全文获取类型
收费全文 | 986篇 |
免费 | 26篇 |
国内免费 | 8篇 |
专业分类
化学 | 785篇 |
晶体学 | 7篇 |
力学 | 10篇 |
数学 | 68篇 |
物理学 | 150篇 |
出版年
2022年 | 7篇 |
2021年 | 14篇 |
2020年 | 8篇 |
2019年 | 19篇 |
2018年 | 16篇 |
2017年 | 6篇 |
2016年 | 18篇 |
2015年 | 14篇 |
2014年 | 29篇 |
2013年 | 49篇 |
2012年 | 54篇 |
2011年 | 57篇 |
2010年 | 31篇 |
2009年 | 22篇 |
2008年 | 44篇 |
2007年 | 59篇 |
2006年 | 50篇 |
2005年 | 62篇 |
2004年 | 65篇 |
2003年 | 45篇 |
2002年 | 56篇 |
2001年 | 14篇 |
2000年 | 13篇 |
1999年 | 10篇 |
1998年 | 9篇 |
1997年 | 11篇 |
1996年 | 24篇 |
1995年 | 11篇 |
1994年 | 12篇 |
1993年 | 7篇 |
1992年 | 13篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 15篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 18篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1976年 | 5篇 |
1974年 | 4篇 |
1971年 | 4篇 |
1970年 | 5篇 |
1967年 | 5篇 |
排序方式: 共有1020条查询结果,搜索用时 15 毫秒
41.
Rieko Furuyama Junji Saito Sei-ichi Ishii Makoto Mitani Shigekazu Matsui Yasushi Tohi Haruyuki Makio Naoto Matsukawa Hidetsugu Tanaka Terunori Fujita 《Journal of molecular catalysis. A, Chemical》2003,200(1-2):31-42
This contribution reports ethylene and propylene polymerization behavior of a series of Ti complexes bearing a pair of phenoxy–imine chelate ligands. The bis(phenoxy–imine)Ti complexes in conjunction with methylalumoxane (MAO) can be active catalysts for the polymerization of ethylene. Unexpectedly, this C2 symmetric catalyst produces syndiotactic polypropylene. 13C NMR spectroscopy has revealed that the syndiotacticity arises from a chain-end control mechanism. Substitutions on the phenoxy–imine ligands have substantial effects on both ethylene and propylene polymerization behavior of the complexes. In particular, the steric bulk of the substituent ortho to the phenoxy–oxygen is fundamental to obtaining high activity and high molecular weight for ethylene polymerization and high syndioselectivity for the chain-end controlled propylene polymerization. The highest ethylene polymerization activity, 3240 kg/mol-cat h, exhibited by a complex having a t-butyl group ortho to the phenoxy–oxygen, represents one of the highest reported to date for Ti-based non-metallocene catalysts. Additionally, the polypropylene produced exhibits a Tm, 140 °C, and syndioselectivity, rrrr 83.7% (achieved by a complex bearing a trimethylsilyl group ortho to the phenoxy–oxygen) that are among the highest for polypropylenes produced via a chain-end control mechanism. Hence, the bis(phenoxy–imine)Ti complexes are rare examples of non-metallocene catalysts that are useful for the polymerization of not only ethylene but also propylene. 相似文献
42.
43.
Murahashi S Imada Y Kawakami T Harada K Yonemushi Y Tomita N 《Journal of the American Chemical Society》2002,124(12):2888-2889
A chiral titanium complex, Ti(O-i-Pr)(4)/BINOL/tert-butylcatechol, catalyzes enantioselective addition reaction of ketene silyl acetals to nitrones to give optically active beta-amino acid derivatives which are biologically active compounds and useful synthetic intermediates of natural products and pharmaceuticals such as beta-lactam antibiotics. The combined process of catalytic oxidation of secondary amines and enantioselective carbon-carbon bond formation of nitrones thus obtained with ketene silyl acetals provides a useful two-step method for the synthesis of optically active beta-amino acid derivatives and related nitrogen compounds. 相似文献
44.
Duanjun Xu Kunio Miki Yasushi Kai Nobutami Kasai Masanori Wada 《Journal of organometallic chemistry》1985,287(2):265-273
The structure of a nickel(II) complex, trans-[Ni(C6Cl5)(PMe2Ph)2{C(OMe)Me}]BF4, containing the simplest alkyl(alkoxy)carbene ligand has been determined by X-ray crystallography (R = 0.091). The geometry around the nickel atom is square-planar. The comparatively short NiC(1) bond length of 1.843(10) Å showed the presence of π-bonding in the nickel-carbene bond. 相似文献
45.
Takimiya K Kunugi Y Konda Y Niihara N Otsubo T 《Journal of the American Chemical Society》2004,126(16):5084-5085
2,6-Diphenylbenzo[1,2-b:4,5-b']dichalcogenophenes including thiophene, selenophene, and tellurophene analogues as organic semiconductors for field-effect transistors were effectively synthesized in three steps from commercially available 1,4-dibromobenzene. All three benzodichalcogenophenes acted as good p-type semiconductors, and particularly the selenophene analogue, 2,6-diphenylbenzo[1,2-b:4,5-b']diselenophene, showed high FET mobility of 0.17 cm2 V-1 s-1. 相似文献
46.
Kashiwagi T Ohkoshi S Seino H Mizobe Y Hashimoto K 《Journal of the American Chemical Society》2004,126(16):5024-5025
We have prepared a pillared layer magnetic material containing a noncoordinated aromatic molecule, [{MnII(pyrimidine)(H2O)}2{MnII(H2O)2}{WV(CN)8}2](pyrimidine)2.2H2O. This compound has one-dimensional channels (6.2 x 2.1 A) that are occupied by noncoordinated pyrimidine. The magnetization versus temperature plots showed the magnetic phased transition temperature (TC) was 47 K. The magnetization versus external magnetic field plots showed that the saturation magnetization (MS) value was 13.0 muB at 2 K. This MS value indicates that an antiferromagnetic interaction operates between the WV (S = 1/2) and MnII (S = 5/2) ions. The magnetic hysteresis loop showed that the coercive field (HC) was 17 G at 2 K. 相似文献
47.
Jun-ichi Yamada Yasushi Nishimoto Satoru Tanaka Rie Nakanishi Kenji Hagiya Hiroyuki Anzai 《Tetrahedron letters》1995,36(52):9509-9512
The synthesis of BEDT-TTF derivatives fused with heterocycles (3–7) has been accomplished via the BF3-promoted reaction of organotin thiolates (8 and 9) with electrophiles (10 and 11). Electrical conductivities of radical cation salts derived from some of them were also investigated. 相似文献
48.
Iridium complexes show high catalytic activity in intermolecular additions of acid chlorides to terminal alkynes to afford valuable (Z)-β-chloro-α,β-unsaturated ketones. Ligands in the catalytic system play a crucial role in this reaction. An N-heterocyclic carbene (NHC) is an efficient ligand for the addition of aroyl chlorides, while dicyclohexyl(2-methylphenyl)phosphine (PCy(2)(o-Tol)) is indispensable for the reaction of aliphatic acid chlorides. The addition reactions proceed regio- and stereoselectively with suppression of decarbonylation and β-hydrogen elimination, which have been two major intrinsic problems in transition-metal-catalyzed reactions. Stoichiometric reactions of active iridium catalysts with aroyl chlorides and aliphatic acid chlorides are carried out to gain insights into the reaction mechanisms. 相似文献
49.
Shuntaro Chiba Yasushi Okuno Teruki Honma Mitsunori Ikeguchi 《Journal of computational chemistry》2019,40(29):2577-2585
We propose a novel force-field-parametrization procedure that fits the parameters of potential functions in a manner that the pair distribution function (DF) of molecules derived from candidate parameters can reproduce the given target DF. Conventionally, approaches to minimize the difference between the candidate and target DFs employ radial DFs (RDF). RDF itself has been reported to be insufficient for uniquely identifying the parameters of a molecule. To overcome the weakness, we introduce energy DF (EDF) as a target DF, which describes the distribution of the pairwise energy of molecules. We found that the EDF responds more sensitively to a small perturbation in the pairwise potential parameters and provides better fitting accuracy compared to that of RDF. These findings provide valuable insights into a wide range of coarse graining methods, which determine parameters using information obtained from a higher-level calculation than that of the developed force field. © 2019 The Authors. Journal of Computational Chemistry published by Wiley Periodicals, Inc. 相似文献
50.
Improvements in drug design have historically been centered around structure-based optimization of molecule specificity for a targeted protein, in an effort to reduce unintentional binding to other proteins and off-target effects. Although the "one-to-one" drug design strategy has been successful in impairing function of targets associated with a number of diseases, recent reports of drug promiscuity, which are a potential source of adverse reactions in patients, make a case to refine the drug design strategy such that it includes an awareness of multiple interactions from both ligand and protein perspectives. Polypharmacology and chemical biology studies are amassing interaction data at rapid rates, and the integration of such data into an interpretable model requires zooming our perspective out from the single ligand-target level to the larger network-wide level. We review some of the recent developments in systems-level research for drug design and discovery, and discuss the directions that some drug design efforts are heading toward. 相似文献