全文获取类型
收费全文 | 19303篇 |
免费 | 1354篇 |
国内免费 | 1212篇 |
专业分类
化学 | 5484篇 |
晶体学 | 55篇 |
力学 | 1044篇 |
综合类 | 320篇 |
数学 | 11334篇 |
物理学 | 3632篇 |
出版年
2023年 | 203篇 |
2022年 | 351篇 |
2021年 | 331篇 |
2020年 | 458篇 |
2019年 | 533篇 |
2018年 | 483篇 |
2017年 | 527篇 |
2016年 | 426篇 |
2015年 | 355篇 |
2014年 | 587篇 |
2013年 | 1265篇 |
2012年 | 649篇 |
2011年 | 936篇 |
2010年 | 949篇 |
2009年 | 1288篇 |
2008年 | 1255篇 |
2007年 | 1087篇 |
2006年 | 1047篇 |
2005年 | 824篇 |
2004年 | 636篇 |
2003年 | 637篇 |
2002年 | 705篇 |
2001年 | 526篇 |
2000年 | 486篇 |
1999年 | 467篇 |
1998年 | 448篇 |
1997年 | 316篇 |
1996年 | 299篇 |
1995年 | 247篇 |
1994年 | 204篇 |
1993年 | 183篇 |
1992年 | 149篇 |
1991年 | 134篇 |
1990年 | 80篇 |
1989年 | 77篇 |
1988年 | 71篇 |
1987年 | 67篇 |
1985年 | 468篇 |
1984年 | 328篇 |
1983年 | 254篇 |
1982年 | 258篇 |
1981年 | 197篇 |
1980年 | 146篇 |
1979年 | 160篇 |
1978年 | 99篇 |
1977年 | 96篇 |
1976年 | 100篇 |
1975年 | 95篇 |
1974年 | 128篇 |
1973年 | 140篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
The crystal structure of a second high-pressure copper vanadate phase, CuVO3(II), has been determined and refined by full-matrix least-squares procedures using automatic diffractometer data to a residual R = 0.042 (Rw = 0.051). The space group is rhombohedral, , with hexagonal unit cell a = 4.966(2) and c = 14.084(5) Å [aR = 5.501(2) Å and α = 53.66(3)°]. The structure is the fully ordered ilmenite-type and, on the basis of published magnetic data and the interatomic distances, the valence distribution Cu+V5+O3 is proposed. This represents a unique example of Cu+ in an octahedral environment. 相似文献
32.
James Y. Becker Larry L. Miller Frank R. Stermitz 《Journal of Electroanalytical Chemistry》1976,68(2):181-191
The electrochemical oxidation of the alkaloid laudanosine (Ia) to O-methylflavinantine (II) has been studied in acetonitrile solvent. Using cyclic voltammetry, rotating disc voltammetry and preparative electrolyses on several alkaloids, simple aliphatic amines and aromatic compounds, some aspects of the mechanism of this coupling reaction are elucidated. The first anodic wave for laudanosine at platinum has Ep=0.55 V vs. Ag/Ag+. The electrode rapidly becomes partially passivated at potentials above 0.5 V. This is due to a film which “dissolves” below 0.5 V, at a rate independent of the potential. It is shown that the reaction (Ia)→(II) proceeds at 0.5 V by initial oxidation of the amine moiety. If acids such as sodium bicarbonate are added to the anolyte the amine is protonated causing the first wave to disappear. Oxidation at 1.1 V under these acidic conditions produces the same product, but more rapidly and in significantly higher yield because electrode filming and side reactions resulting from the amine oxidation are abrogated. 相似文献
33.
C. Mioskowski A. Solladie-Cavallo G. Solladie 《Journal of organometallic chemistry》1978,149(3):C63-C65
Attachment of a Cr(CO)3 moiety to one of the aromatic rings in rigid diaryl ketones of very low prochirality followed by addition of a chiral and optically pure reagent leads to satisfactory asymmetric induction ~40%. Asymmetric induction is smaller in the case of acyclic systems. 相似文献
34.
The number of products and the H2/CH4 ratio obtained from the flow pyrolyses of (CH3)3GeH and (CH3)3SiH were very different. The (CH3)3GeH decomposition is consistent with the following mechanism:
The pyrolysis of (CH3)3SiH was found to be much more complex, presumably due to the formation of silicon-carbon double bonded intermediates and the (CH3)2Si(H)CH2 radical. We also present data which supports the presence of a H atom chain sequence during this pyrolysis. 相似文献
Full-size image
35.
The degradation of acenaphthylene, acenaphthene, 2-methylnaphthalene, 2-methylindene, 3-methylindene and indene in water solutions was studied. These compounds at the 25-150 mug/l. level were almost totally degraded at ambient temperature within three days. The microbial population responsible for the degradation occurs naturally in ground-water taken from an aquifer in Ames, Iowa, which is contaminated with coal-tar products. These unidentified micro-organisms adapt readily to other waters when used as an inoculant for the degradation of aromatic compounds. The preservation of water to prevent such degradation was also investigated. Filtration through a 0.45-mum filter was found the most effective procedure for preserving the hydrocarbons in these waters. 相似文献
36.
The cis fused bicyclic sulfones 1a, 1c and 3a are lithiated in benzene with n-butyllithium under concomitant cis/trans isomerization of the ring fusion, involving intramolecular proton transfer. H/D exchange of the three α-hydrogens in protic solvents proceeds with retention of configuration. The lithiated sulfones are chlorinated with hexachloroethane (HCE) and show a strong preference for introduction of halogen at an equatorial α-position. 相似文献
37.
The Cr(CO)3(CH3CN)3 complex is found to catalyze the 1,4-addition of hydrogen to 1,3-dienes such as 2-methyl-1,3-butadiene, trans-1,3-pentadiene, and trans, trans-2,4-hexadiene at low temperature (40°) and low H2 pressure (20 psi). For trans, trans-2,4-hexadiene the only product obtained when D2 is used is 2,5-dideuterio-cis-3-hexene. The catalytic 1,4-hydrogenation can be carried out in neat dienes, and turnover numbers for the catalyst of greater than 3000 have been observed. 相似文献
38.
The phenylseleno group (PhSe) has evolved in recent years as a very useful and versatile functionality. Its facile introduction into organic molecules and its subsequent oxidative or reductive removal, has allowed many important synthetic transformations.1–7 Due to the fact that, similarly to halogens, it can exist either as an electrophilic species (PhSe+) or as a nucleophilic one (PhSe-), this group can be introduced either via nucleophilic substrates (e.g. carbanions, olefins), or via electrophilic ones (e.g. epoxides, halides), as illustrated in Scheme 1. Another valuable aspect of the phenylseleno group is that it can be readily oxidized to the corresponding selenoxide(PhSe(O)—), which undergoes β-hydrogen abstraction and syn-elimination to form olefins, under relatively mild conditions (Scheme 2(a)). Furthermore, this group can be substituted with hydrogen, upon the action of an appropriate reducing agent (Scheme 2(b)).The great synthetic utility of the phenylseleno group is apparent from its extensive utilization in numerous natural products syntheses,1 as well as many other synthetic studies.2–7 相似文献
39.
The cyclic sulfones1,2, and3 are lithiated in benzene with n-butyllithium. Lithiation is demonstrated to lead to α-mono, α,α′-di and/or α,α-di and α,α,α′ trimetallation. The lithiated sulfones are chlorinated with hexachloroethane (HCE). Some mechanistic aspects of the reaction of the lithiosulfones with vicinal dihalides are discussed. 相似文献
40.
1-Aryl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolines were prepared according to a one-pot procedure involving the reaction of 2-(3,4-dimethoxyphenyl)-ethylamine with aromatic aldehydes in TFA at reflux. The tetrahydroisoquinolines were treated with H2O2-WO42− in methanol at room temperature to give the corresponding 3,4-dihydroisoquinoline-2-oxides. Treatment of these cyclic nitrones with DMAD in toluene at room temperature gave the corresponding isoxazolo[3,2-a]isoquinolines. These compounds were heated in toluene at reflux to give the corresponding ylides in high yields (Method A). The effect of the substituents on the rate of the rearrangement of such compounds prompted us to discuss a new mechanism involving consecutive C-C bond heterolysis and 1,3-sigmatropic shift. A one-pot reaction involving the treatment of the nitrones with equimolar amounts of DMAD in refluxing toluene also gave the ylides (Method B). The structures of the prepared compounds were elucidated by spectral means and elemental analyses. 相似文献