全文获取类型
收费全文 | 2384篇 |
免费 | 74篇 |
国内免费 | 16篇 |
专业分类
化学 | 1593篇 |
晶体学 | 16篇 |
力学 | 43篇 |
数学 | 466篇 |
物理学 | 356篇 |
出版年
2022年 | 23篇 |
2021年 | 28篇 |
2020年 | 26篇 |
2019年 | 36篇 |
2018年 | 23篇 |
2017年 | 25篇 |
2016年 | 37篇 |
2015年 | 31篇 |
2014年 | 42篇 |
2013年 | 107篇 |
2012年 | 83篇 |
2011年 | 131篇 |
2010年 | 79篇 |
2009年 | 64篇 |
2008年 | 115篇 |
2007年 | 92篇 |
2006年 | 110篇 |
2005年 | 106篇 |
2004年 | 118篇 |
2003年 | 102篇 |
2002年 | 87篇 |
2001年 | 20篇 |
2000年 | 33篇 |
1999年 | 23篇 |
1997年 | 20篇 |
1996年 | 52篇 |
1994年 | 29篇 |
1993年 | 21篇 |
1992年 | 25篇 |
1991年 | 22篇 |
1990年 | 28篇 |
1989年 | 21篇 |
1988年 | 20篇 |
1987年 | 18篇 |
1986年 | 21篇 |
1985年 | 43篇 |
1984年 | 34篇 |
1983年 | 28篇 |
1982年 | 32篇 |
1981年 | 32篇 |
1980年 | 31篇 |
1979年 | 27篇 |
1978年 | 35篇 |
1977年 | 32篇 |
1976年 | 22篇 |
1975年 | 29篇 |
1974年 | 34篇 |
1973年 | 30篇 |
1970年 | 20篇 |
1968年 | 19篇 |
排序方式: 共有2474条查询结果,搜索用时 187 毫秒
71.
Daniel Ammann Ren Bissig Marc Güggi Ern Pretsch Wilhelm Simon Irving J. Borowitz Louis Weiss 《Helvetica chimica acta》1975,58(6):1535-1548
The preparation of a series of non-cyclic, uncharged ligands able to selectively complex alkali and alkaline earth metal cations is described. These molecules are designed to be used as carriers for cations through membranes. Some of the compounds show high Ca2+ and Na+ selectivity, respectively, in liquid membrane electrodes. 相似文献
72.
Reactions of HgCl2 with η5-C5H5Fe(CO)2R (R CH2CHCH2 and CH2C(CH3)CH2) in THF at 25°C rapidly afford adducts of the two reactants. These adducts were converted to the corresponding PF6? salts, [η5-C5H5Fe(CO)2(η2-CH2C(R)CH2HgCl)]+ PF6? (R H and CH3), for characterization. Slower reactions with cleavage of the ironcarbon σ bond and elimination of the R group from η5-C5H5Fe(CO)2R occur for R CH2CHC(CH3)2, CH2CHCHC6H5, and CH2CCC6H5. Both elimination and adduct formation are observed when R CH2CHCHCH3. The kinetics of the cleavage reactions are presented and possible mechanisms for both cleavage and adduct formation are discussed. 相似文献
73.
We report three highly stereoselective pericyclic reactions occurring in cascade leading to the synthesis of Decalins skeletons possessing two contiguous quaternary centers. The tandem reaction is triggered by an oxy-Cope rearrangement to create in situ a 10-membered ring enol ether macrocyle 6, which immediately rearranges via a Claisen [3,3] shift to the corresponding E-cyclodec-6-en-1-one 7. The latter spontaneously cyclizes via a transannular ene reaction to produce Decalin 5. Analysis of the mechanism with respect to the origin of the high diastereoselectivity of the tandem oxy-Cope/Claisen/ene reaction is presented. 相似文献
74.
Arafat A Schroën K de Smet LC Sudhölter EJ Zuilhof H 《Journal of the American Chemical Society》2004,126(28):8600-8601
This communication presents the first functionalization of a hydrogen-terminated silicon-rich silicon nitride (Si3Nx) surface with a well-defined, covalently attached organic monolayer. Properties of the resulting monolayers are monitored by measurement of the static water contact angle, X-ray photoelectron spectroscopy (XPS), and infrared reflection absorption spectroscopy (IRRAS). Further functionalization was performed by reaction of Si3Nx with a trifluoroethanol ester alkene (CH2=CH-(CH2)8CO2CH2CF3) followed by basic hydrolysis to afford the corresponding carboxylic acid-terminated monolayer with hydrophilic properties. These results show that Si3Nx can be functionalized with a tailor-made organic monolayer, has highly tunable wetting properties, and displays significant potential for further functionalization. 相似文献
75.
Chlorine trioxide, Cl(2)O(6), reacts with Au metal, AuCl(3), or HAuCl(4).nH(2)O to yield the well-defined chloryl salt, ClO(2)Au(ClO(4))(4). The crystal and molecular structure of ClO(2)Au(ClO(4))(4) was solved by a Rietveld analysis of powder X-ray diffraction data. The salt crystallizes in a monoclinic cell, space group C2/c, with cell parameters a = 15.074(5), b = 5.2944(2), and c = 22.2020(2) A and beta = 128.325(2) degrees. The structure displays discrete ClO(2)(+) ions lying in channels formed by Au(ClO(4))(4)(-) stacks. Au is located in a distorted square planar environment: Au-O = 1.87 and 2.06 A. [ClO(4)] groups are monodentate with ClO(b) = 1.53 and ClO(t) = 1.39 A (mean distances; O(b), oxygen bonded to Au; O(t), free terminal oxygen). A full vibrational study of the Au(ClO(4))(4)(-) anion is supported by DFT calculations. 相似文献
76.
Louis S. Hegedus 《Angewandte Chemie (International ed. in English)》1988,27(9):1113-1126
The use of transition-metal complexes as reagents for the synthesis of complex organic compounds has been under development for at least several decades, and many extraordinary organic transformations of profound potential have been realized. However, adoption of this chemistry by the practicing synthetic organic chemist has been inordinately slow, and only now are transition-metal reagents beginning to achieve their rightful place in the arsenal of organic synthesis. Several factors contributed to the initial reluctance of synthetic organic chemists to use organometallic reagents. Lacking education and experience in the ways of elements having d electrons, synthetic chemists viewed organometallic processes as something mysterious and unpredictable, and not to be discussed in polite society. Organometallic chemists did not help matters by advertising their latest advances as useful synthetic methodology, but restricting their studies to very simple organic systems lacking any serious functionality (e.g., the “methyl, ethyl, butyl, futile” syndrome). Happily, things have changed. Organometallic chemists have turned their attention to more complex systems, and more recently trained organic chemists have benefited from exposure to the application of transition metals. This combination has set the stage for major advances in the use of transition metals in the synthesis of complex organic compounds. This review deals with one aspect of this area, the use of transition metals in the synthesis of indoles. 相似文献
77.
Species produced by electron transfer to variously substituted bicyclo[3.1.0]hex-3-en-2-ones provide a better insight into the origin of 1,6-addition products sometimes observed by reaction of lithium diorganocuprates with β-cyclopropyl α-enones. Cyclic voltammetry of eight such bicyclohexenones show that the half-lives of the corresponding anion radicals are very short (t?10?4s) except when the initial molecule is phenyl substituted at C-4. In such cases the anion radicals are very stable (t?6s) owing to the greater charge delocalization and we observe a second wave corresponding to the formation of the Dianion. The reduction of the same substrates by the solvated electron in liquid ammonia exhibits the same difference of behaviour. The molecules giving strongly reactive anion radicals only lead to the straight-forward product expected by conjugate reduction; while the 4-phenyl substituted substrates yield a mixture of products from normal conjugate reduction and rearrangement. This correlation strongly suggest that these last products arise from the rearrangement of the dianion. 相似文献
78.
Woodhouse SL Ziolkowski EJ Rendina LM 《Dalton transactions (Cambridge, England : 2003)》2005,(17):2827-2829
A series of novel, dinuclear (2,2':6',2'-terpyridine)platinum(ii) complexes containing bis(thioalkyl)-dicarba-closo-dodecaborane(12)(carborane) ligands were prepared and characterised, and their preliminary anti-cancer characteristics have been determined in vitro; the complexes are the first examples of bis-intercalator complexes containing a boron-rich carborane cage. 相似文献
79.
Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus pyramid in a phosphole as caused by a large, sterically demanding P-substituent. Earlier PE spectra had shown no difference in ionization energies (IE) for simple phospholes and their tetrahydro derivatives (both around 8.0-8.45 eV). Calculations of the Koopmans IE at the Hartree-Fock 6-31G level for 1-methylphospholane showed that, as is known for nitrogen, planarization at phosphorus markedly reduced the ionization energy value (8.74 to 6.29 eV). A reduction in IE also occurred on planarizing 1-methylphosphole, but to a lesser extent, being offset by increased electron delocalization (8.93 to 7.16 eV). This suggests that experimental comparison of IE for the unsaturated and saturated systems could be used to detect the presence of electron delocalization in the former. The IE experimentally determined for the crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was 7.9 eV, the lowest ever recorded for a phosphole. The corresponding phospholane had IE 7.55 eV. The difference in the values is attributed to electron delocalization in the phosphole. Calculations performed on the related model 1-(2-tert-butyl-4,6-dimethylphenyl)phosphole showed that the P-substituent adopted an angle of 55.7 degrees (DFT/6-31G level; 57.6 degrees at the HF/6-31 level) with respect to the C(2)-P-C(5) plane (for P-phenyl, 67.1 degrees and 68.3 degrees, respectively). 相似文献
80.