全文获取类型
收费全文 | 9688篇 |
免费 | 455篇 |
国内免费 | 21篇 |
专业分类
化学 | 6835篇 |
晶体学 | 130篇 |
力学 | 183篇 |
数学 | 1121篇 |
物理学 | 1895篇 |
出版年
2023年 | 69篇 |
2022年 | 101篇 |
2021年 | 130篇 |
2020年 | 241篇 |
2019年 | 221篇 |
2018年 | 103篇 |
2017年 | 120篇 |
2016年 | 333篇 |
2015年 | 312篇 |
2014年 | 365篇 |
2013年 | 451篇 |
2012年 | 582篇 |
2011年 | 675篇 |
2010年 | 394篇 |
2009年 | 342篇 |
2008年 | 504篇 |
2007年 | 471篇 |
2006年 | 448篇 |
2005年 | 366篇 |
2004年 | 358篇 |
2003年 | 300篇 |
2002年 | 283篇 |
2001年 | 178篇 |
2000年 | 173篇 |
1999年 | 129篇 |
1998年 | 113篇 |
1997年 | 128篇 |
1996年 | 127篇 |
1995年 | 124篇 |
1994年 | 133篇 |
1993年 | 105篇 |
1992年 | 98篇 |
1991年 | 90篇 |
1990年 | 79篇 |
1989年 | 96篇 |
1988年 | 85篇 |
1987年 | 78篇 |
1986年 | 66篇 |
1985年 | 52篇 |
1984年 | 40篇 |
1983年 | 54篇 |
1982年 | 44篇 |
1981年 | 73篇 |
1980年 | 60篇 |
1979年 | 67篇 |
1978年 | 75篇 |
1977年 | 57篇 |
1976年 | 52篇 |
1974年 | 34篇 |
1973年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
103.
104.
[reaction: see text] A novel cycloaldol approach to the isobenzofuran core common to many of the eunicellin diterpenes is described. The cycloaldol precursor was prepared by aldol addition of (S)-(+)-carvone and methacrolein followed by etherification to a glycolate ester. Chemoselective enolization of the glycolate ester led to the cycloaldol adduct in high yield and diastereoselectivity. An oxidative rearrangement-allylic diazene rearrangement sequence established the requisite cis ring fusion. 相似文献
105.
Russell SC Czerwieniec G Lebrilla C Tobias H Fergenson DP Steele P Pitesky M Horn J Srivastava A Frank M Gard EE 《Journal of the American Society for Mass Spectrometry》2004,15(6):900-909
The appearance of informative signals in the mass spectra of laser-ablated bio-aerosol particles depends on the effective ionization probabilities (EIP) of individual components during the laser ionization process. This study investigates how bio-aerosol chemical composition governs the EIP values of specific components and the overall features of the spectra from the bio-aerosol mass spectrometry (BAMS). EIP values were determined for a series of amino acid, dipicolinic acid, and peptide aerosol particles to determine what chemical features aid in ionization. The spectra of individual amino acids and dipicolinic acid, as well as mixtures, were examined for extent of fragmentation and the presence of molecular ion dimers, which are indicative of ionization conditions. Standard mixtures yielded information with respect to the significance of secondary ion plume reactions on observed spectra. A greater understanding of how these parameters affect EIP and spectra characteristics of bio-aerosols will aid in the intelligent selection of viable future biomarkers for the identification of bio-terrorism agents. 相似文献
106.
The isotopic exchange of35S between thiosulfate/sulfite and tetrathionate/thiosulfate is examined. The kinetic data are determined. In both cases the
reactions are of second order. As the isotopic exchange between tetrathionate/thiosulfate is very fast, these reaction components
are used in high dilution. Furthermore, the extent of exchange during the separation is determined.
相似文献
107.
By injection of the proton bound homodimer [DMF.H+.DMF] of N,N-dimethylformamide (DMF) generated in an external ion source into a mixture of DMF and a second base within the cell of a Fourier transform ion cyclotron resonance (FT-ICR) spectrometer the equilibria between [DMF.H+.DMF] and the other possible proton bound dimers [DMF.H+.base] and [base.H+.base] have been studied for 13 different bases. Strongly polar bases like aliphatic amides and dimethyl sulfoxide (DMSO) exchange both DMF in [DMF.H+.DMF] by a two step process, while the almost non-polar amines exchange only one DMF. If the base is a primary or secondary amine, the proton bound heterodimer [DMF.H+.amine] reacts further by the addition of one DMF to create a proton bound trimer [(DMF)2.H+.amine]. The affinity deltaG(DMFH+) of the bases towards protonated DMF relative to neutral DMF depends linearly on the difference deltaGB of the gas phase basicity of DMF and the other base, but different correlation lines are obtained for polar and non-polar ligands (deltaGDMFH+ = 0.44GB(base)-375 [kJ/mol] (r = 0.97) and deltaGDMFH+ = 0.46GB(base)-397 [kJ/mol] (r = 0.99), respectively). This different behavior is explained by a different character of the proton bridge in the heterodimers containing only polar ligands and those incorporating a non-polar ligand besides DMF. The former dimers contain a more or less symmetric proton bridge while the latter can be viewed as a protonated base solvated by DMF. The available data have been used to calculate the molecular pair gas phase basicity of DMF and the 13 bases used and to estimate the dissociation energies of the bonds of the proton bridge in various proton bound heterodimers. 相似文献
108.
Computerized quantification of components under overlapping chromatographic peaks is done by calibration of chromatograms against component mixtures. For conventional (single-channel) detectors, the limitations of earlier methods based on ordinary multiple regression, can be circumvented by data reduction with the aid of principal component analysis with the partial least-squares approach. Simulation studies show that the method can be applied even when there is severe peak overlap, unstable baseline, noisy chromatograms or non-linear detector response. Advantages in the quantification of fused peaks by means of multichannel detectors are outlined. Present limitations on the quantitative evaluation of several overlapping component peaks from a single spectro-chromatogram by means of the partial least-squares method combined with multiple regression on the pure component spectra, are discussed with respect to practical high-performance liquid chromatography. 相似文献
109.
A short synthesis of the chiral oxazoline 10 (>95% e.e.) in six steps from chloroacetylferrocene is described. The ligand can be used successfully in an asymmetric Pd(0)-catalyzed allylic alkylation reaction. 相似文献
110.
Won Sick Woo Sam Sik Kang Otto Seligmann V. Mohan Chari Hildebert Wagner 《Tetrahedron letters》1980,21(44):4255-4258
From the seeds of Phytolacca americana two new lignans, Americanin B and D, have been isolated and their structures elucidated mainly by spectroscopic methods 相似文献