全文获取类型
收费全文 | 2842篇 |
免费 | 60篇 |
国内免费 | 51篇 |
专业分类
化学 | 2004篇 |
晶体学 | 13篇 |
力学 | 56篇 |
数学 | 511篇 |
物理学 | 369篇 |
出版年
2021年 | 21篇 |
2017年 | 17篇 |
2016年 | 50篇 |
2015年 | 29篇 |
2014年 | 57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 | 81篇 |
2005年 | 104篇 |
2004年 | 87篇 |
2003年 | 68篇 |
2002年 | 56篇 |
2001年 | 20篇 |
2000年 | 25篇 |
1999年 | 22篇 |
1998年 | 23篇 |
1997年 | 31篇 |
1996年 | 30篇 |
1995年 | 29篇 |
1994年 | 25篇 |
1992年 | 25篇 |
1991年 | 32篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 38篇 |
1987年 | 48篇 |
1986年 | 37篇 |
1985年 | 46篇 |
1984年 | 49篇 |
1983年 | 35篇 |
1982年 | 39篇 |
1981年 | 44篇 |
1980年 | 39篇 |
1979年 | 39篇 |
1978年 | 68篇 |
1977年 | 50篇 |
1976年 | 53篇 |
1975年 | 24篇 |
1974年 | 40篇 |
1973年 | 38篇 |
1972年 | 24篇 |
1971年 | 16篇 |
1969年 | 16篇 |
1885年 | 23篇 |
1884年 | 26篇 |
排序方式: 共有2953条查询结果,搜索用时 31 毫秒
91.
Tetranuclear V(III) complexes, [HB(pz)(3)](4)V(4)(&mgr;-C(6)H(5)OPO(3))(4) (I), its acetonitrile solvate (I.4CH(3)CN), and [HB(pz)(3)](4)V(4)(&mgr;-O(2)NC(6)H(4)OPO(3))(4).4C(7)H(8).H(2)O (II), and tetranuclear vanadyl complexes, (t-Bupz)(4)V(4)O(4)(&mgr;-C(6)H(5)PO(3))(4).2H(2)O (III) and (t-Bupz)(5)V(4)O(4)(&mgr;-C(6)H(5)PO(3))(4).4CH(3)CN.0.6 H(2)O (IV), have been prepared and characterized by spectroscopic, magnetic, and electrochemical methods (pz = pyrazole, t-Bupz = tert-butylpyrazole). The use of organic solvents and bulky organic groups as ancillary ligands leads to formation of neutral species instead of the anionic clusters commonly found in the hydrothermal synthesis of vanadium organophosphate/phosphonate systems. Complexes I.4CH(3)CN and IV have also been characterized by single-crystal X-ray diffraction. Crystal data: I.4CH(3)CN, triclinic, P&onemacr;, a = 15.495(3) ?, b = 17.000(3) ?, c = 17.949(4) ?, alpha = 89.17(3) degrees, beta = 86.00(3) degrees, gamma = 78.60(3) degrees, Z = 2; IV, triclinic, P&onemacr;, a = 15.541(3) ?, b = 16.340(2) ?, c = 19.069(5) ?, alpha = 83.58(2) degrees, beta = 79.67(2) degrees, gamma = 63.68(1) degrees, Z = 2. Both are closed clusters, the core structure of the first consisting of a cubane-like arrangement of metal octahedra and phosphate tetrahedra and the core structure of the second consisting of a distorted, collapsed variant of the first. Unlike other vanadium phosphate clusters, these compounds form in the absence of a central, templating agent. As such they represent the simplest form of a closed cluster in which steric forces and cluster connectivity requirements play the primary role in organizing the cluster framework. 相似文献
92.
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
93.
94.
95.
Z. V. I. Zaretskii J. M. Curtis A. G. Brenton J. H. Beynon Carl Djerassi 《Journal of mass spectrometry : JMS》1988,23(6):453-459
The mechanism of water elimination from metastable molecular, [M ? CH3˙]+ and [M ? ring D]+˙ ions of epimeric 3-hydroxy steroids of the 5α-series has been elucidated. Deuterium labelling, the measurement of the translational energy released during the loss of water, and collision-induced decomposition mass-analysed kinetic energy spectrometry were the techniques used. It was found that the mechanisms of water loss from metastable M+˙ and [M ? ring D]+˙ ions is different from that from [M ? CH3˙]+ ions. 相似文献
96.
97.
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
98.
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
99.
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
100.
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