyzer. CZE with ultraviolet (CZE-UV) and with mass detection (CZE-MS) were compared by analyzing gadobenate dimeglumine and the free ligand diluted in water and in biological fluids (i.e., human serum and urine). The optimization of some relevant CZE-MS parameters was accomplished, like CE buffer composition, sheath liquid composition and flow, and type and length of the separation capillary. CZE-UV was highly influenced by the biological sample components, which hindered a reliable quantification of both gadobenate and free ligand in serum and urine. In CZE-MS, on the other hand, the electrophoretic runs turned out to be independent of the clinical matrices, due to the informative potential and to the selectivity of MS detection. 相似文献
63.
Huang X Margulis CJ Li Y Berne BJ 《Journal of the American Chemical Society》2005,127(50):17842-17851
When supercritical CO2 is dissolved in an ionic liquid, its partial molar volume is much smaller than that observed in most other solvents. In this article we explore in atomistic detail and explain in an intuitive way the peculiar volumetric behavior experimentally observed when supercritical CO2 is dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim+] [PF6(-)]). We also provide physical insight into the structure and dynamics occurring across the boundary of the CO2 ionic liquid interface. We find that the liquid structure of [Bmim+] [PF6(-)] in the presence of CO2 is nearly identical to that in the neat ionic liquid (IL) even at fairly large mole fractions of CO2. Our simulations indicate, in agreement with experiments, that partial miscibilities of one fluid into the other are very unsymmetrical, CO2 being highly soluble in the ionic liquid phase while the ionic liquid is highly insoluble in the CO2 phase. We interpret our results in terms of the size and shape of spontaneously forming cavities in the ionic liquid phase, and we propose that CO2 occupies extremely well-defined locations in the IL. Even though our accurate prediction of cavity sizes in the neat IL indicates that these cavities are small compared with the van der Waals radius of a single carbon or oxygen atom, CO2 appears to occupy a space that was for the most part a priori "empty". 相似文献
64.
Magnani A Marchettini N Ristori S Rossi C Rossi F Rustici M Spalla O Tiezzi E 《Journal of the American Chemical Society》2004,126(37):11406-11407
The present work deals with the spatially extended oscillatory Belousov Zhabotinsky reaction-diffusion system carried out in an anisotropic environment of phosphatidylcholines/water binary system, which presents layered aqueous domains separated by lipid bilayers. We report the occurrence of stable Turing patterns, spiral waves, and other exotic structures in phospholipids bilayers that are generally used as a models for cell plasma membranes. 相似文献
65.
Quassinoids from Picrasma crenata 总被引:1,自引:0,他引:1
From woods of Picrasma crenata, a new stereoisomer dihydronorneoquassin was obtained together with others well knowns dihydronorneoquassin, parain, alpha-neoquassin, beta-neoquassin and quassin. The structures were determined by spectroscopic data and chemical evidence. 相似文献
66.
A series of acyclic vinyl ethers have been prepared and treated with Grubbs' ruthenium catalyst ((Cy3P)2RuCl2=CHPh). Contrary to previous findings, it has been shown that certain vinyl ethers are excellent substrates for the ring closing metathesis with ruthenium based catalysts. 相似文献
67.
Cecilia Bartolucci Luciano Cellai Concetta Martuccio Andrea Rossi Anna Laura Segre Simona Rizea Savu Luigi Silvestro 《Helvetica chimica acta》1996,79(6):1611-1619
Glycosylrifamycins, a new type of semisynthetic rifamycin derivatives, can be easily obtained by reaction of 3-(2-aminoethylthio)rifamycin SV ( 2 ) with a glycosyl compound carrying a coupling group, such as isothicyanate or carboxy. We prepared O-acetylated and free glucopyranosyl and arabinopyranosyl derivatives of rifamycin S and SV (see 3–10 ). Additionally, derivatives with D -saccharo-1,4-lactone and with shikimic acid were obtained (see 11–15 ). Glycosylrifamycins show an interesting inhibitory power on Gram-positive bacteria (Table). 相似文献
68.
S. De Rossi E. Iguchi M. Schiavello R. J. D. Tilley 《Reaction Kinetics and Catalysis Letters》1977,6(2):201-208
The oxidation of propene was studied on several tungsten oxides which contained small amounts of Ti, Ta, Nb and Sn. Only the Sn-containing specimen was found to be selective in the conversion of propene to acrolein. The catalytic results are correlated with crystal structures determined by electron microscopy.
, Ti, Ta, Nb Sn. , , Sn, . , .相似文献
69.
Marco Vincenti Claudio Minero Stefania Lago Costanza Rovida 《Journal of separation science》1995,18(6):359-362
A rapid and effective procedure, developed for the determination of polar compounds in water, has been applied to hydroxycarbamates. In few minutes, it is possible to perform a catalyzed derivatization of such compounds directly in the aqueous medium, using n-hexyl chloroformate. Extraction with n-hexane and injection into a benchtop GC-MS system are the only two further steps to complete the analysis. Chemical ionization proved to be more effective than electron impact to produce valuable mass spectra in terms of selectivity and sensitivity. Linear quantitative responses were demonstrated over a two-order-of-magnitude range. The minimum detectable concentration was 3 ppb for acetohydroxamic acid and 10 ppb for hydroxyurethane, obtained from Po river water samples spiked with the two hydroxycarbamate standards. 相似文献
70.
Calorimetric measurements were carried out on ester-solvent systems where the esters were H(CH2)xCOO(CH2)yH, with x and y varying from 1 to 5, and the solvents were n-hexane and 1,2-dichloroethane. Calculation of the enthalpies of cavity formation enabled the enthalpies of interaction to be determined. Both enthalpies correlated with number of carbon atoms N, equal to x+y in the esters, giving for 1,2-dichloroethane
相似文献 |