全文获取类型
收费全文 | 7641篇 |
免费 | 94篇 |
国内免费 | 18篇 |
专业分类
化学 | 3954篇 |
晶体学 | 139篇 |
力学 | 243篇 |
综合类 | 3篇 |
数学 | 1181篇 |
物理学 | 2233篇 |
出版年
2021年 | 51篇 |
2019年 | 76篇 |
2018年 | 78篇 |
2017年 | 82篇 |
2016年 | 115篇 |
2015年 | 105篇 |
2014年 | 123篇 |
2013年 | 310篇 |
2012年 | 240篇 |
2011年 | 284篇 |
2010年 | 162篇 |
2009年 | 195篇 |
2008年 | 266篇 |
2007年 | 250篇 |
2006年 | 205篇 |
2005年 | 184篇 |
2004年 | 187篇 |
2003年 | 147篇 |
2002年 | 166篇 |
2001年 | 131篇 |
2000年 | 138篇 |
1999年 | 132篇 |
1998年 | 85篇 |
1997年 | 95篇 |
1996年 | 96篇 |
1995年 | 97篇 |
1994年 | 96篇 |
1993年 | 82篇 |
1992年 | 106篇 |
1991年 | 96篇 |
1990年 | 100篇 |
1989年 | 77篇 |
1988年 | 73篇 |
1987年 | 92篇 |
1986年 | 89篇 |
1985年 | 107篇 |
1984年 | 114篇 |
1983年 | 112篇 |
1982年 | 108篇 |
1981年 | 123篇 |
1980年 | 137篇 |
1979年 | 107篇 |
1978年 | 114篇 |
1977年 | 138篇 |
1976年 | 102篇 |
1975年 | 77篇 |
1974年 | 69篇 |
1973年 | 57篇 |
1971年 | 53篇 |
1970年 | 50篇 |
排序方式: 共有7753条查询结果,搜索用时 15 毫秒
51.
The Raman and infrared spectra of (CH3)3SiSi(CH3)3 (I), (CD3)3SiSi(CD3)3, (C6H5)3SiSi(C6H5)3 (II), (CH3)3SiSi(C6H5)3 (III), and (CD3)3SiSi(C6H5)3 are reported. Assignments are based on symmetry G
36
+
(free internal rotation) forI, D3d forII, and C3v forIII. Normal coordinate treatment has been done using some simplifications in the phenyl coordinates. The SiSi stretching force constant inI amounts to 1,65 N/cm. In compoundsII andIII strong vibrational coupling is elucidated byPED calculations. 相似文献
52.
Sulfur(IV) Compounds as Ligands. II. The Crystal and Molecular Structure of Pentacarbonyl--(sulfur dioxide)chromium The structure of pentacarbonyl(sulfurdioxide)chromium ( 1 ) has been determined from single crystal X-ray data. The compound crystallizes with eight formula units in the rhombic unit cell (space group Pbn21) of the dimensions a = 657.8(2) pm, b = 1245.2(4) pm, c = 2177.4(5) pm (at 180 K). The sulfur dioxide is η1-coplanar coordinated, the Cr? S distance is 219 pm, the shortest bond reported so far between chromium(0) and sulfur. The Cr? C(ax) bond (189 pm) was found only marginally shorter than the Cr? C(eq) bonds (190 pm) providing proof of the high π-acceptor capacity of SO2. 相似文献
53.
G. Vassilev F. Perales Ch. Miniatura J. Robert J. Reinhardt F. Vecchiocattivi J. Baudon 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1990,17(2):101-107
A metastable hydrogen (deuterium) atom source in which groundstate atoms produced by a RF discharge dissociator are bombarded by electrons, provides a relatively large amount of slow metastable atoms (velocity 3–5 km/s). Total integral cross sections for H*(D*)(2s) + H2(X 1Σ g + ,v=0) collisions have been measured in a wide range of relative velocity (2,5–30 km/s), by using the attenuation method. A significant improvement of accuracy is obtained, with respect to previous measurements, at low relative velocities. Total cross sections for H* and D*, as functions of the relative velocity, are different, especially in the low velocity range. H* + H2 total differential cross sections have also been measured, with an angular spread of 3.6°, for two different collision energy distributions, centered respectively at 100 meV and 390 meV. A first attempt of theoretical analysis of the cross sections, by means of an optical potential, is presented. 相似文献
54.
Ehret R Baumann W Brischwein M Lehmann M Henning T Freund I Drechsler S Friedrich U Hubert ML Motrescu E Kob A Palzer H Grothe H Wolf B 《Fresenius' Journal of Analytical Chemistry》2001,369(1):30-35
The identification of drug targets for pharmaceutical screening can be greatly accelerated by gene databases and expression studies. The identification of leading compounds from growing libraries is realized by high throughput screening platforms. Subsequently, for optimization and validation of identified leading compounds studies of their functionality have to be carried out, and just these functionality tests are a limiting factor. A rigorous preselection of identified compounds by in vitro cellular screening is necessary prior to using the drug candidates for the further time consuming and expensive stage, e.g. in animal models. Our efforts are focused to the parallel development, adaptation and integration of different microelectronic sensors into miniaturized biochips for a multiparametric, functional on-line analysis of living cells in physiologically environments. Parallel and on-line acquisition of data related to different cellular targets is required for advanced stages of drug screening and for economizing animal tests. 相似文献
55.
The emission Mössbauer spectra of 125I labeled iodobenzene, methyl iodide, and of their dilute solutions in benzene and hexane, were computer analyzed. Two species were observed, in one of which tellurium is presumably attached to two organic moieties, while in the other, tellurium is attached to a single organic moiety. 相似文献
56.
S. Chandrasekhar G. Pavan Kumar Reddy Ch. Nagesh Ch. Raji Reddy 《Tetrahedron letters》2007,48(7):1269-1271
The first deaminative homologation of amines (-CH2NH2) to esters (-CH2CH2COOEt) in one-pot is reported. The reaction proceeds through, formation of an aldehyde from an amine in the presence of Pd/C as catalyst followed by Wittig reaction and catalytic hydrogenation using poly(ethylene glycol) as the solvent in one-pot. 相似文献
57.
The electroreduction of camphorquinone in DMF, at mercury electrodes, was investigated by a variety of techniques. In DMF, in the absence of proton donor, camphorquinone exhibits two one-electron waves: the first, a one-electron reversible wave to be due to a reversible charge transfer without a coupled chemical reaction. After the first charge transfer, the semidione anion radical is reduced to the dianion. The irreversibility of the second wave derives from a fast irreversible protonation of the dianion. A wide variety of changes in behaviour is observed in the reduction of camphorquinone as increasing amounts of benzoic acid are added: a new two-electron irreversible wave appears at a potential less negative than the original wave. A proton donor to substrate ratio of 2 is required to completely suppress the two original waves. A mechanism for the electroreduction of camphorquinone is proposed and discussed on the basis that the prewave current is controlled by the diffusion of the undissociated acid species and that the undissociated acid, rather than the solvated proton, takes part in the protonation, prior to the charge transfer. 相似文献
58.
Chiral carbenium ions can be attacked by arene nucleophiles with high facial diastereoselectivity (dr >/= 94/6). Benzylic cations, such as 2, were generated under acidic conditions and reacted with arenes in intra- and intermolecular Friedel-Crafts alkylation reaction. The depicted reaction 1 --> 3 represents one example for the unprecedented, highly diastereoselective intermolecular Friedel-Crafts alkylation reactions which were observed in this study. 相似文献
59.
The successive reaction of (CO)6M with Na[NCR21] and [Et3O]BF4 yields (CO)5M[C(NCR21)OEt] (II: M = Cr; III: M = W; CR21 = C(C6H4Br-p)2 (a), CPh2 (b), C(C6H4OMe-p)2 (c), C(C6H4)2O (d), CBu2tt (e)). Hexacarbonyltungsten, (CO)6W, reacts with Na[NCPh2] and MeOSO2F to give (CO)5W[C(NCPh2) OMe] (IV). X-Ray analysis of IIe shows that: (1) the CNC fragment is almost linear (171.7°); (2) the two NC bond lengths are equal within experimental error; and (3) the O,C,Cr,N plane is perpendicular to the C(Me3),C,N,C(Me3) plane (90.0°). Therefore compounds II–IV are best described as 1-alkoxy-2-azaallenyl complexes. 相似文献
60.
R. Mateva Ch. Konstantinov V. Kabaivanov 《Journal of polymer science. Part A, Polymer chemistry》1970,8(12):3563-3575
The influence of the acidity of organoaluminum compounds and their complexes on the catalytic activity in polymerization of trioxane has been studied. It is found that the catalytic activity of organoaluminum compounds of the type RnALX3?n depends on n as well as the nature of R or X. Catalytic activity of the complexes also depends on the type of cocatalyst used. The change of electroconductivity in the course of formation of some initiator complexes has been studied and an attempt has been made to determine the relation between conductivity and polymerization activity. 相似文献