全文获取类型
收费全文 | 41406篇 |
免费 | 1503篇 |
国内免费 | 1450篇 |
专业分类
化学 | 29668篇 |
晶体学 | 322篇 |
力学 | 797篇 |
综合类 | 10篇 |
数学 | 5713篇 |
物理学 | 7849篇 |
出版年
2023年 | 306篇 |
2022年 | 531篇 |
2021年 | 705篇 |
2020年 | 680篇 |
2019年 | 698篇 |
2018年 | 781篇 |
2017年 | 772篇 |
2016年 | 1500篇 |
2015年 | 1210篇 |
2014年 | 1286篇 |
2013年 | 2791篇 |
2012年 | 3033篇 |
2011年 | 3285篇 |
2010年 | 1897篇 |
2009年 | 1914篇 |
2008年 | 2550篇 |
2007年 | 2488篇 |
2006年 | 2177篇 |
2005年 | 2052篇 |
2004年 | 2155篇 |
2003年 | 1544篇 |
2002年 | 1427篇 |
2001年 | 880篇 |
2000年 | 749篇 |
1999年 | 577篇 |
1998年 | 477篇 |
1997年 | 370篇 |
1996年 | 453篇 |
1995年 | 337篇 |
1994年 | 293篇 |
1993年 | 352篇 |
1992年 | 304篇 |
1991年 | 207篇 |
1990年 | 198篇 |
1989年 | 170篇 |
1988年 | 171篇 |
1987年 | 155篇 |
1986年 | 157篇 |
1985年 | 248篇 |
1984年 | 218篇 |
1983年 | 173篇 |
1982年 | 150篇 |
1981年 | 150篇 |
1980年 | 122篇 |
1979年 | 121篇 |
1978年 | 104篇 |
1977年 | 102篇 |
1976年 | 96篇 |
1975年 | 90篇 |
1973年 | 96篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
L De Boni L T Fran?a H P Grieneisen M Janowicz T B Kist A R Consiglio J R Schoffen V Stefani C Termignoni 《Electrophoresis》1999,20(12):2493-2500
The phenomenon of electrophoresis in free solution has been studied theoretically down to the molecular level for decades. In addition, intermolecular photo-induced proton transfer reactions, which occur in a wide class of molecules (phenols and aminoarenes) as well as proteins (green fluorescent protein), were also studied extensively. However, the study of the effect of light-induced electrophoretic mobility changes of the analytes in electrophoresis was begun only recently. In the present work, capillary zone electrophoresis was chosen as the environment to measure the magnitude of these electrophoretic mobility shifts induced by light. Background electrolytes (running electrolytes) with high refractive indices were developed, allowing the capillary to work like an optical fiber. The experimental conditions for obtaining stable coupling and guided laser light along the liquid core are discussed. Experimental evidence of band compression is observed, leading to a solitary wave behavior of the analyte band (2-naphthol). These solitary waves result from competition between thermal diffusion (dispersion mechanism) and a nonlinear (band compression) effect due to the combined electrophoresis phenomenon and absorption of guided light by the molecules of the band (which are subjected to a "reversible intermolecular proton transfer reaction" as one of their decay routes). The possibilities of applying this effect to different methods and techniques are also discussed. 相似文献
102.
103.
Treatment of a solution of excess PCl(3) and PS (PS = "proton sponge" = 1,8-dimethylamino naphthalene) with arachno-4-CB(8)H(14) (1) in CH(2)Cl(2), followed by hydrolysis of the reaction mixture, resulted in the isolation of the eleven-vertex diphosphacarbaborane nido-7,8,9-P(2)CB(8)H(10) (2) (yield 34%) as the main product. Other products isolated from this reaction were the phosphacarboranes nido-7,8,9,10-P(3)CB(7)H(8) (3) (yield 5%) and closo-2,1-PCB(8)H(9) (4) (yield 15%). Compound 2 can be deprotonated by PS in CH(2)Cl(2) or NaH in diethyl ether to give the [nido-7,8,9-P(2)CB(8)H(9)](-) (2(-)()) anion, which gives back the original compound, 2, upon re-protonation. Thermal rearrangement of anion 2(-) (Na(+) salt) at 350 degrees C for 2 h produced the isomeric [nido-7,8,10-P(2)CB(8)H(9)](-) (5(-)()) anion, which was isolated as a PPh(4)(+) salt (yield 86%). Multinuclear ((1)H, (11)B, (31)P, and (13)C), two-dimensional [(11)B-(11)B] COSY, (1)H{(11)B(selective)}, (1)H{(31)P(selective)}, and gradient-enhanced ([(1)H-(13)C] HSQC) magnetic resonance measurements led to complete assignments of all resonances which are in excellent agreement with the structures proposed. Coupling constants, (1)J((31)P,(13)C), (2)J((31)P,C,(1)H), and (1)J((31)P,(31)P), were calculated using the DFT method B3LYP/6-311+G(d,p). The molecular geometries of all compounds were optimized ab initio at a correlated level of theory (RMP2(fc)) using the 6-31G basis set, and their correctness was assessed by comparison of the experimental (11)B and (13)C chemical shifts with those calculated by the GIAO-SCF/II//RMP2(fc)/6-31G method. The computations also include the structures and chemical shieldings of the still unknown isomers [nido-7,10,8-P(2)CB(8)H(9)](-) (6(-)) and [nido-7,9,8-P(2)CB(8)H(9)](-) (7(-)). 相似文献
104.
1,5-Bis(dicarboxymethylaminomethyl)-2,6-dihydroxynaphthalene (BDDN) forms fluorescent complexes with aluminium, barium, beryllium, calcium, magnesium and strontium. All the complexes have 2:1 metal :ligand ratios. Barium, calcium, magnesium and strontium exhibit maximal fluorescence at pH 11.7, with excitation and emission maxima at 385 and 445 nm respectively. Aluminium and beryllium show maximal fluorescence at pH 5.8 and 5.2 respectively, the excitation and emission maxima being at 370 and 405 nm. The formation of the calcium complex provides a highly sensitive and selective determination of calcium in the range 10/2-500 ng. The fluorescence measurement of calcium should be made within 5 min of mixing the solutions because of the instability of the reagent at the given pH. Potassium cyanide may be used as a masking agent and ter- or quadrivalent cations should be removed by preliminary extraction with 8-hydroxyquinoline in chloroform at pH 6.0. In such conditions, of 33 cations studied, only magnesium (>20 ng), strontium (>70 ng) and barium (>150 ng) caused interference in the determination of 300 ng of calcium. Among 16 anions examined, only EDTA interfered seriously. 相似文献
105.
V. Goliáš D. Pittauerová R. Procházka Z. Třískala 《Journal of Radioanalytical and Nuclear Chemistry》2005,266(3):461-470
Summary The activities of218Po,214Po and211Bi were determined in samples obtained of soil/gas. Sampling work was taken in Jáchymov (Czech Republic) at the outcrop of
the Geister-vein,by electrostatic precipitation from filtered soil gas on stainless steel disks.The samples were measured
in a field laboratory using a semi-conductor alpha-spectrometer.The activities of218Po,214Po and211Bi were calculated.Samples taken from active dump material (near-by radon source) exhibited a high211Bi/214Po ratio, while those of the vein outcrop (a relatively deeper source) had a low ratio.A mathematical model was employed to
determine the radon age calculated from the actinon/radon input ratio.This varied in a range of 5.6 to -7.7 seconds.Negative
age values are probably caused by the preference for actinon, which rapidly comes into equilibrium with the source of this
gas. 相似文献
106.
The transfer constants (Cs) of the polystyrene radical with some derivatives of phthalic acid have been determined. Among the agents used, tetrachlorophthalanhydride (TCPA) differs distinctly from other compounds by its value of Cs 3·1 × 10?3 for thermal and 3·4 × 10?3 for initiated polymerization of styrene. The values of Cs for phthalanhydride, dimethyl phthalate, and tetrachlorodimethyl phthalate are lower by two decimal orders. The considerable decrease in the degree of polymerization of styrene prepared in the presence of TCPA is mainly attributed to the increased reactivity of chlorine atoms in TCPA induced by the acceptor effect of anhydride ring. Participation of a TCPA-styrene complex in transfer reaction has been assumed but not proved. 相似文献
107.
Six new polyamides have been prepared either by polymerization in solution or by interfacial polymerization by reacting perchloroterephthaloyl dichloride with aromatic diamines. The polymers were characterized by i.r. spectroscopy, elemental analysis, DSC, and TGA. The glass transition temperature, the melting temperature, and the thermal decomposition temperature have been determined. In order to estimate the molecular weights, intrinsic viscosities were determined. Some of the polymers showed high thermal stability. 相似文献
108.
Formulations of the tin etiopurpurin (SnET2) have been observed to undergo a spectral change on storage in dimethylformamide solution. This results in an alteration in the action spectrum, with enhanced photodynamic activity at lower wavelengths and decreased activity at 660 nm. On the basis of structural considerations, a reduced analog of SnET2 was prepared with an absorbance maximum at 640 nm. Formation of this product, termed SnET2H(2) relieves steric strain inherent in the parent molecule. Nuclear magnetic resonance, spectral and photodynamic data are consistent with the formation of SnET2H(2) during SnET2 storage. Slight modifications in the original synthesis are also reported, resulting in improved yields of intermediate products. 相似文献
109.
Leon P. Jenner Mickael V. Cherrier Patricia Amara Luis M. Rubio Yvain Nicolet 《Chemical science》2021,12(14):5269
The nitrogenase MoFe protein contains two different FeS centers, the P-cluster and the iron–molybdenum cofactor (FeMo-co). The former is a [Fe8S7] center responsible for conveying electrons to the latter, a [MoFe7S9C-(R)-homocitrate] species, where N2 reduction takes place. NifB is arguably the key enzyme in FeMo-co assembly as it catalyzes the fusion of two [Fe4S4] clusters and the insertion of carbide and sulfide ions to build NifB-co, a [Fe8S9C] precursor to FeMo-co. Recently, two crystal structures of NifB proteins were reported, one containing two out of three [Fe4S4] clusters coordinated by the protein which is likely to correspond to an early stage of the reaction mechanism. The other one was fully complemented with the three [Fe4S4] clusters (RS, K1 and K2), but was obtained at lower resolution and a satisfactory model was not obtained. Here we report improved processing of this crystallographic data. At odds with what was previously reported, this structure contains a unique [Fe8S8] cluster, likely to be a NifB-co precursor resulting from the fusion of K1- and K2-clusters. Strikingly, this new [Fe8S8] cluster has both a structure and coordination sphere geometry reminiscent of the fully reduced P-cluster (PN-state) with an additional μ2-bridging sulfide ion pointing toward the RS cluster. Comparison of available NifB structures further unveils the plasticity of this protein and suggests how ligand reorganization would accommodate cluster loading and fusion in the time-course of NifB-co synthesis.The K-cluster of NifB as a key intermediate in the synthesis of the nitrogenase active site supports [Fe4S4] cluster fusion occurs before carbide and sulfide insertion and displays ligand spatial arrangement reminiscent to that of the P-cluster. 相似文献
110.
de Farias Robson Fernandes Martínez Leandro Airoldi Claudio 《Transition Metal Chemistry》2002,27(7):748-750
This paper describes the synthesis, characterization and a thermogravimetric (t.g.) study of 11 adducts of general formula MCl2·nL (M = Cu, Co and Sn; n = 1 and 2 for Sn, and 2 for Cu and Co; L = ethyleneurea eu, ethylenethiourea etu, and propyleneurea pu). The i.r. results shows that the eu and pu adducts are coordinated through oxygen, whereas for etu nitrogen is the coordination site. The (C=O) or (C=S) trend is: Cu > Co > Sn. The t.g. curves show that the adducts release ligand molecules in a single mass loss step. As regards considering adducts with the same stoichiometry, the observed thermal stability trend is: etu > pu > eu. 相似文献