全文获取类型
收费全文 | 2530篇 |
免费 | 132篇 |
国内免费 | 16篇 |
专业分类
化学 | 1624篇 |
晶体学 | 8篇 |
力学 | 110篇 |
数学 | 596篇 |
物理学 | 340篇 |
出版年
2024年 | 3篇 |
2023年 | 24篇 |
2022年 | 25篇 |
2021年 | 48篇 |
2020年 | 51篇 |
2019年 | 41篇 |
2018年 | 50篇 |
2017年 | 24篇 |
2016年 | 83篇 |
2015年 | 97篇 |
2014年 | 97篇 |
2013年 | 148篇 |
2012年 | 231篇 |
2011年 | 232篇 |
2010年 | 142篇 |
2009年 | 149篇 |
2008年 | 162篇 |
2007年 | 193篇 |
2006年 | 164篇 |
2005年 | 147篇 |
2004年 | 122篇 |
2003年 | 96篇 |
2002年 | 94篇 |
2001年 | 36篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1998年 | 13篇 |
1997年 | 16篇 |
1996年 | 16篇 |
1995年 | 13篇 |
1994年 | 20篇 |
1993年 | 11篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1909年 | 2篇 |
1891年 | 2篇 |
1890年 | 2篇 |
排序方式: 共有2678条查询结果,搜索用时 31 毫秒
51.
Frédéric Blanc Christophe Copéret Bernard Fenet 《Journal of organometallic chemistry》2005,690(23):5014-5026
The reaction of Mo(N)(CH2tBu)3 (1) and SiO2-(700) generates (SiO)Mo(NH)(CHtBu)(CH2tBu) (2) when performed in C6H6 (material [1/SiO2-(700)]C6H6). The grafting occurs presumably by protonation of the nitrido ligand to form an intermediate (SiO)Mo(NH)(CH2tBu)3 (3), a pentacoordinated complex, which decomposes into 2 and 2,2-dimethylpropane. While [1/SiO2-(700)]C6H6 is highly active in olefin metathesis, [1/SiO2-(700)]CH2Cl2 and [1/SiO2-(700)]THF are poorly active or inactive catalysts respectively. In contrast, when Mo(N)(CH2tBu)3 reacts with a molecular silanol derivative, a soluble model of the surface of SiO2-(700), it yields a very stable complex, (c-C5H9)7Si7O12SiO-Mo(NH)(CH2tBu)3 (3m), which does not spontaneously generate 2,2-dimethylpropane and an alkylidene complex in contrast to the surface complex. Moreover, 3m does not catalyse olefin metathesis at room temperature as it does not already contain the initiating carbene ligand, and it is necessary to heat up the reaction mixture to 110 °C to obtain low catalytic activity. Nevertheless, the complex 3m generates well-defined metallocarbenes when heated in the presence of PMe3: (c-C5H9)7Si7O12SiO-Mo(N)(CHtBu)(P(CH3)3)2 (4m) as a 10:1 mixture of its syn and anti rotamers with the loss of 2 equiv. of 2,2-dimethylpropane. 相似文献
52.
Axially disymmetric tertiary amines or quaternary ammonium salts are synthesized by double alkylation of primary or secondary amines with racemic or optically pure 2,2′-bis (bromomethyl)-1,1′-binaphthyl. Their reductive cleavage by lithium aluminium hydride in refluxing THF leads to chiral secondary or tertiary amines , substituted by a binaphthyl unit, with high yields and absence of racemization. 相似文献
53.
[reaction: see text] Regioselective metalation of pyrazines and cross-coupling reactions provides an easy access to botryllazines A and B and to an isomer of botryllazine A with good yields from chloropyrazine. 相似文献
54.
Physical adsorption of the title compounds on rhombic sulfur of 0.4 to 0.5 m2/g is reported. The isotherms are of type II for N2, Ar and C5H12, of type III for SO2 and CO2, and linear for SF6. There is no hysteresis. The method of Ross & Olivier shows that the surface is relatively heterogenous (γ 17). Isosteric heats of adsorption and c values of the B.E.T. equation are also reported. 相似文献
55.
Magali Szlosek-PinaudPhilippe Diaz Jean MartinezFrédéric Lamaty 《Tetrahedron letters》2003,44(48):8657-8659
A novel one-pot palladium-catalyzed cascade between 2-iodophenol, methyl bromomethylacrylate and an arylboronic acid provides an efficient access to heterocycles possessing the 3,3-disubstituted-2,3-dihydrobenzofuran skeleton via allylation/carbopalladation/Suzuki cross-coupling. 相似文献
56.
Microwave irradiation effectively accelerates cross-coupling metathesis reactions between deactivated olefins. Reactions have been carried out with the phosphine-free Hoveyda-Grubbs catalyst and the "second generation Grubbs' catalyst." While there have been reports that a "microwave effect" is observed in various transformations, the accelerations we observe are due to the efficient and rapid heating and increased pressure in the microwave apparatus. 相似文献
57.
Gruy F 《Journal of colloid and interface science》2001,237(1):28-39
Aggregation of silica powder in water has been experimentally studied by turbidimetry. Aggregation was carried out in a stirred tank under physicochemical conditions corresponding to attractive interparticle forces. The effects of different primary particle sizes and stirring rates on aggregation dynamics have been studied. The scattering cross sections of silica aggregates were calculated in the framework of the anomalous diffraction approximation of light scattering theory. Aggregation has been studied by using Kusters's and Brakalov's approaches. By comparison between experimental and theoretical turbidity changes with time it has been shown that aggregates are small and slightly porous. The aggregation process is characterized by a weak fractal dimension D(wf) and an aggregate limit size L.D(wf) is found in the range 2.4-2.5. D(wf) (respectively L) is a weakly increasing (respectively decreasing) function of the stirring rate or of the shear rate. Copyright 2001 Academic Press. 相似文献
58.
Erich Dubler Helmut W. Schmalle Frdric Arod Alain Schneider 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(2):m111-m115
The syntheses and structures of two mixed‐ligand complexes of platinum(II) with deprotonated oxopurine bases and triphenylphosphine are reported, namely the theophyllinate complex cis‐bis(1,2,3,6‐tetrahydro‐1,3‐dimethylpurine‐2,6‐dionato‐κN7)bis(triphenylphosphine‐κP)platinum(II), [Pt(C7H7N4O2)2(C18H15P)2], (I), and the theobrominate complex cis‐chloro(1,2,3,6‐tetrahydro‐3,7‐dimethylpurine‐2,6‐dionato‐κN1)bis(triphenylphosphine‐κP)platinum(II) ethanol hemisolvate, [PtCl(C7H7N4O2)(C18H15P)2]·0.5C2H5OH, (II). In (I), the coordination geometry of Pt is square planar, formed by the two coordinating N atoms of the theophyllinate anions in a cis arrangement and two P atoms from the triphenylphosphine groups. In (II), there are two crystallographically independent molecules. They both exhibit a square‐planar coordination geometry around Pt involving one Cl atom, the coordinating N atom of the theobrominate anion and two P atoms from the triphenylphosphine groups. The two triphenylphosphine groups are arranged in a cis configuration in both structures. The heterocyclic rings are rotated with respect to the coordination plane of the metal by 82.99 (8) and 88.09 (8)° in complex (I), and by 85.91 (16) and 88.14 (18)° in complex (II). Both structures are stabilized by intramolecular stacking interactions involving the purine rings and the phenyl rings of adjacent triphenylphosphine moieties. 相似文献
59.
Two-stage Off-Gel isoelectric focusing: protein followed by peptide fractionation and application to proteome analysis of human plasma 总被引:1,自引:0,他引:1
Heller M Michel PE Morier P Crettaz D Wenz C Tissot JD Reymond F Rossier JS 《Electrophoresis》2005,26(6):1174-1188
This paper presents the recently introduced Off-Gel electrophoresis (OGE) technology as a versatile tool to reproducibly fractionate intact proteins and peptides into discrete liquid fractions. The coupling of two stages of OGE, i.e., the separation of intact proteins in a first-stage followed by fractionation of peptides derived from each protein fraction after proteolysis in a second stage, results in an array of 15 x 15 fractions that are directly amenable to additional peptide fractionation like reverse-phase liquid chromatography (RPC). The analysis of all second-stage peptide fractions from only the first-stage protein fraction representing pH 5.0 -5.15 by on-line reverse-phase LC-tandem mass spectrometry resulted in the identification of 53 proteins (337 peptides), of which 10 were on different immunoglobulin (Ig) chains, with an input of only 1.5 mg human blood plasma proteins. Increasing the protein load to approximately 12 mg increased the number of identified proteins in the same protein fraction to 73 proteins (449 peptides), of which 15 were Ig-related. Immunodepletion of six of the most abundant proteins (albumin, transferrin, haptoglobin, IgG, IgA, and alpha-1-antitrypsin) prior to first-stage OGE with an input of 1.5 mg of protein (equivalent to approximately 10 mg nondepleted plasma) resulted in the identification of 81 proteins (660 peptides), of which three were still Ig fragments. The pI-based separation of peptides appears to be nonuniform based on the theoretically determined pI values of identified peptides. This observation specifically accounts for the neutral zone (pI 5-8) and can be accounted for by the physicochemical properties of the peptides given by their amino acid composition. The power of OGE separation of proteins and peptides is discussed with a focus on the use of the knowledge about the pI of proteins and peptides that assist the validation of correct identifications together with the retention time of peptides on RPC. 相似文献
60.
Dupont A Eastoe J Martin L Steytler DC Heenan RK Guittard F Taffin de Givenchy E 《Langmuir : the ACS journal of surfaces and colloids》2004,20(23):9960-9967
Hybrid fluorocarbon-hydrocarbon (F-H) sulfate surfactants are shown to be efficient stabilizers in water-in-CO2 (w/c) microemulsions. The chain structure and F-H ratio affect the regions of P-T phase stability and aggregation structure in these w/c phases. High-pressure near-infrared spectroscopy and small-angle neutron scattering measurements of microemulsified water provide evidence for the stabilization of w/c microemulsion droplets. The relative lengths of the two chains were found to influence the favored aggregation structure: for symmetric chain surfactants (F8H8, F7H7) spherical reverse micelles are present, but for asymmetric chain surfactants (F7H4, F8H4) extended cylinder aggregates form. These changes in aggregation are consistent with different surfactant packing parameters owing to the controlled variations in molecular structure. Furthermore, the general order of w/c phase transition pressures (F8H8 < F7H7 and F8H4 < F7H4) is in line with estimations of surfactant fractional free volume, as proposed by Johnston et al. (J. Phys. Chem. B 2004, 108, 1962-1966). Studies of adsorption at the poly(dimethylsiloxane)-water interface are shown to be valuable for assessing the CO2-philicity of new surfactants. All in all, the symmetric F8H8 and F7H7 analogues are seen to be the most efficient compounds from this class for applications in CO2. 相似文献