全文获取类型
收费全文 | 6930篇 |
免费 | 1167篇 |
国内免费 | 742篇 |
专业分类
化学 | 4655篇 |
晶体学 | 72篇 |
力学 | 407篇 |
综合类 | 41篇 |
数学 | 646篇 |
物理学 | 3018篇 |
出版年
2024年 | 16篇 |
2023年 | 149篇 |
2022年 | 168篇 |
2021年 | 175篇 |
2020年 | 262篇 |
2019年 | 217篇 |
2018年 | 205篇 |
2017年 | 185篇 |
2016年 | 292篇 |
2015年 | 288篇 |
2014年 | 356篇 |
2013年 | 502篇 |
2012年 | 540篇 |
2011年 | 609篇 |
2010年 | 408篇 |
2009年 | 411篇 |
2008年 | 434篇 |
2007年 | 370篇 |
2006年 | 363篇 |
2005年 | 335篇 |
2004年 | 276篇 |
2003年 | 204篇 |
2002年 | 222篇 |
2001年 | 173篇 |
2000年 | 168篇 |
1999年 | 176篇 |
1998年 | 130篇 |
1997年 | 128篇 |
1996年 | 151篇 |
1995年 | 107篇 |
1994年 | 113篇 |
1993年 | 108篇 |
1992年 | 100篇 |
1991年 | 83篇 |
1990年 | 63篇 |
1989年 | 74篇 |
1988年 | 52篇 |
1987年 | 46篇 |
1986年 | 38篇 |
1985年 | 34篇 |
1984年 | 19篇 |
1983年 | 18篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1976年 | 4篇 |
1973年 | 3篇 |
1970年 | 3篇 |
1957年 | 3篇 |
排序方式: 共有8839条查询结果,搜索用时 46 毫秒
951.
952.
953.
Stacked models that include 9,9'-bis(6'-N,N,N-trimethylammonium)hexyl]fluorene-co-alt-4,7-(2,1,3-benzothiadiazole)dibromide (F(BT)F) monomer sandwiched between two stacked 2,1,3-benzothiadiazole (BT) units were explored using theoretical approaches. Molecular structures and the optical characteristics of the investigated species were investigated at the M06-2X/6-311G(d,p)//TD-M06-2X/6-311G(d,p) level of theory. In all models, the electronic excitation to the lowest singlet ππ* excited state (S1(ππ*)) is governed by the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO → LUMO) transitions. The obtained results suggest that stacking interaction might have only minor effects on the transition energy for both absorption and emission processes. Instead, the reduction in the excitation energy of the stacked complexes should be attributed to the dipole-dipole interaction. The larger the interaction energy of the stacked models, the bigger the observed differences between absorption-emission energies. The presence of the solvation medium with small dielectric constant may increase the absorption-emission energy differences. It is expected that the largest absorption-emission shift can be observed in the benzene solution. 相似文献
954.
The effect of substituents on the oxidation potential for the one-electron reaction of 1,4-dimethoxybenzene was understood with a theoretical calculation based on density functional theory (DFT) at the level of B3LYP/6-311+G(d). It is found that the oxidation potential for the one-electron reaction of 1,4-dimethoxybenzene is 4.13 V (vs Li/Li(+)) and can be changed from 3.8 to 5.9 V (vs Li/Li(+)) by substituting electron-donating or electron-withdrawing groups for the hydrogen atoms on the aromatic ring. These potentials are in the range of the limited potentials for the lithium ion batteries using different cathode materials, and thus the substituted compounds can be selected as the redox shuttles for the overcharge prevention of these batteries. The oxidation potential of 1,4-dimethoxybenzene decreases when the hydrogen atoms are replaced with electron-donating groups but increases when replaced with electron-withdrawing groups. The further oxidation of these substituted compounds was also analyzed on the basis of the theoretic calculation. 相似文献
955.
Treatment of various anilides with 1.5 equiv. of phenyliodine bis(trifluoroacetate) (PIFA) and 1.0 equiv. of BF(3)·OEt(2) in AcOH at room temperature afforded the corresponding para-acetoxylated products with high regioselectivity. In addition, this reaction could be expanded to the etherification of anilides. In the presence of 2.0 equiv. of PIFA and 2.0 equiv. of BF(3)·OEt(2), the reaction of anilides with alcohols provided the corresponding para-etherified products in good yields. 相似文献
956.
Hybrids of vinblastine and phomopsin, designed by a molecular modelling study, were elaborated in order to target tubulin. The key step of the synthesis (fragmentation and insertion of vindoline) was mediated by an internal N-carboxyanhydride (or O-acylcarbamate). This reaction was diastereospecific and addition of silver salts could reverse the diastereoselectivity. Even if the synthesized compounds are inactive, this synthesis represents an original example of a C-N fragmentation mediated by a N-carboxyanhydride. 相似文献
957.
Chen GJ Gao CY Tian JL Tang J Gu W Liu X Yan SP Liao DZ Cheng P 《Dalton transactions (Cambridge, England : 2003)》2011,40(20):5579-5583
By changing the ratio of reactants, two mononuclear Dy complexes, [Dy(phen)(acac)(3)] (1) and [Dy(phen)(2)(NO(3))(2)(acac)]·H(2)O (2) have been synthesized and structurally characterized. In 1, a Dy atom bearing square-antiprism coordination geometry exhibits SMM behaviour, while compound 2 with a bicapped-square-antiprism geometry does not show such SMM properties. The different magnetic behaviours seen in 1 and 2 are probably due to a different coordination environment and ligand field around the Dy(III) ions. The results proved the important influence of the structural environment of a SMM on its magnetic behaviour. 相似文献
958.
Lewis basic DABCO-functionalized 3D-like metal-organic framework, Zn-MOF, catalyzes nitroaldol (Henry) reaction of 4-nitrobenzaldehyde with nitroalkanes in a size-dependent manner. Small nitroalkanes give rise to higher conversion than larger ones. This MOF-based heterogeneous catalyst is very robust and can be recycled several times without losing its activity. 相似文献
959.
A sensitive, specific and rapid liquid chromatography-mass spectrometry (LC-MS/MS) method has been developed and validated for enantioselective determination of darusentan enantiomers, orally active potent endothelin-A receptor antagonist, in rat plasma. The plasma samples were pretreated by protein precipitation with methanol and baseline chromatographic separation was performed on a Chiralcel OD-RH column with a mobile phase consisting of acetonitrile/water/formic acid (50:50:0.1, v/v/v) at a flow rate of 0.5 mL/min. The detection was accomplished by multiple-reaction monitoring (MRM) scanning via electrospray ionization (ESI) source operating in the negative ionization mode. The calibration curve was linear over the investigated concentration from 0.500 to 2500 ng/mL (r≥0.995) for each enantiomer using 50 μL of rat plasma. The lower limit of quantitation (LLOQ) for each enantiomer was 0.500 ng/mL. The intra- and inter-day precisions were not more than 10.2% and the accuracy was within the range from -5.4 to 6.3% for darusentan enantiomers. No chiral inversion was observed during the plasma preparation, storage and analysis. The method proved adequate for enantioselective pharmacokinetic studies of darusentan enantiomers after oral administration of three different doses of racemic darusentan. 相似文献
960.
Min Gao Xiaolei Wang Ming Gu Zhiguo Su Ye Wang Jan‐Christer Janson 《Journal of separation science》2011,34(15):1853-1858
A porous polyamide resin is shown to possess hydrogen bond acceptor properties suitable for the separation of polyphenolic solutes such as phenolic acids, flavonols and flavonoids. The separation is achieved in the presence of solvent mixtures of acetic acid and ethanol. The extent of hydrogen bond adsorption is reviewed based on data obtained from the elution behaviour of a variety of simple polyphenolic solutes. Polyamide adsorption chromatography was applied for the purification of resveratrol and polydatin from Polygonum cuspidatum Sieb. & Zucc. 相似文献