全文获取类型
收费全文 | 4901篇 |
免费 | 240篇 |
国内免费 | 51篇 |
专业分类
化学 | 3403篇 |
晶体学 | 35篇 |
力学 | 144篇 |
数学 | 910篇 |
物理学 | 700篇 |
出版年
2023年 | 42篇 |
2022年 | 61篇 |
2021年 | 133篇 |
2020年 | 120篇 |
2019年 | 92篇 |
2018年 | 91篇 |
2017年 | 82篇 |
2016年 | 178篇 |
2015年 | 172篇 |
2014年 | 207篇 |
2013年 | 354篇 |
2012年 | 333篇 |
2011年 | 429篇 |
2010年 | 284篇 |
2009年 | 229篇 |
2008年 | 323篇 |
2007年 | 333篇 |
2006年 | 268篇 |
2005年 | 236篇 |
2004年 | 212篇 |
2003年 | 161篇 |
2002年 | 163篇 |
2001年 | 75篇 |
2000年 | 45篇 |
1999年 | 52篇 |
1998年 | 33篇 |
1997年 | 44篇 |
1996年 | 51篇 |
1995年 | 33篇 |
1994年 | 31篇 |
1993年 | 26篇 |
1992年 | 23篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 18篇 |
1986年 | 14篇 |
1985年 | 23篇 |
1984年 | 26篇 |
1983年 | 18篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1979年 | 15篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 10篇 |
1973年 | 14篇 |
1972年 | 9篇 |
排序方式: 共有5192条查询结果,搜索用时 31 毫秒
141.
Busch BB Paz MM Shea KJ Staiger CL Stoddard JM Walker JR Zhou XZ Zhu H 《Journal of the American Chemical Society》2002,124(14):3636-3646
Trialkyl and aryl organoboranes catalyze the polymerization of dimethylsulfoxonium methylide (1). The product of the polymerization is a tris-polymethylene organoborane. Oxidation affords linear telechelic alpha-hydroxy polymethylene. The polymer molecular weight was found to be directly proportional to the stoichiometric ratio of ylide/borane, and polydispersities as low as 1.01-1.03 have been realized. Although oligomeric polymethylene has been the most frequent synthetic target of this method, polymeric star organoboranes with molecular weights of 1.5 million have been produced. The average turnover frequency at 120 degrees C in 1,2,4,5-tetrachlorobenzene/toluene is estimated at >6 x 10(6) g of polymethylene (mol boron)(-1) h(-1). The mechanism of the polyhomologation reaction involves initial formation of a zwitterionic organoborane.ylide complex which breaks down in a rate-limiting 1,2-alkyl group migration with concomitant expulsion of a molecule of DMSO. The reaction was found to be first order in the borane catalyst and zero order in ylide. DMSO does not interfere with the reaction. The temperature dependence of the reaction rate yielded the following activation energy parameters (toluene, DeltaH(++) = 23.2 kcal/mol, DeltaS(++) = 12.6 cal deg/mol, DeltaG(++) = 19.5 kcal/mol; THF, DeltaH(++) = 26.5 kcal/mol, DeltaS(++) = 21.5 cal deg/mol, DeltaG(++) = 20.1 kcal/mol). 相似文献
142.
Reversed phase high performance liquid chromatography (RPLC) is currently the method of choice for the analysis of basic compounds. However, with traditional silica materials, secondary interactions between the analyte and residual silanols produce peak tailing which can negatively affect resolution, sensitivity, and reproducibility. In order to reduce these secondary interactions, which comprise ion exchange, hydrogen bonding, and London forces interactions, chromatographic analyses can be carried out at low or high pH values where silanol groups and basic compounds are mostly uncharged. The chromatographic behaviour of a particular bidentate stationary phase, Zorbax Extend C18, was studied with a set of basic and neutral compounds. Thanks to a higher chemical stability than traditional silica based supports, analyses were carried out with a high pH mobile phase, which represents a good alternative to the acidic mobile phases generally used to reduce ion exchange interactions. The performance of this bidentate stationary phase was also compared with that of other supports and it was proved that it is advantageous to work with high pH mobile phases when analyzing basic compounds. 相似文献
143.
Basallote MG Estevan F Feliz M Jesús Fernández-Trujillo M Hoyos DA Llusar R Uriel S Vicent C 《Dalton transactions (Cambridge, England : 2003)》2004,(4):530-536
The novel incomplete cuboidal cluster [W3Se4H3(dmpe)3](PF6), [1](PF6), has been prepared by reduction of [W3Se4Br3(dmpe)3](PF6) with LiBH4 in THF solution. The trihydroxo complex [W3Se4(OH)3(dmpe)3](PF6), [2](PF6), was obtained by reacting [W3Se4Br3(dmpe)3](PF6) with NaOH in MeCN-H2O solution. The complexes [1](PF6) and [2](PF6) were converted to their BPh4- salts by treatment with NaBPh4. Recrystallisation of [1](BPh4) in the presence of traces of water affords the mixed dihydride hydroxo complex [W3Se4H2(OH)(dmpe)3](BPh4). The crystal structures of [1](BPh4), [2](BPh4) and [W3Se4H2(OH)(dmpe)3](BPh4) have been resolved. Although the [1]+ trihydride does not react with an excess of halide salts, reaction with HX leads to [W3Se4X3(dmpe)3]+ (X = Cl, Br). The kinetics of this reaction has been studied at 25 degrees C in MeCN-H2O solution (1:1, v/v) and found to occur with two consecutive kinetic steps. The first step is independent of the nature and concentration of the X(-) anion but shows a first order dependence on the concentration of acid (k1 = 12.0 mol(-1) dm(3) s(-1)), whereas the second one is independent of the nature and concentration of both the acid and added salts (k2 = 0.024 s(-1)). In contrast, the reaction of [2]+ with acids occurs in a single step with kobs = 0.63 s(-1)(HCl) and 0.17 s(-1)(HBr). These kinetic results are discussed on the basis of the mechanism previously proposed for the reactions of the analogous [W3S4H3(dmpe)3]+ cluster, with special emphasis on the effects caused by the change of S by Se on the rate constants for the different processes involved. 相似文献
144.
Vicente M Bastida R Lodeiro C Macías A Parola AJ Valencia L Spey SE 《Inorganic chemistry》2003,42(21):6768-6779
The synthesis of a new oxaaza macrocyclic ligand, L, derived from O(1),O(7)-bis(2-formylphenyl)-1,4,7-trioxaheptane and tren containing an amine terminal pendant arm, and its metal complexation with alkaline earth (M = Ca(2+), Sr(2+), Ba(2+)), transition (M = Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+)), post-transition (M = Pb(2+)), and Y(3+) and lanthanide (M = La(3+), Er(3+)) metal ions are reported. Crystal structures of [H(2)L](ClO(4))(2).3H(2)O, [PbL](ClO(4))(2), and [ZnLCl](ClO(4)).H(2)O are also reported. In the [PbL] complex, the metal ion is located inside the macrocyclic cavity coordinated by all N(4)O(3) donor atoms while, in the [ZnLCl] complex, the metal ion is encapsulated only by the nitrogen atoms present in the ligand. pi-pi interactions in the [H(2)L](ClO(4))(2).3H(2)O and [PbL](ClO(4))(2) structures are observed. Protonation and Zn(2+), Cd(2+), and Cu(2+) complexation were studied by means of potentiometric, UV-vis, and fluorescent emission measurements. The 10-fold fluorescence emission increase observed in the pH range 7-9 in the presence of Zn(2+) leads to L as a good sensor for this biological metal in water solution. 相似文献
145.
Miguel F. Braa Jos M. Castellano Maria L. Lpez Rodriguez Magdalena Glvez Manuel R. Amil Enrique Rubio 《Journal of heterocyclic chemistry》1987,24(2):369-371
Hydrolysis of N,N'-diacyl-1,2-di(4-pyridyl)ethylenediamines 1 in aqueous sulfuric acid gave the corresponding imidazolines 3. 1,2-Di-(4-pyridyl)ethylenediamine 2 was prepared in 61 % yield by treating N,N'-di-t-butyl-oxycarbonyl-1,2-di(4-pyridyl)ethylenediamine 4 with trifluoroacetic acid or in 94% yield by the hydrolysis under basic conditions of N,N'-diphthaloylglycyl-1,2-di(4-pyridy)ethylenediamine 13. 相似文献
146.
The treatment of polybutadienes with iron carbonyls results in formation of polymers containing tricarbonyl(conjugated diene)iron units [C8H12Fe(CO)3] and also results in geometrical isomerization of free double bonds. Heating of the iron carbonyl-containing polymers gives ferromagnetic products with enhanced thermal stability. The incorporation of iron carbonyl groups into the polymer is favored by basic solvents and high temperatures, the geometrical isomerization by acidic solvents and low temperatures. Steric factors are powerful in determining the rate of isomerization. 相似文献
147.
Gabriela Flores-Ramírez Manuel Rivera Alfredo Morales-Pablos Joel Osuna Xavier Soberón Paul Gaytán 《BMC chemical biology》2007,7(1):1
Background
The effect of single and multiple amino acid substitutions in the green fluorescent protein (GFP) from Aequorea victoria has been extensively explored, yielding several proteins of diverse spectral properties. However, the role of amino acid deletions in this protein -as with most proteins- is still unknown, due to the technical difficulties involved in generating combinatorial in-phase amino acid deletions on a target region. 相似文献148.
The metal binding properties of the dinucleotide duplex, dCG x dCG, were analyzed in the gas phase with ion mobility mass spectrometry. Both MALDI and ESI were used to generate [M(dCG x dCG)]+ complexes. The collision cross section of each complex was measured in helium using ion mobility based methods and compared to calculated cross sections of theoretical structures. When metal cations classified as hard acids were combined with dCG x dCG, the [M(dCG x dCG)]+ complex organized into a globular structure. However, when soft acid metal cations were examined, a structure was observed where the two C-G base pairs were Watson-Crick bound. 相似文献
149.
Frutos LM Castano O Andres JL Merchan M Acuna AU 《The Journal of chemical physics》2004,120(3):1208-1216
Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of "nonvertical" behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion parameter gamma=(2g(2)/kappa(1))(1/2) in which g stands for the norm of the energy gradient in the PES of the acceptor triplet state and kappa(1) is a combination of vibrational force constants of the ground-state acceptor in the gradient direction. The application of the model to existing experimental data for the triplet energy transfer reaction to COT from a series of pi,pi(*) triplet donors, provides a detailed interpretation of the parameters that determine the transfer rate constant. In addition, the model shows that the observed decrease of the acceptor electronic excitation energy is due to thermal activation of C=C bond stretchings and C-C bond torsions, which collectively change the ground-state COT bent conformation (D(2d)) toward a planar triplet state (D(8h)). 相似文献
150.
A kinetic method is presented for the determination of 0.5–5 μg ml?1 gallium based on its activating effect on the copper(II)-catalyzed oxidation of 4,4′-dihydroxybenzophenone thiosemicarbazone by hydrogen peroxide. The reaction is monitored spectrophotometrically at 415 nm. Two sets of reaction conditions are established; one for the direct determination of gallium, and another, in which indium affects the gallium response, for determination of indium. Mixtures of these cations can be determined at μg ml?1 levels and in gallium/indium ratios from 7.5:1 to 1:1.6, with an accuracy and precision of ca. 4.5%. 相似文献