全文获取类型
收费全文 | 68218篇 |
免费 | 3435篇 |
国内免费 | 7943篇 |
专业分类
化学 | 41267篇 |
晶体学 | 863篇 |
力学 | 1032篇 |
综合类 | 403篇 |
数学 | 11322篇 |
物理学 | 9951篇 |
综合类 | 14758篇 |
出版年
2024年 | 92篇 |
2023年 | 459篇 |
2022年 | 1035篇 |
2021年 | 1019篇 |
2020年 | 1330篇 |
2019年 | 1359篇 |
2018年 | 1221篇 |
2017年 | 1780篇 |
2016年 | 1792篇 |
2015年 | 1692篇 |
2014年 | 2187篇 |
2013年 | 4385篇 |
2012年 | 4320篇 |
2011年 | 3241篇 |
2010年 | 2677篇 |
2009年 | 3841篇 |
2008年 | 3965篇 |
2007年 | 4551篇 |
2006年 | 4160篇 |
2005年 | 3744篇 |
2004年 | 3716篇 |
2003年 | 2977篇 |
2002年 | 3658篇 |
2001年 | 2542篇 |
2000年 | 2325篇 |
1999年 | 2229篇 |
1998年 | 1769篇 |
1997年 | 1494篇 |
1996年 | 1268篇 |
1995年 | 1183篇 |
1994年 | 1130篇 |
1993年 | 896篇 |
1992年 | 916篇 |
1991年 | 643篇 |
1990年 | 534篇 |
1989年 | 567篇 |
1988年 | 401篇 |
1987年 | 323篇 |
1986年 | 303篇 |
1985年 | 222篇 |
1984年 | 218篇 |
1983年 | 98篇 |
1982年 | 188篇 |
1981年 | 153篇 |
1980年 | 197篇 |
1979年 | 202篇 |
1978年 | 183篇 |
1977年 | 141篇 |
1976年 | 110篇 |
1973年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Dwayne R. Senn 《Journal of polymer science. Part A, Polymer chemistry》1994,32(6):1175-1183
Random copoly(p-phenylene sulfide sulfone/ketone)s (PPSS/K) are prepared in high yield by the polycondensation of sodium hydrosulfide (NaSH) with bis(4-chlorophenyl) sulfone (BCPS) and 4,4′-dichlorobenzophenone (DCBP). The polymerization is conducted between 200–220°C, depending on the composition of the copolymer, and in the presence of water without any detrimental effects to the molecular weight. The copolymers with sulfone/ketone mole ratios (S : K) > 25 : 75 are amorphous, while the copolymers with S : K ratios ≤ 25 : 75 are crystalline. These materials form tough, creaseable films and exhibit a linear increase in the glass transition temperature with increasing sulfone content. Sulfuric acid solutions of the copolymers are dark orange to red and display an increasing λmax in the uv-visible spectra as the S:K ratio of the copolymers decreases. © 1994 John Wiley & Sons, Inc. 相似文献
992.
H. Balard M. Sidqi E. Papirer J. B. Donnet A. Tuel H. Hommel A. P. Legrand 《Chromatographia》1988,25(8):712-716
Summary The surface properties of silicas modified with – diols having between 4 and 16 carbon atoms were investigated by inverse gas chromatography. It is shown, that the value of the dispersive component of the surface energy does not change monotonously: a minimum value is recorded when the surface is covered by a monolayer of methylene groups. Further, when measuring the adsorption enthalpies of polar probes, an alternation of their H values is observed: grafts having an odd number of carbon atoms systematically show higher H values than the others. A trans-trans configuration of the grafted chains which allows optimum interactions is proposed, suggesting a preferential diesterification reaction of both terminal hydroxyl groups of odd diols. 相似文献
993.
The polarographic behaviour of salicylaldehyde-2-pyridylhydrazone (SAPH) has been studied in aqueous buffer solution containing 40% ethanol using DC and DP polarographic methods. In the pH range 1.8–7.5 the observed single irreversible reduction wave is assigned to the splitting of the N-N bond and reduction of C=N centre. In alkaline medium, a second wave appears at a more negative potential due to the reduction of the salicylaldehyde which is formed by hydrolytic decomposition of the SAPH molecule. The effect of pH on the limiting current andE
1/2 as well as the reduction mechanism are discussed and compared with similar compounds. The kinetic parameters of the electrode reaction have been calculated.The analytical properties of the copper(II)-SAPH system is described. The complex gave rise to a single irreversible well-defined wave (E
1/2=–0.58 V at pH=5.2). The reaction process is diffusion controlled. A method is suggested for the determination of Cu(II) in presence of different metal ions as the difference in theirE
1/2 values is sufficient for the purpose.This work is taken partly from the M. Sc. Thesis of Jamal S. Shalabi 相似文献
994.
The parallel between orbital first and second electric moments and statistical first and second central moments is noted. Three measures of orbital spatial distribution in terms of their moments are proposed, and applied to the LMO's in a series of ten-electron hydrides. Consistent differences between bond and lone pair distributions are found. Using the statistical interpretation, for each LMO an effective solid angle around the central atom is postulated. 相似文献
995.
In distinction to Extended Hückel Theory which predicts as the most stable conformation of free zwitterionic GABA a totally extended form, PCILO and SCF ab initio studies show that the intrinsically preferred conformation of the isolated molecule is a highly folded one, resulting from strong interactions between the two charged ends. Computations are also carried out for hydrated GABA in the supermolecule approach allowing moreover for the flexibility of binding of some of the water molecules of the first hydration shell. They predict the coexistence in solution of a large number of conformations showing different degrees of folding (or extension), a result confirmed by recent NMR studies. This and a number of similar results show that we have to adapt our thinking on the role of conformations in pharmacological activity to this situation, which was frequently obscured by the more abundant results of X-ray crystallography yielding a single conformation. 相似文献
996.
997.
Shin-ichi Naya 《Tetrahedron》2004,60(23):4953-4958
The synthesis and properties of a novel type of 7,7-bis(heteroazulen-3-yl)-8,8-dicyano-1,4-quinodimethanes (9a-c) are studied. The synthetic method is based on a TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes with 4-(dicyanomethyl)benzaldehyde to afford the corresponding methane derivatives, followed by oxidative hydrogen abstraction with DDQ. The polarization of 9a-c is evaluated by the inspection of their 13C NMR and IR spectra. Based on the investigation of the UV-Vis spectra of 9a-c and protonated cations 10a-c, conformational changes of the heteroazulene-moiety and (dicyanomethyl)phenyl group are suggested. In the CV measurements of 9a-c, two reversible reduction waves are observed, indicating the stabilizing ability of heteroazulenes toward the corresponding radical and anion species. Furthermore, 9a-c exhibit two irreversible oxidation waves, which suggest a conformational change in the radical cation during the redox process. The conformational change is rationalized on the basis of the MO calculations. 相似文献
998.
Xiao-jun Wang Li Zhang Yibo Xu Dhileepkumar Krishnamurthy Chris H. Senanayake 《Tetrahedron letters》2004,45(38):7167-7170
A practical protocol for synthesis of 2-(N-substituted)-aminobenzimidazoles was developed. N-(2-Aminoaryl)thioureas undergo a CuCl-promoted intramolecular cyclization to give the corresponding 2-(N-substituted amino)benzimidazoles in good to excellent isolated yields. 相似文献
999.
变温ERP研究揭示氟烷基化富勒烯C_(60)(R_f)_n与四(N,N-二甲基)氨基乙烯 (TDAE)间稳定的电荷转移络合物TDAE-C_(60)(R_f)_n的居里温度高达190K[R_f = C_3F_7,H(CF_2)_4]。而非氟烷基化络合物TDAE-C_(60)的相应值仅为16.1K。 这种铁磁性可能是络合物分子中氟烷基化富勒烯多价阴离子基在相应居里温度下具 有某种有序排列的缘故。 相似文献
1000.