全文获取类型
收费全文 | 6059篇 |
免费 | 1046篇 |
国内免费 | 649篇 |
专业分类
化学 | 3994篇 |
晶体学 | 64篇 |
力学 | 312篇 |
综合类 | 37篇 |
数学 | 770篇 |
物理学 | 2577篇 |
出版年
2024年 | 3篇 |
2023年 | 135篇 |
2022年 | 123篇 |
2021年 | 193篇 |
2020年 | 234篇 |
2019年 | 199篇 |
2018年 | 192篇 |
2017年 | 183篇 |
2016年 | 306篇 |
2015年 | 260篇 |
2014年 | 340篇 |
2013年 | 411篇 |
2012年 | 519篇 |
2011年 | 587篇 |
2010年 | 359篇 |
2009年 | 363篇 |
2008年 | 386篇 |
2007年 | 360篇 |
2006年 | 344篇 |
2005年 | 285篇 |
2004年 | 214篇 |
2003年 | 192篇 |
2002年 | 204篇 |
2001年 | 159篇 |
2000年 | 141篇 |
1999年 | 176篇 |
1998年 | 138篇 |
1997年 | 121篇 |
1996年 | 120篇 |
1995年 | 81篇 |
1994年 | 63篇 |
1993年 | 59篇 |
1992年 | 55篇 |
1991年 | 61篇 |
1990年 | 38篇 |
1989年 | 27篇 |
1988年 | 26篇 |
1987年 | 18篇 |
1986年 | 26篇 |
1985年 | 20篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有7754条查询结果,搜索用时 578 毫秒
71.
Nishiura M Hou Z Wakatsuki Y Yamaki T Miyamoto T 《Journal of the American Chemical Society》2003,125(5):1184-1185
Various terminal alkynes have been cleanly dimerized into the corresponding head-to-head (Z)-enynes by use of the half-metallocene lutetium alkyl complexes Me2Si(C5Me4)(NAr)Lu(CH2SiMe3)(THF) (Ar = Ph, C6H3Me2-2,6, C6H2Me3-2,4,6) as catalysts. Aromatic C-Cl, C-Br, and C-I bonds, which are known to be extremely susceptible to reductive cleavage by transition metals, survived in the present reactions. The corresponding dimeric alkynide species [Me2Si(C5Me4)(NAr)Lu(mu-CCR)]2 are thought to be the true catalysts, some of which have been isolated and structurally characterized. These alkynide species were thermally stable and soluble at the reaction temperatures (80-110 degrees C), but they precipitated upon cooling to room temperature after completion of the reaction. Therefore, this catalyst system works homogeneously but can be separated and reused, thus constituting the first example of a recyclable catalyst system for the dimerization of terminal alkynes and also the first example of (Z)-selective head-to-head dimerization of aromatic terminal alkynes. 相似文献
72.
AQC柱前衍生反相高效液相色谱法测定土壤中氨基酸 总被引:14,自引:1,他引:14
以2-氨基丁酸为内标物,6-氨基喹啉基-N-羟基琥珀酰亚氨基甲酸酯(AQC)为柱前衍生试剂,用XTerraC18柱,1.8mmol/L甲酸铵溶液(pH=3.0)和乙腈为流动相,采用梯度洗脱,光电二极管(PDA)检测器在248nm波长处检测,建立了一种利用反相高效液相色谱同时测定土壤中17种氨基酸的分析方法。方法重现性好,精密度高,氨基酸浓度在25~600μmol/L范围内,线性相关系数均大于0.99;各氨基酸检出限≤0.5pmol。并且在该色谱条件下,获得了灵敏度较高,彼此分离良好的17种AQC衍生氨基酸的LC-MS质谱总离子流图,为今后土壤氨基酸的深入研究提供了更加有利的手段。 相似文献
73.
74.
75.
76.
This Letter shows explicitly that the descent sequence of cocycles in a gauge group realizes the ech-de Rham double complex. Meanwhile the ech complex corresponds to finite gauge transformations. This Letter also shows how the indices of each order of cocycles characterizes the obstructions on overlapping spheres in different dimensions and how these indices are equal to a common one. 相似文献
77.
Two acetylene polymers containing cyanobiphenyl-based mesogens,poly { 10-[ ((4'- cyano-4-biphenylyl ) oxy ) carbonyl] - 1 -decyne } (PA8CN ), which has a relativelyfiexible polyalkyne backbone, and poly {[4-(((12-((4'-cyano-4-biphenylyl)oxy)dodecyl)oxy)carbonyl) phenyl]-acetylene} (PB12CN), which has a fairly rigid poly (phenylacetylene )backbone, were synthesized using respectively WCl_6 and [Rh(nbd)Cl]_2 as the catalysts.PA8CN exhibits enantiotropic interdigitated smectic A phase (S_(Ad)) over a temperaturerange as wide as ca. 100℃, whereas PB12CN is non-mesomorphic, demonstrating thatthe backbone rigidity plays an important role in determining the liquid crystallinity of thepolyacetylenes. 相似文献
78.
Using FcCOONa (Fc = (eta(5)-C(5)H(5))Fe(eta(5)-C(5)H(4))) as starting material, we obtained an unprecedented metal-organic coordination polymer containing ferrocenecarboxylate components [[Pb(2)(FcCOO)(eta(2)-FcCOO)(mu(2)-eta(2)-FcCOO)(mu(3)-eta(2)-FcCOO)(CH(3)OH)].1.5CH(3)OH.H(2)O](n) (1), tetramer [Zn(4)(mu(2)-FcCOO)(6)(mu(4)-O)] (2), and coordination polymers [Pb(FcCOO)(mu(2)-FcCOO)(bpe)](n) (3) (bpe = 1,2-bis(4-pyridyl)ethene), [[Zn(FcCOO)(2)(bpt)].2.5H(2)O](n) (4) (bpt = N,N'-bis(3-pyridylmethyl)thiourea), and [Zn(FcCOO)(eta(2)-FcCOO)(bbp)](n) (5) (bbp = 4,4'-trimethylene-dipyridine). Compounds 1 and 2 are formed by ferrocenecarboxylate units coordinating with Pb(II) or Zn(II). In polymer 1, ferrocenecarboxylate units have four kinds of coordinate modes; just these novel coordinate modes lead to the unprecedented one-dimensional polymer where two kinds of rhomboids are arranged alternatively along the chain. Compound 2 is a tetramer, in which a distinct connectivity of the six ferrocene units is established through the four Zn atoms. Compounds 3-5 are obtained by organic ligands bridging Pb(II) or Zn(II), leading to a new type of metal-organic coordination polymer. 相似文献
79.
Two new dicyanamide coordination polymers, {Mn(dmpz)[N(CN)2]2}2 (1) and {Cu(dmpz)[N(CN)2]2}2 (2)(dmpz=3,5-dimethylpyrazole), were synthesized and characterized by single crystal X-ray diffraction analysis and IR spectroscopy. In 1 and 2 the metal ions have two different coordination modes, where one is coordinated to four dicyanamide anions and two monodentate dmpz molecules to form a slightly distorted octahedral geometry, while the other adopts octahedral geometry, surrounded by four nitrile N atoms and two amide N atoms of the dicyanamide anions. Both complexes contain two alternating chains that are parallel to each other. 相似文献
80.