全文获取类型
收费全文 | 4370篇 |
免费 | 323篇 |
国内免费 | 388篇 |
专业分类
化学 | 4768篇 |
晶体学 | 5篇 |
力学 | 10篇 |
综合类 | 8篇 |
数学 | 78篇 |
物理学 | 212篇 |
出版年
2024年 | 3篇 |
2023年 | 33篇 |
2022年 | 61篇 |
2021年 | 88篇 |
2020年 | 109篇 |
2019年 | 140篇 |
2018年 | 116篇 |
2017年 | 133篇 |
2016年 | 163篇 |
2015年 | 153篇 |
2014年 | 280篇 |
2013年 | 421篇 |
2012年 | 277篇 |
2011年 | 331篇 |
2010年 | 250篇 |
2009年 | 290篇 |
2008年 | 302篇 |
2007年 | 317篇 |
2006年 | 273篇 |
2005年 | 233篇 |
2004年 | 233篇 |
2003年 | 200篇 |
2002年 | 158篇 |
2001年 | 69篇 |
2000年 | 60篇 |
1999年 | 51篇 |
1998年 | 42篇 |
1997年 | 45篇 |
1996年 | 39篇 |
1995年 | 39篇 |
1994年 | 30篇 |
1993年 | 28篇 |
1992年 | 30篇 |
1991年 | 22篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有5081条查询结果,搜索用时 15 毫秒
21.
An efficient and stereodefined process is described for the first preparation of a new prenyl-benzoylfuranone type sesquiterpenoid, (±)-3-(2,4-dihydroxybenzoyl)-4,5-dimethyl-5-(4,8-dimethyl-3(E),7(E)-nonadien-1-yl)tetrahydro-2-furanone. The synthetic strategy is based on nucleophilic addition of organometallic reagents to the functionalized ketoamides elaborated from dihydroxyacetone dimer for the stereoselective construction of the key quaternary carbon center in the target compound. 相似文献
22.
Zois Syrgiannis 《Tetrahedron letters》2006,47(17):2961-2964
The thermodynamic parameters, ΔΔH# and ΔΔS#, were determined for the interception of an intermediate, with the structural characteristics of an aziridinium imide, by nucleophilic solvents during the reaction of 2-methyl-2-butene with N-phenyltriazolinedione. The experimentally measured parameters were found to be in favor of an SN2-‘like’ transition state and showed strong dependence on the bulkiness of the incoming molecule of the nucleophile-solvent. 相似文献
23.
E. V. Gert 《Cellulose (London, England)》1996,3(1):217-228
For preparation of various modified forms of microcrystalline cellulose, an optional combination is proposed of hydrolytic activity with other properties of nitric acid such as, adduct forming ability (manifested as Knecht compound formation), oxidizing, bleaching and nitrating. The manifestation of a chosen acid function is reached by variation of the reaction conditions: concentration, temperature, time, etc. The sole-reagent continuous procedure involving several operations made in a predetermined sequence allows the use of a more simple technological scheme and saves energy, time and materials. The features of transformation of fibrous and microcrystalline forms of native cellulose into cellulose-II through the Knecht compound are discussed. Purely acidic transformation schemes of native cellulose into microcrystalline cellulose hydrate and amorphous low-nitrate powder cellulose according to the above scheme are considered. Morphological features as well as distinguishing properties — enhanced hydrophilicity, sorption activity and degree of whiteness — of the prepared materials are pointed out. The possibility of obtaining uniformly oxidized forms of powder cellulose according to this scheme and the outlook for the use of nitrogen(IV) oxide as a multifunctional reagent are discussed. 相似文献
24.
The selenenylation of racemic and optically active α-phosphoryl sulfoxides is a key step leading efficiently to α-phosphorylvinyl sulfoxides or α-phosphorylvinyl selenides depending on the reaction conditions. Oxidation of α-phosphorylvinyl selenides and subsequent thermolysis of selenoxides afford alkynylphosphonates. Studies of the stereochemical course of nucleophilic addition to α-phosphoryl sulfoxides show high facial stereoselectivity of the reaction, however, epimerisation at the α-carbon atom leads to mixtures of diastereomers. 相似文献
25.
Guoyi Bai Chenfang Zhang Yuecheng Zhang Haijun Yu Fei He Huisen Ning Ligong Chen 《Reaction Kinetics and Catalysis Letters》2007,90(2):373-380
The racemization of R-(-)-2-amino-1-butanol in a reaction using Co/γ-Al2O3 catalysts and catalysts modified by Mg or Ca was investigated in this paper. Complete racemization was achieved with a yield
of over 83% at using the Mg modified Co/γ-Al2O3 catalyst under optimized reaction conditions of 170°C and 2.5 MPa of H2. The catalysts were thoroughly characterized by XRD, XPS, TPR, SEM and TEM. The addition of Mg and Ca may be advantageous
for dispersing and stabilizing the active species of the Co/γ-Al2O3 catalyst, protecting from sintering, significantly improving its catalytic activity and stability. 相似文献
26.
Seiichi Nakamura Jun Inagaki Tomohiro Sugimoto Yasuyuki Ura Shunichi Hashimoto 《Tetrahedron》2002,58(52):10375-10386
An efficient, highly stereoselective synthesis of the C10–C31 (BCDEF ring) portion of pinnatoxin A has been achieved utilizing tandem double hemiketal formation/intramolecular hetero-Michael addition to construct the 6,5,6-dispiroketal (BCD ring) system and subsequent intramolecular ketalization to form the 5,6-bicycloketal (EF ring) system as key steps. 相似文献
27.
Nitrostyrene derivatives are widely used in organic syntheses as a substrate for Michael addition, photoisomerization and cycloaddition. In contrast, ortho-hydroxy derivatives exhibit unusual behaviors in these reactions. Conjugate addition proceeded upon treatment of the ortho-hydroxy-β-nitrostyrene with an amine; however, subsequent C–C bond cleavage readily occurred to afford the corresponding imine. Moreover, conversion of the trans-isomer to a cis-isomer did not occur efficiently, even when UV light was irradiated. We studied these unusual behaviors of β-nitrostyrene, focusing on the role of the ortho-hydroxy group. 相似文献
28.
Phthalimide derivatives as nitrogen nucleophiles with α,β‐unsaturated aldehydes for asymmetric aza‐Michael additions have been reported. The reactions proceed smoothly to afford corresponding Michael adducts in good yields (up to 98%) and enantioselectivities (up to 95% ee). 相似文献
29.
D. D. Enchev 《Phosphorus, sulfur, and silicon and the related elements》2013,188(9):2141-2147
Abstract The synthesis and reactivity of the titled compounds towards electrophilic reagents have been discussed. 相似文献
30.
Junting Li Brian C. Benicewicz 《Journal of polymer science. Part A, Polymer chemistry》2013,51(17):3572-3582
Monoalkynyl‐functionalized fullerene was precisely synthesized starting with pristine fullerene (C60) and characterized by multiple techniques. Methyl methacrylate and 6‐azido hexyl methacrylate were then randomly copolymerized via reversible addition fragmentation chain transfer polymerization to build polymer backbones with well‐controlled molecular weights and copolymer compositions. Finally, these two moieties were covalently assembled into a series of well‐defined side chain fullerene polymers (SFPs) via the copper‐mediated click reaction which was verified by Fourier transform infrared spectroscopy and 1H NMR. The fullerene loadings of the resultant polymers were estimated by thermogravimetric analysis and UV–vis spectroscopy, demonstrating consistent and high conversions in most of the samples. The morphology studies of the SFPs were performed both in solution and on solid substrates. Very intriguing self‐aggregation behaviors were detected by both gel permeation chromatography and dynamic light scattering analyses. Furthermore, the scanning electron microscopic images of these polymers showed the formation of various supramolecular nanoparticle assemblies and crystalline‐like clusters depending on the fullerene contents and polymer chain lengths. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3572–3582 相似文献