全文获取类型
收费全文 | 57547篇 |
免费 | 13003篇 |
国内免费 | 4590篇 |
专业分类
化学 | 56542篇 |
晶体学 | 840篇 |
力学 | 1662篇 |
综合类 | 297篇 |
数学 | 4322篇 |
物理学 | 11477篇 |
出版年
2024年 | 54篇 |
2023年 | 346篇 |
2022年 | 656篇 |
2021年 | 892篇 |
2020年 | 1933篇 |
2019年 | 3292篇 |
2018年 | 1665篇 |
2017年 | 1367篇 |
2016年 | 4207篇 |
2015年 | 4442篇 |
2014年 | 4543篇 |
2013年 | 5602篇 |
2012年 | 4998篇 |
2011年 | 4218篇 |
2010年 | 4405篇 |
2009年 | 4335篇 |
2008年 | 4163篇 |
2007年 | 3294篇 |
2006年 | 3065篇 |
2005年 | 3023篇 |
2004年 | 2579篇 |
2003年 | 2199篇 |
2002年 | 2910篇 |
2001年 | 2095篇 |
2000年 | 1842篇 |
1999年 | 840篇 |
1998年 | 348篇 |
1997年 | 248篇 |
1996年 | 227篇 |
1995年 | 217篇 |
1994年 | 201篇 |
1993年 | 168篇 |
1992年 | 106篇 |
1991年 | 111篇 |
1990年 | 91篇 |
1989年 | 73篇 |
1988年 | 57篇 |
1987年 | 34篇 |
1986年 | 30篇 |
1985年 | 55篇 |
1984年 | 28篇 |
1983年 | 27篇 |
1982年 | 26篇 |
1981年 | 29篇 |
1980年 | 21篇 |
1979年 | 15篇 |
1978年 | 11篇 |
1977年 | 12篇 |
1975年 | 6篇 |
1974年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
2,6-二羟甲基吡啶(1)经活性MnO_2氧化得到2,6-二甲酰基吡啶(2)。邻硝基苯酚与N-取代的二(氯乙基)胺在DMF溶液中反应,得到N-取代的1,5-二(邻硝基苯氧基)-3-氮杂戊烷(3a~3c),再经水合肼/Raney Ni还原,获得N-取代的1,5-二(邻氨基苯氧基)-3-氮杂戊烷(4a~4c)。利用Ba~(2 )作为模板离子,(2)分别与(4a~4c)反应,合成了一类新的含吡啶环系西佛碱大环配合物Ⅰ~Ⅲ,配合物Ⅰ、Ⅲ经与NaBH_4的乙醇溶液还原解络,得到氮杂大环自由配体Ⅳ和Ⅴ。所有西佛碱大环配合物和氮杂大环自由配体均经元素分析、IR、~1H NMR、MS等证实了它们的结构和组成。 相似文献
992.
Hans‐Peter Kormann Günter Schmid Katrin Pelzer K. Philippot Bruno Chaudret 《无机化学与普通化学杂志》2004,630(12):1913-1918
Nanoporous alumina membranes, loaded with palladium and ruthenium nanoparticles of various size, were used for gas phase hydrogenation of 1, 3‐butadiene and for oxidation of carbon monoxide, respectively. Those membranes contain 109 ‐ 1011 pores per cm2, all running perpendicular to the surface. Membrane discs of 20 mm in diameter and only 60 μm thick, incorporated in a reactor in which the reactants can be pumped in a closed circuit through the pores, turned out to very actively catalyze hydrogenation of butadiene (Pd) and oxidation of CO (Ru). The activity of the Pd catalysts depends characteristically on the particles size, the gas flow, and of the educts ratio. As could be expected, larger particles are less active than smaller ones, whereas increasing gas flows in case of hydrogenation accelerates the reactions. Excessive hydrogen reduces selectivity with respect to the various butenes, but favours formation of butane. 相似文献
993.
Mn( )能与许多氮氧化合物形成配合物 ,Wilde对 Mn( )与 2 ,2 -联吡啶及 1 ,1 -二氮杂菲的均配配合物作过详细研究 [1,2 ] ;Mn( )与 2 ,2 -联吡啶 - 1 ,1 -二氧化物 ( bipy O2 )的配合物也有综述 [3] .Mn( )与 bipy O2 的配合物大多是以 Cl O- 4、NO- 3、[Pt Cl4 ]2 -为阴离子 ,少数是卤离子 .它们均形成配位体数目为 3的单核螯合物 ,这些配合物是在水或乙醇中合成的 .Mn( )与 bipy O2 的多核聚合物还未见报道 .本文用 DMF为溶剂 ,以无水 Mn Cl2 和 2 ,2 -联吡啶 - 1 ,1 -二氧化物为原料 ,合成了 Mn( )与bipy O2 的三聚… 相似文献
994.
A. Subbiah Pandi V. Rajakannan D. Velmurugan Masood Parvez Moon‐Jib Kim A. Senthilvelan S. Narasinga Rao 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(3):o164-o167
N‐(2‐Chlorobenzyl)‐1,2,3,4‐tetrahydroisoquinoline‐1,3‐dione, C16H12ClNO2, crystallizes in P21/n with three crystallographically independent molecules in the asymmetric unit, which differ slightly in conformation, N‐(2‐bromo‐4‐methylphenyl)‐1,2,3,4‐tetrahydroisoquinoline‐1,3‐dione, C16H12BrNO2, crystallizes in P21/n with one molecule in the asymmetric unit andN‐(2,3‐dichlorophenyl)‐1,2,3,4‐tetrahydroisoquinoline‐1,3‐dione, C15H9Cl2NO2, crystallizes in P21/c with one molecule in the asymmetric unit. In all three structures, the heterocyclic rings adopt approximately planar conformations. The pyridine rings are orthogonal to the substituted phenyl rings. In all three structures, the crystal packing is stabilized by intermolecular C—H?O hydrogen bonds. 相似文献
995.
Zhaoyang Cheng Shipei Xing Jun Guo Biao Cheng Lan‐Fang Hu Xing‐Hong Zhang Zhan Lu 《中国化学》2019,37(5):421-421
The cover picture shows that sequential 1,1‐dihydrosilylation of terminal aliphatic alkynes with primary silanes enabled by one earth‐abundant cobalt catalyst has been developed. This protocol is operationally simple using readily available aliphatic alkynes, including simple acetylene and complex drug derivative, for efficient access to valuable gem‐bis(dihydrosilyl)alkanes in highly regioselective and atom‐economic manners. Corresponding asymmetric transformations are achieved with excellent enantioselectivities. More details are discussed in the article by Lu et al. on page 457–461.
996.
Highly dispersed α-Fe_2O_3/NaY,NiO/NaY,and CuO/NaY catalyst systems were pre-pared by impregnation method.Dispersion thresholds of the transition metal oxides on NaY" zeolitewere determined by XRD phase analysis.The dispersion capacities of the transition metal oxides on NaYzeolite are much lower than that estimated on the basis of a closed packed monolayer in the micropores.The catalytic activity and selectivity of the highly dispersed oxide catalyst systems for ethylben-zene and cyclohexane dehydrogenation reactions were reported. 相似文献
997.
A new method for indirect determination of captopril (CPT) with NaNO3 and NH4SCN by floatation and separation of copper has been studied. In the weak acid, a small amount of Cu(II) can be reduced to Cu(I) by CPT, then Cu(I) reacted with the SCN, which can float on the surface of the liquid phase with NaNO3. A good linear relationship is observed between the floatation yield (E%) of Cu(II) and the amount of captopril. The linear range is 2?32 mg/L. On the ground, captopril can be indirectly determined by determining E(%) of Cu(II). The method is simple, rapid, reliable and has good selectivity. The developed method can be applied to indirect determination of captopril with satisfactory results. 相似文献
998.
Protocatechuic acid esters (= 3,4‐dihydroxybenzoates) scavenge ca. 5 equiv. of radical in alcoholic solvents, whereas they consume only 2 equiv. of radical in nonalcoholic solvents. While the high radical‐scavenging activity of protocatechuic acid esters in alcoholic solvents as compared to that in nonalcoholic solvents is due to a nucleophilic addition of an alcohol molecule at C(2) of an intermediate o‐quinone structure, thus regenerating a catechol (= benzene‐1,2‐diol) structure, it is still unclear why protocatechuic acid esters scavenge more than 4 equiv. of radical (C(2) refers to the protocatechuic acid numbering). Therefore, to elucidate the oxidation mechanism beyond the formation of the C(2) alcohol adduct, 3,4‐dihydroxy‐2‐methoxybenzoic acid methyl ester ( 4 ), the C(2) MeOH adduct, which is an oxidation product of methyl protocatechuate ( 1 ) in MeOH, was oxidized by the DPPH radical (= 2,2‐diphenyl‐1‐picrylhydrazyl) or o‐chloranil (= 3,4,5,6‐tetrachlorocyclohexa‐3,5‐diene‐1,2‐dione) in CD3OD/(D6)acetone 3 : 1). The oxidation mixtures were directly analyzed by NMR. Oxidation with both the DPPH radical and o‐chloranil produced a C(2),C(6) bis‐methanol adduct ( 7 ), which could scavenge additional 2 equiv. of radical. Calculations of LUMO electron densities of o‐quinones corroborated the regioselective nucleophilic addition of alcohol molecules with o‐quinones. Our results strongly suggest that the regeneration of a catechol structure via a nucleophilic addition of an alcohol molecule with a o‐quinone is a key reaction for the high radical‐scavenging activity of protocatechuic acid esters in alcoholic solvents. 相似文献
999.
使用射频磁控溅射方法在不同衬底温度下(ts=室温,350,500℃)于Si(001)衬底上沉积了CNx膜,并利用拉曼(Raman)光谱、傅里叶变换红外光谱(FTIR)及X射线衍射光电子能谱(XPS)对CNx膜的化学结合状态与温度的关系进行了研究。Raman光谱结果表明,随衬底温度(ts) 增加,D带向低频方向移动,G带向高频方向移动;它们的半高宽分别由375和150cm^-1减小至328和142cm^-1;ID/IG由3.76减小至2.88。FTIR谱中除无序D带(1400cm^-1)和石墨G带(1570cm^-1)外,还有-700cm^-1,~2210cm^-1(C=N),2330cm^-1(C-O)及3255-3351cm^-1(N-H)等峰。XPS测试结果表明:随衬底温度增加,N与C的物质的量比由0.49下降至0.38,sp^2(C-N)组分与sp^3(C-N)组分强度比呈增大趋势。低温(350℃)退火并未对CNx膜的化学结合状态产生较大影响;高温(900℃)退火样品则显示出较好的结晶化程度。 相似文献
1000.
A new pulse sequence is proposed for the determination of scalar coupling correlation in small- and medium-sized organic compounds. The method uses a combination of the double pulsed field gradient spin-echo (DPFGSE) and the selective population transfer (SPT) techniques and is shown to be useful in the analysis of complex spectra with many overlapped signals. The usefulness of this method in the structural elucidation of natural substances is demonstrated using strychnine and digitoxin as examples. 相似文献