全文获取类型
收费全文 | 2789篇 |
免费 | 60篇 |
国内免费 | 54篇 |
专业分类
化学 | 1955篇 |
晶体学 | 15篇 |
力学 | 56篇 |
数学 | 508篇 |
物理学 | 369篇 |
出版年
2021年 | 22篇 |
2017年 | 17篇 |
2016年 | 50篇 |
2015年 | 29篇 |
2014年 | 59篇 |
2013年 | 109篇 |
2012年 | 97篇 |
2011年 | 135篇 |
2010年 | 58篇 |
2009年 | 46篇 |
2008年 | 108篇 |
2007年 | 119篇 |
2006年 | 80篇 |
2005年 | 104篇 |
2004年 | 87篇 |
2003年 | 67篇 |
2002年 | 57篇 |
2001年 | 20篇 |
2000年 | 24篇 |
1999年 | 22篇 |
1998年 | 23篇 |
1997年 | 31篇 |
1996年 | 30篇 |
1995年 | 29篇 |
1994年 | 28篇 |
1993年 | 17篇 |
1992年 | 26篇 |
1991年 | 35篇 |
1990年 | 25篇 |
1989年 | 21篇 |
1988年 | 38篇 |
1987年 | 48篇 |
1986年 | 36篇 |
1985年 | 47篇 |
1984年 | 49篇 |
1983年 | 35篇 |
1982年 | 39篇 |
1981年 | 44篇 |
1980年 | 37篇 |
1979年 | 39篇 |
1978年 | 70篇 |
1977年 | 50篇 |
1976年 | 53篇 |
1975年 | 24篇 |
1974年 | 41篇 |
1973年 | 38篇 |
1972年 | 22篇 |
1969年 | 16篇 |
1885年 | 23篇 |
1884年 | 26篇 |
排序方式: 共有2903条查询结果,搜索用时 15 毫秒
71.
Patrick Koch Carl Djerassi Vijai Lakshmi Francis J. Schmitz 《Helvetica chimica acta》1983,66(8):2431-2436
Thirty-two sterols are identified in the sponge Hyrtios sp. In addition to fourteen monohydroxylated compounds, the sterol fraction contains eight functionalized sidechain sterols, three of them new: 3 β-hydroxy-24-norchol-5-en-23-al (19) , (22-trans)-3 β-hydroxycholest-5, 22-dien-24-one (20) and (22R, 23R, 24S) or (22S, 23S, 24S)- 22,23-epoxy-24-methylchoest-5-en-3 β-ol (24). The probable biological origin, rather than artifact production, of these undescribed components is discussed. 相似文献
72.
73.
74.
Robert E. Linder Hannah Weiler-Feilchenfeld Günter Barth Edward Bunnenberg Carl Djerassi 《Theoretical chemistry accounts》1974,36(2):135-143
CNDO calculations of the MCD spectra of 7- and 9-methyladenine have been carried out in two approximations. A minor modification of the CNDO/S method of Jaffé and coworkers shows an improvement over Jaffé's parameterization with respect to band separations and the signs of the MCDB terms, but is not applicable ton→ π* transitions. 相似文献
75.
Claude Delseth Luhata Tolela Carl Djerassi Paul J. Scheuer Robert J. Wells 《Helvetica chimica acta》1979,62(1):101-109
The steroidal components of 2 marine sponges, Terpios zeteki (from Hawaii) and Dysidea herbacea (from Australia) were fractionated through a combination of chromatographic methods, including reversed phase HPLC., and were analyzed by a combination of physical methods, including high resolution GC.-MS. and 360 MHz 1H-NMR. T. zeteki contains 6 conventional 5α-stanols which comprise 91% of the sterol mixture, and traces (0.5%) of a new C26 sterol, 5α-24-norcholestan-3β-ol. Minor amounts of conventional Δ5-sterols (6.5%) and of a single Δ4-3-ketosteroid (1.5%) were also present. In contrast, the Australian sponge (D. herbacea) contains 3 Δ5,7-sterols which comprise 1.5% of the sterol mixture, and one new C29 sterol, (24 Z)-stigmasta-5,7,24(28)-trien-3β-ol, as the major component (75%). In addition, minor amounts of conventional 5α-stanols (0.5%), Δ5-sterols (5%) and 5α-Δ7-sterols (18%) were present in this complex sterol mixture. The possible dietary or endosymbiotic origins of these sterols are discussed. 相似文献
76.
Mark SS Sandhyarani N Zhu C Campagnolo C Batt CA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(16):6808-6817
We report here a multistep route for the immobilization of DNA and proteins on chemically modified gold substrates using fourth-generation NH(2)-terminated poly(amidoamine) dendrimers supported by an underlying amino undecanethiol (AUT) self-assembled monolayer (SAM). Bioactive ultrathin organic films were prepared via layer-by-layer self-assembly methods and characterized by fluorescence microscopy, variable angle spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). The thickness of the AUT SAM base layer on the gold substrates was determined to be 1.3 nm from ellipsometry. Fluorescence microscopy and AFM measurements, in combination with analyses of the XPS/ATR-FTIR spectra, confirmed the presence of the dendrimer/biopolymer molecules on the multilayer sensor surfaces. Model proteins, including streptavidin and rabbit immunoglobulin proteins, were covalently attached to the dendrimer layer using linear cross-linking reagents. Through surface plasmon resonance measurements, we found that sensor surfaces containing a dendrimer layer displayed an increased protein immobilization capacity, compared to AUT SAM sensor surfaces without dendrimer molecules. Other SPR studies also revealed that the dendrimer-based surfaces are useful for the sensitive and specific detection of DNA-DNA interactions. Significantly, the multicomponent films displayed a high level of stability during repeated regeneration and hybridization cycles, and the kinetics of the DNA-DNA hybridization process did not appear to be influenced by surface mass transport limiting effects. 相似文献
77.
Jrgen Mller Carl Th. Pedersen Helge Egsgaard Elfinn Larsen 《Journal of mass spectrometry : JMS》1980,15(9):456-458
Electron impact spectra of thermolysis products of organic salts heated in the ion source of a mass spectrometer may give rise to organic ions corresponding to the cation of the salt. Field ionization mass spectrometry has been used as a facile method for detemining whether such an ion is due to ionization of the corresponding radical present in the gas phase, or to an electron impact induced fragmentation of a reaction product of higher mass. By comparison of the electron impact and field ionization spectra of a series of N-methyl pyridinium, tropylium and 1,2-dithiolylium salts it has been found possible to identify the radicals formed thermolytically, when present. 相似文献
78.
Pronoy K. Chatterjee Carl M. Conrad 《Journal of polymer science. Part A, Polymer chemistry》1966,4(3):459-474
Sakurada's equation and fundamental kinetic laws were applied to the heterogeneous cyanoethylation of cellulose, performed by reacting fiber with liquid acrylonitrile, with sodium hydroxide as the catalyst. The data fit Sakurada's equation better at higher temperatures; deviation occurs at the initial stage, and the rate of reaction falls abruptly at a later stage. The degree of substitution at which the abrupt rate change occurred decreased as the temperature increased from 31 to 60°C. and also as the crystallinity of the fiber decreased. Diluting the reagent with different solvents decreased the rate of reaction and changed its transition points, but did not change the essential nature of the reaction, each segment of which fits Sakurada's equation very well. A uniform distribution of the catalyst (sodium hydroxide) throughout the fiber was attempted, and then the reaction was studied at 50°C. Diffractograms of the samples provided further evidence that the position of the rate change is associated with the change of cellulose (I) crystalline structure. Approximate energy of activation has been calculated, from the specific rate constants, between 31 and 40°C. as 10.6 kcal. and between 45 and 50°C. as 16.7 kcal. At other temperatures the determination was handicapped, due to temperature dependence of the order of reaction. An empirical relation between the constants of Sakurada's equation and the reaction temperature has been sought and correlated with the Arrhenius equation. Energies of activation, determined from this relationship, have been found to be very close to the above values. The change of order of reaction with temperature suggests that the reaction is affected by diffusion and the mechanism is interpreted as a diffusion-controlled reaction where hydrogen bonds play a significant role in diffusion. 相似文献
79.
Ganesh K. Mothukuri Sangram S. Kale Carl L. Stenbratt Alessandro Zorzi Jonathan Vesin Julien Bortoli Chapalay Kaycie Deyle Gerardo Turcatti Laura Cendron Alessandro Angelini Christian Heinis 《Chemical science》2020,11(30):7858
Macrocycles provide an attractive modality for drug development, but generating ligands for new targets is hampered by the limited availability of large macrocycle libraries. We have established a solution-phase macrocycle synthesis strategy in which three building blocks are coupled sequentially in efficient alkylation reactions that eliminate the need for product purification. We demonstrate the power of the approach by combinatorially reacting 15 bromoacetamide-activated tripeptides, 42 amines, and 6 bis-electrophile cyclization linkers to generate a 3780-compound library with minimal effort. Screening against thrombin yielded a potent and selective inhibitor (Ki = 4.2 ± 0.8 nM) that efficiently blocked blood coagulation in human plasma. Structure–activity relationship and X-ray crystallography analysis revealed that two of the three building blocks acted synergistically and underscored the importance of combinatorial screening in macrocycle development. The three-component library synthesis approach is general and offers a promising avenue to generate macrocycle ligands to other targets.Combination of three efficient chemical reactions allows for solution-phase synthesis of 3780 macrocycles and identification of potent thrombin inhibitor. 相似文献
80.
Electrothermal stirring for heterogeneous immunoassays 总被引:1,自引:0,他引:1
A technique is proposed to enhance microfluidic immuno-sensors, for example, immunoassays, in which a ligand immobilized on a microchannel wall specifically binds analyte flowing through the channel. These sensors can be limited in both response time and sensitivity by the diffusion of analyte to the sensing surface. In certain applications, the sensitivity and response of these heterogeneous immunoassays may be improved by using AC electrokinetically-driven microscale fluid motion to enhance antigen motion towards immobilized ligands. Specifically, the electrothermal effect is used to micro-stir analyte near the binding surface. Numerical simulations of antigen in a microchannel flow subjected to the electrothermal effect show that 6 V(rms) applied to electrodes near a binding region can increase binding in the first few minutes by a factor of seven. The effectiveness of electrothermal stirring is a strong function of the Damk?hler number. The greatest binding enhancement is possible for high Damk?hler numbers, where the reaction is limited by diffusion. Based on these results, the utility of this technique for diffusion-limited microfluidic sensor applications is demonstrated. 相似文献