全文获取类型
收费全文 | 16901篇 |
免费 | 391篇 |
国内免费 | 142篇 |
专业分类
化学 | 11578篇 |
晶体学 | 136篇 |
力学 | 533篇 |
数学 | 2578篇 |
物理学 | 2609篇 |
出版年
2022年 | 138篇 |
2021年 | 152篇 |
2020年 | 220篇 |
2019年 | 207篇 |
2018年 | 187篇 |
2017年 | 152篇 |
2016年 | 319篇 |
2015年 | 275篇 |
2014年 | 324篇 |
2013年 | 802篇 |
2012年 | 784篇 |
2011年 | 1063篇 |
2010年 | 495篇 |
2009年 | 448篇 |
2008年 | 853篇 |
2007年 | 903篇 |
2006年 | 947篇 |
2005年 | 874篇 |
2004年 | 800篇 |
2003年 | 722篇 |
2002年 | 696篇 |
2001年 | 234篇 |
2000年 | 211篇 |
1999年 | 168篇 |
1998年 | 193篇 |
1997年 | 206篇 |
1996年 | 236篇 |
1995年 | 182篇 |
1994年 | 187篇 |
1993年 | 153篇 |
1992年 | 158篇 |
1991年 | 159篇 |
1990年 | 138篇 |
1989年 | 118篇 |
1988年 | 154篇 |
1987年 | 149篇 |
1986年 | 112篇 |
1985年 | 222篇 |
1984年 | 249篇 |
1983年 | 178篇 |
1982年 | 248篇 |
1981年 | 233篇 |
1980年 | 226篇 |
1979年 | 207篇 |
1978年 | 204篇 |
1977年 | 187篇 |
1976年 | 194篇 |
1975年 | 158篇 |
1974年 | 171篇 |
1973年 | 163篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
James H. Brewster 《Tetrahedron》1974,30(13):1807-1818
An expression for potential energy as a function of torsion angle about a saturated CC bond is developed analytically. This equation is used in the development of two methods for the conformational analysis of paraffins-one based on consideration only of exactly staggered conformers and one giving consideration to energy at 1° intervals of torsion angle. Two methods of assigning molecular rotations to hydrocarbon conformers are considered. Various combinations of results so obtained are compared as ways to calculate rotations for paraffins. 相似文献
32.
Wuest JD 《Chemical communications (Cambridge, England)》2005,(47):5830-5837
Detailed structures of molecular crystals cannot yet be predicted with consistent accuracy, but the strategy of molecular tectonics offers crystal engineers a powerful tool for designing molecules that are predisposed to form crystals with particular structural features and properties. 相似文献
33.
James Y. Becker Larry L. Miller Frank R. Stermitz 《Journal of Electroanalytical Chemistry》1976,68(2):181-191
The electrochemical oxidation of the alkaloid laudanosine (Ia) to O-methylflavinantine (II) has been studied in acetonitrile solvent. Using cyclic voltammetry, rotating disc voltammetry and preparative electrolyses on several alkaloids, simple aliphatic amines and aromatic compounds, some aspects of the mechanism of this coupling reaction are elucidated. The first anodic wave for laudanosine at platinum has Ep=0.55 V vs. Ag/Ag+. The electrode rapidly becomes partially passivated at potentials above 0.5 V. This is due to a film which “dissolves” below 0.5 V, at a rate independent of the potential. It is shown that the reaction (Ia)→(II) proceeds at 0.5 V by initial oxidation of the amine moiety. If acids such as sodium bicarbonate are added to the anolyte the amine is protonated causing the first wave to disappear. Oxidation at 1.1 V under these acidic conditions produces the same product, but more rapidly and in significantly higher yield because electrode filming and side reactions resulting from the amine oxidation are abrogated. 相似文献
34.
James L. Charlton 《Tetrahedron letters》1985,26(29):3413-3416
Asymmetric inductive effects have been measured on the Diels-Alder reaction of dimethyl fumarate with o-quinodimethane bearing a chiral α-alkoxy group. The chiral substituents used were 1-phenylethoxy, 2-(1-phenyl)propoxy, 1-(2-phenyl)propoxy, 2-(4-phenyl)-butoxy and 1-cyclohexylethoxy. The greatest asymmetric induction was found with the first of these chiral substituents (47% ee). A π-stacking effect, previously suggested as the rationale for asymmetric induction in a similar system, is shown to be inconsistent with the results from this study. 相似文献
35.
A comparison of some results from Xα-scattered wave (Xα-SW) and extended Hückel (EH) calculations for metal clusters is given. It is found that small clusters of atoms (≈ 13 atoms) using the Xα-SW method reproduce many of the features of the electronic structure of the bulk metals, whereas this is not the case for the same clusters using the EH method. A more systematic approach to EH parametrizations is suggested in order to make this method a more viable approach to treating metal clusters. 相似文献
36.
Kazimiera J. L. Paciorek James H. Nakahara Reinhold H. Rratzer 《Journal of mass spectrometry : JMS》1976,11(11):1217-1219
Electron impact fragmentation patterns were obtained for perfluoroalkylether nitriles, C,F,O[CF(CF3)CFIO]ICF(CF3)CN (x = 1 and 2), perfluoroalkylether acyl halides, CSF,OCF(CF,)-CF2OCF(CF3)COX (X = F and Cl), n-perfluorooctanonitrile and n-perfiuorooctanoyl chloride. The perfluoroalkyl and perfluoroalkylether nitriles afforded ions characteristic of the nitrile function. The major fragment from the acyl chlorides was the [COClJ]+ ion; the presence of chlorine was evidenced also by rearrangement ions of the general form [RfCl] t. The perfluoroalkylether compounds appeared to undergo a typical fragmentation governed by the cleavage of the carbon-oxygen bond. 相似文献
37.
A procedure is described for fitting tabulated theoretical current functions to linear-sweep voltammograms. The method, suitable for a small microcomputer, employs linear extrapolation between current function values, and non-linear regression for minimizing deviations between experimental and theoretical currents. Examples of its use for confirmation of simple electrode reaction mechanisms and estimation of electrochemical parameters are described. 相似文献
38.
Wei Q Nieuwenhuyzen M Meunier F Hardacre C James SL 《Dalton transactions (Cambridge, England : 2003)》2004,(12):1807-1811
The metal-organic framework [Co(INA)(2)]x0.5EtOH (INA = isonicotinate, NC(5)H(4)-4-CO(2)(-)), was synthesised under solvothermal conditions. Its X-ray crystal structure shows channels containing ethanol guests which are hydrogen-bonded to carboxylate oxygens of the framework. The pyridyl rings of the framework alternate between 'open' and 'closed' positions along the channels resulting in large variation in the channel cross-sectional area from ca. 1.4 by 2.3 A at the narrowest point to 4.9 by 5.3 A at the widest. Despite the very small windows, the ethanol guests (of van der Waals diameter ca. 4.2-6.1 A) may be reversibly desorbed and sorbed into the structure quantitatively, as shown by in situ variable-temperture IR spectroscopy and XRPD. The single-crystal structure of the desolvated form [Co(INA)(2)] shows that there is no change in the overall connectivity on desolvation, but the rotational positions of the pyridine rings are altered. This suggests that pyridyl rotation may occur to allow guests to pass in and out. When the synthesis was conducted in 1-propanol solvent [Co(INA)(2)]x0.5Pr(n)OHxH(2)O, was obtained, and a single-crystal X-ray structure revealed the same overall connectivity as in but with pyridine rings disordered over closed and open positions. There was no evidence of included guests from X-ray crystallography, suggesting that they are also highly disordered. Variable-temperature XRPD performed on bulk samples showed peaks which were unsymmetrical and exhibited shoulders, suggesting that for each pattern obtained the material actually consisted of several closely-related phases. The movements of the peaks during desolvation showed the presence of intermediate phases before the final desolvated product was formed. The peak positions of the intermediate phases matched more closely with the calculated pattern for than with or, suggesting that they may have disordered structures similar to. The results also suggest that the intermediate phase represents an initial increase in volume before a larger decrease in volume occurs to give the final desolvated material. 相似文献
39.
A computational study on the intermolecular reaction of protonated oxirane with methylpropene, as a model for initiation of oxidosqualene cyclisation, shows that the SN2-like ring opening is strongly exothermic with a low barrier to reaction and establishes the geometry of the intermolecular reaction. 相似文献
40.
James C. Tou 《Journal of mass spectrometry : JMS》1972,6(8):833-841
The EI Mass spectra of the polyfunctional pentaerythritol derivatives show that the molecular ions [M]+· exhibit extensive ion fragmentation. No Information about [M]+·. can be obtained. In Contrast to this, the FI Mass spectra of these compounds show intense [M]+· and/or [M + 1]+, and a characteristic ion at m/e 31, which is assumed to be the oxonium ion \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_2 = \mathop {\rm O}\limits^ + {\rm H} $\end{document}. Because of surface adsorption and field attraction, FI mass spectrometry presents a serious problem in quantitative analysis of a mixture containing compounds with quite different degrees of polarization. 相似文献