全文获取类型
收费全文 | 564篇 |
免费 | 22篇 |
国内免费 | 2篇 |
专业分类
化学 | 446篇 |
晶体学 | 6篇 |
力学 | 8篇 |
数学 | 57篇 |
物理学 | 71篇 |
出版年
2023年 | 6篇 |
2022年 | 3篇 |
2021年 | 13篇 |
2020年 | 16篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 6篇 |
2016年 | 22篇 |
2015年 | 25篇 |
2014年 | 24篇 |
2013年 | 49篇 |
2012年 | 37篇 |
2011年 | 49篇 |
2010年 | 27篇 |
2009年 | 16篇 |
2008年 | 26篇 |
2007年 | 35篇 |
2006年 | 32篇 |
2005年 | 29篇 |
2004年 | 26篇 |
2003年 | 17篇 |
2002年 | 15篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 8篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1986年 | 2篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1960年 | 1篇 |
1954年 | 1篇 |
1937年 | 1篇 |
1936年 | 1篇 |
1932年 | 2篇 |
排序方式: 共有588条查询结果,搜索用时 828 毫秒
151.
Cornelia E Hoesl 《Tetrahedron》2004,60(2):307-318
Various nipecotic acid derivatives are known to be potent GABA uptake inhibitors thus being useful in the treatment of a number of neurological and psychological disorders. In this paper, the first asymmetric syntheses of 6-substituted nipecotic acid derivatives are presented. The synthetic strategy was designed to provide access to a large variety of enantiomerically pure 6-substituted nipecotic acid derivatives. The synthesis starts from the chiral N-acyldihydropyridines 15 and 16 obtained via asymmetric electrophilic α-amidoalkylation reaction of a chiral N-acylpyridinium ion. These were utilized for the preparation of enantiomerically pure 6-(4,4-diphenylbutyl)nipecotic acids and 6-(4,4-diphenylbutenyl)nipecotic acids in a multistep synthesis, including the removal of the dimethylphenylsilyl blocking group from the dihydropyridine ring, the reduction of the dihydropyridine heterocycle, a Horner-Wittig reaction and the removal of the chiral auxiliary. The obtained target molecules, however, showed only negligible affinity to the GAT-1- and GAT-3 transport proteins. 相似文献
152.
Jackstell R Harkal S Jiao H Spannenberg A Borgmann C Röttger D Nierlich F Elliot M Niven S Cavell K Navarro O Viciu MS Nolan SP Beller M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(16):3891-3900
The telomerization reaction of 1,3-butadiene with alcohols to give alkyl octadienyl ethers in the presence of palladium-carbene catalysts has been studied in detail. Unprecedented catalyst efficiency with turnover numbers (TON) up to 1,500,000 and turnover frequencies (TOF) up to 100,000 h(-1) have been obtained after optimization for the reaction of methanol in the presence of an excess of in situ generated carbene ligands. High yields (75-97 %) and catalyst productivities (TON 15,000-100,000) are observed for other aliphatic alcohols and phenols. For comparison five carbene-palladium(0) complexes have been synthesized and characterized by X-ray crystallography. Both electronic and steric effects on the stability and reactivity of the catalysts have been discussed on the basis of density functional theory calculations. 相似文献
153.
Ede Kapuy Zoltn Cspes Cornelia Kozmutza 《International journal of quantum chemistry》1983,23(3):981-990
Diagrammatic formulation of the many-body perturbation theory is investigated when both the occupied orbitals and the virtual ones are localized, i.e., they are unitary transforms of the canonical Hartree–Fock orbitals. All diagrams representing ground state correlation energy can be generated through fifth order. For cyclic polyenes C6H6 and C10H10 as model systems, the energy corrections are calculated in the Pariser–Parr–Pople approximation for a wide range of the coupling constant β?1, through fourth order including some fifth order terms. The results are compared to those obtained by other methods: perturbation theory by using canonical orbitals and full CI. The effect of neglecting contributions from orbitals localized into neighboring sites is also studied. 相似文献
154.
13C-kinetic isotope effects (KIEs) of four cinnamyl alcohol oxidations and a xylose reductase-catalyzed cinnamyl aldehyde reduction have been determined by 13C NMR using competition reactions with reactants at natural 13C-abundance. Differences in KIEs among oxidations indicate dissimilarities between the respective hydrogen transfers. Their mechanistic implications are discussed. A low primary KIE of the enzymatic reduction is consistent with a kinetically complex mechanism in which steps other than the chemical step of hydride transfer from NADH are slow. 相似文献
155.
An asymmetric synthesis of the anti-metastatic prostacyclin analogue cicaprost and a formal one of its isomer isocicaprost by a new route are described. A key step of these syntheses is the coupling of a chiral bicyclic C6-C14 ethynyl building block with a chiral C15-C21 omega-side chain amide building block with formation of the C14-C15 bond of the target molecules. A highly stereoselective reduction of the thereby obtained C6-C21 intermediate carrying a carbonyl group at C15 of the side chain was accomplished by the chiral oxazaborolidine method. The chiral phosphono acetate method was used for the highly stereoselective attachment of the alpha-side chain to the bicyclic C6-C21 intermediate carrying a carbonyl group at C6. Asymmetric syntheses of the bicyclic C6-C14 ethynyl building blocks were carried out starting from achiral bicyclic C6-C12 ketones by using the chiral lithium amide method. In the course of these syntheses, a new method for the introduction of an ethynyl group at the alpha-position of the carbonyl group of a ketone with formation of the corresponding homopropargylic alcohol was devised. Its key steps are an aldol reaction of the corresponding silyl enol ether with chloral and the elimination of a trichlorocarbinol derivative with formation of the ethynyl group. In addition, a new aldehyde to terminal alkyne transformation has been realized. Its key steps are the conversion of an aldehyde to the corresponding 1-alkenyl dimethylaminosulfoxonium salt and the elimination of the latter with a strong base. Two basically different routes have been followed for the synthesis of the enantiomerically pure C15-C21 omega-side chain amide building block. The first is based on the chiral oxazolidinone method and features a highly stereoselective alkylation of (4R)-N-acetyl-4-benzyloxazolidin-2-one, and the second encompasses a malonate synthesis of the racemic amide and its efficient preparative scale resolution by HPLC on a chiral stationary phase containing column. 相似文献
156.
157.
Cornelia Borgmann Christian Limberg Elisabeth Kaifer Hans Pritzkow Lszl Zsolnai 《Journal of organometallic chemistry》1999,580(2):10099
The reaction of Mo(η3-C3H4(CH3))(CH3CN)2(CO)2Cl with AgBF4 in THF yields the cationic complex [Mo(η3-C3H4(CH3))(CH3CN)2(CO)2(THF)]+[BF4]−, 1, whose X-ray structure has been determined. Oxo nucleophiles are capable of replacing the weakly bound THF molecule in 1 and under simultaneous loss of CH3CN the resulting complexes aggregate to oligonuclear compounds. Accordingly, the reactions with NaOMe and KOH yield [Na(THF)4]+[(η3-C3H4(CH3))(CO)2Mo(μ-OCH3)3Mo(CO)2(η3-C3H4(CH3))]−, 2 and [K(18-crown-6)]+[[Mo(η3-C3H4(CH3))(CO)2]3(μ2-OH)3(μ3-OH)]−, 3, which were characterized by means of single crystal X-ray diffraction. Due to fluoride abstraction from BF4− the reaction of 1 with KOH also yields fluorinated derivatives of 3 but incorporation of fluorine in 3 can be avoided if AgO3SCF3 rather than AgBF4 is used to generate the cation of 1. For purposes of comparison the dinuclear complex [K(18-crown-6)]+[[Mo(η3-C3H4(CH3))(CO)2]2(μ2-F)3]−, 4, has been prepared, too, showing fluoride bridges and KF bonding. The chemical properties and the structures of these compounds in solution as well as their role as structural models for intermediates during molybdenum oxide catalysed propene oxidation are discussed. 相似文献
158.
We discuss the creation of an array of 9 x 9 photorefractive spatial screening solitons in a strontium barium niobate crystal. We investigate the waveguide properties of each channel with a beam of different wavelength and find that the waveguides guide the probe beam independently. A supplementary beam is used to influence the paths of the array solitons and to effectively combine two channels by use of mutual attraction of solitons. To our knowledge this is the first all-optical control of an array of photorefractive solitons. Furthermore, we show that in principle image procession is possible with parallel propagation of photorefractive solitons. 相似文献
159.
We report on the experimental observation of a dynamic instability in the interaction of counterpropagating self-trapped beams in a photorefractive strontium barium niobate crystal. While the interaction of copropagating spatial optical solitons exhibits only transient dynamics, resulting in a final steady state, the counterpropagating geometry supports a dynamic instability mediated by intrinsic feedback. Experimental observations are compared with and found to be in qualitative agreement with numerical simulations. 相似文献
160.
We experimentally observed a counterpropagating dipole-mode vector soliton in a photorefractive SBN:60Ce crystal. We investigated the transient formation dynamics and show that the formation process differs significantly from the copropagating geometry. The experimental results are compared with fully anisotropic numerical simulations and show good qualitative agreement. 相似文献