全文获取类型
收费全文 | 2616篇 |
免费 | 103篇 |
国内免费 | 392篇 |
专业分类
化学 | 2290篇 |
晶体学 | 21篇 |
力学 | 21篇 |
综合类 | 20篇 |
数学 | 454篇 |
物理学 | 305篇 |
出版年
2024年 | 6篇 |
2023年 | 70篇 |
2022年 | 54篇 |
2021年 | 82篇 |
2020年 | 160篇 |
2019年 | 107篇 |
2018年 | 97篇 |
2017年 | 124篇 |
2016年 | 115篇 |
2015年 | 77篇 |
2014年 | 146篇 |
2013年 | 563篇 |
2012年 | 227篇 |
2011年 | 158篇 |
2010年 | 118篇 |
2009年 | 127篇 |
2008年 | 130篇 |
2007年 | 134篇 |
2006年 | 128篇 |
2005年 | 99篇 |
2004年 | 94篇 |
2003年 | 65篇 |
2002年 | 16篇 |
2001年 | 17篇 |
2000年 | 18篇 |
1999年 | 13篇 |
1998年 | 14篇 |
1997年 | 18篇 |
1996年 | 12篇 |
1995年 | 12篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有3111条查询结果,搜索用时 15 毫秒
51.
Previous studies have established that the extended coordination model of solvation can satisfactorily account for the variation in the transfer enthalpies of solutes in mixed-solvent systems. The model parameter relating to the solute-induced disruption of the solvent structure shows a marked dependence on the nature of the mixed solvent. In the present paper we report the transfer enthalpies of acetonitrile from water to aqueous methanol, ethanol and dimethylsulphoxide (DMSO) systems. Analysis of these in terms of the extended coordination model confirms both the model's ability to account for the experimental data, and the variability of the structural disruption parameter. The solvation parameters recovered from the analyses indicate that the net effect of acetonitrile on the solvent structure is a breaking of solvent-solvent bonds. The extent of bond breaking of the solvent increases from MeOH to EtOH. 相似文献
52.
53.
Gerd-Volker Röschenthaler Michael Yu. Belik Alexander E. Sorochinsky 《Tetrahedron》2006,62(42):9902-9910
Addition of diethyl lithiodifluoromethylphosphonate to enantiomerically pure aromatic, heteroaromatic, and aliphatic aldehyde-derived sulfinimines afforded N-sulfinyl α,α-difluoro-β-aminophosphonates with generally good enantioselectivity and in high yield. The reaction with acetophenone-derived sulfinimine resulted in the formation of the addition product with high diastereoselectivity and in only moderate yield. A two-step deprotection involving treatment of diastereomerically pure N-sulfinyl α,α-difluoro-β-aminophosphonates with trifluoroacetic acid in EtOH followed by refluxing with 10 N HCl provided enantiopure α,α-difluoro-β-aminophosphonates and α,α-difluoro-β-aminophosphonic acids. The N-Cbz derivative of (R)-2-amino-1,1-difluoro-2-phenylethylphosphonate was a convenient starting point for the preparation of corresponding difluorophosphonate monoester, difluorophosphonic acid, and difluorophosphonamidic acid. At 21 °C difluorophosphonamidic acid was stable in aqueous solution at pH above 5. 相似文献
54.
Antonella Leggio 《Tetrahedron》2007,63(34):8164-8173
A highly efficient and practical synthesis of peptides in solution phase has been developed. The procedure is based on the use of p-nitrobenzenesulfonyl (nosyl) group for the protection of the amino function of α-amino acids. Every step of the procedure, protection of the amino function by the nosyl group, formation of the peptide bond, and removal of the sulfonamide group, is characterized by high yields and excellent purity of the final products. The described strategy allows the preparation of short peptide sequences keeping the chiral integrity of amino acid precursors. Compatibility of nosyl group with the side-chain protecting groups used in Fmoc-based strategy is demonstrated. The method here presented is an alternative strategy that could provide advantages for future peptide synthesis. 相似文献
55.
用ab initio方法, 在MP2/6-31G**水平下讨论了α-乙酰氧基-亚硝基吡咯烷(α-Acetoxy-NPYR)在各种条件下的解离反应机理, 并对形成终致癌物B, C, D的代谢机理进行研究. 发现在OH-和H2O作用下的解离都遵循羟基进攻羰基机理, OH-作用下是一个经四面体中间体阴离子的无位垒过程, H2O作用下有相对高的活化能(165.36 kJ/mol). H3O+作用下是先形成阳离子产物的SN1过程, 并没有发现遵循两种综合的解离情形. 同时, 羟基化产物异构化为终致癌物B, C, D是一个相对容易进行的过程. 相似文献
56.
57.
58.
59.
Michael D. Swift 《Tetrahedron letters》2007,48(22):3771-3773
A new approach for the stereoselective synthesis of (2R,3S)-2-amino-3,4-dihydroxybutyric acid, an α-amino acid from Lyophyllum ulmarium, has been accomplished using an ether directed aza-Claisen rearrangement. On investigation of optimal conditions for this key step it was shown for the first time that Au(I) can be used to catalyse this transformation. 相似文献
60.
In the research on metallic glass, there are arguments against the "rigid band charge transfer", model which assumes a charge transfer from metalloid atoms to transition metal atoms to explain the experimental evidence of linear reduction of average atomic magnetic moment with the increasing concentration of metalloid atoms, but they could not explain the experimental relation of the reduction. In the present work, spin-polarized SCC-DV-X_a calculation for atomic clusters for metallic glass Co-B has been empoloyed to investigate the local electronic structure and magnetic property of the metallic glass. As opposed to the "rigid band charge transfer" model, calculation in the present work indicates that charge transfers from Co4s to both B and Co3d. It is found that there is Co3d4s-B2p hybird bonding in Co-B, which leads to the linear reduction of average atomic magnetic moment. Thus the explanation removes the above controversy. 相似文献