全文获取类型
收费全文 | 26694篇 |
免费 | 1180篇 |
国内免费 | 3154篇 |
专业分类
化学 | 19048篇 |
晶体学 | 150篇 |
力学 | 936篇 |
综合类 | 60篇 |
数学 | 3908篇 |
物理学 | 6926篇 |
出版年
2023年 | 131篇 |
2022年 | 381篇 |
2021年 | 446篇 |
2020年 | 306篇 |
2019年 | 352篇 |
2018年 | 287篇 |
2017年 | 282篇 |
2016年 | 674篇 |
2015年 | 627篇 |
2014年 | 634篇 |
2013年 | 1472篇 |
2012年 | 1573篇 |
2011年 | 2818篇 |
2010年 | 1557篇 |
2009年 | 1646篇 |
2008年 | 1260篇 |
2007年 | 1213篇 |
2006年 | 1130篇 |
2005年 | 1404篇 |
2004年 | 2479篇 |
2003年 | 1604篇 |
2002年 | 1430篇 |
2001年 | 956篇 |
2000年 | 540篇 |
1999年 | 475篇 |
1998年 | 491篇 |
1997年 | 378篇 |
1996年 | 329篇 |
1995年 | 296篇 |
1994年 | 229篇 |
1993年 | 577篇 |
1992年 | 602篇 |
1991年 | 381篇 |
1990年 | 400篇 |
1989年 | 368篇 |
1988年 | 138篇 |
1987年 | 72篇 |
1986年 | 95篇 |
1985年 | 119篇 |
1984年 | 121篇 |
1983年 | 84篇 |
1982年 | 77篇 |
1981年 | 70篇 |
1980年 | 60篇 |
1979年 | 42篇 |
1978年 | 48篇 |
1977年 | 44篇 |
1976年 | 35篇 |
1975年 | 35篇 |
1974年 | 36篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
Víctor Yañez Rodríguez Miguel Ángel del Águila María José Iglesias Fernando López Ortiz 《Tetrahedron》2012,68(36):7355-7362
Directed ortho lithiation of diphenylphosphinic acid and subsequent electrophilic trapping provides mono ortho-functionalized derivatives including enantiopure γ-aminophosphinic acids in moderate yields. Copper catalyzed coupling of the ortho anion leads to biphenyl-2,2′-diylbis(phenylphosphinic acid), a phosphorus analogue of biphenyl-2,2′-dicarboxylic acid. Preliminary studies of the metal-binding abilities of this O,O-chelating ligand towards a series of metal cations are included. 相似文献
132.
133.
134.
R. S. Corrêa S. R. Souza e Silva L. P. Duarte G. D. F. Silva L. C. A. Barbosa J. Ellena A. C. Doriguetto 《Journal of Structural Chemistry》2012,53(1):156-163
The structural study of two (C30H48O2) pentacyclic triterpene (PCTT) isomers is presented. These terpenes, known as 30-hydroxy-lup-20(29)-en-3-one (1) and (11α)-11-hydroxy-lup-20(29)-en-3-one (2), were isolated from Maytenus imbricata Mart. Ex Reissek (Celastraceae). The molecular structure of 1 and 2 differs in the position of the hydroxyl group. Both compounds crystallize in non-centrosymmetric space groups with two molecules in the asymmetric unit. The crystal structure of 1 shows a triclinic P1 space group (a = 9.5518(1) ?, b = 9.7083(1) ?, c = 14.4696(2) ?, α = 93.832(1)°, β = 102.833(1)°, and γ = 103.307(1)°), while compound 2 crystallizes in a monoclinic P21 one (a = 13.4439(16) ?, b = 14.4463(14) ?, c = 13.5224(9) ? and β = 99.703(8)°). The two molecules independent by symmetry of 1 differ slightly due to the presence of static disorder in oxygen atoms. In addition, the intermolecular geometries of 1 and 2 were analysed, and in each isomer the crystal packing is stabilized by O-H…O intermolecular hydrogen bonds and van der Waals forces. 相似文献
135.
C. Lavilla A. Alla A. Martínez de Ilarduya E. Benito M.G. García‐Martín J.A. Galbis S. Muñoz‐Guerra 《Journal of polymer science. Part A, Polymer chemistry》2012,50(16):3393-3406
Random poly(hexamethylene terephthalate‐co‐galactarate)s and poly(dodecamethylene terephthalate‐co‐galactarate)s copolyesters covering the whole range of compositions were obtained with weight‐average molecular weights of ~30,000–50,000 g mol?1 by melt polycondensation. They were thermally stable above 300 °C, and displayed Tg in the +20 to ?20 °C range with values steadily decreasing with the content in galactarate units. All the copolyesters were semicrystalline with Tm between 50 and 150 °C and those made from dodecanediol were able to crystallize from the melt at a crystallization rate depending on composition. Copolyesters containing up to 50% of galactaric units retained the crystal structure of their respective polyterephthalate homopolyesters, whereas they adopted the structure of the respective polygalactarates when the content in Galx units reached 70%. Stress‐strain essays revealed decay in the mechanical parameters as the aromatic units were replaced by Galx. Incubation in aqueous buffer revealed that hydrolysis of the polyesters were largely enhanced by copolymerization and evidenced the capacity of the Galx unit for making aromatic polyesters susceptible to biodegradation. A detailed NMR analysis complemented by SEM observations indicated that hydrolysis took place by preferred splitting of galactarate ester bonds with releasing of alkanediol and Galx‐diacid. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
136.
137.
In order to advance the knowledge of prototropic tautomerism from the physicochemical point of view, the purine derivative hypoxanthine has been selected and studied. The overall purpose has been to explore thermodynamic aspects of the heterocycle tautomerism under the influence of both its protonation level and the surrounding dielectric constant. A Density Functional Theory study (at the B3LYP/6-31++G** level) was performed, in which the energetic and thermodynamic stabilities, the electric dipole moment values, the tautomeric equilibrium constants and the tautomeric populations were obtained for several hypoxanthine tautomers under systematically modified heterocyclic protonation levels, considering both isolated and ideal aqueous solution states. Among the interesting results obtained are changes in the tautomeric populations for several heterocyclic protonation states and with the increase of the dielectric constant. Several of the predictions made for an aqueous solution show good agreement with recently reported experimental conclusions. Also, the ionizable groups that contribute to the different hypoxanthine ionization steps in the main tautomers have been established. These and other related results are presented and discussed. Finally, the confidence developed in the predicted tautomeric populations in a modeled-ideal aqueous solution allows us to propose that the methodology applied here can be used for the study of prototropic tautomerism in heterocycles belonging to this class, particularly when the experimental work is challenging in both performance and physicochemical data analysis. 相似文献
138.
139.
140.
为了测量黑腔靶X光引光效率、转换效率,我们设计了相应的分解靶(泄漏靶)实验,1989~1992年在″神光″装置上共进行了四次漏靶分解实验,利用灵敏度作了绝对标定的平响应X光二极管对漏靶注入孔、引光孔流出的X光角分布进行了测量,测出了x光角分布,得到了黑腔靶X光转换效率为50%~60%,引光效率约6%,为以后辐射驱动内爆研究的理论计算和靶的优化设计提供了重要的数据。 相似文献