全文获取类型
收费全文 | 3306篇 |
免费 | 74篇 |
国内免费 | 292篇 |
专业分类
化学 | 3251篇 |
晶体学 | 16篇 |
力学 | 5篇 |
综合类 | 1篇 |
数学 | 19篇 |
物理学 | 380篇 |
出版年
2024年 | 6篇 |
2023年 | 108篇 |
2022年 | 18篇 |
2021年 | 32篇 |
2020年 | 30篇 |
2019年 | 80篇 |
2018年 | 79篇 |
2017年 | 82篇 |
2016年 | 76篇 |
2015年 | 73篇 |
2014年 | 121篇 |
2013年 | 211篇 |
2012年 | 147篇 |
2011年 | 200篇 |
2010年 | 187篇 |
2009年 | 225篇 |
2008年 | 199篇 |
2007年 | 234篇 |
2006年 | 248篇 |
2005年 | 260篇 |
2004年 | 241篇 |
2003年 | 96篇 |
2002年 | 60篇 |
2001年 | 76篇 |
2000年 | 80篇 |
1999年 | 55篇 |
1998年 | 62篇 |
1997年 | 48篇 |
1996年 | 48篇 |
1995年 | 46篇 |
1994年 | 31篇 |
1993年 | 31篇 |
1992年 | 29篇 |
1991年 | 31篇 |
1990年 | 26篇 |
1989年 | 10篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 12篇 |
1985年 | 3篇 |
1984年 | 12篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1967年 | 2篇 |
排序方式: 共有3672条查询结果,搜索用时 15 毫秒
61.
The liquid chromatographic enantiomer separation of N-fluorenylmethoxycarbonyl (FMOC) protected alpha-amino acids and their ethyl ester derivatives was performed on polysaccharide-derived chiral stationary phases, Chiralcel OD, Chiralpak AD, and Chiralpak AS. In general, Chiralcel OD and Chiralpak AD showed good performance for resolution of N-FMOC alpha-amino acids and their ethyl esters, respectively. All investigated N-FMOC alpha-amino acid enantiomers were baseline separated on Chiralcel OD or Chiralpak AD, whereas N-FMOC alpha-amino acid ethyl ester enantiomers were baseline resolved (alpha = 1.15-3.03) on Chiralpak AD, except for two analytes. The L-enantiomers of all examined FMOC alpha-amino acid ethyl ester derivatives are preferentially retained on Chiralpak AD, while the elution orders of the other enantiomer separations are not consistent. 相似文献
62.
Xing Yong Wang Jing Nan Cui Wei Min Ren Feng Li Chun Liang Lu Xu Hong Qian State Key Laboratory of Fine Chemicals Dalian University of Technology Dalian China School of Chemical Engineering Technology China University of Mining & Technology Xuzhou China Shanghai Key Laboratory of Chemical Biology East China University of Science Technology Shanghai China 《中国化学快报》2007,18(6):681-684
Baker's yeast mediated reduction of acenaphthenequinone within 4-10 h afforded mono-hydroxyacenaphthenone mainly with low enantioselectivity, the substrate and mono-hydroxyacenaphthenone product almost converted to dihydroxyacenaphthene after 48 h. By control of the reaction time and in the presence of DMF as co-solvent, the reduction of 6-substituted acenaphthenequinones under vigorous agitation afforded the corresponding 2-hydroxyacenaphthenones in 24-84% yields with 10-93% ee. 相似文献
63.
Masaru Mitsuda 《Tetrahedron letters》2006,47(46):8073-8077
Kinetic resolution of N-benzoylated vic-amino alcohols was achieved by benzoylation in the presence of copper triflate and (R,R)-Ph-BOX as catalysts. The observed enantioselectivity was moderate to high. The method was applied to a kinetic resolution of racemic prolinol and piperidinemethanol derivatives as well as an asymmetric desymmetrization of 2-amino-1,3-diol derivatives. 相似文献
64.
The separation of diastereomeric esters derived from (±)-pyridylethanols and 3β-acetoxyetienic acid were achieved by an extraction technique using diethyl ether and aqueous hydrochloric acid. A formal total synthesis of PNU-142721 was effectively carried out to prepare the chiral, non-racemic synthon 1-furo[2,3-c]pyridin-5-yl-ethanol (1) by means of this technique. The structure optimized using MOPAC calculations on each diastereomer suggested the presence of intramolecular CH/π interaction in only the (S)-isomer of the diastereomers. 相似文献
65.
Makoto Minato Takefumi Kaneko Shogo Masauji Takashi Ito 《Journal of organometallic chemistry》2006,691(11):2483-2488
We describe the synthesis of a new asymmetric P,N,N′-tridentate ligand (bis(pyrid-2-ylethyl) menthylphosphine, BPEMP), containing two pyridyl rings and (1S,2R,5S)-menthylphosphino group. The ligand is obtained in five steps from natural abundant l-menthol. The coordination behavior of the ligand toward cationic (allylic)Pd(II) moiety and its first application in palladium-catalyzed asymmetric allylic alkylation are presented. Crystallographic and spectroscopic analyses reveal that [(η3-allylic)Pd(BPEMP)]+ complex forms only one isomer in the solid state as well as in solution. 相似文献
66.
A series of chiral aromatic imides and diimides were synthesized and their electrochemical, absorption, fluorescent, and chiroptical properties were examined for their potential application as molecular chiroptical switches. These compounds exhibit strong UV-vis absorptions, and can be electrochemically reduced to radical anions that absorb in the near infrared (NIR) region. Further reduction to the dianionic states results in new absorptions in the visible region. The changes in circular dichroism upon redox switching were apparent in the UV-vis region but were absent in the NIR region. 相似文献
67.
《Tetrahedron letters》2004,45(30):5799-5801
(+)-β-Hydroxymethylenecamphor 1 and enamines 2a-e were transformed into chiral camphor-based pyridine derivatives 3a-e via a tandem condensation reaction in good yields. 相似文献
68.
O. J. Scherer 《Angewandte Chemie (International ed. in English)》1969,8(11):861-876
Amines with mixed substituents containing two or three El? N bonds 1 El = a higher IVa, Va, or VIa element; by IVa, Va, and VIa elements are meant elements of the IV, V, and VI main groups of the periodic system . are relatively stable if one or two of these bonds are (CH3)3 Si? N bonds. IR and 1H? NMR studies indicate that the (p → d)π bond components of the element-nitrogen bonds steadily decrease from silicon, phosphorus, and sulfur toward their higher homologs. Because of the differences in the polarities of the element-nitrogen bonds, these substances can be used for selective insertion and cleavage reactions. The reaction of metalated N-silylaminoarsines with methyl chloride as well as the reaction of metalated N-trimethyl(IVa)-element-substituted amino-tert-butylphosphines with halogenotrimethyl(IVa) element compounds open new, simple routes for the conversion of elemento-organic amine systems into imine systems. The problem of reversible and irreversible (CH3)3 El ligand migration (1,3 shift) is discussed for trimethyl(IVa)element-substitued benzamidines, diaminophosphines, aminoiminophosphoranes, sulfinamides, and aminosulfimines. 相似文献
69.
A new series of thiophene‐ and furan‐containing chromophores with a chiral prolinol donor and a sulfone acceptor has been synthesized. The UV‐vis absorptions, second‐order nonlinear optical properties, and X‐ray crystal structures are described. 相似文献
70.
A chiral stationary phase (CSP) based on (-)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was evaluated for the direct resolution of the enantiomers of dipeptides and tripeptides. The type and concentration of the acid and the methanol content were optimized with regard to retention time and resolution using Ala-Phe as model peptide. A mobile phase consisting of 10 mM sulfuric acid in 70% aqueous methanol was applied to the separation of a set of 16 structurally diverse dipeptides and tripeptides. Generally, the configuration of the amino acid at the N-terminus determined the enantiomer elution order. With a few exceptions the LL- and LD-enantiomers interacted stronger with the CSP compared to the corresponding DD- or DL-enantiomers. The experimental conditions also allowed the simultaneous separation of all four stereoisomers of Ala-Phe. Addition of ammonium sulfate generally reduced retention times and enantiomer resolution. Addition of triethylamine as modifier led to an overall increase of the retention times while the resolution did not show a general trend, increasing in the case of Ala-Ala but decreasing in the case of Ala-Phe. 相似文献