全文获取类型
收费全文 | 95610篇 |
免费 | 5544篇 |
国内免费 | 12372篇 |
专业分类
化学 | 71783篇 |
晶体学 | 1189篇 |
力学 | 3060篇 |
综合类 | 938篇 |
数学 | 15484篇 |
物理学 | 21072篇 |
出版年
2024年 | 127篇 |
2023年 | 727篇 |
2022年 | 1968篇 |
2021年 | 1919篇 |
2020年 | 2356篇 |
2019年 | 2320篇 |
2018年 | 1995篇 |
2017年 | 2856篇 |
2016年 | 2985篇 |
2015年 | 2490篇 |
2014年 | 3477篇 |
2013年 | 7104篇 |
2012年 | 6404篇 |
2011年 | 5198篇 |
2010年 | 4389篇 |
2009年 | 6015篇 |
2008年 | 6233篇 |
2007年 | 6474篇 |
2006年 | 5948篇 |
2005年 | 5000篇 |
2004年 | 4653篇 |
2003年 | 3900篇 |
2002年 | 5108篇 |
2001年 | 3010篇 |
2000年 | 2795篇 |
1999年 | 2617篇 |
1998年 | 2278篇 |
1997年 | 1796篇 |
1996年 | 1517篇 |
1995年 | 1452篇 |
1994年 | 1282篇 |
1993年 | 1065篇 |
1992年 | 1005篇 |
1991年 | 679篇 |
1990年 | 592篇 |
1989年 | 550篇 |
1988年 | 427篇 |
1987年 | 339篇 |
1986年 | 314篇 |
1985年 | 254篇 |
1984年 | 264篇 |
1983年 | 150篇 |
1982年 | 233篇 |
1981年 | 189篇 |
1980年 | 202篇 |
1979年 | 191篇 |
1978年 | 186篇 |
1977年 | 127篇 |
1976年 | 113篇 |
1973年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Twelve compounds each having a six membered Si-contained heterocycle, have been successfully synthesized. Of these eleven compounds have never been reported in the literature. The structures were confirmed by Spectral analysis and elementary analysis. The stability and properties of these compounds is also discussed. 相似文献
942.
薄层扫描法测定冬眠过程中刺猬活性组织酶的周期变化冯金城(天津师范大学天津300074)孙金生(天津水产研究所天津300221)1前言为了跟踪观察刺猬在冬眠过程中体内乳酸脱氢酶的周期性变化,利用薄层扫描仪测定了其相关的组织同工酶的差异。实践证明,此法简... 相似文献
943.
The properties of thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogel in tetrahydrofuran/H2O mixtures were studied. Scanning electron microscopic (SEM) images demonstrate that the hydrogel changes from homogeneous to heterogeneous microstructure upon the addition of tetrahydrofuran to water. This heterogeneous PNIPAAm hydrogel in the mixture solvent exhibits a very slow response rate at temperatures above its lower critical solution temperature. The decreased response rate is attributed to the formation of special ternary complexes including the polymer and the two solvents in the tetrahydrofuran/H2O mixture. Factors controlling the thermoresponse rate are discussed further and several suggestions are provided for designing and developing fast-response PNIPAAm hydrogels in the future. 相似文献
944.
945.
Marit Rolandsgard 《Tetrahedron》2005,61(16):4129-4140
Preparation of α-oxo derivatives of spiro[4.4]nonane, spiro[4.5]decane and spiro[5.5]undecane derivatives is described. An efficient method for spiroannulation by Rh(I)-catalysed intramolecular hydroacylation provides α,α′-difunctionalised spiro[4.5]decanes. The α,α′-dioxo groups have been converted into vinyl triflates for arylation by Pd-catalysed cross-coupling reactions under Stille, Negishi or Suzuki conditions depending on relative reactivities. Stereoselective saturation of the conjugated aryl olefinic bonds by catalytic hydrogenation over Pd-carbon provides methodology for stereoselective preparation of α-aryl- and α,α′-cis,cis-diaryl spiranes, the latter with a sandwich like structure. Single crystal X-ray analyses have been used in the structural assignments. 相似文献
946.
947.
A facile and short synthesis of 2-oxo-4-sec-amino-2,3,5,6-tetrahydrobenzo[f]isoquinoline-1-carbonitriles has been delineated through base catalyzed ring transformation of 5,6-dihydro-2-oxo-4-sec-amino-2H-benzo[h]chromene-3-carbonitrile by cyanoacetamide in excellent yields. 相似文献
948.
A series of novel bidentate pyrazolone based Schiff base ligands were synthesized by interaction of 4-benzoyl-3-methyl-1-(4′-methylphenyl)-2-pyrazolin-5-one with various aromatic amines like aniline, o-,m-,p-chloroaniline and o-,m-,p-toluidine in a ethanolic medium. All of these ligands have been characterized on the basis of elemental analysis, IR and 1H NMR data. The molecular geometries of five of these ligands have been determined by single crystal X-ray study. Crystallographic study reveals that these ligands exist in the amine-one tautomeric form in the solid state. NMR study also suggests the existence of the amine-one form in solution at room temperature. Ab initio calculations for representative ligand HL1 has been carried out to know the coordination site of the ligand. Novel vanadium Schiff base complexes of these ligands with general formula [OV(L1–7)2(H2O)] have been prepared by interaction of aqueous solution of vanadyl sulfate pentahydrate with DMF solution of the appropriate ligands. The resulting complexes have been characterized on the basis of elemental analysis, vanadium determination, molar conductance and magnetic measurements, thermo gravimetric analysis, infrared and electronic spectral studies. Suitable distorted octahedral structures have been proposed for these complexes. 相似文献
949.
吡啶-2-甲酸镍的合成及其晶体结构 总被引:1,自引:0,他引:1
以吡啶 2 甲酸和Ni(ClO4)2·6H2O为原料反应制得配合物[Ni(C5H4NCOO)2(H2O)2]2H2O,并通过X 射线衍射法测定其晶体结构.该化合物属于单斜晶系,空间群P21/n,晶胞参数:a=0.97351(15)nm,b=0.52254(8)nm,c=1.4483(2)nm,β=90.119(3)°,V=0.7367(2)nm3,Z=2,Dc=1.690g/cm3,μ=1.362mm 1,F(000)=388,结构偏离因子R1=0.0316,wR2=0.0902,共收集到4771个强度数据,其中1700个独立衍射点,1489个(I>2σ(I))可观测点.X 射线分析表明晶体是由分子间氢键和π π弱相互作用堆积而成的二维层状结构. 相似文献
950.
Khoroshen'kov G. V. Fag A. A. Bochkarev M. N. Dechert S. Schumann H. 《Russian Chemical Bulletin》2003,52(8):1715-1719
The reactions of LnI2 (Ln = Nd (1) or Dy (2)) with cyclopentadiene (CpH) in THF at 0 °C afforded the CpLnI2(THF)3 complexes in 65—67% yields. The reaction of thulium diiodide (3) with an excess of CpH at 60 °C produced CpTmI2(THF)3, Cp2TmI(THF)2, and TmI3(THF)3 in 21, 58, and 63% yields, respectively. The reactions of 1 and 2 with pentamethylcyclopentadiene (Cp*H) in THF were accompanied by disproportionation giving rise to the Cp*2LnI(THF)2 and LnI3(THF)
x
complexes. Neodymium triiodide was isolated in the ionic form [NdI2(THF)5]+[NdI4(THF)2]–. Its structure and the structure of CpTmI2(THF)3 were established by X-ray diffraction analysis. 相似文献