全文获取类型
收费全文 | 24448篇 |
免费 | 2314篇 |
国内免费 | 6461篇 |
专业分类
化学 | 29434篇 |
晶体学 | 386篇 |
力学 | 431篇 |
综合类 | 193篇 |
数学 | 592篇 |
物理学 | 2187篇 |
出版年
2024年 | 24篇 |
2023年 | 210篇 |
2022年 | 464篇 |
2021年 | 653篇 |
2020年 | 857篇 |
2019年 | 729篇 |
2018年 | 739篇 |
2017年 | 818篇 |
2016年 | 1136篇 |
2015年 | 1110篇 |
2014年 | 1434篇 |
2013年 | 2404篇 |
2012年 | 1670篇 |
2011年 | 1715篇 |
2010年 | 1450篇 |
2009年 | 1666篇 |
2008年 | 1724篇 |
2007年 | 1804篇 |
2006年 | 1835篇 |
2005年 | 1727篇 |
2004年 | 1652篇 |
2003年 | 1416篇 |
2002年 | 936篇 |
2001年 | 654篇 |
2000年 | 555篇 |
1999年 | 564篇 |
1998年 | 521篇 |
1997年 | 467篇 |
1996年 | 354篇 |
1995年 | 356篇 |
1994年 | 308篇 |
1993年 | 294篇 |
1992年 | 279篇 |
1991年 | 158篇 |
1990年 | 104篇 |
1989年 | 100篇 |
1988年 | 82篇 |
1987年 | 45篇 |
1986年 | 34篇 |
1985年 | 43篇 |
1984年 | 33篇 |
1983年 | 13篇 |
1982年 | 17篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
891.
B. M. Bulychev V. N. Verbetskii A. I. Sizov T. M. Zvukova V. K. Genchel’ V. N. Fokin 《Russian Chemical Bulletin》2007,56(7):1305-1312
Crystallization of non-solvated aluminum hydride from a diethyl ether-benzene mixed solvent was studied. The desolvation of
AlH3·(Et2O)x etherate in solution and the crystallization of α-AlH3 during polythermal heating of the solution occur only in the presence of ≥10 wt.% LiAlH4. The process is multistage, and the crystallization begins with the formation of the AlH3·0.25Et2O solvate, which recrystallizes in the solid phase into γ-AlH3 and then α-AlH3. Four crystalline modifications of aluminum hydride were characterized by X-ray diffraction and electron microscopy.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1259–1265, July, 2007. 相似文献
892.
Ralf Otte Roland Fröhlich Shuichi Nakamura Takeshi Toru Dieter Hoppe 《Tetrahedron letters》2007,48(49):8636-8642
The enantio- and diastereoselective synthesis of allenyl aryl sulfides by asymmetric lithiation of 2-alkynyl (2-hetero)aryl sulfides is described. A dynamic thermodynamic resolution by selective crystallization of the intermediate lithium complexes derived from deprotonation, applying a bis(oxazoline) ligand, was achieved to give enantioselectivities up to 85% ee. Subsequent stereospecific nickel-catalyzed cross-coupling reactions with arylzinc reagents established a versatile access to threefold carbon-substituted allenes. 相似文献
893.
标题化合物C19H23O4Cl(4)是由邻氯苯甲醛(1)与5,5-二甲基-1,3-环己二酮(2)、2,2-二甲基-1,3-二氧六环-4,6-二酮(3)在乙醇中反应而得。结构通过单晶X-射线衍射分析确定,其晶体属于单斜晶系,空间群晶体结构用直接法解出,使用全矩阵最小二乘法对原子参数进行修正,最后的偏离因子R=0.034,Rw=0.042。在晶体结构中存在一个共轭的烯醇结构。单晶X-射线分析表明;平面1(C(1)~C(6)、Cl)和平面2(C(8)~C(10)、C(12)、C(13))之间的两面角为97.11°,原子C(7)呈变形的四面体构型。 相似文献
894.
The stereoselective synthesis of the two segments 3 and 4 of macrolactin A 1 is described. Macrolactin A is a 24-membered polyene macrolide antibiotic, which is of interest due to a strong activity against B16-F10 murine tumor cell and HIV-1 virus. The key step of the synthesis is the 1,2-migration reaction of a (diene)Fe(CO)3 complex with a introduction of the thiol group to construct the unstable (E,Z)-conjugated dienic moiety (C8-C11). 相似文献
895.
Abhimanyu S. Paraskar 《Tetrahedron》2006,62(20):4907-4916
Sodium borohydride in combination with a catalytic amount of CoCl2 has been found to be an excellent catalytic system in reductive cyclizations of suitably substituted azido and cyano groups of α,β-unsaturated esters to afford γ and δ-lactams in high yields. The process has been demonstrated for the enantioselective synthesis of (R)-baclofen, (R)-rolipram, and (R)-4-fluorophenylpiperidinone, a key intermediate for (−)-paroxetine. 相似文献
896.
Pyrolysis study of fluorinated sol-gel silica 总被引:1,自引:0,他引:1
Fluorinated silica gels at various fluorine content were prepared via sol-gel by hydrolysis of 3,3,3-trifluoropropyltrimethoxysilane
and tetraethoxysilane mixtures. The gels, of nominal stoichiometry Si(CH2CH2CF3)XO(2-X/2)(X=0.1-1), were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS) and N2 adsorption analysis. The thermal stability of the fluorinated samples was investigated by coupling thermogravimetric measurements
with mass spectrometric and gas chromatographic analyses of the evolved gaseous species. The chemical reactions occurring
in the gel matrices during heating were siloxane chain rearrangements involving condensation between residual hydroxyl and
ethoxyl groups in the 100-350°C temperature range, whereas the thermal decomposition of the fluoroalkyl groups were observed
at higher temperatures (450-600°C). The release of the fluoroalkyl moieties also involved C-F/Si-O bond exchanges inside the
siloxane chains, with gas-phase evolution of different fluorinated silicon units.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
897.
XinYingZHANG XueSenFAN JianJiWANG YanZhenLI 《中国化学快报》2004,15(10):1170-1172
Quinoline derivatives were efficiently prepared through acid-catalyzed Friedlander reaction in ionic liquid ([bmim] [BF4]). It is shown that the proposed method is operationally simple and environmentally benign in that the reaction media and the catalyst can be recovered and be reused effectively for at least four times. 相似文献
898.
The crystal structure of a series of thienylgermatranes has been studied using X-ray diffraction. Disorder of the thiophene
ring in a 2-thienylgermatrane has been established. In the same positions of the unit cell of the crystal there are found
both molecules with an O(2)-Ge-C-S torsional angle of −166.8(4)° and molecules in which this angle is 14.0(4)°. The lengths
of the transannular N→Ge bonds (2.183–2.283 Å) agrees in value with the values of crystalline germatrane structures with a
Ge-C bond. The length of the transannular N→Ge bond (2.248 Å) in 4-(2-thienyl)phenylgermatrane (molecule A) is the largest
amongst germatranes with a Ge-Car bond. Introduction of a second thiophene ring into the 2-thienylgermatrane molecule lowers the acute toxicity of the compound
by about 27 times (LD50 for the 2,2′-bithienylgermatrane = 447 mg/kg).
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 299–306, February, 2007. 相似文献
899.
900.
ZHANG Bi-Song② 《结构化学》2005,(4)
1 INTRODUCTION Molecular assembly and supramolecular che-mistry are young and rapidly growing subjects andhave attracted great attention. Microporous materialsbuilt up by organic ligands and metal ions have po-tential applications in catalysis, chemical absorption,magnetism and electronic conductivity[1~3]. Multi-dentate organic ligands are good building blocks,such as 1,2,4,5-benzene tetracarboxylic acid (BT-CA), 1,3,5-benzene tricarboxylic acid, terephthalicacid, 1,10-phenanthrol… 相似文献