全文获取类型
收费全文 | 4637篇 |
免费 | 485篇 |
国内免费 | 723篇 |
专业分类
化学 | 4448篇 |
晶体学 | 49篇 |
力学 | 60篇 |
综合类 | 21篇 |
数学 | 61篇 |
物理学 | 1206篇 |
出版年
2024年 | 6篇 |
2023年 | 58篇 |
2022年 | 107篇 |
2021年 | 142篇 |
2020年 | 256篇 |
2019年 | 132篇 |
2018年 | 167篇 |
2017年 | 138篇 |
2016年 | 202篇 |
2015年 | 177篇 |
2014年 | 211篇 |
2013年 | 407篇 |
2012年 | 230篇 |
2011年 | 220篇 |
2010年 | 236篇 |
2009年 | 285篇 |
2008年 | 239篇 |
2007年 | 319篇 |
2006年 | 325篇 |
2005年 | 253篇 |
2004年 | 287篇 |
2003年 | 290篇 |
2002年 | 173篇 |
2001年 | 160篇 |
2000年 | 162篇 |
1999年 | 131篇 |
1998年 | 120篇 |
1997年 | 86篇 |
1996年 | 62篇 |
1995年 | 72篇 |
1994年 | 37篇 |
1993年 | 35篇 |
1992年 | 30篇 |
1991年 | 16篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 13篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有5845条查询结果,搜索用时 15 毫秒
101.
Biju Philip K. Sreekumar 《Journal of polymer science. Part A, Polymer chemistry》2002,40(16):2868-2877
A series of polyesters with π‐conjugated donor–acceptor segments was synthesized by the condensation of azobenzene‐4,4′‐dicarbonylchloride with 1,4:3,6‐dianhydro‐D‐sorbitol ([α] = + 42.5°) and biphenolic chromophores, bis(4‐hydroxyphenylazo)‐2,2′‐dinitrodiphenylmethane and bis(4‐hydroxyphenylazo)‐2,2′‐dinitrodiphenylsulfone. The polymers were characterized by spectral methods (IR, ultraviolet–visible, and NMR), thermal methods (thermogravimetry and differential scanning calorimetry), wide‐angle X‐ray scattering, and polarimetry. The polymers containing isosorbide units were optically active and crystalline. They exhibited glass‐transition temperature values between 100 and 160 °C and were stable up to 400 °C. The second‐harmonic generation (SHG) efficiency of the polymers was experimentally verified by a powder‐reflection technique with 2‐methyl‐4‐nitroaniline as a reference. The SHG efficiencies of the polymers were compared to those of the chromophores and explained as a function of the percentage of chiral composition. The hyperpolarizability β values were also determined by a two‐level model solvatochromic method and computational methods. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2868–2877, 2002 相似文献
102.
Johansson A Håkansson M Jagner S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(18):5311-5318
Stereochemically labile copper and zinc complexes with the N,N'-dimethylethylenediamine ligand (dmeda) have been shown to be promising precursors for the total spontaneous resolution of chiral covalent networks. (N,N')-[Cu(NO3)2(dmeda)]infinity crystallises as a conglomerate and yields either enantiopure (R,R)-1 or enantiopure (S,S)-1. A mixed-valence copper(I/II) complex, [{Cu(II)Br2(dmeda)}3(Cu(I)Br)2]infinity (2), which crystallises as a pair of interpenetrating chiral (10,3)-a nets, is formed from CuBr, CuBr2 and dmeda. One net contains ligands with solely (R,R) configuration and exhibits helices with (P) configuration while the other has solely (S,S)-dmeda ligands and gives rise to a net in which the helices have (M) configuration. The whole crystalline arrangement is racemic, because the interpenetrating chiral nets are of opposite handedness. With zinc chloride (R,S)-[ZnCl(dmeda)2]2[ZnCl4] (3) is obtained, which is a network structure, although not chiral. Total spontaneous resolution of stereochemically labile metal complexes formed from achiral or racemic building blocks is suggested as a viable route for the preparation of covalent chiral networks. Once the absolute structure of the compound has been determined by X-ray crystallography, a quantitative determination of the enantiomeric excess of the bulk product can be undertaken by means of solid-state CD spectroscopy. 相似文献
103.
104.
3类手性配体d-10-樟脑磺酸及其衍生物,α-(-)-羟蒎酮及其衍生物,又恶唑啉6衍生物分别与铜(Ⅱ)形成手性铜配合物,催化重氮乙酸酯与1,1-二苯乙烯或异丁烯的环丙烷化加成反应,得到光学活性环丙羧酸酯,不对称诱导最高可达95%e.e.。 相似文献
105.
Peter LeemingColin A Ray Stephen J SimpsonTimothy W Wallace Richard A Ward 《Tetrahedron》2003,59(3):341-352
Heterodiene [4π+2π] cycloadditions of (S,S)-4,5-diaryl-2-methylene-1,3-dioxolanes 1 to a series of β-amido-α,β-unsaturated carbonyl compounds are diastereoselective (d.r.≥4:1). The products can be purified by trituration or crystallisation and hydrolysed with acid to generate the corresponding β-amido carbonyl compounds, the overall sequence effecting an auxiliary-based enantioselective conjugate addition of an acetate enolate, leading to β-aminoacid derivatives. 相似文献
106.
Amphiphilic diblock copolymers consisting of a hydrophobic polystyrene block (PS) and a hydrophilic poly(ethylene oxide)
block (PEO) with block sizes of 1000 or 3000 g/mol for both blocks were studied at the air/water and toluene/water interface.
Measurements of the film pressure π of spread monolayers at the water surface reveal two limiting regimes of the π−a
m isotherms, in which the mean molecular area a
m is determined either by the size of the hydrophilic or the hydrophobic blocks of the PS-PEO molecules. The interfacial activity
of the block copolymers at the toluene/water interface was studied by measuring the interfacial tension σ over a wide range
of concentrations. Pronounced differences in the temperature dependence of the interfacial tension were observed, depending
mostly on the block length of the hydrophilic PEO block. From the temperature dependence of σ it is inferred that for the
block copolymers with the PEO block size of 3000 g/mol the phase inversion temperature (PIT) is well above 60 °C while for
those with a PEO block size of 1000 g/mol the PIT is below or near 25 °C in the toluene/water system.
Received: 5 February 1998 Accepted: 16 February 1998 相似文献
107.
以万古霉素为流动相添加剂的高效液相色谱法手性分析系统的建立 总被引:2,自引:0,他引:2
以丹酰化苯甘氨酸、丹酰化苯丙氨酸、丹酰化丝氨酸、丹酯化丙氨酸为模型化合物,通过对色谱柱类型、缓冲液类型、pH和盐浓度、万古霉素用量、甲醇用量、柱温等色谱参数进行考察,建立了以万古霉素为手性流动相添加剂在普通色谱柱上进行手性分析的色谱系统,并在该系统中成功地拆分了酮洛芬对映体。 相似文献
108.
A chemically bonded cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase (CSP) was prepared by a radical polymerization reaction. The prepared CSP was packed into fused-silica capillaries with inner diameter of 75 microm to perform enantiomer separations in CEC. The electrochromatographic behavior of the CSP was investigated. On the prepared CSP, high EOF could be generated under acidic mobile phases, which represented an advantage for the separation of acidic enantiomers. Several neutral, acidic, and basic enantiomers were resolved on the prepared CSP under aqueous mobile phases. The column efficiencies were between 20,000 and 100,000 plates/m, which were much higher than those of HPLC. In addition, it was observed that the separation of some enantiomers benefited from the adoption of THF as mobile phase modifier. 相似文献
109.
A. V. Kurkin V. V. Nesterov A. V. Karchava M. A. Yurovskaya 《Chemistry of Heterocyclic Compounds》2003,39(11):1466-1477
A new approach has been proposed to the synthesis of indole derivatives containing a chiral substituent at the nitrogen atom, comprizing Fischer indolization of phenylhydrazines with a chiral substituent at the -nitrogen atom. The initial hydrazines were obtained by the alkylation (Mitsunobu reaction applying optically active esters of lactic acid) of anilines containing an electron-accepting substituent at the amino group. Subsequent removal of the activating acceptor grouping was realized by nitrosation of the chiral secondary aniline followed by reduction of the corresponding N-nitroso compound. 相似文献
110.
Interfacial layers have been widely applied to study the formation and stability of emulsion-based systems. However, the application of isolated interfaces to address digestibility of emulsions is often limited because of the complexity of experimental methods and results. This review summarizes the latest developments in analytical methods and literature data on effects of digestion on interfacial layers. Particular emphasis is given to understand the changes on interfacial magnitudes during oral, gastric, and duodenal digestion, either applied separately or sequentially. Limitations of interfacial aspects and key factors that influence emulsion microstructure in bulk and lipid digestion are identified. Understanding the behavior of interfacial layers upon gastrointestinal digestion promotes an accurate tracking of the physiological fate of emulsions. 相似文献