全文获取类型
收费全文 | 4225篇 |
免费 | 133篇 |
国内免费 | 37篇 |
专业分类
化学 | 3145篇 |
晶体学 | 34篇 |
力学 | 107篇 |
综合类 | 1篇 |
数学 | 574篇 |
物理学 | 534篇 |
出版年
2023年 | 17篇 |
2022年 | 28篇 |
2021年 | 37篇 |
2020年 | 63篇 |
2019年 | 63篇 |
2018年 | 40篇 |
2017年 | 40篇 |
2016年 | 96篇 |
2015年 | 97篇 |
2014年 | 106篇 |
2013年 | 222篇 |
2012年 | 218篇 |
2011年 | 343篇 |
2010年 | 129篇 |
2009年 | 137篇 |
2008年 | 274篇 |
2007年 | 273篇 |
2006年 | 266篇 |
2005年 | 271篇 |
2004年 | 240篇 |
2003年 | 175篇 |
2002年 | 169篇 |
2001年 | 44篇 |
2000年 | 49篇 |
1999年 | 47篇 |
1998年 | 42篇 |
1997年 | 43篇 |
1996年 | 66篇 |
1995年 | 36篇 |
1994年 | 43篇 |
1993年 | 49篇 |
1992年 | 36篇 |
1991年 | 39篇 |
1990年 | 26篇 |
1989年 | 23篇 |
1988年 | 26篇 |
1987年 | 17篇 |
1986年 | 19篇 |
1985年 | 47篇 |
1984年 | 39篇 |
1983年 | 30篇 |
1982年 | 54篇 |
1981年 | 43篇 |
1980年 | 41篇 |
1979年 | 33篇 |
1978年 | 40篇 |
1977年 | 34篇 |
1976年 | 25篇 |
1975年 | 32篇 |
1974年 | 20篇 |
排序方式: 共有4395条查询结果,搜索用时 0 毫秒
11.
Fragmentation pathways of the synthetic pyrethroid cypermethrin and four structurally related insecticides were investigated using a tandem quadrupole mass spectrometer incorporating a hexapole collision cell under positive-ion electron impact ionization conditions. Conventional mass spectrometry using the first quadrupole analyser only and tandem mass spectrometry on selected precursor ions and product ions, and also constant neutral loss scan experiments, were used. Mechanisms and fragmentation pathways are proposed to explain the inherent stability of ions associated with the benzylphenoxy portion of this class of insecticide. 相似文献
12.
Ian H. Leaver 《Journal of polymer science. Part A, Polymer chemistry》1982,20(9):2417-2427
A procedure based on diffuse reflectance spectroscopy was developed to determine the UV screening effect of a benzotriazole light stabilizer in wool fabric. Expressions that enable the protective effect of the stabilizer to be calculated directly from reflectance data were derived. The dependence of the screening effect on additive concentration was deduced from measurements of fabric reflectivity in the wavelength range of 300–400 nm. Two reflectometers with quite different optical geometries were used to measure the reflectance spectra and the merits of the two systems are discussed. A theoretical expression for the optical screening phenomenon in highly scattering materials was evaluated for substrates of different reflectivity and the general implications of this relationship are considered. 相似文献
13.
Photolysis of p-benzoquinone in N2 matrices at 12K has been found to give acetylene and CO; 1,4-naphthoquinone and 9,10-anthraquinone, however, are inert under similar conditions. 相似文献
14.
Let: E M be a fiber bundle and let be an infinitesimal Lie transformation group acting onE. We announce various new results concerning the cohomology of the invariant variational bicomplex (
*,*
(J(E)), dH, dV) and the associated invariant Euler-Lagrange complex. As one application of our general theory, we completely solve the local invariant inverse problem of the calculus of variations for finite-dimensional infinitesimal Lie transformation groups. 相似文献
15.
Summary A critical issue in drug discovery utilizing combinatorial chemistry as part of the discovery process is the choice of scaffolds to be used for a proper presentation, in a three-dimensional space, of the critical elements of structure necessary for molecular recognition (binding) and information transfer (agonist/ antagonist). In the case of polypeptide ligands, considerations related to the properties of various backbone structures (-helix, -sheets, etc.; , space) and those related to three-dimensional presentation of side-chain moieties (topography; (chi) space) must be addressed, although they often present quite different elements in the molecular recognition puzzle. We have addressed aspects of this problem by examining the three-dimensional structures of chemically different scaffolds at various distances from the scaffold to evaluate their putative diversity. We find that chemically diverse scaffolds can readily become topographically similar. We suggest a topographical approach involving design in chi space to deal with these problems. 相似文献
16.
A flashlamp-pumped tunable dye laser has been used to study the photochemical production of Br* (42P1/2) atoms from Br2 and IBr exc 相似文献
17.
Hydroxylamine derivatives of (S)-prolinol have been generated using a Cope elimination. These undergo reverse-Cope elimination onto a pendant double bond to give morpholine N-oxides containing three contiguous chiral centres. 相似文献
18.
The α,β:γ,δ-unsaturated tosylhydrazone lithium salts (4) undergo photolysis at ?60°C to give vinylcyclopropenes (10) and [l,2]diazeto[l,4-a] pyrroles (11) and/or (12). The formation of (11, X=H) establishes the intermediacy of (8, X=H) in the formation of the 3-1,2-diazepine (9). The diazoalkene (5, X=Me) showed opposite electrocyclisation periselectivity to its thermal cyclisation and gave (11, X=Me) (8, X=Me), rather than the pyrazoles (2) and (3). 相似文献
19.
3-Mercapto-2(1H)-pyridinone (1) can be synthesized in three simple high-yielding steps from readily available 2-tert-butylthiazolo[4,5-b]pyridine (2). Its disodium salt condenses with o-chloronitrobenzene, 2-chloro-3-nitropyridine, and 3-chloro-4-nitropyridine 1-oxide to give respectively 4-azaphenoxathiine (10), 1,6-diazaphenoxathiine (12), and 2,6-diazaphenoxathiine 2-oxide (14) which reduces to 2,6-diazaphenoxathiine (15). The structures of these previously unreported azaphenoxathiine systems were confirmed by assignment of their respective (13)C NMR spectra. 相似文献
20.
The constrained Hartree-Fock method using experimental X-ray diffraction data is extended and applied to the case of noncentrosymmetric molecular crystals. A new way to estimate the errors in derived properties as a derivative with respect to added Gaussian noise is also described. Three molecular crystals are examined: ammonia [NH(3)], urea [CO(NH(2))(2)], and alloxan [(CO)(4)(NH)(2)]. The energetic and electrical properties of these molecules in the crystalline state are presented. In all cases, an enhancement of the dipole moment is observed upon application of the experimental constraint. It is found that the phases of the structure factors are robustly determined by the constrained Hartree-Fock model, even in the presence of simulated noise. Plots of the electron density, electrostatic potential, and the electron localization function for the molecules in the crystal are displayed. In general, relative to the Hartree-Fock model, there is a depletion of charge around hydrogen atoms and lone pair regions, and a build-up of charge within the molecular framework near nuclei, directed along the bonds. The electron localization function plots reveal an increase in the pair density between vicinal hydrogen atoms. 相似文献