全文获取类型
收费全文 | 21793篇 |
免费 | 1254篇 |
国内免费 | 3160篇 |
专业分类
化学 | 17237篇 |
晶体学 | 136篇 |
力学 | 716篇 |
综合类 | 178篇 |
数学 | 4819篇 |
物理学 | 3121篇 |
出版年
2024年 | 50篇 |
2023年 | 436篇 |
2022年 | 1315篇 |
2021年 | 1133篇 |
2020年 | 1122篇 |
2019年 | 912篇 |
2018年 | 870篇 |
2017年 | 880篇 |
2016年 | 984篇 |
2015年 | 897篇 |
2014年 | 1102篇 |
2013年 | 2257篇 |
2012年 | 1388篇 |
2011年 | 1230篇 |
2010年 | 902篇 |
2009年 | 1310篇 |
2008年 | 1287篇 |
2007年 | 1141篇 |
2006年 | 942篇 |
2005年 | 877篇 |
2004年 | 702篇 |
2003年 | 542篇 |
2002年 | 678篇 |
2001年 | 321篇 |
2000年 | 383篇 |
1999年 | 358篇 |
1998年 | 298篇 |
1997年 | 265篇 |
1996年 | 241篇 |
1995年 | 200篇 |
1994年 | 183篇 |
1993年 | 143篇 |
1992年 | 128篇 |
1991年 | 98篇 |
1990年 | 60篇 |
1989年 | 89篇 |
1988年 | 60篇 |
1987年 | 41篇 |
1986年 | 34篇 |
1985年 | 59篇 |
1984年 | 67篇 |
1983年 | 30篇 |
1982年 | 42篇 |
1981年 | 32篇 |
1980年 | 25篇 |
1979年 | 24篇 |
1978年 | 17篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
The Knoevenagel condensation of Meldrum's acid with aromatic aldehydes proceeded efficiently in a reusable ionic liquid, ethylammonium nitrate, at room temperature in the absence of any catalyst with high yields. 相似文献
142.
AbstractA series of imidazole derivatives were synthesized from two-component condensation reaction of phenylgloxal monohydrate with guanylhydrazone. They were characterized by spectroscopic and analytical techniques. The in vitro anticancer evaluation of these compounds was done on human breast cancer (MCF-7) and human liver cancer (HepG2) cell lines using the MTT assay method. Most of the newly synthesized compounds displayed cytotoxic activity against these cancerous cells. In fact, compounds 3a, 3e, and 3?h exhibited more cytotoxic activities than the positive control drugs, fluro-5, and irinocam, against the MCF-7 cell line. Almost all the compounds, except for three, 3b, 3d, and 3f, gave more cytotoxic results than cisplatin. Therefore, these compounds could be considered for further development as anticancer drug candidates. 相似文献
143.
Meldrum's acid (1) on bisacylation with succinyl chloride afforded a neutral diketo product (2); however, the dimer enolic products (3) with acidic nature resulted in glutaryl and adipyl dichlorides. Monoacylation of (1) with acetyl chloride gave acidic 5‐acetyl Meldrum's acid (4) in enol form. 相似文献
144.
A bicyclic diamine with defined and stable conformation in solution was prepared from Kemp's triacid. The efficient four-step synthesis of the Boc-protected diamine requires only a single purification by column chromatography. X-ray analysis and NMR spectroscopy confirm the structure of the diamine in the solid state and in solution. 相似文献
145.
A series of highly functionalized piperidine derivatives was synthesized through one-pot, five-component reaction of aldehydes, amines, and β-ketoesters. Silica sulfuric acid efficiently catalyzes the reaction to afford the corresponding piperidine derivatives in good yields. As a representative example, heating of 4-methylaninle, 4-fluorobezaldehyde, and methyl-acetoacetate in methanol in the presence of silica sulfuric acid furnished the corresponding ethyl 2,6-bis(4-fluorophenyl)-1-p-tolyl-4-(p-tolylamino)-1,2,5,6-tetrahydropyridine-3-carboxylate in excellent yield (85%). Most of the synthesized compounds were screened in vitro for their antibacterial and antifungal activities. Most of compounds showed significant antibacterial activity. 相似文献
146.
Bei‐Bei Zhang Qian Jiang Zhi‐Xin Liao Chao Liu Shi‐Jun Liu 《Helvetica chimica acta》2013,96(7):1281-1289
Phytochemical investigation of the EtOH extract of Euphorbia sieboldiana led to the isolation of four new oleanane‐type triterpenoids, (1β,2α,3β,19β)‐1,2,3,19‐tetrahydroxyolean‐12‐en‐28‐oic acid, (1β,3β,19β)‐1,3,19‐trihydroxyolean‐12‐en‐28‐oic acid, (1β,2α,3β,16β,19β)‐1,2,3,16,19‐pentahydroxyolean‐12‐en‐28‐oic acid, and (1β,2α,3β,19β,23)‐1,2,3,19,23‐pentahydroxyolean‐12‐en‐28‐oic acid, along with 16 known compounds. Their structures were established by extensive 1D‐ and 2D‐NMR, as well as other spectral analyses. Biological evaluation of the four new triterpenoids revealed potent cytotoxic activities against HeLa and Hep‐G2 cells. 相似文献
147.
A sequential three-component reaction of aromatic aldehydes with Meldrum's acid and N-methyl indole in the presence of choline chloride/urea ionic liquid as green catalyst has been described. In this one-pot multicomponent reaction, a series of indole-3-propanamide derivatives were synthesized with good to excellent yields. This methodology shows several advantages including fast reaction, easy isolation, operational simplicity that make it a useful and attractive option for the library generation of indole-3-propanamides (5a–l) for drug discovery. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resources: Full experimental and spectral details.] 相似文献
148.
Redox-active polyelectrolyte-surfactant complexes (PSC) were prepared via the ionic self-assembly of sodium polyacrylate (PAAS) and ferrocenyl surfactant, n-alkyl (ferrocenylmethyl)ammonium bromide (Fcn, n = 8, 12, 16, where n is the carbon number of the alkyl chain), in solution. The obtained PAAS-Fcn complex exhibited crystalline and lamellar mesomorphous structure. Moreover, the ferrocenyl moieties formed H-aggregation in the solid complex as known from the blue shift in the ultraviolet-visible spectrum. Cyclic voltammogram (CV) measurements indicated that the reversibility of the electrode process became worse when the surfactant tail length increases because of the more ordered packing in the complex film formed by longer surfactant tails. The present results demonstrate that the electrochemical activity of the redox-active polyacrylate-ferrocenyl surfactant complex can be easily tuned by changing the surfactant tail length. Our work provides a simple and facile approach to the preparation of redox-active polymers with ordered mesomorphous structure by the ionic self-assembly. 相似文献
149.
The title compound, neogeodin hydrate (C17H14C1208, CAS: 94540-50-8), was derived from marine fungus Aspergilhts terreus CRIM301. It was unequivocally characterized by IR, NMR spectroscopies, and single-crystal X-ray crystallography and tested for various biological activities. Neogeodin hydrate crystallizes in the triclinic space group P1 with a = 8.1159(5) A, b = 8.2472(4) A, c= 14.1278(7) A, a = 81.448(2)°, β = 84.860(2)°, γ= 70.400(2)°, V = 880.13(8) A3; Z = 2. It comprises a diphenyl ether, asterric acid skeleton and dichloro substituents. The methoxyphenoxy rings of the inversely related molecules form a ribbon-like structure that is stabilized by O-H...O hydrogen bonds through the doubly disordered carboxyl groups and by C-H...O interactions, generating the same R22(8) ring motif. The chlorinated methylbenzoate rings, making mostly a right angle, link the parallel upper and lower ribbons via bifurcated O-H...O and C-H...O hydrogen bonds, yielding endless channels. The channels formed are further sustained by C-H...O and π...π interactions Neogeodin hydrate exhibits inhibition against superoxide anion radical formation in the xanthine/xanthine oxidase (XXO) assay, but has no aromatase inhibitory activity. 相似文献
150.
Given a reference random variable, we study the solution of its Stein equation and obtain universal bounds on its first and second derivatives. We then extend the analysis of Nourdin and Peccati by bounding the Fortet–Mourier and Wasserstein distances from more general random variables such as members of the Exponential and Pearson families. Using these results, we obtain non-central limit theorems, generalizing the ideas applied to their analysis of convergence to Normal random variables. We do these in both Wiener space and the more general Wiener–Poisson space. In the former space, we study conditions for convergence under several particular cases and characterize when two random variables have the same distribution. In the latter space we give sufficient conditions for a sequence of multiple (Wiener–Poisson) integrals to converge to a Normal random variable. 相似文献