全文获取类型
收费全文 | 5262篇 |
免费 | 179篇 |
国内免费 | 21篇 |
专业分类
化学 | 3879篇 |
晶体学 | 47篇 |
力学 | 116篇 |
数学 | 562篇 |
物理学 | 858篇 |
出版年
2023年 | 27篇 |
2022年 | 26篇 |
2021年 | 65篇 |
2020年 | 88篇 |
2019年 | 85篇 |
2018年 | 54篇 |
2017年 | 63篇 |
2016年 | 138篇 |
2015年 | 98篇 |
2014年 | 115篇 |
2013年 | 245篇 |
2012年 | 345篇 |
2011年 | 409篇 |
2010年 | 173篇 |
2009年 | 152篇 |
2008年 | 312篇 |
2007年 | 345篇 |
2006年 | 344篇 |
2005年 | 299篇 |
2004年 | 294篇 |
2003年 | 210篇 |
2002年 | 207篇 |
2001年 | 60篇 |
2000年 | 61篇 |
1999年 | 51篇 |
1998年 | 74篇 |
1997年 | 73篇 |
1996年 | 74篇 |
1995年 | 44篇 |
1994年 | 76篇 |
1993年 | 65篇 |
1992年 | 43篇 |
1991年 | 46篇 |
1990年 | 55篇 |
1989年 | 38篇 |
1988年 | 47篇 |
1987年 | 27篇 |
1986年 | 38篇 |
1985年 | 61篇 |
1984年 | 51篇 |
1983年 | 25篇 |
1982年 | 44篇 |
1981年 | 47篇 |
1980年 | 29篇 |
1979年 | 31篇 |
1978年 | 28篇 |
1977年 | 20篇 |
1976年 | 27篇 |
1974年 | 25篇 |
1973年 | 22篇 |
排序方式: 共有5462条查询结果,搜索用时 15 毫秒
91.
James J. P. Stewart 《Journal of computational chemistry》1989,10(2):209-220
A new method for obtaining optimized parameters for semiempirical methods has been developed and applied to the modified neglect of diatomic overlap (MNDO) method. The method uses derivatives of calculated values for properties with respect to adjustable parameters to obtain the optimized values of parameters. The large increase in speed is a result of using a simple series expression for calculated values of properties rather than employing full semiempirical calculations. With this optimization procedure, the rate-determining step for parameterizing elements changes from the mechanics of parameterization to the assembling of experimental reference data. 相似文献
92.
Stewart N. Anderson David L. Hughes Raymond L. Richards 《Transition Metal Chemistry》1985,10(1):29-30
Summary The compoundtrans-[MoCl2(PMe2Ph)4] has been prepared by the reduction of MoCl5 (by Mg) or of [MoCl3(PMe2Ph)3] (by LiBun) in the presence of PMe2Ph in tetrahydrofuran (THF). It has eff=2.84 B.M. and crystallises in space group P1 witha=11.591(3),b=12.931(3),c=12.703(3) Å, = 95.28(2), =105.97(2), =103.54(2)°. Refinement of the structure gave R=0.036. The Mo-Cl and Mo-P distances average 2.443(6) and 2.534(8) Å, respectively.Low-valent phosphine complexes of the Group VI metals continue to attract much attention because of their involvement in studies of the catalytic activation of dinitrogen(1), dihydrogen(2, 3), alkenes and alkynes(4). As a by-product during our studies of dinitrogen(1) and hydride(2) complexes of molybdenum and tungsten, we obtainedtrans-[MoCl2- (PMe2Ph)4] as yellow, paramagnetic crystals (eff= 2.84 B.M.). We first obtained the compound during the attempted synthesis ofcis-[Mo(N2)2(PMe2Ph)4] by reduction of MoCl5 with Mg in the presence of PMe2Ph (see Experimental). Upon identification of the compound we found that it could be readily synthesised by treatment of [MoCl3(PMe2Ph)3](5) with LiBun in THF in the presence of PMe2Ph (experimental).The complex was shown to have thetrans structure by x-ray analysis (Figure). Analogues oftrans-[MoCl2(PMe2Ph)4] have been prepared, namely [CrCl2(Me2PCH2CH2PMe2)2](6),trans- [MoCl2(PMe3)4](7), [WCl2(PMe2Ph)4](8) and [WCl2(PMe3)4](4), of which onlytrans-[MoCl2(PMe3)4] has been examined by X-rays(7). Its principal structural parametersi.e. d(Mo-Cl)= 2.420(6), d(Mo-P)av=2.496(3) Å(6) are close to those found here fortrans-[MoCl2(PMe2Ph)4]. 相似文献
93.
94.
In situ derivatization of a variety of alpha-substituted aldehydes via reaction with chiral amines allows convenient and efficient determination of enantiomeric excess. (1)H NMR analysis of the imine diastereomer ratio can be conducted immediately after the aldehyde and amine have been mixed. The results correlate well with ee values determined by more traditional (and slower) methods. This approach may be broadly applicable to alpha-substituted aldehydes. [reaction: see text] 相似文献
95.
Complete basis set extrapolations of alkali halide (LiF, LiCl, NaF, NaCl) energy, dipole, and polarizability surfaces are performed with and without applied fields along the internuclear axis using state-averaged multireference configuration interaction. Comparison between properties (equilibrium separation, dissociation energy, crossing distance, diabatic coupling constant, dipole, and polarizability) derived from the extrapolated potential energy (or dipole) surfaces are made with those obtained from direct extrapolation from the basis set trends. The two extrapolation procedures are generally found to agree well for these systems. Crossing distances from this work are compared to those of previous work and values obtained from the Rittner potential. Complete basis set extrapolated crossing distances agree well with those derived from the Rittner potential for LiF, but were significantly larger for LiCl, NaF, and NaCl. The results presented here serve as an important set of benchmark data for the development of new-generation many-body force fields that are able to model charge transfer. 相似文献
96.
Polytriazolylamines were synthesized by the copper(I)-catalyzed ligation of azides and alkynes. The C3-symmetric derivative, TBTA, was shown to be a powerful stabilizing ligand for copper(I), protecting it from oxidation and disproportionation, while enhancing its catalytic activity. 相似文献
97.
Two highly emissive conjugated polymers with tethered rotaxane repeat units are reported. Hydrogen bonding between acidic alcohols and the N-heteroaromatic groups in the rotaxanes attenuates polymer fluorescence. In addition, the rotaxane groups create precise three-dimensional pockets for metal binding, which results in fluorescence quenching. Exposing thin films of Zn-doped polymers to alcohol vapors reverses the quenching by up to 25%. 相似文献
98.
Turney K Drake TJ Smith JE Tan W Harrison WW 《Rapid communications in mass spectrometry : RCM》2004,18(20):2367-2374
Nanoparticles for the extraction of peptides and subsequent analysis using atmospheric pressure matrix-assisted laser desorption/ionization (APMALDI) have been evaluated. The atmospheric pressure source allows for particles to be directly introduced in the liquid matrix, minimizing sample loss and analysis time. Described in this work are two sample preparation procedures for liquid APMALDI analysis: a C18 functionalized silica nanoparticle for hydrophobic extractions, and an aptamer functionalized magnetite core nanoparticle for rapid, affinity extractions. The C18 particles provide a non-selective support for rapid profiling applications, while the aptamer particles are directed towards reducing the complexity in biological samples. The aptamer functionalized particles provide a more selective analyte-nanoparticle interaction whereby the tertiary structure of the analyte becomes more critical to the extraction. In both cases, the liquid APMALDI matrix provides a support for ionization, and acts as the releasing agent for the analyte-particle interaction. Additionally, analyte enrichment was possible due to the large surface-to-volume ratio of the particles. The experiments conducted with functionalized nanoparticles, in an atmospheric pressure liquid matrix, present a basis for further methodologies and utilities of silica nanoparticles to be developed. 相似文献
99.
The source of the effect of N-alkylation on the redox properties of Ni(II/I) and Cr(III/II) cyclam complexes has been investigated using DFT calculations. The structures of the anhydrous and hydrated complexes were optimized in the gas phase, and single point calculations were performed in a polarized continuum. The main results are the following: the decrease in outer sphere solvation upon N-alkylation is the major source of the relative stabilization of the lower oxidation state complexes by the tertiary amine ligands; tertiary amine nitrogen donors are stronger sigma-donors than the secondary amines, as predicted from the inductive effect of alkyls; steric strain elongates the metal-nitrogen bonds in the tertiary complexes and decreases the ligand strain energies; and the site of water binding to the complexes differs because of their different electronic structures (i.e., in the Ni complexes, the water molecules bind to the M[bond]N[bond]H sites, whereas in the Cr complexes they bind to the central metal cation). Outer sphere hydrogen bonding of water to the ligands in the coordination sphere lowers the ionization potentials by charge delocalization. 相似文献
100.
Stewart ED 《Physical review D: Particles and fields》1995,51(12):6847-6853