全文获取类型
收费全文 | 8847篇 |
免费 | 351篇 |
国内免费 | 61篇 |
专业分类
化学 | 5955篇 |
晶体学 | 47篇 |
力学 | 218篇 |
数学 | 1791篇 |
物理学 | 1248篇 |
出版年
2024年 | 13篇 |
2023年 | 76篇 |
2022年 | 203篇 |
2021年 | 241篇 |
2020年 | 162篇 |
2019年 | 200篇 |
2018年 | 163篇 |
2017年 | 143篇 |
2016年 | 318篇 |
2015年 | 270篇 |
2014年 | 309篇 |
2013年 | 529篇 |
2012年 | 626篇 |
2011年 | 693篇 |
2010年 | 421篇 |
2009年 | 446篇 |
2008年 | 568篇 |
2007年 | 578篇 |
2006年 | 529篇 |
2005年 | 477篇 |
2004年 | 388篇 |
2003年 | 270篇 |
2002年 | 271篇 |
2001年 | 112篇 |
2000年 | 98篇 |
1999年 | 82篇 |
1998年 | 66篇 |
1997年 | 69篇 |
1996年 | 99篇 |
1995年 | 52篇 |
1994年 | 55篇 |
1993年 | 52篇 |
1992年 | 69篇 |
1991年 | 47篇 |
1990年 | 42篇 |
1989年 | 38篇 |
1988年 | 34篇 |
1987年 | 30篇 |
1986年 | 38篇 |
1985年 | 53篇 |
1984年 | 48篇 |
1983年 | 32篇 |
1982年 | 36篇 |
1981年 | 30篇 |
1980年 | 28篇 |
1979年 | 24篇 |
1978年 | 17篇 |
1977年 | 10篇 |
1974年 | 14篇 |
1971年 | 12篇 |
排序方式: 共有9259条查询结果,搜索用时 15 毫秒
111.
112.
Quassinoids from Picrasma crenata 总被引:1,自引:0,他引:1
From woods of Picrasma crenata, a new stereoisomer dihydronorneoquassin was obtained together with others well knowns dihydronorneoquassin, parain, alpha-neoquassin, beta-neoquassin and quassin. The structures were determined by spectroscopic data and chemical evidence. 相似文献
113.
Pérez-Mendoza M Gonzalez J Wright PA Seaton NA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(18):7653-7658
We have measured the adsorption of methane and ethane to high pressure on SBA-2, a structured mesoporous silica composed of spheres connected by narrow channels. The experimental data were analyzed by carrying out Monte Carlo simulations of adsorption in pore structure models of different complexity and then adjusting the parameters of the models to match the Monte Carlo results to the experimental data. We found that a model based on single-sized spherical cavities was inadequate and that it is necessary to explicitly account for the interconnecting channels. Further, we found that despite the basic regularity of the SBA-2 structure, it is necessary to allow for a distribution of the sizes of both the cavities and the channels. These size distributions were obtained by fitting the parameters of the model to the experimental adsorption data, revealing detailed structural information not previously known for this material. The channels were found to be 5-15 A in diameter, while the cavities were 40-50 A in diameter. There is some evidence that the distribution of channel sizes leads to a percolation effect whereby the pore structure is not equally accessible to all adsorptives. 相似文献
114.
Gold-gold interactions in small polynuclear complexes are analysed using extended Hückel calculations. They are influenced by the nature of the ligand donor atoms, by the bridging ligands, but most by the formal oxidation state of the metal. Au---Au bonds are much stronger in complexes of Au(II) and Au(III), but a weak interaction between two d10 centres exists for Au(I) complexes, owing to mixing of the s and p orbitals with the d orbitals. Phosphines induce stronger metal-metal bonds when coordinated trans to the Au---Au bond in [Au(II)[(CH2)2PPh2]L]2 (Ph = phenyl), but have the opposite effect when bonded orthogonally to the metal-metal axis in Au(I) binuclear species. When two gold atoms are bridged by a single carbon atom, belonging either to mesityl (Mes = 2,4,6-Me3C6H2) or CR2, the former produces stronger Au(I)---Au(I) interactions, reflected in shorter distances. Formal oxidation states are proposed for the gold atoms in two mixed-valence clusters, [Au4(C6F5)2((PPh3)2CH})2(PPh3)2](ClO4)2 and [{(2,4,6-C6F3H2)Au(CH2PPh2CH2)2Au{in2-Au(CH2PPh2CH2)2Au](ClO4)2. The results suggest a higher oxidation state for the outer gold atoms, in both the T-shaped tetranuclear cluster and the Au6 linear chain. 相似文献
115.
[reaction: see text]. A very simple methodology to stereoselectively achieve tricyclic isonucleosides (nucleobase = thymine, uracil, and 5-fluoruracil) and 3'-C-branched nucleosides (nucleobase = theophylline) was performed by means of a DBU-mediated addition process using a readily available 2-bromo sugar. The mechanism for these transformations implies the loss of both substituents at C-2 and C-3 on the sugar moiety, and although it seems that DBU is probably involved, its involvement has not yet been ascertained. Cytosine did not react under these conditions. 相似文献
116.
Carmona D Lahoz FJ Atencio R Edwards AJ Oro LA Lamata MP Esteban M Trofimenko S 《Inorganic chemistry》1996,35(9):2549-2557
Treatment of the metallo ligands [ML(pz)(2)(Hpz)] (pz = pyrazolate; L = C(5)Me(5), M = Ir (1); L = mesitylene, M = Ru (3)) with [M'Cl{HB(3-i-Pr-4-Br-pz)(3)}] (M' = Co (4), Ni (5)) yields heterodinuclear complexes of formula [LM(&mgr;-pz)(2)(&mgr;-Cl)M'{HB(3-i-Pr-4-Br-pz)(3)}] (L = C(5)Me(5); M = Ir; M' = Co (6), Ni (7). L = mesitylene; M = Ru; M' = Co (8)). The related complex [Ru(eta(6)-p-cymene)(pz)(2)(Hpz)] (2) reacts with equimolar amounts of 4 or 5 to give mixtures of the corresponding bis(&mgr;-pyrazolato) &mgr;-chloro complexes [(eta(6)-p-cymene)Ru(&mgr;-pz)(2)(&mgr;-Cl)M'{HB(3-i-Pr-4-Br-pz)(3)}] (M' = Co (9), Ni (10)) and the triply pyrazolato-bridged complexes [(eta(6)-p-cymene)Ru(&mgr;-pz)(3)M'{HB(3-i-Pr-4-Br-pz)(3)}] (M' = Co (11), Ni (12)). Complex 1 reacts with 5 in the presence of KOH to give the IrNi complex [(eta(5)-C(5)Me(5))Ir(&mgr;-pz)(3)Ni{HB(3-i-Pr-4-Br-pz)(3)}] (13) whereas its reaction with 4 and KOH rendered the bis(&mgr;-pyrazolato) &mgr;-hydroxo complex [(eta(5)-C(5)Me(5))Ir(&mgr;-pz)(2)(&mgr;-OH)Co{HB(3-i-Pr-4-Br-pz)(3)}] (14). The molecular structure of the heterobridged IrCo complex (6) and those of the homobridged RuNi (12) and IrNi (13) complexes have been determined by X-ray analyses. Compound 6 crystallizes in the monoclinic space group P2(1)/n, with a = 10.146(5) ?, b = 18.435(4) ?, c = 22.187(13) ?, beta = 97.28(4) degrees, and Z = 4. Complex 12 is monoclinic, space group P2(1), with a = 10.1169(7) ?, b = 21.692(2) ?, c = 11.419(1) ?, beta = 112.179(7) degrees, and Z = 2. Compound 13 crystallizes in the monoclinic space group Cc, with a = 13.695(2) ?, b = 27.929(6) ?, c = 13.329(2) ?, beta = 94.11(4) degrees, and Z = 4. All the neutral complexes 6, 12, and 13 consist of linear M.M'.B backbones with two (6) or three (12, 13) pyrazolate ligands bridging the dimetallic M.M' units and three substituted 3-i-Pr-4-Br-pz groups joining M' to the boron atoms. The presence in the proximity of the first-row metal M' of the three space-demanding isopropyl substituents of the pyrazolate groups induces a significant trigonal distortion of the octahedral symmetry, yielding clearly different M'-N bond distances on both sides of the ideal octahedral coordination sphere of these metals. 相似文献
117.
The global electrophilicity power, ω, of a series of dipoles and dipolarophiles commonly used in 1,3-dipolar cycloadditions may be conveniently classified within a unique relative scale. The effects of chemical substitution on the electrophilicity of molecules have been evaluated using a representative set of electron-withdrawing and electron-releasing groups for a series of dipoles including nitrone, nitrile oxide and azide derivatives. The absolute scale of electrophilicity is used to rationalize the chemical reactivity of these species as compared to the static reactivity pattern of the reagents involved in the Diels-Alder reactions. 相似文献
118.
Guerrero-Alvarez JA Moncayo-Bautista A Ariza-Castolo A 《Magnetic resonance in chemistry : MRC》2004,42(6):524-533
The relative configuration of 11 1,4-diazaspiro[4.5]decanes (1a-1j and 1m), 15 1,4-oxazaspiro[4.5]decanes (2a-2o) and 10 1,4-dioxaspiro[4.5]decanes (3a-3n) substituted at the 2-, 6-, 7- or 8-position by a methyl group or using the tert-butyl group as a model for the ananchomeric structure is reported. The relative stereochemistry was analyzed by 1H, 13C, 15N and 17O NMR and all isomers present were characterized spectroscopically. Compounds with a methyl group in the six-membered ring show a chair conformation preference with the methyl group in the equatorial position. Compounds with one or two nitrogens exhibit a tautomeric equilibrium between the imine-diazolidine forms, as demonstrated by IR and 13C NMR. 相似文献
119.
Alvarez S Aullón G Fandos R García Fierro JL Ocón P Otero A Rojas S Terreros P 《Dalton transactions (Cambridge, England : 2003)》2005,(5):938-944
The reaction of [Rh(micro-Cl)(COD)]2 with 4,6-dimethyl-pyrimidinethiolate (Me2-pymt) and subsequent substitution of COD by CO yields [Rh(Me2-pymt)(CO)2]. The stacking pattern found in this compound is in contradiction with previously studied comparable square-planar complexes of type d8-[M(chelate)(monodentate)2] in which each ligand has different pi-acidic character. A theoretical study of the intermolecular interactions and conformation of the title compound has been carried out, combining semi-empirical band calculations on the real chains and ab initio(MP2 level) calculations on a model dimer. The combination of electronic and steric effects determines the rotation of the successive monomers and the deviation from linearity of the one-dimensional stacks. Its behaviour in solution is also special, developing a blue colour and forming micelles, when adding water to acetone solutions. 相似文献
120.
Bayó N Jiménez JC Rivas L Nicolás E Albericio F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(5):1096-1103
An optimized solid-phase strategy for the preparation of the cyclic lipononadepsipeptide [N-Mst(L-Ser1), D-Ser4, L-Thr6, L-Asp8, L-Thr9]syringotoxin is reported. The strategy is based on the use of a mild orthogonal protection scheme and the incorporation of the nonproteinogenic amino acid (Z)-Dhb into the peptide chain as the dipeptide Fmoc-Thr(tBu)-(Z)-Dhb-OH. The didehydrodipeptide was synthesized by a water-soluble carbodiimide-induced beta-elimination of a protected dipeptide containing a residue of Thr with its free hydroxy side chain unprotected. 相似文献