全文获取类型
收费全文 | 847篇 |
免费 | 72篇 |
国内免费 | 56篇 |
专业分类
化学 | 524篇 |
晶体学 | 7篇 |
力学 | 27篇 |
综合类 | 5篇 |
数学 | 173篇 |
物理学 | 239篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 19篇 |
2020年 | 15篇 |
2019年 | 16篇 |
2018年 | 17篇 |
2017年 | 14篇 |
2016年 | 28篇 |
2015年 | 31篇 |
2014年 | 28篇 |
2013年 | 46篇 |
2012年 | 56篇 |
2011年 | 59篇 |
2010年 | 35篇 |
2009年 | 39篇 |
2008年 | 45篇 |
2007年 | 57篇 |
2006年 | 31篇 |
2005年 | 47篇 |
2004年 | 31篇 |
2003年 | 46篇 |
2002年 | 37篇 |
2001年 | 30篇 |
2000年 | 33篇 |
1999年 | 17篇 |
1998年 | 19篇 |
1997年 | 12篇 |
1996年 | 9篇 |
1995年 | 9篇 |
1994年 | 13篇 |
1993年 | 14篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 2篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1972年 | 2篇 |
1942年 | 3篇 |
1928年 | 2篇 |
排序方式: 共有975条查询结果,搜索用时 31 毫秒
881.
Rodríguez G Gallardo A Fernández M Rebuelta M Buján J Bellón JM Honduvilla NG Escudero C San Román J 《Macromolecular bioscience》2004,4(6):579-586
Hydrophilic polymeric drugs bearing "Triflusal" (4-trifluoromethylsalicylic acid), a drug widely used as antithrombogenic agent (Disgren), have been prepared by free radical copolymerization of methacryloyloxyethyl [2-(acetyloxy)-4-(trifluoromethyl)] benzoate (HTRF) and N,N'-dimethylacrylamide (DMA). The reactivity ratios of both monomers have been determined by 1H NMR spectra by applying non-linear least square treatments to the copolymerization equation (terminal model), and the kinetic parameters obtained indicated that the microstructure of copolymer chains is homogeneous, with a random distribution of the active HTRF units along the copolymer chains. That means that for the copolymer system THDMA22 used in this work, HTRF units are mainly isolated in relatively long DMA sequences. Therefore, in this structure the intramolecular interactions between adjacent HTRF units are negligible. Release of Triflusal from THDMA22 has been studied in vitro using buffered solutions at pH = 2, 7.4 and 10 and 37 degrees C. The system showed an interesting pseudo-zero order release profile at pH = 7.4 during several months. It has been also evaluated the pharmacological activity and the behavior of the system in contact with biological media. In this sense, we have carried out some in vitro studies about the antiaggregant properties and biocompatibility of THDMA22. Results demonstrate that this copolymer inhibits platelet aggregation in its macromolecular form and presents a good biocompatibility with Human Osteoblastic Cells (HOS). 相似文献
882.
The triflate salt resulting from the treatment of diphenyl-S-prolinol with trimethylsilyl triflate, catalyzes the addition of nitrones to α,β-unsaturated aldehydes to provide isoxazolidines in high yields and excellent diastereo- and enantioselectivities. 相似文献
883.
Raymond D. Baechler Lynn James San Filippo Alayne Schroll 《Tetrahedron letters》1981,22(52):5247-5250
Mixtures of sulfides and disulfides are obtained upon reaction of boron trisulfide with a series of allyl aryl suit oxides, with the product distributions dependent upon the structures of the intermediate thiosulfoxides. 相似文献
884.
Hua‐Jie Zhu Yi Ren Jie Ren San‐Yan Chu 《International journal of quantum chemistry》2005,101(1):104-112
The gas‐phase ionic SN2 reactions NCS‐ + CH3F and ion pair SN2 reaction LiNCS + CH3F with inversion mechanism were investigated at the level of MP2(full)/6‐311+G**//HF/6‐311+G**. Both of them involve the reactants complex, inversion transition state, and products complex. There are two possible reaction pathways in the ionic SN2 reaction but four reaction pathways in the ion pair SN2 reaction. Our results indicate that the introduction of lithium significantly lower the reaction barrier and make the ion pair displacement reaction more facile. For both ionic and ion pair reaction, methyl thiocyanate is predicted to be the major product, but the latter is more selective. More‐stable methyl isothiocyanate can be prepared by thermal rearrangement of methyl thiocyanate. The theoretical predictions are consistent with the known experimental results. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
885.
Cyril Párkányi Hui Liang Yuan Annette Sappok-Stang Adolfo Ruiz Gutiérrez San Ardi Lee 《Monatshefte für Chemie / Chemical Monthly》1992,123(6-7):637-645
Summary An improved procedure for the synthesis of several isomeric methoxynitronaphthalenes is described. The key intermediates in the synthesis are the respective nitronaphthylamines and nitronaphthols. The syntheses of 1-methoxy-3-nitronaphthalene (1), 1-methoxy-5-nitronaphthalene (2), and 2-methoxy-5-nitronaphthalene (3) are reported.Part of this work was carried out at the University of Texas at El Paso, El Paso, TX, during the tenure of one of the authors there (C.P.) 相似文献
886.
The crystal growth process of Zn(Len)SO4·0.5H2O from water and acetone was investigated using a Calvet microcalorimeter. The heat and the rate of heat production during the crystal growth process at 293.15 K, 295.15 K, 298.15 K and 300.15 K were measured. On the basis of experimental and calculated results, the rate constant and the kinetic parameters (the activation energies, the pre‐exponential) during the crystal growth process were obtained. The results show that the crystal growth process accorded with the Burton‐Cabrera‐Frank dislocation theory. 相似文献
887.
U Myint J. Tölgyessy Naing Win Khaing San Ba Han Khin Mg Myoe 《Journal of Radioanalytical and Nuclear Chemistry》1994,187(5):345-349
Radiometric Flow Injection Analysis of a radioactive (131I) sample is described. For analysis an ASIA (Ismatec) analyzer with a NaI (Tl) scintillation detector was used. 相似文献
888.
在无水乙醇中, 使低水合氯化稀土 (RE = Ho, Er, Tm, Yb, Lu) 与吡咯烷二硫代氨基甲酸铵 (APDC)和1,10-菲咯啉 (o–phen•H2O) 反应, 制得其三元固态配合物. 用化学分析和元素分析确定它的组成为RE(C5H8NS2)3(C12H8N2) (RE = Ho, Er, Tm, Yb, Lu). IR光谱说明RE3+ 分别与3个PDC–的6个硫原子双齿配位, 同时与o–phen的2个氮原子双齿配位, 配位数为8. 用精密转动弹热量计测定了它们的恒容燃烧热△cU分别为(-16788.46 ± 7.74), (-15434.53 ± 8.28), (-15287.80 ± 7.31), (-15200.50 ± 7.22)和(-15254.34 ± 6.61) kJ•mol-1; 并计算了它们的标准摩尔燃烧焓△cHmθ和标准摩尔生成焓△fHmθ分别为( -16803.95 ± 7.74), (-15450.02 ± 8.28), (-15303.29 ± 9.28), (-15215.99 ± 7.22), (-15269.83 ± 6.61) kJ • mol-1和 (-1115.42 ± 8.94), (-2477.80 ± 9.15), (-2619.95 ± 10.44), (-2670.17 ± 8.22), (-2650.06 ± 8.49) kJ•mol-1. 相似文献
889.
Six new prenylated phloroglucinol derivatives, hypersampsones A–F ( 1 – 6 ), were isolated from the aerial part of Hypericum sampsonii, together with 2,4,6‐trihydroxybenzophenone, 2,4,6‐trihydroxybenzophenone 4‐O‐geranyl ether, 2,4,6‐trihydroxybenzophenone 3‐C‐geranyl ether, sampsoniones D and H. Their structures were elucidated by spectroscopic methods, mainly 1D‐ and 2D‐NMR spectroscopy and mass spectrometry. 相似文献
890.
A Ligand-Directed strategy has been adopted to synthesize three novel Copper(І) coordination polymers, [Cu2(bpe)(N3)2]
n
(1), [Cu2(bpy)(N3)2]
n
(2), and [Cu4(py)(N3)4]
n
(3), by the reactions of NaN3 with bpe, bpy and py (bpe = 1,2-trans-(4-pyridyl)ethene, bpy = 4,4′-bipyridine, py = pyrazine) in the presence of H3PO3 under hydrothermal conditions, which proved that the length of ligands had subtle effects on overall network and the coordination
mode of azide. 相似文献