全文获取类型
收费全文 | 41184篇 |
免费 | 1611篇 |
国内免费 | 144篇 |
专业分类
化学 | 27181篇 |
晶体学 | 267篇 |
力学 | 816篇 |
数学 | 5339篇 |
物理学 | 9336篇 |
出版年
2023年 | 290篇 |
2022年 | 298篇 |
2021年 | 490篇 |
2020年 | 661篇 |
2019年 | 578篇 |
2018年 | 466篇 |
2017年 | 442篇 |
2016年 | 1097篇 |
2015年 | 983篇 |
2014年 | 1090篇 |
2013年 | 1891篇 |
2012年 | 2263篇 |
2011年 | 2622篇 |
2010年 | 1371篇 |
2009年 | 1178篇 |
2008年 | 2218篇 |
2007年 | 2142篇 |
2006年 | 2074篇 |
2005年 | 1881篇 |
2004年 | 1584篇 |
2003年 | 1304篇 |
2002年 | 1303篇 |
2001年 | 815篇 |
2000年 | 669篇 |
1999年 | 604篇 |
1998年 | 562篇 |
1997年 | 547篇 |
1996年 | 561篇 |
1995年 | 496篇 |
1994年 | 508篇 |
1993年 | 523篇 |
1992年 | 480篇 |
1991年 | 365篇 |
1990年 | 370篇 |
1989年 | 327篇 |
1988年 | 326篇 |
1987年 | 312篇 |
1986年 | 306篇 |
1985年 | 443篇 |
1984年 | 458篇 |
1983年 | 353篇 |
1982年 | 387篇 |
1981年 | 384篇 |
1980年 | 362篇 |
1979年 | 326篇 |
1978年 | 383篇 |
1977年 | 353篇 |
1976年 | 288篇 |
1975年 | 263篇 |
1974年 | 287篇 |
排序方式: 共有10000条查询结果,搜索用时 562 毫秒
991.
Wolfgang Schwack Bernd Brüger Steven Nyanzi 《Analytical and bioanalytical chemistry》1995,351(2-3):297-300
The differential pulse-polarographic (DPP) determination of both CS2 and COS gases, after their absorption in a methanolic piperidine reagent and the subsequent application of this technique to the residue analysis of dithiocarbamate fungicides is described. Rectilinear calibration curves for both CS2 and COS in the respective regions of 1.5–9.2 and 2.1–12.6 μmol/l were obtained. The DPP method has been successfully applied to the determination of thiram residues on apples after a hot acid hydrolysis of the fruits. 相似文献
992.
Tian Yu-Peng Duan Chun-Ying Lu Zhong-Lin You Xiao-Zeng Thomas C.W. Mak 《Polyhedron》1996,15(24):4415-4420
The crystal structure of the 1,10-phenanthroline bis(S-methyl-β-N-(4-methyloxyphenyl)methylendithiocarbazide)nickel(II) adduct, (Ni(SN)2phen) [SN = S-methyl-β-N-(4-methyloxyphenyl)methylendithiocarbazide, PHEN = phenanthroline], has been determined by single crystal X-ray diffraction. The nickel atom is in an octahedral environment, surrounded by two chelating SN ligands and one chelating phen molecule. The nitrogen atoms from phen are in the cis configuration. The other chelating diamines adducts of the parent complex (Ni(SN)2) were also studied, where the chelating diamnies are 5-nitro-1,10-phenanthroline(NO2phen), 2,2′-bipyridine (bipy), 4,4′-methyl-2,2′-bipyridine (Mebipy). The equilibria were determined by UV-vis spectrometry in dichloromethane. The coordination ability of the added ligands were influenced by substitute groupings and steric factors. From the structure and addition equilibrium studies, the possible addition mechanisms are also discussed. 相似文献
993.
994.
Let A denote a decomposable symmetric complex valued n-linear function on Cm. We prove , where · denotes the symmetric product and ? the tensor product. As a consequence we have per , where M is a positive semidefinite Hermitian matrix and per denotes the permanent function. A sufficient condition for equality in the matrix inequality is that M is a nonnegative diagonal matrix. 相似文献
995.
Wolfgang Stadlbauer Otto Schmut Prof. Dr. Thomas Kappe 《Monatshefte für Chemie / Chemical Monthly》1977,108(2):367-379
The reaction of N-(2-pyridyl)-carbaminates (3), 1-phenyl-3-(2-pyridyl)-urea (5 a) or alkyl and aryl substituted pyridopyrimidinediones (2) synthesized from 2-amino-pyridine with arylisocyanates (4 a, b) yields 3-aryl substituted pyrido-[1,2-a]-s-triazine-2,4-diones (1). 2-Aminopyridine and azamalonic derivatives (ethoxycarbonyl isocyanate, ethyl iminodicarboxylate) react to give the 3-unsubstituted triazine system (1 c). The isolation of monocyclic triazinediones obtained from the reaction of N-phenylbenzamidine (9) with aryl isocyanate (4 a) or ethoxycarbonyl isocyanate failed because of hydrolytic ring opening. The mechanism of the reaction of 3-substituted pyrido-pyrimidinediones (2) and phenyl isocyanate (4 a) is discussed. 相似文献
996.
Kai-Shue Lam I. Harold Zimmerman Jian-Min Yuan John R. Laing Thomas F. George 《Chemical physics》1977,26(3):455-486
Collion-induced emission in molecular systems in an intense laser field is studied using the semiclassical approach, with a view towards cooperative chemical and optical pumping in laser production. The formalism is developed with the electronic-field representation, which treats collision and radiative interaction on the same footing. Electronic-field surfaces can be regarded as forming spectra for spontaneous emission; and particular emission events can be accounted for by propagating classical trajectories on emission electronic-field surfaces. Pre-emission loss from the excited state is dealt with by propagating classical trajectories on a loss surface along a complex contour of emission branch points. This loss surface is derived on the basis of localized radiative couplings between electronic-field states and provides a framework to treat the general problem of discrete state-continuum interactions. The formalism is applied to a two-state, collinear exponential model to compute S-matrix elements and transition probabilities between asymptotic states. 相似文献
997.
A new procedure for the preparation of high aspect ratio Au nanowires utilizing gold electroless reduction in the hexagonally ordered, thiol-modified nanosized channels of the SBA-15 material is reported. Two different Au precursors were adsorbed onto pedant thiol groups, covalently bonded to the mesoporous silica surface, and used as seeds to grow extended Au nanostructures by treatment in Au electroless reduction bath. It is shown that the dimensions and the assembly of the Au seeds are important parameters for the subsequent electroless reduction process. The [AuCl4]- ions complexed to the TOAB molecules assembled on the thiol-modified mesoporous surface of the SBA-15 material are suitable precursors for the subsequent gold electroless reduction. The resulting structures are several micrometer long Au nanowires with uniform diameters of about 5 nm, having large single-crystalline domains. The TEM results clearly show that the growth of the Au nanowires is templated by the channel structure of the SBA-15 material. 相似文献
998.
Selective addition to iridium of aryl C-H bonds ortho to coordinating groups. Not chelation-assisted
Zhang X Kanzelberger M Emge TJ Goldman AS 《Journal of the American Chemical Society》2004,126(41):13192-13193
Precursors of the pincer-ligated iridium species, (PCP)Ir, react with nitrobenzene or acetophenone at ambient temperature to give O,C-chelated complexes resulting from addition of an aryl C-H bond and coordination of a nitro or acetyl oxygen. The C-H additions appear to be completely regioselective for the position ortho to the functional group; however, structural characterization and low-temperature NMR studies demonstrate that the reaction does not proceed via coordination of the functional group followed by C-H addition. In the case of nitrobenzene, kinetic preference for the para and meta positions is demonstrated at low temperature. Addition occurs more slowly at the ortho position, without assistance by the functional group; the ortho-C-H addition product is then trapped by chelation. 相似文献
999.
This work presents a method for extracellular and intracellular pH measurements in live cells based on a combination of the bead injection (BI) technique and fluorescence microscopy. For extracellular pH measurement, cells are grown on fluorescent beads, packed into a small column by a sequential injection instrument, and fluorescence intensity from the beads stained by the indicator is recorded by a fluorescence microscope. The method is applied to quantifying carbachol stimulation of Chinese hamster ovary (CHO) cells transfected with the m1 muscarinic receptor and is verified by a glucose depletion experiment. The results yield an EC(50) value of 1 muM for carbachol, which is in reasonable agreement with the literature value 3 muM determined by an existing potentiometric technique for measuring acid release. The intracellular measurement utilizes CHO M1 cells growing on non-fluorescent beads. For this method the cells rather than the beads are stained by incubating them in a solution of the fluorescent pH indicator BCECF. The cells are also stimulated with carbachol and the intracellular pH dependent fluorescence from the cells is recorded. The results show dependence between intracellular pH changes and carbachol concentration and yield an EC(50) value of 4 muM. 相似文献
1000.
Johannes Häusler 《Monatshefte für Chemie / Chemical Monthly》1982,113(10):1213-1216
Meldrum's acid (2) is acylated by diketen affording the acylMeldrum's acid3. In aqueous bicarbonate its 3-oxogroup is selectively reduced by sodium tetrahydroborate giving the alcohol4, which readily undergoes cyclization in refluxing dioxane leading to the title pyrone7. Under identical conditions3 produces the pyrone6 with the pyrone carboxylic acid5 as intermediate. 相似文献