全文获取类型
收费全文 | 4544篇 |
免费 | 765篇 |
国内免费 | 558篇 |
专业分类
化学 | 4736篇 |
晶体学 | 10篇 |
力学 | 215篇 |
综合类 | 32篇 |
数学 | 151篇 |
物理学 | 723篇 |
出版年
2024年 | 5篇 |
2023年 | 44篇 |
2022年 | 101篇 |
2021年 | 121篇 |
2020年 | 253篇 |
2019年 | 205篇 |
2018年 | 218篇 |
2017年 | 168篇 |
2016年 | 250篇 |
2015年 | 268篇 |
2014年 | 313篇 |
2013年 | 557篇 |
2012年 | 259篇 |
2011年 | 314篇 |
2010年 | 246篇 |
2009年 | 234篇 |
2008年 | 267篇 |
2007年 | 275篇 |
2006年 | 256篇 |
2005年 | 198篇 |
2004年 | 201篇 |
2003年 | 208篇 |
2002年 | 153篇 |
2001年 | 129篇 |
2000年 | 95篇 |
1999年 | 72篇 |
1998年 | 74篇 |
1997年 | 73篇 |
1996年 | 45篇 |
1995年 | 34篇 |
1994年 | 45篇 |
1993年 | 46篇 |
1992年 | 31篇 |
1991年 | 14篇 |
1990年 | 16篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 4篇 |
排序方式: 共有5867条查询结果,搜索用时 15 毫秒
101.
Fluorescence decay and quenching of pyrene labels on copolymers of 2-acrylamido-2-methylpropanesulphonic acid (AMPS) and N,N-dimethylacrylamide were observed in dilute salt-free aqueous solutions as a function of the mole fraction FAMPS of AMPS from 0 to 0.896. Monoexponential decay was found for the samples of FAMPS<0.35 and biexponential decay for the samples of FAMPS>0.35. The fast decay component is 80%, and the averaged lifetime 〈τ〉 and lifetime τ1 of the fast decay is decreased with increasing FAMPS. Quenching efficiency of Cu2+, I−, CH3NO2, and dinitrobenzene to the pyrene label was investigated in the framework of Stern-Volmer plot. With increasing FAMPS the quenching efficiency of Cu2+ is increased while that of I− decreased. For the neutral quenchers, the quenching rate constant kq increases when FAMPS<0.449 then decreases, showing a decline of accessibility to the pyrene label. These results were interpreted consistently with the counterion condensation concept, where condensed counterions caused the polyelectrolyte chains to aggregate. The existence of less-polar “temporal aggregated domain” in highly charged polyelectrolytes appears to lead to the slower decay and lower accessibility of the pyrene labels. 相似文献
102.
Joan Fraga-DubreuilJean Pierre Bazureau 《Tetrahedron》2003,59(32):6121-6130
The first report of the use of task-specific ionic liquid as synthetic equivalent of ionic liquid-phase matrice for the preparation of a small library of 4-thiazolidinones is reported in this paper. The starting (ethyleneglycol)ionic liquid-phase is functionalized in good yields with 4-(formylphenoxy)butyric acid by using usual esterification reaction conditions (DCC/DMAP as catalyst). The synthesis of the ionic liquid-phase bound 4-thiazolidinones was performed by a one-pot three-component condensation under microwave dielectric heating. The final cleavage under microwave/catalysis strategy provides the expected 4-thiazolidinones in high purity after flash-chromatography purification. According to the ionic liquid-phase organic synthesis (IoLiPOS) methodology, it was found that optimized reaction conditions were performed by standard analytical methods (NMR, TLC). The 1H, 13C NMR spectrum of some representative 4-thiazolidinones and ionic liquid-phase bound benzaldehyde are also reported. 相似文献
103.
Nan Yan FU Mei Li PANG Yao Feng YUAN Ji Tao WANG* Department of Chemistry National Key Laboratory of Elemento-organic Chemistry Nankai University Tianjin 《中国化学快报》2002,13(10)
A highly efficient catalyst indium (III) tribromide is used to synthesize 5-alkoxy- carbonyl-4-hydrocarbyl-3,4-dihydropyrimidin-2(1H)-ones by a three-component coupling of β-keto ester, aldehydes and urea through improved Biginelli reaction. 相似文献
104.
105.
Katrin?Anhalt Ines?Sprung Klaus?SchulzeEmail author 《Monatshefte für Chemie / Chemical Monthly》2003,134(12):1593-1606
Summary. Optically active dimethylcyclohexenones, potential building blocks for enantioselective syntheses of various naturally active substances, were prepared. These compounds were obtained by oxidation with KMnO4/Pb(OAc)4 or ozonolysis of the corresponding cyclopentenic precursors, followed by aldol condensation. During the course of the preparation intermediate diols and chiral polyfunctional carbonyl derivatives were separated and identified analytically. 相似文献
106.
107.
《Tetrahedron letters》2003,44(16):3255-3257
The synthetic diphosphate Na2CaP2O7 is a new basic catalyst for the synthesis of unsaturated arylsulfones. The condensation was carried out under mild conditions at room temperature in methanol or ethanol. The optimum of the catalyst's activity is reached when water is added, leading to good yields. 相似文献
108.
M. L. Scheepers P. J. Adriaensens J. M. Gelan R. A. Carleer D. J. Vanderzande N. K. De Vries P. M. Brandts 《Journal of polymer science. Part A, Polymer chemistry》1995,33(6):915-920
1H and 13C NMR have been used for the quantitative determination of methylene-ether bridges in melamine-formaldehyde (MF) resins. The amount of methylene-ether bridges was determined by 13C NMR from the number of monomethylolated amino groups consumed in the condensation reactions and is in agreement with that calculated from the condensation water contents. This latter method, which involves a combination of 1H and 13C NMR, is based on the amount of condensation water released during the formation of both methylene and methylene-ether bridges. © 1995 John Wiley & Sons, Inc. 相似文献
109.
The new chromophore compounds with NLO properties were prepared by Knoevenagel condensation from forrnyl or diformyl calix[4/arene and isophorone derivatives in the presence of piperidine and acetic acid, respectively. In these chromophore calix[4]arenes, the ring locked trienes were employed as the conjugation bridge and electron acceptor in D-π-A units. The NMR spectra demonstrated that they existed in a cone conformation and consequently non-conjugated D-π-A units could be oriented at nearly the same direction. 相似文献
110.
B. S. Fedorov N. I. Golovina L. S. Barinova V. V. Arakcheeva G. V. Lagodzinskaya M. V. Loginova R. F. Trofimova V. Z. Laishev V. F. Lazarev S. P. Smirnov I. Sh. Abdrakhmanov A. I. Firkin L. O. Atovmyan 《Russian Chemical Bulletin》1992,41(12):2185-2188
1,1,4,4-Tetranitrobutane-2,3-diol reacts with formaldehyde, forming 2,2-dinitropropane-1,3-diol or a cyclic ether — 4,4-dinitro-2,3-dihydroxytetrahydrofuran — as a function of the reaction conditions.Institute of Chemical Physics, Chernogolovka, Russian Academy of Sciences, 142432 Chernogolovka. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 12, pp. 2755–2759, December, 1992. 相似文献