全文获取类型
收费全文 | 2510篇 |
免费 | 132篇 |
国内免费 | 16篇 |
专业分类
化学 | 1591篇 |
晶体学 | 9篇 |
力学 | 110篇 |
数学 | 602篇 |
物理学 | 346篇 |
出版年
2024年 | 3篇 |
2023年 | 25篇 |
2022年 | 30篇 |
2021年 | 49篇 |
2020年 | 50篇 |
2019年 | 41篇 |
2018年 | 50篇 |
2017年 | 24篇 |
2016年 | 84篇 |
2015年 | 97篇 |
2014年 | 97篇 |
2013年 | 159篇 |
2012年 | 231篇 |
2011年 | 229篇 |
2010年 | 142篇 |
2009年 | 150篇 |
2008年 | 162篇 |
2007年 | 192篇 |
2006年 | 164篇 |
2005年 | 147篇 |
2004年 | 120篇 |
2003年 | 95篇 |
2002年 | 93篇 |
2001年 | 36篇 |
2000年 | 21篇 |
1999年 | 18篇 |
1998年 | 14篇 |
1997年 | 16篇 |
1996年 | 15篇 |
1995年 | 12篇 |
1994年 | 19篇 |
1993年 | 11篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 8篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1985年 | 2篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1920年 | 1篇 |
1909年 | 2篇 |
1903年 | 1篇 |
1895年 | 1篇 |
1891年 | 2篇 |
1890年 | 2篇 |
1889年 | 1篇 |
排序方式: 共有2658条查询结果,搜索用时 437 毫秒
51.
[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
52.
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
53.
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
54.
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
55.
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
56.
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
57.
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
58.
The syntheses, crystal structures determined by single-crystal X-ray diffraction, and characterizations of new Mo(6) cluster chalcobromides and cyano-substituted compounds with 24 valence electrons per Mo(6) cluster (VEC = 24), are presented in this work. The structures of Cs(4)Mo(6)Br(12)S(2) and Cs(4)Mo(6)Br(12)Se(2) prepared by solid state routes are based on the novel [(Mo(6)Br(i)(6)Y(i)(2))Br(a)(6)](4)(-) (Y = S, Se) discrete units in which two chalcogen and six bromine ligands randomly occupy the inner positions, while the six apical ones are fully occupied by bromine atoms. The interaction of these two compounds with aqueous KCN solution results in apical ligand exchange giving the two first Mo(6) cyano-chalcohalides: Cs(0.4)K(0.6)(Et(4)N)(11)[(Mo(6)Br(6)S(2))(CN)(6)](3).16H(2)O and Cs(0.4)K(0.6)(Et(4)N)(11)[(Mo(6)Br(6)Se(2))(CN)(6)](3).16H(2)O. Their crystal structures, built from the original [(Mo(6)Br(i)(6)Y(i)(2))(CN)(a)(6)](4)(-) discrete units, will be compared to those of the two solid state precursors and other previously reported Mo(6) cluster compounds. Their redox properties and (77)Se NMR characterizations will be presented. Crystal data: Cs(4)Mo(6)Br(12)S(2), orthorhombic, Pbca (No. 61), a = 11.511(5) A, b = 18.772(5) A, c = 28.381 A (5), Z = 8; Cs(4)Mo(6)Br(12)Se(2), Pbca (No. 61), a = 11.6237(1) A, b = 18.9447(1) A, c = 28.4874(1) A, Z = 8; Cs(0.4)K(0.6)(Et(4)N)(11)[(Mo(6)Br(6)S(2))(CN)(6)](3).16H(2)O, Pm-3m (No. 221), a = 17.1969(4) A, Z = 1; Cs(0.4)K(0.6)(Et(4)N)(11)[(Mo(6)Br(6)Se(2))(CN)(6)](3).16H(2)O, Pm-3m (No. 221), a = 17.235(5) A, Z = 1. 相似文献
59.
Cline Schneider Frdric Doucet Stanislav Strekopytov Christopher Exley 《Polyhedron》2004,23(18):3185-3191
Hydroxyaluminosilicates (HAS) are critical secondary mineral phases in the biogeochemical cycle of aluminium. They are formed from the reaction of silicic acid (Si(OH)4) with an aluminium hydroxide template and act as a geochemical control of the biological availability of Al. There are two main forms of HAS which we have called HASA and HASB and which of these will predominate will depend upon the Si(OH)4 to Al ratio in any one environment. In all but the most heavily weathered environments or those undergoing a progressive acidification Si(OH)4 will be present in significant excess to Al and HASB will be the dominant secondary mineral phase. We have tried to determine the solubility of HASB(s) so that its contribution to Al solubility control might be compared with other secondary minerals such as Al(OH)3(gibbsite). In preliminary experiments, the dissolution of HASB(s) was found to be non-congruent with almost no Al being released during 18 months ageing. We then demonstrated that HASB(s) was significantly less soluble than Al(OH)3(s) prepared under identical experimental conditions. We have used this information to describe a solubility expression for HASB(s) at a predefined quasi-equibrium and to calculate a solubility constant.
K*Al2Si2O5(OH)4=[Al2O4+][SiO2]2[OH-]4