全文获取类型
收费全文 | 1378篇 |
免费 | 47篇 |
国内免费 | 6篇 |
专业分类
化学 | 968篇 |
晶体学 | 6篇 |
力学 | 25篇 |
数学 | 213篇 |
物理学 | 219篇 |
出版年
2023年 | 9篇 |
2022年 | 33篇 |
2021年 | 35篇 |
2020年 | 30篇 |
2019年 | 24篇 |
2018年 | 33篇 |
2017年 | 20篇 |
2016年 | 50篇 |
2015年 | 34篇 |
2014年 | 41篇 |
2013年 | 74篇 |
2012年 | 63篇 |
2011年 | 72篇 |
2010年 | 44篇 |
2009年 | 39篇 |
2008年 | 72篇 |
2007年 | 53篇 |
2006年 | 45篇 |
2005年 | 43篇 |
2004年 | 39篇 |
2003年 | 43篇 |
2002年 | 39篇 |
2001年 | 19篇 |
2000年 | 11篇 |
1999年 | 7篇 |
1998年 | 12篇 |
1997年 | 18篇 |
1996年 | 18篇 |
1995年 | 10篇 |
1994年 | 7篇 |
1993年 | 18篇 |
1992年 | 11篇 |
1991年 | 17篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1987年 | 8篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1981年 | 12篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1976年 | 8篇 |
1918年 | 8篇 |
1901年 | 11篇 |
1895年 | 6篇 |
1894年 | 6篇 |
1888年 | 6篇 |
1885年 | 9篇 |
排序方式: 共有1431条查询结果,搜索用时 31 毫秒
31.
Olivier Mentre Anne-Claire Dhaussy Francis Abraham Hugo Steinfink 《Journal of solid state chemistry》1998,140(2):120
The solid solution Sr2−xPbxV3O9, 0≤x≤2, was prepared by solid state reactions and characterized by X-ray diffraction, IR spectroscopy, and magnetic susceptibility measurements. Single crystals of the pure strontium phase and mixed Sr/Pb compounds were prepared by high temperature treatment of the respective powder compositions. Pb2V3O9crystals could only be obtained by the electrochemical reduction of molten PbV2O6. These crystals were always twinned. The previously reported crystal structure of Sr2V3O9was confirmed. It was refined toR=0.050,Rw=0.057, in space group C2/c,a=7.555(1) Å,b=16.275(2) Å,c=6.948(1) Å,β=119.78(1)°. The single crystal structural studies of the Sr1.02Pb0.98V3O9and Sr0.67Pb1.33V3O9members of the series show that the introduction of lead gives rise to a progressively complicated splitting of Sr2+/Pb2+and the tetrahedral vanadium ion crystallographic sites. As a consequence the vanadium framework distorts and beyond the Sr0.5Pb1.5V3O9composition the crystal symmetry becomes triclinic. This distortion is ascribed to the stereochemical effect of the 6s2lone pair of Pb2+. The crystallographic parameters of Pb2V3O9area=7.598(1) Å,b=16.393(3) Å,c=6.972(2) Å,α=91.38(1)°,β=119.35(1)°,γ=90.47(1)°. Pb2V3O9exhibits a more complex IR spectrum than the monoclinic phases. Despite the similarity between the triclinic and monoclinic phases the magnetic susceptibilities indicate differences in the coupling between V4+ions at low temperatures. 相似文献
32.
Emmanuel Blas Patricio-Rangel Margarita Tlahuextl Hugo Tlahuext Antonio Rafael Tapia-Benavides 《Acta Crystallographica. Section C, Structural Chemistry》2020,76(4):322-327
The synthesis and characterization of two new 1,3,5‐triazines containing 2‐(aminomethyl)‐1H‐benzimidazole hydrochloride as a substituent are reported, namely, 2‐{[(4,6‐dichloro‐1,3,5‐triazin‐2‐yl)amino]methyl}‐1H‐benzimidazol‐3‐ium chloride, C11H9Cl2N6+·Cl? ( 1 ), and bis(2,2′‐{[(6‐chloro‐1,3,5‐triazine‐2,4‐diyl)bis(azanediyl)]bis(methylene)}bis(1H‐benzimidazol‐3‐ium)) tetrachloride heptahydrate, 2C19H18ClN92+·4Cl?·7H2O ( 2 ). Both salts were characterized using single‐crystal X‐ray diffraction analysis and IR spectroscopy. Moreover, the NMR (1H and 13C) spectra of 1 were obtained. Salts 1 and 2 have triclinic symmetry (space group P) and their supramolecular structures are stabilized by hydrogen bonding and offset π–π interactions. In hydrated salt 2 , the noncovalent interactions yield pseudo‐nanotubes filled with chloride anions and water molecules, which were modelled in the refinement with substitutional and positional disorder. 相似文献
33.
This stability of Sb(III) and Sb(V) species was studied during single extraction from soils by water. EDTA, diluted H2SO4 and H3PO4, and oxalic acid/oxalate solutions, with and without ascorbic acid, were used as stabilizing reagent of both Sb species. Antimony redox speciation in soil extracts was performed by selective hydride generation-atomic fluorescence spectrometry. Simulated extraction procedures (without soil) showed that, except in oxalate medium, Sb(III) was oxidized to Sb(V), and this reaction was avoided with ascorbic acid. Recovery studies from a spiked agricultural soil showed that no oxidation but sorption of Sb(III) occurred during the extraction process in water and H2SO4 medium, and quantitative oxidation in EDTA and oxalate medium. With ascorbic acid, this oxidation was totally avoided in EDTA and partially avoided in oxalate solution. A new sequential extraction procedure was proposed and applied to the fractionation and redox speciation of antimony in agricultural soils, using EDTA + ascorbic acid, pH 7 (available under complexing and moderately reducible conditions); oxalic acid/oxalate + ascorbic acid (extractable in reducible conditions) and HNO3 + HCl + HF (residual fraction). The proposed extraction scheme can provide information about the availability and mobility of antimony redox species in agricultural soils. 相似文献
34.
Claudete J. Valduga Hugo S. Braibante Mara E. F. Braibante 《Journal of heterocyclic chemistry》1997,34(5):1453-1457
The reactivity of the β-enamino ketones, 3-amino-1-(p-phenyl-substituted)-2-buten-1-ones 1a-d and β-enamino esters, ethyl 3-amino-3-(p-phenyl-substituted)-2-propenoates 5a-d was systematically studied when allowed to react with hydrazine and methylhydrazine under solid support K -10/ultrasound conditions and in homogeneous media (reflux in ethanol or dichloromethane). The products were pyrazoles 2a-d , N-methylpyrazoles 3a-d, 4a-d and N-methylpyrazolinones 6a-c and 7a-c . The regiochemistry of the cyclization reactions showed dependence upon the reaction conditions employed as well as upon the sub-stituent in the aromatic ring. 相似文献
35.
Sem DS Bertolaet B Baker B Chang E Costache AD Coutts S Dong Q Hansen M Hong V Huang X Jack RM Kho R Lang H Ma CT Meininger D Pellecchia M Pierre F Villar H Yu L 《Chemistry & biology》2004,11(2):185-194
Genomics-driven growth in the number of enzymes of unknown function has created a need for better strategies to characterize them. Since enzyme inhibitors have traditionally served this purpose, we present here an efficient systems-based inhibitor design strategy, enabled by bioinformatic and NMR structural developments. First, we parse the oxidoreductase gene family into structural subfamilies termed pharmacofamilies, which share pharmacophore features in their cofactor binding sites. Then we identify a ligand for this site and use NMR-based binding site mapping (NMR SOLVE) to determine where to extend a combinatorial library, such that diversity elements are directed into the adjacent substrate site. The cofactor mimic is reused in the library in a manner that parallels the reuse of cofactor domains in the oxidoreductase gene family. A library designed in this manner yielded specific inhibitors for multiple oxidoreductases. 相似文献
36.
Although heme protein-based, amperometric nitric oxide (NO) biosensors have been well documented in previous studies, most have been conducted in anaerobic conditions. Herein we report a novel hemoglobin-based NO biosensor that is not only very sensitive but also usable in air. The heme protein was entrapped in a sodium montmorillonite film, which was immobilized at a pyrolytic graphite electrode surface. Film-entrapped hemoglobin can directly exchange electrons with the electrode, and this process has proven to favor the catalytic reduction of oxygen. In addition, NO induced a cathodic potential shift of the catalytic reduction peak of oxygen. This potential shift was proportional to the logarithm of NO concentration ranging from 4.0 × 10−11 to 5.0 × 10−6 mol/L. The detection limit has been estimated to be 20 pM, approximately four orders lower than previously reported amperometric detectors. 相似文献
37.
We computed the free energy of solvation for a series of ions and neutral molecules using two different continuum approaches. First, we used the AM1–SM1 technique, where the AM1 Fock matrix is modified to include a generalized Born contribution. Second, we applied the DelPhi approach, where the electrostatic component of the free energy of solvation is evaluated by resolving the Poisson–Boltzman equation by a finite difference method. Both methods appear equally reliable for ionic systems. For neutral compounds, AM1–SM1 performs better than DelPhi; however, the differences become less pronounced for compounds with larger free energies of solvation. In parallel, both methods were applied to study the influence of the solvation process in the overall drug receptor interaction for a series of closely related ligands for the D1 dopamine receptor. An inverse linear relationship was found between the free energy of solvation and the logarithm of the affinity of the ligands; nevertheless, electrostatic properties are likely to modulate affinity as well. © 1993 John Wiley & Sons, Inc. 相似文献
38.
39.
Reactions of [M(SR)(3)(PMe(2)Ph)(2)] (M = Ru, Os; R = C(6)F(4)H-4, C(6)F(5)) with CS(2) in acetone afford [Ru(S(2)CSR)(2)(PMe(2)Ph)(2)] (R = C(6)F(4)H-4, 1; C(6)F(5), 3) and trans-thiolates [Ru(SR)(2)(S(2)CSR)(PMe(2)Ph)(2)] (R = C(6)F(4)H-4, 2; C(6)F(5), 4) or the isomers trans-thiolates [Os(SR)(2)(S(2)CSR)(PMe(2)Ph)(2)] (R = C(6)F(4)H-4, 5; C(6)F(5), 7) and trans-thiolate-phosphine [Os(SR)(2)(S(2)CSR)(PMe(2)Ph)(2)] (R = C(6)F(4)H-4, 6; C(6)F(5), 8) through processes involving CS(2) insertion into M-SR bonds. The ruthenium(III) complexes [Ru(SR)(3)(PMe(2)Ph)(2)] react with CS(2) to give the diamagnetic thiolate-thioxanthato ruthenium(II) and the paramagnetic ruthenium(III) complexes while osmium(III) complexes [Os(SR)(3)(PMe(2)Ph)(2)] react to give the paramagnetic thiolate-thioxanthato osmium(III) isomers. The single-crystal X-ray diffraction studies of 1, 4, 5, and 8 show distorted octahedral structures. (31)P [(1)H] and (19)F NMR studies show that the solution structures of 1 and 3 are consistent with the solid-state structure of 1. 相似文献
40.