全文获取类型
收费全文 | 2421篇 |
免费 | 105篇 |
国内免费 | 414篇 |
专业分类
化学 | 2333篇 |
晶体学 | 19篇 |
力学 | 5篇 |
综合类 | 20篇 |
数学 | 256篇 |
物理学 | 307篇 |
出版年
2024年 | 6篇 |
2023年 | 68篇 |
2022年 | 50篇 |
2021年 | 74篇 |
2020年 | 151篇 |
2019年 | 105篇 |
2018年 | 94篇 |
2017年 | 114篇 |
2016年 | 106篇 |
2015年 | 66篇 |
2014年 | 131篇 |
2013年 | 509篇 |
2012年 | 167篇 |
2011年 | 144篇 |
2010年 | 115篇 |
2009年 | 125篇 |
2008年 | 118篇 |
2007年 | 132篇 |
2006年 | 140篇 |
2005年 | 111篇 |
2004年 | 115篇 |
2003年 | 63篇 |
2002年 | 14篇 |
2001年 | 17篇 |
2000年 | 23篇 |
1999年 | 10篇 |
1998年 | 21篇 |
1997年 | 19篇 |
1996年 | 24篇 |
1995年 | 20篇 |
1994年 | 19篇 |
1993年 | 13篇 |
1992年 | 9篇 |
1991年 | 18篇 |
1990年 | 8篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有2940条查询结果,搜索用时 31 毫秒
81.
82.
通过类Mannich 反应“一锅法”合成了10个未见文献报道的含吡啶基的环状α-氨基膦酸酯, 其结构经1H NMR, 31P NMR, IR, MS和元素分析确证. 测定了该系列化合物的杀虫、杀菌和除草活性, 结果表明, 所测化合物的杀虫活性不太好, 但所有化合物在药液浓度为1.0×10-4 g/mL 时都具有较好的除草活性, 部分化合物在5.0×10-5 g/mL时显示出良好的杀菌活性. 相似文献
83.
84.
Madan G. Barthakur 《Tetrahedron letters》2006,47(28):4925-4927
An efficient one-pot synthesis of substituted 4H-thiopyrans has been accomplished from a three-component reaction of α,β-unsaturated ketones, Lawesson’s reagent and alkynes under microwave irradiation. 相似文献
85.
α-取代氨基氟代苯基膦酸酯衍生物的合成、晶体结构与抗癌活性 总被引:4,自引:0,他引:4
利用席夫碱与亚磷酸酯反应, 合成了新型O,O'-二烷基-α-(6-甲氧苯并噻唑-2-基氨基)-4-氟苯基膦酸酯化合物, 结构经元素分析, IR, 1H NMR, 13C NMR和X单晶衍射确认. X单晶衍射测试结果表明: 化合物3d分子属于四面体晶系, 空间群I4(1)/a, a=2.1055(3) nm, b=2.1055(3) nm, c=2.0521(5) nm, α=90.00°, β=90.00°, γ=90.00°, V=0.9098(3) nm3,
Z=16, Dc=1.321 mg/m3, =0.250 mm-1, F(000)=3808. 化合物还存在着1个分子内氢键[N(2)—H(2)…O(1)]. 生物测定表明化合物3f在20 g/mL浓度下对PC3细胞的抑制率为84.3%. 相似文献
86.
Treatment of phenyl 2,3-O-cyclic ketene acetal- and 2,3-O-thionocarbonyl-1-thio-mannopyranosides with TMSOTf and MeOTf, respectively, gave the corresponding 2,3-O-cyclic dioxonium intermediates, which proceeded via 1→2 migration and concurrent glycosidation in the presence of alcohols to provide the corresponding 2-S-phenyl glycosides stereoselectively. While the former donors were too labile, the latter donors have proved superior for the present purpose. The X-ray crystallographic structures of phenyl 4-O-methyl-2,3-O-thiocarbonyl-1-thio-α-l-rhamnopyranoside (1), a typical donor for the present reaction, and its anomeric azide analogue (6), which could not undergo the present reaction under similar conditions, are provided. 相似文献
87.
The Diels-Alder reaction of enantiomerically pure (E)-γ-keto-α,β-unsaturated p-tolylsulphoxides 3 with cyclopentadiene give four easily separable diastereomers. The effect of several Lewis acids on the reaction was studied, finding a high endo selectivity with respect to the carbonyl group and moderate π-diastereoselectivity using BF3·Et2O as catalyst. The reactivity of compounds 3 as well as their endo selectivity are both higher than those observed for the corresponding (E)-3-sulfinylacrylates. 相似文献
88.
N. Suryakiran 《Tetrahedron letters》2007,48(5):877-881
The synthesis of α-halo β-keto-sulfones using potassium halide and hydrogen peroxide as a chemoselective mono halogenation reagent and the synthesis of α,α-symmetrical and asymmetrical dihalo β-keto-sulfones and α-halo, α-alkyl and β-keto-sulfones is described. Base induced cleavage of α-halo β-keto-sulfones, α,α-dihalo β-keto-sulfones, and α-halo, α-alkyl β-keto-sulfones afforded the corresponding halomethyl sulfones, dihalomethyl sulfones and haloalkyl sulfones. 相似文献
89.
Liping Li 《Journal of solid state chemistry》2007,180(10):2896-2904
Highly crystalline one-dimensional (1D) α-MnO2 nanostructures were synthesized by a hydrothermal method. All samples were characterized by X-ray diffraction, transmission electron microscope, thermogravimetric and differential scanning calorimeter, and infrared spectroscopy. During the formation reactions, the tunnel structure of 1D α-MnO2 was simultaneously modified by NH4+ species and water molecules. The amount of NH4+ species that were trapped in the tunnels is almost independent on the reaction temperature, while the total water content increased with the reaction temperature. The average diameter of α-MnO2 nanorods increased from 9.2 to 16.5 nm when the reaction temperature increased from 140 to 220 °C. 1D α-MnO2 was destabilized by a subsequent high-temperature treatment in air, which is accompanied by a structural transformation to 1D Mn2O3 of a cubic structure. At low temperatures, all 1D α-MnO2 nanorods showed two magnetic transitions that were characterized by a decreased Néel temperature with rod diameter reduction. According to the effective magnetic moments experimentally measured, Mn ions presented in the nanorods were determined to be in a mixed valency of high spin state Mn4+/Mn3+. 相似文献
90.
C60和C70等碳笼烯的发现及其在新物质、新材料等研究领域中的重要性,激励人们去探索合成更多新型多面体碳簇化合物与碳元素类似物[1,2].最近,两类新型多面体碳管笼烯的设想提出来了[3,4]应用HMO和MNDO方法对其稳定性变化规律、结构和成键特征进行了讨论.关于多苯的vanderWaals簇实验上已有广泛的调查[5,6],理论上对其二聚物(C6H6)。的不同几何构型与稳定性进行了深入的研究[7].本文采用abinitio计算有效势(effectivecorePotential)方法,对C12碳管元素簇和C12H12,碳管烷的平衡结构、稳定性和价键特征进行了理论预测.1… 相似文献